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Internal membrane compartments vastly outweigh the cell plasma membrane, T or F



What is the role of Rab proteins in exocytosis

Rab proteins identify target membranes for fusion and Rab-GTPs play a role in tethering and docking of vesicles to the membrane


Explain the mechanism of action of the botulinum toxin

Botulinum binds first to gangliosides on neuronal membranes. It then enters the luminal space of recycling synaptic vesicles and following endocytosis, one subunit, the SNARE Protease escapes the vesicle and enters the synaptic cytosol. Here the SNARE protease subunit cleaves a specific SNARE protein. The cleaved SNARE protein cannot support the fusion of synaptic vesicles which results in a long blockade of neurotransmission


SNARE proteins help to overcome the force the prevents membrane fusion, what are the two main types of SNAREs

Vesicular or v-SNAREs such as synaptobrevin are found on the vesicle membrane. Target SNAREs or t-SNAREs are found on the target membrane of the compartment


From which organelle(s) does the COPI coat allows the budding of vesicles from

The Golgi cisterne (or cis-Golgi)


Give an example of endocytosis in nutrient uptake

The protein portion of LDL is recognised by LDL receptor on the cell surface which binds with high affinity to LDL. The adapter molecule called adaptin binds to the intracellular domain of the LDL receptor. Adaptin recruits clathrin molecules which binds to it and coats the inside of the membrane. Assembly of the clathrin coat causes the membrane to invaginate. This forms a vesicle that buds off the inside of the cell taking the LDL receptors and the bound LDL with it along with clathrin and adaptin. Inside the cell the clathrin coated vesicle uncoats and fuses with the endosome. The endosome has an acidic pH which causes the LDL receptors to release the LDL cargo. Empty LDL receptors are recycled back to the plasma membrane in recycling vesicles that bud off from the endosome. Meanwhile the endosomal content delivered to the lysosome where hydrolytic enzymes digest the LDL releasing cholesterol, amino acids and small peptides


Give an example of a human disease characterised by defects in an endocytotic mechanism

Defects in endocytosis can cause atherosclerosis. This occurs due to mutations in LDL receptor account for familial cases of atherosclerosis. This results in an accumulation of lipoproteins in blood and the formation of atheromatous plaques which block arteries


Phagocytosis is a different type of endocytosis, how is it classified and why

Phagocytosis is an example of actin-driven vesicle formation. This is because actin drives membrane engulfment in phagocytosis by forming a pseudopod around the bacterium


Newly-synthesized ER lipids and proteins are packaged into COPI vesicles which snip off the SER, T or F

F – these vesicles are COPII coated


Explain how SNAREs act to force membrane fusion

SNARE proteins form a tight 4-helical coiled-coil with hydrophobic faces (containing leucine, valine etc.) coming together away from the cytosol. This coiled-coil structure acts as a zipper all the way to the target membrane and the force produced by SNARE protein coiling into two opposing membranes is translated and forces their fusion


Describe the process of lipid synthesis in the SER membrane

Fatty acid binding protein delivers a fatty acid to the plasma membrane. Here, CoA transferase allows synthesis of two fatty acids at the glycerol head group. A phosphatase then releases the phosphate bound to the glycerol molecule which allows for hydroxylation. This hydroxyl group is then replaced by the lipid head group (i.e. choline, serine, ethanolamine).


What is seen in mutants in dynamin

Mutated dynamin cannot hydrolyse GTP and thus cannot pinch off endocytic vesicles


How do the botulinum and tetanus toxins lead to muscular paralysis

Botulinum and tetanus toxins attack SNARE proteins with high specificity. SNAREs are responsible for acetylcholine release at the neuromuscular junction


Which subtypes of SNAREs are Syntaxin and SNAP25

Syntaxin and SNAP25 are t-SNAREs


Exocytosis is responsible for secretion of hormones, digestive enzymes, recycling of plasma membrane receptors and neuronal communication. What are the 2 exocytotic pathways and how do they differ

Constitutive exocytosis is always occurring with little to no regulation and occurs in non-polarised cells. In contrast, regulated secretion involves the accumulation and storage of a molecule before a signal-triggered release


How is botulism and tetanus contracted

Botulism happens upon consumption of contaminated food whereas tetanus infections can happen after skin cuts, during childbirth or dirty needle injections


Other than the cell membrane, what structure do clathrin coats mediate the budding of vesicles from

The trans-golgi network


Which type of vesicle coat allows the budding of vesicles from the endoplasmic reticulum



When are LDL receptors actively synthesised

When blood cholesterol is low


What is the role of adapter proteins in endocytosis

Adapter proteins such as adaptin link selected cargo to the clathrin lattice


What different roles do the SER and RER have

The RER is involved in protein synthesis whereas the SER is responsible for lipid synthesis and the formation of vesicles


What is the problem that prevents the fusion of vesicles with the target membrane

The target membranes and the membrane of the vesicle are negatively charged due to the presence of phosphatidylserine. This causes a repulsion between the two membranes that prevents fusion


Insulin release is an example of one type of exocytosis. Which type is it and how does it work

Release of highly-concentrated insulin from pancreatic beta cells is an example of regulated exocytosis and happens only in response to high glucose. Insulin is tightly packaged inside secretory granules before secretion. Vesicles move from compartment to compartment with excess lipid material being removed and recycled back to the golgi. This acts to concentrate insulin so much that it is nearly crystalline. Upon release, insulin binds to its receptor and triggers delivery of glucose transporters into the plasma membrane of muscle cells


Give examples of cells that are phagocytotic

Neutrophils and macrophages


How long does the paralysis caused by botulinum toxin last for if untreated

4-6 months


What is autophagy and how does it work

Autophagy is the third pathway towards lysosomal digestion. It acts to help eliminate malfunctioning cell elements and occurs via vesicle fusion and engulfment of organelles


What is the name of the mechanism by which LDL is transported into cells

Receptor-mediate endocytosis


Outline the process of phagocytosis

The microbe adheres to the phagocyte which forms pseudopods that eventually engulf the particle. The phagocytic vesicle is then fused with a lysosome forming a phagosome. The microbe within the vesicle is killed and digested by lysosomal enzymes within the phagosome leaving a residual body. The ingestible and residual material is removed by exocytosis


What are the effects of the tetanus and botulinum neurotoxins produced by some bacterial infections

They can result in complete neuromuscular paralysis


What is the role of dynamin in endocytosis

Dynamin is a large molecular weight GTPase that is required for coated pit scission to form coated vesicles during endocytosis. GTP hydrolysis allows the vesicle to bud off from the membrane