How is the structure of a gill related to its function

Gills enhance diffusion between aquatic creatures' blood and the surrounding water. They have a large surface area to increase the area available for gas exchange and a lot of capillaries to increase the bloodflow through the gills. The cells in the capillaries are flattened, which further increases their surface area and the rate of gas exchange.

Gills takes in diffusion between water creatures blood and the water around..A larger surface area is available for gas exchange. the cells are flattened which increases their surface area and the rate of gas exchange. their blood goes in the opposite direction of the h20 over the gills 

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.