Biological Membranes Flashcards
Why do membranes form bilayers?
Phospholipids are amphipathic
What are the four membrane protein types and their examples?
- Tansporters- Na/K pump
- Linkers- cytoskeleton to membrane
- Receptors- for GFs
- Enzymes- adenyl cyclase- catalyses production of cAMP
What are the differences between archaea and eubacteria/eukaryota phospholipids?
- A= branched isoprene chains, E= unbranched fatty acids
- A= ether linkage (C-O), E= Ester linkage (O=C-O)
- A= L-glycerol, E= D-glycerol
What are retinal rod cells made up of?
Discs
How can rhodopsin content in rod cell discs be measured and how much of the membrane protein does it make up?
- Low angle X-ray diffraction of isolated discs
* 80% of disc membrane protein
Why is the diffraction pattern of rhodopsin evidence for the fluid mosaic model?
The pattern is reinforced by the binding of antibodies to the cell membrane.
This pattern changes with temperature- incompatible with planar crystalline lattice structure
How do somatic cell fusion experiments provide evidence for fluid mosaic model?
- Mouse and human cell fused- mix of antibodies
* Incubated to 40*c -> proteins have diffused as membrane is fluid- no longer two clear halves
How do FRAP experiments support the fluid mosaic model?
Fluorescence recovery after photobleaching:
• Certain membrane components marked by fluorescent labels
• Defined membrane areas bleached by laser
• Area will be depopulated with fluorescent components as they’re free to move
What’s the width of the lipid bilayer?
5nm
What is a nascent polypeptide?
Polypeptide still attached to ribosome synthesising it
What happens to cytosolic nascent polypeptides?
- Ribosome associates with ER membrane, cotranslational import into lumen, then transport to destination
- OR remains in cytosol and so does protein product. Post-translational import into organelles can then occur
What is the signal hypothesis?
- First amino acids synthesised are a signal peptide
- Recognises by receptor protein with associated pore on ER membrane
- Signal peptide may later be cleaved
Outline cell fractionation
- Homogenisation- no outer membrane
2. Centrifugation- separates organelles
What do scintillation counters do?
Detect newly synthesised proteins as they are radioactive ^3H
What is a pulse chase experiment?
Exposing cells to a labelled compound and and then to an unlabelled form
• Position of ^3H labelled protein/amino acid tracked by cell fractionation/EM autoradiography
What does the Golgi apparatus do?
- Receives entire output of newly synthesised proteins and lipids from ER. processes in stacked cisternae and distributes inside/outside cell
- modification of polypeptides and membrane lipids
- glycolisation
Which is the trans face of the Golgi?
Exit side of the organelle
Which is the Cis-face of the Golgi?
Recruits from RER
What do scaffold proteins do in the Golgi?
Maintain the polarised stacks of cisternae
What is constitutive exocytosis?
Mucus and glycoproteins of extracellular matrix
What is regulated exocytosis?
- Neurotransmitters
* Zymogen granules (storage organelles for digestive enzymes)
What are the requirements of a good experimental cell system?
- Grow in lab under simple conditions
- Grow on agar for single cell manipulation
- Genetics- ready isolation of mutants
- Express asymmetry of from- polarised
Eg budding yeast
What do SNARE proteins do?
Matching pairs on vesicles and targets
v-SNAREs and t-SNAREs
How is directional flow of vesicles ensured?
Ran proteins and NTP hydrolysis- prevents anything other than correct sequence of events
What is the plasmalemma?
The plasma membrane that bounds a cell
