1.3 Membrane structure

Question 1

What is the fluid mosaic model made up of (plasma membrane)

Answer 1

• hydrophobic fatty acid tail + hydrophilic phosphate head = phospholipid
• Phospholipid bilayer = 2 phospholipids
• Integral proteins = glycoproteins, channel proteins
• Peripheral proteins
• Polysaccharide chain attached to glycoprotein
• Cholesterol

Question 2

What makes up the bilayer

Answer 2

phospholipids form bilayers due to their amphipathic properties. (charged heads and non-polar tails)

Question 3

What do glycoproteins do

Answer 3

glycoproteins are involved in cell adhesion, cell signalling and immune system recognition

Question 4

what do channel proteins do

Answer 4

channel proteins are involved in facilitated diffusion protein pumps are involved in active transport

Question 5

What are glycoproteins involved in?

Answer 5

Glycoproteins are involved in cell adhesion, cell signalling and immune system recognition

Question 6

What are channel proteins involved in

Answer 6

-Channel proteins are involved in facilitated diffusion, protein pumps are involved in active transport

Question 7

What are peripheral proteins

Answer 7

-Peripheral proteins are often membrane bound enzymes

Question 8

Hydrophobic

Answer 8

-Hydrophobic: not attracted to water

Question 9

Hydrophilic

Answer 9

-Hydrophilic: attracted to water

Question 10

Amphipathic

Answer 10

-Amphipathic: substances which are both hydrophobic and hydrophilic (phospholipids)

Question 11

-The hydrophilic part of the phospholipid is the phosphate group.

Answer 11

-The hydrophilic part of the phospholipid is the phosphate group.

Question 12

-The hydrophobic part of the phospholipid consists of two hydrocarbon chains.

Answer 12

-The hydrophobic part of the phospholipid consists of two hydrocarbon chains.

Question 13

Membrane proteins

Answer 13

-Membrane proteins are diverse in terms of structure, position in the membrane and function.
Their primary function is to form a barrier through which ions and hydrophilic molecules cannot easily pass. This is carried out by the phospholipid bilayer. Almost all other functions are carried out by proteins in the membrane.
-Example: Pumps for active transport which use ATP to move particles across the membrane.
-Because of these varied functions, membrane proteins are very diverse in structure and in their position in the membrane.

Question 14

Two types of membrane proteins

Answer 14

intergral proteins

peripheral proteins

Question 15

Integral proteins

Answer 15

Integral proteins are hydrophobic meaning they are embedded within the hydrocarbon chains in the centre of the membrane. Many integral proteins are transmembrane with hydrophilic heads on either side of the membrane. (phosphate heads)

Question 16

Peripheral proteins

Answer 16

Peripheral proteins are hydrophilic on their surface, not embedded in the membrane. Most of them are attached to the surface of integral proteins. Some of them have a single hydrocarbon chain inserted into the membrane anchoring the protein to the membrane surface.

Question 17

membrane content in membranes

Answer 17

The more active a membrane the higher its protein content

Question 18

Cholesterol in membranes

Answer 18

Cholesterol is a component of animal cell membranes
Cholesterol provides stability by reducing the fluidity of proteins through the membrane
Cholesterol also maintains fluidity which is particularly important in colder environments
Membranes are made of phospholipids and proteins + animal membranes have cholesterol
Cholesterol is a type of lipid which belongs to the steroid substance group. Cholesterol is mostly hydrophobic with a singular hydrophilic group which is attracted to the phosphate heads on the periphery of the membrane.

Question 19

Why did they think the DAVSON danielli model was correct?

Answer 19

Why did they think this was correct?

1. Chemical analysis showed their composition = protein + phospholipid
2. Evidence from red blood cells showed they have 2x as much phospholipid as that required to form a single phospholipid membrane layer = bilayer
3. Experiments showed that despite its thin layer the membrane acted as a barrier to some molecules
4. When able to view electron micrograph it appears as two dark lines with a lighter part in the middle (since they knew proteins stain darker this fitted their model)

Question 20

In falsifying this theory, the following evidence was incorporated.

Answer 20

1. Freeze fracture electron micrographs showed globular proteins in the centre of the bilayer
2. Analysis showed some parts of certain proteins were hydrophobic therefore would embed in the lipid bilayer
3. Fusion of cells with membrane proteins tagged with coloured fluorescent markers showed the proteins could move within the membrane.

Question 21

The role of cholesterol in mammalian membranes - to reduce membrane fluidity and permeability to solutes

Answer 21

1. Cell membranes do not correspond exactly to any of the three states of matter. The hydrophobic hydrocarbon tails usually behave as a liquid, but the hydrophilic phosphate heads act more like a solid. Overall the membrane is fluid as components of the membrane are
   free to move.
2. The fluidity of animal cell membranes needs to be carefully controlled. If they were too fluid they would be less able to control what substances pass through, but if they were not fluid enough the movement of the cell and substances within it would be restricted.
3. Cholesterol disrupts the regular packing of the hydrocarbon tails of phospholipid molecules, so prevents them crystallizing and behaving as a solid. However, it also restricts molecular motion and therefore the fluidity of the membrane. It also reduces the permeability to hydrophilic particles such as sodium ions and hydrogen ions.
4. Due to its shape cholesterol can help membranes
   to curve into a concave shape, which helps in the formation of vesicles during endocytosis.