Module 2 Section 5 - Biological Membranes Flashcards
Pyper
Give four functions of cell surface membranes (or five if you split one up).
- partially permeable barriers between and within cells & organelles maintaining different conditions (this also allows compartmentalisation where substances are held for chemical reactions) and can control the exchange of substances passing through them
- sites of chemical reactions (e.g. respiration)
- sites of cell communication (cell signalling, messenger molecules bind to receptors which can lead to changes in a cell)
- cell recognition
What are cell membranes made of?
- Lipids (mainly phospholipids)
- Proteins
- Carbohydrates (which are usually attached to the two above)
fluid mosaic model
A model that describes the fluid and flexible nature of the cell membrane and the components it is made from.
Describe the fluid mosaic model. [4]
- Phospholipids form a continuous bilayer
-> this is fluid as they are constantly on the move - Proteins are scattered throughout the bilayer (like a mosaic/plum pudding)
- Some lipids have carbohydrate (sugar) chains bonded to them - glycolipids
- Some proteins have carbohydrate (sugar) chains bonded to them - glycoproteins
- Some cholesterol molecules are also in the bilayer
Why does the fluid mosaic model describe membranes as ‘fluid’?
The phospholipids & proteins can move around
Why does the fluid mosaic model describe membranes as a ‘mosaic’?
The different types of proteins randomly scattered between the phospholipids float within the bilayer (although some may be fixed in position)
OR
The plasma membrane is made of lots of different molecules
Is the phospholipid tail hydrophobic or hydrophilic?
Hydrophobic
Role of phospholipids in the fluid mosaic model of cell membranes
Centre of bilayer is hydrophobic, so it acts as a barrier to stop water soluble substances (e.g. ions/polar substances) from diffusing through
Give two functions of proteins in the plasma membrane.
- Receptors
- Antigens/cell recognition/cell markers
- Carriers
- Channels
- Enzymes
Why can water diffuse through the cell membrane?
It’s polar, however it’s small enough to diffuse through
What can pass through the phospholipid bilayer (only through the phospholipids)?
Fat/lipid soluble substances (but not water soluble ones like ions/polar molecules which diffuse through more slowly).
Role of cholesterol in the fluid mosaic model of cell membranes
They give the membrane stability, as at high temperatures, they bind to the phospholids’ hydrophobic tails, causing them to pack more closely together. This makes the membrane less fluid and more rigid.
Some parts of cholesterol are hydrophobic, so it can create a further barrier to water soluble substances to stop them from moving through the membrane - cholesterol affects the permeability of the membrane.
Describe and explain the diffusion of polar molecules directly across a membrane.
There is still some simple diffusion, however it is much slower than with non-polar molecules. This is because the hydrophobic phospholipid tails repel polar molecules/ions.
What does cholesterol do when it’s cold?
Cholesterol prevents the phospholipids from packing too closely together, making the membrane more fluid and less rigid.
Role of proteins in the fluid mosaic model of cell membranes
- Control the movement of substances into and out of the cell
- Some form protein channels (pores) which allow small charged molecules through
- Some are carrier proteins which help transport bigger molecules and charged particles across the membrane (by active transport and facilitated diffusion).
- Some act as receptors for molecules (e.g. hormones) triggering a chemical reaction for cell signalling
glycolipid vs. glycoprotein
glyco = sweet (sugar)
glycolipid - carbohydrate bonded to a lipid
glycoprotein - carbohydrate bonded to a protein
3 roles of glycolipids & glycoproteins in the fluid mosaic model of cell membranes
- Form hydrogen bonds with surrounding water molecules to stabilise the membrane
- Act as receptors for messenger molecules in cell signalling
- Are antigens (which are involved with self-recognition)
The hormone adrenaline binds to cardiac cells. Describe how adrenaline does this. [2]
It binds to a glycoprotein (1) receptor on the cell surface (1).
Two factors that affect membrane permability
- Solvent
- Temperature
Explain why the phospholipid bilayer can melt.
Above 45°C, the increasing kinetic energy allows the phospholipids to move far away from each other, destroying the membrane’s structural integrity (ability to hold together) causing it to “melt”.
Explain how the type of solvent affects membrane permeability.
- Membrane is more soluble in the solvent
- Lipids dissolve in the cell membrane -> membrane loses its structure
- Increases membrane permeability
Explain how low temperatures affects membrane permeability.
- Phospholipids have little energy -> don’t move around lots -> low permeability
- Channel & carrier proteins denature -> increases permeability as they can no longer control the movement of substances
- Ice can penetrate the membrane, leaving big pores when it melts -> increases permeability
Explain how temperatures between 0 and 45°C affects membrane permeability.
- Phospholipids aren’t too closely packed together and so can move around
- Increasing temp increases kinetic energy -> permeability increases
Explain how high temperatures affects membrane permeability.
- Above 45°C, phospholipid bilayer melts -> membrane becomes more permeable
- Water expands, putting pressure on the membrane
- Channel and carrier proteins denature -> can’t control the transfer of substances -> permeability increases
Graph of alcohol concentration on membrane permeability
https://cdn.discordapp.com/attachments/1293960808828506224/1295071505683582996/image.png?ex=670d50d4&is=670bff54&hm=66ac284f71797a5cfc32b8104882a56e876736e025b575bf9698fe08d5685211&
pg 127 of CGP textbook
Graph of temperature on membrane permeability
https://cdn.discordapp.com/attachments/1293960808828506224/1295071805337239654/image.png?ex=670d511b&is=670bff9b&hm=18e48c8e45d3aae7be247ce1f2705dbf84b708f2d247170e16a29310c6b8c602&
pg 127 of CGP textbook
diffusion
The passive, net movement of molecules from an area of high concentration to an area of low concentration, down the concentration gradient, until equilibrium is reached. No external energy is required.
Simple diffusion
Where particles simply diffuse through the membrane.
Where can diffusion occur?
In solutions or gases
Diffusion is ____ to the concentration gradient.
proportional
osmosis
The net movement of water molecules from a high water potential to a lower water potential, across a partially permeable membrane down the water potential gradient.
water potential (general definition)
The likelihood of water molecules to diffuse into or out of a solution.
water potential (pressure definition)
Pressure exerted by free water molecules on a membrane.
What is water potential measured in?
kPa
What is the water potential of:
a) distilled water?
b) a solution of glucose?
a) 0
b) negative
The more –ve the water potential, the ____ the concentration of the solutes.
stronger
hypotonic solution
A more dilute solution than the cells - has less concentrated solutes
hypertonic solution
A more concentrated solution than the cells - has more concentrated solutes
isotonic solution
The same concentration of solutes as the cell
What happens when a cell is placed in a hypotonic solution?
The solution has a higher water potential than the cell, so the net water movement is into the cell through the partially permeable plasma membrane down the water potential gradient via osmosis.
Animals cells swell and eventually burst, while plant cells’ vacuoles & cytoplasms swell and push against the cell wall. The cell becomes turgid.
What happens when a cell is placed in a hypertonic solution?
The solution has a lower water potential than the cell, so the net water movement is out of the cell through the partially permeable plasma membrane down the water potential gradient via osmosis.
Animals cells shrink, while plant cells become flaccid as the cytoplasm & plasma membrane pull away from the cell wall (plant cellsplasmolyse).
plasmolysis
When a plant cell is placed in a hypertonic solution, the cytoplasm & plasma membrane pull away from the cell wall
Explain why plant cells don’t burst when left in pure water. [2]
They have a strong (1) cell wall (1) which limits the uptake of water (1).
ψ meaning
Greek letter psi
symbol for water potential
Carrot cylinders of equal length are left in a beaker of different concentrations of solution for 18 hours. Explain why they are left for a long time (2). [1]
So the maximum change in length can be measured.
Explain why the potato cylinders are each blotted dry before reweighing (RP). [2]
Water will affect the mass, and the amount of water on each cylinder will differ.
Explain why the potato cylinder skins are removed (RP). [2]
The potato skin is waterproof, but you want to have osmosis occuring throughout the potato pieces.
Explain why the tubes with beetroot in solution are shaken (RP). [4]
- Beetroot pieces may stick together/to the test tube and so affect the rate of diffusion (not related to the concentration of alcohol used)
- Pigment builds up in the solution surrounding the beetroot pieces, decreasing the concentration gradient and so decreasing the rate of diffusion (also not related to the concentration of alcohol used)
When a piece of young carrot is added to a beaker of 0.6 mol/dm3 sucrose solution, there is a decrease in the length of the piece of carrot.
Young carrots store sugars in their tissues, but in older carrots, some of this is converted to starch.
How would using older carrot pieces affect the result obtained for this specific sucrose concentration? GIve a reason for your answer. [2]
Greater water potential gradient (as there’s a lower concentration of sucrose) so this will lead to a greater decrease in length of the carrot piece.
What is the dependent variable for the beetroot practical?
The solutions’ absorbance of blue light
model for how animal cells react to different water potentials of solutions
chickens’ eggs that have had their shells dissolved
How can you investigate how animal cells are affected by water potential?
- Make sodium chloride solutions of different concentrations (e.g. 0.2, 0.4, 0.6, 0.8 and 1.0M)
- Pour an equal volume of each concentration into five different beakers
- Take the deshelled chickens’ eggs and pat dry with a paper towel to remove any excess moisture
- Use a mass balance to measure the mass of each egg
- Place each egg into a different beaker, so that the sodium chloride completely covers the eggs
- Leave all the eggs for 24 hours
- Remove the eggs and pat each egg dry with new dry paper towels
- Reweigh each egg
- Percentage change in mass = (final mass - initial mass) / initial mass * 100
- Plot a graph of percentage change in mass against sodium chloride concentration. You can use this to estimate how solutions of different water potential affect the mass of the egg
facilitated diffusion
The passive movement of large or charged molecules (diffusion) through the plasma membrane via carrier proteins and channel proteins (transport proteins) from an area of high concentration to an area of low concentration, down the concentration gradient
Why do some bigger molecules go through facilitated diffusion?
They would diffuse very slowly through the phospholipid bilayer
How is facilitated diffusion triggered?
A chemical messenger binds to the transport protein, causing it to change shape to allow a specific substance to pass through the membrane
Transport proteins are open all the time. T/F and why?
False - they open & close to only let certain molecules pass through
Can polar molecules diffuse through membranes?
Yes, but only at a slow rate.
How are glucose molecules transported across a membrane? Be as specific as possible, explaining your answer.
Facilitated diffusion with carrier proteins
They are too big to be transported with channel proteins
Carrier proteins are specific. T/F and why?
True - they are specific to an ion, molecule, or group of substances (as there are specifically-shaped binding sites)
Give examples of some molecules that diffuse through facilitated diffusion.
- Glucose
- Amino acids
- Ions
- Polar molecules
How potassium ions transported across the membrane? Is this passive or active?
Facilitated diffusion - channel protein. Passive
Most ions are transported this way (can you give one that isn’t?)
How are calcium ions transported across the membrane? Is this passive or active?
Active transport - carrier protein. Active
How are bacteria transported across the membrane? Is this passive or active?
Endocytosis. Active
How are enzymes and hormones transported across the membrane? Is this passive or active?
Exocytosis. Active
How are ions transported through a membrane?
Facilitated diffusion/active transport (charged so can’t be transported through hydrophobic fatty acids in phospholipid bilayer via simple diffusion)
The tails of phospholipids are polar/non-polar.
non-polar (& hydrophobic)
Identify the type of particle that could cross the membrane using simple diffusion.
hydrophobic & non-polar
Why will only uncharged particles pass through the membrane?
The phosphate heads are -vely charged, but the fatty acids are uncharged. Fatty acids make up most of the membrane structure, so the membrane is uncharged
What route do particles that pass through the membrane via simple diffusion take?
They go through little gaps formed from the tilting of phospholipids with unstaturated fatty acids
Look at a molecule of triglyceride. Why can’t it diffuse through a cell membrane?
It is too large to diffuse through the membrane by simple diffusion so the phospholipids act as a barrier
Look at a molecule of citric acid. Why can’t it diffuse through a cell membrane?
It is too charged and hydrophilic to diffuse through the membrane by simple diffusion so the phospholipids act as a barrier
Explain why glucose can’t just diffuse through the phospholipid bilater. Suggest a way that it can travel through a membrane.
Glucose is polar due to having hydroxyl groups, hydrophilic (water soluble), lipophobic (not lipid soluble) and large so the phospholipids act as a barrier
Facilitated diffusion
Look at a diagram of a fatty acid molecule. Describe its properties and how it can travel through a membrane.
Fatty acids are non-polar, hydrophobic (not water soluble) and lipophilic (lipid soluble) so the phospholipids act as a barrier
Simple diffusion
For simple diffusion, the rate of diffusionis/isn’t meaningfully affected by the presence of transport proteins.
isn’t
Diffusion & osmosis need energy. T/F and why?
Both diffusion and osmosis rely on the random movement (kinetic energy) of particles, which requires energy. Cells don’t need additional energy from metabolic sources e.g. ATP to transport them though.
Thylakoid membranes contain electron transport chain proteins. T/F and why?
True - electrons are charged so need to be transported across membranes via transport proteins
Explain why a highly folded membrane may enable a higher rate of facilitated diffusion, compared to a membrane with no folds.
- Increasing the number of folds increases the surface area to volume ratio.
- This provides more space for a larger number of transport proteins on the membrane.
- If all other factors are constant, an increase in the number of transport proteins increases the rate of facilitated diffusion.
Are carrier proteins intrinsic or extrinsic proteins?
intrinsic - they span both bilayers of the plasma membrane
Are channel proteins intrinsic or extrinsic proteins?
intrinsic - they span both bilayers of the plasma membrane
Are glycoproteins intrinsic or extrinsic proteins?
extrinsic (for the A Level at least) - they are bound to only one of the bilayers (outer or inner) of the plasma membrane
How do carrier proteins work?
Large/polar molecule binds to carrier protein
Carrier proteins change shape to be complementary to the molecule
This moves and releases the molecule to the other side of the membrane
Carrier proteins do not require an external source of energy to function. T/F and why?
False - they can act in passive (facilitated diffusion) and active (active transport) methods of transport.
For active transport, carrier proteins require energy in the form of ATP to change shape, in order to move the solute against the concentration gradient.
Proteins are hydro____ and polar/nonpolar.
hydrophilic and polar - think of the OH and NH2 groups
A student carried out an investigation involving the uptake of the stain methylene blue by yeast cells, which involved adding methylene blue to a suspension of yeast cells. Samples of the stained yeast cells were heated to different temperatures. The student then observed the cells at high power under a light microscope.
As the temperature increased from 10°C to 80°C, the percentage of cells observed stained blue decreased from 98 to 0 and the colour of the solution surrounding the cells changed from colourless to light blue at 60°C to blue.
Using only the information provided in the paragraph above, outline the evidence that supports the statement that yeast cells take up methylene blue by active transport. [2]
At low temperatures, all the methylene blue stain in the cells, against the concentration gradient (if it moved in by diffusion, then there would be some stain in the cells and some in the surrounding solution).
A student carried out an investigation involving the uptake of the stain methylene blue by yeast cells, which involved adding methylene blue to a suspension of yeast cells. Samples of the stained yeast cells were heated to different temperatures. The student then observed the cells at high power under a light microscope.
As the temperature increased from 10°C to 80°C, the percentage of cells observed stained blue decreased from 98 to 0 and the colour of the solution surrounding the cells changed from colourless to light blue at 60°C to blue.
Yeast cells take up methylene blue by active transport.
Suggest why some cells did not stain blue at 20°C. [1]
The cells are dead or not respiring (so therefore weren’t releasing energy to absorb the methylene blue).
How do really big molecules enter cells?
Endocytosis
Give three examples of molecules that enter cells through bulk transport.
- Some carbohydrates
- Lipids
- Proteins
- Microogranisms & dead cells can also be taken in by phagocytes.
Describe how endocytosis occurs.
- Cell surrounds a substance with a bit of its membrane
- The membrane (bends inwards and) pinches off to form a vesicle
- The vesicle moves into the cytoplasm for further processing of the substance
Describe how exocytosis occurs.
- Vesicles containing substances are formed by the Golgi apparatus
- The vesicles move to and fuse with the plasma membrane (which then bends inwards)
- The vesicles then release their contents out of the cell (or sometimes into the membrane e.g. membrane proteins)
Does bulk transport need an external energy source?
Yes - ATP.
Describe how cell signalling works.
- A cell releases a messenger molecule
- Messenger molecule travels (e.g. in the blood) to another cell
- This cell has got the complementary membrane-bound receptor, so the molecule binds to the receptor
- This triggers a change in the cell
How do antihistamine drugs work to relieve pain?
- Antihistamines are a complementary shape to the membrane-bound receptors that bind to histamines, so they bind to them
- This means the messenger molecules (histamines) can no longer bind to their complementary receptors as they’ve been blocked
- This means the histamines can’t trigger a change or response in the cells
Give three examples of membrane-bound receptors.
Don’t necessarily need to know
- Proteins
- Glycoproteins
- Glycolipids
How is ATP broken down?
ATP is hydrolysed to form ADP and 1 phosphate (separated from the others)
The energy for direct active transport is provided by the ATP molecule, stored in the bond between…
two phosphates
active co-transport
The coupled movement of substances across a cell membrane via a carrier protein
How does active co-transport work?
- the transport protein binds to the first particle and one phosphate in ATP
- it hydrolyses ATP into ADP and one phosphate ion
- this opens up the transport protein; the transport protein changes shape
- this transfers the first particle across the membrane and reveals a second binding site
- the specific desired second particle binds to the second binding site in the transport protein
- this causes the phosphate ion to be released, triggering the transport protein to revert back to its original shape and releasing the second particle through the membrane
Don’t think you need to know this
What are intrinsic proteins also called?
integral proteins
Don’t know if you need to know this
two ways active co transport can get its energy
hydrolysis of ATP
moving a particle down its concentration gradient
indirect active transport
The movement of one type of particle uses energy, and this movement maintains the concentration gradient needed for transportation of a different particle(s)
indirect active transport process
- one particle moves against its concentration gradient by active transport with a carrier protein
- this means the concentration gradient is in the right direction for both particles
- another transport protein moves the two particles down their concentration gradient via facilitated diffusion
intrinsic protein (+ examples)
a protein embedded in both layers of the membrane with their arrangement determined by their hydrophilic and hydrophobic regions (they are more hydrophobic)
e.g. carrier/channel proteins
Don’t know if you need to know this
extrinsic protein (+ examples)
a hydrophilic protein found on the surface (outer or inner) of the plasma membrane
e.g. enzymes, glycoprotein (for the A Level at least)
Don’t know if you need to know this
What are extrinsic proteins also called?
peripheral proteins
Don’t know if you need to know this
intercellular vs. intracellular
Intercellular means between cells
Intracellular means inside the cell
Why is agar jelly used to mimic diffusion through a membrane?
Has a similar consistency to cells’ cytoplasms
Why shouldn’t you increase the temperature of the agar jelly in the RP to above 65°C?
The agar jelly will start to melt
Millie is investigating the rate of A scientist uptake of a solute into bacterial cells by simple diffusion and active. Explain why, after a certain point, the graph for active transport plateaus.
All the carrier proteins are in use, so no more solute can travel through the bacterial membranes via active transport at one time.
Cubes of agar jelly contain universal indication. They are cut to different side lengths and placed in a beaker contain hydrochloric acid and the time taken for cubes to turn red was measured. What was the role of the universal indicator in this experiment.
- To act as an end point
- To check the presence of H+ ions
unit for surface area to volume ratio
no unit (can do SA:V of you really want, but you don’t need to)
I think this is correct, but dont’ quote me
An agar jelly cube of side length 20mm contains universal indicator. The time taken for the block to turn entirely red after being submerged in acid is measured to be 28.8 minutes. Calculate the rate of diffusion of acid from the outer surface to the centre of the cube.
Distance to the centre of the cube = 20 / 2 = 10mm
Rate = distance travelled / time
= 10 / 28.8
~ 0.347 mm/min
Agar jelly cubes of varying side lengths contain universal indicator. The time taken for the blocks to turn entirely red after being submerged in acid is measured.
Identify two limitations in the procedure that may cause the results to be inaccurate and explain which cube’s results are most likely to have been affected by these limitations. [3 marks per limitation]
Limitation: inconsistency in cutting - affects the smallest cube.
It is the smallest cube so a small error in cutting will have proportionately larger effect in a small cube.
Limitation: end point is measured by eye- affects the biggest cube
It is the largest cube so harder to see through a larger depth of jelly (to judge when the cube is entirely red).
Suggest and explain how a low pH affects cell membranes. [2]
denatures/changes tertiary structure of membrane proteins, which then increases membrane permeability
