5.1Chapter 5: Respiration and the Respiratory System

Diffusion in biology — Alhussan Biology - Stage 8 Revision Notes & Topical Questions

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Biology 5.1 Diffusion in biology — Particle Movement Across Membranes — Alhussan International Academy — Biology (Stages 7–9) notes: Define diffusion, explain the factors that affect its rate, and give examples from living organisms.Matter — it is made of moving particles too sma…

Passive process
Net movement
Smelling food
Digestive system
Pollination
Cell membrane

Part 1 of 6What is diffusion

1Matter

Matter it is made of moving particles too small to see.

2Diffusion

Diffusion the net (overall) movement of particles from an area of high concentration to an area of lower concentration, down a concentration gradient.

It takes place in gases and liquids as a result of the random movement of particles.
It happens because more particles move out of the crowded region than into it, until they are evenly spread.

3Passive process

Passive process diffusion needs no energy from the cell.

4Net movement

Net movement particles move in both directions, so the word "net" must be used, and they do not stop moving at equilibrium.

Part 2 of 6Diffusion in everyday life

Diffusion in everyday life

Smelling food

Smelling food particles escape from cooking food, where they are at a high concentration, and spread through the air down a concentration gradient.
You breathe some in and your nose sends signals to the brain, which recognises the food.

Digestive system

Digestive system digested food moves into the blood by diffusion down a concentration gradient, so it reaches the cells.

Pollination

Pollination flowers produce a scent that diffuses through the air to tell pollinators they are there.
Insects such as butterflies and moths follow the scent up the concentration gradient until they reach the flower.

Sharks

Sharks blood from an injured animal diffuses through the water, and sharks swim up the concentration gradient, smelling it at very low concentrations.

Part 3 of 6Diffusion across cell membranes

Cell membrane

Cell membrane it is partially permeable,
- so small molecules such as oxygen, carbon dioxide and water pass through but large molecules such as starch cannot.

Oxygen

Oxygen it diffuses from the alveoli into the blood and from the blood into respiring cells, where its concentration is low because it is used up.

Carbon dioxide

Carbon dioxide it diffuses the opposite way, from respiring cells to the blood and then to the alveoli.

Leaves

Leaves carbon dioxide diffuses in through the stomata for photosynthesis, and oxygen diffuses out.

Digested food

Digested food glucose diffuses from the small intestine into the blood.

Link

Link diffusion is important for gas exchange in the respiratory system and for transport in the blood.

Part 4 of 6Factors affecting the rate of diffusion

Factor	Effect on rate	Reason
Concentration gradient	A steeper gradient gives a faster rate	More particles move in one direction
Temperature	Higher gives faster	Particles have more kinetic energy
Surface area	Larger gives faster	More space for particles to cross
Distance	Shorter gives faster	Particles have a shorter way to travel

Exchange surfaces
- alveoli, villi and root hairs have a large surface area, a thin wall and a good blood supply that keeps the gradient steep by carrying the substance away.

Part 5 of 6Investigating diffusion

Agar cubes

Agar cubes cubes of agar jelly with indicator, of different sizes, are placed in acid and the time for the colour change to reach the centre is measured.
Small cubes change colour faster because they have a larger surface area compared with their volume.

Potassium permanganate

Potassium permanganate a crystal placed in water at different temperatures shows how far the purple colour spreads in a given time.

Planning

Planning state the variable changed, the variable measured and the variables controlled, and repeat to get a mean.

Part 6 of 6Diffusion compared with other processes

1Osmosis

Osmosis the diffusion of water across a partially permeable membrane.

2Active transport

Active transport movement of substances against a concentration gradient, which needs energy from respiration.
A root hair cell takes in mineral ions from soil where their concentration is low.

3Identifying the process

Identifying the process check whether the movement is down or against the concentration gradient and whether energy is needed.

Key terms

Term	Meaning
Diffusion	The net movement of particles from high to low concentration.
Concentration gradient	The difference in concentration between two regions.
Partially permeable membrane	A membrane that lets some substances pass but not others.
Passive process	A process that does not need energy from the cell.
Surface area	The total area of a surface.
Equilibrium	An even spread of particles.