CH 5:The Cell Membrane Movement and Mechanics

Ch 5A: Cell membrane separates cell from nonliving surroundings Controls movement in & out of the cell selectively permeable Made of phospholipids, proteins & other macromolecules

Phospholipids 2 Fatty acid tails Phosphate group head bilayer hydrophobic Phosphate group head hydrophilic bilayer Fatty acid

Phospholipid bilayer polar nonpolar polar hydrophilic heads hydrophobic tails polar hydrophilic heads

Fluid Mosaic Model

Membrane is a collage of proteins & other molecules embedded in the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Transmembrane proteins The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins. Phospholipids Filaments of cytoskeleton Cholesterol Peripheral protein Cytoplasm

Membrane Fats affects flexibility % unsaturated fatty acids varies Unsaturated = fluid Saturated = viscous Cold-adapted organisms, like winter wheat increase % unsaturated fats in autumn cholesterol in membrane

Membrane Proteins Ex: channels, permeases (pumps) determine membrane’s specific functions types: peripheral proteins surface ex: identity marker (antigens) integral proteins Embedded transmembrane protein transport proteins Ex: channels, permeases (pumps)

Why are proteins the perfect molecule to build structures in the cell membrane? 2007-2008

2 classes of amino acids  NONPOLAR/HYDROPHOBIC vs. POLAR/HYDROPHILIC nonpolar & hydrophobic

Proteins Within membrane On surfaces nonpolar amino acids hydrophobic anchors protein On surfaces polar amino acids hydrophilic Polar areas of protein Nonpolar areas of protein

Many Functions of Membrane Proteins Outside Plasma membrane Inside Transporter Enzyme activity Cell surface receptor Signal transduction - transmitting a signal from outside the cell to the cell nucleus, like receiving a hormone which triggers a receptor on the inside of the cell that then signals to the nucleus that a protein must be made. Cell surface identity marker Cell adhesion Attachment to the cytoskeleton

Membrane carbohydrates role in cell-cell recognition Identify Antigens basis for rejection of foreign cells by immune system The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

5B:Movement across the Cell Membrane 2007-2008

Diffusion 2nd Law of Thermodynamics universe tends toward disorder (entropy) Movement from high concentration of that substance to low concentration of that substance.

Diffusion Move from HIGH to LOW concentration movement of water “passive transport” no energy needed movement of water diffusion osmosis

Diffusion across cell membrane OUT waste ammonia salts CO2 H2O products IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O OUT IN cell needs materials in & products or waste out

Diffusion What molecules can get through directly? fats & other lipids [nonpolar] What molecules can NOT get through directly? polar molecules H2O ions salts, ammonia large molecules starches, proteins lipid inside cell outside cell salt NH3 sugar aa H2O

Facilitated Diffusion Diffusion through protein channels channels move specific molecules across no energy needed facilitated = with help open channel = fast transport high low Donuts! Each transport protein is specific as to the substances that it will translocate (move). For example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer. Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane -- simply provide corridors allowing a specific molecule or ion to cross the membrane. These channel proteins allow fast transport. For example, water channel proteins, aquaporins, facilitate massive amounts of diffusion. “The Bouncer”

specific channels allow specific material across cell membrane inside cell H2O aa sugar salt outside cell NH3

conformational change Active Transport Cells may need to move molecules against concentration gradient Conformational shape change transports solute from one side to other protein “pump” “costs” energy = ATP conformational change low high Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the protein changes shape. These shape changes could be triggered by the binding and release of the transported molecule. This is model for active transport. ATP

How about large molecules? use vesicles & vacuoles Endocytosis [in] phagocytosis = “cellular eating” pinocytosis = “cellular drinking” Exocytosis [out] exocytosis

The Special Case of Water 2007-2008

Osmosis is diffusion of water high concentration of water to low concentration of water

Concentration of water Direction of osmosis is determined by comparing total solute concentrations Hypertonic - more solute, less water Hypotonic - less solute, more water Isotonic - equal solute, equal water hypotonic hypertonic water net movement of water

Managing water balance Cell survival depends on balance freshwater balanced saltwater

Isotonic Solution no net movement of water volume of cell is stable flows across membrane equally, in both directions volume of cell is stable animal cell immersed in mild salt solution example: blood cells in blood plasma balanced

Hypotonic Solution a cell in fresh water example: Paramecium problem: gains water, swells & can burst water continually enters Paramecium cell solution: contractile vacuole to pump (ATP) water out of cell ATP freshwater

Hypertonic Solution a cell in salt water plant cells example: shellfish problem: lose water & die solution: take up water or pump out salt plant cells plasmolysis = wilt saltwater

Aquaporins 1991 | 2003 Water moves rapidly into & out of cells evidence that there were water channels Peter Agre John Hopkins Roderick MacKinnon Rockefeller

Osmosis… .05 M .03 M Cell (compared to beaker)  hypertonic or hypotonic Beaker (compared to cell)  hypertonic or hypotonic Which way does the water flow?  in or out of cell