Exchange Surfaces and Breathing
Explain, in terms of surface area:volume ratio, why multicellular organisms need specialised exchange surfaces and single-celled organisms do not;
Organisms need to absorb certain substances, (e.g. oxygen, glucose, proteins, fats, water and minerals) from the surrounding environment and remove waste products (carbon dioxide, oxygen and other wastes).
Single celled organisms have a large surface-area-to-volume ratio so they can exchange the necessary gases, nutrients and wastes.
Multicellular organisms not only need more supplies as they have more cells, but they also have a smaller surface-area-to-volume ratio, meaning that the outer surface is not large enough to enable gases and nutrients to enter the body fast enough to keep all of the cells alive. Nutrients and gases also have to travel a larger distance to the centre of the organism. So, larger organisms need a large area to exchange more substances, so often they combine this with a transport system to move substances around the body.
Describe the features of an efficient exchange surface, with reference to diffusion of oxygen and carbon dioxide across an alveolus;
- Large surface area to provide more space for molecules to pass through
- Thin barrier to reduce the diffusion distance
- Fresh supply of molecules on one side to maintain the diffusion gradient
- Carbon dioxide is brought in the blood to the lungs. The concentration is higher in the blood than in the alveoli, so it diffuses across.
- Breathing fills the lungs with air, so there is more oxygen in the alveolus than in the blood
- Removal of required molecules on the other side to maintain the steep diffusion gradient
- Blood carries oxygen away from the lungs
- Breathing removes Carbon Dioxide from the lungs
Describe the features of the mammalian lung that adapt it to efficient gaseous exchange;
- Many, many alveoli meaning that the total surface area is about 70m2.
- Alveolus wall is one cell thick
- Capillary wall is one cell thick
- Both walls consist of squamous cells
- Capillaries in close contact with the alveolus wall
- Narrow capillaries
- Red blood cells are closer to the capillary wall
- Closer to air in the alveoli
- Reducing the rate at which the red blood cells flow past in the blood
- Total barrier is only two flattened cells, or 1μm thick
Describe the distribution of cartilage, ciliated epithelium, goblet cells, smooth muscle…