Cell Structure and Function (Unit Overview) — AP Biology Study Guide

For: AP Biology candidates sitting AP Biology.

Covers: Full unit overview of AP Biology Unit 2: Cell Structure and Function, including all 10 core sub-topics from subcellular components to the origins of eukaryotic cell compartmentalization, aligned to the 2020 CED.

You should already know: Basic cell theory, the structure and properties of biological macromolecules, fundamental differences between prokaryotic and eukaryotic cells.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Biology style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. Why This Matters

This unit accounts for 10–13% of the total AP Biology exam score, and concepts from this unit appear across both multiple-choice (MCQ) and free-response (FRQ) sections, often as a foundation for questions on other core topics like cell communication, metabolism, cell division, and organismal physiology. Cell Structure and Function is where AP Biology’s core enduring understanding—structure determines function—is first systematically explored across all levels of cellular organization.

All biological processes occur at the cellular level, so mastering this unit is required to make sense of every subsequent topic in the course. For example, the compartmentalization of eukaryotic cells allows for specialized, separated metabolic reactions that could not occur in an undivided cytoplasm, enabling the complexity of multicellular life. Foundational concepts like surface area-to-volume ratio introduced here reappear across the course, from gas exchange in animal lungs to nutrient uptake in plant roots. This unit also builds directly on the macromolecule chemistry you learned in Unit 1, connecting monomer structure to large-scale cellular structure and function.

2. Unit Concept Map

The 10 sub-topics of this unit build sequentially from a foundational parts list to evolutionary explanation, following a logical progression where each topic depends on mastery of the previous:

1. Cell Structure: Subcellular Components first establishes the identity and basic location of all organelles and subcellular structures in prokaryotic and eukaryotic cells, providing the core "parts list" for the rest of the unit.
2. Cell Structure and Function connects each structure’s shape and chemical properties to its specific role in the cell, formalizing the core structure-function theme.
3. Cell Size scales this to whole-cell properties, deriving how surface area-to-volume ratio limits cell size and shapes exchange efficiency, a cross-cutting concept for the entire course.
4. Plasma Membranes describes the fluid mosaic structure of the cell’s outer boundary and eukaryotic internal membranes, establishing the physical barrier that regulates molecular exchange.
5. Membrane Permeability follows by explaining which molecules can cross the phospholipid bilayer directly vs what requires protein assistance, building directly on membrane structure.
6. Membrane Transport introduces the two broad categories of transport (passive and active) across membranes, explaining how molecules move against or down their concentration gradients.
7. Facilitated Diffusion breaks out this common specialized passive transport mechanism, which relies on membrane proteins to move large or charged molecules that cannot cross the bilayer.
8. Tonicity and Osmoregulation applies transport principles to whole-cell water balance, explaining how solute concentration gradients drive water movement and shape cell survival.
9. Cell Compartmentalization connects organelle structure and membrane function to explain the adaptive advantage of internal membranes in eukaryotes, tying together all prior topics.
10. Origins of Cell Compartmentalization concludes with the evolutionary explanation for how compartmentalization first arose in eukaryotes, via endosymbiosis and plasma membrane infolding.

3. A Guided Tour of a Unit-Style Exam Problem

AP Biology almost always tests this unit by asking questions that require combining concepts from multiple sub-topics. Below is a typical exam-style question, with a step-by-step tour of which sub-topics apply at each stage:

Question: A researcher places a spherical, non-walled animal cell with an internal non-penetrating solute concentration of 0.2 M into a 0.4 M solution of the same non-penetrating solute. The cell has a diameter of 10 μm. Predict the direction of net water movement, explain how cell size affects the rate of this movement, and justify your prediction based on membrane properties.

We use three core sub-topics from across the unit to answer this fully:

1. Step 1: Apply Cell Size to explain rate of movement: The question asks how cell size affects rate, so we use the surface area-to-volume relationship from this sub-topic. For a spherical cell, surface area is $SA=4\pi r^2$ and volume is $V=\frac{4}{3}\pi r^3$, which simplifies to: $$\frac{SA}{V}=\frac{3}{r}$$ This cell is small (10 μm diameter = 5 μm radius), so it has a high SA:V ratio. A high SA:V means more membrane surface per unit of cytoplasm that requires water exchange, so water will move rapidly to reach equilibrium.
2. Step 2: Apply Membrane Permeability to justify your prediction: Before we can predict water movement, we need to confirm which molecules can cross the membrane. The problem states the solute is non-penetrating: this aligns with permeability rules, which tell us large polar solutes cannot cross the phospholipid bilayer without specific transporters (which are not present here). Water, however, can cross freely via aquaporins for facilitated diffusion. This separation of penetrating vs non-penetrating molecules is required to correctly predict water movement.
3. Step 3: Apply Tonicity and Osmoregulation to predict net movement: The extracellular solution has a higher concentration of non-penetrating solute (0.4 M > 0.2 M), so it is hypertonic to the cell’s cytoplasm. Water moves from areas of higher water potential (lower solute concentration) to lower water potential (higher solute concentration), so net water movement is out of the cell, and the cell will shrink (crenate).

Unit-level exam tip: Always look for opportunities to connect multiple sub-topics in FRQ answers; full credit almost always requires justifying your answer with a concept from a different sub-topic, not just stating a prediction.

4. Cross-Cutting Common Pitfalls

These are recurring cross-unit traps that trip up students across multiple sub-topics, rooted in common misunderstandings:

• Wrong move: Confusing selective permeability of the whole membrane with permeability of the pure phospholipid bilayer, claiming all large/polar molecules cannot cross any biological membrane. Why: Students memorize bilayer permeability rules but forget that membrane proteins can enable permeability to specific large/polar molecules. Correct move: Always explicitly distinguish between permeability of the phospholipid bilayer alone vs permeability of the entire biological membrane (which includes embedded transport proteins) when answering questions.
• Wrong move: Stating that surface area-to-volume ratio increases as cell size increases. Why: Students mix up absolute surface area (which does increase with size) with the ratio of surface area to volume. Correct move: For any cell size question, remember this rule: as cell diameter increases, total surface area increases, but the ratio $\frac{SA}{V}$ decreases.
• Wrong move: Defining active transport solely as movement from low to high concentration. Why: Students memorize the common correlation between active transport and moving against gradients, but this is not a definition. Correct move: Always define active transport by its requirement for energy input (ATP or electrochemical gradient energy), regardless of the direction of the concentration gradient.
• Wrong move: Calculating tonicity based on total solute concentration, ignoring whether solutes can cross the membrane. Why: Students confuse osmolarity (total solute concentration) with tonicity (effect on cell water balance, determined only by non-penetrating solutes). Correct move: For any tonicity question, first sort solutes into penetrating (can cross the membrane) and non-penetrating (cannot cross) before comparing concentrations across the membrane.
• Wrong move: Claiming prokaryotes have no compartmentalization because they lack membrane-bound organelles. Why: Students only associate compartmentalization with eukaryotic organelles, missing that prokaryotes also organize and separate cellular processes. Correct move: Recognize that prokaryotes have limited but functional compartmentalization (e.g., the nucleoid region, thylakoids in cyanobacteria, the cell wall boundary) that organizes cellular work.
• Wrong move: Attributing the origin of all eukaryotic organelles to endosymbiosis. Why: Students overgeneralize from the well-studied examples of mitochondria and chloroplasts to all organelles. Correct move: Only associate mitochondria and chloroplasts with endosymbiotic origin; other organelles (ER, Golgi, nuclear envelope) arose from infolding of the ancestral prokaryote’s plasma membrane.

5. Quick Check: Do You Know When to Use Which Sub-Topic?

For each question below, identify which of the 10 unit sub-topics you would use to answer it:

1. Explain why mitochondria and chloroplasts have their own circular, prokaryote-like genomes.
2. Predict the net movement of water when a plant cell with 0.1 M internal non-penetrating solute is placed in 0.3 M NaCl solution, where NaCl is non-penetrating.
3. Justify why most cells are less than 100 μm in diameter.
4. Explain how the structure of the phospholipid bilayer allows small nonpolar molecules to cross easily but blocks large charged molecules.
5. Explain how the folded structure of the rough ER supports its function in protein synthesis and processing.

Answers:

1. Origins of Cell Compartmentalization
2. Tonicity and Osmoregulation
3. Cell Size
4. Membrane Permeability
5. Cell Structure and Function

6. Sub-Topics in This Unit (See Also)

Click through to each in-depth sub-topic study guide:

• Cell Structure: Subcellular Components
• Cell Structure and Function
• Cell Size
• Plasma Membranes
• Membrane Permeability
• Membrane Transport
• Facilitated Diffusion
• Tonicity and Osmoregulation
• Cell Compartmentalization
• Origins of Cell Compartmentalization