Biology by Bradford
Biology by Bradford · AP Biology · Unit 2.1

Cell Structure & Size

Membranes buy a cell the right to run incompatible chemistry at the same time — and geometry decides how big it is allowed to get while doing so.

AP · U2.1CED 2.1–2.2Interactive lesson
Concept 01 · CED 2.1

Not a bag of soup

A eukaryotic cell is not one reaction vessel. It is dozens of them, each held at its own pH, its own ion concentration, its own enzyme set — separated by membranes.

Why compartments

Incompatible chemistry

Digestive enzymes at pH 5 and cytosolic enzymes at pH 7 cannot share a space. Membranes let both run at once.

The production line

Endomembrane system

Nuclear envelope, rough ER, Golgi, vesicles, lysosomes and plasma membrane — one connected system, not a list.

Shared inheritance

What every cell has

Plasma membrane, cytoplasm, DNA and ribosomes. Everything else is a eukaryotic addition.

PROKARYOTEEUKARYOTECELL WALLPLASMA MEMBRANENUCLEOID · CIRCULAR DNAPLASMIDFLAGELLUMPLASMA MEMBRANE · NO WALLNUCLEUS · DOUBLE MEMBRANE, PORESMITOCHONDRIONROUGH ER1–5 µm across10–100 µm acrossTHE SAME TWO CELLS, DRAWN TO ONE SCALEEUKARYOTE · 30 µmPROKARYOTE · 3 µm10 µmTEN TIMES WIDER1000× THE VOLUME
The headline difference is the nucleus, but the useful one is scale. A eukaryotic cell is typically ten times wider — and therefore about a thousand times greater in volume — which is precisely why it cannot rely on diffusion across its outer surface alone.
PROKARYOTEEUKARYOTEDDNANot wound on histonesWound around histonesOne circular chromosomeSeveral linear chromosomesIntrons rareIntrons commonUNIT 6OORGANELLESNo nucleusNucleusNo membrane-bound organellesMembrane-bound organelles70S ribosomes80S ribosomesRREPRODUCTIONBinary fissionMitosis, or meiosis for gametesUNIT 4Chromosome not pairedChromosomes often in pairsUNIT 5AAVERAGE SIZEAbout 1–5 µmAbout 10–100 µmTAGGED ROWS ARE PROPERLY MET IN A LATER UNIT
Four families of difference, and the mnemonic runs down the left: DNA, Organelles, Reproduction, Average size. Two rows are worth reading carefully. Prokaryotic DNA is often called “naked”, but the real contrast is that it is not wound around histones — bacteria do have proteins associated with the nucleoid. And a prokaryote is not “haploid”: haploid and diploid describe sets of paired chromosomes, which a single circular chromosome does not have. The tagged rows depend on material from later units, so treat them as signposts rather than as things to master now.
Diagram · interactive

Take the cell apart

Tap each organelle. Watch how many of them are really the same delivery system seen at different stages.

PLASMA MEMBRANENUCLEOLUSNUCLEUSROUGH ERSMOOTH ERGOLGIMITOCHONDRIONLYSOSOMEFREERIBOSOMESTAP A PART
Diagram

Start with the membrane

Tap the plasma membrane, nucleus, rough or smooth ER, Golgi, mitochondrion, lysosome or the free ribosomes.

Signature interactive · CED 2.2

The cube bench

Four cubes, one variable. Watch the ratio collapse — and watch the shaded shell become a smaller and smaller share of the whole.

Concept 02 · CED 2.2

Why cells stay small

Supply crosses a surface. Demand fills a volume. Those two things scale differently, and that mismatch sets a hard ceiling on cell size.

The mismatch

Squares against cubes

Double the diameter and area rises fourfold while volume rises eightfold. The ratio halves every time.

The consequence

Diffusion runs out

Diffusion is quick across micrometres and useless across centimetres. Beyond a certain size the centre simply cannot be served.

The workaround

Fold, flatten, divide

Microvilli and cristae fold. Red blood cells flatten. Everything else just stays small and multiplies.

1234560246SIDE LENGTH / cmSURFACE AREA : VOLUMERATIO = 6 / LFALLS AWAY FAST
The relationship is not a gentle decline. Between 1 cm and 2 cm the ratio drops by three whole units; between 5 cm and 6 cm it drops by 0.2. Almost all of the penalty for being large is paid early, which is why cells cluster at the small end of this curve.
Live model · particles

Watch the middle starve

Molecules enter from every face and wander at random. Same rules, same clock, two different sizes — and the shading records everywhere they have been.

Live model · interactive

Grow the cell yourself

Drag the slider and watch three things at once: the cube scaling with the side length, the shaded shell staying the same thickness because diffusion reaches the same distance regardless of size, and the marker sliding down the curve.

Concept map · interactive

How it all hangs together

Tap a node to light up its connections. The pink dashed link joins the two halves of this lesson.

gains organellesdivided intoincludesmakesor exportslimited byset byraisessame trick, smaller scaleProkaryoteEukaryoteCompartmentsEndomembraneLysosomeSecretionCell sizeSurface : volumeCristae & microvilli
Concept map

Tap a node

Compartments and cell size look like separate topics. They are the same problem: how much membrane can you get, and where.

Retrieval · drag and drop

Fill the gaps

Drag a term into a gap, or tap a term and then tap a gap. Two terms belong nowhere.

compartmentsendomembranehydrolyticdouble membranesurface areavolumediffusionfolded covalentisotonic

Eukaryotic cells divide their interior into , so that incompatible reactions can run at once. The system modifies and ships proteins from the rough ER through the Golgi to their destination. A lysosome holds enzymes at a pH far below that of the cytosol, and the nucleus is enclosed by a pierced by pores. Exchange with the environment happens across the , while demand for oxygen and nutrients depends on the . Because volume rises faster as a cell grows, can no longer supply the centre — so exchange surfaces are usually to buy back area.

Check yourself · AP-style

Six questions

Single best answer. You get the reasoning as soon as you commit.