exchanging substances

• 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.