2.3 Membranes Flashcards
What is the cell-surface made of?
lipids and proteins
phospholipid molecules
What do the phosphate heads contain?
glycerol and phosphate
what do the phospholipid tails (fatty acid tails) contain?
a long hydrocarbon chain of fatty acids
What is the role of a cell-surface membrane?
they surround cells and act as a barrier between the cell and its environment controlling which substances enter and leave the cell. They are selectively permeable = choses substances
What is the role of membranes around organelles?
the membranes around organelles divide the cell into different components and act as a barrier between the organelle and the cytoplasm
What is the problem with the cell-surface membran being small?
cannot see all the details of its structure even under an electron microscope
What is the fluid Mosaic model?
describes how the molecules of the different substances that make up the membrane are arranged in a mosaic and not all these molecules stay in place
How are phospholipid molecules arranged in the membrane?
in a bilayer
What are receptor proteins?
allow the cell to detect chemicals released from other cells. The chemicals signal to the cell to respond in some way.
What are glycoproteins?
proteins with a carbohydrate chain
What is a glycolipid?
lipid with a carbohydrate chain
What are the functions of membrane proteins?
may act as enzymes
may act as channels
act as carrier proteins
act as receptors for hormones
act as molecules that are important in cell recognition
What is the role of cholesterol in the membrane?
adds strength and prevents the movement of other molecules in the membrane
helps maintain the shape of animal cells - important for cell that aren’t supported by other cells (red blood cells)
prevents the movement of other molecules in the membrane
prevents bursting
controls membrane fluidity
How does cholesterol make the membrane less fluid?
it fits between the phosopholipids and binds to the hydrophobic tails = causes them to pack more closely together
this retricts the movement of phospholipids making the membrane more rigid = less fluid
Why doesnt the membrane allow water soluble substances through?
the centre of the bilayer is hydrophobic and it acts as a barrier to these dissolved substances
What practical is used to test the permeability of the membrane?
Beetroot cells:
1. 10 test tubes, x5 labelled A, x5 labelled B and intials on all, assign each test tube a temp (30,40,50,60,70)
2. 5cm3 of distilled water in A test tubes
3. place each A test tube in appropriate thermostatically controlled water bath for 5 mins
4. add 3 beetroot discs to test tube A and start timer
5. leave in water bath for 2 mins then put a bung in and invert the test tube x10
6. pour the liquid only into corresponding test tube B for that temp
7. repeat steps 4-6 until you have done all temps
9. take 5 test tubes to a colorimeter
10. staring with lowest temp B test tube, place test tube in colorimeter and take the reading of % absorption = press T on colorimeter
What is diffustion?
the net movement of ions, atoms or molecules that make up a substance from an area of high concentration to an area of low concentration down a concentration gradient
What are the surfaces where diffusion takes place called?
exchange surfaces
What is the rate of diffusion?
the amount diffused through the surface / the time taken
What are the factors affecting diffusion rate?
temperature (increase KE) - higher = faster diff.
surface area (microvilli) - larger = faster diff.
difference in concentration (concentration gradient) - higher = faster diff.
diffusion distance - thinner = faster diff.
lipid solubility - higher = increased diff
membrane fluidity - more fluid = increased diff
What is simple diffusion?
molecules diffusing directly through a cell membrane
What type of molecules do carrier proteins diffuse?
large, water-soluble molecules
Why do charged and water-soluble molecules diffuse slowly through the membrane?
the centre of the bilayer is hydrophobic
How do carrier proteins facilitate diffusion?
differenr carrier proteins facilitate the diffusion of different molecules:
1. molecule attaches to a carrier protein on the carrier’s binding site which is specific to the molecule
2. the protein changes shape when they bind to the molecule
3. this releases the molecules on the opposite side of the membrane
How do channel proteins carry out facilt. diffusion?
forms pores in the membrane for charged particles to diffuse through
1. gates open for ions to pass through according to the needs of the cell = pore formed
2. ions pass through the channel
Why does the conc. gradient affect the rate of diffusion?
the higher the conc.gradient, the faster the rate of diffusion
as diffusion takes place, the difference in conc. decreases until it reaches an equilibrium, diffusion slows down over time
What factors affects facilitated diffusion?
conc.gradient - higher = faster diff.
no. of channel and carrier proteins - more = faster diff.
How do you calculate the rate of diffusion from a graph?
the gradient
How is water potential formed?
water molecules are in constant motion and move randomly
some will hit the membrane = collison creates pressue - water potential
What are the units for water potential?
units for pressure - KPa
What increases water potential?
the more water molecules that are presnt and can move about freely, the greater the water potential
What substance has the highest water potential?
water - 0KPa
all other solutions will have a negative water potential
Why is the water potential higher outside of the cell than inside it?
there is net movement of water molecules from distilled water into the cell
What is osmosis?
the diffusion of water molecules across a selectively permeable membrane from an area of higher water potential to an area of lower water potential
What is water potential?
the liklelihood of water molecules to diffuse out or into a solution/ cell
What are the factors affecting osmosis?
water potential gradient - higher = faster rate
thickness of exchange surface - thinner = faster rate
surface area - larger = faster rate
What proteins are involved in Active Transport?
carrier = Co-transporters
What is the process of Active transport using co-transporters?
- co-transporter binds two molecules at the same time
- the conc.gradient of one of the molecules is used to move the other molecule against its own conc.gradient
What are the factors affecting Active Transport?
the speed of induvidual carrier proteins - faster =faster rate
no. of carrier proteins - more = faster rate
rate of respiration in the cell and availablity of ATP
What makes up the membrane?
Intrinsic proteins - channel and carrier proteins = spans the entire bilayer
extrinsic proteins
glycoprotein
glycolipid
cholesterol
phospholipids
surface proteins
What is the role of the glycoprotein in the membrane?
enables cells to recognise another cell as familiar or foreign
allows WBC to move around the body and initiate immune repsonses and identifys other cells
What is the role of the glycolipids in the membrane?
act as recognition sites and antigens
What is the role of an extrinsic protein in the membrane?
mechanical support, it is connected to carbohydrate chains to make glycoproteins
What is the role of a surface protein in the membrane?
provides effective communication between the cell and its environment
What is a micelle?
the structure that phospholipids on the surface of the water
it forms a monolayer - hydrophyllic head on the outside, hydrophobic tails in the centre and forms a circle
it helps the body absorb lipid and fat soluble vitamins