Prokaryotic vs. Eukaryotic Cells

You’ll be able to

Distinguish prokaryotic and eukaryotic cells by their defining structural features
Explain how compartmentalization by internal membranes benefits eukaryotic cells
Relate the surface-area-to-volume ratio to limits on cell size

Two blueprints for a cell

Every cell fits one of two floor plans. Prokaryotic cells — bacteria and archaea — are small and simple: their DNA floats free in a region called the nucleoid, and they have no membrane-bound organelles. Eukaryotic cells — those of plants, animals, fungi, and protists — are larger and partitioned: their DNA is sealed inside a nucleus, and the cytoplasm is filled with membrane-bound organelles, each a specialized compartment. Both types share four features: a plasma membrane, cytoplasm, ribosomes, and DNA.

The power of compartments

The single biggest advantage of the eukaryotic plan is compartmentalization. By wrapping different jobs inside separate membrane-bound spaces, a cell can run reactions that would interfere with one another at the same time — digesting molecules in a lysosome while building proteins on the ER, for instance. Compartments also let a cell hold incompatible conditions side by side (an acidic lysosome next to a neutral cytosol) and concentrate the right enzymes and substrates in one place, making reactions far more efficient.

Why cells stay small: surface area vs. volume

A cell exchanges everything it needs — nutrients in, wastes out — across its surface (the plasma membrane), but its metabolic demand grows with its volume. As a cell enlarges, volume grows faster than surface area, so the membrane can no longer service the interior. The surface-area-to-volume ratio (SA:V) captures this limit: a high ratio means plenty of membrane per unit of cargo. This is why cells stay microscopic, and why cells that need fast exchange (like intestinal cells) fold their membranes into finger-like microvilli.

Surface-area-to-volume ratio (cube)

SA:V = 6s² / s³ = 6 / s

For a cube of side length s, surface area is 6s² and volume is s³. As s increases, the ratio 6/s falls — bigger cells have relatively less membrane per unit of contents.

Worked example

Compare a cube-shaped cell of side length 2 µm with one of side length 4 µm. Which has the higher surface-area-to-volume ratio, and what does that mean for exchange?

1.Small cell: surface area = 6 × 2² = 24 µm²; volume = 2³ = 8 µm³; SA:V = 24 / 8 = 3 (or 6/2 = 3).
2.Large cell: surface area = 6 × 4² = 96 µm²; volume = 4³ = 64 µm³; SA:V = 96 / 64 = 1.5 (or 6/4 = 1.5).
3.The smaller cell has the higher ratio (3 vs. 1.5): it has twice as much membrane per unit of volume to move materials in and out.

Answer: The 2 µm cell wins with SA:V = 3 versus 1.5 for the 4 µm cell. More surface per unit volume means it can exchange nutrients and wastes fast enough to supply its whole interior — the reason cells stay small.

Checkpoint

Which feature is found in eukaryotic cells but never in prokaryotic cells?

Watch out

Prokaryotes are not "cells without DNA" or "cells without ribosomes" — they have both. What they lack is a nucleus and other membrane-bound organelles. Their ribosomes are simply free in the cytoplasm.

Checkpoint

As a spherical cell grows larger, why does it become harder for the cell to survive?

On the exam

When an AP question links cell size to function, argue from SA:V: small cells and highly folded membranes (microvilli, root hairs, alveoli) maximize surface area per volume for faster exchange. Naming the ratio and its direction earns the point.

Answer the 2 checkpoints as you read.