Animal cell structure
An animal cell has a nucleus, cytoplasm and a plasma membrane, with organelles such as mitochondria and ribosomes, but no cell wall, chloroplasts or large vacuole.
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The animal cell is the basic unit of animals. Knowing each organelle and its job is the foundation for cell biology, transport and respiration.
Under a light microscope you can see the nucleus, cytoplasm and plasma membrane of a cheek cell. The smaller organelles, mitochondria, ribosomes, the endoplasmic reticulum, the Golgi apparatus and lysosomes, appear only under an electron microscope, which is why exam diagrams of an animal cell are usually drawn from an electron micrograph.
An animal cell is a eukaryotic cell: its DNA is enclosed in a nucleus and its organelles are bound by membranes, which sets it apart from a bacterium.
Parts and functions
| Part | Function |
|---|---|
| Nucleus | Controls the cell and contains the genetic material (chromosomes) inside a nuclear membrane |
| Cytoplasm | Jelly-like medium where most chemical reactions of the cell take place |
| Plasma membrane | Controls what enters and leaves the cell; semi-permeable |
| Mitochondria | Site of aerobic respiration, releasing energy as ATP |
| Ribosomes | Where proteins are made from amino acids |
| Rough endoplasmic reticulum | Studded with ribosomes; transports proteins made on them |
| Smooth endoplasmic reticulum | Makes lipids and steroids; no ribosomes attached |
| Golgi apparatus | Modifies, packages and secretes proteins in vesicles |
| Lysosome | Contains digestive enzymes that break down worn-out organelles and foreign material |
| Centrioles | Form the spindle fibres during cell division |
How structure suits function
An animal cell has no cell wall, so its outer boundary is the flexible plasma membrane. It has no chloroplasts because animals do not photosynthesise, and any vacuoles are small and temporary.
Individual organelles are also shaped for their jobs, and the DSKP expects you to link the shape to the function.
- Mitochondrion, the inner membrane is folded into cristae, giving a large surface area for the enzymes of aerobic respiration; cells with high energy demand, such as muscle and sperm cells, contain more mitochondria.
- Nucleus, the nuclear membrane has pores that let messenger RNA pass out to the ribosomes while keeping the chromosomes inside.
- Rough endoplasmic reticulum, its extensive folded membranes carry ribosomes and form channels that transport the newly made proteins; cells that secrete enzymes or hormones have a lot of it.
- Golgi apparatus, its stacked flattened sacs pinch off vesicles, so it is prominent in secretory cells such as those of the pancreas.
- Plasma membrane, the phospholipid bilayer with embedded proteins is flexible and semi-permeable, letting the cell change shape (as a white blood cell does when it engulfs bacteria) and controlling exchange by diffusion, osmosis and active transport.
- Lysosome, the enzymes are kept inside a membrane so they do not digest the cell itself.
Related processes
Every organelle in the table is the site of a process you meet elsewhere in the syllabus. Aerobic respiration takes place in the mitochondria, where glucose and oxygen are used to make ATP.
Protein synthesis runs from the nucleus (where the gene is transcribed) to the ribosomes (where the protein is assembled), then through the rough endoplasmic reticulum and Golgi apparatus for transport and packaging before secretion.
The plasma membrane is the site of passive transport (diffusion and osmosis) and active transport. Because an animal cell has no cell wall, it responds differently to osmosis from a plant cell: in a hypotonic solution water enters, the cell swells and may burst (haemolysis), while in a hypertonic solution water leaves and the cell shrinks (crenation).
The nucleus is where mitosis begins, with the centrioles organising the spindle that separates the chromosomes.
Common labelling errors
Worked question
Question (in the style of Paper 2 Section A): Diagram Y shows an animal cell seen under an electron microscope. Structure P has a double membrane with the inner membrane folded.
Structure Q is a stack of flattened sacs with small vesicles around it. (a) Name P and Q. [2 marks] (b) State the function of P and explain how its structure suits that function. [3 marks] (c) A sperm cell contains far more of structure P than a skin cell does.
Explain why. [2 marks]
Model answer: (a) P is a mitochondrion; Q is the Golgi apparatus. (b) P is the site of aerobic respiration, which releases energy in the form of ATP.
The inner membrane is folded into cristae, which increases the surface area for the enzymes involved in respiration. (c) A sperm cell must swim using its tail, which needs a large amount of energy; more mitochondria produce more ATP for this movement.
Marking note: in part (b) the mark for "suits its function" requires the phrase increases surface area. Stating cristae alone earns the structure mark but not the explanation mark.
Source:SRC-DSKP-EN
Frequently asked questions
What organelles are found in an animal cell?
Which organelles are in an animal cell but not a plant cell?
Why does an animal cell burst in distilled water while a plant cell does not?
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