Paper 1 - 3.2.3 transport across cell membranes (req prac 3) Flashcards
why is a cell membrane described to have a ‘fluid mosaic’ structure?
the bilayer is fluid because the phospholipids are constantly moving and proteins are scattered through the bilayer, like a mosaic.
what do receptor proteins do?
allow the cell to detect chemicals released from other cells
which proteins allow large molecules like ions through the bilayer?
channel and carrier
can proteins in the bilayer move?
some can and some are fixed
what are glycoproteins?
polysaccharide (carbohydrate) chain attached to a protein
what are glycolipids?
polysaccharide chain attached to lipids
what is the structure and function of the bilayer?
phospholipids with a hydrophilic head facing outwards and a hydrophobic tail facing inwards. the centre of the bilayer is hydrophobic so water soluble substances can’t pass through it.
what is the role of cholesterol and where are they found?
cholesterol helps to maintain the shape of animal cells (no cell wall) cholesterol molecules fit between phospholipids causing them to pack more tightly which restricts the movement of of the phospholipids and makes the membrane less fluid and more rigid.
what conditions affect cell membrane permeability?
temperature and solvent concentrations
what happens to the permeability of the membrane at temperatures below 0 degrees?
increases because channel and carrier proteins deform.
what happens to the permeability of the membrane between 0 and 45 degrees?
increases - the membrane is partially permeable so with higher temps the phospholipids have more energy so they move around more and aren’t as tightly packed together.
what happens to the permeability of the membrane at temperatures above 45 degrees?
increases because the phospholipid bilayer starts to melt, and the channel and carrier proteins deform due to excess water inside the cell putting pressure on them.
describe simple diffusion
net movement of molecules from an area of high concentration to an area of lower concentration down the concentration gradient. it is a passive process so no ATP is needed.
what 3 properties does a molecule need to have to pass through the phospholipid bilayer by simple diffusion?
lipid soluble, small and non-polar
describe facilitated diffusion
carrier/channel proteins move particles down a concentration gradient through the phospholipid bilayer. it is a passive process - no ATP
when are carrier proteins used?
active transport/large molecules
when are channel proteins used?
moving large polar molecules across the membrane
how do carrier proteins work?
large molecule attaches to carrier protein in the membrane then the protein changes shape then releases the molecule on the opposite side of the membrane
how do channel proteins work?
proteins form pores in the membrane for charged particles to diffuse down the concentration gradient
what factors affects the rate of simple diffusion?
size of the concentration gradient, the thickness of the exchange surface, the surface area
what is the function of microvilli?
they give the cell a larger surface area so more particles can be exchanged in the same amount of time - increasing the rate of diffusion.
what is the limiting factor in facilitated diffusion?
the number of channel/carrier proteins - once all the proteins are in use the diffusion rate cannot increase
what are aquaporins and why are there lots of them found in the kidneys?
they are special channel proteins that allow the facilitated diffusion of water. some kidney cells are adapted to have lots of them to allow the body to reabsorb a lot of the water that would otherwise be excreted
describe osmosis
diffusion of water across a partially permeable membrane from an area of higher water potential to an area of lower water potential down the concentration gradient.
what are the factors that affect the rate of osmosis?
the water potential gradient, thickness of the exchange surface and the surface area of the exchange surface
steps of required practical 3 - using serial dilutions to find the water potential of potato cells
1) use cork borer to cut identical sized chips (1cm diameter)
2) divide chips into 3 groups. measure the mass of the group using a mass balance
3) place one group in each sucrose solution (0.5m, o.25m, 0.125m)
4) leave chips in the solution for at least 20 minutes
5) remove chips, pat dry with paper towel
6) weigh each group again and record results
7) calculate the % change in mass for each group
8) use the results to make a calibration curve, showing % change in mass (y) against sucrose concentration (x)
required practical 3 - at what point on the graph is the water potential of the sucrose solution the same as the water potential of the potato cells
where the curve crosses the x-axis (percentage change is 0%)
describe active transport
active transport uses energy (ATP) and carrier proteins to move molecules and ions across the cell membrane from a low to a high concentration against the concentration gradient.
what are the differences between active transport and facilitated diffusion?
active transport moves solutes from an area of low concentration to an area of high concentration whereas facilitated diffusion moves solutes from an area of high concentration to an area of low concentration.
active transport requires energy however facilitated diffusion is a passive process.
what are co-transporters?
they are a type of carrier protein
how do co-transporters work in co-transport
they bind to 2 molecules at a time
the concentration gradient of of one of the molecules is used to move the other molecule against its own concentration gradient
why do we use percentage change and not change in grams for practical?
fairly compare because starting masses will be slightly different
what serial dilution technique do you use when creating the solutions starting with initial sucrose concentration of 2m ? (req prac)
1) line up test tubes
2) add 10cm3 of initial 2m sucrose solution to the first tube and 5cm3 of distilled water to the other 4 test tubes
3) then using a pipette, draw 5cm3 of the solution from the first test tube. add it to the distilled water in the second test tube and mix the solution thoroughly. you now have 10cm3 of solution that’s half as concentrated as the solution in the first test tube. (1M)
4) repeat process 3 more times
