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 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 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
What are the qualities of the phosphate head?
hydrophyllic
polar
contains glycerol and phosphate
What does hydrophyllic mean?
attracted to water
What are the qualities of the fatty acid tails of a phospholipid?
non-polar
hydrophobic
contains long hydrocarbon chains and fatty acids
What determines the allignment of the phospholipids?
the way they react with water
How is a barrier formed with the phospholipids?
the centre of the bilayer is hydrophobic which doesnt allow water soluble substances through it and acts as a barrier to these dissolved substances
What is the role of a channel protein?
facilitated diffusion of ions
What is a uniport?
transports 1 specific ion/ molecule down a conc.gradient (positive diffusion)
What are the two types of Co-transport?
symport and antiport
What is symport?
two different substances transported simultaneously at the same time in the same direction
What is antiport?
transports two different substances in opposite directions at the same time
Why do intrinsic proteins in the membrane have a hydrophyllic lining?
the tails create hydrophobic area which polar (water-soluble) molecules cant pass through
What are the effects temperatures below 0’C on the permeablility of the membrane?
the phospholipids dont have much energy so they cant move very much. They are packed closely together and the membrane is rigid. But channel and carrier proteins deform, increasing the permeability
Ice crystals may form and pierce the membrane making it highly permeable
What are the effects temperatures between 0’C and 45’C on the permeablility of the membrane?
the phospholipids can move around and aren’t packed as tightly together = the membrane is partially permeable
as the temp increases the phospholipids move around more as they have more energy = increases permeablilty
What are the effects temperatures above 45’C on the permeablility of the membrane?
the phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable
water inside the cell expands - puts pressure on the membrane
Channel and carrier proteins start to deform so they cant control what enters/ leaves the cell = increases permeablilty.
What is the relationship between cholesterol and the membranes fluidity?
increase in cholesterol = decrease in membrane fluidity
What does a reduce in fluidity cause?
the membrane can no longer contain the contents of the cell
How does cholesterol make the membrane more rigid?
it binds to the hydrophobic tails which causes them to pack more closely together restricting the movement of the phospholipids
Where does simple diffusion take place in the membrane?
molecules pass through the phospholipid bilayer and the molecules pass between the phospholipids
What are the qualities of substances of simple diffusion in the membrane?
small
non-polar
lipid soluble
eg: Oxyegn, gases, carbon dioxide
gas exchange
How do molecules pass through the membrane by facilitated diffusion?
through a protein
Ions travel through a channel protein
molecules travel through a carrier protein
What are the qualities of molecules that dissuse by facilitated diffusion in the membrane?
polar (ions)
water soluble
large
eg. glucose, amino acids, ions
What is the relationship of the rate and conc.gradient of simple diffusion?
as the conc.grad increases the rate of diffusion increases
What is the relationship of the rate and conc.gradient of facilitated diffusion?
intially, as the conc, grad increases the rate of diffusion increases then over time, the conc,grad increases, the rate of diffusion become constant as the no. and availablility of transport proteins becomes a limiting factor
Why do we start with a temp of 30’C for the beetroot prac?
a beetroots normal temp is 25’C underground
What is the definition of diffusion?
the net movement of molecules/ions down a conc.gradient until an equililbrium is reached. It is a passive process
Why are beetroot cells used to investigate the permeablility of the membrane?
beetroot cells contain a purple dye called anthocyanin which can diffuse across the beetrtoot membrane via the channel proteins
What are 3 variables that need to be controlled in the beetroot prac?
the volume of water in the test tubes
the length of the beetroot
time for diffusion to take place
Why must the beetroot cylinders need to be rinsed fully?
it removes pigment off the side of the beetroot
it removes excess pigment was leaked when it was cut so the results are from diffusion
What is the effect of increasing temp on the permeablility of cell membranes?
increase temperature = increases kinetic energy of the phospholipids and proteins.
phospholipid will move move = membrane becomes more fluid.
Increased KE of the pigment = results in increased diffusion rate
denatures the carrier proteins ans pigment leaks out of the membrane, uncontrollably
What is Active Transport?
the movement of a substance from a low conc. to a high conc using metabollic energy and a carrier protein
What is the process of Active Transport?
- transport through carrier proteins
- molecule binds to a receptor complementary in shape on the protein
- ATP used
- this causes the protein to change shape and release the molecule to the other side
Why is Co-transport used in the ileum?
to absorb glucose from the lumen to the gut there must be a higher conc. of glucose in the lumen compared to the epithelial cell - for facilt.diff
but there is usually more glucose in the epithelial cells = co-transport and active transport used
What is the process of co-transport in the ileum?
- carrier proteins enable sodium ions to be actively transported from the epithelial cell into the blood in the capillary = reduces sodium ion conc. in epithelial cell compared the lumen in the ileum
- sodium ions diffuse from the lumen of the ileum into the epithelial cell - facilt.diff
- the protein the sodium ions diffuse through is a co-transport protein = glucose/amino acids also attach and are transported into the epithelial cell against their conc,grad
- glucose moves by facilit,diff from the epithelial cell into the blood
How is the epithelial cell adapted for co-transport?
microvilli:
increases S.A for co-transport proteins
it is a folded membrane with lots of carrier proteins embedded in the membrane for max apsorbsion of glucose
Why is the conc. of glucose in the blood lower then in the epithelial cells?
the blood flows and carries away absorbed glucose maintaining a conc.grad of glucose between epithelial cell and capillary
What limits Active transport?
anything that limits aerobic respiration = oxygen availability, respiratory inhibitant (cyanide)
the number and availability of carrier proteins
Why does more active transport happen in a higher tempreture?
the ions have more kinetic energy so they are more likely to collide with carrier proteins
How does a respiratory inhibitant decrease the rate of Active transport?
inhibits repiration = reduces the energy produced by aerobic respiration
What does a plato on a graph show for transporting molecules?
an equilibrium has been reached = facilt.diff only
what causes water potential to lower?
adding solutes to water forming a solution
What are free water molecules?
water molecules in a solution that are not surrounding a solute molecule
How do you make solutions from different concentration?
start with a known conc.
scale factor = dividing the conc of the know conc by the conc you want to make
then divide the volume you want by the scale factor = volume of conc needed
volume wanted - volume of conc = volume of water to top up solution
What is the method of the potato prac for osmosis?
- use a cork boere to cut potato dics into identically sized cylinder = 1cm in diameter
- divide potatos in groups of 3 and measure the mass of each group using a mass balance
- place one group into each sucrose solution
- leave potatos in solution for about 20 mins (should all have the same amount of time for osmosis)
- remove and blot potatoes with paper towel= removes excess water
- weigh each group again and record results
- calulate % change in mass for each group
- use results to draw a calibration curve
What are the potato prac findings?
potato chips will gain water (therefore mass) in solutions with a higher water potential than the chips and lose water in solutions witha lower water potential
What does isotonic mean?
the water potential of the external solution is the same as the cell
no net movement of water molecules
no change in size of the cell
What does hypertonic mean?
the water potential of external solution is lower than in the cell
What does hypotonic mean?
the water potential of the external solution is higher than the cell
What happens to a plant cell in a hypetonic solution?
water diffuses out by osmosis = cell shrinks, shrivels and dies
cell becomes plamolysed = the cell membrane pulls away from the cell wall
What happens to a plant cell in a hypotonic solution?
the cytoplasm swells and expands until it bursts
the cell becomes turgid as water enters the cell by osmosis
What happens to animal cells in a hypertonic solution? (rbc)
cytoplasm shrinks = water leaves cell by osmosis
crenation
What happens to a animal cell (rbc) in a hypotonic solution?
water enters the cell by osmosis - cell swells and expands until it bursts
cell lysis = cell burts - no cell wall
What is the diluting from a stock solution equation?
C1 X V1 = C2 X V2
c = concentration
v = volume
1 = original/ stock solution
2 = solution we want to make
What are the precautions of the potato prac?
use the same potato
the time the cylinders were left in the solution should be the same
potato discs need to be blotted before weighed
tempreture of solutions should be the same
potato should be fully semerged = osmosis SA should be the same
make sure there was no skins on the potatoes = has different permeablility to water - would alter results
How would you make the potato prac more accurate?
have smaller intervals of sucrose solution conc
Why will a droplet of sucrose solution move up in another sucrose solution with different concentrations?
the drop is less dense than the solution
so the conc of the drop is less than the solution - it is more dilute
as water moved out of the cell into the droplet solution
so the water potential of the droplet solution is higher than in the cell so the droplet solution is hypertonic to the cell
Why will a droplet of sucrose solution move down in another sucrose solution with different concentrations?
the droplet is more dense than the solution
the concentration of the drop is higher than the conc of the solution - less dilute
water moved into the cell by osmosis
so the water potential of the droplet solution is hypotonic to the cell
What is endocytosis?
the process by which cells take in substances from outside of the cell by engulfing them in a vesicle (endosome)
active process
What happens if endocytosis happens repeatedly?
the cell size decreases because the membrane surface area decreases
What is exocytosis?
the process by which materials are removed from, or transported out of cells
active process
How can the water potential of a cell/tissue be determined?
place into a dilution series of different concentrations, compare the changes in mass,
plot a graph and where the line crosses the x axis is the water potential of the cell/tissue
How is the bilayer formed of the membrane?
the phospholipid heads are hydrophyllic so they attract water around them and the tails are hydrophobic so they repell water
Why would the cell-surface membrane appear as two dark lines under a microscope?
Membrane has phospholipid bilayer
Stain binds to phosphate / glycerol
On inside and outside of membrane
Describe binary fission in bacteria
Replication of circular DNA
Replication of plasmids
Division of cytoplasm to produce 2 daughter cells
What happens when acid is added to a membrane?
the acid denatures the membrane proteins
What happens when you add an organic solvent to a membrane?
phospholipids dissolve
how may cell-surface membranes be adapted for rapid transport?
folded = large surface area
large no. of protein channels for facilitated diffusiom
large no. of protein carriers for active transport
