Exchange and transport


Surface area to volume ratio

The higher the SA:Vol ratio is the more efficient diffusion is.

Ficks Law equation:

Rate of diffusion= area of diffusion x difference in concentration / thickness of surface over which diffusion takes place

Diffusion is faster when

  • Surfaces have a large area and are thin
  • The higher the difference in concentration

Small single-celled organisms and very small multicellular organisms can obtain all they need and get rid of CO2 by diffusion due to the short diffusion pathway, high SA:Vol ratio, metabolic demands are low

Large organisms have a low SA:Vol ratio. Diffusion can not supply and remove wastes due to the large distances. They have therefore evolved specialised gas exchange systems which have a large SA:Vol ratio.

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Gas exchange in mammals

An efficient gas exchange system must have a large SA, thin diffusion pathway and maintained steep concentration gradient.

The main organ of mammalian gas exchange is the lungs. Air is drawn into the lungs in the process of breathing.

In the lungs the gas exchange occurs in millions of small sacs called alveoli.

Alveoli are ideal for gas exchange because:

  • Large surface area
  • Made up of single layer of flattened epithelial cells to provide a short diffusion pathway
  • Surrounded by capillaries containing deoxygenated blood so steep oxygen concentration gradient
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Gas exchange in insects

Insects are invertebrates with relatively high oxygen demands

Insect bodies have a branching system of tubes running through them (Tracheae and tracheoles). Tracheae are lined with spirals of chitin which keeps the structure but is also impermeable to gases.

Smaller tracheoles have no chitin so are freely permeable to gases.Tracheae open to the outside by pores called spiracles which are found along the thorax and abdomen.

The tracheal system:

Oxygen diffuses into the tracheae through the spiracles and then into tracheoles to eventually end up in cells of organs. CO2 leaves in the opposite way.

In large, very active insects the abdomen can be pumped in and out to draw in more air. This is called mechanical ventilation.

The insect gas exchange system therefore provides a large SA, with its network of tiny tracheoles, which have very thin surfaces.

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Gas exchange in fish


  • Pressure in mouth decreases as operculum moves out
  • Waters enters through the mouth to equalise the pressure
  • Pressure in mouth is increased and volume decreased by raising floor of moth
  • The increased pressure causes the operculum to open and water leaves and passes between the gill filaments where gas exchange occurs

Gas exchange

The gills have filaments with lots of little branches called lamellae which have a rich blood supply.

-This means there is a very large surface area for gas exchange and a short diffusion pathway for gases.

There is a steep concentration gradient maintained by the countercurrent flow system where blood flows in the opposite direction to water. This means that the blood always meets the water, across the lamella, that is more saturated with O2 than it. 

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Gas exchange in plants

Gas exchange in the leaf:

Leaves provide large surface area exposed to the external environment.

The irregular shape and spacing of the spongy mesophyll layer also increases surface area for gas exchange. They have thin walls so gas exchange can occur quickly.

Gases move in and out of the leaf by diffusion through the stomata.


Plants need to allow evaporation of water from leaves to maintain water uptake from soil which requires the stomata to be open.

They also need to conserve water for when its in short supply so the stomata close.

Guard cells around the stomata control the process of opening and closing 

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