AH Lectures 1-5 Flashcards
What is exocytosis?
Transport of proteins and lipids that are made within the cell to the exterior of the cell –> Via vesicle membrane fusion with P.M.
Note –> When vesicles are released from elsewhere in the cell this is called vesicle budding rather than exocytosis
What two categories can exocytosis be divided into? Outline how each of the two types work.
- One type happens by default without signals –> constitutive exocytosis –> happen all the time without signal (almost by default) –> E.g. mucus in the lung lining.
- The other requires signals to occur –> non-constitutive exocytosis –> Pool of vesicles wait by the membrane and only undergo fusion in response to a signal –> i.e. synaptic vesicle fusion between two nerve cells –> this process is Ca2+ dependent –> It interacts with PM proteins to trigger snare entwining –> Reserve pool waiting behind those for the next impulse.
Regulated secretory vesicles are warped along microtubules using motor proteins –> Primed e.g. snares touching and just waiting for a calcium influx
Number two –> Especially relevant for specialised secretory cells e.g. hormones (insulin), digestive enzymes, neurotransmitters
What is the function of exocytosis?
Functions:
- Releasing proteins into the extracellular space
Whatever is in the lumen of the vesicle will be exposed to the exterior to the plasma membrane when it fuses.
- Insertion of membrane proteins into P.M –> proteins within the membrane of the vesicle will add to the P.M.
- Vesicles fusing with P.M is also important for cell growth –> when a cell grows the membrane has to get bigger so exocytosis can insert more lipids and proteins
Explain the process of vesicle formation.
The process is not well understood –> Domains in lipid P.M components are thought to be involved e.g. lipid rafts used to concentrate proteins.
However, there are a couple of key processes that tend to occur before exocytosis:
- Vesicle Maturation: removal of clathrin as it can stop fusion to PM, this is recycled then reused elsewhere.
- Vesicles are constantly taken back –> to replenish Golgi –> otherwise it will shrink.
- Cargo within vesicle tends to be highly concentrated –> maximise transport
- Active processing proteins –> Cleavage can activate the protein just before it is released –> E.g. cleavage of pro-peptides at the N terminal.
- Kiss+run –> when a little bit of fusion occurs and vesicle content is released –> the vesicle can then be used again –> Key in the immune system, especially mast cells.
Can exocytosis be localised? How is this achieved?
Exocytosis can be localised or all-around PM
Polarised cells have a direction –> Mainly controlled by cell adhesion blocking off parts of the cell (preventing exocytosis in particular regions) or cytoskeleton leading vesicles only to certain ends.
Example of this:
- Nerve cells –> exocytosis only occurs at the synapses.
- Epithelial cells –> the sides can be fused through tight seals in cell adhesion –> forces the movement of the content from the bottom of the cell upwards.
What is endocytosis?
Endocytosis is a cellular process in which substances are brought into the cell.
What are the three main types of endocytosis?
- Pinocytosis –> small molecules suspended in a fluid are brought into the cell (drinking).
- Phagocytosis –> cell eating and taking in larger material. E.g. recycling of material/ removal of apoptotic cells
- Receptor-mediated –> ligand has bound to specific receptor and cell brings it in to pass on signal or down regulate signal.
Difference between micro and macro endocytosis?
Endocytosis which is on a small scale with small vesicles –> micro
Endocytosis which is on a large scale with larger vesicles –> macro
Macro is present in ‘hungry’ cells which could be a sign of cancer.
How does cell know what type of endocytosis to use?
Different cell types will use these types in different ways –> This may change depending on whether the cell is dividing, starving or becomes cancerous.
Role of endocytosis in the immune system?
Important for immune system –> method used for surveillance of surroundings –> what is happening to the surrounding tissues –> if pathogen present –> macrophages can engulf them using phagocytosis.
However, Pathogens have also evolved ways to use endocytosis to enter the cells.
What are the characteristics of pinocytosis?
- Fluids and solutes are taken up
- Small vesicles about 100nm
- Most eukaryotic cells do it continuously without signals —> continuous monitoring of environment or boosting nutrient uptake (epithelial cells) –> This is constitutively
- Can remove damaged membrane?
Outline the process of vesicle formation –> i.e. when a vesicle is formed during endocytosis.
Two key proteins –> Clathrin and Caveolae
- Clathrin forms coats and allows the vesicles to form and take in extracellular fluid –> once vesicle is formed –> coat is shed to form a naked vesicle.
- Caveolae don’t form coats –> They are made up of caveolins and cavin proteins and are present in the PM of most cells, mainly in lipid rafts –> Responsible for PM bending into flask like shapes –> Caveolae are not shed.
- Lastly –> Dynamin wraps around the neck of the vesicle and pinches it off
- After this the vesicle can fuse with endosomes –> Endosomes can be classified into early/ late/ recyclable depending on how long they have been present in the cell and the markers present.
Definition of Transcytosed?
Transcytosed = taken right across the cell –> E.g. the epithelial cells in the gut, inside to the circulatory system.
Apart from vesicle formation what other role do caveolae play?
Cells use caveolae to know if they are being stretched or squashed
For example:
- Caveolin helps the cell know if it is being stretched –> Cavin proteins popped off –> to signal the inside of the cell –> response could increase cytoskeleton around the area to resist changes
Why is it difficult to prevent viruses from entering into the cell?
What are the characteristics of phagocytosis?
Phagocytosis
- Taking up larger material + some fluid will enter
E.g. microorganisms and dead cells
- Contents are broken down to make it safe for the cell and recycle
- Size >250nm phagosomes
Outline what ‘professional phagocytes’ (macrophages and neutrophils) do with the phagosome once it enters the cell.
Macrophages have a role in wound healing and regeneration response
- Contents are taken in via endocytosis
- Phagosome fuses with a lysosome
- Enzymes within the lysosome to break down the material.
- Permeases will allow molecules to be reused in the cytoplasm.
- Residual R bodies will be removed via exocytosis
Outline the characteristics of receptor-mediated endocytosis.
- Involves a ligand binding to a receptor in order to trigger endocytosis
- Receptors are concentrated within a particular area –> Sometimes this has already occurred in a lipid raft.
- This type of endocytosis is used when specific molecules/substances are required from circulation.
- All receptors to undergo this process use clathrin
- Sometimes a mix of cargo can be present in a single vesicle.
Explain step by step the process of receptor-mediated endocytosis using LDL cholesterol as an example.
LDL cholesterol –> cholesterol associated with a protein –> so that it can be transported through the blood.
- Protein portion recognised by LDL receptors
- Adaptin binds to the inside of the receptors
- Adaptin recruits clathrin which coats the membrane
- Bending of membrane and formation of vesicle
- Inside the cell it uncoats and fuses with endosome
- Low internal pH causes LDL receptor to release cargo
- Returned in a vesicle to the PM?
- Cycle occurs every 10 mins
- Protein in LDL cholesterol Delivered to lysosome which has proteases to digest protein portion
- Cholesterol is then released into the cytosol for membrane synthesis.
Extra:
Some cells cannot take up cholesterol in disorders
Why? no/low receptors, other steps could not work
Result = high blood cholesterol, risk of coronary artery disease
How is iron taken up into cells?
Receptor mediate endocytosis - Iron uptake
- Transferrin and iron bind to a receptor on the cell
- Fuses with endosome
- Low pH releases iron
What are peroxisomes? General characteristics?
Peroxisomes are small, membrane-enclosed organelles that contain enzymes involved in a variety of metabolic reactions, including several aspects of energy metabolism.
Characteristics:
- Single membrane-bound
- Do not contain DNA or Ribosomes
- All their proteins are encoded in the nucleus
- These proteins are obtained by selective import from the cytosol. Some enter the membrane via the ER
- Contain oxidative enzymes e.g. catalase and urate oxidase –> Super high concentrations
- Major sites of oxygen utilisation
- They are self-replicating
Outline how Peroxisomes use molecular oxygen and hydrogen peroxide to perform oxidation reactions.
- Use molecular oxygen to remove hydrogen atoms from organic substrates –> in an oxidation reaction producing H2O2
RH2 + O2 –> R + H2O2
- Catalase uses the hydrogen peroxide generated by the other enzymes to generate other substrates
E.g. formic acid, formaldehyde and alcohols
This is through the peroxidation reaction H2O2 + R’H2 –> RI + 2H2O
- Hydrogen peroxide is broken by catalase: 2H2O2 –> O2+2H2O –> bad for cells as it forms reactive oxygen species.
Examples of peroxisome action?
- Key in liver and kidney cells to detoxify harmful molecules present in the blood
E.g. ethanol –> acetaldehyde
- A major function in the breakdown of fatty acids to acetyl CoA which is called β oxidation, by blocking off 2x carbon atoms at a time –> used for the citric acid cycle.
- Catalyse formation of plasmalogens = phospholipids in myelin –> Plasmalogen deficiencies from peroxisomal disorders lead directly to neurological diseases
How are proteins imported into peroxisomes?
- Ser-Lys-Leu located at c terminal functions as the import signal –> If the sequence is attached to cytosolic proteins it will be imported into the peroxisome after being recognised by soluble receptors in the cytosol.
- Transport involves –> Peroxin proteins + ATP hydrolysis-driven process
- Six different peroxins form a protein translocator at the peroxisome membrane where unfolding of proteins does not need to occur –> The pore is dynamic, adapting in size for each cargo.
- Pex5 accompanies cargo into the peroxisome before returning to the cytosol –> The return is due to the removal of ubiquitin


















