GRADE 8 Science LIVING THINGS AND THEIR ENVIRONMENT – Movement of materials in and out of the cell Notes

LIVING THINGS AND THEIR ENVIRONMENT — Movement of materials in and out of the cell

Cells need to take in useful substances (like water, oxygen and food molecules) and remove wastes (like carbon dioxide). The movement of materials across the cell membrane keeps the cell alive and working. Below are the main ways materials move in and out of cells.

Key ideas & words

• Cell membrane (selectively permeable): lets some things pass, blocks others.
• Solute: the substance dissolved (e.g. salt, sugar).
• Solvent: the liquid that dissolves the solute (usually water).
• Concentration gradient: difference in concentration between two places.
• Passive transport: movement without using cell energy (ATP).
• Active transport: movement that needs energy (ATP), often against the gradient.

1. Diffusion (passive)

Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration until they are evenly spread (equilibrium).

Examples: oxygen moving from air in the lungs into blood; smells spreading in a room.

2. Osmosis (a special diffusion of water)

Osmosis is the movement of water molecules across a selectively permeable membrane from an area of low solute concentration (more water) to high solute concentration (less water).

Important for plant cells and animal cells: it affects cell size and shape.

3. Facilitated diffusion (passive but uses protein channels)

Large or charged molecules (e.g. glucose, ions) cannot pass directly through the membrane. They move through special proteins called channels or carriers — still without energy, and down their concentration gradient.

Example: glucose entering cells after digestion (through carrier proteins).

4. Active transport (needs energy)

Active transport moves substances from low concentration to high concentration (against the gradient) and requires energy (ATP).

Examples: root hair cells absorbing mineral ions from soil; sodium–potassium pump in animal cells.

5. Bulk transport — Endocytosis and Exocytosis

Large particles or large amounts of substances are moved in vesicles:

• Endocytosis: cell membrane folds in to bring materials inside (e.g. white blood cells engulf bacteria).
• Exocytosis: vesicles fuse with membrane to release substances outside (e.g. cells release hormones or wastes).

Simple school experiments (easy & cheap)

1. Potato and salt water: Place equal potato slices in fresh water and in salty water. After 30–60 minutes observe size/firmness — the fresh water slice becomes firmer (water in), salt water slice becomes limp (water out).
2. Onion epidermis + salt: Place a thin onion skin on a slide with fresh water and observe cells. Then add a drop of concentrated salt solution and watch cells shrink (plasmolysis) under a microscope.
3. Sugar cube test: Drop a sugar cube in water and observe diffusion of sugar into the water (too small to see molecules, but you can taste sweet water around the cube over time).

Why this matters (importance)

• Maintains cell health (homeostasis).
• Allows plants to take up water and minerals for growth and to stay upright.
• Allows animals to obtain oxygen and nutrients and remove wastes like carbon dioxide and urea.

Quick summary

Passive transport (diffusion, osmosis, facilitated diffusion) needs no energy and moves substances down their concentration gradient. Active transport needs energy and moves substances against the gradient. Bulk transport moves large particles by special cell membrane actions.

Short quiz (check yourself)

1. What is osmosis? (One line answer)
2. Give one example of diffusion in animals.
3. Why do root hair cells use active transport? (short)
4. What happens to a plant cell in very salty water?
5. Is facilitated diffusion active or passive?

Answers

1. Movement of water across a selectively permeable membrane from low solute to high solute concentration.
2. Oxygen moving from alveoli (lungs) into blood; carbon dioxide moving out of blood into the lungs.
3. To absorb mineral ions from dilute soil where the ion concentration in the soil is lower than inside the root cells.
4. The plant cell loses water and shrinks (plasmolysis), the cell becomes flaccid.
5. Facilitated diffusion is passive (no energy used).

Tip to remember: "Down with the flow = passive (no energy); Up the hill = active (needs energy)."