Cell Biology — Class 7 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 7 Science, Term 2 — Chapter 4. The cell and its parts.
1. About this chapter
This chapter covers the cell and its organelles, their functions, and the differences between plant and animal cells.
2. The cell and its organelles
- The cell is the basic unit of life. Its main parts are the cell membrane, the cytoplasm and the nucleus.
- The cell membrane is the outermost layer of an animal cell; a plant cell has, in addition, a tough cell wall outside the membrane.
| Organelle | Function / nickname |
|---|---|
| Nucleus | the "brain" and control room; the largest organelle; carries heredity |
| Mitochondria | the "powerhouse"; carry out respiration (release energy from food) |
| Chloroplast | carries out photosynthesis (in plant cells only) |
| Centriole | helps in cell division (in animal cells) |
3. Plant cell vs animal cell
- A plant cell has a cell wall, a large vacuole and chloroplasts; an animal cell does not have a cell wall or chloroplasts but has a centriole.
4. Worked examples
Example 1. Which organelle is the "brain" of the cell? The nucleus.
Example 2. Which organelle is the "powerhouse"? The mitochondria.
Example 3. What does the centriole help with? Cell division (in animal cells).
5. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
- The part of the cell called the brain of the cell is the — (a) ribosome / (b) nucleus. Ans: (b) nucleus.
- The largest organelle is the — (a) chloroplast / (b) nucleus. Ans: (b) nucleus.
- The outermost layer of an animal cell is the — (a) cell wall / (b) cell membrane. Ans: (b) cell membrane.
- The organelle that helps in cell division is the — (a) Golgi complex / (b) centriole. Ans: (b) centriole.
II. Analogy 5. Chloroplast : photosynthesis :: mitochondria : respiration. 6. Cell wall : plant cell :: centriole : animal cell.
III. Fill in the blanks (match) 7. Powerhouse of the cell — mitochondria. 8. Control room of the cell — nucleus.
IV. Answer briefly 9. Why is the nucleus called the brain of the cell? — It controls all the processes in the cell and carries heredity. 10. Give one difference between a plant cell and an animal cell. — A plant cell has a cell wall and chloroplasts; an animal cell does not.
6. Common mistakes
- Mistake: Saying an animal cell has a cell wall. Fix: Only plant cells have a cell wall; the animal cell's outer layer is the cell membrane.
- Mistake: Confusing mitochondria and chloroplast. Fix: Mitochondria = respiration (powerhouse); chloroplast = photosynthesis (plants only).
- Mistake: Calling the nucleus the powerhouse. Fix: The nucleus is the brain/control room; the mitochondria is the powerhouse.
7. Quick revision
- Term 2 · Ch 4 · cell biology.
- Cell = basic unit of life; cell membrane (outermost in animal cell), cytoplasm, nucleus.
- Nucleus = brain/control room, largest organelle; mitochondria = powerhouse (respiration); chloroplast = photosynthesis; centriole = cell division.
- Plant cell: cell wall + chloroplasts + large vacuole; animal cell: no cell wall, has a centriole.
