The principles of exchange and transport

diffusion and surface area Understand the relationship between an organism’s size and its surface area to volume ratio activity diffusion and surface area Froggy page 52

and it affects the rate of exchange of materials at exchange surfaces surface area is the total number of cells in direct contact with the surrounding environment and it affects the rate of exchange of materials at exchange surfaces

and it influences the demand for metabolites. volume is the total three-dimensional space occupied by metabolically active tissues and it influences the demand for metabolites.

surface area influences of metabolites to tissues the rate of supply of metabolites to tissues As an organism’s size increases its surface area increases less than its volume as many cells are not in direct contact with the surrounding environment.

have a small surface area to volume ratio large animals have a small surface area to volume ratio small animals have a large surface area to volume ratio

exchange surfaces are adpated by passive and active methods to maximise exchange by passive and active methods This includes having methods of increasing surface area thin separating surface concentration gradients

Look at the following examples and discuss how each is adapted for exchange.

Root hair cell Leaf Alveoli Erythrocytes Capillaries

leaf mesophyll root hairs capillaries erythrocytes alveoli Explain how the following exchange surfaces are adapted to maximise exchange. Include: Function Adaptation Diagram leaf mesophyll root hairs capillaries erythrocytes alveoli

Main site of photosynthesis Mesophyll Main site of photosynthesis Palisade mesophyll tall & thin arranged end on less cell wall for light to cross large volume to contain chloroplasts Spongy mesophyll Lots air spaces Allow diffusion CO2 to palisade cells As cell PS they use CO2, maintaining a high concentration gradient for CO2 uptake

Root hair cell Uptake water by osmosis Uptake mineral ions by active transport Long thin cytoplasmic extension creates large surface area to volume ratio

Capillaries Site of gas and nutrient exchange with all cells in body Walls one cell thick, reduces diffusion distance Cells flattened (squamous / pavement epithelium)

Erythrocytes Biconcave shape maximises SA:Vol for exchange oxygen No nucleus to maximise volume to carry haemoglobin

Alveoli Site of gas exchange in lungs Walls one cell thick, reduces diffusion distance Squamous epithelial cells Breathing movements maintain concentration gradients Carbon dioxide conc always higher in blood Oxygen conc always higher in alveolus

The principle of mass flow ANIMALS Animals need a constant supply of oxygen and nutrients. And they need to get rid of waste products such as carbon dioxide. Simple animals, such as sea anemones, flatworms and nematodes can do this by diffusion across their moist body surfaces, because they have a really large SA:VOL ratio.

But in larger animals diffusion is too slow to supply all the body cells efficiently. They need a transport system to carry oxygen, nutrient, carbon dioxide, waste products and hormones to and from the special exchange surfaces. Our respiratory system carries oxygen rich air to the exchange surfaces in the lungs and removes carbon dioxide laden air from the body. Our own circulatory system transports large volumes of fluid to all parts of our bodies. This is an example of a mass flow system.

PLANTS Carbon dioxide is able to diffuse into the leaves from the atmosphere through the stomata. Water and mineral nutrients have to reach the leaves from the roots and photosynthetic products, such as sugars and amino acids, need to be transported away from the leaves to maintain concentration gradients. In simple organisms, such as algae, these processes take place by diffusion.

But in complex, multicellular plants, a mass flow system is needed. There are two such systems:- Xylem tissue – which transports water and mineral salts up the xylem from the roots to the leaves. Phloem tissue – transports the materials made in photosynthesis to all other parts of the plants.