Biology (UP) - Gas Exchange, SA and Volume


Working out SA, Volume and the Ratio

Surface Area:

  • Find area of one face
  • Multiply that by the number of sides of the shape
  • In cm2


  • Multiply height, length and width
  • In cm3


  • SA:V -> must be SA first
  • Try to simplify it as much as possible
  • e.g. 96cm2:64cm3 = 3:2 (some may say 1.5:1 although this isn't technically allowed)
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Why is the SA:V ratio vital for gas exchange?

  • The SA:V ratio decreases as the size of the organism increases
  • This makes gas diffusion less efficient for larger, multicellular organisms
  • Unicellular organisms tend to use their membranes. This is because the smaller the organism the bigger the SA:V ratio so diffusion via membrane is most efficient
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Gas Exchange requirements

  • Needed for respiration, which produces ATP/energy
  • Some organisms have specialised structures e.g. gils, lungs, stomata. Others just use membranes


  • Large SA:V ratio e.g. alveoli have a round shape to increase SA
  • Short diffusion pathway e.g. in capilliaries
  • Moist surface - moisture aids diffusion because gas dissolves into moisture for easier and faster diffusion
  • Maintain concentration gradient (high to low for diffusion). If it reaches equilibrium then gases stop moving - in lungs the blood takes oxygen away so this is maintained
  • Diffusion is passive. No energy is needed.
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What is gas exchange needed for and what the diffe

Needed for:

  • Respiration and photosynthesis. 

Types of plants:

  • Monocotyledonous = plants like grass, wheat, barley etc. They are long, with veins going upwards
  • Dicotyledonous = plants with leaves like you find on average trees, with one long vein and then many branching off.
  • Weedkillers work by focusing on either monocots or dicots
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What adaptations are there for gas exchange in pla

  • The pallisade mesophyll is the main photosynthetic tissue. The cells here are vertical, rather than horizontal, so more can be packed in for maximum light absorption
  • The lower epidermis has large amounts of stomata and guard cells. They open and close to allow gas diffusion in and out of the leave. 
  • The spongy mesophyll has loosely packed cells so that there are gaps between them, and the surface of these cells is moist. The air spaces are important so that gases coming in from the stomata can move through the leaf to the pallisade mesophyll for photosynthesis. They also diffuse into the cells here. 
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What is Fick's Law

The rate of diffusion 

Is proportional to

Surface area X difference in concentration

Divided by

Length of diffusion path/membrane thickness

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Gas Exchange in insects overview

  • Single celled organisms are small, with a large SA:V ratio. Gases can therefore be exchanged by diffusion across a permeable membrane, and if they have a cell wall, this is permeable too
  • Insects are mostly terrestrial, and face a constant battle to make sure they don't dry out.
  • Water evaporates easily and could dehydrate them easily, which would kill them. This means water conservation is key.
  • However, for efficient gas exchange a thin permeable membrane is needed a large SA. But to prevent water loss, they have a coating and a small SA (see water loss cards).
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How does gas exchange in insects work?

  • Insects have developed an internal network of tubes called the trachea and tracheoles. This brings air directly to respiring tissues
  • Trachea are the tubes running around the outside of the interior of the insect. Gases leave and enter the trachea by pores called spiracles, which can be opened and closed by a valve. When open, water can evaporate. They are mostly kept closed but most do so periodically for G.E. Air is pushed around the trachea by contracting muscles.
  • Tracheoles are branches off the trachea. They don't connect to anything, but provide better coverage for the air to get to all cells in the body. It means that gas doesn't have to diffuse far off the main system. It also prevents diffusion from cell to cell, which would gradually decrease as it went on.
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How does gase move in and out of the trachea syste

  • Along the diffusion gradient
  • Mass transport: muscle contraction in insects squeeze the trachea, enabling mass movement of air in and out. This speeds up gas exchange.
  • Tracheoles are filled with water: During inactive periods, muscle cells surrounding tracheoles respire. They carry out some anaerobic respiration which produces lactate. This lactate is soluble and decreases water potential of the cells. Therefore, water moves into the cells from the tracheoles via osmosis. The water in the tracheoles decreases in volume, and therefore draws air into them. This means the final diffusion pathway is in gas, not liquid. Therefore, it is more rapid. It increases the rate at which air moved into tracheoles, but leads to greater water evaporation.
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