PH1123 - Membranes Flashcards
where is cholesterol found and its function? (2)
- sits on the inner and outer membrane
- reduces degree of fluidity and regulates rigidity
where are there many drug targets?
- proteins on the plasma membrane
what are lipid rafts?
- rich collections cholesterol and sphingolipids and other similar lipids with or without associated proteins that serve as attachment points for other biomolecules
what do lipid rafts do? (2)
- cell signalling
- endocytic pathways
- cell signalling
- endocytic pathways
- separate environments to optimize the course of chemical reactions
- incompatible processes can go on simultaneously inside the same cell
what is the first stage of endocytosis?
- part of the membrane falls into the cytoplasm of the cell
how do secretory vesicles release their contents?
- exocytosis
what is endocytosis?
- the taking in of matter by a living cell by invagination of its membrane to form a vacuole
what molecules are endocytosis used for? (3)
- cell uptake of extracellular nutrients
- cell recovery through endocytic recycling
- cholesterol homeostasis
what are the different endocytic pathways? (5)
- macropinocytosis
- clathrin coated vesicles
- clathrin independent endocytosis
- cavaeolae; platform is lipid raft
- phagocytosis
what is clathrin? (2)
- triskeleton composed of three clathrin heavy chains and three light chains which form a polyhedral lattice around the vesicle
- protein that plays a major role in the formation of coated vesicles
what are adaptins?
- they link clathrin to receptors in coated vesicles
where do the molecules go to after fusion in clathrin endocytosis? (3)
- to early endosome (pH6.5)
- then late endosome (pH 5.5)
- then lysosome H+ enters (pH 4.5)
how does clathrin endocytosis work? (5)
- cargo binds to cargo receptor
- adaptin links clathrin to receptors in coated vesicles
- vescicle forms and fission occurs
- uncoating (opening/splitting of complex) occurs and are transported via transport vesicles
- fusion and sorting endosome
in the acidic cell stomach what is inside and what is the optimum pH? (4)
- nucleases (break up DNA)
- proteinases (break up proteins)
- lipases (break up lipids)
- optimum pH is low (4.5)
what is receptor mediated endocytosis? (4)
- tranferrin (ion carrying molecule) binds to transferrin receptor
- epidermal growth factor (EGF) binding to EGF receptor
- LDL binding to LDL receptor (endocytic pathway traversed by receptor and ligand)
- all these rely on clathrin mediated endocytosis
what is the condition when you have too little iron?
anaemia
what is the condition when you have too much iron?
- haemochromatosis
what is the function of iron in cells? (3)
- cofactor in enzymes necessary for metabolic processes
- essential element for blood production and important for haemoglobin and myoglobin
- iron transports oxygen in blood from lungs to tissues
how does iron get into cells? (7)
- transferrin (ligand) binds and transports iron in blood serum
- transferrin-receptor complexes concentrate in clathrin coated pits and are internalized in an endycotic vesicle
- the clathrin coat is removed and the complexes are directed into an endosome
- the pH in the endosome drops to pH 6 due to the action of (H+)-ATPase pump
- the pH drop weakens the affinity of transferrin for iron, leading to Fe3+ dissociation from the protein where Fe3+ is reduced to Fe2+
- as the pH drops the affinity of the receptor for apotransferrin increases
- remains bound to the receptor in the endosome, the complex recycles to the cell surface and ready for another round of iron uptake
how does receptor mediated endocytosis of epidermal growth factor work (EGF)? (2)
- lLigand binding to the receptor’s extracellular domain leads to dimerization (joining) of adjacent EGFR monomers which activates the cytoplasmic tyrosine kinase domains
- after ligand binding and receptor activation the EGFRs are endocytosed and recycled or directed to lysosomes for degradation
what happens if there is no regulation of epidermal growth factor? (2)
- binding of an EGF ligand to the receptor’s extracellular domain initiates signal transduction pathways that regulate cell proliferation and differentiation
- over-expression and aberrant activity of EFGR is involved in the development of cancer
what is HER2 a target for?
- herceptin
why do fats need specialised transport?
- they are hydrophobic
what are triglycerides transported by?
- b globulins
what is cholesterol transported by?
- b globulins
what are phospholipids transported by?
- a globulins
what are free fatty acids transported by?
albumin
what is the receptor mediated endocytosis of LDL receptors? (4)
- LDL contains chloesterol and cholesterol esters and binds to LDL receptors
- invagination of LDL to form a clathrin coated vesicle
- lysosomes fuse with the vesicle and hydrolyses it where LDL falls off the receptor at low pH
- free cholesterol is released and the endosome is recycled
what is autophagy?
- eating from the inside
what usually goes through autophagy?
- old and damaged organelles are wrapped in membrane and delivered to the lysosomes for degradation
what is the secretory pathway? (2)
- a series of steps a cell uses to move proteins out of the cell
- deliver membrane proteins like tranferrin receptors and LDL receptors
what are soluble proteins that are excreted via exocytosis? (2)
- insulin (hormone)
- pepsin
what are small molecules that are excreted via exocytosis? (3)
- neurotransmitters
- acetycholine
- noradrenaline
how do proteins travel from the ER to the extracellular fluid in vesicles? (3)
- budding of vesicle from ER
- at golgi modification of proteins occur
- vesicle fuses with plasma membrane to release contents
what is apoptosis? (2)
- programmed cell death
- allows removal of specific cells for various reasons
what are the two pathways of apoptosis? (2)
- intrinsic (cell decides) pathways
- extrinsic (told by other cells) pathways
what is the intrinsic pathway of apoptosis? (2)
- cellular stress
- regulation of BCL-2 proteins by transcriptional and post-transcriptional mechanisms
- cell death
what is the process of apoptosis? (4)
- pro-apoptotic stimulus
- execution
- engulfment
- clearance
what happens to red blood cells in a hypotonic solution? (2)
- the concentration of solutes inside the cell is higher (lower water potential inside the cell) than the outside
- osmosis will occur from outside of cell into the cell and cause it to swell and then burst
what happens to red blood cells in a hypertonic solution? (2)
- higher concentration of solutes in solution compared to cell (less water potential outside cell)
- cell shrinks and dies
what is the difference between an ion channel and an ion pump? (2)
- an ion channels allows ion to rush down gradients of concentration or electrical potential (PASSIVE)
- an ion pump actively (uses ATP) pushes ions against a gradient and therefore further build up the gradient (ACTIVE)
what are nicotinic acetylcholine receptors an example of?
- ionotropic receptors (ligand-gated ion channels)
what are g proteins? (3)
- proteins bind GTP rather than ATP
- they are active when bound to GTP and inactive when bound to GDP
- can hydrolyse GTP to GDP
what are g protein coupled receptors? (2)
- don’t have ion channels but G-proteins
- have 2 domains where intracellular binds to G-proteins and extracellular binds to neurotransmitters
what is signal transduction? (2)
- the mechanism whereby a command is executed within a cell
- an external signal generates an intracellular message
what are examples of signal transduction? (4)
- how an action potential casues transmitter release
- how acetylcholine phosphorylates ion channels
- how hormone release is controlled (eg insulin)
- how agonists modify cell growth (eg EGF)
what is a symport and an example? (2)
- two molecules working together driving each other from one place to another
- glucose-Na+ coupled transport
