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
Co - translational translocation into ER
- requires an ER signal sequence
- large + small ribosomal subunits come together and bind to mRNA bound
- complex can then interact with translocator complex (closed until ribosome binds)
- protein passes through translocator until fully made, signal sequence is then cleaved off by signal peptidase
- mature polypeptide chain is then released into the ER lumen
- ribosome released and translocator closes
Polysomes
- a single mRNA can be simultaneously bound by several ribosomes at a time
- sucrose gradient separates mRNA with different numbers of attached ribosomes
Protein glycosylation in ER
- glycosylation = the addition of complex sugars to proteins to give them different functions
- lipid linked sugar anchored in membrane interacts with enzyme causing the sugar to be cleaved from the lipid and added onto the protein
- side chain sugars are added/removed/trimmed in the Golgi
Features of the Golgi apparatus
- major site of carbohydrate synthesis e.g. pectin in plants and glycosaminoglycans in animals
- further glycosylation of proteins
- plays part if the secretory pathway - sorts and dispatches proteins made in the ER
- made of interconnected, flattened cisternae
- cis face + trans face through medial cisternae
Movement from ER to Golgi
- bidirectional movement
- vesicles labelled with COPII proteins which act as signals and direct them from ER to the Golgi
- COPI protein directs vesicles back to the ER e.g. if incorrectly folded
Movement from CIS Golgi to ER
- proteins that should be in ER have KDEL at the end
- COPII vesicles shed COPII coat and fuse to vesicular tubular clusters, then fuse with CIS Golgi
- within the Golgi there are KDEL receptors which retrieve the protein and send it back to the ER
2 mechanisms for movement of proteins through Golgi
1) Cisternal Maturation - cis face of Golgi matures and becomes the trans face, taking the proteins with it
2) Vesicles transport - budding off of vesicles from Golgi which transport proteins as they bind to the next cisternae
- could be both mechanisms
Complex glycosylation of proteins in Golgi
- different enzymes are found in different parts of the Golgi - all have different roles in glycosylation
- sequence of amino acids determines what happens to the protein and what it is like after glycosylation
