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 components of many lipids. Glycogen is a glucose polysaccharide used for energy storage in animals.

The central vacuole stores nutrients and metabolic waste and helps maintain pH and turgor pressure. Pumping excess water out of a cell is the role of a contractile vacuole in certain protists, not the central vacuole of a plant cell.

Gap junctions form channels between adjacent animal cells, allowing ions and small molecules to pass directly between their cytoplasms. They help coordinate activity in tissues such as cardiac muscle.

Plasmodesmata are channels connecting the cytoplasm of adjacent plant cells.

Cell membranes grow through the synthesis and addition of lipids and proteins. Flippases move phospholipids between membrane leaflets; they do not synthesize lipids.

Metabolism includes all chemical reactions in a cell. Catabolic pathways break down complex molecules, whereas anabolic pathways build complex molecules from simpler units.

Enzymes accelerate chemical reactions by lowering activation energy without being consumed.

In the cytosol, glycolysis converts one glucose molecule into two pyruvate molecules. It consumes 2 ATP and produces 4 ATP, for a net gain of 2 ATP. Other products include 2 NADH, 2 H⁺, and 2 H₂O.

Enzyme enolase converts 2-phosphoglycerate to phosphoenolpyruvate.

Standard DNA and RNA nucleotides contain carbon, hydrogen, oxygen, nitrogen, and phosphorus. Sulfur is not normally a constituent of these nucleotides.

Cellular DNA is generally double-stranded, whereas RNA is generally single-stranded. A single-base mutation may be silent or may affect a protein; it does not always cause serious problems.

RNA and DNA are nucleic acids. RNA typically contains ribose, uses uracil rather than thymine, and is single-stranded. Both DNA and RNA have nucleotides linked by phosphodiester bonds.

Endocrine cells release hormones into the bloodstream, allowing signaling to distant target cells. Paracrine signals act locally by diffusing to nearby cells.

Steroid hormones typically bind intracellular receptors because they can cross the plasma membrane. Ligand-gated ion channels, G protein-coupled receptors, and receptor tyrosine kinases are cell-surface receptors.

Active transport uses energy to move solutes against their electrochemical gradients. The sodium-potassium pump is an example of active transport.

In a hypotonic solution, water enters the cell by osmosis. Plant cells become turgid as the cell wall limits expansion.

Negative feedback counteracts an initial change and helps return a regulated variable toward its set point. Positive feedback amplifies an initial change.

Statements I, II, and III are true. Statement IV is false: many enzymes consist of a single polypeptide and do not have quaternary structure.

Enzyme activity can be affected by substrate and product concentrations, pH, and temperature. Enzymes do not change a reaction’s Gibbs free-energy change or equilibrium constant.

Light-dependent reactions in thylakoid membranes produce O₂, ATP, and NADPH. The Calvin cycle in the stroma uses ATP and NADPH to make G3P, which can be used to synthesize glucose.

Water is oxidized at photosystem II, releasing O₂, H⁺, and electrons. Electron transport builds a proton gradient that drives ATP synthase; photosystem I ultimately supplies electrons for NADPH formation. During chloroplast ATP synthesis, H⁺ flows from the thylakoid lumen into the stroma.

Glycolysis produces pyruvate and a net gain of ATP in the cytosol. Fermentation regenerates NAD⁺, allowing glycolysis to continue. In typical aerobic eukaryotic cells, pyruvate is converted to acetyl-CoA in the mitochondrial matrix before entering the citric acid cycle.

Fermentation oxidizes NADH to NAD⁺. ADP is phosphorylated to ATP during the associated glycolysis; this is not a reduction reaction.

Phospholipids have hydrophilic heads and hydrophobic tails, allowing them to form bilayers in cell membranes.

Steroids have four fused carbon rings. Cholesterol helps regulate membrane fluidity. Phospholipids form membrane bilayers, triacylglycerols store energy, cellulose forms plant cell-wall microfibrils, and chitin is a nitrogen-containing polysaccharide in fungal cell walls.

Microtubules form parts of centrosomes, centrioles, and many cilia and flagella. Actin microfilaments interact with myosin in muscle contraction. Intermediate filaments include keratins and the lamins of the nuclear lamina.

Protein kinases add phosphate groups to proteins; protein phosphatases remove them. Phosphorylation or dephosphorylation can activate or inhibit a protein, depending on the protein.

All statements A–D are correct. Ligand binding activates a G protein-coupled receptor, promoting GDP–GTP exchange on the Gs alpha subunit. Activated Gs stimulates adenylyl cyclase to produce cAMP, which activates protein kinase A. Therefore, none of the listed statements is false.

All statements A–D apply to diffusion. Random molecular motion produces net movement down a concentration gradient. Oxygen and carbon dioxide can diffuse across cell membranes. Diffusion is consistent with the second law of thermodynamics, and osmosis is the diffusion of water across a selectively permeable membrane.

Endocytosis brings extracellular material into a cell through phagocytosis, pinocytosis, or receptor-mediated endocytosis. Autophagy breaks down cellular components, including damaged organelles, using lysosomes. Secretion releases materials from a cell, often by exocytosis.

Extreme pH and excessive heat can denature enzymes. Moderate cooling usually slows enzyme activity without denaturing the enzyme.

The Calvin cycle produces G3P, a three-carbon sugar that can be used to make glucose. The citric acid cycle produces CO₂, NADH, FADH₂, and ATP or GTP.

Rubisco catalyzes carbon fixation in the Calvin cycle, adding CO₂ to RuBP and producing 3-PGA. As an enzyme, it lowers activation energy. Citrate synthase catalyzes the combination of acetyl-CoA and oxaloacetate in the citric acid cycle.

The equation summarizes oxygenic photosynthesis, which occurs in chloroplasts in plants and algae.

Fermentation occurs in the cytosol and regenerates NAD⁺ by transferring electrons to an organic acceptor. It differs from anaerobic respiration, which uses an electron transport chain. Muscle cells produce lactate; alcoholic fermentation produces ethanol and CO₂.

Both organelles have double membranes, DNA, and ribosomes and are explained by endosymbiotic origins. Mitochondrial aerobic electron transport ends with O₂; chloroplast linear electron transport ends with NADP⁺.

A single polypeptide can form a functional protein with tertiary structure. Quaternary structure describes the association of multiple polypeptide subunits and is not required for every protein.

Primary structure is the linear amino acid sequence linked by peptide bonds. Secondary structure includes alpha helices and beta-pleated sheets stabilized by hydrogen bonds between backbone groups. Tertiary structure is the overall three-dimensional shape of a polypeptide, stabilized by interactions such as disulfide bonds, ionic interactions, hydrophobic interactions, and hydrogen bonds.

The endomembrane system participates in protein processing, transport, and other cellular functions. It includes the nuclear envelope, rough and smooth ER, transport vesicles, Golgi complex, lysosomes, vacuoles, and plasma membrane. Peroxisomes are not part of this system.

Ribosomes and cytoskeletal structures occur in both prokaryotic and eukaryotic cells. Cell walls occur in prokaryotes and in some eukaryotes. A nucleoid is characteristic of prokaryotic cells; centrioles and lysosomes occur in many animal cells.

Pinocytosis takes in extracellular fluid, whereas phagocytosis engulfs large particles. Endocytosis brings material into a cell in vesicles; exocytosis releases material when vesicles fuse with the plasma membrane.

Producing glucose and oxygen from carbon dioxide and water requires an energy input and is endergonic. Exergonic reactions have a negative Gibbs free-energy change under the specified conditions.

All four statements are true. Chlorophyll absorbs light during photosynthesis, helping convert light energy into chemical energy. Chlorophyll b and carotenoids serve as accessory pigments. Chlorophyll’s hydrophobic hydrocarbon tail helps anchor it in the thylakoid membrane, and its ring structure contains a central Mg²⁺ ion.

Chlorophyll a is the primary photosynthetic pigment and appears blue-green. Chlorophyll b and carotenoids are accessory pigments that broaden the range of light absorbed.

At photosystem II, water is oxidized, releasing electrons, H⁺, and O₂.

All four statements are true. Both plant and animal cells carry out aerobic respiration. At the end of the mitochondrial electron transport chain, oxygen accepts electrons and protons to form water. During fermentation, ATP comes from glycolysis, while fermentation regenerates NAD⁺. In aerobic eukaryotic cells, pyruvate oxidation produces acetyl-CoA.

Gluconeogenesis synthesizes glucose from noncarbohydrate precursors, including glycerol, lactate, and glucogenic amino acids.

O₂ is the final electron acceptor to produce H₂O in cellular respiration.

Correct steps in the scientific method:

Observation → hypothesis → predictions → controlled experimentation → analysis → conclusion

Ribosomes bind to the cytosolic surface of rough ER, where they synthesize proteins destined for secretion, membranes, or the endomembrane system.

Chloroplasts, mitochondria, and the nucleus are enclosed by two membranes. Lysosomes, smooth and rough ER, the Golgi complex, and peroxisomes have a single membrane. Centrioles and ribosomes lack surrounding membranes.

In competitive inhibition, an inhibitor and a substrate compete for the enzyme’s active site. The substrate cannot occupy that site while the inhibitor is bound.

C₄ plants such as sugarcane and corn initially fix CO₂ in mesophyll cells and deliver it to the Calvin cycle in bundle-sheath cells. This spatial separation helps reduce photorespiration in hot conditions.

CAM plants separate initial CO₂ fixation and the Calvin cycle in time. They open stomata at night to take in CO₂ and store it in organic acids. During the day, stomata close and stored CO₂ is released for the Calvin cycle. C₄ plants instead use spatial separation.

Oxidative phosphorylation produces most ATP during aerobic glucose breakdown. Electron transport establishes a proton gradient, and ATP synthase uses that gradient to phosphorylate ADP.

Pyruvate oxidation produces acetyl-CoA in the mitochondrial matrix. Glycolysis produces ATP and pyruvate in the cytosol; fermentation in the cytosol can produce lactate or ethanol, depending on the organism.

Amino acids are monomers of proteins. Each typical amino acid has an amino group, a carboxyl group, a hydrogen atom, and an R group attached to an alpha carbon. Dehydration reactions join monomers or smaller components to form polysaccharides, disaccharides, proteins, and fats.

Prokaryotic cells lack a membrane-bound nucleus and the membrane-bound organelles typical of eukaryotic cells. They do contain ribosomes and DNA in a nucleoid region, and they commonly reproduce by binary fission.

Chloroplasts produce ATP during photosynthesis and are absent from animal cells. Their internal fluid is the stroma. The matrix is the compartment enclosed by the inner mitochondrial membrane.

A granum is a stack of thylakoids. Each thylakoid consists of a membrane surrounding a lumen. The thylakoid membrane contains photosystems and chlorophyll. The stroma lies outside the thylakoids.

Photosystem II oxidizes water, releasing O₂, H⁺, and electrons. Electron flow supports the proton gradient used for photophosphorylation. Photosystem I supplies energized electrons for NADPH formation.

Typical alcoholic fermentation releases ethanol and CO₂. Typical lactic acid fermentation produces lactate without releasing CO₂. Statements I, III, and IV are true.

PGAL, also called G3P, is an intermediate of glycolysis. Complete aerobic glucose oxidation yields CO₂ and water, with some released energy captured in ATP.

Both chloroplasts and mitochondria contain their own DNA and ribosomes and are enclosed by two membranes. Mitochondria carry out aerobic cellular respiration, whereas chloroplasts carry out photosynthesis. NADPH production during the light-dependent reactions of photosynthesis occurs in chloroplasts. Mitochondria can also generate NADPH through other reactions.

All are true.

IP₃ is generated from PIP₂ and binds to IP₃ receptors on the endoplasmic reticulum. Opening these channels releases Ca²⁺ into the cytosol, where it can regulate proteins.

Reduction is the gain of electrons. NADP⁺ gains electrons to form NADPH in choice B. The conversions in A, C, D, and E are oxidations, involving electron loss.

Mitochondrial oxidative phosphorylation couples electron transport to chemiosmosis across the inner mitochondrial membrane. Ubiquinone and cytochrome c carry electrons. The analogous light-driven ATP synthesis in chloroplasts is photophosphorylation.

The citric acid cycle, also called the Krebs cycle, occurs in the mitochondrial matrix. The Calvin cycle occurs in the chloroplast stroma.

Grana are stacks of thylakoids in chloroplasts. Mitochondria have an outer membrane, a folded inner membrane forming cristae, and a matrix.

Proton movement through ATP synthase down an electrochemical gradient is facilitated diffusion. During ATP synthesis in chloroplasts, H⁺ moves from the thylakoid lumen to the stroma. In mitochondria, H⁺ moves from the intermembrane space into the matrix.