Topic 2B - Year 1 - Cell Membranes - The basics

Question 1

In which type of cells eukaryotic or prokaryotic are the organelles surrounded by membranes?

Answer 1

Eukaryotic

Question 2

What is the purpose of a cell surface membrane?

Answer 2

The cell surface membrane acts as a barrier between the cell and its environment , it controls what enters and leaves the cell due to the partially permeable nature of its membrane.

Question 3

By which 3 processes can substances move across the cell surface membrane?

Answer 3

Osmosis
Diffusion
Active Transport

Question 4

Why are plasma membranes that surround organelles important?

Answer 4

The plasma membranes that surround the organelles in eukaryotic cells are important as they help compartmentalise the cell and thus keep its organelles separate from the cytoplasm.

Question 5

What types of molecules is the plasma membrane composed of?

Answer 5

The plasma membrane is composed of

• phospholipids
• proteins
• carbohydrates
• cholesterol

Question 6

What phrase would be used to describe formation in which the molecules lie to form a plasma membrane?

Answer 6

“A fluid mosaic model”

Question 7

What other word could be used as an alternative to cell membrane?

Answer 7

Plasma membrane

Question 8

Why is the phospholipid bilayer said to be fluid?

Answer 8

The bilayer is said to be fluid as the phospholipids are constantly moving

Question 9

Name the 3 types of protein that lie within or on the surface of a plasma membrane

Answer 9

Receptor proteins
Channel proteins
Carrier proteins

Question 10

What do receptor proteins in the cell surface membrane do ?

Answer 10

Receptor proteins on the cell surface of the membrane allows the cell to detect chemicals released from other cells and when triggered by this they can signal the cell to respond in some way.

Question 11

What type of molecules do carrier proteins help cross the cell surface membrane?

Answer 11

Large molecules

Question 12

What type of molecules do channel proteins help cross the cell surface membrane?

Answer 12

Charged (ions)

Question 13

If a protein has a carbohydrate chain attached to it what is it referred to as?

Answer 13

Glycoprotein

Question 14

If a lipid has a carbohydrate chain attached to it what is it referred to as?

Answer 14

Glycolipid

Question 15

Which part of a phospholipid is hydrophilic?

Answer 15

The phosphate head

Question 16

Which part of a phospholipid is hydrophobic?

Answer 16

The hydrocarbon tail

Question 17

What do phospholipid molecules act as a barrier to?

Answer 17

Water soluble substances

Question 18

How do phospholipids arrange themselves as they form a plasma membrane ?

Answer 18

Phospholipids arrange themselves with their phosphate heads pointing outwards and their hydrophobic tails pointing inwards

Question 19

In consequence of the middle of a phospholipid bilayer being hydrophobic which type of substances cannot pass through?

Answer 19

Water soluble molecules like ions and polar molecules cannot passively diffuse across the membrane.

Question 20

What type of substances can pass through the plasma membrane?

Answer 20

Small , non-polar molecules can pass through the plasma membrane.

Question 21

What attribute does cholesterol give to a cell membrane?

Answer 21

Stability

Question 22

FACT TO REMEMBER

Answer 22

Cholesterol is a type of lipid

Question 23

Cholesterol is found in all cell membranes with the exception of one, which cell membrane does not contain cholesterol?

Answer 23

Bacterial cell membrane

Question 24

Where do the molecules of cholesterol fit in relation to the phospholipids?

Answer 24

The molecules of cholesterol fit between the phospholipid.

Question 25

What do cholesterol molecules do to make the phospholipid bilayer more stable?

Answer 25

The cholesterol molecules bind to the hydrophobic tails of the phospholipids , this causes the phospholipids to pack more tightly together making the membrane less fluid.

Question 26

In cells that don’t have a cell wall why is cholesterol particularly important?

Answer 26

Cholesterol binds to the hydrophobic tails of the phospholipids pulling them closer together , which reduces the fluid nature of the membrane making it more rigid. having a cell that is rigid enough to hold its shape is particularly important if there is no cell wall.

Question 27

Cholesterol has hydrophobic regions , hence does it add or detract from the bilayers nature asa barrier to water?

Answer 27

Cholesterol has hydrophobic regions so it is able to create a further barrier to polar substances moving through the membrane.

Question 28

How would being in a very cold environment (0 degrees) impact the permeability of a plasma membrane?

Answer 28

At really cold temperatures phospholipids have very little energy and thus are packed tightly together making the structure very rigid.

At excessively cold temperatures channel and carrier proteins may denature , the denaturing of enzymes embedded in the phospholipid bilayer will make the membrane more permeable.

At sub zero temperatures ice crystals may form , this may pierce holes in the plasma membrane making it highly permeable when it thaws.

Question 29

How do membranes change as temperature increases?

Answer 29

As temperature increases phospholipids attain more energy , if phospholipids have more energy they tend to move around more and consequently arn’t packed as tightly together , if phospholipids are not packed as tightly together the permeability of the membrane increases.

Question 30

At temperatures which exceed 45 degrees how does the phospholipid bilayer behave?

Answer 30

At extremely high temperatures the phospholipid bilayer begins to melt (breakdown) increasing permeability.

At extremely high temperatures water inside of the cell begins to expand , this puts pressure on the membrane further encouraging its break down and thus increase in permeability.

At extremely high temperatures channel and carrier proteins de-nature , this means they cannot control what enters and leaves the cell - this increases the permeability of the membrane.

Question 31

How does putting a plasma membrane in a solvent affect its permeability ?

Answer 31

If a plasma membrane is put in a solvent its lipids will be dissolved which will break down the membrane increasing permeability , the more concentrated the solvent the greater the permeability will be increased as the more membrane will be broken down.