Cell Membranes & Endomembrane System Flashcards
Why are cell membranes needed?
- define boundaries
- enclose organelles
- generation of electrical signals
- cell signalling
- attachment sites
What is the endomembrane system?
Internal membrane delimiting organelles
Features of the lipid bilayer
- made of hydrophilic heads (polar, choline + phosphate + glycerol) and hydrophobic tails (non-polar, fatty acid, contain 14-24 carbons in length, contain cis double bond which effects membrane packing)
- contains embedded + attached proteins
- contains cholesterol to establish stability and fluidity of the membrane
Function of lipids in the cell membrane
- 50% of membrane mass
- self organising, form spherical shapes to protect fatty acid tails
- form micelles ( 1 layer) when lipids are in low concentration
Cholesterol in cell membranes
- polar hydroxyl heads sit close to polar heads of the bilayer
- only found in membranes of animal cells
- affect membrane fluidity:
At low temperatures, spacing is increased between hydrocarbons, increased fluidity
At high temperatures, hydrocarbon tails are pulled together, decreased fluidity
How does temperature affect membrane fluidity?
- increased temperature, hydrocarbons are less packed, membrane is more fluid
- decreased temperature, hydrocarbons are more packed, membrane is less fluid
- at very low temperatures the membrane can enter a crystalline state
Other factors affecting membrane fluidity
- lateral movement - phospholipids rapidly moving laterally within the plane
- flip-flopping - movement of phospholipids between planes (very rare)
- rotation of phospholipids
- flexion - movement of hydrocarbon tails
- cis double bonds - unsaturated, more spread out as chains are more difficult to pack
What are lipid rafts?
- areas enriched with cholesterol and sphingolipids form rafts and move laterally - can pick up proteins
- means that not all areas of the membrane have the same mobility
- roles include cell signalling + uptake of extracellular molecules
What is the nuclear envelope and what are its functions?
- contains and inner and outer lipid bilayer
- controls what enters and exits the nucleus via the nuclear pores
- selective movement in and out
What are nuclear pore complexes (NPCs)
- embedded in the membrane of the nucleus
- bidirectional movement
- octagonal symmetry
- 3000-4000 per cell
- transport 500 macromolecules per second
- Movement in = DNA & RNA building blocks, molecules to provide energy, ribosomal proteins
- Movement out = ribosomal subunits, synthesised by nucleolus
Nuclear localisation signal (NLS) & nuclear export signal (NES)
- NLS = amino acid sequence that tags a protein fir entry to the nucleus
- NES = amino acid sequence that tags a protein for exit from the nucleus
- both recognised by NPCs
- small proteins move freely
- larger proteins move slower
Features of the endoplasmic reticulum
- Rough - protein synthesis in ribosomes
- smooth - lipid synthesis
- storage of Ca2+ - involved in cell signalling
- hormone production
- conversion of glycogen to glucose
Co - translational protein import to ER
- ribosome bound to ER membrane
- protein imported to ER lumen as it is translated
- transmembrane proteins are only partially translocated across the membrane
- water soluble proteins fully translocate across the membrane
After co - translational import to ER
IN ER:
IN ER:
- mRNA bound by ribosome
- 2 ribosomal subunits bind together then to mRNA
- complex then binds to translocation machinery embedded within the ER
- ribosome moves along the mRNA, translating it, making 1 amino acid at a time (continuous signal allows the protein to pass through as it is being translated)
Co - translational protein import
IN OTHER STRUCTURES e.g. mitochondria
(POST TRANSLATIONAL)
IN OTHER STRUCTURES:
- Ribosome binds to mRNA
- translates it to a protein
- different signal sequence which also binds, allowing the protein to pass through
- signal sequence is cleaved off once protein has been transported
