What you'll learn
Cells are the basic building blocks of all living things, and understanding their structure is the foundation of biology. For AQA GCSE Biology you need to know the differences between animal, plant, and bacterial cells, the functions of the sub-cellular structures, the difference between eukaryotic and prokaryotic cells, and how cells are specialised for their jobs. This guide covers the two cell types, the parts of animal and plant cells, bacterial cells, and cell specialisation and differentiation. By the end you should be able to identify cell structures, state their functions, and explain how cells are adapted to their roles.
Key terms and definitions
Eukaryotic cell — A cell with a nucleus and other membrane-bound structures (animal and plant cells).
Prokaryotic cell — A cell without a true nucleus (bacteria).
Nucleus — The structure that controls the cell and contains the genetic material (DNA).
Cytoplasm — The jelly-like substance where chemical reactions happen.
Cell membrane — The layer that controls what enters and leaves the cell.
Mitochondria — The structures where aerobic respiration takes place, releasing energy.
Ribosome — The site of protein synthesis.
Specialised cell — A cell adapted to carry out a particular function.
Core concepts
Eukaryotic and prokaryotic cells
There are two main types of cell. Eukaryotic cells have a true nucleus and other membrane-bound structures; animal and plant cells are eukaryotic. Prokaryotic cells, such as bacteria, are smaller and simpler, and do not have a true nucleus — their genetic material floats freely in the cytoplasm. Knowing this distinction is fundamental.
Structures in animal cells
Animal cells contain these key sub-cellular structures:
- Nucleus — controls the cell's activities and contains the DNA.
- Cytoplasm — the jelly-like substance where most chemical reactions take place.
- Cell membrane — controls what substances enter and leave the cell.
- Mitochondria — where aerobic respiration happens, releasing energy for the cell.
- Ribosomes — where proteins are made (protein synthesis).
All of these are also found in plant cells.
Extra structures in plant cells
Plant cells have everything an animal cell has, plus some extra structures:
- Cell wall — made of cellulose, it strengthens and supports the cell.
- Chloroplasts — contain chlorophyll and are where photosynthesis takes place.
- Permanent vacuole — a large space filled with cell sap, which helps keep the cell firm.
These extra structures reflect the plant's need to make its own food by photosynthesis and to support itself.
Bacterial (prokaryotic) cells
A bacterial cell is a prokaryote and differs from plant and animal cells:
- It has a cell wall, cell membrane, cytoplasm and ribosomes.
- It does not have a true nucleus; instead its genetic material is a single loop of DNA in the cytoplasm.
- It may also have small rings of DNA called plasmids.
Bacterial cells are also much smaller than plant and animal cells.
Cell differentiation and specialisation
Most cells become specialised to carry out a particular job — this process is called differentiation. As a cell differentiates, it develops features that adapt it to its function. Examples include:
- Sperm cells — have a tail to swim and many mitochondria for energy.
- Nerve cells — are long, to carry impulses over distances.
- Root hair cells — have a large surface area to absorb water and minerals.
- Red blood cells — have no nucleus, giving more room for haemoglobin to carry oxygen.
- Muscle cells — contain many mitochondria to release energy for contraction.
Each cell's structure is closely linked to its function, and linking an adaptation to its purpose is a common exam skill.
Cell size and microscopy
Cells are very small and are studied using microscopes. Light microscopes allow cells and larger structures to be seen, while electron microscopes, with much higher magnification and resolution, reveal smaller structures inside cells. This is why our understanding of cell structure improved as microscopes developed.
The size of cells and orders of magnitude
Cells and their structures vary enormously in size, and you need to be able to work with these differences. Plant and animal cells are typically around 10 to 100 micrometres (µm) across, while bacteria are much smaller, only a few micrometres. Structures inside cells are smaller still. Because the sizes span such a wide range, they are often compared using orders of magnitude — how many times bigger one thing is than another, in powers of ten. For example, an animal cell about 100 times larger than a bacterium is said to be two orders of magnitude bigger. Being comfortable with these size comparisons, and with the units millimetre, micrometre and nanometre, is important for the calculations linked to this topic.
Stem cells and differentiation
Most cells become specialised through differentiation, but some cells remain unspecialised and can develop into different cell types — these are stem cells. In animals, stem cells are found in the early embryo (where they can become any type of cell) and in some adult tissues such as bone marrow (where they can become a limited range of cell types). In plants, stem cells are found in areas called meristems, allowing plants to keep growing throughout their lives. Stem cells are important because they can be used to replace damaged or diseased cells, which has medical potential. Understanding that differentiation produces specialised cells from unspecialised stem cells connects cell structure to growth and repair.
In animals, most cells differentiate at an early stage and then can only divide to make more of the same type for repair, whereas in plants many cells keep the ability to differentiate throughout life. This difference is why plants can grow new roots and shoots from cuttings, and it is another example of how cell structure and function are linked to the way an organism grows.
Worked examples
Example 1: Comparing cell types
Give two ways a bacterial cell differs from an animal cell. A bacterial cell has no true nucleus (its DNA is a loop in the cytoplasm), and it has a cell wall, which an animal cell does not. It is also much smaller and may contain plasmids.
Example 2: Function of mitochondria
State the function of the mitochondria and name one cell that has many of them. Mitochondria are where aerobic respiration takes place, releasing energy. Cells that need a lot of energy, such as muscle cells or sperm cells, have many mitochondria.
Example 3: A specialised cell
Explain how a root hair cell is adapted to its function. A root hair cell has a long, thin extension that gives it a large surface area, which increases the rate at which it can absorb water and mineral ions from the soil.
Example 4: Identifying a plant cell
A cell has a nucleus, a cell wall, chloroplasts and a permanent vacuole. What type of cell is it, and how do you know? It is a plant cell. The cell wall, chloroplasts and permanent vacuole are found in plant cells but not in animal cells, and the nucleus shows it is eukaryotic.
Common mistakes and how to avoid them
A common error is saying bacteria have a nucleus. Bacteria are prokaryotic and have no true nucleus — their DNA is a single loop in the cytoplasm. Only eukaryotic cells (plant and animal) have a nucleus.
Students often give cell-wall and chloroplasts to animal cells. Remember: cell wall, chloroplasts and permanent vacuole are found only in plant cells (the cell wall is also in bacteria), not in animal cells.
Another mistake is confusing the functions of ribosomes and mitochondria. Ribosomes make proteins; mitochondria release energy in respiration. Learn each function clearly.
When explaining specialised cells, do not just describe them — link the adaptation to the function (for example, a root hair cell's large surface area increases absorption).
Finally, remember that plant cells have both a cell wall and a cell membrane — the wall gives support, while the membrane controls what enters and leaves.
Exam technique for "Cell structure and function"
Be ready to label animal, plant and bacterial cells, and to state the function of each structure. A table of structures and functions is the most useful thing to memorise.
For comparison questions, give clear differences — for example, plant cells have a cell wall, chloroplasts and a vacuole that animal cells lack; bacteria have no true nucleus. Know the eukaryotic/prokaryotic distinction.
For specialised-cell questions, always link the adaptation to the function. Use precise terms — nucleus, cytoplasm, mitochondria, ribosomes, differentiation — throughout, and be ready to relate cell study to microscopy.
Quick revision summary
- Eukaryotic cells (animal and plant) have a true nucleus; prokaryotic cells (bacteria) do not.
- All these cells have a cell membrane, cytoplasm, mitochondria and ribosomes; the nucleus contains the DNA.
- Plant cells also have a cell wall (cellulose), chloroplasts (photosynthesis) and a permanent vacuole.
- Bacterial cells have a cell wall, membrane, cytoplasm and ribosomes, DNA as a loop in the cytoplasm, and sometimes plasmids; they are much smaller.
- Differentiation produces specialised cells (sperm, nerve, root hair, red blood, muscle) adapted to their functions.
- Always link a cell's adaptation to its function, and study cells using light and electron microscopes.