Unit 1: Section 2 - Cell Membranes

Question 1

What do plasma membranes do?

Answer 1

Control which substances enter and leave the cell. They are partially permeable. Allow recognition by other cells. They allow cell communication.

Question 2

What do membranes within cells do?

Answer 2

• Membranes around organelles divide the cell into different compartments. Making different functions more efficient.
• Membranes of some organelles are folded to increase surface area and so make chemical reactions more efficient.
• Can form vesicles
• Control what enters and leaves the organelle.

Question 3

What structure do cell membranes have?

Answer 3

‘Fluid mosaic’

Question 4

What are the 4 components of a cell membrane?

Answer 4

Phospholipids
Cholesterol
Proteins
Glycolipids/Gylcoproteins

Question 5

Describe the structure of a phospholipid bilayer

Answer 5

Head is hydrophilic - attracts water
Tail is hydrophobic - repels water

2 layers, heads face outward and tails face inwards with water on either side

The centre of the bilayer is hydrophobic therefore water-soluble molecules cannot pass through the bilayer.

Question 6

What does cholesterol do?

Answer 6

Fit between the phospholipids, they bind to the tails causing them to pack more closely together, this makes the membrane less fluid and more rigid.

Question 7

What do proteins do?

Answer 7

1) Form channels in the membrane for small or charged particles to pass through
2) Carrier proteins allow molecules and ions across the membrane by active transport or facilitated diffusion.
3) Act as receptors for molecules in cell signalling.

Question 8

What do glycolipids and glycoproteins do?

Answer 8

Stabilise the membrane by forming hydrogen bonds with surrounding water molecules

Act as sites where drugs, hormones and antibodies bind

Act as receptors for cell signalling

Question 9

What is cell signalling?

Answer 9

How cells communicate with each other to control processes inside the body and to respond to changes in the environment.

Question 10

How do cells communicate with each other?

Answer 10

Messenger molecules:

1) One cell releases a messenger molecule
2) This molecule travels to another cell
3) The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane

Question 11

Why do receptor proteins have specific shapes?

Answer 11

So that only messenger molecules with a complementary shape can bind to them.

Question 12

What is the name given to a cell that responds to a particular messenger molecule?

Answer 12

A target cell

Question 13

Why are drugs designed to bind to cell membrane receptors?

Answer 13

When they bind to the cell membrane they either trigger a response in the cell or black the receptor and prevent it from working

Question 14

What happens to a cell membrane at temperatures below 0?

Answer 14

The phospholipids don’t have as much energy, so they cant move very much. They’re packed closely together and the membrane is rigid.

Channel proteins and carrier proteins denature increasing the permeability of the membrane. Ice crystals may pierce the membrane making it highly permeable when it thaws

Question 15

What happens to a cell membrane at temperatures between 0 and 45 degrees?

Answer 15

The phospholipids can move around and aren’t packed as tightly together - membrane is partially permeable. As temperature increases the phospholipids move more because they have more energy - this increases the permeability of the membrane.

Question 16

What happens to a cell membrane at temperatures above 45 degrees?

Answer 16

Phospholipid bi-layer melts and the membrane becomes more permeable. Water inside the cell expands putting pressure on the membrane.

Channel proteins and carrier proteins in the membrane denature - increases permeability

Question 17

What is diffusion?

Answer 17

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.

Question 18

What is a concentration gradient?

Answer 18

The path from an area of higher concentration to an area of lower concentration - particles diffuse down a concentration gradient.

Question 19

3 factors that affect the rate of diffusion

Answer 19

1) Concentration gradient
2) Thickness of the exchange surface
3) Surface area

Question 20

What is osmosis?

Answer 20

Diffusion of water molecules across a partially permeable membrane, from an area of high concentration to an area of low concentration.

Question 21

Does pure water have the highest or lowest water potential?

Answer 21

Highest - all solutions have a lower water potential than water

Question 22

What is a hypotonic solution?

Answer 22

A solution with a higher water potential than the cell

Question 23

What is an isotonic solution?

Answer 23

A solution with the same water potential as the cell

Question 24

What is a hypertonic solution?

Answer 24

A solution with a lower water potential than the cell

Question 25

What do carrier proteins do?

Answer 25

Move large molecules into or out of the cell, down their concentration gradient

Question 26

How does a carrier protein work?

Answer 26

1) A large molecule attaches to a carrier protein
2) Then, the protein changes shape
3) This releases the molecule on the opposite side of the membrane

Question 27

What do channel proteins do?

Answer 27

Form pores in the membrane for charged particles to diffuse through.

Question 28

What does active transport do?

Answer 28

Move substances against a concentration gradient.

Question 29

How does it do this?

Answer 29

It uses the same method as carrier proteins but uses ATP in order to move the substance against the concentration gradient.

Question 30

How does endocytosis work?

Answer 30

Some molecules are way too big to be taken into a cell by carrier proteins, a cell can surround a substance with a section of it’s plasma membrane. The membrane then pinches off to form a vesicle inside the cell containing the substance.

Question 31

How does exocytosis work?

Answer 31

Some substances produced by the cell need to be released, vesicles containing the substance pinch off from the sacs of the golgi apparatus and move towards the plasma membrane. The vesicles fuse with the plasma membrane and release their contents outside the cell. Some substances may not leave the cell, they may be inserted straight into the plasma membrane.