Sticky Biology — Companion Study Material

David Shusterman, MD · 2026 edition

Selected Worked Answers — Check the Mechanism

These answers model reasoning for a selection of the book’s central problems. They are not the only acceptable wording, and they do not replace the attempt that comes before checking. Compare the causal steps in your explanation with the steps here. If you omitted a condition, revise the explanation and try a changed case.

1. Why Can Water Dissolve Salt Without Dissolving Everything?

Return to Chapter 2 for the full explanation.

Water molecules are polar, so their partial charges can interact with ions and other polar groups. In a soluble ionic compound, interactions with water can help separate and stabilize ions in solution. Whether a substance dissolves depends on the overall energetic and entropic balance under the conditions, not simply on whether water is present. Nonpolar substances have different interactions and often do not mix readily with water.

The common error is to treat “universal solvent” as a literal promise. A better prediction asks what particles are involved and how they interact with water and one another. Temperature, concentration and chemical identity matter. The memory image of water surrounding particles is useful only when those interactions are plausible.

2. Why Does Breaking a Bond Require Energy?

Return to Chapter 2 for the full explanation.

Separating atoms joined by a chemical bond requires an energy input. Forming new bonds releases energy, and the net change across a reaction depends on the full set of changes. Biological energy explanations also depend on free energy under the relevant conditions. Saying that energy simply comes out when a bond breaks leaves out the accounting that makes a reaction favorable overall.

For ATP hydrolysis, describe the overall reaction and its coupling to another process. Do not imagine the phosphate bond as a tiny container that spills usable energy when cut. The products, interactions with water and cellular conditions matter. The useful memory cue is a coupled reaction, not a broken spring with no chemical context.

3. What Happens to an Animal Cell in a Hypotonic Solution?

Return to Chapter 5 for the full explanation.

If the surrounding solution is hypotonic relative to the cell, water tends to enter under the specified conditions because of the effective difference in nonpenetrating solutes. The cell can swell and may lyse if the imbalance is sufficiently large and uncorrected. The prediction depends on membrane permeability and which solutes can cross. Total solute concentration alone is not always enough to establish tonicity.

A plant cell has a wall that changes the mechanical outcome of water entry. Pressure can build against that wall rather than producing the same response as an unsupported animal cell. The membrane transport principle remains related while the structural context changes. This is why a changed example is a stronger test than repeating “water enters.”

4. Can an Enzyme Change Where a Reaction Ends at Equilibrium?

Return to Chapter 6 for the full explanation.

An enzyme lowers an activation barrier and changes the rate at which a reaction approaches equilibrium. It does not change the reaction’s equilibrium position by itself. Both forward and reverse processes remain subject to the same thermodynamic relationships. A catalyst can make a process faster without changing the free-energy difference between its initial and final states.

Cells can change reaction behavior by altering concentrations, removing products or coupling processes. Those changes should not be attributed to catalysis alone. If your drawing shows an enzyme lowering the products’ energy rather than the activation barrier, redraw it. The height of the barrier and the difference between endpoints answer different questions.

5. Why Does Respiration Depend on a Membrane Gradient?

Return to Chapter 7 for the full explanation.

In oxidative phosphorylation, electron transfer supports proton movement across a membrane. The resulting electrochemical gradient can drive proton flow through ATP synthase, coupling that flow to ATP production. The membrane helps maintain a separation that would otherwise dissipate. A proton gradient is therefore both a chemical and an electrical relationship across a physical boundary.

If protons leak back by a route that bypasses ATP synthase, less of the gradient’s energy may support ATP synthesis. Electron transport and oxygen consumption need not stop in exactly the same way or at the same moment. The effect depends on the disturbance and regulatory responses. Follow electron movement, proton movement and ATP formation as connected but distinct processes.

6. Does a Plant Stop Respiring in the Light?

Return to Chapter 8 for the full explanation.

No, plant cells continue to use respiration while photosynthesis occurs in suitable cells and conditions. Photosynthesis captures light energy and fixes carbon, while respiration supports cellular work through metabolic pathways. Net gas exchange reflects the combined rates of several processes. A net uptake of carbon dioxide does not mean that no carbon dioxide is being produced internally.

To test your explanation, consider a root cell without photosynthetic chloroplasts. It can receive organic compounds produced elsewhere and use them in metabolism. The plant is an integrated organism rather than a collection of cells all doing the same thing. Green appearance is not a substitute for tracing the actual routes.

7. Why Do Cells with the Same Genome Behave Differently?

Return to Chapter 12 for the full explanation.

Different cells can express different sets and amounts of genes, process molecules differently and respond to different signals. Existing cell structures, regulatory proteins and developmental history also influence behavior. Shared DNA sequence does not imply identical molecular activity at every moment. Cell identity depends on how inherited information is used in a particular context.

The common shortcut is to say that unused genes are absent. In many differentiated cells, much of the same genome remains present while expression differs. There are biological exceptions to simple genome-sharing statements, so the general explanation should remain appropriately qualified. The central distinction is between possessing information and expressing it in a particular way.

8. Why Does Meiosis Increase Variation Among Gametes?

Return to Chapter 10 for the full explanation.

Homologous chromosomes can exchange corresponding DNA segments through crossing over, and homologous pairs are distributed into daughter cells through their orientations and separation. These processes produce different combinations of inherited material. Fertilization then combines gametes from parents. Variation arises through several stages rather than from chromosomes deliberately choosing new traits.

Crossing over usually rearranges existing sequence combinations rather than creating every variant from nothing. Mutation provides new sequence variants, while recombination reshuffles them. Keep these sources of variation separate in your explanation. The word “mixing” helps only if you can name what is mixed and by which process.

9. What Does a One-in-Four Inheritance Probability Mean?

Return to Chapter 13 for the full explanation.

In a specified Mendelian cross, a one-in-four probability describes the chance for each relevant offspring under the model’s assumptions. It does not guarantee exactly one affected offspring in every group of four. Small samples can differ from expected proportions by chance. The probability calculation and the observed family outcome are related but not identical.

If the question asks about several independent offspring, combine probabilities according to the event described. “All three have a particular outcome” differs from “at least one has that outcome.” Define the event before multiplying or subtracting. Also check whether the genetic model, penetrance and independence assumptions are appropriate to the problem.

10. Does a Mutation Appear Because an Organism Needs It?

Return to Chapter 15 for the full explanation.

Mutation does not generally arise as a targeted response that supplies the particular adaptive change an organism needs. Mutational processes have biases and can vary in rate, but natural selection acts on the consequences of inherited variation already arising through those processes. A beneficial variant can become more common when it contributes to reproductive success. The population’s later adaptation should not be projected backward as the mutation’s intention.

Antimicrobial resistance provides a useful context for separating origin from selection. Resistant variants can survive and reproduce differently when exposed to an antimicrobial, changing population composition. Other routes, including gene transfer in microbes, can also contribute. Explain the source of variation and the selective environment as distinct parts of the story.

11. Why Can Two Similar Organisms Be Distant Relatives?

Return to Chapter 16 for the full explanation.

Similar environments can favor similar features in different lineages, producing convergent evolution. Similarity alone therefore does not establish recent shared ancestry for a particular trait. Phylogenetic inference compares multiple kinds of evidence and distinguishes shared ancestry from independent origins. A familiar body shape is a clue, not a complete family tree.

When reading a tree, identify shared nodes rather than judging relatedness by which tips are drawn nearest on the page. Rotating branches around a node does not change the ancestry represented. Branch lengths carry time or amount-of-change information only when the diagram defines them that way. The drawing’s geometry must be interpreted through its convention.

12. How Does Water Reach a Leaf?

Return to Chapter 18 for the full explanation.

Water enters and moves through plant tissues along water-potential relationships, and transpiration can help generate tension in xylem. Cohesion among water molecules supports a continuous column under suitable conditions. Roots, vascular tissues, leaf surfaces and the atmosphere participate in the route. The process is not a tiny muscular pump lifting water one drop at a time.

Stomatal closure can reduce water loss while also limiting carbon dioxide entry. The tradeoff links water balance with photosynthesis rather than separating them into independent plant functions. Environmental conditions and regulation influence the outcome. A correct answer follows the route and identifies the competing demands.

13. Why Do Many Large Animals Need Internal Transport?

Return to Chapter 19 for the full explanation.

As a body becomes larger, distances between many cells and the external environment increase. Diffusion is effective over short distances but becomes too slow for many needs across large distances. Bulk transport can move materials between exchange surfaces and tissues, while diffusion still operates across short final gaps. The two mechanisms cooperate rather than replacing one another completely.

Body shape and activity influence the demands, so size alone does not specify one universal design. Thin or highly branched forms can maintain short exchange distances in ways a compact body cannot. Compare geometry and metabolic requirements before predicting the system. A surface-area-to-volume ratio is a starting relationship, not the whole organism.

14. Why Is Digestion Different from Cellular Respiration?

Return to Chapter 21 for the full explanation.

Digestion breaks food into forms that can be absorbed and processed by the organism. Cellular respiration uses metabolic reactions to transfer energy from suitable substrates and support ATP production. Digestion can supply those substrates, but breaking food down in the gut is not identical to making ATP in a cell. Absorption, transport and metabolism connect the two processes.

An amino acid from food might enter a new protein rather than immediately serve as a respiratory substrate. Carbon from a sugar might be stored, incorporated into another molecule or oxidized. Nutrients have multiple possible destinations. A complete explanation distinguishes obtaining material from deciding, through regulated metabolism, where it goes.

15. Why Can Blood Flow Change Without a New Heartbeat Pattern?

Return to Chapter 22 for the full explanation.

Blood flow depends on pressure differences and resistance, among other aspects of the circulation. Changes in vessel diameter can alter resistance and redistribute flow. The heart supplies the pumping action, but local and systemic regulation also influence delivery. A single heart-rate measurement cannot describe every tissue’s blood supply.

Gas exchange adds another layer because oxygen must move between air, blood and tissues under appropriate gradients. Circulation and ventilation are connected but distinct. A problem in one route cannot always be inferred from a measurement in the other. Trace the entire path before assigning the limiting step.

16. Why Does an Action Potential Not Get Smaller as It Travels Normally?

Return to Chapter 23 for the full explanation.

In normal propagation, local currents bring neighboring membrane regions toward threshold and trigger regenerative channel activity. The signal is renewed along the excitable membrane rather than merely drifting as a fading chemical packet. Action-potential amplitude is not the usual way a neuron represents a stronger sustained stimulus. Firing frequency and patterns can carry information, depending on the system.

The refractory properties of recently active membrane influence timing and propagation. They arise from channel behavior and membrane conditions, not from a neuron consciously refusing another message. A useful drawing shows successive membrane regions and their states. It should not depict a single ion racing the entire length as the message itself.

17. What Makes an Adaptive Immune Response Specific?

Return to Chapter 24 for the full explanation.

Different lymphocytes possess receptors with different recognition properties. An appropriate antigenic encounter can contribute to activation and expansion of selected populations under the required signaling conditions. The response therefore changes the abundance and activity of cells with particular recognition capacities. Antibodies are molecules produced by differentiated B-cell descendants, not tiny immune cells themselves.

Memory can support a changed response during later encounters, but protection is not automatically complete or lifelong. Pathogen variation, waning responses and other factors can matter. Keep specificity, memory and guaranteed outcome as separate ideas. The first two help explain a response without establishing the third.

18. Why Is Filtration Only the Start of Urine Formation?

Return to Chapter 25 for the full explanation.

Filtration moves water and suitable small substances from blood into the nephron’s initial filtrate. Reabsorption returns selected substances toward the blood, while secretion moves other substances into the tubular fluid. Excretion reflects the combined result of these processes. A filtered substance is not necessarily lost from the body.

For a simplified accounting, excreted amount equals filtered amount minus reabsorbed amount plus secreted amount over a matched interval. The equation organizes the routes but does not describe every regulatory detail. A change in urine concentration can involve water handling as well as solute handling. Follow both rather than treating concentration as an amount by itself.

19. What Does Negative Feedback Actually Oppose?

Return to Chapter 25 for the full explanation.

Negative feedback acts so that a response tends to reduce the initiating deviation in a regulated variable. The word “negative” describes the loop’s relationship, not whether the outcome is unpleasant. A variable can fluctuate around a working range while feedback remains active. Perfect constancy is not required for homeostasis.

Identify the variable, sensing relationship, integrating processes and effectors in your chosen example. Then trace how the response changes the original disturbance. If the response amplifies that disturbance, the loop has a different sign. Labels should follow the causal arrows rather than the emotional meaning of positive and negative.

20. Why Might a Population Count Fall Without More Deaths?

Return to Chapter 26 for the full explanation.

Emigration can reduce local membership, and reduced recruitment can change future counts even if the death rate is unchanged. A survey can also detect fewer individuals because conditions or methods alter visibility. The interpretation depends on the population boundary, life stages counted and sampling design. A lower observation is not automatically a measured increase in mortality.

To distinguish explanations, track movement, reproduction, survival and detectability where possible. Repeated standardized surveys can help, but they do not eliminate every uncertainty. State whether your conclusion concerns observed counts or estimated abundance. This keeps the measurement attached to what it can support.

21. Does Mutualism Mean Both Partners Always Benefit?

Return to Chapter 27 for the full explanation.

Mutualism describes a beneficial relationship under the conditions and outcomes being considered. Resource supply, partner identity and environmental stress can change its balance. A familiar example should not be treated as an unconditional contract between species. Organisms respond through their biology rather than through conscious promises of cooperation.

To test a proposed mutualism, specify benefits such as improved survival, growth or reproduction for each participant. Compare appropriate conditions with and without the interaction while accounting for other changes. Measuring a resource transfer alone may not establish the net consequence. Benefits and costs can occur through several routes at once.

22. Why Is There No Universal Ten-Percent Transfer Rule?

Return to Chapter 28 for the full explanation.

Trophic transfer depends on how much production is consumed, assimilated and allocated to new consumer production. Organisms and environments differ in these processes. A supplied efficiency can be used in a teaching calculation, but it should not be mistaken for a constant governing every ecosystem. State the basis and interval of the compared quantities.

Also distinguish biomass from production. A small producer stock can turn over quickly and support a larger consumer stock at a given moment. That does not create energy or overturn the limits of transfer. The flow through the stock explains what the snapshot alone cannot.

23. What Would Count as Evidence That Restoration Worked?

Return to Chapter 29 for the full explanation.

The answer depends on the stated goal. Planting, removing a barrier or changing management records an intervention, while survival, reproduction, habitat use or restored function may measure its intended outcomes. Monitoring should connect the action to those outcomes over an appropriate interval. A completed activity is not automatically a demonstrated ecological result.

Comparison sites and repeated observations can help separate intervention effects from changing background conditions. Unintended consequences and effects on people may also need assessment. A useful conclusion identifies what improved, how it was measured and what remains uncertain. That precision makes the result more actionable rather than less impressive.

24. How Do You Know Whether an Idea Has Become Sticky?

Return to Chapter 30 for the full explanation.

Try to reconstruct the mechanism without looking, explain each major arrow and apply it to a changed case. Then check against the source and correct the weak points. Familiarity with the original page is a different experience from being able to use the idea. Both may feel comfortable, but the changed case provides a more demanding test.

If the attempt fails, identify the missing relationship rather than rereading everything indiscriminately. You may know the parts but not their order, or the order but not the reason for a step. A focused repair gives the next practice attempt a clear purpose. Learning continues through these corrections rather than ending with a single successful recall.

Glossary — Words with Relationships

This glossary gives compact working meanings for terms used throughout the book. Use each entry as a route back to an explanation, not as a replacement for the process itself. When two terms are easy to confuse, compare what changes, what stays the same and the scale at which each term applies.

Activation Energy

The energy barrier associated with reaching a reaction’s transition state. Enzymes can lower an activation barrier without changing the overall free-energy difference between reactants and products. See Chapter 6.

Active Transport

Membrane transport that moves a substance against its electrochemical gradient through coupling to an energy source. The source may be ATP hydrolysis or another gradient, depending on the transport mechanism. See Chapter 5.

Adaptation

An inherited feature shaped by natural selection because of its consequences for reproductive success in a context. The term can also describe the evolutionary process producing such fit, rather than an individual’s short-term adjustment. See Chapter 15.

Allele

A version of a DNA sequence at a particular genomic location. Different alleles can contribute to variation, but their effects depend on the locus, other genetic factors and environmental context. See Chapter 13.

Antibody

An immunoglobulin molecule that binds particular molecular features of an antigen. Antibodies are produced by differentiated B-cell descendants and are molecules rather than cells. See Chapter 24.

Antigen

A molecular structure recognized by components of adaptive immunity. Recognition involves particular features, called epitopes, and does not automatically establish that the source is harmful. See Chapter 24.

ATP

A nucleotide whose reactions help couple energy-releasing and energy-requiring cellular processes. ATP is continually regenerated and used, and its hydrolysis must be understood as an overall chemical reaction. See Chapter 6.

Biodiversity

Biological variation within species, among species and across ecosystems. Different measures capture different aspects, so species richness alone does not describe every dimension of diversity. See Chapter 29.

Biomass

The amount of biological material present in a defined group or area at a specified time. Biomass is a stock, whereas production describes a rate of making new biological material. See Chapter 28.

Carrying Capacity

A model parameter representing the population level supported under specified environmental conditions. It can change with resources and interactions and is not a fixed guarantee about a real population’s future. See Chapter 26.

Cell

A membrane-bounded unit of life with organized molecular processes. Cells differ greatly in structure and capabilities, and some depend extensively on other cells or organisms. See Chapter 4.

Cellular Respiration

Metabolic processes that transfer energy from suitable substrates and support ATP production. Aerobic respiration uses oxygen as a final electron acceptor in its electron-transport pathway, while other respiratory pathways use different acceptors. See Chapter 7.

Chromosome

An organized DNA-containing structure carrying genetic information. Its physical organization differs among organisms, and a replicated chromosome contains sister chromatids until their separation under the relevant counting convention. See Chapter 10.

Community

Populations of different species occurring together in a defined setting. Community ecology examines their composition and interactions, including effects that pass indirectly through other species. See Chapter 27.

Concentration Gradient

A difference in a substance’s concentration across space. For ions, electrical conditions also matter, so the electrochemical gradient combines concentration and electrical contributions to movement. See Chapter 5.

Control

A comparison condition used to help interpret the effect being tested. An appropriate control addresses alternative explanations, and its design depends on the specific experiment rather than on one universal recipe. See Chapter 30.

Cytoplasm

Cellular material within the plasma membrane, with usage in eukaryotes generally excluding the nucleus. Cytosol is the fluid component, while cytoplasm includes additional structures and contents. See Chapter 4.

Diffusion

The spreading of particles through random molecular motion. A concentration difference can produce net diffusion; at equilibrium, particles continue moving even when opposing flows balance. See Chapter 5, “Random Movement Can Have a Net Direction.”

DNA

Deoxyribonucleic acid, a polymer that stores inherited sequence information in cells and many viruses. Complementary base pairing supports copying and other interactions, but molecular machinery is required to carry out those processes. See Chapter 11.

Dominant

In complete dominance for a specified trait and conditions, an allele is called dominant when the heterozygote has the same phenotype as the corresponding dominant homozygote. This describes a phenotype relationship, not strength, frequency or benefit. Incomplete dominance and codominance describe other relationships. See Chapter 13, “Dominance Describes a Phenotype Relationship.”

Ecosystem

Organisms and the nonliving environment with which they interact within a defined boundary. Ecosystems exchange matter and energy with surrounding systems and need not be closed. See Chapter 28.

Enzyme

A biological catalyst, usually a protein and sometimes RNA, that accelerates a reaction. Its activity depends on molecular interactions and conditions, and catalysis alone does not change equilibrium. See Chapter 6.

Epigenetic Regulation

Regulation associated with molecular features such as chromatin modifications that can affect gene activity without changing the underlying DNA sequence. Persistence and inheritance vary by mechanism and biological context. See Chapter 12.

Eukaryote

An organism whose cells belong to the lineage characterized by nuclei and other membrane-bounded compartments. Some specialized eukaryotic cells lose particular structures during development, so a lineage definition is more reliable than one isolated cell feature. See Chapter 4.

Evolution

Change in inherited characteristics of populations across generations. Natural selection, genetic drift, mutation and gene flow can contribute, and evolution is not equivalent to an individual learning or developing. See Chapter 15.

Feedback

A relationship in which a process’s consequences influence the process or its initiating variable. Negative feedback tends to oppose a deviation, while positive feedback tends to amplify it under the stated conditions. See Chapter 25.

Fertilization

The joining of gametes and their genetic contributions in sexual reproduction. Its cellular details vary among organisms, and it is distinct from the later processes of embryonic development. See Chapter 20.

Fitness

Reproductive contribution in a specified evolutionary context, often considered relative to alternatives. Fitness does not simply mean strength, health or longevity outside the conditions affecting reproduction. See Chapter 15.

Gene

A DNA sequence whose expression contributes to a functional RNA or, through RNA, a protein product. Gene boundaries and regulation can be complex, and one gene does not necessarily correspond to one simple trait. See Chapter 12.

Gene Expression

The use of genetic information to produce functional molecular products. Regulation can occur at several stages, including transcription, RNA processing, translation and product stability or activity. See Chapter 12.

Gene Flow

Movement of genetic variants among populations through successful migration and reproduction or other relevant transfer. Movement of an individual without a genetic contribution does not necessarily produce the same evolutionary effect. See Chapter 15.

Genetic Drift

Changes in variant frequencies caused by chance sampling in population processes. Drift can be especially influential in small populations and does not require a variant to improve reproductive success. See Chapter 15.

Genome

The complete genetic material of an organism or relevant biological entity under the stated usage. A genome includes more than protein-coding genes and does not by itself specify every outcome independently of context. See Chapter 14.

Genotype

The genetic constitution at one or more specified loci or across a broader scope. Always check the intended scale, because a genotype label in a cross may describe only a small part of the genome. See Chapter 13.

Habitat

The place and environmental setting in which an organism lives. Habitat differs from niche, which includes conditions, resource use and relationships associated with persistence and reproduction. See Chapter 27.

Homeostasis

Dynamic regulation that helps maintain internal conditions within workable limits. It involves continuing responses and fluctuations rather than perfect constancy or a single unchanging numerical target. See Chapter 25.

Homologous Chromosomes

Chromosomes corresponding in gene arrangement, typically one inherited from each parent in a diploid organism. They can carry different alleles and are not the same thing as sister chromatids. See Chapter 10.

Hormone

A signaling molecule that influences responsive cells through appropriate receptors and pathways. In animal endocrine signaling, hormones commonly travel through circulation, while effects depend on target-cell context. See Chapter 23.

Hypothesis

A proposed explanation that generates expectations open to evaluation with evidence. A hypothesis is not strengthened merely by sounding plausible; it must be compared with observations and alternatives. See Chapter 1.

Ion

An atom or molecule with a net electrical charge from an imbalance of electrons and protons. Ion movement depends on electrical as well as chemical conditions, which matters strongly across cell membranes. See Chapter 2.

Meiosis

A specialized division process that reduces chromosome-set number and contributes to sexual life cycles. Homolog separation and sister-chromatid separation occur in distinct divisions, with recombination adding further variation. See Chapter 10.

Membrane Potential

An electrical potential difference across a membrane. It reflects charge separation near the membrane and should not be pictured as an entire cell interior containing only negative charges. See Chapter 23.

Metabolism

The connected chemical reactions of a living system. It includes pathways that build molecules and pathways that break down or transform substrates, linked through energy and material exchanges. See Chapter 6.

Mitosis

Nuclear division that separates replicated chromosomes into daughter nuclei in eukaryotic cells. Cytokinesis divides the cell’s cytoplasm and is related to, but distinct from, mitosis itself. See Chapter 10.

Mutation

A change in genetic sequence. Its consequences depend on location, molecular effect, cell lineage and environment, and it is not automatically harmful, beneficial or visible in a trait. See Chapter 14.

Natural Selection

Differential reproductive contribution associated with inherited variation under particular conditions. Selection can change variant frequencies without foresight, intention or a goal of making organisms universally better. See Chapter 15.

Niche

The conditions, resource use and relationships associated with an organism’s way of persisting and reproducing. A niche is broader than a physical address and does not imply an assigned ecological duty. See Chapter 27.

Organelle

A specialized cellular structure with particular functions, commonly referring to membrane-bounded compartments in eukaryotic cells. Usage varies for some structures, so the relevant definition should be stated when precision matters. See Chapter 4.

Osmosis

Net water movement across a selectively permeable membrane driven by water-potential differences. Solutes and pressure can influence that relationship, and the outcome depends on membrane properties and the surrounding conditions. See Chapter 5.

Phenotype

An observable or measurable characteristic of an organism. Phenotypes arise through genetic, environmental and developmental influences, so an observed trait does not always identify a unique genotype. See Chapter 13.

Photosynthesis

Processes that use light energy to support chemical transformations, including carbon fixation in oxygenic photosynthetic organisms. The released oxygen in oxygenic photosynthesis comes from water, while carbon in new organic material comes from carbon dioxide. See Chapter 8.

Phylogeny

An inferred history of evolutionary relationships among lineages. A phylogenetic tree represents hypotheses about shared ancestry, and its branch lengths carry additional meaning only when explicitly defined. See Chapter 16.

Population

Individuals of the same species within a defined place and time. Population descriptions depend on boundaries, counted life stages, movement and the methods used to estimate membership. See Chapter 26.

Primary Production

The rate at which primary producers build organic material from inorganic carbon using suitable energy sources. Net primary production accounts for producer respiration and differs from a snapshot of standing biomass. See Chapter 28.

Protein

A polymer of amino acids whose structure and interactions support particular functions. Many proteins require folding, modification or partners, so sequence-to-function relationships involve more than simply making a chain. See Chapter 3.

Receptor

A molecule that recognizes a signal or other binding partner and can participate in a response. Binding specificity and downstream effects depend on molecular structure and cellular context. See Chapter 9.

Recessive

Describing an allele-associated phenotype not expressed in a specified heterozygous comparison under a simple dominance model. Recessive does not mean rare, weak or evolutionarily unimportant. See Chapter 13.

Replication

Copying genetic material through a molecular process, usually referring here to DNA replication. In semiconservative DNA replication, each resulting double-stranded molecule contains one parental strand and one newly synthesized strand. See Chapter 11.

Ribosome

An RNA-and-protein complex that translates an RNA message into a polypeptide. It works with transfer RNAs and other factors and does not directly translate DNA into protein. See Chapter 12.

RNA

Ribonucleic acid, a polymer with roles in information transfer, regulation, structure and catalysis. Messenger RNA is one type, so RNA should not be defined only as a temporary protein-making message. See Chapter 12.

Species

A grouping of organisms or lineages defined using a concept suited to the biological question. Reproductive, morphological and evolutionary criteria have different strengths and limitations across living and fossil organisms. See Chapter 16.

Tonicity

The effect of a solution on cell volume through water movement, determined by effectively nonpenetrating solutes under the conditions. It differs from total osmotic concentration when solutes cross the membrane differently. See Chapter 5.

Transcription

Synthesis of RNA using a DNA template in cellular gene expression. The RNA product may be processed and can have several possible functions, including serving as a message for translation. See Chapter 12.

Translation

Synthesis of a polypeptide according to an RNA sequence through the genetic code. Ribosomes and transfer RNAs connect nucleotide triplets with amino acids, while initiation and termination define the translated region. See Chapter 12.

Trophic Level

A position in a feeding-based account of energy and material transfer. Omnivory and changing diets can place an organism across more than one simple level, so real food webs often resist a single ladder. See Chapter 28.

Variation

Differences among individuals or biological entities in a defined characteristic. Variation can have genetic and environmental sources, and only the relevant inherited component supports evolutionary responses through selection across generations. See Chapter 14.

Virus

A biological entity whose replication depends on host-cell machinery and suitable interactions. Viruses vary in genome type and replication strategy, and their dependence distinguishes them from independently functioning cellular organisms. See Chapter 17.