Cell Structure and Function
Organelles, membranes and transport; surface-area-to-volume ratio.
The cell is the smallest unit that carries out all the processes of life. Prokaryotic cells, such as bacteria, are small and have no nucleus; their DNA floats in the cytoplasm. Eukaryotic cells, found in plants, animals, fungi and protists, keep their DNA in a nucleus and divide their work among membrane-bound organelles.
Each organelle has a job: mitochondria release energy from food through cellular respiration, chloroplasts capture light energy in photosynthesis, ribosomes build proteins and the endoplasmic reticulum and Golgi apparatus modify and ship them. The cell membrane, a phospholipid bilayer, controls what enters and leaves.
Key ideas
- Plant cells have a cell wall, chloroplasts and a large central vacuole; animal cells do not.
- Diffusion and osmosis move substances down a concentration gradient without energy; active transport uses ATP to move them up it.
- Surface-area-to-volume ratio limits how large a cell can grow.
- Cell theory: all living things are made of cells, and all cells come from existing cells.
Worked example
A cell is placed in a salt solution more concentrated than its cytoplasm. What happens and why?
- The outside solution is hypertonic: lower water concentration outside.
- Water moves by osmosis from high water concentration (inside) to low (outside).
- The cell loses water and shrinks; a plant cell's membrane pulls away from its wall (plasmolysis).
Answer: The cell shrinks as water leaves by osmosis
Common mistakes
- Saying salt moves into the cell, when it is water that moves across the membrane.
- Claiming animal cells have cell walls.
- Calling mitochondria the place where energy is made rather than converted.
Practice questions
Try each one, then open the answer. Numerical answers on this page are verified by an automated calculation check.
1.Which organelle carries out photosynthesis?
Show answer
The chloroplast
2.Does diffusion require energy from ATP?
Show answer
No; it is passive transport down a gradient
3.A cube-shaped cell has side 2 µm. What is its surface-area-to-volume ratio (per µm)?
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