3.1 MOVEMENT OF SUBSTANCESACROSS THE PLASMA MEMBRANE
substances that arerequired by cells• Glucose• H2O• O2• Mineralsubstances to beeliminated from cells• ammonia• urea• CO2Cell membranes are like gates.
The main function of a plasma membrane is tocontrol what enters and what exits a cell.• keeps certain substances inside and othersubstances on the outside.• This function is critical. If needed molecules(such as those used in protein synthesis) werefree to leave, then death of the cell would quicklyoccur.the necessity for movement ofsubstances across the plasmamembrane:
PLASMA MEMBRANEMembrane lipids are organized in a bilayer (twosheets of lipids making up a single membrane)approximately 60 to 100 Å thick
Plasma Membrane StructurePlasma membrane phospholipid bilayer combination of phospholipids & proteinshttp://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swfFLUID MOSAICMODEL• not fixed in any rigid pattern• float around in the membraneProteins
These molecules form a phospholipid bilayer• hydrophilic (polar) heads facing their aqueoussurroundings (e.g., the cytosol)• hydrophobic tails facing each otherHydrophobic end (repels water)Hydrophilic polar heads (attracted to water)the charges on thephosphate and aminogroups (in red) make thatportion of the moleculehydrophilic.Amphiphilic lipids~major constituents of cell membranes
Integral and peripheral membrane proteinsPERIPHERAL PROTEINS - lie on the surface of the membrane.INTEGRAL PROTEINS - extend into and sometimes completely through themembrane.• marker proteins ~ like nametags that identify the cell to other cells• transport proteins ~ responsible for shipping and receiving. They move materials in andout of the cell• receptor proteins ~ serve as binding or attachment sites, especially for hormones orother molecular messengers
thin film on the inside of an eggSemipermeable membrane – lipid bilayerselectively permeable membranepartially permeable membranedifferentially permeable membrane• Some materials freely pass - the membrane ispermeable to such molecules and whether they areinside or outside of the cell depends on other factors• Some materials are excluded• Some materials enter or leave the cell only by the usingcell energy
1. Small hydrophobic molecules, such as CO2, O2 and smalllipids, dissolve in the membrane and pass through readily.2. Tiny polar molecules, such as H2O and alcohol, can alsoslip between the phospholipid molecules.3. Ions and most nutrient molecules do not move freelythrough membranes, but are often carried by thetransportprotein channels, either with or without the use of energy.4. Most large molecules are excluded and must bemanufactured within the cell, or moved by significantalterations of the membrane itself.
• FluidAny substance that can move or change shape in response toexternal forces without breaking apart. Gases and liquids arefluids.• ConcentrationThe number of molecules of a substance in a given volume• GradientA physical difference between two regions so that molecules willtend to move from one of the regions toward the other.Concentration, pressure and electrical charge gradients arecommon in cells.
Passive Transportinvolves moving thingsthrough membraneswithout the expenditure ofcell energy down gradients.e.g: diffusion.TRANSPORTATIONActive Transportthe pumping ofmolecules or ionsthrough a membraneagainst theirconcentrationgradient.
to wherethere isless of itDiffusionthe net movement of a substance fromwhere there ismore of it alonga concentrationgradientuntil molecules areequally distributed(and the gradientno longer exists).Passive Transport
• passive transport (requires no energy)• without the involvement of specific carrier proteins• The Rate of Diffusion can be affected by:-- Temperature (Higher temperature, faster molecule movement)- Molecule size (Smaller molecules often move more easily)- Concentration (Initial rate faster with higher concentration)- Electrical and pressure gradients of the two regions (Greaterthe gradient differential, the more rapid the diffusion (again, initially))• Materials that may move freely through membranes bysimple diffusion include:- H20 (water)- CO2 (carbon dioxide)- O2 (oxygen)- Uncharged & some small lipid-soluble molecules (alcohol)http://www.bbc.co.uk/education/asguru/biology/01cellbiology/05pathways/06passivesimple/index.shtmlPassive TransportSimple Diffusion
Simple Diffusion* Small polar moleculescannot readily dissolvein the hydrophobicbilayer but can passthrough the membranevia "holes" formed byintrinsic proteins formingchannels.* The proteins are thuscalled channel/ transportproteins.Passive Transport
Facilitated Diffusion• passive transport (No energy is involved)• with the help of membrane transport proteins, which temporarily bindto the substance to be moved through the membrane• carrier proteins and channel proteins are involved (has a specificreceptor site for that substance)• Materials that move through membranes by facilitated diffusion include:- small polar molecules (e.g. glucose)- Many small ions- Amino acidsPassive Transport
the substancebeing depositedon the otherside of themembrane.The proteinthen returnsto its originalshape readyto operateagainthe substancebinds to thecarrier proteinundergoes aconformational(shape) changePassive Transport
Osmosis• a special case ofdiffusion.• diffusion of watermolecules from an area ofhigh water concentrationto an area of low waterconcentration,• i.e. from an area of lowsolute concentration to anarea of high soluteconcentrationPassive TransportWater moleculeselectively permeable membranesolute
• A hypotonic solution has a LOWER concentrationof solutes• A hypertonic solution has a HIGHERconcentration of solutes• An isotonic solution is a solution that has aconcentration of solutes equal to its environmentSo how does this affect cells?Passive Transport
Human Red Blood Cells Typical Plant CellHypertonicHypotonic Isotonic HypertonicHypotonic IsotonicThe net movement of water isalways FROM a hypotonic solution(higher concentration of water) TOa hypertonic solution (lowerconcentration of water )There is NO NETMOVEMENT in isotonicsolutionsPassive Transport
Active Transport• against the concentration gradient• typically requires two carrier protein active sites:-- to recognize the substance to be carried, and- to release ATP (adenosine triphosphate) to providethe energy for the protein carriers or "pumps".Active Transporthttp://www.bbc.co.uk/education/asguru/biology/01cellbiology/05pathways/08active/index.shtml
Comparison of passive and active transportTELL ME THEANSWER PLEASESimilarities? Differences?
Discuss the processes of passive transportand active transport in living organisms:a) gaseous exchange in thealveoli and blood capillariessimple diffusionb) absorption of digested food in thevillusfacilitateddiffusion
c) absorption of water by root hairs of a plantDiscuss the processes of passive transportand active transport in living organisms:
d) ion intake by root hairs of a plantactive transportDiscuss the processes of passive transportand active transport in living organisms:
Further information:http://search.yahoo.com/search?ei=UTF-8&fr=yfp-t-501&fp_ip=MY&p=plashttp://www.phschool.com/science/biology_place/labbench/index.htmlTHANK YOU!http://www.flickr.com/photos/charliebrown8989/338719803/