Monosaccharides are glucose, fructose, galactose, and ribose. Disaccharides are sucrose, maltose, and lactose. Polysaccharides are starch, glycogen, cellulose, and chitin.

Cholesterol and other steroids are lipids. Glycerol and fatty acids are building blocks of many lipids. Glycogen is a glucose polysaccharide used for energy storage in animals.

B is incorrect. A plant central vacuole stores substances and helps regulate pH, osmotic balance, and turgor pressure. Contractile vacuoles in many freshwater protists collect and expel excess water; this is a different structure and function.

Gap junction channels connect adjacent animal cells and allow ions and small molecules to pass between their cytoplasms. Plasmodesmata provide cytoplasmic connections between plant cells.

Plasmodesmata are perforated channels in plant cells.

Cell membranes can grow through the synthesis and insertion of membrane lipids and proteins. Flippases move certain phospholipids between the two leaflets of a membrane; they do not synthesize phospholipids.

Metabolism is the set of chemical reactions in an organism. Catabolic pathways break down complex molecules and can release usable energy; anabolic pathways build complex molecules from simpler units.

Enzymes lower activation energy without being consumed. They do not change the reaction’s overall free-energy change or equilibrium constant.

During glycolysis, one glucose is converted into two pyruvates in cytosol. 2ATP are invested and 4ATP are harvested, so net 2ATP are harvested. Other harvesting products are 2NADH, 2H⁺, 2H₂O.

Enzyme enolase converts 2-phosphoglycerate to phosphoenolpyruvate.

Standard DNA and RNA nucleotides contain carbon, hydrogen, oxygen, nitrogen, and phosphorus. Sulfur is not a normal constituent of these nucleotides. Sulfate and phosphate are polyatomic ions, rather than element names.

D is incorrect. Cellular DNA is generally double-stranded, and cellular RNA is generally single-stranded, although RNA can form paired regions. A single-base mutation may be silent or may alter protein sequence or function; it does not always cause a serious problem.

RNA and DNA strands are nucleic acids but RNA has unique characteristics, which are ribose sugar, uracil of bases, and single strand. Both DNA and RNA strands are linked by phosphodiester bonds.

B describes paracrine rather than endocrine signaling. Endocrine cells release hormones into the bloodstream; hormones act on target cells with suitable receptors. Hormones may circulate dissolved in plasma or bound to carrier proteins; blood cells themselves are not the general transport mechanism.

Receptors binding hydrophobic hormones such as steroid hormones are found inside of a cell and called intracellular reception.

Active transport uses energy to move solutes against a concentration or electrochemical gradient. The sodium-potassium pump is an example, so E (C and D) is the best answer.

In hypotonic conditions water moves from outside the cell to inside the cell, and plants become turgid.

Negative feedback counteracts a deviation from a regulated condition and helps maintain homeostasis. Positive feedback amplifies an initial change. These mechanisms are defined by their response to the change, not simply by whether internal and external conditions move in the same direction.

I, II, and III are true, so C is correct. Enzymes can be reused, most are proteins, and excessive heat can denature bacterial enzymes. IV is false: many enzymes function as a single polypeptide with tertiary structure; quaternary structure is not required for every enzyme.

C is false. Substrate and product concentrations can affect reaction rate, including through substrate availability, reversibility, and product inhibition. Active-site changes can decrease, increase, or leave activity unchanged; a decrease is not inevitable or necessarily immediate. Enzymes do not change ΔG or Kₑq.

The light-dependent reactions at the thylakoid membrane produce O₂, ATP, and NADPH. The Calvin cycle in the stroma uses ATP and NADPH to make G3P from CO₂; G3P can then be used to synthesize glucose and other carbohydrates.

D is correct: H⁺ flows from the thylakoid lumen to the stroma through ATP synthase, driving ATP synthesis. Water oxidation at photosystem II releases O₂, H⁺, and electrons (2H₂O → O₂ + 4H⁺ + 4e⁻). Electron transport helps build the proton gradient. ATP is synthesized by ATP synthase, not directly by photosystem II. Electrons re-excited at photosystem I are ultimately used to reduce NADP⁺ to NADPH. In aerobic mitochondrial respiration, O₂ is the final electron acceptor and H⁺ returns from the intermembrane space to the matrix.

Glycolysis occurs in the cytosol and yields ATP by substrate-level phosphorylation. In alcoholic fermentation, ethanol formation regenerates NAD⁺; lactate fermentation similarly regenerates NAD⁺ by reducing pyruvate. These fermentation steps do not themselves make additional ATP. During aerobic respiration, pyruvate is converted to acetyl-CoA, which enters the citric acid cycle in the mitochondrial matrix.

NADH is oxidized to NAD⁺ when an organic molecule accepts its electrons. NAD⁺ regeneration allows glycolysis to continue. ATP is synthesized by phosphorylation of ADP during glycolysis; ATP formation is not reduction of ADP and does not occur in the NAD⁺-regenerating fermentation step.

Cell membranes are composed of phospholipids forming bilayers (hydrophilic and hydrophobic).

Steroids contain four fused carbon rings. Cholesterol helps regulate animal cell membrane fluidity. Phospholipids form membrane bilayers, triacylglycerols store energy, cellulose is a plant cell-wall polysaccharide, and chitin is a structural polysaccharide found in fungal cell walls.

Microtubules are hollow polymers of tubulin and form the 9+2 axoneme of most motile eukaryotic cilia and flagella. Intermediate filaments include nuclear lamins and keratins. Microfilaments are actin polymers that interact with myosin during muscle contraction; myosin is a motor protein, not a microfilament.

Protein kinases transfer phosphate groups to proteins, often using ATP. Protein phosphatases remove phosphate groups. Depending on the target protein, phosphorylation can activate or inhibit its function.

In this specified pathway, a ligand activates a G-protein-coupled receptor, Gs exchanges GDP for GTP, activated Gs stimulates adenylyl cyclase, and cAMP activates protein kinase A. This is one G-protein pathway; other G proteins may inhibit adenylyl cyclase or activate different effectors.

Endocytosis means the movement of liquid, particles, and molecules by pinocytosis, phagocytosis, and receptor-mediated endocytosis. Autophagy means breaking down damaged organelles by lysosomes containing hydrolytic enzymes. Secretion means the movement of molecules from the inside to the outside of a cell.

Extreme pH changes and excessive heat can disrupt an enzyme’s folded structure. Modest cooling usually slows activity without denaturing the enzyme. The response depends on the enzyme and conditions; special cases of cold denaturation also exist.

The Calvin cycle produces glyceraldehyde-3-phosphate (G3P), a three-carbon sugar phosphate. G3P is not glucose, but it can be used to synthesize glucose and other carbohydrates. The citric acid cycle produces CO₂, NADH, FADH₂, and ATP or GTP.

Rubisco lowers activation energy and catalyzes CO₂ fixation to RuBP in the Calvin cycle, forming 3-phosphoglycerate. Citrate synthase, not Rubisco, catalyzes the combination of acetyl-CoA and oxaloacetate.

This reaction is the main concept of chloroplasts.

These fermentation pathways occur in the cytosol and regenerate NAD⁺ by transferring electrons from NADH to an organic acceptor. Muscle cells can produce lactate; yeast can produce ethanol. Fermentation does not use a respiratory electron transport chain. Anaerobic respiration does use an electron transport chain, with a final electron acceptor other than O₂.

Both organelles occur in plant cells, have double-membrane envelopes, contain DNA and ribosomes, and have an endosymbiotic origin. Typical aerobic animal cells contain mitochondria but do not contain chloroplasts, so E is not a shared feature.

E is false. A single polypeptide can fold into a functional tertiary structure. Quaternary structure describes the association of multiple polypeptide subunits and is not required for every functional protein. Tertiary structure is stabilized by hydrophobic interactions, hydrogen bonds, ionic interactions, and, in some proteins, disulfide bonds.

Primary structure uses only peptide bonds to form the linear of amino acids sequences, and secondary structure forms alpha helix and beta-pleated sheets using hydrogen bonds. Tertiary structure is formed by disulfide bonds, ionic bonds, hydrophobic interaction, and hydrogen bonds.

Endomembrane systems are involved in transport of protein and metabolic functions. This system contains nuclear envelope, rough ER, smooth ER, transport vesicles, Golgi complex, lysosomes, vacuoles, and cell membrane. Peroxisomes are not part of endomembrane system.

Both groups have ribosomes and cytoskeletal proteins. Prokaryotes have a nucleoid rather than a membrane-bound nucleus. Centrioles are characteristic of many animal cells and are not shared by both groups.

Pinocytosis internalizes extracellular fluid; phagocytosis internalizes large particles. Receptor-mediated endocytosis uses membrane receptors to internalize specific molecules. Endocytosis brings material into the cell in vesicles, whereas exocytosis releases material outside the cell.

An exergonic reaction has a negative ΔG. A reaction with a positive ΔG is endergonic. Spontaneity describes thermodynamic favorability, not reaction speed; an activation-energy barrier may remain. Enzymes lower this barrier without changing ΔG or the equilibrium constant.

All four statements are true after correcting IV: chlorophyll contains a central Mg²⁺ ion coordinated by its ring, not a magnesium molecule. Chlorophyll a participates in reaction centers; chlorophyll b and carotenoids are accessory pigments. The hydrophobic tail anchors chlorophyll in the thylakoid membrane.

Chlorophyll a is the primary reaction-center pigment and appears blue-green. It absorbs strongly in the violet-blue and red regions and reflects or transmits much of the green region. Chlorophyll b is an accessory pigment that appears yellow-green and has absorption peaks shifted relative to chlorophyll a. Carotenoids absorb mainly blue and blue-green light and appear yellow, orange, or red. Exact peak wavelengths depend on the pigment environment.

Water supplies electrons to photosystem II. Water oxidation can be written as 2 H₂O → O₂ + 4 H⁺ + 4 e⁻. These electrons replace electrons lost by the PSII reaction center.

Aerobic respiration occurs in plant and animal cells and produces water when O₂ is reduced at the end of the electron transport chain. Glycolysis supplies the ATP associated with fermentation; the NAD⁺-regenerating fermentation reactions themselves do not produce additional ATP. Pyruvate oxidation produces acetyl-CoA in the mitochondrial matrix of eukaryotic cells.

Gluconeogenesis synthesizes glucose from non-carbohydrate precursors such as glycerol, lactate, and glucogenic amino acids. Not all amino acids are glucogenic; leucine and lysine are exclusively ketogenic.

O₂ accepts electrons at the end of the mitochondrial electron transport chain and combines with H⁺ to form H₂O.

A common sequence is observation → hypothesis → predictions → experimentation → analysis and conclusion. Controls are incorporated into the experimental design and used during the experiment; they are not a separate step performed only after experimentation.

Ribosomes are bound at rough ER for protein and membrane synthesis.

The lysosome is bounded by a single membrane. The nucleus, chloroplast, and mitochondrion each have a double-membrane envelope. Therefore B only is correct, corresponding to answer E.

Competitive inhibitors compete with substrates for the active site; substrates do not bind the allosteric site in this mechanism. Allosteric activators can increase enzyme activity, positive cooperativity can amplify responses, and feedback inhibition can regulate anabolic pathways. In noncompetitive inhibition, an inhibitor binds at a site distinct from the substrate-binding site and reduces activity.

C4 plants adapted in hot and dry conditions have spatial separations to fix CO₂ in mesophyll cells for the working of the Calvin cycle in bundle sheath cells such as sugarcane and corn.

Spatial separation occurs in C4 plants. CAM plants have temporal separation (day and night) that CO₂ enters into stomata during the night and then the Calvin cycle works with closed stomata during the day.

Oxidative phosphorylation produces most of the ATP during aerobic glucose oxidation. A common modern estimate for eukaryotic cells is about 30–32 ATP per glucose in total: about 2 net ATP from glycolysis, 2 ATP (or GTP equivalents) from the citric acid cycle, and about 26–28 ATP from oxidative phosphorylation. Actual yields vary with NADH shuttle systems, proton leakage, transport costs, and cellular conditions. The electron transport chain builds the proton gradient; ATP synthase uses that gradient to synthesize ATP.

During aerobic glucose oxidation in typical eukaryotic cells, pyruvate is transported into the mitochondrial matrix and converted to acetyl-CoA. Glycolysis produces pyruvate and ATP in the cytosol. Lactate or ethanol can be produced by cytosolic fermentation. Cells can also synthesize cytosolic acetyl-CoA through other pathways, so the question is specifically about glucose oxidation.

Amino acid, a subunit of protein, is composed of an amino group, a carboxyl group, a functional group, α carbon, and hydrogen. It is not formed by dehydration synthesis.

Prokaryotic cells lack a membrane-bound nucleus and generally lack the membrane-bound organelles characteristic of eukaryotes. Their DNA occupies a nucleoid, and they contain ribosomes; neither the nucleoid nor ribosomes are surrounded by a membrane. Binary fission is asexual reproduction, not mitosis, meiosis, or sexual reproduction.

The organelle described is a chloroplast, which occurs in plant and algal cells and carries out photosynthesis. Its internal fluid compartment is the stroma. The matrix is the internal fluid compartment of a mitochondrion. Mitochondria occur in both typical plant and animal cells.

Substructures of chloroplasts are grana and stroma. The thylakoid disk, a single structure of the granum, contains thylakoid membrane and lumen. The thylakoid membrane contains photosystems and chlorophyll.

Water oxidation at photosystem II releases O₂, H⁺, and electrons. PSII and electron transport contribute to the proton gradient; ATP is synthesized by ATP synthase, not directly by PSII. Electrons passing through PSI ultimately support reduction of NADP⁺ to NADPH. 3-PGA is formed during Calvin-cycle carbon fixation. Thus E is the incorrect match.

I, III, and IV are true. Classic lactate fermentation regenerates NAD⁺ by reducing pyruvate to lactate without releasing CO₂. Alcoholic fermentation releases CO₂ when pyruvate is converted to acetaldehyde, which is then reduced to ethanol. This distinction does not imply that every CO₂-producing fermentation pathway must produce ethanol.

Complete aerobic glucose oxidation yields CO₂ and H₂O, conserves energy in ATP, and releases heat. G3P (also called PGAL) is an intermediate in glycolysis and a carbohydrate product of the Calvin cycle; it is not a net final product of complete aerobic glucose oxidation.

Both mitochondria and chloroplasts contain their own DNA and ribosomes and have double-membrane envelopes. Aerobic glucose oxidation is a mitochondrial function, whereas linear photosynthetic electron transport is a chloroplast function. NADPH production is not exclusive to chloroplasts: mitochondria also have pathways that generate NADPH, so it cannot serve as a distinguishing feature here.

All are true.

Phospholipase C cleaves PIP₂ into IP₃ and DAG. IP₃ binds IP₃ receptors in the ER membrane and opens Ca²⁺-release channels. The released cytosolic Ca²⁺ regulates target proteins, often through Ca²⁺-binding proteins such as calmodulin. IP₃ acts on the ER receptor; Ca²⁺ mediates the subsequent regulation of these proteins.

Reduction is a gain of electrons; oxidation is a loss of electrons. NADP⁺ gains electrons to become NADPH. Carbon is oxidized when glucose or CH₄ is converted to CO₂. Water is oxidized when O₂ is produced during photosynthesis. NADH is oxidized when it becomes NAD⁺. Oxygen gain or loss is not a reliable description of every redox reaction.

Mitochondrial oxidative phosphorylation couples electron transport to chemiosmotic ATP synthesis at the inner mitochondrial membrane. Ubiquinone and cytochrome c transfer electrons; ATP synthase uses the proton gradient. The outer mitochondrial membrane is not the membrane that houses this electron transport chain and ATP synthase. ATP generation in chloroplast thylakoids is photophosphorylation.

The citric acid cycle takes place primarily in the mitochondrial matrix, not in the chloroplast stroma. Although the cycle does not directly consume O₂, its sustained operation in typical aerobic cells depends on regeneration of NAD⁺ and FAD through respiratory electron transport. The Calvin cycle occurs in the chloroplast stroma.

Grana are stacks of thylakoid disks in chloroplasts. Mitochondria have a double-membrane envelope, inner-membrane folds called cristae, and an internal matrix in which most citric-acid-cycle reactions occur.

II and III are true. During ATP synthesis, H⁺ moves down its electrochemical gradient through ATP synthase: from thylakoid lumen to stroma in chloroplasts and from intermembrane space to matrix in mitochondria. Proton pumping by appropriate electron transport complexes moves H⁺ against this gradient and is active transport. Thus I and IV are false.