Animal and Plant Cell Structure

Cells as organised living units

Cells are the basic units from which living organisms are built. Each cell contains smaller parts called sub-cellular structures. Their structures suit particular jobs, and the cell works because these jobs are connected. For example, genetic instructions in the nucleus can be used to make proteins at ribosomes, while respiration in mitochondria transfers the energy needed for cell activities.

Animal and plant cells are eukaryotic: their genetic material is enclosed inside a nucleus. The diagrams used in biology are usually generalised representations, so the exact outline and number of structures can differ between real cells.

Structures shared by most animal and plant cells

The nucleus contains the cell's genetic material. Genes provide instructions that control the cell's activities, including which proteins it makes. Enclosing this genetic material in a nucleus is a defining feature of eukaryotic cells.

The cytoplasm is the jelly-like material filling much of the cell. Many of the cell's chemical reactions take place there. It also contains other sub-cellular structures.

The cell membrane forms the cell's boundary and controls which substances enter and leave. A cell must take in useful materials such as oxygen and glucose and remove substances such as carbon dioxide. The membrane therefore helps maintain the conditions needed inside the cell.

Mitochondria are the site of most reactions of aerobic respiration. Respiration transfers energy that the cell can use for its processes. Both animal and plant cells respire, so both may contain mitochondria.

Ribosomes are the sites of protein synthesis: this is where proteins are made. Proteins have many roles, including forming cell structures and acting as enzymes that control chemical reactions.

A typical animal-cell diagram should therefore show a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. The specification says most animal cells have these parts because real cells can vary.

The additional structures of plant cells

Plant cells have the structures described above, but they often also have chloroplasts and a permanent vacuole. Plant and algal cells also have a cellulose cell wall.

Chloroplasts are the site of photosynthesis. They contain chlorophyll, which absorbs light energy used to make glucose. Chloroplasts are found in photosynthetic plant tissues, particularly green tissues exposed to light; not every plant cell needs them.

The permanent vacuole is a fluid-filled space containing cell sap, a solution of substances including sugars and salts. Water in the vacuole helps press the cell contents against the cell wall, helping the cell remain rigid.

The cell wall is a rigid layer made of cellulose outside the cell membrane. It supports and strengthens the cell and helps it keep its shape. The wall does not replace the membrane: the membrane still controls movement into and out of the cell. Algal cells also have a cell wall made of cellulose.

These structures work together. Water in the vacuole produces pressure against the strong cellulose wall, so the cell gains support without bursting or losing its shape easily.

Plasmids in bacterial cells

Bacteria are prokaryotic, so their genetic material is not enclosed in a nucleus. Their main genetic material forms a single DNA loop in the cytoplasm. Some bacterial cells also contain one or more plasmids: small rings of DNA carrying additional genes that are separate from the main DNA loop.

A plasmid's function is to store extra genetic information. Like other genes, plasmid genes provide instructions that can affect the bacterium's characteristics. Plasmids are therefore associated with bacterial cells, not with the nuclei of animal or plant cells.

Comparing the three cell types

A useful comparison begins with shared structures. Animal, plant and bacterial cells all have a cell membrane, cytoplasm and ribosomes. Animal and plant cells have a nucleus and mitochondria, whereas bacteria do not. Plant cells often add chloroplasts and a , and plant and algal cells have a cellulose . Bacteria also have a cell wall, but it is made from different substances, and some bacteria possess .

Animal and plant cells

Most animal cells: nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.