Permeability & Transport Flashcards
What is the primary function of the plasma membrane ?
- Fat containing barrier that separates the internal chemical components of the cell from the external environment
- No membranes = no cells = no life!
- Import of nutrients/molecules, export of waste/molecules
- Relay of cell signals through ‘sensor’ proteins
- Permits motility through flexible size and shape
What is the common structure of the cell membrane ?
- Two closely opposed sheets – lipid bilayer
- Interspaced with membrane spanning proteins with varying functions
Membrane lipids are ?
Amphipathic
What is the most abundant lipids in membrane ?
Phospholipids
Structure of phosphatidylcholine ?
Choline joined to glycerol via phosphate with two hydrocarbon chains
what does a double bond between two carbon atoms produce ?
‘Kink’ in hydrocarbon chain – important for membrane fluidity
Hydrophilic molecules contain charged atoms or polar groups with uneven distribution of positive and negative charges. Charged atoms can form ?
Favourable electrostatic or hydrogen bond interactions with water
Hydrophobic molecules contain uncharged atoms or non polar groups. Therefore, cannot ?
Form favourable interactions with water
Lipid bilayer is a 2D fluid. Explain the movements ?
- Molecules move rapidly within a layer – thermal motions
- Exchange of phospholipids between layers ‘flip-flop’ is rare
- Crucial for membrane function and integrity
How do we know that there is membrane fluidity?
FRAP = Fluorescence recovery after photobleaching
What is a membrane fluidity factor ?
All about how close and regular are the packing of the hydrocarbon tails:
• Regular and close = less fluid
• Irregular and loose = more fluid
What is an another membrane fluidity factor?
The tail length
- Long tail length = less fluid
- Short tail length = more fluid
What is an another membrane fluidity factor?
Double bond number
- Less double bonds = less fluid
- More double bonds = more fluid
What is an another membrane fluidity factor?
Cholesterol content
- Fill spaces left by kinks in unsaturated hydrocarbon tails
- Increases membrane rigidity and reduces permeability
- Also roles in cell signalling (lipid rafts)
Explain how the membrane assembly begins in ER ?
- ER synthesises new phospholipids from free floating fatty acids
- New phospholipids deposited exclusively in cytosolic layer
- Phospholipids transferred to opposite layer through scramblases
- Some new membrane retained in ER, rest transferred to other membranes
How is asymmetry maintained in the bilayer?
- Golgi membrane contains enzymes called flippases
- Flippase enzymes maintain bilayer asymmetry
- Catalyses the transfer of specific phospholipids from exterior to interior layer
Protein directly attached to lipid bilayer – integral membrane proteins – can only be removed by?
Disrupting with detergents (e.g. Triton X-100, SDS)
Remaining proteins known as peripheral membrane proteins – removed with ?
More gentle extraction procedures
Explain the Membrane reinforcement – Cell cortex?
- Membranes thin and fragile – require support
- Support framework provided by meshwork of fibrous proteins
- Attached to cytosolic surface of membrane – affects shape, strength and motility
What is a glycoprotein ?
Protein with bound oligosaccharides
What is a proteoglycans?
Other proteins with one or more polysaccharide chains
All sugar containing molecules on outside of cells form carbohydrate layer. Why is this crucial ?
Crucial for lubrication (motility, protection) and cell recognition
How would you avoid cell damage by electrical damage ?
Electrical charge must be balanced on both inside and outside of cell
Briefly explain a membrane transporter proteins ?
- Undergoes conformational changes
- Molecules/ions bind sites
- Highly selective
Briefly explain a membrane ion proteins ?
- Discriminates solutes on size and charge
- Forms pore across bilayer in open or closed confirmation
- Opening controlled by external stimulus or cell conditions
Explain the different transporter mechanisms ?
- Direction of transport often depends on relative concentrations of solute
- Simple diffusion - molecules naturally flow down a concentration gradient
- Passive transport (facilitated diffusion) – molecules move down a concentration gradient facilitated by channels and transporters
- Active transport – molecules move against a concentration gradient in energy- (ATP) dependent process using transporters (pumps)
Simple diffusion – Oxygen/Carbon dioxide diffusion
- Simple movement of O2 or CO2 to areas of lower partial pressure (reciprocal directions)
- Particularly important on alveoli membranes where gaseous exchange takes place
Passive transport – Glucose
- Transporter crosses membrane 12 times
- Changes in conformational state facilitate glucose binding, transport and release
- Highly specific for glucose isoform (D-glucose, not L-glucose)
- Important in liver cells where glycolysis takes place
Active transport – Na+-K+ pumps
- To maintain osmotic balance and correct electrochemical gradient Na+ and K+ ions need to be pumped against a massive concentration gradient
- Outward transport of Na+ coupled with inward transport of K+ requiring conformational change
- ATP hydrolysed to ADP + phosphate – pump serves as ATPase
