Cells and cell structure

Every living thing is made of cells: some of just one, and you of about 30 trillion. A cell is the smallest unit of life. Animal and plant cells share the same basic parts, plant cells have a few extra ones, and bacteria are much smaller and simpler. Many cells are specialised: their shape and parts suit one job.

Key facts

  • Animal cells have a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes.
  • Plant cells have all of these, plus a cell wall made of cellulose, chloroplasts and a permanent vacuole filled with cell sap.
  • Nucleus: controls the cell and holds the genetic material (DNA). Cell membrane: controls what goes in and out. Cytoplasm: where most chemical reactions happen. Mitochondria: where aerobic respiration releases energy. Ribosomes: where proteins are made.
  • Chloroplasts absorb light for photosynthesis. The cell wall strengthens and supports the cell. The vacuole keeps the cell firm.
  • Plant and animal cells are eukaryotic (their DNA is inside a nucleus). Bacteria are prokaryotic: no nucleus, just one loop of DNA and small rings called plasmids.
  • Specialised cells have features that suit their job: a sperm cell has a tail, a red blood cell has no nucleus, a root hair cell has a large surface area.
  • Cells → tissues → organs → organ systems → organism. A tissue is a group of similar cells doing the same job.

Worked examples

Example 1

Name three parts that a plant cell has but an animal cell does not.

  1. Both cells have a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes.
  2. Round the outside of a plant cell is a stiff cell wall made of cellulose.
  3. Green chloroplasts absorb light for photosynthesis.
  4. A large permanent vacuole full of cell sap keeps the cell firm. So the three extra parts are the cell wall, chloroplasts and the vacuole.

Answer: cell wall, chloroplasts and a permanent vacuole

Example 2

A cell has a cell wall, a single loop of DNA and small rings of DNA, but no nucleus. What kind of cell is it?

  1. There is no nucleus, so the DNA is loose in the cytoplasm.
  2. The small rings of DNA are plasmids. Only bacteria have them.
  3. A cell with no nucleus is prokaryotic: it is a bacterium. Some bacteria also have a flagellum (a tail) to move.

Answer: a bacterial (prokaryotic) cell

Example 3

Explain how a root hair cell is adapted to take in water from the soil.

  1. The long thin hair sticks out into the soil.
  2. This gives a large surface area, so water (and mineral ions) can be taken in faster.
  3. It lives underground with no light, so it has no chloroplasts.

Answer: its long hair gives a large surface area; it has no chloroplasts (it is underground)

Example 4

Put these in order from smallest to largest: organ, cell, organism, tissue, organ system.

  1. Cells are the smallest unit, for example a muscle cell.
  2. A group of similar cells doing the same job is a tissue: muscle tissue.
  3. Several tissues working together make an organ: the stomach has muscle and glandular tissue.
  4. Organs working together make an organ system (the digestive system), and organ systems make the organism. cell → tissue → organ → organ system → organism

Answer: cell → tissue → organ → organ system → organism