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Membranes Cell Membranes - all cells are delineated by a membrane - play a crucial role in almost all cellular activity - separate the cell from its surroundings - regulates the movement of substances in and out of the cell o (i.e. a selective barrier – controls concentration variation of molecules in cell) Internal membranes - enclose: o nucleus o mitochondria o chloroplasts etc. o (i.e. compartmentalise special metabolic activities) Analysis of extracts of cell membranes (a) made of lipid (b) associated with proteins which contribute to their function i. selective permeability ii. trans-membrane transport iii. structural integrity The cell membrane is a structure common to both animal and plant cells. There used to be a theory of the structure as shown on the following page. However, this was later not accepted and a new theory was created which is now accepted to be correct. Davson Danielli (1930) model – unit membrane Original model phospholipid bilayer 3.5nm The problem seen in this model was that it did not explain how the substances got through the membrane. (a) (b) (c) hydrophilic bond 2nm Total: 7.5 nm 7.5 nm thick not seen under light microscope forms anchorage for component protein Phospholipids (a) (b) (c) Composed of fatty acid chains attracted to glycerol have polar heads (hydrophilic) non polar tails (hydrophobic – do not mix with water) If a thin layer of polar lipids is spread on the surface of water, the molecules orientate into a monolayer. water polar heads Singer and Nicholson – Fluid Mosaic Model – 1972 – model now accepted cholesterol glycoprotein protein phospholipidic tail carrier protein head pore protein • “proteins flowing in a sea of lipids” • fluid mosaic model – i.e. membrane is not static • both proteins and lipids have considerable freedom of movement: mainly lateral Even with an electron microscope it is not possible to see he molecular structure of a cell membrane. Thus it is necessary to construct a model to explain its various properties. The matrix of the membrane is composed of a bilayer of phospholipids and cholesterol molecules. Proteins are embedded in the bilayer. (a) some penetrate only part of the way through, while others penetrate all the way through (b) Lipids functions: i. structure ii. transport iii. act as enzymes - composition similar to olive oil variation affects fluidity and permeability i. unsaturated lipids have curled tails to prevent close packing and to make membrane structure more open and fluid ii. cholesterol is important in regulating fluidity The structure is asymmetric as there are different proteins in either half of the bilayer. Carbohydrates close t exterior forming “glycocalyx” – receptor molecule / cell recognition Proteins are involved in the transport of molecules across membrane specific receptors for hormones or act as enzymes. Evidence for the structure is proposed from: (a) (b) (c) freeze fracture studies isotopic labelling of proteins electron microscope studies Movement through membranes (a) (b) (c) (d) (e) (a) diffusion facilitated diffusion active transport osmosis exocytosis / endocytosis Diffusion - “the net movement of uncharged molecules (e.g. CO2, O2, urea) from a region where they are in high concentration to a region where they are in low concentration” - i.e. DOWN THE CONCENTRATION GRADIENT - it occurs wherever a concentration gradient exists and would continue until the diffusing substance is evenly distributed - e.g. O2 through cell membrane of plant and animal cells or released from photosynthesising chloroplasts. Rate of diffusion depends on… - concentration gradient - the distance – shorter the distance, the faster the rate: rate α 1 distance 2 - area – the larger the surface area, the greater the rate of diffusion - nature of the structure – the greater the number/size of pores, the greater the rate of diffusion - the size and nature of the diffusing molecule – the smaller the molecule, the greater the rate of diffusion. e.g. fat soluble molecule – diffuses fast - size of charge on ion – positive (+) ions move in more readily than negative (-)ions. Ions of a higher charge are attracted more into the cell Fick’s law of diffusion rate of diffusion α surface area × concentration difference thickness of membrane Routes of diffusion (a) (b) - - CO2, O2, H2O(even though it is polar), urea, ethanol (i.e. small uncharged molecules) - the smaller / the more fat soluble the molecule, the faster the rate (e.g. O2, CO2, urea, ethanol – very rapid) - molecules squeeze between polar phospholipids heads – they dissolve on the lipid on one side and emerge from the other O2 - Thus, K+, Na+, Cl-, HCO3- and glucose cannot cross in this way. They have to be aided by proteins. Glucose, amino acids and ions travel through in different ways: through water filled pores in the channel proteins - pores are selective in determining which substance will move across (by size of pore) - pores may be gated – open / closed by nerves. Conclusion Diffusion: down a concentration gradient no energy needed (b) Facilitated Diffusion - for substances (e.g. glucose) the rate of diffusion is speeded up by the presence of protein carriers in the membrane rate of reaction glucose concentration limiting rate number of carrier molecules limits rate concentration of glucose - down a concentration gradient (i.e. from high conc. → low conc.) - there are 2 types of protein membrane for facilitated diffusion: i. specific carrier proteins forming a “gate” allowing solute to go through (e.g. to transport glucose) carrier protein for glucose – permease glucose binds to permease on one side of the membrane and is released from the other carrier protein specific to particular molecules the binding of the solute molecule to the carrier protein alters the conformation of the carrier so that its position in the membrane changes and the solute molecule is discharged to the other side of the membrane ii. ion channels protein pores that open / close to control the passage of selected ions (e.g. Na+, K+ hydrophilic channel allows solutes through Summary – facilitated diffusion: (a) Carrier proteins (b) No energy (c) down a gradient (c) Active Transport - the movement of substances across a membrane using energy (usually A.T.P.) - occurs against a concentration gradient – involves a specific carrier protein - typically, A.T.P. is hydrolysed and the binding of the phosphate group to the carrier molecule changes the protein’s confirmation in such a way that the solute is delivered / transferred across the membrane. - the specific carrier protein also acts as ATPase – releasing the energy for A.T.P. Key facts - factors that affect the rate of respiration (and therefore A.T.P. production) will affect the rate of active transport - in cells where active transport is particularly important, there are large numbers of mitochondria (to produce A.T.P.) - active transport is important in many aspects of physiology. E.g.:o uptake of products of digestion o uptake of mineral ions from the soil o nerve impulses o reabsorption in kidney tubules Summary - AGAINST concentration gradient uses energy (A.T.P.) specific carrier protein More on active transport… - there are 3 methods of carrier proteins: (a) Uniport - where a single substance is transported across a membrane (i.e. in one direction) (e.g. calcium pumps in the muscles) (b) Symport - where 2 substances are transported in the same direction (e.g. glucose/sodium pumps) (c) Antiport - where 2 substances are transported in opposite directions at the same time (e.g. sodium/potassium pumps - nerve impulse) uniport symport antiport