Explain cell theory and compare prokaryotic and eukaryotic cells.

Cell theory, the shared features of all cells, and the key differences between prokaryotic and eukaryotic cells, for TCE Biology Unit 1.

Generated by Claude Opus 4.87 min answerUpdated 2026-06-02

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What this dot point is asking

Cell theory

The cell is the foundation idea of biology because every organism, from a single bacterium to a blue whale, is built from cells. The modern cell theory has three core statements:

All living things are made of one or more cells.
The cell is the smallest unit that shows all the characteristics of life.
All cells come from pre-existing cells by division.

This theory developed over time as microscopes improved. Robert Hooke first described cork cells, and later workers such as Schleiden, Schwann, and Virchow added the ideas that all organisms are cellular and that cells reproduce only from other cells. The third statement directly rejects spontaneous generation, the old belief that living things could form from non-living matter.

Features shared by all cells

Despite huge variety, every cell has a few features in common, which reflects their shared evolutionary origin:

A plasma membrane that separates the inside of the cell from its surroundings and controls what enters and leaves.
Cytoplasm (or cytosol), the watery internal environment where chemical reactions occur.
Genetic material in the form of DNA, which carries the instructions for building proteins.
Ribosomes, the structures that build proteins.

Because all cells share these features, biologists treat them as strong evidence that all life descends from a common ancestor.

Prokaryotic cells

Prokaryotic cells are the cell type found in bacteria and archaea. The word prokaryote means before the nucleus, and the defining feature is that they have no membrane-bound nucleus. Key features include:

A single circular chromosome free in the cytoplasm, in a region called the nucleoid.
No membrane-bound organelles such as mitochondria or chloroplasts.
Generally very small, around 1 to 10 micrometres across.
A cell wall outside the membrane, and sometimes extra DNA in small rings called plasmids.

Prokaryotes usually live as single cells. They still carry out respiration and, in some cases, photosynthesis, but these processes happen at the plasma membrane rather than inside organelles.

Eukaryotic cells

Eukaryotic cells are found in animals, plants, fungi, and protists. The word eukaryote means true nucleus. Their defining feature is a membrane-bound nucleus that contains the DNA, packaged with proteins into linear chromosomes. They also have:

Membrane-bound organelles such as mitochondria, the endoplasmic reticulum, and the Golgi apparatus, which separate different chemical jobs into compartments.
A larger size, typically 10 to 100 micrometres.
Often a complex internal skeleton (the cytoskeleton) that gives shape and moves materials.

Plant cells add a cellulose cell wall, chloroplasts for photosynthesis, and a large central vacuole. Animal cells lack these but may have features such as centrioles.

Why cell size matters

Cells stay small because they rely on diffusion across the membrane to exchange materials. A small cell has a large surface area relative to its volume, so materials move in and out quickly enough to meet the cell's needs. This is why most cells, even in large organisms, are microscopic, and why large organisms are built from many small cells rather than a few giant ones.

Exam-style practice questions

Practice questions written in the style of TASC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

TCE 20226 marksA student examines two cells under a light microscope. Cell X is approximately

2\ \mu m

long, has DNA arranged in a single ring and no internal membrane-bound structures. Cell Y is approximately

50\ \mu m

long with a distinct nucleus and several membrane-bound organelles. Identify each cell type and explain three structural features that allowed you to classify them.

Show worked answer →

A 6 mark answer needs both identifications plus three justified features.

Identification: Cell X is prokaryotic (likely a bacterium); cell Y is eukaryotic.
Feature 1 - nucleus: Cell Y has a membrane-bound nucleus enclosing its DNA, the defining eukaryotic feature; cell X has its DNA loose as a single ring in the nucleoid, with no nuclear membrane.
Feature 2 - organelles: Cell Y has membrane-bound organelles that compartmentalise reactions; cell X lacks them, so its reactions (including respiration) occur at the plasma membrane.
Feature 3 - size: Cell X at $2\ \mu m$ sits in the typical prokaryotic range ( $1$ to $10\ \mu m$ ); cell Y at $50\ \mu m$ sits in the eukaryotic range ( $10$ to $100\ \mu m$ ).

Markers reward correct classification plus three clearly linked features (nucleus, organelles, size).

TCE 20245 marksState the three principles of the modern cell theory and explain how the third principle contradicts the older idea of spontaneous generation.

Show worked answer →

A 5 mark answer lists the three principles and connects the third to spontaneous generation.

Principle 1: All living things are made of one or more cells.
Principle 2: The cell is the smallest unit that shows all the characteristics of life.
Principle 3: All cells arise from pre-existing cells by division.
Link to spontaneous generation: Spontaneous generation claimed that living things could form from non-living matter. The third principle states cells come only from other cells, so life cannot arise spontaneously from non-living material. This directly rejects spontaneous generation.

Markers reward all three principles stated correctly and a clear explanation that "cells only from cells" rules out life forming from non-living matter.