Cell Structure and Organisation

Every living thing is built from cells, and every cell carries the machinery of life: taking in nutrients, releasing energy, removing waste and reproducing. The cell theory states that all living organisms are made of cells, the cell is the basic unit of structure and function, and new cells arise only from existing cells.

• Robert Hooke first saw cells in cork in 1665
• Schwann and Schleiden extended the idea to all living things
• Virchow showed that cells come only from pre-existing cells

Key point: examiners love the cell theory statement, so learn all three parts as one sentence each.

The nucleus houses chromosomes and controls all cell activity through DNA. The mitochondrion releases energy from glucose during aerobic respiration, which is why metabolically active cells are packed with them. Ribosomes build proteins and are the smallest organelles you are required to know.

• Chloroplasts trap light energy for photosynthesis; they contain chlorophyll
• The cell membrane is partially permeable and controls what enters and leaves
• The sap vacuole keeps plant cells firm by storing water and dissolved substances
• Lysosomes digest worn-out organelles and invading microbes

Plant versus animal cells

Plant cells carry a rigid cellulose cell wall, chloroplasts and a large sap vacuole; animal cells have none of these three. That trio is the cleanest way to answer any "state two differences" question.

Key point: cell wall, chloroplasts, large vacuole are present in plants and absent in animals.

Cells specialise, then cooperate. Similar cells form a tissue, different tissues form an organ, organs working together form a system, and all the systems together make the organism. In the exam, trace the ladder in order; a common trap is jumping from cell straight to organ.

• Cell -> muscle cell; tissue -> muscle tissue; organ -> heart; system -> circulatory system
• Amoeba and Chlamydomonas are unicellular: one cell does every job