3.2 Cell Membranes Flashcards
Covers cell membrane structure, Exchange across cell membranes by osmosis, diffusion and active transport.
What is the purpose of a cell membrane? What are the different ways it can be used?
Cell membranes surround cells. Serve as barrier between cell and environment.
Purpose: Control substances that enter and leave the cell - they’re partially permeable - let molecules through some not - substances move across membrane via diffusion, osmosis or active transport
Describe in detail of the structure of the cell membranes. Give details of the components - look at the textbook/revision guide and if you miss anything, add it in in a different colour.
Cell membranes have fluid mosaic structure. Phospholipid form a double layer - fluid because the phospholipids are continuously moving.
- Phospholipid: Hydrophilic heat and hydrophobic tail. Range themselves so heads facing to water and tails are face to face. Centre is hydrophobic so membrane doesn’t allow water-soluble substances (like ions) through it - acts as a barrier to these dissolved substances.
- Cholesterol: - type of lipid. Present in all cell membranes. Cholesterol molecules fit between the phospholipids. Maintain shape of cells that aren’t supported by others e.g. blood cell.
- Protein carrier and channel allow substances through like ions and other molecules.
- Glycolipids & Glycoproteins can act as receptors and binding sites.
Investigating permeability of cell membrane
Look at required practical.
Give details about diffusion.
- Net movement of molecules from a higher concentration to a lower concentration.
- molecules can diffuse both ways, but there’s a net movement.
- Passive process - no energy is needed for it to happen.
- Particles can diffuse across cell membranes as long as they can move freely through the membrane.
Give details of facilitated diffusion. Details about carrier protein and channel protein e.g. how they work, what sorts of substances they diffuse.
Large molecules that can’t diffuse through phospholipid bilayer, charged particles e.g. ions and polar molecules can’t go through therefore require facilitated diffusion - also passive process.
Carrier proteins: Move large molecules across membrane.
1) attaches to protein
2) Protein changes shape
3) Release molecule onto other side.
Channel: Form pores in the membrane allows charged particles to diffuse through. Different channels facilitates diffusion of different charged particles
What factors does rate of diffusion depend on?
Simple diffusion:
- Concentration gradient - higher it is the faster the diffusion
- Thickness of exchange surface - thinner the faster the diffusion
- The surface area - larger the surface area, faster the rate of absorption eg. microvilli.
What does facilitated depend on?
The concentration gradient - the higher the concentration gradient, the faster the rate of facilitated diffusion. Once all carrier and channel proteins are all in use, they can’t increase any more.
What’s osmosis? Give details about it
Osmosis is the diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential.
Water potential is the likelihood that the water molecules will diffuse out or into solution. Pure water has the highest water potential - all solution have a lower water potential than pure water.
What are the factors that can affect the rate of osmosis?
Water potential gradient - the higher the water potential gradient, the faster the rate of osmosis.
Thickness of exchange surface - the thinner the exchange surface the faster the rate of osmosis.
- The surface area of the exchange surface - the larger the surface are the faster the rate of osmosis
How do you do a serial dilutions experiment to investigate water potential?
1) Line up 3 test tubes.
2) Add 10cm3 of inital 2M sucrose solution to first test tube and 5cm3 of distilled water to other 4 test tubes.
3) draw 5cm3 using pipette of first solution and add to distilled water in the second tube and mix thoroughly. Now have 10cm3 of solution that’s half as concentrated as the solution in the first test tube.
4) repeat 3 more times to create 0.5M, 0.25M and 0.125M solutions.
How do you use solutions to find the water potential of potato cells?
- cork borer cut potatoes 1cm
- divide to 3 groups, measure mass
- one group in each solution
- leave for 20 mins
- gently remove and pat dry
- weigh
- calculate % change in mass
- draw calibration curve % change in mass against sucrose conc.
- 0% change in mass is potato water potential.
How does active transport work?
Carrier proteins involved in active transport. Process is pretty similar to facilitated diffusion - molecule attaches to carrier protein, protein changes shape and moves molecule across.
- a.t. usually moves substances from low conc to high conc.
What are the factors affecting the rate of active transport?
1) The speed of individual carrier proteins - the faster they work the faster the rate of active transport.
2) The number of carrier proteins present - the more proteins there are, the faster the rate of active transport.
3) The rate of respiration in the cell and the availability of ATP. If respiration is inhibited, active transport can’t take place.
How is glucose absorbed into the blood using the co-transport mechanism? Give details of the 4 steps involved.
Glucose absorbed into the bloodstream in the small intestine by cotransport protein and mechanism.
1) Sodium ions are active transported out of the ileum epithelial cells, into the blood, by the sodium potassium pump. This creates a concentration gradient - there’s now a higher conc of sodiums in the lumen of the ileum than inside the cell.
2) Causes Na+ ions to diffuse from the lumen into epithelial cell, down conc. gradient - do this via sodium-glucose cotransporter proteins.
3) The co-transporter carries glucose into the cell with the sodium. As a result the concentration of glucose inside the cell increases.
4) Glucose diffuses out of the cell, into the blood down it’s concentration gradient through a protein channel by facilitated diffusion.
