BIOLOGY · Stage 8 · Chapter 5: Respiration and the Respiratory System
Biology 5.1 Diffusion in biology — Particle Movement Across Membranes — Alhussan International Academy - Biology (Stages 7-9) 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
Section 1 of 6What is diffusion
- Matter
- it is made of moving particles too small to see.
- 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.
- Passive process
- diffusion needs no energy from the cell.
- Net movement
- particles move in both directions, so the word "net" must be used, and they do not stop moving at equilibrium.
Section 2 of 6Diffusion in everyday life
- 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
- digested food moves into the blood by diffusion down a concentration gradient, so it reaches the cells.
- 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
- blood from an injured animal diffuses through the water, and sharks swim up the concentration gradient, smelling it at very low concentrations.
Section 3 of 6Diffusion across cell membranes
- 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
- 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
- it diffuses the opposite way, from respiring cells to the blood and then to the alveoli.
- Leaves
- carbon dioxide diffuses in through the stomata for photosynthesis, and oxygen diffuses out.
- Digested food
- glucose diffuses from the small intestine into the blood.
- Link
- diffusion is important for gas exchange in the respiratory system and for transport in the blood.
Section 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.
Section 5 of 6Investigating diffusion
- 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
- a crystal placed in water at different temperatures shows how far the purple colour spreads in a given time.
- Planning
- state the variable changed, the variable measured and the variables controlled, and repeat to get a mean.
Section 6 of 6Diffusion compared with other processes
- Osmosis
- the diffusion of water across a partially permeable membrane.
- 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.
- Identifying the process
- check whether the movement is down or against the concentration gradient and whether energy is needed.
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