transport across cell membranes

• the cell membrane is made up of proteins and phospholipids
• selectively permeable means that some substances can cross the cell membrane in or out of the cells, whilst others cant
• since the cell membrane is selectively permeable it can create a in balance of the concentration of substances on either side of the cell membrane creating a concentration gradient.
• Substances can move from a HIGH concentration to a LOW concentration; this movement is DOWN a concentration gradient
• Substances can move from an area of LOW concentration to a HIGH concentration, this is AGAINST the concentration gradient
• If there is no difference between the substance on one side compared to the other, then a concentration gradient will NOT exist
• passive transport involves the movement of molecules from HIGH concentration to a LOW concentration DOWN a concentration gradient. This process does NOT require energy.
• an example of passive transport is diffusion
• oxygen, glucose and amino acids move into a cell using diffusion
• carbon dioxide and waste leaves the cell through diffusion
• Diffusion is important as it allows living cells to gain oxygen and glucose but also allows carbon dioxide and waste to leave the cell.

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• Osmosis is the movement of water from an area of high water concentration to an area of low water concentration down a water concentration gradient across a selectively permeable membrane.
• • lower water solution in plat cell = plasmolysed
• • equal water solution in plants = stays the same
• plant cell with a higher water solution = turgid
• higher water solution (animal cell) = cell bursts
• red blood cell with equal water conc = stays the same
• red blood cell with lower water conc = shrinks
• Active transport is the movement of molecules from an area of LOW concentration to an area of HIGH concentration against the concentration gradient. This process requires energy.