Vesicular transport Flashcards
How are vesicles formed?
- Trigger (ligand, presence of object etc)
- Indentation of the membrane
- Pouch formed on membrane.
- Neck of pouched sealed off
- Vesicle detached from the membrane.
Same for endo and exo and if the vesicle forms at cell membrane or other structure.
What is vesicular transport?
Sometimes cells need to transport unusual or large substances through the membrane for which there isn’t a specific carrier protein. Or it may need to store a substance prior to release to the ECF. Materials are transferred between ECF and ICF or between structures within the cell in vesicles.
Requires energy from ATP.
Endocytosis - transport into the cell
Exocytosis - transport out of the cell
What is receptor-mediated endocytosis?
The uptake of specific substances bound to membrane surface receptors.
step 1: Substance attaches to membrane receptor.
step 2: membrane pockets inwards.
step 3: pocket pinches off as endocytic vesicle containing target molecule.
This doesn’t occur unless all the receptors fill up.
A secondary lysosome makes the receptors release the ligand and receptors (primary vesicle) goes back and forms part of the cell membrane again.
What is pinocytosis?
Cell drinking. Uptake of extracellular fluid. Vesicle forms filled with ECF. Similar process to receptor-mediated endocytosis but not as selective and not initiated by ligand binding. Target seems to be general ECF contents. Performed by most cells.
What is phagocytosis?
Only performed by certain cells.
Involves engulfing solid objects - may be as large as the cell itself.
Makes a little pocket like other type of endocytosis but has little feet that reach out and grab object.
The vesicle that forms is called a phagosome. It fuses with a lyosome. The lysosomal enzymes digest the contents. The left overs from enzymes eating the foreign body are expelled from the cell by exocytosis.
What is exocytosis?
Functional reverse of endocytosis.
Vesicles are created in endoplasmic reticulum (ER) or Golgi apparatus within cytoplasm.
Basically four types of vesicles are produced only some of which are directly involved in exocytosis. Some are used to transport things within the cell eg. lysosome.
What are the four types of vesicles formed in ER and golgi?
Transfer vesicles - Transfers enzymes between ER and Golgi apparatus and between saccules of Golgi apparatus.
Lysosomes - contain intracellular enzymes.
Secretory vesicles - allow cell to secrete products such as hormones.
Membrane renewal vesicles - contain new/recycled membrane components.
What is the endoplasmic reticulum?
Factory number 1!
Produces transfer vesicles.
Network of intracellular fluid-filled tubules and flattened sacs.
Synthesises and stores proteins, carbohydrates and lipids
Detoxifies drugs or toxins
Transfers products to the Golgi bodies (via transfer vesicles) for further processing
What is the Golgi apparatus?
Factory number 2!
Produces lysosomes, membrane renewal vesicles, secretory vesicles and transfer vesicles.
Sets of stacked, flattened membranous discs called saccules.
Processes material from ER into final form.
Synthesises and packages enzymes for use within the cell.
Synthesises and packages secretions such as hormones for release through exocytosis.
Produces new membrane and membrane structures.
What are lysosomes?
Really important for breaking down other substances - digestive enzymes.
Produced by the Golgi apparatus, containing digestive enzymes.
Bind to vesicles to form secondary lysosomes.
Digest or modify the contents of other vesicles which may be released into the cytosol (ICF) or be exocytosed.
How do membrane renewal vesicles?
Adds new lipids and proteins to cell membrane.
Allows cell to change number and types of carrier proteins or receptors in cell membrane thus changing sensitivity of cell.
Secretory vesicles
Produced by Golgi apparatus of cells specialised for secretion of hormones or enzymes.
Vesicles bud off from GA and then fuse with cell membrane to release products to ECF.
Secretory vesicles may release products constitutively (unregulated) or may remain in the cell until release is triggered (regulated). (This is how action potentials happen - exocytosis of neurotransmitter).
What is important about the rate of endocytosis and exocytosis?
Needs to be balanced so that the cell doesn’t change shape.
