lecture 8 Flashcards
What is the Golgi apparatus?
- The golgi is a major site of carbohydrate synthesis as well as a sorting and dispatching station for products of the endoplasmic reticulum.
- a series of flattened sacs often referred to as cisternae
- it has two faces: cis face is the one closest to the ER, trans face closest to the plasma membrane
- has three set stacks: cis cisterna, medial cisterna, trans cisterna, and then the diffuse cis Golgi network and trans golgi network
How is the Golgi apparatus functionally compartmentalised?
- each of the different cisternae have different enzymes which give them different functions:
1. cis golgi network: phosphorylation of oligosaccharides on lysosomal proteins
2. cis: removal of Man
3. medial: removal of Man, addition of GlcNAc
4. trans: addition of Gal, addition of NANA
5. trans golgi network: sulfation of tyrosines and carbohydrates
What two types of exocytosis are there?
some secretory vesicles continually traffic out to the membrane, bring things like newly synthesized plasma membrane lipids (need to be continally removed)
there is also regulated exocytosis: some vesicles are not continuously transported to the plasma membrane, will send it after receiving a certain signal e.g. insulin: transported from ER to golgi and packaged in a secretory vesicle. This vesicle doesn’t release contents until stimulated by hormone/neurotransmitter
What is the pathway of endocytosis?
- starts out at plasma membrane and buds into the cytosol (clathrin coated pit)
- transported first to the early endosome: sorts the inward bound vesicles as opposed to outward which are sorted by TGN
- one of two options: either shunts vesicles onward to late endosome (microtubule-mediated transport) - multivesicular body transported along microtubule
- late endosome becomes endolysosome and eventually fuse with lysosome to form lysosome where products will be degraded.
- Or it can initiate a retrieval process. Some things such as receptors - e.g. for some signalling processes the receptor actually needs to be endocytosed, that receptor after the signalling event will actually be shunted back out to the plasma membrane by the early endosome
Describe receptor-mediated endocytosis of LDL
A phospholipid monolayer filled with cholesteryl ester molecule. Cholesterol molecule in monolayer.
Low-density lipoprotein is bound by LDL receptor proteins –> forms clathrin coated pit and is endocytosed
naked vesicle fuses with early endosome but adaptor protein still part of membrane
protein buds off early endosome in transport vesicles and returns LDL receptors to plasma membrane while the LDL particle ends up in the lysosome to release free cholesterol
approx 1 in 500 individuals inherits one defective LDL receptor gene, and as a result, is likely to die prematurely from a heart attack caused by atherosclerosis - LDL receptor protein still concentrates cholesterol at the plasma membrane but it is not endocytosed. You cannot have mutations in both of these genes.
How do exocytosis and endocytosis facilitate signalling across the neuronal synapse?
- delivery of synaptic vesicle components to plasma membrane
- endocytosis of synaptic vesicle components to form new synaptic vesicles directly
- endocytosis of synaptic vesicle components and delivery to endosome
- budding of synaptic vesicle from endosome
- loading of neurotransmitter into synaptic vesicle
- secretion of neurotransmitter by exocytosis in response to an action potential
What must cells of metazoans do?
Associate to form organs
What do the cytoskeletons of cells in epithelial tissue do?
they are linked - mechanical stresses are transmitted from cell to cell by cytoskeletal filaments anchored to cell-matrix and cell-cell adhesion sites
What is the main stress-bearing component of connective tissue?
The extracellular matrix - directly bears mechanical stresses of tension and compression
What are the four functional classes of cell junctions found in animal tissues?
- anchoring junctions
- -actin filament attachment sites: 1. cell-cell junctions (adherens junctions) 2. cell-matrix junctions (actin-linked cell-matrix adhesions)
- occluding junctions: 1. tight junctions (in vertebrates), 2. septate junctions (in invertebrates)
- channel-forming junctions: 1. gap junctions (animals), 2. plasmodesmata (in plants)
- signal-relaying junctions: 1. chemical synapses (in the nervous system), 2. immunological synapses (in the immune system), 3. transmembrane ligand-receptor cell-cell signaling contacts (Delta-Notch, ephrin-Eph, etc.). Anchoring, occluding, and channel-forming junctions can all have signaling functions in addition to their structural roles.
What are tight junctions?
- specialised junctions of epithelial/endothelial cells
- also known as occluding junctions
- they facilitate transcellular transport
- they prevent molecules from diffusing between cells (i.e. the gap between two cells from apical to basal surface) so if a molecule wants to get from one side of the cell to the other it has to go through the cell and be recognised by specific receptors/channels etc
- tight junctions forms barriers to diffusion of: solutes - we can see this using dye solutions, and membrane proteins - this tells us that newly synthesised proteins are directed to specific regions of the cell membrane
- most apical of the junctions
How are tight junctions formed?
- by a meshwork of sealing strands of transmembrane proteins
- points of tight contact are called focal connections - tight junctions consist of a series of focal connections
- the proteins that produce these are claudin and occludin - hydrophilic adhesion molecules
- one claudin binds one claudin in the other cell , one occludin one occludin
- they are associated with other cytoplasmic proteins such as JAM ( adhesion molecule)
What are anchoring junctions?
- allow the cytoskeleton to adhere to ECM or other cells
- include: adherens junctions (connect to actin filaments), desmosomes, hemidesmosomes (connect to intermediate filaments)
- by anchoring adhesion molecules to the cytoskeleton it allows the cell to maintain robust cell-cell or cell-matrix adhesion: the cell changes behaviour under stress as opposed to simply ‘losing’ the adhesion molecules
Of what are anchoring junctions composed?
consist of:
- an intracellular plaque that attaches to the cytoskeleton
- transmembrane proteins that bind to adjacent proteins on other cells/ECM
What are adherens junctions?
- form a continuous belt below the tight junctions, the zonula adherens, in epithelial cells that bring actin filaments into alignment
- cadherins form the transmembrane linkages
