Adaptations Of Gas Exchange


Surface Area Tp Volume Ratio

  • When a cell increases in size, the diffusion pathway becomes longer. This means that the diffusion from the outer cell surface to the inner cell is longer. 
  • When the cell becomes larger, diffusion won't be able to meet the cells needs such as :
    • Supplying nutrients such as Glucose and Oxygen. 
    • Removing waste such as Urea and Carbon Dioxide
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Gas Exchange

  • Gas Exchange is the process by which Oxygen reaches cells and Carbon Dioxide is removed from them
  • Unicellular organisms have a large surface area to volume ratio.
  • Gas exchange occurs across the whole surface.
  • The permeable membrane allows gases to diffuse.
  • Special organs are not required as diffusion is sufficient to meet the oxygen requirements of the organism.
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Simple Cellular Organisms (Flatworms)

  • Flatworms have adapted a flattened shape to overcome the problem of an increase in size. 
  • This increases their surface area to volume ratio therefore no cell in the body is far from the surface meaning there is no need for specialised gas exchange organs. 
  • They exchange gases directly with the environment via diffusion. 
  • Diffusion across the permeable membrane meets the oxygen requirements of the organism. 
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Simple Multicellular Organisms (Earthworm)

  • Earthworms have developed a tubular shape and is restricted to damp enviroments.
  • They secrete mucus to keep the body surface cells moist. 
  • This allows gases to diffuse and dissolve
  • The elongated shape provides a large surface area to volume ratio compared with organisms of similar volume. 
  • They exchange gases directly with the environment via diffusion across the moist surface
  • Blood vessels are close to the body surface so gases can diffuse in and out of the blood across the body surface. 
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Large Multicellular Organisms

  • As size increases surface area to volume ratio decreases.
  • Diffusion across the body surface is insufficient to provide enough oxygen to survive. 
  • Large organisms are more metabolically active meaning so they require more oxygen.
  • Diffusion pathways are too large so rate of diffusion is very slow. This means that they need a specialised gas exchange surface and a method of circulation. 
  • Many animals have a toughened body surface so they have internal gas exchange surfaces.
  • Blood circulates in the vessels, this maintains a concentration gradient for the diffusion of oxygen into the cell and carbon dioxide out of the cells. 
  • Blood contains the respiratory pigment haemoglobin to carry oxygen to body cells. 
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Specialised Gas Exchange Surfaces

  • Many animals and plants have specialised gas exchange surfaces so tat gas exchnage is rapid and efficient. 
  • Gill lamella in fish, Alveoli in Lungs and Tracheoles are examples. 
  • To macimise the rate of diffusion, these surfaces nust contain 
    • Large SA:V ratio. 
    • Thin so diffusion pathways are short.
    • Partially permeable. 
    • Moist to allow gases to disolve and diffuse. 
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  • Insects fly so they need alot of energy so they need a good supply of oxygen.
  • Air diffuses into the insect through paired holes called spiracles running along each side of the body.
  • The spiracles lead to a system of branched, chitin-lined air tubes called Trachae. 
  • Spiracles can open and close to allow oxygen and carbon dioxide to enter and leave. 
  • When resting, diffusion meets oxygen demands, whereas during flight, abdomen movement ventilates the trachae.
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Insects Gas Exchange Surface

  • The Trachae branch repeatedly until they end as very fine, thin walled, Tracheoles. 
  • Oxygen diffuses from the ends of tracheoles into the cells and Carbon Dioxide out of the cells and into the tracheoles. 
  • Ends of tracheoles are fluid filled and close to the muscle fibres. The fluid is where gas exchange takes place. 
  • This fluid provides a moist surface for oxygen to dissolve and when muscles contract fluid is drawn to the muscle cells. 
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Advantages of Insect Gas Exchange Surfaces

  • Oxygen is supplied directly to the tissues. 
  • No respiratory pigment is required. 
  • Oxygen diffuses faster in the air than blood.
  • Spiracles close in order to reduce water loss. 
  • Chitin is a structural feature which allows the trachae to remain open. 
  • Gas exchange organs are inside the body of a terrestrial organism as it reduces oxygen and water loss. It is protected by ribs and exoskeletons in insects. 
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Gas Exchange in Fish

  • Fish can be caategorised into 2 groups determined by the material that makes up the skeleton and their gill ventilation mechanisms. 
  • There are many problems caused by living in water:
    • Water contains less oxygen than air.
    • Rate of diffusion is slower in water than in air
    • Water is a dense medium so it doesnt flow as freely
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Cartilagenous Fis

  • They have a skeleton made entirely of cartilage. 
  • Nearly all live in Sea water
  • Just behind the head are 5 Gill Clefts which open at Gill Slits. 
  • Water is taken in by the mouth and forced through the Gill Slits when the floor of the mouth is raised. 
  • Gas exchange involnves parralell flow. This means that blood circulates the same direction as water travels over the gills. 
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Bony Fish

  • These types of fish have an internal skeleton made up of bones. 
  • The gills are covered by a flap called the operculum. 
  • Gas exchaneg involves counter current flow. This means that blood in the gill capillaries circulates in the opposite direction as water flowing over the gills. 
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Fish Anatomy

  • The head of a bony fish with the operculum pulled back will reveal gills. There are usually 4 on each side. 
  • The flap covering the goll is called the operculum and these enclose the gill arches. 
  • Water is taken in through the mouth, passes over the gills and is expelled out the operculum. 
  • Movements of the buccal cavity floor and operculum allow a one way current of water to flow through the gills for a gas exchange of Oxygen and Carbon Dioxide.
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Bony Fish Gills Stucture

  • Along each gill arch there are many filaments and on these there are gill lamellae. 
  • The gill filaments have a large surface area for gas exchange
  • Blood circulates through the gill lamellae creating a concentration gradient. Oxygen diffuses through the gill lamellae into the capillaries and carbon dioxide diffuses out into the water. 
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Counter Current Flow

  • Blood always meets oxygen at a higher concentration. 
  • The gradient for diffusion of oxygen into the blood from the water is maintaines over the whole length of the gill lamellae. 
  • Counter current flow is more efficient as it results in a higher oxygen saturation level. 
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Paralell Flow

  • Water is taken into teh mouth and blood is flowing through the gill capillaries in the same direction as the water. 
  • Gas exchange is very effiecent at first as there is a very steep concentration gradient. 
  • However halfway along the gill lamellae, equilibrium is reached and diffusion of Oxygen and Carbon Dioxide is no longer possible.
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Ventilation Mechanism

  • When water enters the fish, the mouth opens and the operculum closes. The buccal cavity lowers and volume inside the buccal cavity increases. However pressure decreases.
  • When water leaves the fish the mouth is closed and the operculum opens. The buccal cavity rises and volume inside the buccal cavity decreases. Therefore pressure increases. 
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  • Vertebrates include five classes: 
    • Amphibians, Reptiles, Birds, Fish and Mammals
  • Life is thought to have evolved in water with animals adapting in order to colonise the land.


  • Tadpoles live in water so they have gills for gas exchange. 
  • In adults, when resting diffusion occurs across the moist skin. When active it happens at the lungs. 
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Ventilation In Human Lungs - Inspiration

  • During Inspiration muscles contract meaning ribs move upwards and out.
  • The outer pleural membrane is pulled outwards, reducing the pressure in the pleural cavity.
  • The inner pleural membrane is pulled outwards and the lung surface is drawn out causing alveoli to fully expand.
  • Pressure in the alveoli is lower than atmospheric pressure so the air moves into the alveoli.
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Ventilation In Human Lungs - Expiration

  • During expiration muscles relax meaning the ribs move downwards and inwards. 
  • The outer pleural membrane returns inwards, increasing the pressure in the pleural cavity. 
  • The inner pleural membrane returns inwards. 
  • The lungs retract so the alveoli deflate. 
  • The alveolar pressure is higher than the atmospheric pressure so the air moves out. 
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Pressure changes in Inspiration

  • During inspiration the diaphragm flattens and the rib cage expands pulling on the outer pleural membrane which lowers pressure in the pleural cavity. 
  • The inner pleural membrane pulls on the lungs which increases the volume of the lungs 
  • This decreases the pressure in the alveoli. 
  • Alveolar pressure is lower than atmospheric pressure so air moves in. 
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Alveolar Gas Exchange

Alveoli are suited for gas exchange because:

  • Alveoli have a large surface area for gas diffusion

  • Moist so that gases can disolve and diffuse easily
  • Permeable so oxygen and carbon dioxide can diffuse
  • One call thick to provide a short diffusion pathway. 
  • Extensive capillary network provides blood circulation with a maintained concentration gradient for CO2 and O2 diffusion. 
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Gas Exchange in Plants - Leaf Adaptations

  • The leaf blade is flat so that the diffusion pathway is short and SA:V is also increased. 
  • The spongy mesophyll is permeated with air spaces to allow diffusion and circulation of gases which maintains concentration gradients. 
  • Mesophyll walls are moist to allow dissolving and diffusion of gases. 
  • Stomata pores allow exchange of oxygen and carbon dioxide. 
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Photosynthesis Leaf Adaptations

  • Large SA to absorb as much light as possible. 
  • Leaves can orientate themselves towards sunlight. 
  • Leavs are thin to allow light to reach lower levels. 
  • Cuticle and epidermis are transparent so light can pass to the mesophyll below. 
  • Palisade cells are long and densely packed and have many chloroplasts. 
  • Chloroplasts rotate and move in the mesophyll to maximise light absorption. 
  • Intercellular air spaces allow CO2 to diffuse into the cells and O2 to diffuse out of the cells. 
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  • The tomata is found at the lower epidermis. 
  • Their role is to allow water and gases to pass through. 
  • The wicth of the pores can change to control gas exchange between atmosphere and internal tissues of the leaf. 
  • There are always 2 guard cells which may contain chloroplasts and have unevenly thickened cell walls. 
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Stomatas Opening and Closing

  • The inner cell wall is thick and outer is thin. When guard cells are rigid the pore opens and when guard cells are flaccid the pore closes. 
  • K+ ions are transported from epidermal cells to guard cells. 
  • Insoluble starch from the guard cells are converted to soluble maltase by an enzyme in the cytoplasm. 
  • Water potential in the guard cells are lowered so water enters via osmosis. 
  • The guard cells become turgid and curve apart because outer cell walls are thinner than inner cell walls.
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