exchanging substances

• cells can use diffusion to take in a substance they need to get rid of waste products

  in humans, urea diffuses from cells into blood plasma for removal from the body via the kidneys
• how easy a organism can exchange substances depends on the oraganisms surface area to volume ratio ( SA:V )
• multicellular organisms need exchange surfaces to allow substances to move in and out of the cell
• in single-celled organisms, gases and dissolved substances can diffuse in and out of the cell across the cell membrane due to the large surface area compared to the volume of the cell
• the larger the organism, the smaller the SA is compared to the volume of the organism
• multicellular organisms have smaller SA compared to volume - SA:V ratio, not enough substance can diffuse from their outside to supply volume. so they need some sort to exchange surface for efficient diffusion . the exchage surface area structure have to allow enough necessary substance to pass through
• exchange surface are adapted to maximise effectiveness by: having a large surface area, having a thin membrane, having a large diffusion gradient
• thin membrane = less distance for molecules to travel
• large diffusion gradient = concentration difference between inside and outside of the organism
• gas exchange in lungs- alveoli are surrounded by capillaries which are connected to the bloodstream and are specialised to allow for gas exchange by large SA, very thin walls, a good blood supply
• oxygen enters lungs via air sacs (alveoli) into capillary beds where it dissolves in plasma and then binds with haemoglobin in red blood cells
• blood carries oxygen around body
• carbon dioxide leaves body via air sacs (alveoli)
• heart has four chambers: right atrium, left atrium, right ventricle, left ventricle
• the small insestine is covered in millions of villi
• villi increase the surface area of the small intestine for absorption of nutrients quickly into the blood
• villi have a single layer of surface cells, a good blood supply
• small intestine absorbs glucose, amino acids, vitamins, minerals, water
• the structure of leaves lets gases diffuse in and out of cells
• carbon dioxide diffuse into the air spaces within the leaf, then it diffuses into the cell where photosynthesis happens
• 1. underneath the leaf is a exchange surface that covered in little holes called the stomata which CO2 diffuses through
• 2) oxygen (produced by photosynthesis) and water vapor also diffuse through the stomata
• 3) the size of the stomata is controlled by the guard cells. they close the stomata if the plant is losing water faster then being replaced by roots, without the the plant would wilt
• 4) the flat shape of the leaf increase the area of the exchange surface so its more effective
• Gills have a large surface area for gas exchange in fish
• water (containing oxygen) enters the fish through the mouth and passes out through the gills. as this happens, oxygen diffuses from water to blood in the gills and CO2 diffuses from blood into water
• each gill is made from thin plates called gill filaments which give a big surface area for exchanging gases
• gill filaments which are covered in gill lamellae (which increase SA even more) and have lots of blood capillaries to speed up diffusion they also have a thin surface layer to minimise distance travelled