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Movement Across the Membrane
Chapter 6
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Phospholipids Phosphate head Fatty acid tails Arranged as a bilayer
“attracted to water” Phosphate head hydrophilic Fatty acid tails hydrophobic Arranged as a bilayer Fatty acid “repelled by water” Aaaah, one of those structure–function examples
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Permeability to polar molecules?
Membrane becomes semi-permeable via protein channels specific channels allow specific material across cell membrane inside cell H2O aa sugar salt outside cell NH3
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Proteins domains anchor molecule
Within membrane nonpolar amino acids hydrophobic anchors protein into membrane On outer surfaces of membrane in fluid polar amino acids hydrophilic extend into extracellular fluid & into cytosol Polar areas of protein Nonpolar areas of protein
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H+ NH2 H+ COOH Cytoplasm Retinal chromophore Nonpolar (hydrophobic) a-helices in the cell membrane Examples aquaporin = water channel in bacteria Porin monomer b-pleated sheets Bacterial outer membrane H2O H+ proton pump channel in photosynthetic bacteria function through conformational change = protein changes shape H2O
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Cell membrane is more than lipids…
Transmembrane proteins embedded in phospholipid bilayer create semi-permeabe channels lipid bilayer membrane protein channels in lipid bilyer membrane
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Membrane is a collage of proteins & other molecules embedded in the fluid matrix of 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 1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
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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 system The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.
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Movement across the Cell Membrane
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Diffusion 2nd Law of Thermodynamics governs biological systems
universe tends towards disorder (entropy) Movement from high concentration of that substance to low concentration of that substance. Diffusion movement from HIGH LOW concentration
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Simple Diffusion Move from HIGH to LOW concentration movement of water
“passive transport” no energy needed movement of water diffusion osmosis
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Facilitated Diffusion
Diffusion through protein channels channels move specific molecules across cell membrane 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”
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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 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 “The Doorman”
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Contractile Vacuole
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Active transport Many models & mechanisms antiport symport ATP ATP
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 antiport symport
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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 Active transport diffusion against concentration gradient LOW HIGH uses a protein pump requires ATP ATP
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Transport summary simple diffusion facilitated diffusion
ATP active transport
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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 membrane
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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
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Managing water balance
Cell survival depends on balancing water uptake & loss freshwater balanced saltwater
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Managing water balance
1 Managing water balance Hypotonic a cell in fresh water high concentration of water around cell problem: cell gains water, swells & can burst example: Paramecium ex: water continually enters Paramecium cell solution: contractile vacuole pumps water out of cell ATP plant cells turgid = full cell wall protects from bursting KABOOM! ATP No problem, here freshwater
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Managing water balance
2 Managing water balance Hypertonic a cell in salt water low concentration of water around cell problem: cell loses water & can die example: shellfish solution: take up water or pump out salt plant cells plasmolysis = wilt can recover I’m shrinking, I’m shrinking! I will survive! saltwater
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