Membrane Structure Flashcards
What is a cell membrane?
- Structure that gives cells it shape.
- Separates extracellular environment from intercellular environment.
- Dynamic in shape and keeps changing
- Allows transport of substances into and out of the cell
- In eukaryotes each organelle has a membrane
- Has multiple functions: signal transduction, cell communication, complex reaction sequences, energy transduction
What is a cell membrane made of?
- phopholipid bilayer
2. proteins
What is the phospholipid bilayer?
Made up of phospholipids. They had a hydrophillic head with a polar group. Also have a hydrophobic fatty acid tail. The head interacts with the extra/inter cellular environments.
The tails provides structure.
Has two leaflets, the outer and inner.
What are Lipids?
Building blocks of membranes.
Non-covalently linked.
Insoluble in water.
Amphipatic - head and tail
What are the different types of lipids?
- Fatty Acids
- Glycerolipids
- Glycerophospholipids
- Sphingolipids
- Sterol Lipids
- Prenol Lipids
- Saccarolipids
- Polyketides
Function of Lipids?
- Structural - membrane components and protein modification.
- Metabolic - energy storage
- Other Functions - cell signalling, hormones, enzyme cofactors, electron carriers, pigments
Effects of chain length on lipids?
The longer the chain the higher the melting point.
The longer the chain = more stable = more rigid.
Solubility in water will decrease as chain length increases.
Effects of double bonds on lipids?
Lipids contain long chains of carbon, therefore there can sometimes be double bonds.
An increase in the number of double bonds decreases the melting point.
Double bonds usually found in cis configuration. Causes kink in chain and unable to pack in as tightly.
Difference between saturate and unsaturated fats?
Saturated fats have no double bonds and are packed in tightly. Have a higher melting point than unsaturated fats.
Unsaturated fats have lipids that contain double bonds. The double bonds cause kinks so the lipids aren’t able to pack in as tightly. This causes a lower melting point than saturated.
Structure of Glycerophospholipids
- Glycerol backbone
- fatty acid tails often unsaturated
- Phosphate and alcohol head group - OH on head group is polar
Glycerophsopholipid head groups?
Serine Ethanolamine Choline Glycerol Inositol
What is the importance of glycerophospholipids?
Structural component for cell membrane - forms bilayer
What are Sphingolipids?
- Major membrane components
2. Derivatives of the amino alcohol sphingosine
Importance of sphingolipids?
- Forms a mechanically stable and chemical resistant outer leflet of the cell membrane.
- Linked to cell signalling and cell recognition
What are steroids?
Mostly of eukaryotic origin.
Most common in cholesterol (a sterol)
Cholesterol is a mjor component of cell membrane.
Has a small head group able to easily bind/interact with membrane
What are the properties of a cell membrane?
- Flexible
- self-sealing
- selectivly permeable
- two dimensional
What is the Fluid Mosaic Model?
- Every membrane differs in lipid composition. Typically have asymmetrical distribution between leaflets except for cholesterol.
- Lipids are in constant motion
- Proteins can move laterally
- Microdomains - within a leaflet different sections with varying distributions of lipids. Sphingolipids and cholesterol form lipid rafts which are involved in signalling proteins
What are the different formations of biomembranes?
- Micelle - individual lipids are wedged shaped and they form a single layer ball.
- Bilayer - most common, two layers of cylindrical lipids, flat
- Vesicle - bilayer but forms a small globular structure
Stability of different formations of biomembranes?
Determined by: size of chains, degree of saturation, size of head group and temperature
How is the bilayer held together?
Ionic bonds between the head groups.
Head groups form hydrogen bonds with water in surrounding envirnment.
Van der Waals interactions between fatty acid tails.
How can lipid bilayers be made in the lab?
- treat cells with solvent to remove proteins
- disperse left over phospholipds in water
- liposome will form
What are the different type of lipid motion in the bilayer?
- Spinning - they can rotate without changing location
- lateral diffusion - move within the same leaflet. This occurs 10^7 times per second.
- transverse diffusion - flip flopping between different leaflets this is very rare
How can we show lateral diffusion occurs experimentally?
- Label cell surface with fluroescence
- Laser beam a patch on the cell surface
- Wait for lipids to start moving
- observe blank patch now contains some fluorescent labelled molecules
What causes bilayer to go from gel like to fluidlike?
Heat disorders the interactions between fatty acids tails
How do lipids effects membrane properties?
- Fatty acid chains effect fluidity/rigidity by length and conformation
- Thickness, different type of lipids can alter the thickness of membrane
- Head type effects curvature. Head and tails similar forms flat membrane (PC). Head small and tail large forms curved membrane (PE)
What aspects of membrane function require curvature of membrane?
- viral budding
- formation of vesicle
- stability of curved structure think microvili in small intestine
What causes translocation of lipids?
Enzymes catalyse movement of lipids.
Flippase: moves lipid from outside to inside
Floppase: moves lipid inside to outside
Scramblase: moves lipids in both directions
Basics of membrane proteins
- Located in/on membrane bilayer
- different membranes have different compositions, lipid:protein ratio
- < 1/3 genes code for membrane proteins
Functions of membrane proteins?
- transporters
- receptors
- adhesion molecules
- lipid synthesis
- energy transduction
What are the three types of membrane proteins?
- Integral
- Lipid-anchored
- Peripheral
Features of integral protein
- Firmly associated with membrane - require harsh detergents and organic solvents to remove them.
- Span the membrane - inside and outside of bilayer. Hydrophobic interactions spanning the membrane and hydrophilic surfaces in the extramembrane domains
Features of lipid-anchored proteins
- Protein covalently linked to a lipid
- Lipid is embedded in the membrane.
- the protein does not enter the bilayer
Features of a peripheral protein?
- Released with mild treatment
- does not contact core of bilayer
- hydrophilic interactions with membrane surface or other proteins
Structures of intergral proteins?
- transmembrane alpha-helices
2. transmembrane beta-sheet (beta-barrel)
Structure of alpha-helix
Backbone held together by hydrogen bonds. Side chains of amino acids.
Embedded in the hydrophobic core.
Interactions between hydrophobic amino acids side chains and fatty acid tails.
Ionic interaction between head groups.
Perpendicular to the plane of the bilayers.
Structure of Beta-barrel proteins
- Anti parallel to bilayer.
- Form pores (porins)
- Some side chains face the membrane and are hydrophobic.
- Other side chains face internally creating the pore which is hydrophilic - allows water though.
- Not stable
- makes a polar section of the membrane
Can proteins move in bilayer?
Proteins can move laterally however not as frequently/easily as lipids.
Proteins unable to flip-flop.
Importance of transmembrane transport
Some molecules can’t diffuse through the bilayer.
Cells require some of these molecules that can’t get through to function.
What is the membrane permeable to?
- gasses
2. small unchagred polar molecules e.g.
What is the membrane not permeable to?
- ions
- large, uncharged molecules
- charged polar molecules
Order of permeability easiest to hardest
- gasses (CO2, N2, O2)
- small uncharged polar molecules (Ethanol, water partially)
- large uncharged polar molecules (Glucose, fructose)
- Ions (K+, Ca2+, CL-)
- Charged polar molecules (AMino acids, ATP, glucose, proteins)
What are the different transport types?
- Diffusion
- Facilitated diffusion
- primary active transport
- secondary active transport
- Ion channels
Types of transporter proteins
Uniporter: allows transport in one direction
Symporter: allows two different molecules into/out of the cell at the same time (one direction).
Antiporter: allows one molecule in and one molecule out at the same time
What is passive transport?
Simple diffusion across a cell membrane. Concentration wants to be the same on both sides therefore solutes will move to reach and maintain equilibrium (Chemical gradient).
Electrically charged molecules will move to maintain electrochemical gradient.
What are the two classifications of transporters?
- Carriers. Have a high specificity but are slow.
2. Channel. Have a low specificity but are fast.
What is active transport?
Transport of a molecule across the cell membrane that require energy typically as it is acting against the concentration gradient.
Can get energy from chemical reaction such as ATP hydrolysis or through coupled transport
Classes of ATP-Pumps
- P-Class
- V-Class
- F-Class
- ABC Superfamily
What is a P-Class pump?
Proton Pumps, can be used in ATP synthesis.
Found in plants and fungi (H+ pump), mammalian stomach (Na+/K+ pump).
Cation transporters.
Phosphorylated on Asp as part of cycle.
Have a Transmembrane domain with three other domains (A, P and N) branching off in the inter cellular environment.
What are F and V Class pumps?
Driven by ATP hydrolysis.
Has a rotating component withing the membrane (F0/V0). Other compenent is external from the membrane (F1/V1).
What are ABC transporters?
Pump amino acids, peptides, protein, metal ions, lipids, and compounds (drugs).
Uses ATP.
Has a transmembrane component and component within the cytosol (NBDs).
What is the H+/lactose co-transporter?
Secondary transport.
Lactose goes into cell.
What are Aquaporins?
Transport water across membrane.
Changes in osmotic pressure causes cells to shrink/swell.
Only allows single water molecules through not whole chains.
What are Ion Channels?
Differ from transporters allows unrestricted diffusion. Usually have a ligand gate to stop movement.
Only open for a ms - very fast.
Proton clamp can measure a single ion channel.
What is the role of voltage and ligand gated ion channels in neurons?
Important for muscle contraction. AP in neuron opens Ca2+ channel. This opens nicotinic acetylcholine receptor.
Depolarisation causes voltage-gated Na+ channel to open - generates AP.
What are some consequences of defective ion channels?
Musce stiffness/paralysis.
Cystic fibrosis
Toxins target ion channels.
