Boundaries & membranes Flashcards
What do cell boundaries allow for?
- Formation of suitable semi-fluid matrix (protoplasm) for cell functions (life)
- Formation of specialised microenvironements where concrete solutes dedicated cell processes happen
- Segregation of harmful activities
What is the principle boundary for a cell?
The exterior plasma membrane
How do membranes help eukaryotic cells?
They have internal boundaries which allow compartmentalisation
What are membranes composed of?
They are sheet-like structures composed of lipids & proteins in varying ratios
Also contains carbohydrates
What is the structure of a membrane called?
lipid-bilayer
Composed of two closely touching lipid sheets - very thin 5-8nm
What are membrane lipids like?
Amphipathic
Cont hydrophilic & hydrophobic parts
Effects how molecules orientate in an aqueous environment - leads to formation of lipid bilayer
What is the most common phospholipid in eukaryotic cell membranes?
Phosphatidyl choline
Name 3 types of membrane lipids
- Phosphatidylseriene (a phospholipid)
- Cholesterol (a sterol)
- Galactocerebroside (a glycolipid)
What role does cholesterol play in the membrane?
It inserts itself in the membrane and stiffens it
What is the structure of a cholesterol molecule?
- Polar head group
- Rigid planar steroid ring structure
- Nonpolar hydrocarbon tail
What replaces cholesterol in fungi membranes?
Ergosterol
What is the ‘fluid-mosaic’ model?
Contains INTEGRAL proteins penetrating lipid bilayer floating in ‘lipid sea’
Also PERIPHERAL proteins outside lipid bilayer linked by non covalent bonds
What are the key structural features of a lipid bilayer?
- Self assembling
- Lipid layers are fluid
- Asymmetry
- Integral proteins pass through the lipid bilayer
What does it mean that lipid bilayers are ‘self-assembling’?
They can repair tears in the membrane
What does it mean that lipid bilayers are fluid?
Lipids may move 2um in 1 sec!
However, proteins have more restricted movement & some held in fixed positions by cytoskeleton, ‘rafts’ & membrane ‘domains’
What does it mean that lipid bilayers are asymmetrical?
Inner & outer lipid layers have different components & lipid composition
Lipids rarely cross monolayers unless under action of ‘scrambles’ or ‘flippase’ enzymes
How is the structure of a lipid bilayer linked to membrane functions?
- ‘Semi-permeable’ barrier to movement of solutes/gases
- Transporting solutes
- Flexibility for cell movement & division, membrane fusion
- Respond to external signals
- Intercellular interaction
Describe how the ‘semi-permeable’ membrane works?
- Barrier to movement of solute/gases
- Small non-polar molecules (O2, N2, CO2) & lipid soluble pass thru EASILY
- Ions & larger polar molecules (amino acids, sugars) FAIL
- Water fairly rapid diffusion, ASSISTED by ‘aquaporin’ channels
Allows maintenance of inteerior & compartmentalisation
Describe how the bilayer’s structure allows for transporting solutes?
Integral proteins provide means of accumulating sugars/amino acids/ions –> act as channels often via active processes
Describe how the bilayer’s structure allows for it to respond to external signals?
Protein receptors in membrane respond to external simuli
Describe how the bilayer’s structure allows for intercellular interation?
Carbohydrates on membrane surface provide cell identity, used in self recognition
How can cells adjust their membrane fluidity?
By adjusting their lipid composition
e.g. cells grown at lower temperature have more lipid with double bonds in tails to maintain fluidity
How can the membrane compostiion vary according to its biological function?
Inner mitochondrial membrane has high protein content –> due to transport processes
Whereas nerve myelin sheath has high lipid content –> needed for electrical insulation
Why are cells walls sometimes needed?
Plasma membrae is very thin –> often needs additional support/protection
–> cell walls MUST have mechanisms to allow permeability
