organisms exchange substances with their environment
oxygen and carbon dioxide are transferred between cells and the environment during gas exchange
in humans, urea diffuses from liver cells into the blood plasma for removal from the body by the kidneys
the bigger an object is, the smaller its surface area to volume ratio is
in single-celled organisms, gases and dissolved substances can diffuse directly into or out of the cell across the cell membrane. this is because they have a large surface area to volume ratio
multicellular organisms have a smaller surface area compared to their volume. this means they need some sort of exchange surface for efficient diffusion
exchange surfaces are adapted to maximise effectiveness: they have a thin membrane to allow for rapid exchange of substances ; they have a large surface area ; exchange surfaces in animals have many blood vessels to get substance in and out of the blood quickly and gas exchange surfaces in animals are often ventilated
gas exchange happens in the lungs
the job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it
the lungs contain millions of small air sace called alveoli where gas exchange takes place
the alveoli are specialised to maximise the diffusion of oxygen and carbon dioxide. they have: a big surface area ; moist lining ; very thin walls and a good blood supply
inside the small intestine, there are millions and millions of small projections called villi
villi increase the surface area of the small intestine so that digested food is absorbed faster
villi have a single layer of surface cells and a very good blood supply to assist quick absorption
carbon dioxide diffuses into the air space within the leaf, diffusing into cells where photosynthesis happens. the leaf's structure is adapted so that this can happen easily.
oxygen and water vapour can diffuse out through the stomata
the size of the stomata in leaves is controlled by guard cells
the flattened shape of a leaf increases the area of the exchange surface so that it is more effective
the walls of the cells inside the leaf form another exchange surface.
gills are the gas exchange surface in fish
water enters a fish through its mouth and passes out through the gills. as this happens, oxgen diffuses from the water into the blood in the gills
each gill is made out many thin plates called gill filaments
gill filaments are covered in many small structures called lamellae, which increase the surface area even more
lamellae have many blood capillaries to speed up diffusion
gills also have a thin layer of cells to minimise the distance that the gases have to diffuse
blood flows through the lamellae in one direction and water flows in the opposite direction. this maintains a large concentration gradient between the water and the blood.