Adaptations for gas exchange

revision cards on adaptations for gas exchange for WJEC AS biology

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respiratory surface properties in order to achieve

  • Large surface area compared to the volume of the organism. 
  • Be thin so diffusion path is short.
  • Be permeable to allow respiratory gases through.
  • Be moist to allow a medium in which gases can dissolve.
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  • Single- celled organisms.
  • Live in water.
  • Unicellular organism.
  • Simple diffusion of gases.
  • cell membrane is the gas exchange surface.
  • Diffusion of gases occurs over the whole body surface.
  • have a large surface area:volume ratio.
  • cell membrane is: 'THIN'- diffusion pathway is short and 'MOIST'- gases can dissolve.
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Simple multi- cellular organisms that live in wate

  • Example: flatworms
  • Get sufficient oxygen for their needs by having a flattened shape.
  • Increases the surface area:volume ratio
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  • The random net movement of particles from a region of its higher concentration to a region of its lower concentration until evenly distributed.

Four factors which affect the rate of diffusion:

  • Thickness of membrane 
  • Concentration gradient
  • Surface Area
  • Temperature
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Gas exchange in simple terrestrial organisms: The

Oxygen makes up about 20% of the aIr (plentiful supply of oxygen for terrestral organisms)

  • Trouble is: surfaces that allow gaseous exchange by diffusion are also permeable to water- terrestrial organisms need to conserve water.

The Earthworm lives on land and keeps its thin skin moist by secreting mucus from the epidermis.

  • Provides little protection from desiccation so earthworms tend to stay in moist conditions.  
  • Earthworms have no special organs for gas exchange hence gas exchange takes place by diffusion across the epidermis covering the whole body. 

This is possible because it has:

  • A tubular, elongated shape. (large SA:V ratio)
  • moist skin surface
  • Well adapted capillary network in the skin close to the surface
  • Blood containing haemoglobin to transport more oxygen.
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Earthworms continued...

Lack of activity means it is difficult to maintain the concentration gradient for the diffusion of gases.

Earthworms have a blood vascular system containing the pigment haemoglobin in solution. They also have blood vessels with multiple hearts.

Pumping activity of the major blood vessels circulates blood and dissolved gases round the body and maintains steep diffusion gradients.

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Larger and more advanced multicellular organisms

Examples: Insects, fish, reptiles, birds and mammals.

  • Have a greater demand for energy (higher metabolic rate)
  • Have a smaller surface area:volume ratio.
  • Need a system to transport gases to and from the gas exchange surface.
  • Have to evolve systems and organs to increase the available surface for gas exchange.


  • In aquatic insects and fish the gas exchange surface takes form of the gills.
  • Terrestrial animal groups such as birds, reptiles and mammals have developed lungs.

All of these different mechanisms need a means of ventilation to supply the the respiratory surfaces with a fresh supply of oxygen and to maintain concentration gradients. 

These animal groups have also developed:

  • An internal transport system- provided by a blood circulation system to move gases between the respiring cells and respiratory surface.
  • A respiratory pigment in the blood- to increase its oxygen carrying capacity. 
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Gas exchange in fish

  • Aquatic organisms have a problem with gaseous exchange because water contains far less oxygen then air- also rate of diffusion in water is slower.

Bony Fish

  • large and active. Their needs are supplied by a specialized area. the gills, with a large surface area extended by the gill filaments. 
  • Water is a dense medium with a low oxygen content, therefore to increase efficiency, water needs to be forced over the gill filaments by pressure differences.
  • The gills have extensive network of blood capillaries to allow efficient diffusion and haemoglobin for oxygen carriage. 
  • Compared with parallel flow, counter current flow increases efficiency because the diffusion gradient between the adjacent flows is maintained on the whole. 
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Bony Fish

  • Covered in scales which are impermeable to water so no gas exchange takes place over their external body surface. Instead they have internal gas exchange surface called gills.

The gills of fish have the following features to maximise the rate of diffusion:

  • A large SA, extended by gill filaments and gill plates.
  • A short diffusion path as there in only a thin membrane separating the blood in the gill plates from the surrounding water.
  • A rich supply of blood vessels to transport gases to and from the surface.
  • A ventilation mechanism, aided by counter current, to maintain a concentration gradient between the water flowing over the gills and the blood flowing through.

The surface area of each gill filament is further increased by having many gill plates or lamellae.

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Gill structure

Gill Structure:

  • Gill rakers- traps grit.
  • Gill arch- contains main artery and veins.
  • Capillaries- where gas exchange occurs.
  • Gill filaments- increases surface area and contains smaller filaments called gill plates or lamellae. 

Blood and water flow in opposite direction- counter current flow. 

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Adaptations for vertebrate groups to gaseous excha

  • Problem for all terrestrial organisms is that water evaporates from the body surface resulting in dehydration.
  • Gas exchange surfaces need to be thin and have permeable surfaces with a large surface area. 
  • These features conflict with the need to conserve water.
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The amphibians include frogs, toads and newts.

  • Frogs are typical of amphibians in that they live in moist habitats as they require water for fertilisation. 
  • The larvae (tadpoles) also live in water and have gills.
  • The transition from larva to land living adults involves great changes on the body form. Otherwise known as metamorphosis. 
  • The inactive adult uses the moist skin as its respiratory surface and this provides sufficient oxygen for its needs. 
  • When active, when mating, the frogs use lungs as their respiratory surface.
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The reptiles include: crocodiles, lizards and snakes. 

  • Reptiles can move on all four limbs without the trunk of its body touching the ground. 
  • Pairs of ribs project from the backbone.
  • Ribs provide support and protection to the organs in the body cavity.
  • Ribs are also involved in the ventilation of the lungs.
  • The lung also has a more complex internal structure than that of an amphibian, with the in growth of tissues increasing the surface area for gas exchange.
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  • Have an internal structure, similar to that of mammals.
  • Large volumes of oxygen is however needed to provide the energy for flight. 
  • Ventilation of the lungs is brought about by the movement of the ribs. 
  • During flight, the action of the flight muscles ventilates the lungs.
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  • To reduce water loss and hence prevent desiccation, terrestrial organisms need to have water proof coverings over their body surfaces.
  • Insects have evolved a rigid exoskeleton which is covered by a cuticle.
  • They have a relative small surface area: volume ratio and so cannot use their body structure to exchange gases by diffusion.
  • Gas exchange in insects occurs through paired holes called spiracles, running along the side of the body.
  • The spiracles lead into a system of branched chitin lined air tubed called tracheae.
  • The spiracles can open and close like valves so gaseous exchange can occur and water loss is reduced.
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Mechanism of the opening an closing of the stomata

  • Stomata are small pores found on the lower surface of the leaf.
  • Each pore is bounded by 2 guard cells.
  • Guard cells have chloroplasts and unevenly thickened walls.
  • The stomata allows exchange of gases between the atmosphere and internal tissue of the leaves.
  • Water, however, also evaporates from the stomata. This is known as transpiration.
  • Presence of a waxy cuticle on the upper surface of the leaves reduces water loss significantly.

Stomata opening in the day:

  • Chloroplasts in the guard cells photosynthesis, producing ATP.
  • Using ATP, K+ ion pumps in the cell membranes of the surrounding epidermal cells actively transporting K+ ions into the guard cells.
  • Stored starch in converted into malate.
  • The water potential in the guard cells is lowered and water enters by osmosis.
  • Guard cells become turgid, and curve apart. Because the outer walls are thinner than the inner walls, the pore widens.

The reverse process occurs a night and the stomata closes. (to prevent water loss.)

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