AP Biology Study Guide
This is a free study guide plus a 15-question practice test for AP Biology. Read the unit-by-unit guide below, then test yourself on water and macromolecules, cells, energy, cell division, genetics, gene expression, evolution and ecology. The questions mix multiple choice, select all that apply and fill in the blank. Answer all 15, then check your score and read the explanation for every question.
Which property of water, caused by hydrogen bonding between water molecules, helps water rise through the xylem of a tall tree?
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All 15 questions at a glance
Prefer to read before you play? Every question in the sampler is listed below. Answers and explanations stay collapsed until you open them.
1. Which property of water, caused by hydrogen bonding between water molecules, helps water rise through the xylem of a tall tree?
- A. High density when frozen
- B. Nonpolarity
- C. Cohesion
- D. Low heat capacity
Show answer and explanation
Answer: C. Cohesion
Hydrogen bonds hold water molecules to each other, which is cohesion. Together with adhesion to the walls of the xylem, cohesion lets a column of water be pulled upward as water evaporates from the leaves. Water is polar, not nonpolar, and it has a high heat capacity.
2. A plant cell is placed in pure water. The cell will most likely:
- A. Become turgid as water moves in
- B. Plasmolyze as water moves out
- C. Burst immediately
- D. Stay the same because the cell wall blocks water
Show answer and explanation
Answer: A. Become turgid as water moves in
Pure water is hypotonic to the cell, so water moves in by osmosis. The cell wall resists, which builds turgor pressure and keeps the cell firm without bursting. An animal cell with no wall in the same situation could burst.
3. A competitive inhibitor lowers an enzyme's activity by:
- A. Binding far from the active site and changing its shape
- B. Increasing the substrate concentration
- C. Permanently raising the activation energy of every reaction
- D. Binding to the active site and blocking the substrate
Show answer and explanation
Answer: D. Binding to the active site and blocking the substrate
A competitive inhibitor resembles the substrate and competes for the active site. Adding more substrate can overcome it. An inhibitor that binds elsewhere and changes the enzyme's shape is noncompetitive.
4. Complete the sentence.
The light reactions take place in the _____ membranes of the chloroplast and produce ATP and _____.
Options for blank 1: thylakoid, stromal, cristae, outer
Options for blank 2: pyruvate, glucose, acetyl-CoA, NADPH
Show answer and explanation
Answer: thylakoid, NADPH
Thylakoid membranes hold the photosystems and the electron transport chain. They make ATP and NADPH, which the Calvin cycle in the stroma then uses to build sugar. Cristae belong to the mitochondrion, not the chloroplast.
5. In cellular respiration, which stage produces the most ATP per glucose?
- A. Glycolysis
- B. Oxidative phosphorylation (the electron transport chain and chemiosmosis)
- C. Fermentation
- D. The Calvin cycle
Show answer and explanation
Answer: B. Oxidative phosphorylation (the electron transport chain and chemiosmosis)
Most of the ATP is made when electrons pass down the chain and protons flow back through ATP synthase. Glycolysis nets only a small amount, and fermentation makes none beyond glycolysis. The Calvin cycle belongs to photosynthesis.
6. Which proteins or enzymes work at a DNA replication fork?
Select all that apply.
- A. Primase
- B. RNA polymerase II
- C. Reverse transcriptase
- D. Helicase
- E. DNA ligase
Show answer and explanation
Answer: A. Primase, D. Helicase, E. DNA ligase
Helicase unwinds the helix, primase lays down an RNA primer, DNA polymerase extends it, and ligase joins the fragments on the lagging strand. RNA polymerase II transcribes genes, and reverse transcriptase copies RNA into DNA in some viruses.
7. Which checkpoint is the main point at which a cell commits to dividing, depending on its size, nutrients and DNA condition?
- A. The cytokinesis checkpoint
- B. The G1 checkpoint
- C. The anaphase checkpoint
- D. The prophase checkpoint
Show answer and explanation
Answer: B. The G1 checkpoint
The G1 checkpoint, also called the restriction point, decides whether the cell moves into S phase. Cells that fail it may pause or exit the cycle. Cancer often involves broken checkpoint control, such as a faulty p53 protein.
8. In a cross between two parents that are both AaBb (independent assortment), what phenotypic ratio is expected among offspring?
- A. 3:1
- B. 1:2:1
- C. 1:1:1:1
- D. 9:3:3:1
Show answer and explanation
Answer: D. 9:3:3:1
A dihybrid cross of two heterozygotes gives nine offspring with both dominant traits, three with one dominant and one recessive, three with the reverse, and one with both recessive. A 3:1 ratio comes from a monohybrid cross.
9. Complete the sentence.
During translation, the ribosome reads mRNA in groups of three nucleotides called _____.
Options for blank 1: anticodons, codons, promoters, introns
Show answer and explanation
Answer: codons
Each codon specifies an amino acid or a stop signal. The tRNA's anticodon pairs with the matching codon. Introns are removed before translation, and promoters are DNA sequences where transcription begins.
10. In the lac operon of E. coli, what happens when lactose is present?
- A. A lactose derivative binds the repressor, so the genes are transcribed
- B. The repressor binds tightly and blocks transcription
- C. The operator is deleted from the DNA
- D. Translation stops because ribosomes cannot attach
Show answer and explanation
Answer: A. A lactose derivative binds the repressor, so the genes are transcribed
The lac operon is inducible. A lactose derivative binds the repressor and changes its shape, so it lets go of the operator and RNA polymerase can transcribe the genes that digest lactose. When lactose is gone, the repressor binds the operator again.
11. Which statement correctly describes natural selection?
- A. Individuals change their traits during life and pass those changes on
- B. Populations evolve toward a goal they need
- C. Individuals with heritable traits that help them survive and reproduce leave more offspring
- D. Mutations appear because a population needs them
Show answer and explanation
Answer: C. Individuals with heritable traits that help them survive and reproduce leave more offspring
Selection acts on variation that already exists and is heritable. It does not make traits appear on demand, and traits gained during one life, like larger muscles, are not inherited. Over generations the favored traits become more common.
12. Which conditions must hold for a population to be in Hardy-Weinberg equilibrium?
Select all that apply.
- A. A large population
- B. Frequent new mutations
- C. Random mating
- D. No gene flow
- E. Strong directional selection
Show answer and explanation
Answer: A. A large population, C. Random mating, D. No gene flow
Hardy-Weinberg assumes random mating, no gene flow, no mutation, no natural selection and a very large population. Real populations never meet all five. The model gives a baseline: if allele frequencies move away from it, one of those conditions is being broken.
13. Complete the sentence.
Only about _____ percent of the energy at one trophic level is typically available to the next level.
Options for blank 1: 1, 10, 50, 90
Show answer and explanation
Answer: 10
Most energy is lost as heat through metabolism or stays in material that is not eaten. The 10 percent figure is a rule of thumb that varies by ecosystem, but it explains why food chains are short and top predators are rare.
14. Which are density-dependent limiting factors on a population?
Select all that apply.
- A. Competition for food
- B. A volcanic eruption
- C. Spread of disease
- D. A severe freeze
- E. Predation
Show answer and explanation
Answer: A. Competition for food, C. Spread of disease, E. Predation
Density-dependent factors hit harder as a population becomes more crowded: food competition, disease and predation all fit. Eruptions and freezes affect a population regardless of how many individuals live there, so they are density-independent.
15. In the Calvin cycle, where does carbon fixation occur?
- A. In the thylakoid lumen
- B. In the mitochondrial matrix
- C. In the cytosol
- D. In the stroma of the chloroplast
Show answer and explanation
Answer: D. In the stroma of the chloroplast
Rubisco attaches carbon dioxide to RuBP in the stroma. The ATP and NADPH made in the light reactions then power the steps that build sugar. The mitochondrial matrix hosts the citric acid cycle, not carbon fixation.
AP Biology study guide: the eight units
AP Biology has eight units. In order, they are chemistry of life, cell structure and function, cellular energetics, cell communication and the cell cycle, heredity, gene expression and regulation, natural selection, and ecology. The weights are uneven: natural selection is the heaviest at roughly 13 to 20 percent, while chemistry of life and heredity are the lightest at roughly 8 to 11 percent. Check the exact ranges in the current Course and Exam Description, because the College Board can revise them.
Four big ideas tie the units together: evolution, energetics, information storage and transmission, and systems interactions. A useful study guide is not a list of facts. For every unit, write what the structure does, what energy or information moves through it, and what happens when it changes. That last question is what the free-response section keeps asking.
What the exam asks and how to use this practice test
The exam has two halves of equal weight. Section I is 60 multiple-choice questions in 90 minutes, and Section II is 6 free-response questions in 90 minutes: 2 long questions worth 9 points each and 4 short questions worth 4 points each. The long questions center on experiments and data, so you will read graphs, state a hypothesis and explain what a result means. The College Board also provides a formula sheet, so spend your effort on knowing when to use an equation, not on memorizing it.
This practice test has 15 questions across all eight units. It adds select all that apply and fill in the blank, which the real multiple choice does not use, because recalling a term is harder than recognizing it. Take it in one sitting, read every explanation, and treat a wrong answer as a pointer to a whole unit. Practice the free response separately: take any experiment, name the independent and dependent variables, then write what the data would look like if your explanation were wrong.
How StudyPDF builds a study guide and practice tests from your notes
Fifteen questions can show you which units are shaky. They cannot get you ready on their own, and a generic guide rarely matches what your teacher spent three weeks on.
StudyPDF works from your material instead. You upload your textbook chapter, lab write-ups or the review packet your teacher posted, press New material, and StudyPDF builds a study guide and full practice tests from those exact pages. Every question is grounded in your material, and every explanation cites where the answer comes from. You do not have to upload anything to start: you can also just name a topic, like Unit 3 or the lac operon, and Bo sets it up with its key concepts, so you can build materials on it. Starting is free.
Written by the StudyPDF team. Last updated 2026-08-19.