The Cell: The Fundamental Unit of Life: Class 9 Study Guide
All living things are made of cells. Each part of a cell has a job.
Robert Hooke first saw and named cells in 1665, when he looked at a thin slice of cork under a microscope. The cell theory says that all living things are made of cells, the cell is the basic unit of life, and new cells come from existing cells. Some organisms are unicellular (one cell, like amoeba) and others are multicellular.
A prokaryotic cell, such as a bacterium, has no true nucleus with a membrane around it. A eukaryotic cell has a proper nucleus and membrane-bound organelles.
Parts of a cell and what they do
| Part | Job |
|---|---|
| Cell membrane | Controls what enters and leaves |
| Cell wall (plants) | Gives support and shape |
| Nucleus | Controls the cell; holds the chromosomes with DNA |
| Cytoplasm | Jelly-like fluid where reactions happen |
| Mitochondria | Release energy from food: the 'powerhouse' |
| Endoplasmic reticulum | Makes and moves materials |
| Golgi apparatus | Packs and sends out materials |
| Lysosomes | Digest waste and old parts |
| Chloroplast (plants) | Photosynthesis |
| Vacuole | Stores water and substances |
Plant and animal cells
Plant cells have a cell wall, chloroplasts and a large central vacuole. Animal cells have no cell wall or chloroplasts, and their vacuoles are small. Both have a membrane, nucleus, cytoplasm and mitochondria.
Wall or only a membrane?
Green means plant.
Large or small?
Plant or animal.
Movement across the membrane
Diffusion is the movement of particles from higher to lower concentration. Osmosis is the movement of water through a selectively permeable membrane from a dilute to a concentrated solution. A raisin swells in water because water enters by osmosis, and shrinks in a strong sugar solution as water leaves.
Worked examples and common mistakes
Example: Which organelle makes most of the cell's usable energy? The mitochondrion. Example: Why do plants not burst in water? Their cell walls resist the swelling. Common mistakes: saying that plant cells have no mitochondria (they do), and mixing up the cell wall and the cell membrane. All cells have a membrane; only plants have a wall outside it.
Check your understanding: model answers
1. The nucleus. 2. For example plant cells have a cell wall and chloroplasts, and animal cells do not. 3. Water enters by osmosis because the inside is more concentrated.
✨ Key Takeaways
- The cell is the basic unit of life.
- Each organelle has its own job.
- Plant cells have a cell wall, chloroplasts and a big vacuole.
- Osmosis is the movement of water through a membrane.
💬 Questions for Discussion & Reflection
- Which organelle contains the DNA?
- Name two differences between plant and animal cells.
- Why does a raisin swell in water?
Print a science observation log →