Chapter 7 Exchange Surfaces and Breathing

• Organisms need to exchange substances with their enviroment.
• Smaller animals have a higher surface area: volume ratio.
• An organism needs to supply every cell with oxygen and nutrients. It also needs to remove waste products from every cell to avoid damage.
• In single-celled organisms, these substances can diffuse directly into or out of the cell across the cell surface membrane. The diffusion rate is quick because of the small distances the substances have to travel.
• In multicellular organisms, diffusion across the outer membrane is too slow.

  Some cells are deep within the body, and the distance from them to the outside is too great.

  Larger animals have a larger SA:V ration - difficult to exchange enough to supply all cells through a relatively small outer surface.

  Higher metabolic rate means substances are used up faster.
• Root hair cells have a large surface area:

  They grow into long hairs which stick out into soil. This give the roots a large surface area, which helps to increase the rate of absorption of water and mineral ions.
• Alveoli


  The gas exchange surface in the lungs.
• Alveoli are made of a single layer of thin flat, cells called the alveolar epithelium. This decreases the distance over which oxygen and carbon dioxide must diffuse, making it more efficient.
• Alveoli are surrounded by a large capillary network, giving each alveolus a supply of blood to quickly take oxygen away and bring more carbon dioxide.
• The lungs are ventilated, so the air in each alveolus is constantly replaced.
• Good ventilation helps maintain the concentration gradients of oxygen and carbon dioxide.
• The gills are the gas exchange surface in fish.
• In the gills, oxygen and carbon dioxide are exchanged between the fish's blood and the surrounding water.
• Fish gills also contain a large network of capillaries to keep them well supplied with blood.
• Fish gills are well ventilated - the fish keeps swimming to ensure fresh water is constantly passing over them.