Gas Exchange

Single-celled organisms can exchange gases via diffusion across their outer-membrane

They have a large surface area, a thin surface providing a short diffusion pathway and there are no cells far from the exchange surface. This means they don't need an exchange system

However, other organisms such as plants, fish, humans and insect need an exchange system because:

• some cells are deep in the body and the distance is too big for substances to be reached by diffusion
• larger organisms have a smaller SA:V ratio so would not exchange a sufficient amount of nutrients to survive

Fish can't exchange gases across their outer surface because they have waterproof, gas-tight outer covering. Therefore the exchange surface for fish are the lamellae in the gills

Gills made up of gill filaments with lamellae attached at right angles

The lamellae have a large surface area, are very thin and contain lots of capillaries. The lamellae are found on the gill flaments

Fish use the counter-current exchange system - the blood in the capillaries and the water over the gills flow in opposite directions

This maintains a constant diffusion gradient across the whole of the lamellae as water is always in contact with blood with a lower concentration of oxygen

The gas exchange surface in insects is the tracheoles which branch off from the tracheae

The tracheae are air-filled pipes. The tracheoles go inbetween the individual cells and have a very thin wall so diffusion is fast

Air enters the tracheae via the pores in it's surface called spiracles

Oxygen reaches the cells by moving down the concentration gradient through the tracheae

Carbon dioxide leaves the insect by moving down the concentration gradient out of the spiracles

Muscle contraction moves air through the tracheae

The insects balance gas exchange with water loss. To reduce water loss they have:

• Small SA:V ratio 
• Waterproof waxy coverings

The gas exchange surface for plants are the mesophyll cells in the leaves

Whilst CO2 is used for photosynthesis in day time, O2 is needed for respiration day and night.

The oxygen produced from photosynthesis can be used for respiration but most diffuses out of plant

Plants don't need a transport system for gases as have a very large SA:V ratio

Gas enters the leaves via the stomata, which are stimulated by light to open. The opening and closing of the stomata is controlled by the guard cells

The guard cells open when there is a low water potential in the vacuole in the guard cell and so fillw with water and swell

The leaves contain lots of air spaces around the mesophyll cells so that gases can diffuse to and from them easily

Plants and insects need to balance the need for gas exchange with the problem of water loss, as whenever a spiracle or stomata opens water is lost at a faster rate

Insects reduce water loss by:

• Closing the spiracles using muscle contraction
• Having a waterproof, waxy cuticle on their outer surface
• Having tiny hairs around their spiracles to trap moist air

Plants reduce water loss by:

• Closing the stomata by having the guard cell lose water and turn flaccid
• Some plants in particularly dry, windy or warm habitats have adapted speciically to reduce water loss, these are called xerophytes. They have adaptations such as:
  • Stomata sunk in pits to trap moist air which reduces the water potential gradient reducing transpiration rate
  • Curling up thier leaves to protect from wind and trap moist/still air to reduce the water potential gradient and reducing exposed surace area
  • Hairy leaves to trap moist air around stomata reducing the water potential gradient
  • Have a reduced SA:V by being round (pine needles) to reduce rate of diffusion