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Main idea Organelles work together rather than acting as separate parts. The nucleus stores instructions, while ribosomes use those instructions to build proteins.

Mitochondria Release energy through aerobic respiration. Cells with high energy needs, such as muscle cells, usually contain more mitochondria.

Cell membrane Controls what enters and leaves the cell. This keeps internal conditions stable even when the outside environment changes.

Exam focus Be able to explain the function of each organelle and link structure to function.

Diffusion Particles move from a high concentration to a low concentration. No energy is required because movement follows the gradient.

Osmosis Water moves through a partially permeable membrane from a dilute solution to a more concentrated solution.

Active transport Moves substances against the concentration gradient and requires energy from respiration.

Common mistake Do not describe osmosis as any particle moving. Osmosis is specifically the movement of water.

Q Why does active transport need energy?

A Because substances are moved from low concentration to high concentration, against the normal direction of diffusion.

Q What is the role of the nucleus?

A It contains genetic material and controls many cell activities by carrying instructions for protein production.

Q How are diffusion and osmosis similar?

A Both are passive processes that move substances down a concentration gradient.