Biological membranes Flashcards
Describe the role of cell membranes within cells(5)
- compartmentalises organelles e.g vesicles,lysososmes
- isolates content of organelles e.g lysosomes contain hydrolytic enzymes,would be dangerous if they were released when they’re not needed as cell will be broken down.
- Site of attachment of enzymes,ribosomes & other molecules
- Provide selective permeability. Phospholipid bilayer only let in large,polar,charged molecules and channel and carrier protein are specific to certain molecules ∴ controlling what enters + leaves.
- Creates conc. gradients + specific environments e.g membrane maintains acidic environment inside lysosomes.
Outline the role of membrane bound receptors
- Have a role in cell signalling/communication
- Glycoproteins or glycolipids
- Hormones can bind to receptors which triggers a cascade of reactions in cell
- Drugs- bind to receptors and either TRIGGER a response in cell or BLOCK receptor-prevent it from working.
Outline the role of cell membranes as barriers
cell membranes only let certain substances in and unwanted substances out.It is able to do this due to it’s selective permeability. Selectively permeable due to Phospholipid bilayer + carrier & channel proteins
Describe how channel + carrier proteins work (draw out) and how they make cell membrane selectively permeable
Channel : check google images
form aqueous channels/pores across cell membranes for specific small ions and polar molecules.
Carrier:
- Large molecules binds to carrier protein
- carrier protein changes shape
- molecules released on the other side.
Describe the role of cell membranes as sites of cell communication+signalling
involved with releases signal molecules by exocytosis as well as receiving these signal molecules by receptors on cell membrane.
cell membranes have glycoproteins/lipids which act as receptors with specific complementary shapes.
Only a messenger molecule with complementary shapes can bind to.
Attachment of signal molecules triggers a change in cell.
List components of cell surface membranes.
P phospholipds E xtrinsic protein G lycoprotein I ntrinsic protein G lycolipids C holestrol C arrier protein C hannel proteins
Roles of components of fluid mosaic model
Phospholipids
forms a barrier because phospholipid head hydrophilic and tails are hydrophobic. Ions + Polar molecules can’t come through(water soluble).Allows fat-soluble substances e.g vitamins.
Cholesterol
Gives membrane stability.Fit between phospholipids and binds to tails making them fit closely together and rigid
Proteins
controls what enters/leaves channel + carrier proteins.Acts as receptors, cell signalling.
Glycolipids/proteins
Stabilises membrane by forming HB bonds with surrounding water molecules
Describe role of cell membranes as sites of chemical reactions
Proteins in membranes forming organelles, or present within organelles have to be in particular positions for chemical reactions to take place.
State factors affecting membrane permeability
Draw graph for each factor
- solvents
- Temp
cgp pg.127
Explain how Temp affect permeability
Above 45C- membrane breaks down,becomes permeable.h2o inside cell expands,puttibg pressure on membrane.Channel + carrier proteins denature cant control what enters and leaves cell
0-45C- phospholipids can move around and aren’t packed tightly.Membrane partially permeable.↑ temp,↑ phospholipid movement-↑ permeability
0C- phospholipid little kinetic energy,packed closely,rigid.Channel+ carrier protein denature,increasing permeability .Ice crystals form piercing membrane,when thawed even higher permeability
Explain how solvents affect permeability
Some solvents e.g ethanol increases permeability
Organic solvents will dissolve membrane
Write a method to investigate the effects of temp on beetroot membrane permeability.
- use scalpel to cut 5 equal-sized pieces of beetroot on a cutting board.Rinse/wipe each piece to remove any pigment released when cutting.
- put 5 pieces into 5 different test tubes, each containing 5cm3 of h2o.Use pipette/measuring cylinder to measure h2o.
- Put each test tube into a h2o bath with diff. temps for the same amount of time.
- After the water bath remove the beetroot,water changes from colourless to pink.
- colorimetry.Turn it on and stabilise for 5mins . Put filter on blue.
- Put cuvette containing distilled water into colorimeter and calibrate to 0.
- Use a pipette to transfer solutions in each test tube into diff. cuvettes.Put cuvette in colorimeter one by one and record absorption
- Analyse results.The higher the absorbance the more pigment released ∴ the higher the permeability
Identify the control,independant,dependant variables
control:
- size of beetroot pieces
- beetroot the cube pieces are coming from
- volume of water
- time beetroot stayed in waterbaths
- colorimeter used.
Independant:
Temp
Dependant:
Concentration of pigment released
How can experiment be changed to see effects of organic solvents on membrane permeability
Same procedure but instead of putting beetroot pieces into water, you will have to put to create a serial dilution of ethanol(organic solvent) using water
State the appropriate units for temp,vol,conc,absorbance
temp: C degrees
vol : cm3/dm3
conc: moldm-3 / gdm-3
absorbance: Au
State limitations of procedure and ways to improve these limitations
Difficulty maintaining temp,so SET WATERBATHS FOR HIGHER TEMPS
difficulty in getting accurate beetroot pieces so USE A SCALPEL AND RULER TO CUT EACH PIECE,THEN USE THE ONES WITH MOST SIMILAR SIZES
Describe how experiment could be made more reliable and accurate
reliable
- use more samples of beetroot
- maintain temp thermostatically(controlled water baths)
Accurate:
-use more intermediate temps within existing range
Define diffusion
net movement from an area of high concentrate to an area of low concentration down a concentration gradient
Define facilitated diffusion and why does it take place
: net movement of particles from an area of high concentration to an area of low concentration down a concentration gradient across a partially permeable membrane via carrier/channel proteins
∵ phospholipid bilayer prevents polar, charged,large molecules from diffusing easily,so diffusion is made easier by carrier + channel proteins
state factors affecting diffusion
large S.A
conc. gradient
thickness of exchange surface
temp.
Explain how these factors affect diffusion
the greater the S.A,the higher the number of particles that can diffuse across the cell membrane in a given amount of time
the steeper the conc. gradient the faster the rate of diffusion
the shorter the distance the faster the rate of diffusion
Higher temp- particles have more kinetic energy and so they move more and collide at a faster rate ∴ faster rate of diffusion
low temp- particles will have less kinetic energy and so they move and collide at slower rate ∴ slow rate of diffusion
Write a method to investigate factors affecting diffusion rates on model cells
- make up agar jelly with phenolphthalein and dilute NaOH.This will make the jelly pink
- Preparr 5 test tubes containing HCl in increasing concs.
- Using scalpel, cut out 5 equal-sized cubes from agar jelly.
- Put one of the cubes into the 1st test tube and use a stopwatch to time how long it takes for cube to turn colourless.
- do the same for rest test tubes of HCl us9ng new cube each time
How can experiment be changed to measure affect of temp + S.A on diffusion
- Prepare agar jelly same way
- cut into diff. sized cubes and work out S.A: Vol
- Time how long it takes for cues to get colourless when placed in same conc. of HCl.
- Prepare agar jelly same way
- prepare test tubes of same conc. of HCl and put boiling tubes in water baths of diff. temps
- When HCl in each tube reaches desired temp. place cubes in.
- Time how long it takes for cues to get colourless
Define osmosis
Diffusion of water molecules across a partially permeable membrane from an area of high water potential to an area of low water potential
