Biology Unit 3: Exchange of materials

- Substances are sometimes absorbed against a concentration gradient by active transport.

- Active transport uses energy from respiration.

- With active transport, cells can absorb ions from very dilute solutions and move molecules through partially permeable membranes.

- There are some situations where this process is particularly important. Such as:

         - The mineral ions in the soil are usually found in very dilute solutions. These            solutions are more dilute than the solution within the plant cells, therefore                they cannot pass through the partially permeable membrane.  By using                    active transport, plants can absorb these mineral ions, even though it is                  against a concentration gradient.

         - Glucose is always moved out of the gut and kidney tubules into your blood,            even when it is against a large concentration gradient.

- The breathing system takes air into and out of the body.

- The lungs are in the upper part of the body (thorax), protected by the ribcage and separated from the lower part of the body (abdomen) by the diaphragm.

- Oxygen from the air diffuses into the bloodstream and carbon dioxide diffuses out. 

- The alveoli of the lungs provide a very large, moist surface area with a rich blood supply and thin walls to make diffusion as effective as possible.

- The rich blood supply maintains a concentration gradient for diffusion.

- The villi in the small intestine provide a large surface area with an extensive network of capillaries. This makes them well adapted to absorb the products of digestion by diffusion and active transport.

- In material exchanges, the surface area : volume ratio is always important - a big surface area is vital for successful diffusion.

- Glucose and other dissolved food molesules are moved from the small intestine into the bloodstream by active transport, against the concentration gradient. This makes sure that non of the digested food is wasted and lost in the faeces. 

- The excellent blood supply means that dissolved substances can pass through into the bloodstream and passed around the body as quickly as possible. So, therefore a steep concentration gradient is maintained for diffusion.

- Whatever the organism, gas and solute exchange depends on a large surface area, moist surfaces, short diffusion distances and a large concentration gradient. 

- For instance, fish are provided with oxygen by the oxygen particles in the water that passes through the gills. The surface area of the respiratory surface is increased by the layers of gills.

- Another example is an insect. Their respiratory surface is a tracheole. This is provided with oxygen from the oxygen particles in water from the air. The surface area of the tracheoles is increased by them being highly branched.

- Plants have stomata which allow them to obtain carbon dioxide from the atmosphere.

- Carbon dioxide enters the leaf by diffusion. Leaves have a flat thin shape and internal air spaces to increase the surface area available for diffusion. 

- Most of the water and mineral ions needed by a plant are absorbed by the root hair cells which increase the surface area of the roots. 

- The loss of water vapour from the surface of plant leaves is called transpiration.

- Water is lost through the stomata which are opened and closed by guard cells to let in carbon dioxide for photosynthesis.

- Water is pulled up through the xylem from the roots to replace the water lost from the leaves in the transpiration stream.

- Transpiration is more rapid in hot, dry, windy or light conditions.

Transpiration Stream - The movement of water from roots to leaves as a result of the loss of water from leaves by evaporation.