Currency metabolites vs. current metabolites: What's the right term?

Currency metabolites vs. current metabolites: What's the right term?

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I have seen the two termscurrency metaboliteandcurrent metaboliteused interchangeably. Is there a consensus on which is the right term?

Currency refers to anything that is used in any circumstances, as a medium of exchange [1]. That's why ATP and NAD are considered as currency metabolites [2]. So currency metabolite can be used for any metabolite that has multiple uses in different reactions/pathways.

Current refers to something that is prevalent at the present time [3]. So current metabolite is used to refer a metabolite that is already available for a specific reaction/pathway and doesn't need to undergo supplementary transformations to fit for that specific reaction.

The same metabolite can be simultaneously a currency metabolite and a current metabolite for a specific situation.


  1. Wikipedia contributors, "Currency," Wikipedia, The Free Encyclopedia, (accessed July 12, 2014).
  2. Gerlee P, Lizana L, Sneppen K. Pathway identification by network pruning in the metabolic network of Escherichia coli. Bioinformatics. 2009 Dec 15;25(24):3282-8. doi: 10.1093/bioinformatics/btp575. PubMed PMID: 19808881.
  3. The Free Dictionary by Farlex.

A ‘rule of 0.5’ for the metabolite-likeness of approved pharmaceutical drugs

We exploit the recent availability of a community reconstruction of the human metabolic network (‘Recon2’) to study how close in structural terms are marketed drugs to the nearest known metabolite(s) that Recon2 contains. While other encodings using different kinds of chemical fingerprints give greater differences, we find using the 166 Public MDL Molecular Access (MACCS) keys that 90 % of marketed drugs have a Tanimoto similarity of more than 0.5 to the (structurally) ‘nearest’ human metabolite. This suggests a ‘rule of 0.5’ mnemonic for assessing the metabolite-like properties that characterise successful, marketed drugs. Multiobjective clustering leads to a similar conclusion, while artificial (synthetic) structures are seen to be less human-metabolite-like. This ‘rule of 0.5’ may have considerable predictive value in chemical biology and drug discovery, and may represent a powerful filter for decision making processes.


The prevalence of overweight and obesity has risen dramatically over the past 3 decades and is threatening to become a global epidemic ( 1). A substantial proportion of the population is at increased risk of morbidity and mortality as a result of increased body weight. In affluent countries, excess body fat accounts for ≈30–40% of coronary heart disease ( 2) cancers of the colon, breast, and endometrium and most cases of type 2 diabetes ( 3). Genetic susceptibility predisposes people to the development of body fatness but cannot account for the exponential increase in obesity in nearly all Western countries. Obesity is generally accepted as resulting from an imbalance between food intake and daily physical activity. Obesity is thus the largest nutrition-related problem in the developed world. Despite the overwhelming amount of research and statistical analysis, no clear explanation can be given for the relation between changes in behavior and the rapid increase in obesity prevalence in the past 3 decades.

Health guidelines have been focused on 3 particular lifestyle factors: increased levels of daily physical activity and reductions in the intakes of fat and sugars, particularly added sugars. The urgency of taking public action regarding physical activity is generally accepted, but there is much debate about dietary factors, such as total fat intake, intake of sugars, and intake of rapidly digested carbohydrates. In the 1970s, some nutritionists considered sucrose, particularly added sucrose, as perhaps the most important dietary factor predisposing to weight gain ( 4). Since then, attention has shifted toward fat as the major nutritional component promoting excess energy intake and weight gain ( 5, 6). Evidence that the regulation of fat balance has a lower priority than the regulation of the intakes of carbohydrates, protein, and alcohol has contributed to the general knowledge that fat intake increases the risk of excess energy intake and the promotion of fat storage ( 7). Furthermore, data from national food surveys indicate a pronounced shift in the fat-carbohydrate ratio toward a fattier diet ( 8).

Despite the controversy about the particular role of sugars, the message that fat in the diet is responsible for excess energy intake and weight gain became stronger. As a consequence of recommendations to reduce fat intake, the market for low-fat food expanded rapidly in the 1990s ( 9). The actual intake of fat expressed as a percentage of energy, based on the subject's self-recordings, has decreased significantly over the past decade ( 10). The reduction in absolute fat intake was substantially less. Although a number of meta-analyses on the relation between ad libitum low-fat diets and body weight control showed that dietary fat intake is directly associated with obesity ( 11, 12), the scientific evidence for the relation between dietary fat content and the prevalence of obesity has also been challenged. For example, Katan et al ( 13) questioned the importance of low-fat, high-carbohydrate diets in the prevention and treatment of obesity. Reduction of fat intake resulted in a reduction of only a few kilograms of body weight.

Another important argument concerns the so-called fat paradox ( 14). With the increasing popularity of low-fat products, food intake statistics have shown a decrease in dietary fat intake, although the prevalence of obesity is rising. A direct relation between dietary fat and energy density was also questioned because of the observation that many currently available low-fat foods are based on sugars, and thus they have energy density values similar to those of their high-fat counterparts ( 14). This has renewed interest in sugars as the primary nutritional factor behind the increase in obesity. Many refined carbohydrate foods produce a high glycemic response, thereby promoting postprandial carbohydrate oxidation at the expense of fat oxidation and thus altering fuel partitioning in a way that may be conductive to body fat gain ( 15). This is in contrast to foods that produce a low glycemic response and lower postprandial insulin secretion.

This review examines the role of sugars, particularly sucrose, as dietary factors in relation to body weight control and compares the role of sugars with that of the other important dietary factor, fat.

Metabolomics is the qualitative and quantitative assessment of the metabolites (small molecules < 1.5 kDa) in body fluids. The metabolites are the downstream of the genetic transcription and translation processes and also downstream of the interactions with environmental exposures thus, they are thought to closely relate to the phenotype, especially for multifactorial diseases. In the last decade, metabolomics has been increasingly used to identify biomarkers in disease, and it is currently recognized as a very powerful tool with great potential for clinical translation. The metabolome and the associated pathways also help improve our understanding of the pathophysiology and mechanisms of disease.

While there has been increasing interest and research in metabolomics of the eye, the application of metabolomics to retinal diseases has been limited, even though these are leading causes of blindness. In this manuscript, we perform a comprehensive summary of the tools and knowledge required to perform a metabolomics study, and we highlight essential statistical methods for rigorous study design and data analysis. We review available protocols, summarize the best approaches, and address the current unmet need for information on collection and processing of tissues and biofluids that can be used for metabolomics of retinal diseases. Additionally, we critically analyze recent work in this field, both in animal models and in human clinical disease, including diabetic retinopathy and age-related macular degeneration. Finally, we identify opportunities for future research applying metabolomics to improve our current assessment and understanding of mechanisms of vitreoretinal diseases, and to hence improve patient assessment and care.

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Section II: Effects of diet on the gut microbiome

The link between diet and microbial diversity

Another putative mechanistic pathway of how plant-based diets can affect health may involve the gut microbiome which has increasingly received scientific and popular interest, lastly not only through initiatives such as the Human Microbiome Project 102 . A common measure for characterizing the gut community is enterotyping, which is a way to stratify individuals according to their gut bacterial diversity, by calculating the ratio between bacterial genera, such as Prevotella and Bacteroides 103 . While interventional controlled trials are still scarce, this ratio has been shown to be conclusive for differentiating plant-based from animal-based microbial profiles 36 . Specifically, in a sample of 98 individuals, Wu et al. 36 found that a diet high in protein and animal fats was related to more Bacteroides, whereas a diet high in carbohydrates, representing a plant-based one, was associated with more Prevotella. Moreover, the authors showed that a change in diet to high-fat/low-fiber or to low-fat/high-fiber in ten individuals elicited a change in gut microbial enterotype with a time delay of 24 h only and remained stable over 10 days, however not being able to switch completely to another enterotype 36 . Another strictly controlled 30-day cross-over interventional study showed that a change in diet to either an exclusively animal-based or plant-based diet promoted gut microbiota diversity and genetic expression to change within 5 days 35 . Particularly, in response to adopting an animal-based diet, microbial diversity increased rapidly, even overshadowing individual microbial gene expression. Beyond large shifts in overall diet, already modest dietary modifications such as the daily consumption of 43 g of walnuts, were able to promote probiotic- and butyric acid-producing bacterial species in two RCTs, after 3 and 8 weeks respectively 104,105 , highlighting the high adaptability of the gut microbiome to dietary components. The Prevotella to Bacteroides ratio (P/B) has been shown to be involved in the success of dietary interventions targeting weight loss, with larger weight loss in high P/B compared to low P/B in a 6-month whole-grain diet compared to a conventional diet 106 . Only recently, other microbial communities, such as the salivary microbiome, have been shown to be different between omnivores and vegan dieters 107 , opening new avenues for research on adaptable mechanisms related to dietary intake.

A continuum in microbial diversity dependent on diet

Plant-based diets are supposed to be linked to a specific microbial profile, with a vegan profile being most different from an omnivore, but not always different from a vegetarian profile (reviewed in ref. 15 ). Some specifically vegan gut microbial characteristics have also been found in a small sample of six obese subjects after 1 month following a vegetarian diet, namely less pathobionts, more protective bacterial species improving lipid metabolism and a reduced level of intestinal inflammation 108 . Investigating long-term dietary patterns a study found a dose-dependent effect for altered gut microbiota in vegetarians and vegans compared to omnivores depending on the quantity of animal products 109 . The authors showed that gut microbial profiles of plant-based diets feature the same total number but lower counts of Bacteroides, Bifidobacterium, E. coli and Enterobacteriaceae compared to omnivores, with the biggest difference to vegans. Still today it remains unclear, what this shift in bacterial composition means in functional terms, prompting the field to develop more functional analyses.

In a 30-day intervention study, David et al. found that fermentation processes linked to fat and carbohydrate decomposition were related to the abundance of certain microbial species 35 . They found a strong correlation between fiber intake and Prevotella abundance in the microbial gut. More recently, Prevotella has been associated with plant-based diets 110 that are comparable to low-fat/high-fiber diets 111 and might be linked to the increased synthesis of short-chain fatty acids (SCFA) 112 . SCFAs are discussed as putative signaling molecules between the gut microbiome and the receptors, i.e. free fatty acid receptor 2 (FFA2) 51 , found in host cells across different tissues 113 and could therefore be one potential mechanism of microbiome−host communication.

The underlying mechanisms of nutrient decomposition by Prevotella and whether abundant Prevotella populations in the gut are beneficial for overall health remain unknown. Yet it seems possible that an increased fiber intake and therefore higher Prevotella abundance such as associated with plant-based diets is beneficial for regulating glycemic control and keeping inflammatory processes within normal levels, possibly due to reduced appetite and lower energy intake mediated by a higher fiber content 114 . Moreover, it has been brought forward that the microbiome might influence bodily homeostatic control, suggesting a role for the gut microbiota in whole-body control mechanisms on the systemic level. Novel strategies aim to develop gut-microbiota-based therapies to improve bodily states, e.g. glycemic control 115 , based on inducing microbial changes and thereby eliciting higher-level changes in homeostasis. While highly speculative, such strategies could in theory also exert changes on the brain level, which will be discussed next in the light of a bi-directional feedback between the gut and the brain.

Effects on cognition and behavior linking diet and cognition via the microbiome−gut−brain axis

While the number of interventional studies focusing on cognitive and mental health outcomes after adopting plant-based diets overall is very limited (see Section I above), one underlying mechanism of how plant-based diets may affect mood could involve signaling pathways on the microbiome−gut−brain axis 116,117,118,119 . A recent 4-week intervention RCT showed that probiotic administration compared to placebo and no intervention modulated brain activity during emotional decision-making and emotional recognition tasks 117 . In chronic depression it has been proposed that immunoglobulin A and M antibodies are synthesized by the host in response to gut commensals and are linked to depressive symptoms 120 . Whether the identified gram-negative bacteria might also play a role in plant-based diets remains to be explored. A meta-analysis on five studies concluded that probiotics may mediate an alleviating effect on depression symptomatic 121 —however, sample sizes remained rather small (n < 100) and no long-term effects were tested (up to 8 weeks).

Currently, several studies aim to identify microbial profiles in relation to disease and how microbial data can be used on a multimodal way to improve functional resolution, e.g. characterizing microbial profiles of individuals suffering from type-1 diabetes 122 . Yet, evidence for specific effects of diet on cognitive functions and behavior through changes in the microbiome remains scarce. A recent study indicated the possibility that our food choices determine the quantity and quality of neurotransmitter-precursor levels that we ingest, which in turn might influence behavior, as shown by lower fairness during a money-redistribution task, called the ultimatum game, after a high-carbohydrate/protein ratio breakfast than after a low-ratio breakfast 123 . Strang et al. found that precursor forms of serotonin and dopamine, measured in blood serum, predicted behavior in this task, and precursor concentrations were dependent on the nutrient profile of the consumed meal before the task. Also on a cross-sectional level tryptophan metabolites from fecal samples have been associated with amygdala-reward network functional connectivity 124 . On top of the dietary composition per se, the microbiota largely contributes to neurotransmitter precursor concentrations thus, in addition to measuring neurotransmitter precursors in the serum, metabolomics on fecal samples would be helpful to further understand the functional role of the gut microbiota in neurotransmitter biosynthesis and regulation 125 .

Indicating the relevance of gut microbiota for cognition, a first human study assessing cognitive tests and brain imaging could distinguish obese from nonobese individuals using a microbial profile 126 . The authors found a specific microbiotic profile, particularly defined by Actinobacteria phylum abundance, that was associated with microstructural properties in the hypothalamus and in the caudate nucleus. Further, a preclinical study tested whether probiotics could enhance cognitive function in healthy subjects, showing small effects on improved memory performance and reduced stress levels 127 .

A recent study could show that microbial composition influences cerebral amyloidogenesis in a mouse model for Alzheimer’s disease 128 . Health status of the donor mouse seemingly mattered: fecal transplants from transgenic mice had a larger impact on amyloid beta proliferation in the brain compared to wild-type feces. Translational interpretations to humans should be done with caution if at all—yet the results remain elucidative for showing a link between the gut microbiome and brain metabolism.

The evidence for effects of strictly plant-based diets on cognition is very limited. For other plant-based diets such as the Mediterranean diet or DASH diet, there are more available studies that indicate protective effects on cardiovascular and brain health in the aging population (reviewed in refs. 129,130 ). Several attempts have been made to clarify potential underlying mechanisms, for example using supplementary plant polyphenols, fish/fish-oil consumption or whole dietary pattern change in RCTs 131,132,133,134,135,136,137 , yet results are not always equivocal and large-scale intervention studies have yet to be completed.

The overall findings of this paragraph add to the evidence that microbial diversity may be associated with brain health, although underlying mechanisms and candidate signaling molecules remain unknown.

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By testing these and other hormones at once, clinicians can get a good picture of how well the body’s metabolism as a whole is functioning.

You can take these tests from the comfort of your own home by ordering an at-home metabolic testing kit.


Life as we know it is a wonderfully diverse enterprise. Organisms pass traits from parent to offspring, function in a varied and changing environment, and carry out a myriad of complex biochemical processes. Understanding the principles of biology helps give context and clarity to larger scientific and social issues. Our biology learning modules reveal the current state of scientific understanding on topics like cell structure and function, genetics, taxonomy, evolution by natural selection, and more.

Water is not "food" for plants or animals.

Did you know?

Did you know that carbohydrates, the main source of energy for the human body, are made of different types of sugar molecules? Depending on their molecular structure, some carbohydrates are broken down quickly in the body to release as energy others need a special enzyme to digest and still others cannot be digested at all by humans.


Our bodies are efficient chemical processing plants, breaking down nutrients to use and store for energy. This module introduces carbohydrates, an important macronutrient. It explains how different carbohydrates are used by plants and animals. Simple sugars and complex carbohydrates are identified, and their biochemical structures are compared and contrasted.

Key Concepts
  • Carbohydrates are a class of macronutrients that are essential to living organisms. They are the main energy source for the human body.
  • Carbohydrates are organic molecules in which carbon (C) bonds with hydrogen and oxygen (H2O) in different ratios depending on the specific carbohydrate.
  • Plants harvest energy from the sun and manufacture carbohydrates during photosynthesis. In a reverse process, animals break down carbohydrates during metabolism to release energy.
  • All carbohydrates are made up of units of sugar. There are two types of carbohydrates: simple sugars – the monosaccharides and disaccharides – and complex carbohydrates – the polysaccharides, which are polymers of the simple sugars.
  • Examples of complex carbohydrates are starch (the principal polysaccharide used by plants to store glucose for later use as energy), glycogen (the polysaccharide used by animals to store energy), and cellulose (plant fiber).
Did you know?

Did you know that there are an estimated 100,000 different proteins in the human body? Proteins, one of the major nutrients required by our bodies, are large molecules made up of hundreds, even thousands, of amino acids combined in different ways. Fat is another essential nutrient, providing a reserve supply of energy, insulation and protection for the body, and structure for cells.


Fats and proteins are two of the major nutrient groups that our bodies need. This module provides an introduction to these two macronutrients. The basic chemical structure of fats as triglycerides is presented along with the purposes and types of fat. The module also introduces the amazing structure of protein molecules, including the peptide bond, and explains the purpose of proteins.

Key Concepts
  • In addition to carbohydrates, fats and proteins are the other two macronutrients required by the human body.
  • Fats, a subgroup of lipids, are also known as triglycerides, meaning their molecules are made from one molecule of glycerol and three fatty acids.
  • Fats in the body serve mainly as an energy storage system. They are also used as insulation to conserve body heat and protect internal organs, to form the main structural material in cell membranes, and to manufacture steroids and hormones to help regulate the growth and maintenance of tissue.
  • Fats are classified as saturated or unsaturated. Saturated fats contain no double carbon-carbon bonds in their fatty acid chains and tend to be solid at room temperature. Unsaturated fats contain double carbon-carbon bonds and are generally liquid at room temperature. Unsaturated fats can be either polyunsaturated (many double bonds) or monounsaturated (one or few double bonds).
  • Proteins are polymers of hundreds or even thousands of amino acids. Each protein has a different structure and performs a different function in the body. There are around 100,000 different proteins in the human body, each of which is made up of combinations of only 20 amino acids.
  • Enzymes are proteins that help to carry out specific chemical reactions in the body.
Did you know?

Did you know that the human body contains an estimated 100,000 different proteins, all due to the numerous ways that only 20 amino acids can combine? Proteins are large molecules made up of hundreds, even thousands, of amino acids combined in different ways.


This module explores how proteins are polymers composed of building blocks called amino acids. Using the historic research of Frederick Sanger on insulin as a starting point, the complex structures of proteins, due to molecular bonds like the disulfide bridge and the peptide bond, are explained.

Key Concepts
  • Proteins are vital components to nearly every biological process.
  • Proteins are polymers composed of building blocks called amino acids, of which life on Earth uses just twenty.
  • Molecular bonds determine the structures of amino acids and proteins. Peptide bonds link amino acids together in a chain disulfide bridge bonds hold proteins together.
  • Using techniques like electrophoresis and chromatography, Frederick Sanger discovered that proteins were built of specific amino acid sequences and that changing the sequence would make it a different protein.
  • Proteins can have four types of structures: (1) Primary, the sequence of amino acids, (2) Secondary, hydrogen bonds among the strands of amino acids form beta sheets or alpha-helixes, (3) Tertiary, the three-dimensional, twisted structure based on bonding interactions between amino acid strands, and (4) Quartnerary, the complex structure made up of multiple folded subunits.
Did you know?

Did you know that in the early days of blood transfusions, more people died than survived them? All blood looks pretty much the same to the naked eye, but blood that is lifesaving for one person may be deadly to another. Transfusions became a safe medical procedure only when scientists came to understand the complexity of blood components and different blood types.


Knowledge of blood components brought about a revolution in surgery through safe transfusion. The module traces the development of our understanding of blood over centuries, beginning in 1628 with English physician William Harvey's breakthrough research on circulation. With a focus on early 20th-century experiments by Austrian researcher Karl Landsteiner, the module shows how the identification of clotting factors, blood types, and antigens was critical to medical science. Whole blood, plasma, serum, and different types of blood cells are defined.

Key Concepts
  • Blood is a complex fluid with many different components, but can be divided into solids (red blood cells, white blood cells, and platelets) and liquid (plasma).
  • Blood plasma includes clotting factors (agents that help to form blood clots) and when these are removed, the remaining liquid is known as serum.
  • The main cellular components of blood are: red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).
  • The Austrian researcher Karl Landsteiner studied agglutination, or clumping together of blood cells with certain antigens. Based on his findings, he proposed that there were three types of blood (A, B, O) and later added a fourth type (AB).
  • Antibodies are proteins produced by plasma cells, a type of B-cell lymphocyte, and are present in the blood serum. These antibodies are important for blood transfusion, since the blood type of a patient and the type of antibodies present in the donor’s blood will determine whether or not it agglutinates or clumps.
Did you know?

Did you know that studying lipids can help us understand and treat medical conditions such as heart disease, hormone disorders, multiple sclerosis, and many others? Lipids are necessary for the structure of all living cells. Their chemical composition allows them to have many important functions, from storing energy to regulating metabolism to helping the fur coat of sea otters repel water.


Fats, oils, waxes, steroids, certain plant pigments, and parts of the cell membrane – these are all lipids. This module explores the world of lipids, a class of compounds produced by both plants and animals. It begins with a look at the chemical reaction that produces soap and then examines the chemical composition of a wide variety of lipid types. Properties and functions of lipids are discussed.

Key Concepts
  • Lipids are a large and diverse class of biological molecules marked by their being hydrophobic, or unable to dissolve in water.
  • The hydrophobic nature of lipids stems from the many nonpolar covalent bonds. Water, on the other hand, has polar covalent bonds and mixes well only with other polar or charged compounds.
  • Fats and oils are high-energy molecules used by organisms to store and transfer chemical energy. The distinct structures of different fat molecules gives them different properties.
  • Phospholipids are specialized lipids that are partially soluble in water. This dual nature allows them to form structures called membranes which surround all living cells.
Did you know?

Did you know that the human body consists of trillions of individual cells and 200 distinct types of cells? Human cells range in size from 1/12,000 of an inch (a few micrometers) to over 39 inches (more than a meter) long. All living things are made of cells, but in spite of vast differences in size, shape, and function, these building blocks of life share remarkable similarities.


Cells are the basic structural and functional unit of life. This module traces the discovery of the cell in the 1600s and the development of modern cell theory. The module looks at similarities and differences between different types of cells and the relationship between cell structure and function. The Theory of Universal Common Descent is presented along with evidence that all living things on Earth descended from a common ancestor.

Key Concepts
  • Cells are the basic structural and functional unit of all living things and contain inheritable genetic material.
  • The activity of a cell is carried out by the sub-cellular structures it possesses.
  • Cells possess an outer boundary layer, called a cell membrane, cytoplasm, which contains organelles, and genetic material.
  • There is considerable variety among living cells, including the function of membranes and subcellular structures, and the different types of functions the cells carry out, such as chemical transport, support, and other functions.
Did you know?

Did you know that Benjamin Franklin’s 1774 experiments with pouring oil onto a pond of water was an early step in gaining a scientific understanding of cell membranes? Cell membranes were thought to be passive barriers until the 1960s, but we now know that they are active and responsive structures that serve a critical function as gatekeepers and communicators.


Cell membranes are much more than passive barriers they are complex and dynamic structures that control what enters and leaves the cell. This module explores how scientists came to understand cell membranes, including the experiments that led to the development of the fluid-mosaic model of membrane structure. The module describes how the components and structure of cell membranes relate to key functions.

Key Concepts
  • The outer layer of a cell, or a cell membrane, is a complex structure with many different kinds of molecules that are in constant motion, moving fluidly throughout the membrane.
  • Cell membranes form selective barriers that protect the cell from the watery environment around them while letting water-insoluble molecules like oxygen, carbon dioxide and some hormones pass through.
  • Most of the cell membrane is formed by phospholipids that have a unique structure that causes them to self-arrange into a double layer that is hydrophobic in the middle and hydrophilic on the outside.
Did you know?

Did you know that the absence of one tiny amino acid in cell membranes causes Cystic Fibrosis, a life-threatening disease? And a common aliment, heartburn, is treated with medicine that slows down the rate at which protons are pumped across cell membranes into the stomach. Studying how molecules travel across plasma membranes (cell membranes) is the key to understanding and treating many medical conditions.


For living things to survive, different molecules need to enter and leave cells, yet cell membranes serve as a barrier to most molecules. Fortunately, all living cells have built-in transporters that allow water, glucose, sodium, potassium, chloride, and other molecules to cross the plasma membrane. This module looks at how passive and active transporters work. It highlights the importance of the study of cell membranes by looking at advances in treating cystic fibrosis and common digestive ailments as well as the development of effective pain relievers.

Key Concepts
  • Whether or not a molecule is able to pass easily, or at all, into or out of a cell is largely dependent on its charge and solubility in water.
  • The plasma membrane serves as a semi-permeable barrier to the cell. Only uncharged, non-polar molecules are able to pass into or out of the cell without aid.
  • All plasma membranes possess transporters to help move molecules from one side of the membrane to the other. These transporters can be active (pumps) or passive (channels) and are sometimes regulated by gates.
  • The lack of a specific transporter can interrupt cellular functions and cause diseases like cystic fibrosis.
  • Research into pain relievers provided insight into the most important and universal transporter in the human body, the sodium-potassium pump.
Did you know?

Did you know that “survival of the fittest” is not the only explanation for the success of a species over time? Cooperation can be just as important when it comes to how species adapt in order to survive. According to Lynn Margulis, who proposed that modern-day mitochondria and chloroplasts evolved through endosymbiosis, “Life did not take over the globe by combat, but by networking.”


Evolution isn't always about competition. It can also be about cooperation, as is the case with the development of chloroplasts and mitochondria from free-living bacteria. This module explains the theory of endosymbiosis along with its origins. Convincing evidence in support of the theory is presented. The evolution of the nucleus and other organelles through invagination of the cell membrane is also discussed.

Key Concepts
  • One of the main differences between eukaryotic cells and prokaryotic cells is the presence of a nucleus and other membrane-bound organelles.
  • Chloroplasts and mitochondria have specialized roles in producing energy for the cell and have several unique features including some of their own DNA. Because of this, scientists believe that both of these organelles originated through endosymbiosis when one small cell began to live inside a larger one.
  • Membrane-bound organelles evolved as folds of the plasma membrane this allowed these cells to establish compartments with different environments appropriate for the specific function that the organelle performs.
Did you know?

Did you know that every organ and tissue in your body was formed as the result of individual cells making copies of their DNA and separating themselves into two identical cells? From experiments in the 1870s to research more than 100 years later, scientists have made fascinating discoveries about the complex series of events that allow the cells in plants and animals, including humans, to grow and sustain life.


Cell division is an enormously complex process that must go on millions and millions of times during the life of an organism. This module explains the difference between binary fission and the cell division cycle. The stages of cell division are explored, and research that contributed to our understanding of the process is described.

Key Concepts
  • Most of the cells that make up higher organisms, like vertebrate animals and flowering plants, reproduce via a process called cell division.
  • In cell division, a cell makes a copy of its DNA and then separates itself into two identical cells – each with its own copy of DNA enveloped inside a nucleus.
  • The term mitosis refers specifically to the process whereby the nucleus of the parent cell splits into two identical nuclei prior to cell division.
Did you know?

Did you know that there is a huge variation in the number of chromosomes in living things? While humans have 46 chromosomes and dogs have 78, one kind of ant has only 2 chromosomes and a type of protozoan has nearly 16,000! But what all these life forms have in common is that their genetic code is copied from cell to cell thanks to the process of mitosis, whereby the nucleus of a cell splits into two before the cell divides.


Beginning with the discovery of mitosis, the module details each phase of this cell process. It provides an overview of the structure of cell components that are critical to mitosis. The module describes Clark Noble’s experiments with the Madagascar Periwinkle, which led to the discovery of an effective cancer treatment drug. The relationship between mitosis and cancer is explored as is the mechanism by which anti-cancer drugs work to slow down or prevent cell division.

Key Concepts
  • The term mitosis refers specifically to the process whereby the nucleus of a eukaryotic cell splits into two identical daughter nuclei prior to cell division.
  • Mitosis is a cyclical process consisting of five phases that feed into one another: prophase prometaphase metaphase anaphase telophase.
  • The rate at which mitosis occurs depends on the cell type. Some cells replicate faster and others slower, and the entire process can be interrupted.
  • Chromosomes are made of a material called chromatin, which is dispersed throughout the cell nucleus during interphase. During mitosis, however, the chromatin condenses making individual chromosomes visible under an ordinary light microscope.
Did you know?

Did you know that most chemicals we come into contact with—including the food we eat—must pass through a complex system of cell membranes before they can enter the bloodstream? There are many different ways that chemicals enter the body, depending on the type of chemical and the part of the body with which it comes into contact.


In order for many medicines to work, the chemicals must move from the outside environment into the body. This module discusses the different mechanisms by which chemicals cross the cell membrane and the factors that influence this process. In addition to introducing biological absorption, the module explains how chemicals are stored and distributed within the body.

Key Concepts
  • Chemicals can enter the human body by several methods, but most must pass through living cell membranes before entering the bloodstream.
  • The cell membrane consists mainly of phospholipids and proteins in the form of a lipid bilayer.
  • Mechanisms for moving chemicals through the cell membrane include: passive diffusion, facilitated diffusion, active transport, and endocytosis.
  • Factors such as human anatomy and chemical structures affect the movement of chemicals in the body.
Did you know?

Did you know similar to the way cars are manufactured, chemical compounds in living cells are built up, broken down, and moved around in assembly-line fashion? In living organisms, a series of reactions is needed to get energy from food molecules. One such important chemical pathway is the circular assembly line known as the Krebs cycle.


Food fuels our bodies, but how does our body convert food molecules into usable energy? This module looks at glycolysis and the Krebs cycle, two important stages of cellular respiration, the process by which cells harvest energy from food. It highlights the work of Sir Hans Adolf Krebs and his focus on cyclic pathways as he discovered the main biochemical pathway for breaking down fuel to produce energy.

Key Concepts
  • In a cell, chemical compounds are put together, taken apart, and moved around through pathways that resemble moving assembly lines.
  • The main types of biological macromolecules that cells use for fuel are sugars, fats, and proteins.
  • The main biochemical pathway where the breakdown of biological fuels comes together is called the Krebs cycle. Named for its discoverer, Sir Hans Adolf Krebs, this pathway is like a circular assembly line.
Did you know?

Did you know that the energy in chemical compounds is found in tiny electrons? The electron transport chain is like an assembly line inside of cells that harnesses high-energy electrons so they can be used to make ATP, the energy that organisms need to survive. When Peter Mitchell proposed the way that ATP is made inside cells, other scientists made fun of him – until he was eventually proved correct and won the Nobel Prize in Chemistry.


ATP is the main energy currency of living cells. This module answers the question of how most ATP is generated. A look at two important compounds, NADH and FADH2, reveals their important role in the production of ATP. The module explains the workings of the electron transport chain, which provides high-energy electrons to fuel the ATP-producing process called oxidative phosphorylation.

Key Concepts
  • Adenosine triphosphate (ATP) is the main energy currency of the cell. It is generated from a similar compound, ADP, using energy harnessed from cellular fuels, such as sugars, fats, and proteins.
  • The amount of ATP generated directly during glycolysis (the breakdown of the sugar glucose) is small compared with amount of energy contained within glucose.
  • The energy held by ATP and other energy-holding chemical compounds is contained in electrons. By moving electrons, different molecules move energy around the cell.
  • Two specialized energy currency compounds, NADH and FADH2, are vital to the movement of high-energy electrons from cellular fuels like glucose to an assembly-line system of enzymes called the electron transport chain.
  • Located inside mitochondria, the electron transport chain harnesses energy from NADH and FADH2 to power a process called oxidative phosphorylation, which generates large amounts of ATP. Oxidative phosphorylation requires oxygen.
Did you know?

Did you know that the oxygen we breathe is a waste product? Of photosynthesis, that is. Through this remarkable process, plants capture energy from sunlight and produce the sugars that provide sustenance to nearly every living thing on Earth along with the oxygen we need to survive.


Through photosynthesis, plants harvest energy from the sun to produce oxygen and sugar, the basic energy source for all living things. This module introduces photosynthesis, beginning with experiments leading to its discovery. The stages of photosynthesis are explained. Topics include the role of chlorophyll, the action spectrum of photosynthesis, the wavelengths of light that drive photosynthesis, light-harvesting complexes, and the electron transport chain.

Key Concepts
  • Photosynthesis is a process by which an organism converts light energy from the sun into chemical energy for its sustenance.
  • Photosynthesis occurs in plants, algae, and some species of bacteria.
  • In plants, chloroplasts contain chlorophyll that absorbs light in the red and blue-violet regions of the spectrum.
  • Photosynthesis occurs in two stages: the light-dependent stage that occurs in the thylakoid membrane of the chloroplast and harvests solar energy, and the light-independent stage that takes that energy and makes sugar from carbon dioxide.
Did you know?

Did you know that it is much easier to determine when life appeared on Earth than how life came to exist? Evidence points to life on Earth as early as 3.8 billion years ago, but the question of how life came to be has puzzled scientists and philosophers since prehistoric times. In the 1950s, scientists successfully created biological molecules by recreating the atmosphere of primordial Earth in a bottle and shocking it with lightning. This and other experiments give clues to the origins of life.


Since prehistoric times, people have pondered how life came to exist. This module describes investigations into the origins of life through history, including Louis Pasteur’s experiments that disproved the long-held idea of spontaneous generation and and later research showing that the emergence of biological molecules from a nonliving environment – or abiogenesis – is not only possible, but likely under the right conditions.

Key Concepts
  • Theories about the origins of life are as ancient as human culture. Greek thinkers like Anaximander thought life originated with spontaneous generation, the idea that small organisms are spontaneously generated from nonliving matter.
  • The theory of spontaneous generation was challenged in the 18th and 19th centuries by scientists conducting experiments on the growth of microorganisms. Louis Pasteur, by conducting experiments that showed exposure to fresh air was the cause of microorganism growth, effectively disproved the spontaneous generation theory.
  • Abiogenesis, the theory that life evolved from nonliving chemical systems, replaced spontaneous generation as the leading theory for the origin of life.
  • Haldane and Oparin theorized that a "soup" of organic molecules on ancient Earth was the source of life's building blocks. Experiments by Miller and Urey showed that likely conditions on early Earth could create the needed organic molecules for life to appear.
  • RNA, and through evolutionary processes, DNA and the diversity of life as we know it, likely formed due to chemical reactions among the organic compounds in the "soup" of early Earth.
Did you know?

Did you know that scientists don’t need time travel to mimic conditions on Earth 4 billion years ago? Scientists who want to experiment in an environment like that of primordial Earth need only to visit volcanoes, which have chemical conditions similar to those on Earth long, long ago. That’s just what chemist David Deamer did in his research into the origins of life. Just as had happened in the lab, Deamer’s volcano experiments produced a necessary step toward the formation of living matter.


Building on earlier experiments showing how life’s chemical building blocks could form from nonliving material on early Earth, this module explores theories on the next steps needed for life. These include the formation of long polymers, which then fold into complex macromolecules. The module describes experiments in an environment like that of primordial Earth, resulting in the spontaneous emergence of phospholipids, which could form into membranes, paving the way for RNA duplication and the eventual emergence of living cells.

Key Concepts
  • For life to occur, smaller molecules must join together and form polymers, which then fold into complex shapes. These large molecules are called macromolecules.
  • Simple membranes made of lipids may have served as nature’s test tubes, providing the enclosed environments necessary for RNA enzymes to develop.
  • The possible ancestor to living cells, liposomes, may have been created from phospholipids formed from the gases of Earth’s primeval atmosphere or from free fatty acids delivered to ancient Earth via meteorites.
  • To trigger abiogenesis, a system of molecules would need to develop the ability to copy themselves using polymers.
  • Protocells made of liposomes that exchanged fatty acids between their membranes possibly absorbed RNA enzymes and made copies of themselves, leading to the evolutionary development of living cells.
Did you know?

Did you know that the theory of evolution did not begin with Charles Darwin? The idea of evolution was part of Western thought for more than 2,000 years before Darwin changed the world with his legendary book On the Origin of Species.


The experiences and observations of Charles Darwin significantly contributed to his theory of evolution through natural selection. This module explores those influences and describes evolution as a force for biological change and diversification. The first in a series, it details how the theory challenged the cultural mindset of the time, including the effect of his major works: On the Origin of Species by Means of Natural Selection and Sexual Selection and the Descent of Man.

Key Concepts
  • Charles Darwin played a key role in supporting and explaining the theory of evolution through natural selection.
  • Darwin's skills of observation and ability to record data accurately allowed him to create a comprehensive model of the mechanism by which evolution occurs.
  • The theory of evolution through natural selection explains how all forms of life are related to one another genealogically, and emphasizes that variation within a species is the root for evolutionary change.
Did you know?

Did you know that Darwin's experience with his ten children fueled his thinking about evolution? He theorized that some human behaviors, such as a young child's selfishness, were based upon instincts that were adaptations. These natural differences that always exist among individuals are at the heart of the principle of natural selection as the engine of evolutionary change.


The second in a series discussing the work of Charles Darwin, this module takes a deeper look into the processes that led to Darwin's theory of natural selection and examines specific mechanisms that drive evolutionary change. Key points on which the idea of natural selection rests are outlined. Examples from Darwin's personal life shed light on his thinking about change within a species and the "struggle for existence."

Key Concepts
  • Variation within a species increases the likelihood that at least some members of a population will survive under changed environmental conditions.
  • The common characteristics of individuals within a population will change over time, as those with advantageous traits will come to be most common or widespread.
  • While evidence of evolution by natural selection exists, its effects cannot be predicted.
Did you know?

Did you know that Charles Darwin preferred the phrase descent with modification over the simpler term evolution? In his groundbreaking book On the Origin of Species, Darwin chose his words very carefully. "Evolution" was used in different ways at the time, and Darwin wanted to convey the important concept that life forms descended from a common ancestor.


Our understanding of the term evolution has changed significantly since Darwin's time. This module explains how Darwin's work helped to give evolution the meaning it has today. It details the concept of "descent with modification" that Darwin described with the one figure originally included in Origin of Species. The module discusses how this model revolutionized scientific thinking about the similarities and differences between and within species, laying the foundation for our current understanding of biodiversity.

Key Concepts
  • Darwin's theory of Descent with Modification shows how as organisms reproduce, slight changes create variation, which could lead to new species over time.
  • Darwin provided the first model that could logically account for biodiversity, explaining lineage and the small variations that distinguish one species from another, similar-looking one.
  • Darwin's work radically changed thinking regarding the Scale of Nature, a model that suggested that some species were naturally inferior to one another, and showed species evolved in response to environmental pressures, not because of some hierarchy of order.
Did you know?

Did you know that there is a species of moth with a 12-inch long nectar-gathering tongue? Not by coincidence, this moth feeds on and pollinates a kind of orchid that has an 11-inch long nectar-producing tube. Nature abounds with examples of plants and animals that have adapted to their environment over time to ensure the survival of the species.


This module introduces the concept of evolutionary adaptation. It follows the development of Charles Darwin's ideas on how species adapt to their environment in order to survive and reproduce. The difference between adaptation and natural selection is explained. With a look at penguins and other examples from nature, the module explores the processes that influence the diversity of life.

Key Concepts
  • Natural selection is the mechanism that explains how organisms change.
  • The structure of an organism and many of its features are directly related to the environment in which it lives.
  • Numerous environmental mechanisms, both naturally occurring and man-made, influence adaptive evolution.
Did you know?

Did you know that people started classifying living things as early as 300 BCE? But our modern classification system officially began in the 18th century when Carolus Linneaus listed every plant and animal species known in the world – more than 12,000 in all. He produced one of the great works in the history of science, Systema Naturae, which we still use today.


Modern taxonomy officially began in 1758 with Systema Naturae, the classic work by Carolus Linnaeus. This module, the first in a two-part series on species taxonomy, focuses on Linnaeus’ system for classifying and naming plants and animals. The module discusses the contribution of diverse cultures to the development of our modern biological classification and describes the historical development of a scientific basis for classifying species.

Key Concepts
  • Under Linnaeus's system, every species is known by a unique Latin-sounding genus and species name that distinguishes it from other species.
  • Linnaeus's work organized organisms into logical classes based on their appearance and characteristics, and thus provides a basis for comparing different species.
Did you know?

Did you know that Tyrannosaurus rex could have been called Manospondylus gigas? The rules of scientific nomenclature usually dictate that when more than one name for a species is discovered has been given, the older one prevails. Luckily, common sense won out in the case of T. Rex, and this most famous dinosaur was allowed to keep the newer name that both scientists and the general public had become familiar with.


Carolus Linnaeus, the “father of taxonomy,” developed a uniform system for naming plants and animals to ensure that each species has a unique name. This module outlines rules of forming two-term taxonomic names according to genus and species. The module gives examples of naming controversies and describes how they were resolved, including by bending the rules in regard to certain famous beasts.

Key Concepts
  • The system of binomial nomenclature was Linnaeus' response to the need of a clear, distinct naming of species that would be recognized around the world and reduce the chance of one species being known by multiple names.
  • Scientific names are always written in italics, with the genus capitalized and the species lowercase, and should sound as though they are Latin.
Did you know?

Did you know that bones, stones, and tools are often all scientists have to go by to piece together the human family tree? This is the work of paleoanthropologists, who trace human ancestry by analyzing fossilized bones and teeth, tools made by the creatures, and the surrounding sediment and stones. Although pre-human fossils are rarely uncovered, any such find can give great insight into the evolution of humans.


Paleoanthropology is the study of human ancestry through fossil remains and other evidence. This module explains how paleoanthropologists uncover and evaluate clues to the lineage to modern humans, tracing intermediate forms along the way since the time we diverged from our cousins, the great apes. Key discoveries are highlighted that shed light on the pathway of human evolution. The module describes different ideas through history about how the human family tree is constructed and which characteristics best define humanness.

Key Concepts
  • Paleoanthropology is the study of human ancestry through analysis of fossilized bones and teeth, the stones and sediments in which bones are buried, as well as the tools associated with those ancient creatures.
  • Charles Darwin first proposed that humans and apes shared a common ancestor in his 1871 publication The Descent of Man.
  • Early researchers sought out an ape-man fossil, or a “missing link,” to make a clear lineage from ape to man. However, we know today that the human lineage branches like a tree with many human-like species in a few genera.
  • Key discoveries have helped to provide more detail on human evolution, such as Mary and Louis Leakey’s discovery of Homo habilis, but also brought up difficult questions of taxonomic classification.
Did you know?

Did you know that only half of British children lived to age 21 in Charles Darwin’s time? Thanks to modern medicine and public health measures, most humans in developed countries now survive to adulthood. As survival constantly improves, natural selection is no longer a major force that shapes the human population. However, evolution will likely continue among humans due to other evolutionary forces at work.


Some noted modern scientists have declared that human evolution is over. With advances in medicine and public health, natural selection is no longer a major shaping force for humans. Even so, it doesn’t mean that humans won’t evolve. This module explores the various directions that human evolution might take. Various influences on human evolution are discussed by way of specific examples, including artificial selection through surgical advances and how “bottlenecking” could affect the human gene pool if distant space colonies are formed in the future.

Key Concepts
  • Humans continue to evolve due to a variety of evolutionary forces: natural selection, artificial selection, genetic drift, and via transhuman breakthroughs.
  • Evolution is the gradual genetic change of a species over time due to unequal reproduction among members.
  • Natural selection is the phenomenon that rewards certain advantageous traits and punishes others through better or worse survival or reproduction. Natural selection is one of the forces that moves evolution forward.
  • Artificial selection is the selective breeding of animals or plants by humans to modify an organism.
  • Genetic drift is a change in the frequency of a population's genes and alleles over time, often by founder effects (when a small group of individuals relocate) or bottlenecking (when a large population is decimated, leaving a smaller group to repopulate).
  • Transhumanism is the idea that humans can evolve new physical and mental capabilities, particularly through the use of science and engineering.
Did you know?

Did you know that the Piltdown Man hoax inspired the development of various scientific tests for authenticating paleoanthropological specimens? From the 1960s onward, scientists developed methods for more accurately finding both the date and geography of origin for materials.


The Piltdown Man was once hailed as the "missing link" in the evolution of apes to humans. However, the discovery at Piltdown - human skull fragments, ancient mammal bones, and archaic tools - was an elaborate hoax. The deception took a long time to be revealed due to errors by the discoverers. They succumbed to confirmation bias by accepting any evidence that supported their discovery and rejecting any contradictory evidence.

Key Concepts
  • Confirmation bias is the tendency to accept any evidence that seems to support one’s belief while rejecting all evidence that is contrary.
  • The 1912 discovery of Piltdown Man was widely hailed as the discovery of the "missing link" between humans and apes, but was really an elaborate deception.
  • The hoax perpetrator buried ancient animal bones, flint tools, and both human and ape skull fragments in the English town Piltdown.
  • The discoverers of the Piltdown Man were so enthusiastic about their findings that they ignored contradictory evidence and failed to carefully test the bones.
  • By the 1940s the scientific community were skeptical of the Piltdown discovery and it was officially declared a hoax in 1953.
Did you know?

Did you know people used to believe that fully formed miniature versions of offspring were contained in sperm cells? Early theories of reproduction were later disproven, but inheritance patterns remained a mystery until Gregor Mendel performed his groundbreaking experiments with pea plants in the 1800s.


This module describes the experiments that resulted in Mendel's Laws of Inheritance. A look at specific traits in pea plants over generations shows how Mendel's research methods resulted in an understanding of dominant and recessive genes. Partial dominance is also discussed.

Key Concepts
  • Mendel determined that an organism inherits two copies of the genetic material that determines an individual's physical traits, one copy coming from each the male and female parent.
  • Mendel observed that for each trait, sometimes what is inherited from one parent masks what is inherited from the other. He called the hidden trait recessive and the expressed trait dominant.
  • Since the time of Mendel, other scientists have observed that not all traits are inherited with the simple dominant-recessive pattern incomplete dominance and co-dominance can result in a variety of phenotypes for some traits.
Did you know?

Did you know that Gregor Mendel is known as the “Father of Genetics,” and yet his work was largely ignored by scientists during his lifetime? It was only when three scientists rediscovered Mendel’s work nearly 35 years after it was published that people came to appreciate its implications for the scientific understanding of inheritance.


The power of Mendel’s scientific approach can be seen in the research that led to his Second Law. This module, the second in a series, provides details on Mendel's work with dihybrid crosses and independent assortment. The module describes tests that confirmed Mendel’s ideas about the random and independent segregation of genetic factors.

Key Concepts
  • Genetic markers randomly and independently segregate into a parent’s gametes, some of which are dominant over others.
  • The cross of two organisms that each possess multiple heterozygous pairs is called a dihybrid cross.
  • Dihybrid crosses result in a trait expression ratio of 9:3:3:1 – 9 with both traits dominant, 3 with trait one dominant and trait two recessive, 3 with trait one recessive and trait two dominant, and 1 with both traits recessive.
Did you know?

Did you know that one of the most important discoveries in biology was made while a British army medical officer was trying to develop a vaccine for pneumonia after World War I? Although a vaccine for pneumonia still does not exist, Frederick Griffith discovered “transformation.” This means that organisms can be genetically reprogrammed into a slightly different version of themselves.


This module is the first in a series that discusses the discovery, structure, and function of DNA. Key experiments are discussed: from Griffith’s discovery of genetic “transformation” to Avery, MacLeod, and McCarty’s determination of the “transforming agent” to confirmation by Hershey and Chase of DNA rather than protein as the genetic material.

Key Concepts
  • It required numerous experiments by many scientists to determine that DNA, and not protein, is the genetic material on which life is built.
  • DNA can be “transformed,” or genetically re-programmed, into a slightly different version of itself.
Did you know?

Did you know that the precise combinations of just four nitrogen bases form the billions of nucleotides that make up our own unique DNA molecules? The information stored in the base sequence of a single DNA strand stores all of the genetic information in your body and gives us our individual genetic traits.


Exploration of the structure of DNA sheds light on fascinating properties of the molecule. This module, the second in a series, highlights major discoveries, from the parts of a nucleotide - the building blocks of DNA - to the double helix structure of the DNA molecule. The module describes scientific developments that led to an understanding of the mechanism by which DNA replicates itself.

Key Concepts
  • DNA consist of two strands of repeating units called nucleotides each nucleotide is made up of a five-carbon sugar, a phosphate group, and a nitrogen base.
  • The specific sequence of the four different nucleotides that make up an organism's DNA gives that organism its own unique genetic traits.
  • The four nitrogen bases are complementary – adenine is complementary to thymine, cytosine is complementary to guanine – and the pairs form hydrogen bonds when the 5'/3' ends of their attached sugar-phosphate groups are oriented anti-parallel to one another.
Did you know?

Did you know that DNA in a human body must make exact copies of itself not merely thousands of times, not millions of times, and not even billions of times, but staggeringly, more than a trillion times? Whether a yeast, a bacterium, or a human cell, every living cell must be able to copy all of its DNA with amazing accuracy.


In the field of molecular biology, scientists examine how DNA encodes all the complexities of living things. This third module in the DNA series focuses in the process by which DNA is replicated. The module describes the DNA synthesis assay, where scientists were able to replicate DNA in a test tube. Advancements in understanding the features and properties of DNA replication are discussed.

Key Concepts
  • Once the structure of the DNA molecule was discovered, scientists could immediately envision a possible copying mechanism based on the rules of nucleotide-base pairing.
  • In order to study and observe DNA replication more directly, scientists in the 1950s devised techniques to perform DNA replication in a test tube, called the DNA synthesis assay.
  • By using the DNA synthesis assay, scientists were able to observe the features and properties of DNA replication and test various hypotheses about how the process works.
  • The process of DNA replication was identified by several teams of researchers all working together to break down the process into multiple steps that could more easily be studied individually.
Did you know?

Did you know that blue eyes are the result of defective genes for pigment? Some recessive traits, like eye color, are harmless, while others are deadly. The way that genes translate into physical traits has to do with the particular enzyme that each type of gene makes, a discovery that was made by two scientists by way of the mutant bread mold they created, winning them the Nobel Prize in 1958.


Through a look at the devastating Tay-Sachs disease and other hereditary conditions, this module explores the connection between genes and enzymes. The role of dominance vs. recessivity is examined. The module traces developments in our understanding of gene expression, starting with a rediscovery of Mendel’s laws of inheritance and built upon by the pioneering work of later scientists. The module introduces the Central Dogma of molecular biology, which is the one-way process of using DNA to make RNA and RNA to make proteins.

Key Concepts
  • Genes cannot be used directly by organisms. The information stored in genes must be used to make products, such as enzymes, that cells need to perform different functions. Gene expression is the chemical pathway from genes to the gene products, such as proteins, that organisms can use.
  • Since organisms have two genes for everything, even If one gene of a pair produces a defective enzyme or no enzyme at all, the other gene in the pair will make enough enzyme to do its job. Only an individual with two genes for a defective enzyme will actually show the recessive trait, such as an inherited disease or condition, blue eyes, or a recessive peapod shape.
  • In the mid-1900s, George Beadle and Edward Tatum showed that a defective gene leads to a defective enzyme. Their “one gene, one enzyme” hypothesis was later expanded to “one gene, one RNA."
  • The genetic code is the set of rules that combines amino acids to form polypeptides and is nearly the same for all life-forms on Earth.
  • The genetic code is not a way for cells to translate genetic information in DNA directly into chains of amino acids to make proteins. Rather, RNA molecules must be made as intermediaries along the way from DNA to the polypeptides that fold into proteins.
  • Genetic information moves in one direction, from DNA to RNA to protein. This is known as the Central Dogma of molecular biology.
Did you know?

Did you know that DNA testing of 50,000-year-old bones showed that the original Neanderthal man did not descend from the same mother as modern humans? However, male lineage tests revealed that 8% of the non-African human gene pool consists of Neanderthal DNA. Certain genetic sequences called haplotypes are passed on only by males or by females, so to get a complete picture of the ancestry of a population, scientists generate one family tree based on the male line and another based on the female line.


Using genetic markers passed down through the male or female line, scientists can construct family trees going back thousands of years. This module introduces haplotypes – genetic sequences that we inherit from only one parent. As an example, the module looks at the degree of interbreeding between now-extinct Neanderthals and modern humans as determined through an analysis of Y-chromosome haplotypes (male lineage) and mitochondrial DNA haplotypes (female lineage).

Key Concepts
  • Haplotypes are genetic sequences that we inherit from only one parent. There are two types: Y-chromosomes, inherited from your father, and mitochondrial DNA, inherited from your mother.
  • Y-chromosome haplotypes are subject to random mutation and the discovery of numerous different haplotypes led scientists to construct the Y-chromosome family tree.
  • Mitochondria are organelles in all eukaryotic cells and have some of their own DNA. All of your mitochondria come from your mother and help to build a mtDNA family tree.
  • The most recent common ancestor (MRCA) is someone who exists in everyone's family tree.
  • When scientists study populations of a given ethnic group, they generate one family tree based on the populations' Y-chromosomes and another one based on mtDNA.
  • When constructing family trees, often an ancestor will appear in multiple places - this is a phenomenon known as pedigree collapse.
Did you know?

Did you know that elephant seals in the Northern Pacific have a signature asymmetrical face that is extremely rare among other populations of elephant seals? This is because an evolutionary force called a bottleneck event acted upon their gene pool. Other forces can work to change the gene pool of a population, such as natural selection, gene flow, and the founder effect, among others.


Changes in the genetic makeup of a population affect the incidence of certain traits and diseases within the population. Beginning with a look at the abnormally high rate of a dangerous health condition in US Amish communities, this module explores forces that affect a population's gene pool. Among them are natural selection, gene flow, and two types of genetic drift: founder effects and bottleneck events. The Harvey-Weinberg Equilibrium equation is presented along with sample problems that show how to calculate the frequency of specific alleles in a population.

'Metabolism Drops' Recall: What to Know About TikTok's Latest Dangerous Trend

The maker of Metabolism Drops, Rae Wellness, has issued a recall after teens began promoting this weight-loss supplement on TikTok.

If you’re not sure what “metabolism drops” are, just ask the teenagers of TikTok. Young users of the video-based social platform are going crazy for them, and it’s forced the company behind the product to take drastic action. 

“Wellness solution” brand Rae, founded by former Target execs Angela Tebbe and Eric Carl, has decided to “proactively pause the sale” of their Metabolism Drops and Metabolism Capsules. They posted a lengthy statement on their website to explain their position. 

“We became concerned when we started to notice a conversation emerge [on TikTok]: teenage girls misusing the product alongside conversation about weight loss, at times using more than the recommended dose,” it said. 𠇊ll of our products are formulated for, and marketed to, adult women 18 and older.” 

Dazed Digital reports that hundreds of influencer-style videos have been uploaded to TikTok with teens showing off their bottles of Metabolism Drops, captioned with “let’s get skinny!” and #weightlosschallenge. The product, which retails for $14.99, is so popular, it was already on a three-month backorder at Target before it was pulled from sale. 

The statement from Rae stressed that while the action is listed in Target stores as a recall, there are “no safety concerns with any of our products whatsoever.” 

“There is no risk in taking our Metabolism Drops as directed,” they added. “We took this action simply because we feel it’s the right thing to do as a company. Seeing a groundswell of this kind of conversation was antithetical to our values. The wellbeing [sic] of all women and the promotion of positive body images are essential to the foundation of this brand.”   

The brand’s philosophy does seem commendable. “Women are taking control of their health like never before,” the website states. “We’re committed to supporting them with evidence-based holistic wellness solutions that promote self-love and help them radiate from within.”

But the issue here is that the message is getting lost in translation on its way to teenage girls, who have just as much access to wellness websites as their older counterparts. According to Dazed Digital, Rae’s ingestible drops, which are promoted on the brand’s website as supporting and enhancing a person’s natural metabolism, are being used as appetite suppressants and weight-loss stimulants by the young women spreading the Rae love on TikTok. Reportedly, they 𠇍idn’t crave sweets” and �lt way less bloated after meals” after taking the drops. 

None of Rae’s wellness products𠅊s well as the metabolism products, they sell sleep capsules, energy drops, and a libido booster𠅊re approved by the Food and Drug Administration. However the company points out that the FDA does not approve dietary supplements by law. 

“We follow what the FDA established for dietary supplements in 2007 called 𠆌urrent Good Manufacturing Practice’ (cGMP),” the brand states. “These regulations require that each batch of our vitamins is tested for identity, purity, strength, and composition, so we can be sure that what’s on the label is what’s in our vitamins and supplements. We also use a test each batch for heavy metals, microbes, allergens, gluten, and other contaminants, ensuring they are below levels deemed safe by the cGMP.”

The issue here isn’t whether Rae’s Metabolism Drops (or any of their other products) has FDA approval or the backing of science. What everyone should really be concerned about is that a huge number of teenage girls think weight loss (by whatever means available or necessary) is the route to a healthy body and healthy body image. 

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Closing Summary

From Warburg's data that began in the 1920s, to overwhelmingly similar results from countless of PET scans starting in the 1980s, to the more recent molecular biological confirmation of these observations, it has now become common knowledge that increased glucose metabolism is a universal principle of cancer cells. As shown in Fig. 1, most of the major oncogenic events that are known to be involved with driving a normal cell to a tumor cell have been shown to also be responsible for upregulating glucose metabolism. Thus, 2-DG takes advantage of this by naturally accumulating more in tumors than normal surrounding tissues as well as blocking a central point where oncogenes, or loss of tumor suppressor genes converge. This contrasts to other approaches where small molecules are used to target different aspects of tumor metabolism without benefitting from the natural selectivity that increased glucose uptake offers 102, 103 . In conclusion, 2-DG has been found to have wide-reaching effects on glucose metabolism in tumor cells under hypoxic and normoxic conditions as well as in endothelial cells and viruses which are summarized in Fig. 3.

With the biological trait of increased glucose metabolism so inherent in the make-up of a tumor cell, it appears only a matter of time when 2-DG will become a widely accepted treatment for cancer. Moreover, the fortuitous reality that 2-DG also mimics mannose not only has opened the opportunity to investigate how best to use it for limiting cancer growth but also presents the possibility of developing it as an anti-viral agent. These are just two of the many possible beneficial applications of this remarkable sugar analog.

Additionally, it should be noted that the non-patentable control of the compositional matter of 2-DG has slowed its development. Thus, philanthropic and or governmental resources will be required for 2-DG to reach its full clinical potential.