Membrane structure and function Flashcards
Main function of biological membranes
- Barrier between intracellular and extracellular fluid
- Site of biochemical reactions
- Contains transport proteins
- Detect extracellular signals
- Enable cellular communication (e.g. via adhesion/contact)
Who proposed that cell membranes contain lipids, why?
Overton (1890s): Lipid-soluble substances crossed membranes easily; water-soluble did not
→ Concluded membranes contain lipids
3 problems with a simple lipid bilayer model?
- Surface tension
- Electrical resistance
- Solute permeability (e.g., glucose vs. galactose, ion transport)
What did Gorter and Grendel discover?
1925: Extracted lipids from RBCs
Spread on water → SA was 2x surface area of cells
→ Concluded that membranes are lipid bilayers
What model is updated?
Fluid Mosaic Model
Singer & Nicolson (1973)
→ membrane = dynamic bilayer with embedded non-uniform proteins
Key properties of fluid mosaic model?
- Phospholipid bilayer with amphipathic lipids
- Proteins embedded or attached
- Membrane is fluid and asymmetrical
- Selectively permeable
3 types of membrane proteins
1 - Integral [span/embedded in membrane]
2 - Peripheral [loosely attached via protein interactions]
3 - Lipid-anchored [covalently bonded to lipid in membrane]
Why is membrane fluidity important?
- Membrane fusion (e.g., vesicle secretion)
- Equal distribution during cell division
- Cell migration (e.g., immune response)
What factors influence membrane fluidity?
- FA saturation (un = more fluid)
- Chain length
- Temp
- Cholesterol (in eukaryotes)
How does cholesterol affect membrane fluidity?
At 37°C: Makes membrane less fluid (stabilizes)
At low temp: Prevents phospholipids from packing too tightly
What is membrane asymmetry and how is it maintained?
Different lipids on inner vs outer leaflet
Maintained by:
- Scramblases = Random lipid flipping
- Flippases/Floppases: Specific phospholipid translocators
Predominant membrane lipids
Phosphoglyerides
Sphingolipids (all amphipathic)
What forces drive lipid bilayer formation?
Hydrophobic effect: water exclude non-polar tails
Van de Waals forces: stabilise close packing of tails
How is membrane fluidity experimentally demonstrated?
FRAP (Fluorescence Recovery After Photobleaching)
→ Fluorescent lipids recover in bleached area = evidence of lateral diffusion
What is hereditary spherocytosis and its cause?
Mutation in membrane cytoskeletal proteins (e.g. spectrin)
- RBCs lose biconcave shape → become spherical
- Less efficient gas exchange → anaemia
What is the Langmuir trough used for?
To measure SA occupied by lipids
Helped determine membrane = bilayer (Gorter & Grendel)
Why is the phospholipid bilayer self-sealing?
Exposed hydrophobic edges are energetically unfavourable
→ bilayers spontaneously form sealed spheres
What is the amphipathic nature of phospholipids?
Hydrophobic head & hydrophobic tail
→ drives bilayer formation in aqueous environments
What shapes do phospholipids form and why?
Roughly cylindrical shape
→ Promotes formation of flat bilayers that seal into vesicles
What are flippases and floppases?
- Flippases: Move PS and PE from outer to inner leaflet
- Floppases: Move lipids from inner to outer leaflet
→ Maintain asymmetry in plasma membrane
What are scramblases?
Enzymes that move phospholipids between layers non-specifically
→ promote symmetrical growth in ER membrane
What are the three major types of membrane lipids?
- Phosphoglycerides (e.g., phosphatidylcholine)
- Sphingolipids (e.g., sphingomyelin)
- Sterols (e.g., cholesterol in eukaryotes)
What are the characteristics of integral membrane proteins?
- Span the bilayer/deeply embedded
- Hydrophobic regions → interact with lipid tails
- Often aloha-helical/beta-barrel structures
What tools are used to predict membrane-spanning domains?
Hydropathy plots: Analyze hydrophobic regions in protein sequences
→ Predict number/location of transmembrane segments
