Structure and Transport
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| Biological Membranes | primarily lipid in nature but actual lipid composition may vary between different membranes
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| Membrane Lipids | Polar head group & long nonpolar tails
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| Amphipathic-polar and nonpolar duality is the nature of membrane lipids, allows membrane lipids to form biological membranes
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| Cell Membranes are formed of 3 main lipid components | glycerophospholipids, sphingolipids, cholesterol
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| Glycerophospholipids are the most abundant; 3 main components: glycerol backbone, long chain of fatty acids esters, & phosphate head group
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| phosphatidic acid- class of compounds, but most basic of glycerophospholipid
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| fatty acid group- provide further diversity because they can either be saturate (single bond) or unsaturated (double bond)
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| Triacylglyercol- variation of glyverophospholipids, contains glycerol backbone but wiht 3 fatty acid chains and no phosphate; function: energy reserviors for the cell...no polar heads
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| Sphingolipids-lipid components built on amino alcohols (Sphingosine) rather than glycerol
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| ceremides- formed by liking a fatty acid to the amine group of sphingosine; cerebrosides- sphingolipids & fatty acids; gangliosides- adding a sugar
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| Cholesterol- most abundant steroid in animals, 4 rings identifies it as cholesterol; hydroxyl group is your polar head and rings are nonpolar tail
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| Membrane Structure | Bilayer- two sheets of membrane lipids aligned such that -one layer of polar head groups face the aqueous ext. of cell - other layer of polar head groups faced the aqueous int. of the cell - nonpolar tails are sandwiched in between
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| no two sheets of membranes are identical, each leaflet have a different composition of lipids making them asymmetric
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| Biological Membranes | are not solely lipids; proteins and glycoproteins are also present
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| Peripheral Membrane proteins- proteins in which are only associated with one leaflet , one side cell exterior or cytosol; not signaling molecules
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| Integral Membrane proteins- protein which extend thru the membrane and are exposed to both the cellular ext. and cytosol; signaling molecule; look at pg 10 of notes
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| Fluid Mosaic Model | biological membranes have a shape but aren't rigid, which means they are flexible, able to move and swell up
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| Lateral Diffusion- can move around (lipids and proteins); flip- flop- lipid flip from outside to inside
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| Membrane Transport | primary function of a biological membrane is to maintain a seperation between the cytosol and the ext. of the cell (inside & outside)
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| Two ways in which a molecule can translocate across a membrane- passive diffusion & facilitated transport (facilitated diffusion & active transport)
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| Passive Diffusion- molecules can cross a membrane WITHOUT any external assistance
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| Facilitated Transport- molecules can cross a membrane WITH the assistance of a protein
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| Facilitated Diffusion- protein only opens a channel or pore thru the membrane with NO expenditure of energy
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| Active Transport- protein USES an energy source to move a molecule across a membrane
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| Thermodynamics of Passive Diffusion | Passive diffusion of molecules across a membrane is an equilibrium process (move in or out of cells)
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| formula on page 12 of notes
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| telling about molecules moving in and out of cell not about the rate or speed in which its done at
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| Diffusion- in the direction of (WITH) the concentration gradient DOES NOT require an outside energy source making it SPONTANEOUS
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| Diffusion- opposite (AGAINST)the concentration gradient REQUIRES an outside energy source make it NON SPONTANEOUS
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| C2>C1 (outside higher concentration) G is negative & net transport is spontaneous
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| C1>C2 (inside higher concentration) G is positive make it non spontaneous
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| C2=C1 (inside & outside concentration are equal) at equilibrium and no net transport
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| Kinetics of Passive Diffusion | how fast it happens nothing about energy
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| very large molecules will cross the membrane at a much slower rate than small molecules; thickness of the membrane also affectes the rate of transit
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| a larger concentration gradient will allow molecules to movce more quickly across the membrane (larger driving force)
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| Fick's Law of Diffusion | see page 13 of notes for formula
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| empirically derived- find it out by experiment can't calculate it
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| larger the negative number- moves quicker into the cell
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| larger molecule- smaller DM - moves slower; smaller molecule- larger DM- moves faster; very polar molecules moves slowly; AMPHIPATIC moves best and fastest
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| Passive Diffusion | very slow, inadequate to provide the cell with what it needs, most transport into a cell with the assistance of transport proteins through facilitated diffusion
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| Facilitated Transport | accomplished by incorporating integral membrane proteins (provide assitance to molecule crossing a membrane) into the cell membrane to act as channels for or transporters of particular substances
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| proteins lower activation enery making it easier to move across membrane
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| Facilitated diffusion and active transport
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| facilitated diffusion-with the concentration gradient , doesn't require outside energy, proteins are called channels or pores; fast- line straight up then plateaus out
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| Active transport- against the concentration gradient, Does require outside energy, proteins are called transporters or pumps; line diagonally
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| uniport | transport one molecule
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| symport | transport two molecules going the same direction
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| antiport | one molecule going in and one molecule coming out
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| Molecular Trapping | The cell can prevent equilibrium from establishing for the transport of a molecule which effectively traps the molecule inside the cell
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| The cell does this by covalently modifying the transported molecule once it enters the cell by making the molecule more polar or attaching to a very large macromolecule(protein); this works bcuz each moves across the membrane slowly
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| this also works for facilitated transport because the modified molecule will not be recognized by the protein for reverse transport
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| Equilibrium Modification | modify the equilibrium by lowering the intracellular concentration of the transported molecule which affects the equilibrium
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| amino acids that would be expected to spanning the membrane of integral membrane protein and why | valine,alanine, isoleuine, leucine,methoinine, proline, and glycine- because they are all nonpolar
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