Prof E. Smythe Flashcards
What are the properties of plasma membranes?
- Barrier
- Flexible, self-repairing, continuous
- Selectively permeable
- Fluid
When a membrane is very fluid what are the features of its phospholipids?
- Unsaturated (many double bonds)
- Short tails
When a membrane is more rigid what are the features of its phospholipids?
- Saturated (no double bonds, stiff)
- Long
What kind of double bond is usually present in phosolipids?
Cis
What are the 4 main types of phospholipids?
- Phosphatidylethanolamine
- Phosphatidylserine
- Phosphatidylcholine
- Sphingomyelin
Which phospholipid has a negative charge?
Phosphatidylserine
What is the structure of a phospholipid?
Hydrophilic phosphate head and a hydrophobic tail made of 2 fatty acid chains. These are connected by a glycerol molecule.
What is the structure of cholesterol?
- Polar head
- Rigid steroid ring structure
- Hydrophobic hydrocarbon tail
What is the function of cholesterol?
Packs between phospholipids which locally makes the membrane more rigid/ less permeable
How is bacterial membrane composition more simple?
- No cholesterol
- No sphingomyelin
How are phospholipids amphipathic?
Hydrophilic portion and a hydrophobic portion
What 2 structures can phospholipids form in aqueous solutions?
- Bilayer
- Micelle
(Energetically favourable to form sealed compartments in order to shield the hydrophobic portion)
What is an integral membrane protein?
Directly embedded in the whole bilayer via a hydrophobic domain
What is a peripheral membrane protein?
- Only present on the inner/ outer leaflet of the bilayer. - - Associate non-covalently with integral membrane proteins.
- Some have a fatty acid modification which can associate with the bilayer.
What is a microdomain?
Areas of the plasma membrane which differ because of specialised clusters of proteins/ lipids.
What is a ‘raft’ microdomain?
- Specialised microdomain with a high level of cholesterol and sphingomyelin where the membrane is slightly thicker
- Allows longer transmembrane proteins to embed
What are the 3 key features of membranes?
- Asymmetric
- Proteins always have the same orientation
- Lipid composition of the 2 leaflets of the bilayer is different
What is spectrin?
- Protein which forms mesh cytoskeleton in red blood cells
- Peripheral membrane proteins connect cytoskeleton to the membrane
In what scenarios is membrane asymmetry important?
- Blood groups
- Coagulation
- Cell recognition
How is membrane asymmetry important in blood groups?
Blood group is determined by the oligosaccharides attached to sphingomyelin and proteins in the red blood cell membrane and to proteins in plasma.
How is membrane asymmetry important in coagulation?
- Phosphatidylserine is the nucleation site for coagulation cascade
- PS moved from inner leaflet to outer leaflet to trigger clotting
Where is phosphatidylserine usually found in the plasma membrane?
Inner leaflet
How is membrane asymmetry important in cell recognition?
- Phosphatidylserine and phosphatidylethanolamine are transferred to outer leaflet in apoptotic cells
- These are recognised by receptors on macrophages (signal) which can then clear away the apoptotic cells
Which molecules are lipid bilayers highly impermeable to?
Polar molecules and ions
What are the different types of transport proteins?
- Channels
- Carriers
Which transport proteins mediate passive transport?
Channels and carriers
Which transport proteins mediate active transport?
Carriers (using ATP)
What establishes the electrochemical gradient?
Membrane potential and ionic concentration differences on each side of the membrane
Which transport protein is more efficient?
Channels because they only interact weakly with their solute (form an aqueous pore). Carriers directly bind which is slower.
What are the features of ion channels?
- Hydrophilic pores
- Highly specific for their ion
- Can be open or closed (no energy required)
- Rapid transport
- Regulated by ions binding, membrane potentials, ligands
What are the ways that active transport can be mediated?
- Co-transport
- Energy from ATP
- Energy from light
What is symport?
Co-transport where both molecules are travelling in the same direction
What is antiport
Co-transport where molecules are travelling in opposite directions
What limits carrier-mediated diffusion?
Number of carrier proteins in the membrane
What gradient drives transport in mammals?
Na+ gradient
How is glucose transported into the blood from the lumen of the intestine?
- Co-transport with Na+ into epithelial cell via glucose/Na+ symporter on apical surface
- Diffuses into blood via carrier protein on basal surface
- Na+ leaves via Na+/K+ pump (3 Na+ out for 2 K+ in)
- Tight junctions separate apical and basal carrier proteins
What gradient drives transport in bacteria?
H+
Which enzyme makes sure that there are the same amount of phospholipids in each leaflet?
Scramblase
Which enzyme ensures that membrane asymmetry is maintained?
Flippase
What are the flattened sacs of the Golgi and endoplasmic reticulum called?
Cisternae
What are the functions of the ER?
- Quality control
- Synthesis of lipids/ proteins
- Storage
- Detoxification
How is the ER involved in quality control for proteins?
- Associated ribosomes feed the new protein into the ER lumen via a pore as it is translated
- Chaperone proteins fold the protein
- If proteins are misfolded they are fed back out and ubiquitinated which marks it for destruction
What are the functions of the smooth ER?
- Phospholipid and cholesterol synthesis
- Steroid hormone production
- Synthesis and storage of glycerides
- Synthesis and storage of glycogen
- Calcium store
Where are acinar cells found?
Pancreas
How are molecules transported from the ER to the Golgi?
- Vesicles bud off and fuse with the Golgi membrane
- Asymmetry of the membrane is maintained
What is the role of protein coats in vesicle transport?
- The area of the membrane forming the vesicle is coated with specific peripheral proteins which helps cargo selection
- Protein coat is discarded before fusion
How do vesicles make sure they fuse with the correct target?
- Vesicle membranes contain SNARE proteins (revealed when protein coat is discarded) which are complementary to SNAREs on the target membrane
- v-SNARE=on vesicle
- t-SNARE=on target
- Ensures specifity
What are the functions of the Golgi? (3)
- Modification and packaging of secreted proteins (add sugar)
- Renewal and modification of the plasma membrane
- Delivery of material to other organelles
How do proteins move through the Golgi complex?
- Start at cis face, move through to trans
- Different sugars/modification added at each cisterna section
- Sorted at the Trans-Golgi network for secretion, storage etc.
What is opsonisation?
When bacteria are covered in IGG antibodies
Describe the process of phagocytosis.
- Phagocyte receptors recognise IGG proteins on bacteria and bind
- Pseudopods form (arm projections) using the actin cytoskeleton to engulf the bacteria
- No change in cell size because membrane is recycled
What is the difference between macropinocytosis and phagocytosis?
Phagocytosis engulfs solid material whereas macropinocytosis ingests fluids
Describe macropinocytosis.
- Non-specific uptake
- Pathway used by cancer cells to uptake nutrients
- Actin driven ruffles form in a similar way to phagocytosis
What is LDL and what does it do?
Low-density lipoprotein
Transports cholesterol around the body
How is LDL endocytosed by the clathrin pathway?
- LDL binds to LDL receptors on cell surface
- Clathrin-coated pit forms (specialised microdomain)
- Forms clathrin-coated vesicle
- Coat removed and vesicle fuses with early endosome (has a lower pH which causes the LDL to dissociate from its receptor)
- Receptor recycled and returned to cell surface
- LDL hydrolysed in lysosome
What is the structure of clathrin?
- Triskelia
- Doesn’t associate directly with the membrane, binds to adaptor proteins
How do clathrin-coated vesicles pinch off after formation?
Dynamin (GTPase)
What are Rab proteins?
- Regulate endocytic transport
- Associate with different organelles to give them identity
What are early and late endosomes?
- Early endosome recycles and then matures into late endosome
- Late endosome sorts material for degradation and directs it to lysosomes
What is another name for late endosomes?
MVBs= MultiVesicular Body
What do lysosomes contain?
- Lipases
- Hydrolases
