Biological Membranes

Question 1

What do phospholipids consist of

Answer 1

Two fatty acid molecules bonded to a glycerol which is bonded to a phosphate group.

Question 2

What are the hydrophilic and hydrophobic parts of the phospholipids

Answer 2

The hydrophobic region is the fatty acids and the hydrophilic region consists of the phosphate groups.

Question 3

What happens when phospholipids are put in water

Answer 3

As they contain both hydrophobic and hydrophilic regions, when in water they arrange themselves in a phospholipid bilayer. Where the hydrophilic phosphate groups can interact with the water molecules but the hydrophobic fatty acid tails cluster together away from the water in the centre.

Question 4

What does the phospholipid bilayer make up

Answer 4

It makes up all cell membranes - including the cell surface membrane and membranes surrounding organelles such as lysosomes

Question 5

Functions of cell membranes (1) (barrier)

Answer 5

Act as a barrier between the internal contents of the cell and the environment, or between the contents of an organelle and the cytoplasm.

Question 6

Functions of cell membranes (2) (separation)

Answer 6

Membranes can separate one part of an organelle from another part

Question 7

Functions of cell membranes (3) (reactions)

Answer 7

Membranes can be locations for chemical reactions for example some of the stages in respiration.

Question 8

Functions of cell membranes (4) (indicating)

Answer 8

Membranes are involved in cell signalling

Question 9

Functions of membranes (5) (permeability)

Answer 9

Membranes are partially permeable, allowing some substances to pass but acting as a barrier to others.

Question 10

Why is the cell membrane partially permeable

Answer 10

Because the membrane contains a hydrophobic centre, the hydrophobic molecules such as steroid hormones can easily pass through the cell membrane. However, the hydrophobic centre prevents hydrophilic molecules from easily passing through. (As they are polar and can’t easily pass through the non-polar region of the membrane)

Question 11

What is an exception to the permeability of membranes and why. And what is the downside

Answer 11

Water molecules can easily pass through the cell membrane despite being polar as they are very small. However this occurs at a very slow rate.

Question 12

Functions of cell membranes (6) (electrical)

Answer 12

They can allow electrical signals to pass along them

Question 13

Functions of cell membranes (7) (enzymes)

Answer 13

They provide attachment sites for enzymes and other molecules involved in metabolism.

Question 14

Functions of cell membranes (8)

Answer 14

They provide a surface for attachment (ribosomes on the RER)
Provide a surface for pigments (chlorophyll in thylakoid membranes of chloroplasts).

Question 15

What does the cell surface membrane consist of

Answer 15

It is based on the phospholipid bilayer, however it contains a large number of protein molecules too as well as cholesterol.

Question 16

What is the structure of cholesterol and why does this allow interaction with the cell membrane

Answer 16

They have a polar hydrophilic group at one end, this group attracts to the polar head group of the phospholipid molecules.
The rest of the molecule is non polar and hydrophobic so attracts to the non-polar fatty acids in the phospholipids.

Question 17

What is cholesterols first role and what is the effect

Answer 17

Because cholesterol interacts with phospholipids it increases the strength of the cell surface membrane. Making the membrane more stable and less likely to get damaged by other molecules involved the membrane.

Question 18

What is the second role of cholesterol in the membrane

Answer 18

It controls the fluidity of the membrane by reducing the sideways movement of phospholipids.

Question 19

What does cholesterol help the membrane to be in different conditions

Answer 19

Prevents the membrane from becoming too fluid in warm conditions and too rigid under cool conditions.

Question 20

What is the third function of the cholesterol in membranes

Answer 20

By packing the spaces between phospholipids, cholesterol helps to reduce the movement of water soluble chemicals across the cell surface membranes

Question 21

What is the structure of the cell surface membrane known as

Answer 21

The fluid mosaic model

Question 22

Why is it called the fluid mosaic model

Answer 22

Fluid - as the phospholipid molecules can move around within each layer, so the membrane is flexible and can change shape.
Mosaic - as the membrane is studded with protein molecules of different sizes and the arrangement of these proteins varies.

Question 23

What are intrinsic proteins

Answer 23

Proteins that are fully embedded in the membrane from one side to the other

Question 24

Because they are embedded in the membrane what is a property of the amino acids in the proteins

Answer 24

They have hydrophobic amino acids on the outside surface of the protein. These hydrophobic amino acids can interact with the fatty acid tails in the phospholipid bilayer keeping the intrinsic proteins in place.

Question 25

What are protein channels

Answer 25

A type of intrinsic proteins with a hydrophilic channel running through the centre, the channel is lined with hydrophilic amino acids and is filled with water molecules.

Question 26

What is the purpose of protein Chanel’s

Answer 26

They allow water soluble molecules and ions to diffuse down a concentration gradient through the membrane.

Question 27

What are carrier proteins

Answer 27

A type of intrinsic proteins that can change their shape or position to transfer molecules or ions through the membrane. They are involved in active transport.

Question 28

What are extrinsic proteins

Answer 28

They don’t span the membrane and are instead found on one side of the membrane or the other side.

Question 29

What are the main functions of extrinsic proteins

Answer 29

They play a structural role within the molecule whereas others act as enzymes, some act as receptors for other molecules such as hormones too.

Question 30

What are glycoproteins and where can they be found

Answer 30

Proteins that also contain a carbohydrate molecule attached and they can be found on membranes.

Question 31

what are the roles of glycoproteins

Answer 31

Some allow cells to attach to each other forming tissues (cell adhesion)
Some play a role in the immune system presenting antigens to T cells
Others act as receptors for hormones and when the hormones binds a response is caused in the cell.

Question 32

What are glycolipids and where can they be found

Answer 32

Carbohydrates can be found attached to phospholipid molecules too, these are glycolipids. They can also be found on cell membranes.

Question 33

What are the functions of glycolipids

Answer 33

They are used when cells come in contact with each other, and the glycolipids on the surface of one cell can be recognised by another cell determining whether cells come into contact.
They can act as antigens (e.g in determining blood group)

Question 34

How does temperature affect membranes

Answer 34

As the phospholipids in the membrane are constantly moving. When the temperature is increased the phospholipids with have more kinetic energy and will move more. As a result then membrane becomes more fluid and begins to lose its structure.

Question 35

How does temperature affect membranes (2) Further increase

Answer 35

If the temperature continues to increase then the membrane will break down completely eventually.
The loss of structure increases the permeability of the membrane making it easier for substances to pass through it.

Question 36

How does temperature affect membranes (3)

Answer 36

As well, as this carrier and channel proteins in the membrane will be denatured at higher temperatures. These proteins are involved in transport across membranes, so membrane permeability is affected.

Question 37

What are many organic solvents

Answer 37

They are less polar than water or non polar

Question 38

What do many concentrated organic solvents do

Answer 38

They dissolve membranes, disrupting cells

Question 39

What do less concentrated organic solvents do

Answer 39

They will not dissolve membranes however they can still cause damage.

Question 40

What can Non-polar alcohol molecules do

Answer 40

They can enter the cell membrane and their presence disrupts the membrane

Question 41

What happens when the membrane is disrupted

Answer 41

It becomes more fluid and permeable.