Summary AQA A level Year 1/AS Level Biology - Organisms Exchange Substances with Their Environment Notes

AQA A level Biology - topic 2 Organisms
Exchange Substances with Their Environment
This content is for Year 1/AS Level
All subtopics are underlined
This topic is called Organisms Exchange
Substances with Their Environment


Organisms Exchange Substances with Their
Environment


Exam tips for each subtopic are all the way at the
bottom
Surface Area to Volume Ratio


Overview


All living organisms need to exchange substances with their environment
to survive. These substances include:

 Oxygen (O₂) – for aerobic respiration
 Carbon dioxide (CO₂) – waste product of respiration
 Nutrients – glucose, amino acids, minerals
 Water – for hydration and reactions
 Excretory products – urea, salts



Key idea: The rate of exchange depends on surface area, volume,
diffusion distance, and concentration gradients.

,Principles of Exchange
1. Diffusion – movement of molecules from high → low concentration.
2. Osmosis – diffusion of water across a partially permeable
membrane.
3. Active transport – movement against a concentration gradient using
energy (ATP).
4. Bulk transport – movement of large molecules by
endocytosis/exocytosis.




Factors Affecting Rate of Exchange
1. Surface area to volume ratio (SA:V)
o Smaller cells → high SA:V → efficient exchange
o Larger cells → low SA:V → need adaptations (folds, specialized
surfaces)
2. Diffusion distance
o Short diffusion distance → faster exchange
o Long diffusion distance → slower exchange
3. Concentration gradient
o Steeper gradient → faster diffusion
o Maintained by blood flow in animals or water flow in
plants/fish
4. Temperature
o Higher temperature → molecules move faster → faster
diffusion




Exchange in Different Organisms


Organism Method Adaptations
Single- Diffusion across Large SA:V; thin membrane
celled cell membrane
Small Diffusion across Thin, flattened body; moist surface
multicellula body surface
r
Large Specialized Lungs, gills, root hairs, villi, leaves
multicellula exchange
r surfaces
Plants Diffusion and Stomata for gases; root hairs for

, active transport water/minerals; large SA:V; thin
membranes
Animals Circulatory Alveoli in lungs; villi in small intestine;
system + gill lamellae in fish; thin walls, large
exchange SA, good blood supply
surfaces



Adaptations of Efficient Exchange Surfaces
 Large surface area → more molecules diffuse at once
 Thin barrier → short diffusion distance
 Good blood/air/water supply → maintains steep concentration
gradient
 Moist surfaces → allows gases to dissolve before diffusion
 Specialized structures → folding, villi, alveoli, lamellae



Summary

 All organisms need to exchange substances to survive.
 Small organisms: direct diffusion across the surface is enough.
 Large organisms: require specialized exchange surfaces and
sometimes transport systems.
 Efficiency depends on: SA:V ratio, diffusion distance, concentration
gradients, temperature, and movement of fluids.




Surface Area to Volume Ratio (SA:V)


Definition

 SA:V measures how much surface area is available for exchange per
unit of volume of the organism or cell.



SA:V = surface area / volume

 As size increases, volume increases faster than surface area → SA:V
decreases.