Membrane structure and function

Question 1

What are the 5 membrane features?

Answer 1

1. Flexible
2. Self-sealing
3. Selectively permeable
4. They define the external boundaries
5. Divide the internal space

Question 2

Describe the basic composition of a membrane

Answer 2

Biological membranes are composed of a lipid bilayer.
• Most of the lipids are phospholipids.
• Phospholipids have a hydrophilic head group and a hydrophobic tail.
• Phospholipids and glycolipids are amphipathic (having both hydrophobic and hydrophilic parts).
• Readily form biomolecular sheets in aqueous media.
• Form lipid bilayer (liposomes)/ micelles.
• Non-covalent assemblies.

Question 3

Describe the components in lipid composition

Answer 3

Choline 
Serine 
Ethanolamine
Inositol
Phosphate 
Glycerol 
Fatty acids

Question 4

Describe membrane asymmetry

Answer 4

These phospholipids are not distributed evenly. First two are mainly found on the outside of the cell membrane, opposed to the two that follow which are found on the inside. They are asymmetrically distributed. The cholesterol is relatively distributed throughout the membrane.

Question 5

Describe the fluid mosaic model

Answer 5

• 1972 Singer and Nicolson proposed a model that allowed the passage of molecules through the membranes. This structure explains how cell membranes could be more dynamic and interact more with the cells/ environment.
• Phospholipid bilayer with proteins floating in it, making up a mosaic pattern

Question 6

Describe intrinsic/ integral proteins

Answer 6

• Transmembrane proteins are embedded through both layers of a membrane.
• Have amino acids with hydrophobic R-groups
• Strong non-covalent bonds
• Trans-membrane domain often an α-helix
• On their external surfaces, which interact with the hydrophobic core of the membrane keeping them in place.
• Channel and carrier proteins are intrinsic proteins, which are both involved in transport across the membrane.
• Channel proteins:
o Provides a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes.
o They are held in positions by interactions between the hydrophobic core of the membrane and the hydrophobic r-groups on the outside of the proteins.
• Carrier proteins:
o Have an important role in both passive transport (down a concentration gradient) and active transport (against a concentration gradient) into cells.
o The shape of the protein often changes.

Question 7

Describe extrensicic/ peripherial proteins

Answer 7

• Present on one side of the bilayer- located on the extracellular or cytosolic membrane.
• Associated by non-covalent bonds.
• Have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or intrinsic proteins.
• Present between any layer.
• Glycoproteins and enzymes are extrinsic proteins.

Question 8

Describe glycoproteins

Answer 8

• Glycoproteins are proteins which contain oligosaccharide chains (glycans) covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. Secreted extracellular proteins are often glycosylated.
• Embedded in cell surface membranes with attached carbohydrate chains of varying lengths and shapes.
• Role in cell adhesion (when cells join to form tight junctions in certain tissues) and as receptors for chemical signals.
• When the chemical binds to the receptor, it elicits a response from the cell. This may cause a direct response or set off a cascade of events inside the cell. This process is known as cell communication (cell signaling). For example:
o Receptors for neurotransmitters e.g. acetylcholine at nerve cell synapses. The binding of the neurotransmitter triggers or prevents an impulse in the next neuron.
o Receptors for peptide hormones, including insulin and glucagon, which affects the uptake and storage of glucose by cells.
• Some drugs bind to cell receptors -> Beta-blockers are used to reduce the response of the heart to stress.

Question 9

Describe glycolipids

Answer 9

• Lipids attached with a carbohydrate (sugar) chains.

* Known as cell markers/ antigens and can be recognized by the cells of the immune system as self or non-self

Question 10

Describe lipid anchor proteins

Answer 10

• Covalently linked to a lipid molecule such as glycero-phosphatidylinositol

Question 11

Describe cholesterol

Answer 11

• A lipid with a hydrophilic head and a hydrophobic tail.
• Regulates fluidity of membranes.
• Positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the tails, putting them together. It adds stability to membranes without making them too rigid.
• Prevents the membrane become solid by stopping the phospholipid molecules from grouping too closely and crystallising.

Question 12

Membrane carbohydrates

Answer 12

• Carbohydrates are associated with both membrane lipids and proteins.
• Form 2-10% of the membrane weight.
• Carbohydrate on all membranes faces away from the cytosol.
• They are often involved in cell-cell interactions or cellular recognition.
• In RBCs 8% of the weight is carbohydrate

Question 13

Describe the factors that affect the fluidity of the membrane

Answer 13

• Temperature
• Fatty acid composition – saturation leads to the molecules being packaged very tightly, well as unsaturated fatty acids are less compacted, thus increasing the fluidity of the membrane.
• Chain length
• Degree and extent of saturation
• Cholesterol content – flat, large molecule. Large hydrophobic region and small hydrophilic region.

Question 14

Describe cholesterol and temperature

Answer 14

At low temperatures the cholesterol will disrupt interactions between the fatty acids, causing the fluidity to increase.

At higher temperatures it stabilises the plasma membrane.

Question 15

Describe membrane fluidity

Answer 15

• Lateral movement of lipids in the membrane is rapid

* Transverse movement is slow and requires the action of three enzymes

Question 16

Describe membrane synthesis

Answer 16

• Enzymes responsible for the synthesis of membrane phospholipids are associated largely with the cytosolic side of the ER. Newly synthesized lipids are then move to the luminal side by the flippases.
• Synthesis ER lipids may be altered as the membranes flow from the ER to Golgi to vesicles. Exceptions sphingomyelin and glycolipids started in ER and finished in Golgi.
• Floppase - moves phospholipids from the inner to the outer leaflet
• Flippase (flipase) - moves phospholipids from the outer to the inner leaflet
• Scramblase - bidirectional movement

Question 17

Describe apoptosis

Answer 17

Apoptosis – allows the body to turn over cells without causing an inflammatory response
• Programmed death
• Doesn’t release contents
• Doesn’t cause inflammatory
• Phosphatidylserine (blue) on the outside of cell: ‘Eat me signal’

Question 18

Describe the factors that affect the fluidity of the membrane (2)

Answer 18

Temperature decreases:
• Saturated fatty acids become compressed, kinks in the tail push phospholipid molecules away- maintaining membrane fluidity.
• Cholesterol buffers the effect of lowered temperatures, to prevent a reduction in the membrane’s fluidity. It prevents the phospholipids from packing together too closely by inserting itself between them.
Temperatures increases:
• Phospholipids acquire more KE and move around randomly, increasing membrane fluidity and permeability.
• Affects the way membrane-embedded proteins are positioned and may function.
• Enzymes in the membrane may drift and his could alter the rate of reactions they catalyse.
• Cholesterol molecules buffer increasing heat as it reduces the increase of membrane fluidity, makes it more rigid (holds it together).
Fatty acid composition: chain length: increased chain length means increased rigidity (more interactions) so less fluid.
Degree and extent of saturation:
• More saturated (straight chains) means increased rigidity o less fluid.
Cholesterol content:
• OH (hydroxyl group) forms hydrogen bonds between the phospholipid’s hydrophobic tails
• Disrupts regular interaction between fatty acid chains

Question 19

Describe Spur cell anaemia

Answer 19

• Cholesterol content increased by 25-65% leading to decreased membrane fluidity, more rigid. This causes the shape of the plasma membrane to change, as shown by the RBC with ‘spikes’ in the diagram. The flexibility is lost, and these abnormal RBCs will break down
• Less able to work as a RBC.
• Spur cell anaemia is found in end-stage alcoholic liver disease. The morphologic changes of spur cells are caused by accumulation of excess membrane cholesterol, leading to an impaired deformability and consequently a reduced red cell survival.