The cell as the unit of life
Every living thing is made of one or more cells. A cell is surrounded by a plasma membrane (cell membrane) that controls what enters and leaves. Inside is the jelly-like cytoplasm where many chemical reactions take place, and floating within it are tiny structures called organelles, each with its own function.
Key organelles and their functions
| Organelle | Function |
|---|---|
| Nucleus | Controls cell activities; contains genetic material (chromosomes). |
| Mitochondria | Site of cell respiration; release energy. |
| Ribosome | Site of protein synthesis. |
| Chloroplast | Contains chlorophyll for photosynthesis (plant cells). |
Plant cells versus animal cells
A plant cell has extra structures that an animal cell lacks: a rigid cell wall for support, chloroplasts for photosynthesis, and a large permanent vacuole that stores cell sap. An animal cell has no cell wall or chloroplast and only small, temporary vacuoles.
Example
A muscle cell contains many mitochondria because it needs a large amount of energy from respiration to contract.
Structure suits function
The design of a cell always fits the job it does. A cell that must release a lot of energy, such as a muscle cell, is packed with mitochondria. A cell that makes and exports large amounts of protein has many ribosomes. A leaf cell exposed to sunlight is full of chloroplasts, while a root cell hidden in the soil has none, because there is no light for photosynthesis. Studying an organelle therefore tells you a great deal about what a cell does.
Together these organelles keep the cell alive: the nucleus stores and controls the instructions, the mitochondria supply energy, the ribosomes build proteins, and the plasma membrane guards the border. A cell is like a tiny factory in which each part has one clear responsibility.
Remember
Cell wall, chloroplast and a large vacuole are found in plant cells; both plant and animal cells share the nucleus, cytoplasm, plasma membrane and mitochondria.