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.
The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.
The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.
Movement from high concentration of that substance to low concentration of that substance .
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.
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.
Plants: nitrate & phosphate pumps in roots. Why? Nitrate for amino acids Phosphate for DNA & membranes Not coincidentally these are the main constituents of fertilizer Supplying these nutrients to plants Replenishing the soil since plants are depleting it
The Cell MembraneAP Biology 2007-2008
Phosphate Phospholipids “attracted to water” Phosphate head hydrophilic Fatty acid tails hydrophobic Fatty acid Arranged as a bilayer “repelled by water” Aaaah, one of those structure–function examplesAP Biology
Arranged as a Phospholipid bilayer Serves as a cellular barrier / border sugar H2O salt polar hydrophilic heads nonpolar hydrophobic impermeable to polar molecules tails polar hydrophilic heads waste lipidsAP Biology
Cell membrane defines cell Cell membrane separates living cell from aqueous environment thin barrier = 8nm thick Controls traffic in & out of the cell allows some substances to cross more easily than others hydrophobic (nonpolar) vs. hydrophilic (polar)AP Biology
Permeability to polar molecules? Membrane becomes semi-permeable via protein channels specific channels allow specific material across cell membraneinside cell H2O aa sugar NH3AP Biology salt outside cell
Cell membrane is more than lipids… Transmembrane proteins embedded in phospholipid bilayer create semi-permeabe channels lipid bilayer protein channels membrane in lipid bilyer membraneAP Biology
Why are proteins the perfect molecule to build structures in the cell membrane?AP Biology 2007-2008
Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobicAP Biology
Classes of amino acids What do these amino acids have in common? I like the polar ones the best!AP Biology polar & hydrophilic
Proteins domains anchor molecule Polar areas Within membrane of protein nonpolar amino acids hydrophobic anchors protein into membrane On outer surfaces of membrane in fluid polar amino acids hydrophilic extend into extracellular fluid & Nonpolar areas of proteinAP Biology into cytosol
H+ H + Examples Retinal chromophore NH2aquaporin =water channel in bacteriaPorin monomer H2O β-pleated sheets Bacterial Nonpolar outer (hydrophobic) COOH membrane α-helices in the cell membrane Cytoplasm H+ H + proton pump channel in photosynthetic bacteria function through conformational change = HOAP Biology 2 protein changes shape
Many Functions of Membrane Proteins “Channel” Outside Plasma membrane Inside Transporter Enzyme Cell surface activity receptor “Antigen” Cell surface Cell adhesion Attachment to theAP Biology identity marker cytoskeleton
Membrane Proteins Proteins determine membrane’s specific functions cell membrane & organelle membranes each have unique collections of proteins Classes of membrane proteins: peripheral proteins loosely bound to surface of membrane ex: cell surface identity marker (antigens) integral proteins penetrate lipid bilayer, usually across whole membrane transmembrane protein ex: transport proteins channels, permeases (pumps)AP Biology
Cell membrane must be more than lipids… In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer It’s like a fluid… It’s like a mosaic… It’s the Fluid Mosaic Model!AP Biology
Membrane is a collage of proteins & other moleculesembedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid GlycolipidPhospholipids Cholesterol Transmembrane proteins Peripheral protein Cytoplasm Filaments of cytoskeleton AP Biology 1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
Membrane carbohydrates Play a key role in cell-cell recognition ability of a cell to distinguish one cell from another antigens important in organ & tissue development basis for rejection of foreign cells by immune systemAP Biology
Movement across the Cell MembraneAP Biology 2007-2008
Diffusion 2nd Law of Thermodynamics governs biological systems universe tends towards disorder (entropy) Diffusion movement from HIGH → LOW concentrationAP Biology
Simple Diffusion Move from HIGH to LOW concentration “passive transport” no energy needed movement of waterAP Biology diffusion osmosis
Facilitated Diffusion Diffusion through protein channels channels move specific molecules across cell membrane facilitated = with help no energy needed open channel = fast transport HIGH LOWAP Biology “The Bouncer”
Active Transport Cells may need to move molecules against concentration gradient conformational shape change transports solute from one side of membrane to other protein “pump” “costs” energy = ATP LOW conformational change ATP HIGHAP Biology “The Doorman”
Active transport Many models & mechanisms ATP ATP antiport symportAP Biology
Getting through cell membrane Passive Transport Simple diffusion diffusion of nonpolar, hydrophobic molecules lipids HIGH → LOW concentration gradient Facilitated transport diffusion of polar, hydrophilic molecules through a protein channel HIGH → LOW concentration gradient Active transport diffusion against concentration gradient LOW → HIGH uses a protein pump ATP requires ATPAP Biology
Transport summary simple diffusion facilitated diffusion active ATP transportAP Biology
How about large molecules? Moving large molecules into & out of cell through vesicles & vacuoles endocytosis phagocytosis = “cellular eating” pinocytosis = “cellular drinking” exocytosisAP Biology exocytosis
Endocytosis fuse with phagocytosis lysosome for digestion pinocytosis non-specific process triggered by receptor-mediated molecular endocytosis signalAP Biology
The Special Case of Water Movement of water across the cell membraneAP Biology 2007-2008
Osmosis is just diffusion of water Water is very important to life, so we talk about water separately Diffusion of water from HIGH concentration of water to LOW concentration of water across a semi-permeable membraneAP Biology
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 water hypotonic hypertonicAP Biology net movement of water
Managing water balance Cell survival depends on balancing water uptake & lossAP Biology freshwater balanced saltwater
1 Managing water balance Hypotonic a cell in fresh water high concentration of water around cell problem: cell gains water, swells & can burst KABOOM! example: Paramecium ex: water continually enters Paramecium cell solution: contractile vacuole ATP pumps water out of cell ATP No problem, plant cells here turgid = full cell wall protects from burstingAP Biology freshwater
Pumping water out Contractile vacuole in Paramecium ATPAP Biology
2 Managing water balance Hypertonic I’m shrinking, a cell in salt water I’m shrinking! low concentration of water around cell problem: cell loses water & can die example: shellfish solution: take up water or pump out salt Iwill survive! plant cells plasmolysis = wilt can recoverAP Biology saltwater
3 Managing water balance Isotonic That’s animal cell immersed in perfect! mild salt solution no difference in concentration of water between cell & environment problem: none no net movement of water flows across membrane equally, in both directions cell in equilibrium I could be better… volume of cell is stable example: blood cells in blood plasma slightly salty IV solution in hospitalAP Biology balanced
1991 | 2003 Aquaporins Water moves rapidly into & out of cells evidence that there were water channels protein channels allowing flow of water across cell membrane Peter Agre Roderick MacKinnonAP Biology John Hopkins Rockefeller
Do you understand 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 cellAP Biology
Ghosts of Lectures Past (storage)AP Biology 2007-2008
Diffusion through phospholipid bilayer What molecules can get through directly? fats & other lipids What molecules can lipid inside cell NOT get through NH3 salt directly? polar molecules H2O ions (charged) salts, ammonia sugar aa H2O outside cell large molecules starches, proteinsAP Biology
Membrane fat composition varies Fat composition affects flexibility membrane must be fluid & flexible about as fluid as thick salad oil % unsaturated fatty acids in phospholipids keep membrane less viscous cold-adapted organisms, like winter wheat increase % in autumn cholesterol in membraneAP Biology
Diffusion across cell membrane Cell membrane is the boundary between inside & outside… separates cell from its environment Can it be an impenetrable boundary? NO! IN OUT food waste carbohydrates OUT ammonia sugars, proteins salts amino acids CO2 IN lipids H2O salts, O2, H2O productsAP Biology cell needs materials in & products or waste out