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AP Biology practice questions

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Question 1Chemistry of Life

Which property of water is most directly responsible for its ability to dissolve many ionic compounds?

  1. Its high specific heat
  2. Its low molecular mass
  3. Its high surface tension
  4. Its polarity and ability to form hydrogen bonds Correct answer.

Explanation

Water molecules are polar, with partial charges that surround and stabilize ions, allowing ionic substances to dissociate and dissolve. This ion-dipole interaction depends on polarity and hydrogen bonding, not on mass or heat capacity.

Question 2Chemistry of Life

A solution has a hydrogen ion concentration of 1 x 10^-4 M. What is its pH?

  1. 10
  2. 14
  3. 7
  4. 4 Correct answer.

Explanation

pH is the negative base-10 logarithm of the hydrogen ion concentration. For [H+] = 1 x 10^-4 M, pH = -log(10^-4) = 4.

Question 3Chemistry of Life

Which type of bond holds the two strands of a DNA double helix together?

  1. Hydrogen bonds between complementary bases Correct answer.
  2. Covalent bonds between sugars
  3. Peptide bonds between nucleotides
  4. Ionic bonds between phosphate groups

Explanation

Complementary bases on opposite strands pair through hydrogen bonds, which hold the strands together while allowing separation. The backbone itself is joined by covalent phosphodiester bonds.

Question 4Chemistry of Life

Which functional group is responsible for the acidic properties of carboxylic acids?

  1. Hydroxyl group (-OH)
  2. Amino group (-NH2)
  3. Sulfhydryl group (-SH)
  4. Carboxyl group (-COOH) Correct answer.

Explanation

The carboxyl group can donate a proton from its hydroxyl portion, releasing H+ and leaving a resonance-stabilized carboxylate ion. This proton donation is what makes carboxylic acids acidic.

Question 5Chemistry of Life

Which statement best describes the role of an enzyme in a biochemical reaction?

  1. It changes the overall free energy change of the reaction
  2. It increases the activation energy of the reaction
  3. It lowers the activation energy without being consumed Correct answer.
  4. It permanently binds the substrate and is degraded

Explanation

Enzymes are catalysts that lower the activation energy barrier, speeding the reaction, and they are regenerated after each catalytic cycle. They do not alter the net free energy change between reactants and products.

Question 6Chemistry of Life

Which of the following is a polymer built from amino acid monomers?

  1. Nucleotide chain
  2. Triglyceride
  3. Starch
  4. Polypeptide Correct answer.

Explanation

Amino acids are joined by peptide bonds to form polypeptides, which fold into proteins. The other options are built from sugars, nucleotides, or glycerol and fatty acids.

Question 7Chemistry of Life

A dehydration synthesis reaction joins two glucose molecules. What is produced in addition to the disaccharide?

  1. Oxygen gas
  2. A phosphate group
  3. Carbon dioxide
  4. Water Correct answer.

Explanation

Dehydration synthesis removes a hydroxyl group from one monomer and a hydrogen from another, releasing a water molecule as the covalent bond forms. The reverse process, hydrolysis, consumes water.

Question 8Chemistry of Life

Two solutions are separated by a membrane permeable only to water. Solution A contains 0.5 M solute and Solution B contains 0.1 M solute. Which direction will water move by osmosis?

  1. Equally in both directions with no net movement
  2. From A to B
  3. From B to A Correct answer.
  4. No movement occurs without an energy source

Explanation

Water moves across a semipermeable membrane from the region of lower solute concentration toward the region of higher solute concentration. Solution B has the lower solute concentration, so net water movement is from B to A.

Question 9Chemistry of Life

Which interaction most directly stabilizes the tertiary structure of a soluble protein?

  1. Covalent bonds between adjacent amino acids in sequence
  2. Hydrogen bonds between backbone phosphates
  3. Hydrophobic interactions among nonpolar side chains Correct answer.
  4. Base stacking between aromatic nucleotides

Explanation

In a soluble protein, nonpolar side chains cluster in the interior away from water, and these hydrophobic interactions help stabilize the folded tertiary shape. Hydrogen bonds, ionic bonds, and disulfide bridges also contribute.

Question 10Chemistry of Life

A buffer system contains a weak acid and its conjugate base. What happens when a small amount of strong acid is added?

  1. The pH rises sharply above the starting value
  2. The weak acid releases additional H+
  3. The buffer is destroyed immediately
  4. The conjugate base neutralizes some added H+ Correct answer.

Explanation

A buffer resists pH change because its conjugate base accepts added H+ ions, converting them into the weak acid. This minimizes the shift in hydrogen ion concentration and keeps pH relatively stable.

Question 11Chemistry of Life

Which structural feature distinguishes a saturated fatty acid from an unsaturated one?

  1. The presence of glycerol
  2. The number of phosphate groups
  3. The presence of carbon-carbon double bonds Correct answer.
  4. The number of carboxyl groups present

Explanation

Unsaturated fatty acids contain one or more carbon-carbon double bonds that create kinks in the hydrocarbon chain, while saturated fatty acids have only single bonds. This difference affects packing and membrane fluidity.

Question 12Chemistry of Life

In a phospholipid bilayer, which part of the molecule faces the aqueous interior and exterior of the cell?

  1. The glycerol backbone exclusively
  2. The hydrophilic phosphate head groups Correct answer.
  3. The hydrophobic fatty acid tails
  4. The double bonds in the tails

Explanation

Phospholipids are amphipathic: the hydrophilic phosphate heads orient toward water on both surfaces, while the hydrophobic tails cluster in the middle. This arrangement forms a stable bilayer barrier.

Question 13Chemistry of Life

An enzyme is added to a reaction mixture, and the reaction rate increases until it plateaus even as substrate is added. What most likely explains the plateau?

  1. The activation energy has increased
  2. The substrate has been completely consumed
  3. All enzyme active sites are saturated with substrate Correct answer.
  4. The enzyme has been permanently denatured

Explanation

At high substrate concentrations, the rate levels off because all available active sites are occupied; the enzyme is working at maximum velocity. Adding more substrate cannot increase the rate until enzyme is freed.

Question 14Chemistry of Life

Which property of carbon makes it capable of forming the diverse backbone structures found in biomolecules?

  1. Its high electronegativity relative to oxygen
  2. Its tendency to form only ionic bonds
  3. Its ability to form up to four covalent bonds Correct answer.
  4. Its large atomic radius compared with other elements

Explanation

Carbon has four valence electrons, allowing it to form four covalent bonds and create chains, branches, and rings. This tetravalency underlies the structural diversity of organic molecules.

Question 15Chemistry of Life

A researcher measures the pH of a solution before and after adding a small amount of HCl. The pH decreases only slightly. Which conclusion is best supported?

  1. The solution is pure water
  2. The solution is strongly basic
  3. The added HCl was neutralized by oxygen
  4. The solution contains a buffer system Correct answer.

Explanation

A large addition of strong acid would sharply lower the pH of an unbuffered solution. A small change indicates buffering, where the weak acid-conjugate base pair absorbs added H+.

Question 16Chemistry of Life

Which observation most strongly indicates that a reaction is endergonic?

  1. The activation energy is very low
  2. The reaction releases heat to the surroundings
  3. The reaction proceeds spontaneously without energy input
  4. The products have higher free energy than the reactants Correct answer.

Explanation

An endergonic reaction has a positive change in free energy, meaning products store more free energy than reactants. Such reactions require a net input of energy to proceed.

Question 17Chemistry of Life

A mutation changes a single amino acid in an enzyme's active site, yet the enzyme still binds substrate but catalyzes the reaction very slowly. Which explanation is most consistent?

  1. The mutation altered catalytic residues without preventing substrate binding Correct answer.
  2. The mutation changed the enzyme into a structural protein
  3. The mutation removed the enzyme's peptide bonds
  4. The mutation prevented all substrate from entering the cell

Explanation

Active sites have both substrate-binding and catalytic roles. A change that affects catalysis while leaving binding intact reduces turnover rate without abolishing substrate association.

Question 18Chemistry of Life

Two enzymes catalyze similar reactions but have different optimal pH values. Which experimental result would best support the claim that pH affects enzyme shape?

  1. Activity is unaffected by temperature changes
  2. Activity is constant across the entire pH range
  3. Activity is highest near the optimal pH and declines sharply at extreme pH Correct answer.
  4. Activity increases steadily with substrate concentration at all pH values

Explanation

Extreme pH alters the ionization of side chains and disrupts hydrogen and ionic bonds, changing enzyme shape and lowering activity. A sharp decline away from the optimum supports pH-dependent structural integrity.

Question 19Chemistry of Life

A polypeptide is synthesized and then exposed to a strong reducing agent that breaks disulfide bonds. Which level of protein structure is most directly disrupted?

  1. Tertiary structure Correct answer.
  2. The sequence of nucleotides
  3. Secondary structure
  4. Primary structure

Explanation

Disulfide bonds between cysteine residues are covalent cross-links that stabilize tertiary structure. Breaking them can unfold the three-dimensional arrangement while the amino acid sequence remains intact.

Question 20Cells

Which statement best describes the structure of the plasma membrane according to the fluid mosaic model?

  1. A rigid protein coat surrounding a phospholipid core
  2. A single layer of phospholipids with proteins fixed in place
  3. A phospholipid bilayer with embedded proteins that can move laterally Correct answer.
  4. A double layer of cellulose with pores for transport

Explanation

The fluid mosaic model describes the membrane as a phospholipid bilayer in which proteins are embedded and can move laterally within the layer. This arrangement gives the membrane both fluidity and a mosaic of varied components.

Question 21Cells

A cell placed in a hypertonic solution will most likely:

  1. Lose water and shrink Correct answer.
  2. Gain water and swell
  3. Neither gain nor lose water
  4. Burst from excessive water intake

Explanation

In a hypertonic solution the solute concentration outside the cell is higher than inside, so water moves out by osmosis. The net loss of water causes the cell to shrink.

Question 22Cells

Which organelle is the primary site of ATP synthesis by oxidative phosphorylation in eukaryotic cells?

  1. Golgi apparatus
  2. Mitochondrion Correct answer.
  3. Lysosome
  4. Ribosome

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which perform oxidative phosphorylation. This makes the mitochondrion the main ATP-producing organelle in aerobic eukaryotes.

Question 23Cells

The movement of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration is called:

  1. Phagocytosis
  2. Osmosis Correct answer.
  3. Active transport
  4. Facilitated diffusion of ions

Explanation

Osmosis is the passive diffusion of water across a selectively permeable membrane down its own concentration gradient, which corresponds to moving toward higher solute concentration. It requires no energy input.

Question 24Cells

Which feature distinguishes prokaryotic cells from eukaryotic cells?

  1. Prokaryotic cells lack a plasma membrane
  2. Prokaryotic cells lack a membrane-bound nucleus Correct answer.
  3. Prokaryotic cells lack genetic material
  4. Prokaryotic cells lack ribosomes

Explanation

Prokaryotes keep their DNA in a nucleoid region without a surrounding nuclear envelope, whereas eukaryotes have a true nucleus. Both cell types share a plasma membrane, ribosomes, and genetic material.

Question 25Cells

A molecule that moves across the membrane down its concentration gradient through a transport protein without energy input is best described as undergoing:

  1. Endocytosis
  2. Primary active transport
  3. Osmosis
  4. Facilitated diffusion Correct answer.

Explanation

Facilitated diffusion uses membrane proteins to help polar or charged solutes cross the membrane down their gradient without ATP. It is passive because no energy is expended.

Question 26Cells

Which part of a phospholipid is hydrophobic and faces the interior of the bilayer?

  1. The fatty acid tails Correct answer.
  2. The phosphate group
  3. The choline head group
  4. The glycerol backbone

Explanation

The fatty acid tails are nonpolar and hydrophobic, so they orient inward away from water. The phosphate-containing head is polar and faces the aqueous environment on each side.

Question 27Cells

A researcher treats cells with a drug that blocks ATP synthesis. Which transport process would be inhibited most directly?

  1. Oxygen diffusing across the lipid bilayer
  2. The sodium-potassium pump moving ions against their gradients Correct answer.
  3. Water moving through aquaporins down its gradient
  4. Glucose moving through a channel down its gradient

Explanation

The sodium-potassium pump is an active transporter that uses ATP hydrolysis to move ions against their concentration gradients. Blocking ATP synthesis directly impairs such energy-dependent transport.

Question 28Cells

In a plant cell, the large central vacuole primarily helps the cell by:

  1. Translating mRNA into proteins
  2. Maintaining turgor pressure through water uptake Correct answer.
  3. Packaging proteins for secretion
  4. Synthesizing ATP for cellular work

Explanation

The central vacuole accumulates water and solutes, generating turgor pressure that supports the cell and helps maintain its shape. This is a key function distinguishing plant cells.

Question 29Cells

Which observation would best support the claim that a membrane protein is involved in active transport rather than facilitated diffusion?

  1. Transport of the solute is unaffected by temperature changes
  2. Transport of the solute continues when the solute gradient is reversed Correct answer.
  3. Transport of the solute stops when a channel blocker is added
  4. Transport of the solute increases as its external concentration rises

Explanation

Active transport can move solutes against their gradient, so transport persists even when the gradient is reversed. Facilitated diffusion can only proceed down the gradient and would reverse direction instead.

Question 30Cells

A cell is placed in an isotonic solution. Which outcome is expected for a typical animal cell?

  1. No net movement of water, so cell volume stays roughly constant Correct answer.
  2. Net water exit, causing the cell to shrivel
  3. Net water entry, causing the cell to swell and lyse
  4. Continuous active pumping to prevent water entry

Explanation

In an isotonic solution the solute concentrations inside and outside are equal, so water enters and exits at equal rates. With no net water movement, the cell volume remains stable.

Question 31Cells

Which sequence correctly traces the path of a secreted protein from synthesis to release from the cell?

  1. Smooth ER → nucleus → Golgi apparatus → plasma membrane
  2. Ribosome → mitochondrion → Golgi apparatus → plasma membrane
  3. Golgi apparatus → rough ER → lysosome → plasma membrane
  4. Rough ER → Golgi apparatus → secretory vesicle → plasma membrane Correct answer.

Explanation

Proteins destined for secretion are synthesized on the rough ER, modified and sorted in the Golgi apparatus, packaged into secretory vesicles, and released at the plasma membrane. This order reflects the endomembrane pathway.

Question 32Cells

Which property of the phospholipid bilayer most directly allows small nonpolar molecules such as oxygen to cross the membrane readily?

  1. The glycoprotein coating on the exterior
  2. The high concentration of cholesterol
  3. The hydrophobic interior of the bilayer Correct answer.
  4. The presence of peripheral proteins

Explanation

The hydrophobic core of the bilayer is compatible with nonpolar molecules, allowing them to dissolve through and diffuse across. Polar and charged substances are excluded by this same barrier.

Question 33Cells

A scientist observes that a particular solute enters a cell faster as its external concentration increases until the rate plateaus. This plateau most likely indicates:

  1. The solute is diffusing through the lipid bilayer
  2. The membrane has become impermeable
  3. Transport proteins become saturated Correct answer.
  4. Active transport has stopped

Explanation

When transport depends on a finite number of membrane proteins, increasing solute concentration raises the rate only until all carriers are occupied. The plateau reflects carrier saturation, a hallmark of protein-mediated transport.

Question 34Cells

Which structural feature of the inner mitochondrial membrane directly supports its role in ATP production?

  1. Folded cristae that increase surface area for the electron transport chain Correct answer.
  2. The absence of membrane-bound enzymes
  3. A double lipid bilayer with no proteins
  4. A rigid cell wall surrounding the organelle

Explanation

The inner membrane folds into cristae, greatly increasing the surface area available for electron transport chain complexes and ATP synthase. More surface area supports higher rates of ATP synthesis.

Question 35Cells

A red blood cell is placed in distilled water. Which prediction is most accurate?

  1. It will synthesize more membrane to resist swelling
  2. It will shrink because water leaves the cell
  3. It will remain unchanged because distilled water is isotonic
  4. It will swell and may lyse because water enters by osmosis Correct answer.

Explanation

Distilled water is hypotonic to the cytoplasm, so water enters the cell by osmosis. Without a cell wall to resist expansion, an animal cell can swell and burst.

Question 36Cells

Which best explains why the plasma membrane is described as selectively permeable?

  1. It is completely impermeable to every substance
  2. It allows all molecules to pass freely in both directions
  3. It permits only water to cross under all conditions
  4. It permits some substances to cross while restricting others Correct answer.

Explanation

Selective permeability means the membrane allows certain substances to pass while blocking others, based on size, charge, and lipid solubility. This control is essential for maintaining the cell's internal environment.

Question 37Cells

A cell is treated with a compound that disrupts microtubules. Which process would be most directly impaired?

  1. Diffusion of oxygen across the membrane
  2. Translation of mRNA on ribosomes
  3. Hydrolysis of macromolecules in lysosomes
  4. Separation of chromosomes during mitosis Correct answer.

Explanation

Microtubules form the mitotic spindle, which moves chromosomes during cell division. Disrupting microtubules therefore impairs chromosome segregation.

Question 38Cells

Two solutions are separated by a membrane permeable only to water. Side A contains 0.3 M glucose; side B contains 0.1 M glucose. Which statement describes the net water movement?

  1. Water moves from B to A because B has the higher solute concentration
  2. No net water movement occurs because the membrane is permeable to water
  3. Water moves from A to B because A has the higher solute concentration
  4. Water moves from B to A because A has the higher solute concentration Correct answer.

Explanation

Water moves by osmosis toward the side with the higher solute concentration. Since side A has the greater glucose concentration, net water movement is from B to A.

Question 39Cells

A protein that functions in the cytosol is synthesized by free ribosomes, whereas a protein destined for secretion is synthesized by ribosomes bound to the rough ER. Which statement best explains this difference?

  1. A signal peptide directs the ribosome to the rough ER for proteins entering the endomembrane system Correct answer.
  2. Cytosolic proteins lack amino acids and therefore cannot be made on the rough ER
  3. Secreted proteins are synthesized in the nucleus before reaching the ribosome
  4. Bound ribosomes translate DNA, while free ribosomes translate RNA

Explanation

Proteins destined for the endomembrane system carry an N-terminal signal peptide that directs the ribosome to the rough ER membrane. Proteins without such a signal are completed on free ribosomes and remain in the cytosol.

Question 40Cells

An experiment shows that a solute enters cells only when ATP is present and can move into cells even when the internal solute concentration exceeds the external concentration. Which conclusion is best supported?

  1. The solute crosses by simple diffusion through the bilayer
  2. The solute is moved by an ATP-dependent pump against its gradient Correct answer.
  3. The solute crosses by facilitated diffusion down its gradient
  4. The solute enters by osmosis along with water

Explanation

Movement against a concentration gradient that requires ATP is the defining feature of active transport by a pump. The dependence on ATP and uphill direction together support this conclusion.

Question 41Cells

A freshwater protist lives in a hypotonic environment and must prevent excessive water uptake. Which adaptation would most directly help it maintain water balance?

  1. A thicker phospholipid bilayer that blocks all water entry
  2. A larger nucleus to store additional water
  3. A contractile vacuole that actively expels excess water Correct answer.
  4. Increased numbers of free ribosomes in the cytosol

Explanation

In a hypotonic environment water continually enters by osmosis, so freshwater protists use contractile vacuoles to pump excess water out using energy. This osmoregulation maintains a stable internal volume.

Question 42Cells

Cholesterol is embedded in the plasma membranes of animal cells. Which statement best describes its effect on the membrane?

  1. It serves as the primary energy source for active transport
  2. It forms channels that allow ions to cross freely
  3. It replaces phospholipids to create a rigid, impermeable barrier
  4. It modulates fluidity, reducing it at high temperatures and preventing tight packing at low temperatures Correct answer.

Explanation

Cholesterol acts as a fluidity buffer: it restrains phospholipid movement at higher temperatures and prevents close packing at lower temperatures. This keeps the membrane within a functional range of fluidity.

Question 43Cellular Energetics

Which molecule is the final electron acceptor at the end of the electron transport chain in aerobic cellular respiration?

  1. Pyruvate
  2. Oxygen Correct answer.
  3. Carbon dioxide
  4. NAD+

Explanation

In aerobic respiration, oxygen accepts electrons after they pass through the transport chain, forming water. Without this terminal acceptor, the chain backs up and NADH cannot be reoxidized.

Question 44Cellular Energetics

In glycolysis, one molecule of glucose is converted into which products per net reaction?

  1. 2 pyruvate, 4 ATP, 2 NADH
  2. 1 acetyl-CoA, 2 ATP, 2 NADH
  3. 1 pyruvate, 4 ATP, 1 NADH
  4. 2 pyruvate, 2 ATP, 2 NADH Correct answer.

Explanation

Glycolysis splits glucose into two three-carbon pyruvate molecules while producing a net gain of 2 ATP and 2 NADH. Four ATP are made, but two are consumed during the preparatory phase.

Question 45Cellular Energetics

Where in a eukaryotic cell does the Krebs (citric acid) cycle take place?

  1. Mitochondrial matrix Correct answer.
  2. Cytoplasm
  3. Intermembrane space
  4. Inner mitochondrial membrane

Explanation

The enzymes of the Krebs cycle are located in the mitochondrial matrix, where acetyl-CoA is oxidized. The inner membrane hosts the electron transport chain, a different stage.

Question 46Cellular Energetics

ATP synthase generates ATP by using which energy source?

  1. Direct transfer of a phosphate from NADH
  2. The hydrolysis of glucose to pyruvate
  3. The reduction of oxygen to water
  4. A proton gradient across a membrane Correct answer.

Explanation

ATP synthase is a molecular turbine driven by protons flowing down their electrochemical gradient. This flow powers the rotation that catalyzes ATP formation from ADP and phosphate.

Question 47Cellular Energetics

During photosynthesis, the light reactions produce which molecules that are then used by the Calvin cycle?

  1. Glucose and oxygen
  2. Pyruvate and acetyl-CoA
  3. ATP and NADPH Correct answer.
  4. NADH and FADH2

Explanation

The light reactions convert light energy into chemical energy stored in ATP and NADPH. The Calvin cycle consumes these to reduce carbon dioxide into sugar.

Question 48Cellular Energetics

Which process is the primary source of ATP production in a cell under anaerobic conditions?

  1. The Calvin cycle
  2. Oxidative phosphorylation in the mitochondria
  3. Substrate-level phosphorylation during glycolysis Correct answer.
  4. The electron transport chain

Explanation

Without oxygen, the electron transport chain cannot operate, so the cell relies on glycolysis. Glycolysis generates ATP by substrate-level phosphorylation, transferring phosphate directly from a substrate to ADP.

Question 49Cellular Energetics

The overall purpose of fermentation following glycolysis is to do which of the following?

  1. Regenerate NAD+ so glycolysis can continue Correct answer.
  2. Produce large amounts of ATP
  3. Produce oxygen for the cell
  4. Convert pyruvate into acetyl-CoA

Explanation

Fermentation reoxidizes NADH back to NAD+, keeping the electron carrier available for glycolysis. This allows ATP production to continue when the electron transport chain is unavailable.

Question 50Cellular Energetics

Which statement best describes the role of chlorophyll in photosynthesis?

  1. It absorbs light energy that excites electrons in the photosystems Correct answer.
  2. It stores energy as starch
  3. It transports electrons to the Calvin cycle
  4. It fixes carbon dioxide into sugar

Explanation

Chlorophyll molecules absorb photons and pass the energy to reaction-center pigments, exciting electrons. These excited electrons drive the light reactions.