AP Bio Unit 2.2-2.4 Study GuideAP Bio Unit 2.2-2.4 Study Guide

UNIT 2: CELLS

AP Biology Unit 2: Topics 2.2–2.4

Cell Size · Plasma Membranes · Membrane Permeability

AP Biology Study Guide

Built from the 2025 Unit 2 Workbook (Getting Down with Science), pp. 2–3, 24–36

Source used: 2025 Unit 2 Workbook (Getting Down with Science), pp. 2–3 (topic summary pages: learning objectives 2.2.A, 2.3.A–B, 2.4.A–B and their essential knowledge) and pp. 24–36 (guided notes, the sweet potato SA:V lab, the crocodile icefish mini-FRQ, and review problems). Everything the workbook asks you to know is covered here. Some explanation goes past what the workbook says. That extra material uses standard AP Bio content (for example, why a polar head is hydrophilic), and it's labeled where it matters. One correction to your workbook: the equation box on p. 2 prints the sphere volume as V = 4/3 πr². That's wrong. The correct formula is V = 4/3 πr³, and the workbook's own p. 24 notes have it right. If you use r², every sphere SA:V answer comes out wrong. PART 1: The One Model Under All Three Topics

These three topics are really one idea told three ways:

A cell can only stay alive by exchanging material across its membrane, so everything depends on how much membrane there is (2.2), what the membrane is made of (2.3), and what that makeup lets through (2.4).

2.2 Cell Size asks how much membrane there is compared to the cytoplasm it has to serve.

2.3 Plasma Membranes asks what the membrane is built from and why it holds together.

2.4 Membrane Permeability asks which molecules get across, and why the membrane's structure decides that. If you can explain any membrane question by chaining structure → property → consequence for the cell, you can answer nearly every FRQ prompt from these topics. Examples:

Structure Property it creates Consequence for the cell

Hy drophobic tail interior Ions and polar molecules can't dissolve

through it

Selective permeability , so the cell needs channel and transport

proteins

Unsaturated (kinked) tails Phospholipids can't pack tightly The membrane stay s fluid in the cold

Cholesterol between

phospholipids

Buffers fluidity in both directions The membrane stay s stable over a range of temperatures

Small size (high SA:V) More membrane per unit of cy toplasm Faster exchange of nutrients, wastes, and heat Cell wall outside the

membrane

Rigid structural boundary Shape support and protection from bursting (osmotic ly sis)

How AP phrases this. The verbs in the learning objectives tell you how deep to go. 2.2.A "Explain" means you need cause and effect, not just a definition. 2.3.A/B "Describe" means you need the parts and their roles. 2.4.A "Explain" means you must say why the structure causes the permeability, and 2.4.B "Describe" means you need the cell wall's roles. If the prompt says "explain," an answer that only describes will lose the point.

AP Bio · Topics 2.2–2.4 2 of 12

PART 2: Topic-by-Topic

Topic 2.2: Cell Size and Why Cells Stay Small

The core idea

A cell's surface area (its membrane) is where everything gets in and out: nutrients and O₂ in, wastes and CO₂ out, heat out. Its volume (the cytoplasm) is what demands those things. So the number that matters is the ratio:

SA:V = surface area ÷ volume

High SA:V means lots of membrane for each unit of cytoplasm, so exchange is efficient.

Low SA:V means little membrane for each unit of cytoplasm, so exchange is inefficient.

Why getting bigger lowers SA:V

Surface area grows with the square of the length, and volume grows with the cube. Double a cube's side and SA goes up 4× while V goes up 8×, so SA:V is cut in half.

Shape Surface area Volume SA:V simplifies to

Cube (side s) SA = 6s² V = s³ 6 / s

Sphere (radius r) SA = 4πr² V = 4/3 πr³ 3 / r

Rectangular solid SA = 2lw + 2lh + 2wh V = lwh compute directly

Cy linder SA = 2πrh + 2πr² V = πr²h compute directly

The shortcut is worth memorizing: for a cube SA:V = 6/s, and for a sphere SA:V = 3/r. That's why a bigger cell always has a smaller ratio. The size term sits in the denominator.

What the essential knowledge actually says (in plain language)

1. 2.2.A.1: SA:V controls how well a cell or organism can get nutrients, get rid of waste, gain or lose heat, and exchange chemicals with its surroundings.

2. 2.2.A.2: The membrane has to be big enough to keep up with the cell's needs. So: - Smaller cells have a higher SA:V and exchange more efficiently than larger cells. - As a cell grows, SA:V falls while demand for resources rises. Both changes hurt the cell at once. - Bigger or more complex cells use membrane folds and other structures to add surface area without adding much volume. - Bigger organisms have a lower SA:V, so they lose heat more slowly. Small masses exchange proportionally more heat with their surroundings. - Smaller organisms usually have a higher metabolic rate per unit body mass. A mouse burns more energy per gram than an elephant does.

Trap to avoid: students often write "larger cells have more surface area, so they exchange better." The first half is true; larger cells do have more total surface area. But exchange efficiency depends on surface area relative to volume. A 5 cm cube has 150 cm² of surface but only 1.2 cm² of it per cm³ of volume. A 2 cm cube has just 24 cm² but 3 cm² per cm³. The ratio wins, not the raw number. Trap to avoid: the storage question (workbook p. 2, Q4) flips the usual answer. A cell built for storage (like a fat cell or a starch- storing root cell) benefits from a low SA:V: lots of volume to hold material, and little exchange needed. Of 4.5, 7.2, 3.1, and 5.5, the storage cell is 3.1. Don't just reflexively circle the biggest number.

How cells and organisms get around the limit

Strategy Example How it helps SA:V

Stay small Bacteria (~1–10 μm) High SA:V without any special structures

Divide instead of

growing

All multicellular growth Eight 1 cm cubes have 48 cm² of surface for 8 cm³. One 2 cm cube has only 24

cm² for the same 8 cm³.

Flatten or stretch

out