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Intro to Membranes
Physiology and Pharmacology
| Question | Answer |
|---|---|
| Give examples of Organelles with membranes | Plasma membrane Nucleus Mitochondria Endoplasmic reticulum Golgi Lysosomes Peroxisomes Secretory vesicles |
| Main functions of cell membranes | Compartmentalisation Regulate movement of chemicals Create gradients for electrical signalling etc Permit rapid responses Perform metabolic function for intracellular signalling Define cell shape Cell adhesion Allow cell recognition |
| Why use red blood cells to study membranes | Readily available No organelles- gives a pure sample Easy to manipulate |
| How to manipulate red blood cells | Hypotonic lysis to break cell open Can study the red cell ghosts Can wash haemoglobin away and reseal with know solution inside Can seal smaller areas of membrane Can invert membrane to change proteins on the outside |
| What is the plasma membrane | A mixture of lipid and protein Around at 50:50 ratio The classic model is the fluid mosaic model |
| What lipids are in the plasma membrane | Mostly phospholipids with some cholesterol to provide structural integrity and regulate fluidity |
| What are the 4 main components of phospholipids | Fatty acids Glycerol Phosphate Choline/serine/ethanolamine In sphingomyelin a sphingosine replaces glycerol and one fatty acid |
| How are phospholipids arranged in the membrane | Asymmetrically Loss of this can lead to cell death- PS in the outer leaflet is recognised by macrophages |
| How is asymmetry maintained | By phospholipid transfer proteins Spontaneous switching, no energy needed Flippases move lipids inwards Floppases move lipids outwards Scramblases move lipids in all directions |
| Types of membrane protein | Integral- single pass or multi pass (go multiple times through a small area of membrane) Peripheral- linked to proteins or lipids |
| Roles of proteins in the membrane | Assist passage of solutes Bind to external mediators e.g. generating second messengers Interact with proteins to anchor cell shape Induce a response in the cell Allow cells to recognise each other Adhere cells together |
| What are membrane transport proteins | Proteins change the permeability of the cell membrane e.g. more permeable to water, glucose and ions than the lipid bilayer These can be carriers or channels and can be both passive and active |
| Examples of membrane transport proteins | Ion/water channel Gated ion channel Uniporter (only transports one molecule) Symporter (moves two different molecules in the same direction) Antiporter (moves two different molecules in opposite directions) ATP coupled active transport |
| What are ionotropic receptors | Receptors that activate movement of ions These bind to a substance and open to form a channel e.g. acetylcholine receptors |
| What are metabotropic receptors | Receptors that set of a metabolic process These tend to be G protein coupled receptors When a receptor becomes occupied, a g protein subunit becomes activated and breaks off to move to another protein as a form of intracellular messaging |
| Examples of ionotropic receptors | Neurocrine receptors e.g ACh from motor nerves |
| Examples of metabotropic receptors | Neurocrine receptors e.g ACh from parasympathetic nerves Endocrine receptors e.g. ADH from posterior pituitary Paracrine receptors e.g. histamine from mast cells |
| How do enzymes act as membrane proteins | Activated by metabotropic receptors This generates second messengers e.g phospholipase C > IP3, adenyl cyclase > cAMP |
| Key example of a metabotropic receptor- beta adrenergic receptor | Noradrenaline binds to a membrane protein This activates an enzyme which converts APT to cAMP This activates a protein kinase which goes onto activate other enzymes This increases contraction of the heart |
| Experiment to prove proteins are mobile | Fuse a mouse and human cell with different antigens Label the antigens with antibodies labelled with different substances Incubate at 37 degrees for 40 minutes The proteins will become mixed up |
| Which proteins need restricting to certain areas | post synaptic receptors need localising to pre synaptic membrane where neurotransmitters are released Epithelial cells need to express different receptors at their two faces. They need an asymmetric distribution so certain proteins are facing out. |
| What restricts protein movement | Membrane scaffolds- spectrin, ankyrin, Band 4.1 which link membrane proteins to the cytoskeleton Lipid rafts- densely packed regions of cholesterol and glycosylated phospholipids to give a more ordered membrane with increased rigidity |
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