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cell membranes and signalling ch5-reading.docx

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BIOL 1000
Julie Clark

5-Cell Membranes and Signaling Why it matters  Cystic fibrosis occurs when the protein CFTR has a mutation in genes, hampering its function. This transport protein carries negative chlorine ions out of cells (that are line the lungs and intestinal tract) towards the mucous. As a result, positive sodium ions follow – then, water from the cells follow. This keeps the lining moist. If the lining is dry, then mucous can build up and prevent normal breathing. The mucous buildup can also give individuals bacterial infections 5.1-An Overview of the Structure of membranes  Plasma membrane is semi-permeable, allowing key components to enter cell and others to exit 5.1a- a membrane consists of proteins in a fluid of lipid molecules  Fluid mosaic model suggests the membranes consists of proteins within a mixture of lipid molecules  Lipid bi-layer is dynamic  The proportions of lipids and proteins in a membrane depend on the type and main function of membrane. Eg –mitochindrial membrane contains more protein because of electron transport  Internal and external side of membranes vary in function (membrane asymmetry) 5.1b-experimental evidence in support of the fluid mosaic model  Membranes are fluid – in an experiment, human cell membrane mixed with mouse cell membrane  Membrane asymmetry – in an experiment where a block of ice was fractured, the cell bi-layer split to reveal the inner workings of the lipid bi-layer 5.2-the lipid fabric of a membrane  Lipids refer to fats, phospholipids and steroids (all three of which are water-insoluble)  Certain organisms can adjust lipids in membranes so membranes don’t become too stiff 5.2a-phospholipids are the dominant lipids in membranes  Phospholipids form lipid bi-layer  Head group consists of glycerol attached to a phosphate group which is attached to an alcohol or amino acid. It is also attached to a hydrogen and 2 chains of fatty acids. Phospholipids are amphipathic – fatty acid side is hydrophobic (non-polar) and the head part is hydrophilic (polar)  Unsaturated fatty acids contain 1 or more double bonds between carbons  When placed in aqueous environment – phospholipids take 1 of three forms: micelle, liposome or bilayer. This occurs because of the hydrophobic effect: when polar molecules (such as those in water) tend to exclude the fatty acids 5.2b – Fatty Acid composition and temperature affect membrane fluidity  Saturated fatty acids are linear and pack well together, whereas unsaturated don’t pack so well and are more fluid  Gelling may occur with unsatureated fatty acids – where cool temperatures cause the liquid to turn more solid like 5.2c- organisms can adjust fatty acid composition  Ectotherms (organisms that can’t regulate their body temperatures) may experience gelling in the fatty acids located in the lipid bi-layers of their body cells. Fluidity in the bi-layer is essential as it allows permeability and allowing the function of membrane-bound enzymes (etc). Electron transport chain is hampered.  Unsaturated fatty acids are formed from desaturases (enzymes) which remove two hydrogen atoms from neighboring carbons, causing their double bonding  Desaturase abundance is regulated at the level of gene transcription. Desaturase transcript varies based on the growth temperature of an organism – lower temperatures means more desaturase (so more unsaturated fats are incorporated in cell membranes)  Sterols also influence membrane fluidity. Cholesterol act as buffers – in higher temperatures, they restrain the movement of lipid molecules and at low temperatures, they fill the space between lipid molecules to abate gelling 5.3a – the key functions of Membrane Proteins  Transport: proteins may provide hydrophilic channels to let substances pass through membrane  Enzymatic activity: many enzymes are membrane proteins – eg: enzymes associated with photosynthetic electron transport chains  Signal transduction: cells have receptor proteins on external sides. Hormones attach to these sites and cause internal changes  Attachment/recognition: proteins exposed to both sides of cell membrane act as attachment points for cytoskeleton elements and for cell-to cell communication 5.3b-Integral Membrane Proteins Interact with the Membrane Hydrophobic Core  Integral membrane proteins (a form of transmembrane proteins *remember, Trans means across) span the width of the lipid bi-layer. They consist of non-polar amino acids in the portions that go through the hydrophobic side (forming a structure called the alpha helix) and of polar amino acids in the portions that appear on the aqueous sides.  One can identify amino acids of a transmembrane protein because the amino acids will be non-polar for 17-20 amino acids (matches the length needed to span the bi-layer) 5.3c-peripheral membrane proteins interact with the membrane hydrophilic surface  Peripheral membrane proteins don’t react with hydrophobic core of bi-layer  They attach to integral proteins and lipid molecules through non-covalent (hydrogen and ionic) bonds. They can also be found facing towards the cytoplasm, forming part of the cytoskeleton 5.4- Passive Membrane Transport  Hydrophobic membrane makes it difficult for substances to pass through. Oxygen, however, rapidly diffuses – it is used for cellular respiration 5.4a-passive transport is based on diffusion  One side of the membrane might have a lot of molecules whereas the other might have nothing. This concentration gradient causes molecules to move over, increasing the entropy (disorder) until dynamic equilibrium is reached  Greater the concentration gradient, the greater the rate of diffusion 5.4b-there are two types of passive transport – Simple and facilitated  Simple diffusion : movement of molecules across membrane without need of a transporter (rate of simple diffusion depends on molecule size and lipid solubility)  Small non-polar molecules such as O2 and CO2 can easily pass. Steroid hormones (amphipathic) can pass through the bi-layer. The membrane is, however, impermeable to charged ions  Facilitated diffusion is with the aid of a transporter that carries both charged polar molecules. It occurs when there’s a gradient and stops when the gradient reaches zero  Rate of simple diffusion depends on molecular size and lipid solubility 5.4c-two groups of transport proteins carry out facilitated diffusion  Channel proteins and carrier proteins create a hydrophilic pathway for molecules to travel through  Diffusion of water done through aquaporins – specifically diffuses water molecules  Gated channeles switch between open and closed based on voltage difference across
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