IB biology: topic 1: membrane transport

how do particles move across the membrane?

by simple diffusion

what is diffusion? (definition)

the spreading out of particles in liquids and gases.

what causes diffusion?

particles being in continuous random motion.

what is diffusion? (process)

the net movement of particles move from an area of higher concentration
to an area of lower concentration, down the concentration gradient

what type of process is diffusion? what does this mean?

passive process - no energy required

how do particles move through membranes?

simple diffusion across membranes, where particles pass between the phospholipids.

what must a membrane be in order to allow diffusion to occur?

permeable to the particles.

what type of particles diffuse through the membrane easiest? example?

non-polar particles, oxygen

what type of particles diffuse through the membrane hardest? why?

ions - the centre of the membrane is hydrophobic.

at what rates can polar molecules diffuse at?

low rates

what polar molecules diffuse through membranes easier? example?

small polar molecules, ethanol

how do ions and other particles pass in and out of cells?

facilitated diffusion - diffusing through channels for them in the plasma membrane.

what are the walls of channels in membranes made of?

protein

how can ions diffuse in/out of cells?

through channels for them in the plasma membrane.

how can channels in the membrane control what enters and exits cells in facilitated diffusion?

diameter and chemical properties of channels ensure that only one type of particle can pass through (eg Na+ or K+ but not both).

how can cells control what enters and exits cells in facilitated diffusion?

by controlling which types of channel are synthesised and placed in membrane

describe the stages of facilitated diffusion of K+ ions in axons

the pore is 3nm wide at its narrowest, potassium ions are slightly smaller than 3nm.
when K+ ions are dissolved they become bonded to a shell of water molecules and then are too large.
the bonds between the K+ and water are broken and bonds temporarily form between the ion and a series of amino acids in the pore.
after is has passed through the pore, it can be associated with water again.

why can only K+ ions go through facilitated diffusion in axons?

other molecules are either too large or are too small to form bonds with amino acids in the narrowest part of the pore, and so cannot shed water molecules

what is osmosis?

the net movement of water molecules, from an area of higher to lower water potential, through a partially permeable membrane, down the concentration gradient.

what causes osmosis?

differences in solute concentrations

how do solutes dissolve?

by forming intermolecular bonds with
water molecules - these bonds restrict the movement of water molecules

what type of process is osmosis?

passive process - no energy required

why can osmosis take place in cells?

although they are hydrophilic, water molecules are small enough to pass through the bilayer.

how are H+ ions prevented from passing through aquaporins?

positive charges in the aquaporin repel H+ ions

what are water channels known as?

aquaporins

what do aquaporins do to membranes?

increase membrane permeability to water

what cells have a greater amount of aquaporins ?

root cells and nephrons in kidneys

what happens when an animal cell is placed in a more hypertonic solution?

crenation

what happens when an animal cell is placed in a more hypotonic solution?

lysis

what happens when a plant cell is placed in a more hypertonic solution?

plasmolysis

what happens when a plant cell is placed in a more hypotonic solution?

turgid

what is an example of an isotonic solution?

normal saline - isotonic sodium chloride solution at 0.9% concentration

what is the typical osmolarity of isotonic solutions?

300 mOsm (milliosmoles)

what are examples of medical uses of saline?

basis for eyedrops,
rinse wounds and skin abrasions,
frozen to slush consistency for packing hearts, kidneys and lungs for transplantation.

what is active transport?

the movement of molecules from an area of lower concentration to an area of higher concentration, against the concentration gradient, requiring energy.

what in membranes carries out active transport?

globular proteins known as pump proteins

what takes place in pump protein during active transport?

the molecule or ion enters the pump and reaches as far as the central chamber.
a conformational change to the protein takes place using energy from ATP.
the molecule or ion can now pass through to the other side of the membrane.
the pump returns to its original conformation.

describe the process of active transport of sodium and potassium ions in axons?

the interior of the pump is open to the inside of the axon and 3 Na+ ions can enter the pump and bind to the active site,
ATP transfers a phosphate group form itself to the pump, causing the pump to change shape and close,
the interior of the pump opens to the outside of the axon and the 3 Na+ ions are released,
two K+ ions from outside can then enter and attach to the binding sites,
the binding of the K+ ions triggers the release of the phosphate group, causing the pump to change and close off,
the interior is once again exposed and the 2 K+ ions are released.

what is endocytosis?

the taking in of matter by a living cell by invagination of its membrane to form a vacuole.

what is a vesicle?

a small sac of membrane containing a droplet of fluid.

describe the process of endocytosis?

a small region is pulled from a membrane and pinched off.

this is carried out by proteins in the membrane using energy from ATP.

vesicles formed from the cell surface membrane move substances into the cell from the outside.

what do the vesicles in endocytosis typically contain?

water and solutes and larger molecules (can’t pass across the surface membrane.)

how is endocytosis involved in a growing cell?

phospholipids are synthesised next to RER and inserted

into its membrane.

ribosomes on RER synthesise membrane proteins which

are also inserted into its membrane

vesicles bud off RER, move to surface membrane and

fuse with it.

what is exocytosis?

process by which the contents of a cell vacuole are released to outside the cell

describe the process of exocytosis?

vesicles are used to release materials from cells by fusing with the surface membrane.