Unit 3 – Chapter 1: Surface Area to Volume Ratio and
Gas Exchange
Surface Area to Volume Ratio (SA:V)
As organisms increase in size, their SA:V decreases
Smaller organisms = larger SA:V → exchange substances more easily via diffusion
Larger organisms = smaller SA:V → need specialised exchange surfaces and
transport systems
Why Organisms Need Exchange Systems
To take in substances like oxygen and glucose for respiration
To remove waste products like carbon dioxide and urea
To maintain a stable internal environment (homeostasis)
Features of Efficient Exchange Surfaces
Feature Function/Benefit
Large surface area Increases rate of diffusion
Thin surface Short diffusion distance
Good blood supply Maintains concentration gradient (in animals)
Ventilation Maintains diffusion gradient (in gas exchange)
Fick’s Law of Diffusion
Rate of diffusion ∝ Surface area × concentration difference
thickness of exchange surface
Chapter 2: Gas Exchange in Insects, Fish, and Plants
Gas Exchange in Insects
Insects have a tracheal system for gas exchange.
Structure:
, Spiracles: pores on body surface that open/close to reduce water loss
Tracheae: large tubes leading from spiracles into the body
Tracheoles: small branches that reach individual cells
How It Works:
Oxygen diffuses directly into cells down a concentration gradient
Carbon dioxide diffuses out via the same route
Air can also be moved by rhythmic abdominal movements (ventilation)
Adaptations to Reduce Water Loss:
Spiracles can close
Waterproof cuticle
Small surface area to volume ratio
Gas Exchange in Fish
Fish use gills for gas exchange in water (which has less oxygen than air)
Structure of Gills:
Each gill made of gill filaments, covered in lamellae (thin plates)
Capillaries in lamellae provide a large surface area and short diffusion distance
Counter-Current Flow System:
Blood flows through lamellae in the opposite direction to water flow
Maintains a steep oxygen concentration gradient across the whole length of the gill
Gas Exchange in Plants
Plants exchange gases mainly via stomata on leaves
Leaf Adaptations:
Flat and thin → large surface area, short diffusion path
Stomata open to allow gas exchange (mainly on the lower surface)
Guard cells control the opening/closing of stomata to reduce water loss
During Photosynthesis:
CO₂ moves in, O₂ moves out
Diffusion occurs through air spaces in the spongy mesophyll layer
Gas Exchange
Surface Area to Volume Ratio (SA:V)
As organisms increase in size, their SA:V decreases
Smaller organisms = larger SA:V → exchange substances more easily via diffusion
Larger organisms = smaller SA:V → need specialised exchange surfaces and
transport systems
Why Organisms Need Exchange Systems
To take in substances like oxygen and glucose for respiration
To remove waste products like carbon dioxide and urea
To maintain a stable internal environment (homeostasis)
Features of Efficient Exchange Surfaces
Feature Function/Benefit
Large surface area Increases rate of diffusion
Thin surface Short diffusion distance
Good blood supply Maintains concentration gradient (in animals)
Ventilation Maintains diffusion gradient (in gas exchange)
Fick’s Law of Diffusion
Rate of diffusion ∝ Surface area × concentration difference
thickness of exchange surface
Chapter 2: Gas Exchange in Insects, Fish, and Plants
Gas Exchange in Insects
Insects have a tracheal system for gas exchange.
Structure:
, Spiracles: pores on body surface that open/close to reduce water loss
Tracheae: large tubes leading from spiracles into the body
Tracheoles: small branches that reach individual cells
How It Works:
Oxygen diffuses directly into cells down a concentration gradient
Carbon dioxide diffuses out via the same route
Air can also be moved by rhythmic abdominal movements (ventilation)
Adaptations to Reduce Water Loss:
Spiracles can close
Waterproof cuticle
Small surface area to volume ratio
Gas Exchange in Fish
Fish use gills for gas exchange in water (which has less oxygen than air)
Structure of Gills:
Each gill made of gill filaments, covered in lamellae (thin plates)
Capillaries in lamellae provide a large surface area and short diffusion distance
Counter-Current Flow System:
Blood flows through lamellae in the opposite direction to water flow
Maintains a steep oxygen concentration gradient across the whole length of the gill
Gas Exchange in Plants
Plants exchange gases mainly via stomata on leaves
Leaf Adaptations:
Flat and thin → large surface area, short diffusion path
Stomata open to allow gas exchange (mainly on the lower surface)
Guard cells control the opening/closing of stomata to reduce water loss
During Photosynthesis:
CO₂ moves in, O₂ moves out
Diffusion occurs through air spaces in the spongy mesophyll layer
