Membrane composition

Question 1

Plasma membrane functions

Answer 1

• Selectively permeable barrier, controlling the passage of solutes and water maintaining a specific intracellular composition.
• Maintains a difference in ions, producing an electrochemical gradient.
• Provides proper environment for enzymes to work. -Contains glycoprotein receptors, triggering responses in the cell.
• involved in exocytosis and endocytosis
• molecules essential for adhesion, maintaining cell shape

Question 2

Where are plasma membranes found?

Answer 2

Around cells and sub cellular organelles, certain secretory vesicles

Question 3

Membrane lipids

Answer 3

Phospholipids, including glycerophospholipids and sphingomyelin, glycolipids, including gangliosides and cerebrosides, and cholesterol

Question 4

What is the most important structural feature of lipids + why?

Answer 4

Amphiphatic, with a polar hydrophilic head and a non pola,r hydrophobic hydrocarbon chain tail which allows the molecules to assemble into a bilayer.

Question 5

Why are saturated phospholipids most often found in membranes?

Answer 5

Carbon chains freely rotate around single bonds, oscillating between a zig zag arrangement and a contorted arrangement. Trans is most often found as it has a lower free energy, chains more closely together so more van der Waals, more stable bilayer compared to unsaturated which are commonly found in the contorted configuration.

Question 6

What is the predominant phospholipid in bilayer?

Answer 6

Phosphatidylcholine

Question 7

What happens if ester linkage at position 2 is hydrolysed?

Answer 7

Releases arachidonate, which is an eicosanoid precursor, thus beginning signals of inflammation.

Question 8

Another function of cell membrane explained

Answer 8

Contains phosphatidylinositol which is part of a signal transduction system activated by hormones- releases second messengers .

Question 9

Examples and function of glycolipids

Answer 9

Cerebrosides and gangliosides, carbohydrate head group acts as cell recognition signals.

Question 10

Cholesterol function

Answer 10

Does not form bilayers, is inserted into the membrane, orientating its hydroxyl group toward the polar heads of the amphipathic lipids and the flat cyclic core towards the non polar region of the double membrane. Decreases the mobility of the fatty acid chains, decreasing the permeability of the bilayer.

Tightens packing yet still equally fluid/

Question 11

How do different temperatures influence cholesterol function?

Answer 11

Cold- Cholesterol increases membrane fluidity as it prevent the hydrocarbon chains from freezing/crystallising

hot- cholesterol decreases membrane fluidity as it induces steric hindrance

Question 12

What is lipid bilayer permeable/impermeable to?

Answer 12

Permeable- small hydrophobic molecules, such as oxygen, carbon dioxide and nitrogen. Small, uncharged polar molecules- water, urea, glycerol.

Impermeable- large, uncharged polar molecules- glucose sucrose. Ions

Question 13

Lipid raft definition and function

Answer 13

Specialised domains formed of segregated lipid molecules. Contain mainly glycosphingolipids, cholesterol and protein receptors. Influence membrane fluidity and membrane protein trafficking

Question 14

Caveolae definition and function

Answer 14

Special type of lipid raft with invaginations of the plasma membrane. Structures rich in proteins and lipids. Believed to play a role in mechanoprotection, endocytosis and uptake of pathogenic bacteria.

Question 15

Different types of membrane protein

Answer 15

transmembrane protein, intrinsic and extrinsic, glycoprotein

Question 16

Integral proteins structure and function

Answer 16

Inserted or embedded into the lipid bilayer. Most completely transverse the membrane one of more times, while others are partially inserted into the middle hydrophobic regions. Normally adopt an alpha helical structure or beta barrels.

For integral proteins that transverse more than once, loop structures are exerted on either side of the membrane.

Question 17

How are integral proteins bound to the membrane?

Answer 17

Associated with the lipid bilayer by non covalent interactions.

Question 18

Extrinsic protein structure and function

Answer 18

Do not reach hydrophobic core of lipid bilayer; simply juxtaposed on one side of the membrane, connected by non covalent interactions between the polar amino acid side chains and the polar heads of the lipids.

Question 19

How else are peripheral proteins attached?

Answer 19

Attached to membrane by anchoring structures

Question 20

Explain protein anchoring

Answer 20

Fatty acids used to anchor proteins include myristic and palmitic acid. Myristic acts is bound by an amide link to the amine function of glycine whilst palmitic acid is bound to sulphur of a cysteine residue.

The fatty acid chains, attached to the protein, are then inserted into the bilayer.

Question 21

Isoprenoid chain protein anchoring explained

Answer 21

Farnesyl and geranylgeranyl bind to cysteines near the C-terminal end of the polypeptide, Insert into plasma membrane.

Question 22

Where are these two anchored proteins often found?

Answer 22

Cytoplasmic side

Question 23

Explain another type of protein anchoring system. Example of protein fixed

Answer 23

Glycophosphatidylinositol (GPI) anchors are added to certain proteins that have been transferred into the ER. Anchored to the membrane by a C-terminal transmembrane region. The transmembrane region is then cleaved as the GPI anchor is added, so that the protein remains attached to the membrane to the glycolipid.
Acetylcholineesterase

Question 24

What form of carbohydrates are present in the plasma membrane?

Answer 24

Carbohydrates either linked to proteins or lipids

Question 25

Role of carbohydrates in plasma membrane

Answer 25

Intercellular recognition, binding of different ligands, lectins, carbohydrate binding proteins, bind to them during the process of cell cell adhesion.

Question 26

Where are glycolipids found + related to function?

Answer 26

Exclusively on exposed apical surface, help protect cells from harsh conditions such as low pH and high concentration of degradative enzymes.

Charged glycolipids, such as gangliosides, alter the electrical field across the membrane, altering the diffusion of ions.

Question 27

Fluid mosaic model explained

Answer 27

At high temperatures, lipid bilayers behave as a fluid structure. As temp increases a larger number C-C of fatty acids lose their extended state adopting the gauche formation, increasing fluidity. Different proteins and lipids are embedded like a mosaic.

Question 28

Explain lipid bilayer asymmetric nature

Answer 28

Each layer of the bilayer has a different composition.

Almost all phosphatidyl choline and and sphingomyelin is found in the outer layer.

Inner layer- lipids with terminal primary amino acids (phosphatidylethanolamine and phosphatidylserine) are in inner layer

Question 29

Explain importance of asymmetric lipid bilayer + examples

Answer 29

PKC requires negative phospholipid (phosphatidylserine) for its action on the cytosolic domain

Animals exploit asymmetry to distinguish between live and dead cells

Question 30

How is membrane asymmetry maintained?

Answer 30

Flippases catalyse the flop flop of phospholipids from one lipid bilayer to the next (as phospholipids are formed within the cell).

Question 31

How fast is flip flopping + molecule that is an exception to this?

Answer 31

Very slow- can take hours.

Cholesterol is very rapid

Question 32

What movement occurs rapidly?

Answer 32

Exchanging places within a monolayer- lateral diffusion

Question 33

Floppase v flippase action

Answer 33

Floppase catalyses movement from cytosolic membrane to outer, flippase from outer to inner. Enzymes use ATP

Question 34

Scramblase function

Answer 34

flip flops in both directions using energy from the concentration gradient, passive

Question 35

How are lipids and proteins confined to one region of the membrane?

Answer 35

scaffolding proteins involved in tight and adherent junctions bound to cytoskeleton form a band, preventing the movement of lipids and proteins.