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Assessment 1.2

Structure and Transport

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