Cell structure and function

Biology · Cells. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology topics

The idea

All living things are made of cells. Each part of a cell has its own job, so if you know the job you can work out what happens when that part is missing or stops working.

Animal and plant cells both have a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. Plant cells also have a cellulose cell wall, a large sap vacuole and, in the green parts only, chloroplasts.

A bacterial cell is much smaller and has no nucleus and no mitochondria. Its DNA is one circular loop lying in the cytoplasm, often with extra small rings called plasmids. It still has ribosomes, cytoplasm, a cell membrane and a cell wall.

Rules to remember

Nucleus: contains the DNA (genes) and controls the activities of the cell.
Cell membrane: controls which substances enter and leave the cell. Cytoplasm: where most chemical reactions take place.
Mitochondria: site of aerobic respiration, which releases energy. Ribosomes: where proteins are made.
Chloroplasts: contain chlorophyll and carry out photosynthesis. Sap vacuole: stores cell sap and helps keep the cell firm.
Cellulose cell wall: strong and fully permeable; it supports the cell and stops it bursting.
Bacterial cell: circular DNA and plasmids, ribosomes, cytoplasm, cell membrane, cell wall; no nucleus.
Stains make cell parts easier to see: iodine solution for plant cells, methylene blue for animal cells such as cheek cells.

Common mistake

Students say that every plant cell has chloroplasts. Root cells and onion epidermis cells have none because they receive no light. Also do not confuse the cell wall (support) with the cell membrane (control of entry and exit).

Worked example

A cell seen in an electron micrograph has a cell wall, a large sap vacuole, a nucleus and mitochondria, but no chloroplasts. Which of these could it be: a bacterium, a liver cell, a root cell or a palisade cell?

It has a nucleus and mitochondria, so it is not a bacterium.
It has a cell wall and a large sap vacuole, so it is a plant cell and not a liver cell.
It has no chloroplasts, so it is not a palisade cell, which is packed with them.
A plant cell without chloroplasts comes from a part that gets no light, such as a root.

Answer: A root cell.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Organisation & Maintenance of Organisms

Keep going

Specialised cells, tissues and organs →Take a Biology test