Topic 2: Organisation of the organism

Every living thing — from a tiny bacterium to a blue whale — is built from cells. In this topic you will look inside both animal and plant cells, meet the structures that keep them working, and discover how cells become specialised for particular jobs. You will also see how cells are organised into tissues, organs and systems, and learn to calculate magnification, a skill examiners love to test.

Animal and plant cell structure

Cells contain structures called organelles, each with a function:

• Cytoplasm – jelly-like material where chemical reactions happen.
• Cell membrane – controls what enters and leaves the cell.
• Nucleus – contains DNA and controls the cell’s activities.
• Mitochondria – site of aerobic respiration, releasing energy.
• Ribosomes – site of protein synthesis.
• Vacuoles – fluid-filled spaces.

Plant cells have all the above plus three extra features: a cell wall made of cellulose for support, chloroplasts containing chlorophyll for photosynthesis, and a large permanent vacuole filled with cell sap. Animal cells have none of these three.

Bacterial cells

A typical bacterial cell is much simpler than an animal or plant cell. It has a cell wall (not made of cellulose), a cell membrane, cytoplasm and ribosomes. Crucially, it has no nucleus — instead its DNA is a single circular loop free in the cytoplasm. Many bacteria also have smaller rings of DNA called plasmids. This makes bacteria prokaryotic, unlike the eukaryotic cells of plants and animals.

Specialised cells

Many cells are specialised, meaning their structure is adapted to a particular function:

• Ciliated cells – tiny hairs (cilia) sweep mucus and trapped particles along airways.
• Root hair cells – a long projection increases surface area for absorbing water and mineral ions.
• Xylem vessels – hollow, dead, strengthened tubes that carry water and support the plant.
• Palisade mesophyll cells – packed with chloroplasts for photosynthesis.
• Nerve cells (neurones) – long fibres carry electrical impulses quickly.
• Red blood cells – biconcave and full of haemoglobin to carry oxygen, with no nucleus.
• Sperm and egg cells – adapted for reproduction.

Levels of organisation and magnification

Cells are organised into increasing levels of complexity. A cell is the basic unit; similar cells form a tissue (e.g. muscle); different tissues form an organ (e.g. the stomach); organs working together form an organ system (e.g. the digestive system); and systems make up the whole organism.

To work out magnification, use the formula magnification = image size ÷ actual size. Rearrange it to find any value, and always convert units so both measurements are the same (remember 1 mm = 1000 µm). For example, an image 40 mm long of a cell that is really 0.2 mm gives a magnification of ×200.