Biological Membranes Flashcards
What are the three properties of a biological membrane?
- Flexible
- Continuous
- Self-repairing
What is the point of a membrane?
Provides cell boundary/compartmentalisation, selectively permeable
What is the composition of a biological membrane?
Mainly lipids (phospholipids, cholesterol…), proteins (pumps, ion channels…) and carbohydrates (glycolipids, glycoproteins…)
Give an example of an amphipathic molecule
Phospholipid
How are phospholipids held together?
Non-covalent bonds
Wy are phospholipids described as amphipathic?
They have a polar head (hydrophilic) and a non-polar tail (hydrophobic)
What makes the fatty acid tails present in phospholipids saturated or unsaturated?
Presence of a C=C bond in unsaturated
How is the kink in the tail of a phospholipid caused?
C=C
What affects membrane fluidity?
How the phospholipids pack against each other so the differences in length and saturation of the fatty acids tails in each individual phospholipid
Structure of phospholipid
Choline
Phosphate
Glycerol
Fatty acid tails
What are the four major phospholipids present in mammalian membranes?
- Phophatidylethanolamine
- Phosphatidylcholine
- Phosphatidylserine
- Sphingomyelin
Which major phospholipid present in mammalian membranes has a net negative charge and why?
Phosphatidylserine because it has a negatively-charged phosphate group
Which phospholipid is essential for cell-cell signalling?
The metabolism of phosphatidylinositol is vital for signalling
Will saturated/unsaturated fatty acids cause more fluidity?
Unsaturated as the phospholipids can’t pack together as tightly
What is the role of cholesterol in biological membranes?
Modulate the properties of lipid bilayers
Structure of cholesterol
Polar head
Rigid steroid ring structure
Non-polar hydrocarbon tail
What part of cholesterol’s structure partly immobilises regions of the hydrocarbon tails on the phospholipid?
Rigid steroid ring structure
What is the consequence of more rigid lipid bilayer?
Less permeable
How does cholesterol affect the membrane fluidity?
Doesn’t affect the membrane fluidity but only the rigidity
What can high concentrations of cholesterol in a lipid bilayer prevent?
Can prevent hydrocarbon chains from coming together and crystallising
Examples of movement of phospholipids in a membrane
- Lateral diffusion
- Flexion
- Rotation
- Flip flop
Which phospholipid movement is least common and why?
Flip flop because it requires a lot of energy as the hydrophilic head would have to move across the central hydrophobic core
How is it possible that cholesterol flip flips more commonly that phospholipids do?
Cholesterol has a smaller polar hydroxyl group so less energy required
What are phospholipid bilayers impermeable to?
Charged ions and large water-soluble molecules due to the hydrophobic core
What kind of molecules can diffuse across membranes freely?
Small uncharged polar molecules e.g. CO2
Name a method to illustrate lateral diffusion of membrane proteins
Fluorescence recovery after photobleaching (FRAP)
What is FRAP?
Fluorescence recovery after photobleaching illustrates lateral diffusion of membrane proteins. Can’t follow individual proteins
What are the 2 types of membrane protein?
- Peripherally associated membrane proteins
- Integral membrane proteins
How are peripherally associated membrane proteins part of the membrane?
They are non-covalently bonded with integral proteins
Main difference between integral membrane proteins and peripherally associated membrane proteins
Peripherally associated membrane proteins do not extend into the hydrophobic core of the lipid bilayer
How can peripherally associated membrane proteins be removed from the membrane?
Mild treatments that disrupt ionic interactions/ hydrogen bonds
How are integral membrane proteins part of the membrane?
They are directly inserted into the membrane by hydrophobic domain
How can one remove integral membrane proteins from a membrane?
Use detergents to dissolve them
What are the three types of integral membrane proteins?
- α-helices - transmembrane
- embedded without crossing the bilayer
- not embedded. lipid-anchored via covalently bound fatty acid
What is an example of a lipid anchor? How does it attach the protein to the bilayer?
Phosphotidylinositol via fatty acids
What are 3 benefits of lipid anchoring?
- Mobility at cell surface
- Rapid release of protein into extracellular space
- Regulation of binding and release of proteins to membrane
Are membrane proteins fluid?
Yes but move less rapidly than phospholipids
What are some functions of integral membrane proteins?
- Act as receptor
- Act as adhesive molecule
- Amphipathic α-helices interact to form channels
- Enzymes
- Intracellular signalling
What are some functions of peripherally associated membrane proteins?
- Cell shape determination
- Communication with internal and external environment
- Intracellular transport
What is a lipid raft?
A micro domain which has a higher concentration of cholesterol and sphingolipids
What other molecule along cholesterol is found at a lipid raft?
Sphingolipid
Characteristic of a lipid raft
Membrane is slightly thicker and more rigid at rafts
What is the glycocalyx?
The cell coat
Sugar residues associated with the transmembrane and peripheral proteins
Example of glycocalyx
Slime on fish scales
What is the point of the glycocalyx?
- Protects cell against mechanical and chemical damage
- Prevents unwanted cell-cell interactions
What structure makes it harder for bacterium to be engulfed due to an increase in diameter and receptors being covered?
The glycocalyx
What is the enzyme that drives symmetry in the membrane?
Scramblase
What does scramblase do?
Equilibrate lipid
What does flippase do?
Ensures membrane asymmetry is maintained
Which enzyme drives asymmetry in the membrane?
Flippase
Do scramblase and/or flippase require ATP?
Flippase
What 2 (/3) cellular processes does membrane asymmetry have importance?
- Apoptosis
- Coagulation
(- Phagocytosis)
How is asymmetry of membranes used in coagulation?
Phosphatidylserine provides nucleation site on platelets for coagulation cascade
Translocase moves phosphatidylserine to outside of cell
In coagulation, phosphatidylserine provides what on platelets for a coagulation cascade?
A nucleation site
Which enzyme moves phosphatidylserine to the outisde of a cell?
Translocase
What does translocase do?
Transports phosphatidylserine to outside of cell
Which phospholipid provides a nucleation site on platelets in coagulation?
Phosphatidylserine
