Cell MembranesChapt 5 www.cellsalive.com/
Cell Membrane: At Very High Magnification & in color
Membrane Structure http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellMembranes.html
Cell Membrane Every cell is encircled by a membrane and most cells contain an extensive intracellular membrane system. Membranes fence off the cell's interior from its surroundings. Membranes let in water, certain ions and substrates and they excrete waste substances. They act to protect the cell. Without a membrane the cell contents would diffuse into the surroundings, information containing molecules would be lost and many metabolic pathways would cease to work: The cell would die! www.biologie.uni-hamburg.de/b-online/e22/22.htm
Cell Membranes: • Surround all cells • Fluid-like composition…like soap bubbles • Composed of: • Lipids in a bilayer • Proteins embedded in lipid layer (called transmembrane proteins) • And, Proteins floating within the lipid sea (called integral proteins) • And Proteins associated outside the lipid bilayer (peripheral).
Membrane Lipids • Composed largely of phospholipids • Phospholipids composed of….glycerol and two fatty acids + PO4 group • P-Lipids are polar molecules… P-Lipids are represented like this Text pg. 81
Membrane Lipidsform a Bilayer Outside layer Inside Layer
Quiz • If Phospholipids are polar, which end seeks out water and which avoids water?
Phospholipid Molecule Model phosphate (hydrophilic) glycerol fatty acids (hydrophobic)
Membrane Proteins • Integral: embedded within bilayer • Peripheral: reside outside hydrophobic region of lipids Text pg. 80
Membrane Proteins Text pg 80
Peripheral membrane proteins Integral
Membrane Models Fluid Mosaic Model - lipids arranged in bilayer with proteins embedded or associated with the lipids.
Fluid Mosaic Membrane Text pg 80
Membrane Functions allows for different conditions between inside and outside of cell subdivides cell into compartments with different internal conditions allows release of substances from cell via vesicle fusion with outer membrane: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/exocyt.gif
Membrane Permeability • Biological membranes are physical barriers..but which allow small uncharged molecules to pass… • And, lipid soluble molecules pass through • Big molecules and charged ones do NOT pass through
How to get other molecules across membranes?? • There are two ways that the molecules typically move through the membrane: • passive transport and active transport • Active transport requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane. • Passive transport does not require such an energy expenditure, and occurs spontaneously.
Membrane Transport MechanismsI. Passive Transport • Diffusion- simple movement from regions of high concentration to low concentration • Osmosis- diffusion of water across a semi-permeable membrane • Facilitated diffusion- protein transporters which assist in diffusion Text pg 89
Membrane Transport MechanismsII. Active Transport • Active transport- proteins which transport against concentration gradient. • Requires energy input Text pg 89
Diffusion Movement generated by random motion of particles. Caused by internal thermal energy. Movement always from region of high free energy(high concentration) to regions of low free energy (low conc.) Text pg 86
Osmosis Movement of water across a semi-permeable barrier. Example: Salt in water, cell membrane is barrier. Salt will NOT move across membrane, water will. Text pg 87
cell Osmosis in Hypertonic medium
Endocytosis • Transports macromolecules and large particles into the cell. • Part of the membrane engulfs the particle and folds inward to “bud off.” • Fig. 5.16
Putting Out the Garbage • Vesicles (lysosomes, other secretory vesicles) can fuse with the membrane and open up the the outside…
Membrane Permeability 1) lipid soluble solutes go through faster • smaller molecules go faster 1) uncharged & weakly charged go faster 2) Channels or pores may also exist in membrane to allow transport 1 2
Cellular Membranes REVIEW • Importance of Membranes • Membrane Structure • Proteins • Fluid Mosaic model • Permeability • Types of Transport • Passive and Active
Types of Protein Transporters: Ion Channels • work by facilitated diffusion No E! • deal with small molecules... ions • open pores are “gated”- Can change shape. • How? • How much gets in? • important in cell communication
Ion Channels • Work fast: No conform. changes needed • Not simple pores in membrane: • specific to different ions (Na, K, Ca...) • gates control opening • Toxins, drugs may affect channels • saxitoxin, tetrodotoxin • cystic fibrosis
Cystic Fibrosis • Fatal genetic disorder • Mucus build-up results in lung and liver failure • Patients die between 4 and 30 yrs. • Single gene defect • 1 in 25 Caucasians carry 1 bad gene copy • 1 in 2500 kids has it in Canada • Testing
CF Cont… • ~Proteins for diffusion of salt into the airways don't work. • ~Less salt in the airways means less water in the airways. • ~ Less water in the airways means mucus layer is very sticky (viscous). • ~Sticky mucus cannot be easily moved to clear particles from the lungs. • ~Sticky mucus traps bacteria and causes more lung infections. http://www.the-aps.org/education/lot/cell/HotT.htm
Transport ProteinsFacilitated Diffusion & Active Transport • move solutes faster across membrane • highly specific to specific solutes • can be inhibited by drugs
Types of Protein Transporters A. Facilitated Diffusion Assist in diffusion process. Solutes go from High conc to Low conc. Examples: Glucose transporters http://bio.winona.msus.edu/berg/ANIMTNS/FacDiff.htm Text pg 88
Facilitated DiffusionThe Glucose Transporters • Transport of glucose into cells mediated by proteins in the GLUT (GLUcose Transport) family of transporters. There are 7 different, but related, proteins. But, only four (GLUT1-4) are known to be involved in glucose transport. • All GLUT proteins share a set of similar structural features and are all about 500 amino acids in length (giving them a predicted molecular weight of about 55,000 Daltons) • Glucose uptake shows saturation and glucose uptake can be inhibited by drugs A classic Membrane Transport protein
Glucose TransporterCharacteristics: • integral protein: spans the membrane • 12 alpha helices woven into membrane • 55,000 mol. wt. • Text pg. 88
Glucose Transporter:How it works.. • glucose binds to outside of transporter (exterior side with higher glucose conc.) • glucose binding causes a conform. change in protein • glucose drops off inside cell • protein reassumes 1st configuration
Types of Protein Transporters: Active Transport • carrier proteins • go against the concentration gradients Low to High • require Energy to function (ATP, PEP, light energy, electron transport)
Membrane Transport:Active transport Movement from region of low free energy(low concentration) to regions of high free energy (high conc.) Requires energy input
Active Transport:Sodium-Potassium Pump Na+ low Na+ high K+ low K+ high Balance of the two ions goes hand-in-hand ATP required for maintenance of the pump
The sodium/potassium pump • All nerve and muscle cells have a high internal potassium ion concentration and a low internal sodium ion concentration. [Ki=166 mM; Ko=5 mM; Nai=18 mM; Nao=135 mM]. • Early on, it was thought that the nerve and muscle membranes were relatively impermeable to these ions and that the difference in ionic concentration was set up in early development of the cells. The membrane then became impermeable. • The later availability and use of radioactive Na and K ions showed that this was not true and that there was a metabolic pump that pumped Na out of the cell and K in; the ratio being 3 Na pumped out of the cell for every 2 K pumped into the cell.