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The Cell Membrane
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Overview Cell membrane separates living cell from nonliving surroundings thin barrier = 8nm thick Controls traffic in & out of the cell selectively permeable allows some substances to cross more easily than others hydrophobic vs hydrophilic Made of phospholipids, proteins & other macromolecules
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Aaaah, one of those structure–function
Phospholipids Phosphate Fatty acid tails hydrophobic Phosphate group head hydrophilic Arranged as a bilayer Fatty acid Aaaah, one of those structure–function examples
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Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic
tails polar hydrophilic heads
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It’s like a fluid… It’s like a mosaic… It’s the Fluid Mosaic Model!
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!
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The Fluid Mosaic Model “A sea of phospholipids with floating icebergs of protein” Membrane composed of different components Asymmetrical b/c of different proteins on either side of membrane Lateral protein movement (Frye & Edidin Heterocaryon Expt) proteins do NOT flip-flop membrane is always changing its look
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The Fluid Mosaic Model Freeze-Fracture Technique
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Filaments of cytoskeleton
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
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Membrane Fluidity 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 (less packing) * less intermolecular forces cold-adapted organisms, like winter wheat increase % in autumn
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Membrane Fluidity Fat composition affects flexibility
% of longer vs shorter-chained fatty acids cholesterol in membrane * low temps – increases fluidity (prevents PLs from getting too close) * high temps – decreases fluidity (keeps PLs from getting too far from each other by attracting them in)
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Membrane Proteins Proteins determine membrane’s specific functions
cell membrane & organelle membranes each have unique collections of proteins Membrane proteins: peripheral proteins loosely bound to surface of membrane cell surface identity marker (antigens) integral proteins penetrate lipid bilayer, usually across whole membrane transmembrane protein transport proteins channels, permeases (pumps)
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Why are proteins the perfect molecule to build structures in the cell membrane?
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nonpolar & hydrophobic
Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobic
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I like the polar ones the best!
Classes of amino acids What do these amino acids have in common? I like the polar ones the best! polar & hydrophilic
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Proteins domains anchor molecule
Within membrane nonpolar amino acids hydrophobic anchors protein into membrane On outer surfaces of membrane polar amino acids hydrophilic extend into extracellular fluid & into cytosol Polar areas of protein Nonpolar areas of protein
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Examples water channel in bacteria
NH2 H+ COOH Cytoplasm Retinal chromophore Nonpolar (hydrophobic) a-helices in the cell membrane Examples water channel in bacteria Porin monomer b-pleated sheets Bacterial outer membrane proton pump channel in photosynthetic bacteria function through conformational change = shape change
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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
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Membrane carbohydrates
Oligosaccharides attached to proteins and lipid molecules (“glyco” prefix) 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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Any Questions??
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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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Diffusion Move from HIGH to LOW concentration movement of water
“passive transport” no energy needed movement of water diffusion osmosis
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Diffusion across cell membrane
Cell membrane is the boundary between inside & outside… separates cell from its environment Can it be an impenetrable boundary? NO! 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
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Diffusion through phospholipid bilayer
What molecules can get through directly? fats & other lipids (simple diffusion) water & small polar molecules (osmosis) lipid H2O What molecules can NOT get through directly? Large polar molecules glucose, amino acids ions salts large molecules starches, proteins inside cell outside cell salt NH3 sugar aa
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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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Channels through cell membrane
Membrane becomes semi-permeable with protein channels specific channels allow specific material across cell membrane inside cell H2O aa sugar salt outside cell H+
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conformational change
Active Transport Cells may need to move molecules against concentration gradient 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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Active transport Many models & mechanisms ATP ATP antiport symport
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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How about large molecules?
Moving large molecules into & out of cell through vesicles & vacuoles endocytosis phagocytosis = “cellular eating” pinocytosis = “cellular drinking” exocytosis exocytosis
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Endocytosis fuse with lysosome for digestion phagocytosis
non-specific process pinocytosis triggered by molecular signal receptor-mediated endocytosis
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The Special Case of Water Movement of water across the cell membrane
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Osmosis is 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
Isotonic animal cell immersed in mild salt solution example: blood cells in blood plasma problem: none no net movement of water flows across membrane equally, in both directions volume of cell is stable balanced
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Managing water balance
Hypotonic a cell in fresh water example: Paramecium problem: gains water, swells & can burst water continually enters Paramecium cell solution: contractile vacuole pumps water out of cell ATP plant cells turgid ATP freshwater
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Water regulation Contractile vacuole in Paramecium ATP
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Managing water balance
Hypertonic a cell in salt water example: shellfish problem: lose water & die solution: take up water or pump out salt plant cells plasmolysis = wilt saltwater
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Aquaporins 1991 | 2003 Water moves rapidly into & out of cells
evidence that there were water channels Peter Agre John Hopkins Roderick MacKinnon Rockefeller
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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
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Any Questions??
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