Cell biology and organisation
Biology · WAEC and JAMB · SS2 and SS3
Every later topic — nutrition, respiration, genetics — is cell biology in disguise. Learn the organelles and their jobs properly once and the rest of the syllabus gets easier.
What you need to know
- Cell wall (plants only): rigid cellulose, gives shape and prevents bursting. Fully permeable.
- Cell membrane: partially permeable, controls what enters and leaves. Present in all cells.
- Nucleus: holds DNA, controls all cell activities, directs protein synthesis.
- Mitochondrion: site of aerobic respiration, releases energy as ATP. Abundant in muscle and sperm cells.
- Chloroplast (plants only): contains chlorophyll, site of photosynthesis.
- Ribosome: site of protein synthesis. Found free or on rough endoplasmic reticulum.
- Vacuole: large and permanent in plants, storing cell sap and keeping the cell turgid.
- Levels of organisation: cell -> tissue -> organ -> system -> organism.
Key terms
- Diffusion
- Net movement of particles from high to low concentration, down a gradient, without energy.
- Osmosis
- Movement of water across a partially permeable membrane from a dilute to a more concentrated solution.
- Active transport
- Movement of substances against the concentration gradient, which requires energy from respiration.
- Turgidity
- The firmness of a plant cell when its vacuole is full of water and pressing on the wall.
Worked example
A plant cell is placed in a strong salt solution. Describe what happens and name the process.
- The solution outside is more concentrated than the cell sap, so it is hypertonic.
- Water leaves the vacuole by osmosis, across the partially permeable membrane.
- The protoplasm shrinks away from the cell wall.
Answer: The cell becomes flaccid and then plasmolysed. The process is osmosis.
The mistake to avoid
Plasmolysis happens to the protoplasm, not the cell wall. The wall is fully permeable and does not shrink — saying "the cell wall shrinks" loses the mark.
In the exam
Diagram questions carry marks for clean labelling with straight lines that touch the structure, and for a title. Draw in pencil, label in pen.