F: Cell Membrane Structure

Question 1

What are the properties of membranes?

Answer 1

• Flexible
• Self-sealing
• Selectively permeable

Question 2

Why are biological membranes bilayers?

Answer 2

• Composed largely of lipids + proteins
• The lipids are amphiathic, meaning they have a hydrophilic and hydrophobic part (hydrophilic head, hydrophobic tail = phospholipids), hence forms bilayer
• Assemble spontaneously in aqueous conditions
• Non- covalent assemblies

Question 3

Describe the lipid composition of biological membranes and their distribution

Answer 3

• 60% phospholipids, 30% cholesterol, 10% sphingolipids

Phospholipds- lipids based upon glycerol backbone, with 2 fatty acid tails, fatty acids hydrophobic, glycerol linked to a polar head through phosphate bridge, this is hydrophilic

Cholesterol - Plays important role in regulating membrane fluidity, large flat molecule with very small hydrophilic head group OH

Sphingolipids - Begin as a ceramide (a sphingosine molecule and a fatty acid). Ceramide combines with carbohydrate to become glycolipid. The ceramide combines with carbohydrate + phosphate + choline and this is sphingomyelin (this is a phospholipid but the glycerol backbone has been replaced by sphingosine)

Question 4

What are the polar head groups that can be present in phospholipids?

Answer 4

• Choline
• Serine
• Ethanolamine
• Inositol

Name becomes integrated for example e.g phosphatidylcholine

Question 5

Why is the composition of membranes asymmetrical?

Answer 5

• They don’t have the same composition of lipids on either side of the membrane

Question 6

How are membranes involved in the synthesis of lipids?

Answer 6

• Phospholipid synthesis occurs on cytosolic surface of ER (smooth ER)
• Distribution of newly formed lipid requires enzymes flippase and floppase
• ABC transporter (or ATP-binding cassette) - protein involved in transporting lipids by converting energy gained from ATP hydrolysis into trans-bilayer movement of substrates into or out of the cytoplasm

ABC transporters are a large family of membrane proteins that use ATP hydrolysis to transport various molecules across cellular membranes. They function as either importers (bringing nutrients into cells) or exporters (pumping molecules, including toxins and drugs, out of cells)

Question 7

Describe the function of flippase

Answer 7

Moves phospholipids from outer (extracellular) to inner (cystosolic) leaflet (requires ATP)

Question 8

Describe the function of floppase

Answer 8

Moves phospholipids from inner to outer leaflet (requires ATP)

Question 9

How can chemical composition of cell membranes differ for different types of cells based on their distribution throughout the cell?

Answer 9

• Different cells will have different weighted compositions of protein:lipid:carbohydrate

For example, myelin cell membranes are approx 18%:79%:3% protein:lipid:carbohydrate as opposed to an erythrocyte membrane that is approx 49%:43%:8% protein:lipid:carbohydrate

Question 10

What are the features of integral proteins?

Answer 10

• Single or multi pass
• Strong non-covalent bonds
• Trans-membrane domain often an alpha helix
• Can be predicted form sequence
• Interact extensively with lipid bilayer

Question 11

What are the features of peripheral membrane proteins?

Answer 11

• Located on both extracellular and cytosolic membrane
• Associated by non-covalent bonds
• Interact with integral proteins and lipid polar head groups

Question 12

What are lipid anchored membrane proteins?

Answer 12

• Proteins covalently linked to a lipid molecule such as glycosyl-phosphatidylinositiol or GPI

Question 13

Describe membrane carbohydrates

Answer 13

• Form 2-10% of membrane weight
• Majority of glycolipids and glycol proteins externally facing
• Often involved in cell to cell interactions or cellular recognition
• Blood group antigens are glycolipids
• Selectins important group of membrane glycoproteins

Question 14

What factors effect fluidity?

Answer 14

• Temperature
• Fatty acid composition
• Chain length
• Degree and extent of saturation
• Cholesterol content

Question 15

What is the role of cholesterol in maintaining membrane fluidity?

Answer 15

• In low temp, cholesterol interferes with interactions of phospholipids, increasing fluidity. This is because at low temps, the membrane is rigid, so there’s an increase in cholesterol inserting between phospholipids , increasing the distance between each phospholipid, increasing fluidity
• In high temps, cholesterol stabilises the membrane, decreasing fluidity by pulling phospholipids closer together

The greater the phospholipid interactions the grater the rigidity of the membrane

Question 16

What are the different membrane co-transporter systems?

Answer 16

• Symporter - molecules transported in same direction
• Antiporter - molecules transported in opposite direction

Question 17

What are the differences between simple diffusion and carrier mediated facilitative diffusion?

Answer 17

• Carrier mediated has specificity
• Carrier mediated is fast, simple diffusion is slow
• Simple diffusion has no limit, but carrier mediated is saturated, meaning there’s only a limited number of carriers available for the substrate to bind to

Question 18

What is Ktransport?

Answer 18

• Affinity of transporter for its substrate

Question 19

Describe Glut 1 transport protein

Answer 19

• Located in all mammalian tissues
• Km = 1mM (Km is an indicator of the affinity of the transporter protein for glucose molecules)
• This protein transports glucose into cells from the blood or from other cells

Question 20

Describe GLUT-2

Answer 20

• Located in liver and pancreatic Beta cells
• Km 15-20 mM
• This protein plays an important role in insulin secretion from Beta cells and glucose metabolism in hepatocytes

Question 21

Describe GLUT-3

Answer 21

• Located in all mammalian tissues
• Km = 1mM
• Responsible for the basal transport of glucose to maintain resting-level activity

Question 22

Describe GLUT-4

Answer 22

• Located in muscle and fat cells
• Km = 5mM
• Controls glucose transport into fat and muscles tissues in response to insulin and also into muscle during exercise

Question 23

Describe GLUT-5

Answer 23

• Located in the small intestine
• Primarily a fructose transporter

Question 24

How do carrier mediated transporters function?

Answer 24

• Combines a chemical reaction with a diffusion process to form a solute-carrier complex, which then diffuses through the membrane to release the solute at the permeate side
• Transport of glucose selective
• Transport of glucose can be increased by increasing number of transporters on cell surface

Question 25

Summarise carrier mediated facilitated diffusion

Answer 25

- Driven by concentration gradient - Gradient maintained by phosphorylation - Bidirectional transport - Transporters are selective - Can be saturated (finite number of carriers) - Some transporters are under hormonal regulation

Question 26

Describe active transport

Answer 26

- Movement of molecules against electrochemical gradient from area of low to high concentration of molecules - This requires energy in form of ATP - ATP hydrolysis into ADP + pi allows movement of ions etc. against concentration gradient - Electrochemical gradient can be maintained due to cellular asymmetry

Question 27

What are gated channels?

Answer 27

- Transmembrane proteins that form pores for movement of ions - Activated/opened by different stimuli including: Voltage Ligand Phosphorylation Transmembrane proteins/domain means the protein goes through all layers of the membrane

Question 28

What is the role of the plasma membrane?

Answer 28

- Barrier - Transport - Signal transduction

Question 29

What is the role of the inner and outer membranes of the mitochondria?

Answer 29

- Inner membrane- energy transduction - Outer membrane- Barrier

Question 30

What is the role of the rER membrane?

Answer 30

- Translation protein processing

Question 31

What is the role of the sER membrane?

Answer 31

- Synthesis of complex lipids

Question 32

What is the role of the Golgi membrane?

Answer 32

- Post-translational modification - Processing for secretion

Question 33

What is the role of the nuclear membranes?

Answer 33

- Attachment of chromatin

Question 34

What is the role of the hydrolytic membrane?

Answer 34

A hydrolytic membrane refers to a membrane, often in the context of biological systems, that contains hydrolytic enzymes or is involved in hydrolytic reactions, which are chemical reactions that break down molecules using water

Question 35

What is the role of the peroxisome membrane?

Answer 35

- Fatty acid oxidation

Question 36

What helps to maintain cellular asymmetry?

Answer 36

- Tight junctions which act as 'fences' to separate the molecules, maintaining cell polarity

Question 37

How does compartmentalisation support the function of membranes?

Answer 37

- Intracellular membranes create local environments and separate reactions - Enables electrochemical gradients to be established - Bring enzymes and reactants together and/or keep them apart - Bring reactants together - Required for energy production (mitochondria)

Question 38

What happens in apoptosis?

Answer 38

- Phosphatidylserine is redistributed to the extracellular surface, and this signals phagocytes, which then engulf the cells, leading to cell death, and the cellular components can then be recycled