Prokaryote and eukaryote cell structure

Cells (Unity and diversity) · Cell structure · note 4 of 7

Spec A2.2.5, A2.2.6Download all Cell structure notes (PDF)Download all notes (PDF)12 pages

Prokaryote and eukaryote cell structureSpec A2.2.5, A2.2.6

In short

Prokaryote cells have a cell wall, plasma membrane, cytoplasm, naked DNA in a loop and 70S ribosomes, but no nucleus; Gram-positive eubacteria such as Bacillus and Staphylococcus are the type studied. Eukaryote cells have a compartmentalised cytoplasm with 80S ribosomes, a nucleus with a double membrane and pores, membrane-bound organelles and a cytoskeleton.

Prokaryote cell structure

Prokaryotes have no nucleus. The structure you need is that of Gram-positive eubacteria such as Bacillus and Staphylococcus.

Components of a Gram-positive bacterium
Component	Description and function
Cell wall	A thick layer of peptidoglycan outside the plasma membrane, with no outer membrane beyond it; protects the cell and stops it bursting when water enters by osmosis
Plasma membrane	Phospholipid bilayer with proteins; controls the entry and exit of substances
Cytoplasm	Mainly water; the site of metabolic reactions; not divided into compartments
Naked DNA in a loop	A single circular chromosome, not associated with histones, in a region called the nucleoid
70S ribosomes	Smaller ribosomes, free in the cytoplasm, that carry out protein synthesis

Prokaryote cell structure varies. Many prokaryotes also have plasmids, flagella, pili or a capsule, but the details of these variations are not required.

Gram-positive rod-shaped bacterium as seen in an electron micrograph, labelled cell wall (thick layer of peptidoglycan), plasma membrane, cytoplasm, 70S ribosomes, nucleoid region and naked DNA in a loop, with a 1 µm scale bar. — A Gram-positive bacterium such as *Bacillus*: naked DNA in a loop in the nucleoid region, and 70S ribosomes.

Eukaryote cell structure

A plasma membrane enclosing a compartmentalised cytoplasm with 80S ribosomes.
A nucleus with chromosomes made of DNA bound to histones, contained in a double membrane with pores (the nuclear envelope).
Membrane-bound cytoplasmic organelles: mitochondria, rough and smooth endoplasmic reticulum, the Golgi apparatus and a variety of vesicles or vacuoles, including lysosomes.
A cytoskeleton of microtubules and microfilaments, which supports the cell, moves organelles and allows some cells to move.

Compartments let enzymes and substrates be concentrated where they are needed, keep potentially harmful enzymes (such as those in lysosomes) away from the rest of the cell, and allow different conditions, such as pH, in different organelles.

Prokaryotes and eukaryotes compared
Feature	Prokaryote	Eukaryote
Nucleus	Absent; DNA in a nucleoid region	Present, with a double membrane and pores
DNA	Naked and circular	Linear chromosomes bound to histones
Ribosomes	70S	80S in the cytoplasm
Membrane-bound organelles	Absent	Present: mitochondria, ER, Golgi apparatus, lysosomes
Cytoplasm	Not compartmentalised	Compartmentalised
Typical size	About 0.5–5 µm	About 10–100 µm

Common mistake:

Do not write that prokaryotes have 'no DNA' or 'no chromosome'. They have DNA, as a single naked loop; what they lack is a nucleus.

Written and checked against the IB Biology SL specification · Updated October 2026

Frequently asked questions

What is the difference between prokaryotic and eukaryotic cells?

Prokaryotic cells have no nucleus: their DNA is a naked loop in the cytoplasm, and they have 70S ribosomes and no membrane-bound organelles. Eukaryotic cells have a nucleus with a double membrane and pores, chromosomes of DNA bound to histones, 80S ribosomes, membrane-bound organelles and a cytoskeleton.

How do you calculate magnification in biology?

Magnification equals the size of the image divided by the actual size of the specimen. Measure the image with a ruler, convert both values to the same unit, usually micrometres (1 mm = 1000 µm), then divide. Rearranged, actual size equals image size divided by magnification, which is how cell sizes are found from micrographs.

How do you calculate magnification using a scale bar?

Measure the length of the scale bar on the image with a ruler, convert it to the same unit as the value written on the bar, then divide the measured length by that value. For example, a 20 mm bar labelled 0.5 µm gives 20 000 µm divided by 0.5 µm, a magnification of ×40 000.

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