Cell, Part 2: membrane, osmosis and cell division: visual lesson, CBSE Class 9 Science

Lesson

How cells exchange and divide

Part 2: water and dissolved substances crossing the membrane, and how one cell becomes two.

In this lesson you will

Explain diffusion and osmosis and relate them to the selectively permeable cell membrane
Describe simple activities that show osmosis, and explain endosmosis, exosmosis and plasmolysis
Outline mitosis and meiosis and compute chromosome numbers
Give cautious textbook-level examples of recent advances in cell biology

Table linking four everyday events to the cell idea behind each
Everyday event	Cell idea behind it
Dried raisins swell in water	Osmosis
Wilted palak revives in water	Water enters cells
Salt draws water out of a pickle	Water leaves cells
A cut heals	Cells divide

Concept

The membrane is selective

The plasma membrane is selectively permeable: it lets some substances pass freely, others slowly and some not at all.

Table of four kinds of substance and how easily each crosses the cell membrane
Substance	Crossing the membrane
Oxygen, carbon dioxide	Pass freely
Water	Passes through easily
Glucose, ions	Pass slowly, helped by carriers
Large proteins, starch	Cannot pass

Concept

Diffusion

Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. It needs no energy from the cell.

Table of four examples of diffusion in daily life and in cells
Example	What diffuses	From where to where
Smell of incense	Scent particles	Near the stick to the whole room
Ink in a glass of water	Dye	Drop to all the water
Gas exchange in a leaf	Carbon dioxide	Air to inside the leaf
Cell respiration	Oxygen	Blood to the cell

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