Exchange Surfaces

• It is lined with lots of of finger-like projections called villi which increase the surface area of the small intestine walls and therefore provide quick absorption.
• Each villus has a single layer of surface cells and a vast network of capillaries which means molecules from the small intestine have a short diffusion distance into the blood and the concentration gradient is constantly maintained.

The lungs are responsible for taking in oxygen from the air so it can be carried by the blood to cells that require it for aerobic respiration. They also remove carbon dioxide from the blood and send it back out into the air.

They have lots of tiny air sacs called alveoli which allow for efficient gas exchange.

• They provide a massive surface area for the exchange of gases.
• They have a moist lining for dissolving gases.
• They have very thin walls so a short diffusion distance.
• They have a good blood supply to maintain the concentration gradient.

The water flows in through the fishes mouth and over the gills where oxygen diffuses into the blood from the water and carbon dioxide does the reverse. 

The structure of the gills allows for efficient exchange:

• Each gill has many thin gill filaments which in turn are covered in lamellae which provides a massive surface area.
• Lamellae have excellent blood supply (capillary network) and a thin layer of surface cells to provide a short diffusion distance.
• Blood flows in the opposite direction to the water so a large concentration gradient is maintained and maximum oxygen is absorbed from the water.

The underside of the leaf is an exchange surface between the air and the air spaces inside the cell. It is covered in tiny holes called stomata, which allow for carbon dioxide to enter and oxygen and water vapor to be expelled from the leaf, down concentration gradients. Stomata are controlled by guard cells that can close the stomata when the plant is losing more water than it is receiving, to prevent wilting. 

The cell walls of the cells inside the leaf are the second exchange surface. Carbon dioxide diffuses into the cells for photosynthesis and oxygen and water vapor diffuse out. The air spaces that surround the leaf cells increase the area in which this gas exchange can take place.

Plant roots have specialised cells called root hair cells that are adapted for efficient absorption of water and mineral ions from the soil.

• The root hair structures provide a large surface area.
• The vacuole full of concentrated cell sap creates a steep water potential gradient.
• They contain lots of mitochondria for the active transport of mineral ions.