2.5 Membranes

Question 1

What organelles have membranes?

Answer 1

Mitochondria
Golgi apparatus
Chloroplasts
Lysosomes

Question 2

What is the role of membranes?

Answer 2

To regulate conditions within the cell and organelles by controlling material entry and exit

Question 3

Why is the role of membranes important?

Answer 3

Each of organelles needs specific conditions in order for reactions to occur

Question 4

What do plasma membranes surrounding cells form?

Answer 4

A boundary between the cells environment and the cytoplasm

Question 5

What is the boundary between the cells environment and the cytoplasm needed for with regard to conditions?

Answer 5

• establishing different conditions

- keeping conditions separate

Question 6

Give a property of the cell membrane:

Answer 6

It is partially permeable

Question 7

Why is it necessary for the cell membrane to be partially permeable?

Answer 7

To precisely control exchange between the cell and its environment
Allow some molecules through but not others

Question 8

Where are cell membranes found?

Answer 8

Surrounding cells

Surrounding organelles

Question 9

How is the cell membrane/ boundary vital for cell signalling?

Answer 9

It is vital for receiving messages at receptors bound to the membrane
Allows ends and exocytosis
Allows formation of a membrane network throughout cells

Question 10

What do membranes within cells do?

Answer 10

Divide the cell into different compartments

They act as a barrier between organelle and the cytoplasm

Question 11

What can membranes within the cell form relating to transport?

Answer 11

Vesicles to transport substances between different areas of the cell

Question 12

What do membranes within the cell control? Give an example:

Answer 12

Which substances enter and leave organelles (e.g. RNA leaves the nucleus via the nuclear membrane)

Question 13

What do membranes within organelles act as? Give an example:

Answer 13

Barriers between the membrane contents and the rest of the organelle
E.g. Thylakoid membranes in chloroplasts

Question 14

What can membranes within the cell be the site of? Give an example:

Answer 14

Chemical reactions

E.g. The inner membrane of the Mitochondrion contain enzymes needed for respiration

Question 15

What is exocytosis?

Answer 15

How things get out of the cell

Question 16

What is endocytosis?

Answer 16

How things get into the cell

Question 17

What is the cell membrane made up of?

Answer 17

A phospholipid bilayer

Question 18

What is the structure of a phospholipid?

Answer 18

A phosphate group attached to a glycerol backbone with two fatty acids joined

Question 19

What area of the phospholipid is hydrophilic?

Answer 19

The phosphate heads

Question 20

What area of the phospholipid is hydrophobic?

Answer 20

The fatty acids

Question 21

What do phospholipids automatically form when added to water?

Answer 21

Bilayer or Micelle

Question 22

Describe the arrangement of the bilayer:

Answer 22

Hydrophilic Phosphate heads face outwards toward the water whilst hydrophobic fatty acids stay in the centre away from the water

Question 23

Because the centre of the membrane is hydrophobic what does it not allow through?

Answer 23

Water soluble materials such as ions

Question 24

What is the term for organelles being divided by internal membranes?

Answer 24

Compartmentalisation

Question 25

Why is compartmentalisation important?

Answer 25

Some reactions are incompatible and can not occur in the same environment as they need specific conditions

Question 26

Give an example of a compartmentalised organelle:

Answer 26

Mitochondria - divided by an external and internal membrane

Question 27

The smaller and less polar a molecule…

Answer 27

… The faster it will diffuse across a cell membrane

Question 28

Give examples of small non polar molecules that rapidly diffuse across a cell membrane:

Answer 28

Oxygen

Carbon dioxide

Question 29

Do small polar molecules diffuse across plasma membranes?

Answer 29

Yes but much slower than small non polar molecules

Question 30

Give examples of small polar molecules that diffuse across the membrane:

Answer 30

Water

Urea

Question 31

What molecules are unlikely to diffuse across the membrane?

Answer 31

Charged particles (ions)

Question 32

Describe the fluid mosaic model?

Answer 32

“Cell surface membranes consist of a double layer if phospholipid molecules studded with proteins”

Question 33

In the fluid mosaic model why is the bilayer described as fluid? What does this make the membrane?

Answer 33

• Because the phospholipids can move

- This makes the membrane a flexible structure that constantly changes shape

Question 34

Why is the membrane described as mosaic?

Answer 34

It appears like tiles in a mosaic with proteins embedded in the bilayer

Question 35

What is cholesterol?

Answer 35

A type of lipid

Question 36

Where is cholesterol present?

Answer 36

In all cell membranes (except bacterial)

Question 37

What is cholesterol important in?

Answer 37

Controlling membrane fluidity

Question 38

How are cholesterol molecules arranged in the membrane?

Answer 38

They fit between phospholipids and bind to the hydrophobic fatty acids causing them to pack more closely together

Question 39

The more cholesterol in the membrane…

Answer 39

The less fluid and permeable it is

Question 40

At normal body temperature what is cholesterol important in?

Answer 40

Keeping membranes stable

Question 41

Why is maintaining membrane fluidity important?

Answer 41

Otherwise cells would not be able to function

Question 42

What processes is a fluid membrane necessary for?

Answer 42

• Diffusion of substances across membranes
• Membrane fusion (e.g a vesicl me fusing with the cell membrane during exocytosis)
• cells to move and change shape e.g macrophages during phagocytosis