AP Bio 2.2-2.4 guide with diagramsAP Bio 2.2-2.4 guide with diagrams

Topics 2.2 – 2.4

Cell Size · Plasma Membranes · Membrane Permeability

LO 2.2.A LO 2.3.A LO 2.3.B LO 2.4.A LO 2.4.B

The whole thing on one picture

2.2 CELL SIZE

HOW MUCH membrane?

SA:V ratio

2.3 PLASMA MEMBRANE

WHAT is it made of?

fluid mosaic model

2.4 PERMEABILITY

WHAT gets through?

hydrophobic core + walls

Every answer = STRUCTURE → PROPERTY → CONSEQUENCE for the cell

A cell lives by trading stuff across its membrane. 2.2 = is there enough membrane? 2.3 = what is the membrane built

from? 2.4 = what does that build let through?

How to read this guide

Colored boxes = the 20-second version of a topic

Red "Trap" boxes = where people lose points

Purple "Quick check" = cover the answer and test

yourself

AP verbs matter

Explain (2.2.A, 2.4.A) means you give the cause, not

just a definition. Describe (2.3.A, 2.3.B, 2.4.B) means

you name the parts and what they do. If the prompt

says explain and you only describe, you lose the point.

⚠ Workbook typo: the equation box on p. 2 says sphere volume is V = 4/3 πr². That's wrong. It's V = 4/3 πr³

(cubed). Your p. 24 notes have it right. If you use r², every sphere answer comes out wrong.APBIOLOGY · UNIT2 : CELLS · V I S U A L S T U D Y G U I D EBuilt from your 2025 Unit 2 Workbook (pp. 2–3 learning objectives + essential knowledge, pp. 24–36 notes, lab, mini-FRQ and review). Every essential-knowledge statement is here, each with a picture. Every workbook question

is answered in the key near the end.

Cell Size

2 0 - S E C O N D V E R S I O N

Small cells have a high SA:V, so they exchange materials efficiently. As a cell grows, volume grows faster than

surface area, so SA:V drops while the cell's demand rises. That's why cells stay small, divide, or fold their

membranes.

What has to cross the membrane? (workbook p. 24)

⬇ IN

Nutrients, O₂, and other resources

and chemicals

⬆ OUT

Cellular waste (CO₂ and others)

🌡 OUT / IN

Thermal energy (heat) has to be

given off or taken in

Surface area = the membrane, the "doors." Volume = the cytoplasm, the "customers" who need things. SA:V = doors per

customer. More doors per customer means faster service.

Formulas to memorize

Shape Surface area Volume SA:V shortcut

Cube (side s) 6s² s³ 6 / s

Sphere (radius r) 4πr² 4/3 πr³ 3 / r

Rectangular solid 2lw + 2lh + 2wh lwh calculate SA ÷ V

Cylinder 2πrh + 2πr² πr²h calculate SA ÷ V

The shortcuts show the whole idea: the size (s or r) is in the denominator, so bigger always means a smaller ratio. Units

are 1/length (cm⁻¹, μm⁻¹), so only compare ratios that use the same units.TOPIC2 . 2 · LO2 . 2 . AExplain how surface area-to-volume ratio affects exchange between cells/organisms and the environment.

1 cm cube

SA = 6(1)² = 6 cm²

V = (1)³ = 1 cm³

2 cm cube

SA = 6(2)² = 24 cm²

V = (2)³ = 8 cm³

3 cm cube

SA = 6(3)² = 54 cm²

V = (3)³ = 27 cm³

SA:V ratio (higher = better exchange)

1 cm 6.0

2 cm 3.0

3 cm 2.0

Why it drops

SA grows as side²

V grows as side³

V outruns SA

→ ratio falls

Workbook p. 25 cubes. Double the side and SA goes ×4 but V goes ×8, so the ratio is cut in half. The 2 cm cube (SA:V 3.0) beats the 3 cm (2.0) and the 5 cm (1.2). Above: 1, 2, 3 cm cubes from the sweet potato lab. So why do organisms grow by adding cells?

ONE 2 cm cube

SA 24 cm² · V 8 cm³

SA:V = 3

vs

EIGHT 1 cm cubes (same total volume)

SA 48 cm² · V 8 cm³

SA:V = 6

Same total volume, double the surface area. Splitting into small cells keeps every cell's SA:V high. One giant cell's center would starve and fill up with waste.

Essential knowledge, in plain words

EK What it says

2.2.A.1 SA:V controls how well a cell or organism gets nutrients, gets rid of waste, gains or loses heat, and exchanges chemicals and energy.

2.2.A.2

i

SA:V limits cell size and shape. Smaller cells have a higher SA:V, so they exchange more efficiently. ii As volume goes up, SA:V goes down and demand for internal resources goes up. Both changes hurt the cell at once.

iii Bigger or more complex cells need structures like membrane folds to keep up.

iv Bigger organisms have a lower SA:V, so they exchange heat more slowly. Small masses lose proportionally more heat.

v Smaller organism → higher metabolic rate per unit body mass.

How real cells beat the size limit

Microvilli (electron micrograph,

intestinal cell). Thousands of finger-

like membrane folds add surface

area with almost no added volume,

so the cell absorbs nutrients faster.

Red blood cell (left, red). Its

flattened, dented biconcave shape

puts more surface per volume, so O₂

diffuses in and out quickly.

Mitochondrion. The inner

membrane folds into cristae, which

give more membrane for the

reactions that happen on it.

Chloroplasts do the same with

stacked thylakoids.

Strategy Example Why it works

Stay small Bacteria (~1–10

μm)

High SA:V with no special structures

Divide instead of

growing

All multicellular

growth

Eight 1 cm cubes = 48 cm² of SA vs. 24 cm² for one 2 cm cube

Flatten or stretch Red blood cells,

neurons

A 1×1×8 box has 34 cm² of SA vs. 24 cm² for a 2×2×2 cube with

the same volume

Fold the outer

membrane

Microvilli, root

hairs

Adds SA, barely adds V

Fold internal

membranes

Cristae, thylakoids,

ER

More membrane surface for reactions

Compartmentalize Eukaryotic

organelles

Each organelle has its own membrane for local exchange. That's

how eukaryotes reach 100+ μm (p. 27 Q8).

Organism scale: mouse vs. elephant

mouse

small → HIGH SA:V

loses heat FAST per gram

→ HIGH metabolic rate per gram

elephant

large → LOW SA:V

holds heat → LOW metabolic rate per gram

A mouse has far more skin per gram, so it loses heat fast and has to burn fuel fast to stay warm, which means a higher metabolic rate per gram. The elephant has the opposite problem (shedding heat), which is one reason its ears are huge, thin and full of blood vessels.

TRAP · "PER UNIT BODY MASS"

Don't write "elephants have a lower metabolic rate." Their total rate is far higher. The AP claim is about rate per unit body mass. Leave that phrase out and the statement is wrong.

TRAP · MORE SA ≠ BETTER