2.5 - Biological membranes

Question 1

Why is the cell surface membrane sometimes referred to as the fluid mosaic model?

Answer 1

• The cell surface membrane is made up of mainly phospholipids however there are many more components embedded in the membrane that is held together by hydrophobic and hydrophilic interactions in a mosaic way
• The cell surface membrane is also referred to as fluid as some parts of the membrane can move around each other freely as they are not attached to other parts of the cell

Question 2

What is the structure of a phospholipid?

Answer 2

• Polar head group (Hydrophilic)

- Non polar fatty acid tails (Hydrophobic)

Question 3

Describe the way the phospholipids in a cell surface membrane positioned?

Answer 3

-Outside the cell (Aqueous environment)
-Cell surface membrane with glycoproteins and glycolipids embedded in the membrane (Glycocalyx)
-Hydrophilic head
-Hydrophobic tail
[No H20 present inside the cell surface membrane]
-Hydrophobic tail
-Hydrophilic head
-Cytoskeleton filaments
-Inside the cell (Aqueous environment)

Question 4

What is the function of the cytoskeleton?

Answer 4

-The cytoskeleton gives the cell surface membrane its shape

Question 5

What are the structures that are embedded in the cell surface membrane?

Answer 5

• Carrier proteins (Can be integral or peripheral)

- Channel proteins (Can be integral or peripheral)

Question 6

What is an integral protein?

Answer 6

An integral membrane protein is a type of membrane protein that is permanently embedded in the biological membrane

Question 7

What is a peripheral protein?

Answer 7

• Peripheral proteins are membrane proteins which attach loosely to the inside and outside surfaces
• Peripheral proteins maintain cell shape and support cell membrane to maintain its structure
• Peripheral proteins do not enter into the hydrophobic space within the cell membrane they are just associated with the membrane while positioned on the outside

Question 8

What is the function of channel and carrier proteins?

Answer 8

To allow molecules into and out the cell surface membrane

Question 9

What are channel proteins?

Answer 9

• Channel proteins selectively allow ions to enter and exit the cell membrane
• The channel proteins have R group on amino acids the walls
• The negatively charged R groups stick out and and only allow ions with a positive charge to flow through

Question 10

What is the glycocalyx?

Answer 10

• The carbohydrate layer that sits outside the membrane
• The glycocalyx is made up of many carbohydrates that are projected out from the membrane
• The glycocalyx is involved with cell to cell recognition and interactions with the environment

Question 11

How does the fluorescence recovery after photo bleaching experiment show that the cell surface membrane is fluid?

Answer 11

• The experiment happened as follows:
• The phospholipid molecules in a membrane were stained with a fluorescent dye
• Using a very precise laser, a small section of the phospholipid bilayer was stained white
• After waiting for a second another image was taken of the bilayer
• The white stained phospholipids had moved and spread out across the bilayer
• This shows that the phospholipids are fluid as they can move around eachother

Question 12

How does the fluorescence recovery after photo bleaching experiment show that the cell surface membrane is fluid?

Answer 12

• The experiment happened as follows:
• The phospholipid molecules in a membrane were stained with a fluorescent dye
• Using a very precise laser, a small section of the phospholipid bilayer was stained white
• After waiting for a second another image was taken of the bilayer
• The white stained phospholipids had moved and spread out across the bilayer
• This shows that the phospholipids are fluid as they can move around each other

Question 13

What is the purpose of cholesterol in the cell surface membrane?

Answer 13

Cholesterol regulates the fluidity of the membrane

Question 14

What is the main job of the membrane?

Answer 14

• The membrane is a barrier that is used to compartmentalise the inside of the cell from the outside
• This is done so that there can be different concentrations of molecules in the inside of the cell as there are on the outside the cell

Question 15

What is diffusion?

Answer 15

-Diffusion is the passive movement of particles from a high concentration to a low concentration, down the concentration gradient

Question 16

What molecules can diffuse across the membrane from the outside of a cell to the inside?

Answer 16

-Small non-charged molecules
(For example: Water, oxygen, carbon dioxide)

-Large lipid-soluble molecules can diffuse through the phospholipid bilayer as well because they can dissolve through the centre of the bilayer as they can dissolve in fat and the bilayer is essentially fat
(For example: Hormones like testosterone or oestrogen

Question 17

Why can’t ions pass through the phospholipid bilayer?

Answer 17

Ions are charged so they cannot pass through the membrane

Question 18

Why can’t glucose pass through the phospholipid bilayer via simple diffusion?

Answer 18

Glucose is too large to pass through the membrane via diffusion so it has to use channel/carrier proteins

Question 19

What factors affect the rate of diffusion?

Answer 19

• Temperature
• Diffusion distance
• Surface area
• Size of diffusing molecule
• Concentration gradient

Question 20

How does temperature affect the rate of diffusion?

Answer 20

• As temperature goes up it means that molecules move more quickly as they have a higher kinetic energy
• This INCREASES the rate of diffusion

Question 21

How does diffusion distance affect the rate of diffusion?

Answer 21

• As the diffusion distance increases, it takes longer for the molecule to diffuse along the further distance
• This DECREASES the rate of diffusion

Question 22

How does surface area affect the rate of diffusion?

Answer 22

-A higher surface area INCREASES the rate of diffusion

Question 23

How does the size of diffusing molecule affect the rate of diffusion?

Answer 23

• Smaller molecules diffuse more quickly
• This means that the larger the size of a molecule the slower the diffusion rate
• As size increases, the rate of diffusion DECREASES

Question 24

How does the concentration gradient affect the rate of diffusion?

Answer 24

• As the concentration gradient is steeper, the rate of diffusion is faster
• A higher concentration gradient INCREASES the rate of diffusion

Question 25

How are large molecules such as glucose taken into a cell?

Answer 25

Large molecules such as glucose enter a cell via facilitated diffusion

Question 26

What is facilitated diffusion?

Answer 26

Diffusion that is enabled by a channel or carrier protein

Question 27

What are carrier proteins?

Answer 27

• Carrier proteins are proteins embedded into the phospholipid bilayer that are used to transport ions and molecules that are too big to simply diffuse across the membrane
• Carrier proteins change shape when transporting the molecule (This is sometimes called “flipflopping”)

Question 28

How do carrier proteins help transport glucose and other large molecules into the cell?

Answer 28

• The carrier protein is open on the part that faces out of the cell
• This allows molecules to enter and bind to the carrier protein, this triggers a conformational change in shape of the protein
• This causes the protein to close the side that faces out of the cell and open the end that faces into the cell, this allows the molecule that is bound to the protein to be released into the cell
• The protein then changes back to the original shape to allow the binding of any other molecules

Question 29

Is facilitated diffusion a passive process?

Answer 29

Yes, as no ATP is needed

Question 30

What causes the “Flipflop” movement of the carrier proteins when they are transporting large molecules across a membrane?

Answer 30

-The energy that causes the movement of the proteins comes from the concentration gradient as the molecules are travelling down a concentration gradient

Question 31

What is active transport?

Answer 31

The active movement of molecules up the concentration gradient from low to a high concentration using ATP

Question 32

What is the process of active transport in a sodium-potassium pump?

Answer 32

• Three sodium ions and one ATP binds to the protein on the side that faces the out of the cell
• ATP is hydrolysed and converts to ADP, this causes a conformational shape in the protein
• The sodium ions are released into the membrane
• The binding of two potassium ions triggers a change in shape of the protein back to the original shape

Question 33

What is bulk transport?

Answer 33

• Bulk transport is the active movement of large amounts of molecules across a membrane using ATP
• Exocytosis and endocytosis are both processes of bulk transport

Question 34

What is exocytosis?

Answer 34

• The movement of large quantities out of a cell
• A vesicle inside of the cell moves towards a membrane
• The vesicle fuses with the membrane
• The contents of the vesicle is released into the external environment

Question 35

What is endocytosis?

Answer 35

• The movement of large quantities into a cell
• A food particle or bacterium is engulfed into a cell
• This forms an endocytotic vesicle

Question 36

What is osmosis?

Answer 36

-Osmosis is the passive movement of water from an area of high water potential gradient into an area of low water potential gradient across a partially permeable membrane

Question 37

Why do water molecules surround and stick to solute molecules?

Answer 37

-As water molecules are polar they are attracted to the polar solutes molecules

Question 38

In what liquid would there be a higher water potential?

Answer 38

• In a liquid with less solutes there will be a higher water potential, this is because the solutes cause the water molecules to stick to them meaning that they are not free
• A liquid with a higher water potential has more free water molecules so this means that a liquid with less solutes will have a higher water potential

Question 39

What is the highest possible water potential?

Answer 39

0kPa is pure water which is the highest possible water potential

Question 40

What happens to cells that are in a liquid with a higher WP than theirs?

Answer 40

• Water moves into the cells via osmosis from an area of relatively high WP to a lower WP inside their cells
• In a plant cell, the cell becomes turgid, this is because the cell wall protects the cell from bursting and breaking
• In an animal cell lysis will occur, the cell will burst as it does not have a cell wall to prevent the bursting under the high pressure from the water moving into it

Question 41

What happens to cells that are in a liquid with a lower WP than theirs?

Answer 41

• Water exits the cell via osmosis from an area of relatively high WP inside the cell to a lower WP outside their cells through the cell membrane
• In a plant cell, the cell becomes plasmolysed as the cell wall maintains the shape but the cell membrane separates and shrivels up inside the cell wall
• In an animal cell, the cell is crenated

Question 42

How is osmosis used to determine the WP within a cell?

Answer 42

• Place cylinders of cut potatoes into different solutions of different water potential
• The cylinders of potatoes must be the same size with the same surface area and mass
• The potatoes are placed in solutions with different concentrations of water
• After waiting for 20 minutes, blot the potatoes dry with a paper towel to remove excess surface water and then weigh them
• Work out percentage change between the mass before adding them to the solutions and after they were removed from the different solutions
• The results should show that the percentage change is higher with potatoes from a solution with a lower glucose concentration (Higher water potential)
• This is because the potato will gain more mass in a concentration with a lower glucose concentration as the water will enter the potato from an area of relatively high WP inside the solution to a lower WP inside the potato
• The WP/glucose concentration of the potato is found at the point where the percentage change is 0 because water will have not gone in or out of the potato at that precise glucose concentration
• This means that the WP/glucose concentration in the solution would be equal to the WP/glucose concentration inside the potato, so that is how the WP/glucose concentration inside the potato is found

Question 43

What is the process of the betalain practical which measures the effect of temperature on membrane stability in beetroot, assessed by the degree of pigment leakage?

Answer 43

• Cut disks of beetroot
• Wash the disks of beetroot until the surface colour is faded/gone
• Place the disks in distilled water of different temperatures for one minute
• Measure the colour of the water with a colorimeter once the disks are removed after 1 minute
• The findings should be that when the water is under around 50°C the water doesn’t become pink but when the water is higher than 50/60°C then the water becomes pink from the pigment in the disks of beetroot
• This is because when the temperature in the water is around 50°C/60°C/70°C the membrane structure starts to break down around the pigment
• This releases the pigment from the beetroot which causes the water to change colour from clear to pink

Question 44

Why does temperature of the water cause the membrane to break down in the betalain experiment?

Answer 44

• When the water is hot is causes the phospholipids in the membrane to have higher kinetic energy
• This causes a loss of structure and temporary holes to be made in the membrane where the betalain can leak out and cause the water to turn from clear coloured