F: Cell Membrane Structure Flashcards
What are the properties of membranes?
- Flexible
- Self-sealing
- Selectively permeable
Why are biological membranes bilayers?
- Composed largely of lipids + proteins
- The lipids are amphiathic, meaning they have a hydrophilic and hydrophobic part (hydrophilic head, hydrophobic tail = phospholipids)
- Assemble spontaneously in aqueous conditions
- Non- covalent assemblies
Describe the lipid composition of biological membranes and their distribution
- 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with carbohydrate + phosphate + choline and this is sphingomyelin (this is a phospholipid but the glycerol backbone has been replaced by sphingosine)
What are the polar head groups that can be present in phospholipids?
- Choline
- Serine
- Ethanolamine
- Inositol
Name becomes integrated for example e.g phosphatidylcholine
Why is the composition of membranes asymmetrical?
- They don’t have the same composition of lipids on either side of the membrane
How are membranes involved in the synthesis of lipids?
- 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
Describe the function of flippase
Moves phospholipids from outer to inner leaflet (requires ATP)
Describe the function of floppase
Moves phospholipids from inner to outer leaflet (requires ATP)
How can chemical composition of cell membranes differ for different types of cells based on their distribution throughout the cell?
- 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
What are the features of integral proteins?
- 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
What are the features of peripheral membrane proteins?
- Located on both extracellular and cytosolic membrane
- Associated by non-covalent bonds
- Interact with integral proteins and lipid polar head groups
What are lipid anchored membrane proteins?
- Proteins covalently linked to a lipid molecule such as glycosyl-phosphatidylinositiol or GPI
Describe membrane carbohydrates
- 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
What factors effect fluidity?
- Temperature
- Fatty acid composition
- Chain length
- Degree and extent of saturation
- Cholesterol content
What is the role of cholesterol in maintaining membrane fluidity?
- 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
What are the different membrane co-transporter systems?
- Symporter - molecules transported in same direction
- Antiporter - molecules transported in opposite direction
What are the differences between simple diffusion and carrier mediated facilitative diffusion?
- 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
What is Ktransport?
- Affinity of transporter for its substrate
Describe Glut 1 transport protein
- 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
Describe GLUT-2
- 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
Describe GLUT-3
- Located in all mammalian tissues
- Km = 1mM
- Responsible for the basal transport of glucose to maintain resting-level activity
Describe GLUT-4
- 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
Describe GLUT-5
- Located in the small intestine
- Primarily a fructose transporter
How do carrier mediated transporters function?
- 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
Summarise carrier mediated facilitated diffusion
- 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
Describe active transport
- 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
What are gated channels?
- Transmembrane proteins that form pores for movement of ions
- Activated/opened by different stimuli including:
Voltage
Ligand
Phosphorylation
What is the role of the plasma membrane?
- Barrier
- Transport
- Signal transduction
What is the role of the inner and outer membranes of the mitochondria?
- Inner membrane- energy transduction
- Outer membrane- Barrier
What is the role of the rER membrane?
- Translation protein processing
What is the role of the sER membrane?
- Synthesis of complex lipids
What is the role of the Golgi membrane?
- Post-translational modification
- Processing for secretion
What is the role of the nuclear membranes?
- Attachment of chromatin
What is the role of the hydrolytic membrane?
- Hydrolytic enzymes
What is the role of the peroxisome membrane?
- Fatty acid oxidation
What helps to maintain cellular asymmetry?
- Tight junctions which act as ‘fences’ to separate the molecules, maintaining cell polarity
How does compartmentalisation support the function of membranes?
- 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)
What happens in apoptosis?
- 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
