Edexcel iGCSE Biology-2.16 factors affect the rate of movement- Study Notes- New Syllabus

Factors Affecting Movement of Substances

📝 Introduction

Substances move in/out of cells by diffusion, osmosis, or active transport.
The rate (how fast it happens) depends on several factors:

🔑 Factors

1. Surface Area to Volume Ratio (SA:V)

• Larger surface area → more space for particles to pass through.
• Smaller organisms (e.g. single-celled) have a large SA:V ratio, so exchange is fast.
• Larger organisms have a smaller SA:V ratio, so they need specialised exchange surfaces (lungs, gills, villi).

Example: Amoeba relies on diffusion, but humans need lungs.

2. Distance

• The shorter the distance, the faster the movement.
• Thin membranes = faster diffusion.

Example: Alveoli in lungs have one-cell-thick walls for fast gas exchange.

3. Temperature

• Higher temperature = particles move faster = more collisions = quicker diffusion.
• Low temperature = slower movement.

Example: Oxygen diffuses faster into warm blood than in colder conditions.

4. Concentration Gradient

• Bigger difference in concentration = faster movement.
• Steeper gradient → more collisions across the membrane.
• Once concentrations equalize, movement stops.

Example: In lungs, high O₂ in alveoli + low O₂ in blood → steep gradient → fast diffusion.

📊 Summary Table

⚡ Quick Recap
SA:V ↑ → faster (small organisms fast exchange).
Distance ↓ → faster (thin walls).
Temp ↑ → faster (more kinetic energy).
Gradient ↑ → faster (steeper concentration difference).
👉 Mnemonic: “Some Dogs Take Cookies” (Surface area, Distance, Temperature, Concentration gradient).