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Class 9 Cell: The Building Block of Life: cheat sheet
CBSE Science. The cell is the basic unit of life: its parts and their jobs, plant and animal cells, how water moves in and out, and how cells divide.
Key ideas
- All living things are made of cells, and new cells come only from existing cells.
- Prokaryotic cells (bacteria) have no nuclear membrane; eukaryotic cells have a true nucleus and membrane-bound organelles.
- Plant cells have a cell wall, chloroplasts and a large central vacuole; animal cells do not.
- Diffusion moves substances from high to low concentration; osmosis is the movement of water across a selectively permeable membrane.
- In a hypotonic (more dilute) solution a cell gains water and swells; in a hypertonic one it loses water and shrinks.
- Mitosis gives two identical cells for growth and repair; meiosis gives four cells with half the chromosomes, for gametes.
Remember
| Nucleus | controls the cell; holds the chromosomes |
| Mitochondria / chloroplasts | release energy (ATP) / make food by photosynthesis |
| Ribosomes / Golgi / lysosomes | make proteins / package and send / digest waste |
| Microscope magnification | eyepiece × objective; real size = image size ÷ magnification |
Watch out
- Saying osmosis moves salt or sugar: it moves water.
- Saying the cell wall is selectively permeable (the membrane is; the wall is fully permeable).
- Adding the eyepiece and objective powers instead of multiplying them.
Worked example
An animal cell with 8% sugar in its sap is placed in a 20% sugar solution. Which way does water move? Is the outside solution hypotonic or hypertonic? What happens to the cell?
Working: water moves from the dilute to the concentrated side
Answer: Water moves out of the cell; the solution is hypertonic; the cell shrinks.
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