1. How dissolved substances cross body surfaces

2. Body surfaceDissolved substances Small intestine, via villi, to blood. Alveoli to blood and vice versa. Kidney membranes from blood to urine. Simple sugars/glucose, amino acids, glycerol, water, salts, vitamins. Oxygen into blood and carbon dioxide out. Urea and other waste products. When passing into or out of our bodies, dissolved substances have to cross the surfaces of organs.. To enter or leave cells, dissolved substances have to cross the cell membranes. The table below indicates some body surfaces that dissolved substances pass.

3. Can you list the surfaces through which oxygen has to diffuse to pass from air in the lungs to the sites of aerobic respiration? One of the commonest methods of getting across body surfaces is by diffusion. Diffusion is the spreading of the molecules of a gas, or the molecules of a substance in solution, resulting in a net movement from a region where they are at a higher concentration to a region where they are at a lower concentration. The greater the difference in concentration, the faster the rate of diffusion. Oxygen required for respiration passes through cell membranes and through gas exchange surfaces by diffusion. Other substances such as sugars and ions can also diffuse across cell membranes. Alveolar wall, capillary wall, red blood cell membrane (in and out), capillary wall, cell membranes in tissues, mitochondria membranes. Body defences against diseases

4. How dissolved substances cross body surfaces Oxygen molecules diffuse down the concentration gradient. More molecules move from high to low concentration than move from low to high concentration. e.g. oxygen in alveolus Blood capillary has lower oxygen concentration Diffusion High concentrationLow concentration The diagram below illustrates diffusion. When might diffusion stop? When the concentrations have equalised. This would happen at death, when the blood flow and breathing would stop.

5. How dissolved substances cross body surfaces The roots and root hairs of plants give a huge surface area for the absorption of water and salts from the soil. The leaves and cells inside leaves give a huge surface area for exchange of carbon dioxide and oxygen in photosynthesis and respiration. In humans, in order to exchange substances efficiently, the surface area of the lungs is increased by the alveoli and the surface area of the small intestine is increased by the villi.

6. Root ends: Continued….. How Dissolved substances cross the body surfaces Many roots and root hairs give a huge surface area for absorption of water and salts from the soil solution.

7. Strawberry plant Thin leaves give a large surface area for light absorption. Highly branched root system gives a large surface area for anchorage and root hair attachment. Continued ….. How dissolved substances cross body surfaces

8. Vertical section of leaf: Huge surface area of cells in leaf for exchange of carbon dioxide and oxygen. Air spaces contain oxygen for respiration and carbon dioxide for photosynthesis. How dissolved substances cross body surfaces

9. The gills of fish, also of many aquatic invertebrates, provide a large surface area with a copious blood supply, for efficient exchange of dissolved oxygen and carbon dioxide between water and blood. Fish gills (The gill cover has been cut away to expose the gills). Water in Gill filaments provide a large surface area and have a very good blood supply Supporting gill bar Water out

10. How dissolved substances cross body surfaces The tracheal system of insects branches to form small tracheoles which go to every single cell of the insect body, thus giving a huge surface area for the uptake of oxygen. Part of tracheal system of insect: Tracheae branching throughout body Trachea (air tube) – one of many One of several spiracles (breathing holes) in insect surface Air in Be prepared to explain how gas/solute exchange surfaces are adapted for maximum effectiveness, no matter which organisms the examiners give appropriate information about. Tracheoles – individual air tubes to each cell of body