Diffusion & Exchange surfaces

• • Multiceluar organisms have a smaller surface area compared to their volume - not enough substances can diffuse from their outside surface to supply their entire volume.
  • this means they need some sort of exchange surface for efficient diffusion.
• How are exchange surfaces adapted?

  Thin membrane - so substances have a short distance to diffuse.

  Large Surface Area - so lots of substances can diffuse at once.

  Lots of blood vessels - (exchange surfaces in animals) to get stuff into and out of the blood quickly.

  Gas exchange surfaces ( in animals eg alveoli) are often ventilated too - air moves in and out
• Examples of exchange surfaces include lungs, gills, skin, roots
• What is diffusion


  Diffusion is the spreading out of the particles of any substance in

  solution, or particles of a gas, resulting in a net movement from an area

  of higher concentration to an area of lower concentration.
• Some of the substances transported in and out of cells by diffusion are

  oxygen and carbon dioxide in gas exchange, and of the waste product

  urea from cells into the blood plasma for excretion in the kidney.
• A single-celled organism has a relatively large surface area to volume

  ratio. This allows sufficient transport of molecules into and out of the cell

  to meet the needs of the organism.
• Why does diffusion occur in gases and solutions?

  Because the particles move freely and randomly
• How does the concentration affect the diffusion rate?

  The bigger the concentration gradient (the difference in concentration) the faster the diffusion rate.
• How does the temperature affect diffusion rate?

  A higher temperature will give a faster diffusion rate because the particles have more energy to move around faster.
• How does the surface area affect the diffusion rate?

  The larger the surface area of the membrane, the faster the diffusion rate because more particles can pass through at once.
• How are the alveoli adapted to maximise the diffusion of O2 and CO2?

  An enormous surface area, moist lining for dissolving gases, very thin walls, and a good blood supply.
• The inside of the small intestine is covered in millions of these tiny little projections called villi. They increase the surface area in a big way so that digested food is absorbed much more quickly into the blood.
• Adaptations of villi


  A single layer of surface cells and a very good blood supply to assist quick absorption.
• Oxygen diffuses from the water into the blood in the gills and carbon dioxide diffuses from the blood into the water.
• Adaptation of Gills in Fish

  Each gill is made up of lots of thin plates called gill filaments. which have a big surface area for exchange of gases. The gill filaments are also covered in lots of tiny structures called lamellae, they increase surface area more.
• Adaptations of gill in fish

  They contain lamallae which have lots of blood capillaries that speed up diffusion.

  They also have a thin surface layer of cells to minimise the distance that the gases have to diffuse.
• Adaptations of gills in fishes

  Blood flows through the lamellae in one direction.

  And water flows through in the opposite direction. This maintains a large concentration gradient between the water and the blood.
• Adaptation of gills in fish

  The concentration of oxygen in the water is always higher than that in the blood. so as much as oxygen as possible diffuses from the water into the blood.