cell physiology

• describe simple diffusion


  non polar molecules and very small molecules can pass through the membrane unaided

  water soluble substances generally unable to pass through this way due to the hydrophobic nature of the phospholipid bilayer
• define diffusion


  the net movement of substances from an area of high concentration to an area of low comcentration
• what factors affect diffusion across membranes

  LARGE

  concentration gradient

  temperature

  surface area of the membrane

  SMALL

  size of the molecule

  thickness of the exchange surface
• describe carrier proteins


  they take in the diffusing molecule, change shape and release the molecule on the other side of the membrane

  they have binding sites that match specific molecules as they assist the movement of these molecules across the membrane
• describe ion channel proteins

  they are formed by proteins with a central pore that enables charged particles to pass through

  some are permanently open but some are gated

  gated channels can open or close, allowing control of ion movement
• the rate of facilitated diffusion is dependent on the number of carrier/channel proteins in the membrane
• simple and facilitated diffusion are passive processes
• describe active transport


  energy transported against the concentration gradient

  involves protein carriers, where the substance ti be transported binds to the protein

  the carrier changes shape and releases the transported substance on the other side of the membrane

  these carriers are specific to a particular type of molecule or ion
• in active transport substances are moved against the concentration gradient and metabolic energy in the form of ATP is required, which is provided by the mitochondria

  cells that carry out a lot of active transport and have a high metabolic rate will have a high number of mitochondria
• when does cytosis take place

  large molecules that are too big for carriers

  bulk transport of smaller molecules
• describe endocytosis


  movement of substances into the cell

  the cell surface membrane invaginates around substances entering the cell from the outside

  to form a membrane bound sac or vesicle

  which then pinches off the inside of the cell surface membrane

  when the vesicles are taken into the cell, the fluid nature of the cell surface membrane allows it to reform and close the gap created by cytosis
• what is phagocytosis


  transport of solid material into the cell
• what is pinocytosis


  transport of fluid into the cell
• describe exocytosis


  secretory vesicles move to and fuse wit the cell surface membrane

  contents of the vesicle are released outside the cell

  the creation of a gap in the cell surface membrane is followed by a reforming of the membrane

  important in secretion of many proteins from cells, including digestive enzymes and many hormones
• define osmosis (high and low)


  the net movement of water from a solution of high water potential to a solution of low water potential across a selectively permeable membrane
• define osmosis (negative)


  the net movement of water through a selectively permeable membrane, from a solution of less negative water potential to a solution of more negative water potential
• what is solute potential


  the potential of a solution to take in water
• what is pressure potential


  the effect of pressure on the solution
• water potential of a cell = solute potential + pressure potential
• hypertonic solution


  stronger solution
• hypotonic solution


  weaker solution
• isotonic means


  the two solutions are equal
• water moves from hypotonic to hypertonic solution by osmosis
• what is water potential


  the tendency to take in water by osmosis from pure water across a selectively permeable membrane
• what is water potential measured in

  kilopascals - kPa
• what is the water potential of pure water

  0kPa - unable to take in any more water by osmosis
• why is the water potential of pure water 0kPa

  all the water molecules are free and not forming associations with other molecules
• in solutions some of the water molecules are not free and form hydration shells around the solutes. the presence of the solutes also reduces the ability of the water molecules to diffuse through the solution
• solution always has a ---- water potential and why


  negative

  always have some water in hydration shells

  more concentrated a solution is, the more negative the water potential is as more water is bound up in hydration shells and not free

  and is therefore increasingly more likely to take in water by osmosis
• what do plant cells rely on for support

  turgor
• herbaceous plants are almost totally reliant on turgor pressure
• plant cell wall has a very important role to play as its strength limits the expansion of the cell membrane and cell wall on eachother helps create the turgor pressure itself
• cell short of water

  flaccid
• lots of cells flaccid in a plant

  plant becomes wilted
• cell loses too much water

  plasmolysed

  vacuole can shrink and cell membrane can pull away from cell wall
• what is incipient plasmolysis


  the point where the cell membrane just begins to lose contact with the cell wall
• where in nature does plasmolysis seldom occur

  plants growing in a field that has been given too much fertiliser

  a seed from a woodland tree being carried to a salt marsh and starting to germinate in this environment
• as animal cells do not have a call wall what can this cause

  there is nothing to stop the cell membrane expanding till it bursts - cell lysis

  take place in hypotonic solutions
• what will take place in an animal cell when it is placed in hypertonic solutions

  crenation - the cell loses too much water by osmosis and begins to shrivel up
• in healthy mammals, blood and tissue fluid are kept at the correct water potential to ensure neither lysis nor crenation occur