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Transport in Cells (diffusion (across cell membranes (only very small…
Transport in Cells
diffusion
diffusion - the spreading out of particles from an area of higher concentration to an area of lower concentration
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a higher temperature will also give a faster diffusion rate because the particles have more energy so move around faster
across cell membranes
only very small molecules can diffuse through cell membranes like oxygen, glucose, water and amino acids
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particles flow through the cell membrane from where there is a higher concentration to where there is a lower concentration
they are moving around randomly so they go both ways but there are more particles on one side of the membrane
some of the substances transported in and out of the cells by diffusion are oxygen and carbon dioxide in gas exchange, and the waste product of urea from cells into the blood plasma for excretion in the kidney
a single-celled organism has a relatively large surface area to volume ratio to allow sufficient transport of molecules into and out of the cell, so the larger the surface area of the membrane the faster the diffusion rate as more particles can pass through at once
exchanging substances
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leaves in plants
carbon dioxide diffuses into the air spaces within leaves and then it diffuses into cells where photosynthesis happens, to help this the leaf:
- the underside is an exchange surface, covered in stomata so carbon dioxide diffuses through
- the size of the stomata is controlled by guard cells which can close if too much water is being lost
- air spaces in the leaf increase the surface area so more carbon dioxide can get into cells
gills in fish
- each gill is made up of thin plates called gill filaments which give a big surface area for exchange of gases
- the gill filaments are covered in lots of tiny structures called lamellae which increase the surface area even more
- the lamellae have lots of blood capillaries to speed up diffusion
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active transport
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active transport allows nutrients to be taken into the blood / roots despite the fact that the concentration gradient is the wrong way
osmosis - the movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration