Topic 2B - Cell Membranes

Question 1

Why is it called the fluid-mosaic model

Answer 1

The bilayer is fluid because the phospholipids are constantly moving.
Proteins are scattered throughout the phospholipid bilayer.

Question 2

What does the channel protein do

Answer 2

Span the membrane and make hydrophilic tunnels across it, allowing molecules to pass through via diffusion

Question 3

What does a carrier protein do

Answer 3

Binds to specific solutes and transfer them across the lipid bilayer

Question 4

What do receptor proteins on the cell membrane allow?

Answer 4

Cells to detect chemicals released from other cells, which trigger a response

Question 5

What are glycoproteins?

Answer 5

Proteins that have a carbohydrate attached to it

Question 6

What are glycolipids?

Answer 6

Lipids that have a carbohydrate attached

Question 7

What is a phospholipid?

Answer 7

They have a hydrophilic head and a hydrophobic tail

Question 8

Describe the phospholipid bilayer (3)

Answer 8

Heads face outside and tails face inwards, forming a bilayer
Centre of the bilayer is hydrophobic, so the membrane doesn’t allow water soluble substances to diffuse through
Small non polar substances and water can diffuse through

Question 9

What does cholesterol give to the membrane?

Answer 9

Stability

Question 10

How does cholesterol provide the membrane with stability?

Answer 10

Binds to the hydrophobic tails of the phospholipids, causing them to pack tightly together. This restricts movement and makes membrane less fluid and more rigid

Question 11

How does temperature below 0 degrees affect the membrane? (Phospholipid) (4)

Answer 11

Phospholipids don’t have much energy so they dont move very much
Packed closely together and membrane is rigid
Channel proteins and carrier proteins denature, increasing permeability
Ice crystals may form and Pierce the membrane, making it highly permeable when it thaws

Question 12

How does temperature between 0 and 45 degrees affect the cell membrane? (3)

Answer 12

Phospholipids can move around and aren’t as tightly packed together
Membrane is partially permeable
As temperature increases the energy of the phospholipids increase, making it more permeable

Question 13

How does temperature above 45 degrees affect the cell membrane?

Answer 13

Phospholipid bilayer begins to break down and membrane becomes more permeable
Channel proteins and carrier proteins denature, so they cant control what enters or leaves the cell - increasing permeability

Question 14

What is simple diffusion?

Answer 14

When molecules diffuse directly through a cell membrane

Question 15

What factors affect the rate of diffusion? (3)

Answer 15

The concentration gradient
Thickness of exchange surface
The surface area

Question 16

What is facilitated diffusion?

Answer 16

The use of carrier proteins and channel proteins to transport large or charged molecules across a membrane

Question 17

How do carrier proteins work?

Answer 17

A large molecule attaches to the carrier protein in the membrane
The protein changes shape
This releases the molecule on the other side of the membrane

Question 18

How do channel proteins work?

Answer 18

Form pores in the membrane for charged particles to diffuse through (down conc gradient)

Question 19

What factors affect the rate of facilitated diffusion?

Answer 19

The concentration gradient
The number of channel proteins or carrier proteins

Question 20

What is osmosis?

Answer 20

The movement of water across a partially permeable membrane from an area of high water potential to an area of lower water potential

Question 21

What is the water potential of pure water?

Answer 21

0

Question 22

How can you lower the water potential of pure water?

Answer 22

Adding solutes (makes it more negative)

Question 23

What is isotonic solution?

Answer 23

If 2 solutions have the same water potential
(There is no net movement of water)

Question 24

What is a hypotonic solution?

Answer 24

Solutions with a higher water potential compared to the inside of the cell

Question 25

What is a hypotonic solution?

Answer 25

Solutions with a lower water potential than the cell

Question 26

What factors affect the rate of osmosis?

Answer 26

Water potential gradient
Thickness of exchange surface
Surface area of the exchange surface

Question 27

Briefly explain the osmosis practical?

Answer 27

1. Cut potatoes into identical cubes, measure the mass, divide into 2 groups
2. Place each group into sucrose solutions for equal time, 20 min
3. Remove and gently pat dry, weight each group and record results
4. Calculate percentage change

Question 28

What is active transport?

Answer 28

Uses energy to move molecules and ions across plasma membrane, against a concentration gradient

Question 29

What are the 2 main differences between active transport and diffusion

Answer 29

Active transport mainly moves solutes from an area of low to a high concentration
Active transport requires energy

Question 30

Why is ATP important in active transport?

Answer 30

Active transport required energy
When ATP undergoes hydrolysis, it releases energy which allows the solutes to be transported

Question 31

What are co transporters? (3)

Answer 31

Type of carrier protein
Bind to 2 molecules at a time
The concentration gradient of the other molecules is used against its own concentration gradient

Question 32

What are 3 factors which affect the rate of active transport

Answer 32

Speed of individual carrier proteins
Number of carrier proteins available
Rate of respiration in the cell and the availability of ATP

Question 33

Explain the 3 steps in which glucose is absorbed via co transport

Answer 33

1. Na ions actively transported out of the epithelial cells in the ileum in the blood, by the NaK pump - this creates a conc gradient. More Na in ileum than in the cell
2. This causes Na ions to diffuse from lumen of ileum into the epithelial cells, down the conc gradient, they do this via sodium-glucose co transporter proteins - carries glucose into cell aswell as Na - Glucose in cell increases
3. Glucose diffuses out of cell into blood down the conc gradient via facilitated diffusion