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Biology 9/10/13 Warm-Up What is the function of the cell membrane?
What is the cell membrane composed of? Learning Goal Understand the relationship between the structure of a cell membrane and the transport of molecules
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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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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 Proteins Proteins determine membrane’s specific functions
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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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
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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Cell Membrane Mini-Poster Project
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Movement across the Cell Membrane
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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 IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O OUT waste ammonia salts CO2 H2O products cell needs materials in & products or waste out Can it be an impenetrable boundary? NO!
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Diffusion through phospholipid bilayer
What molecules can get through directly? fats & other lipids 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
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Channels Through the 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 NH3
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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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conformational change
Active Transport Cells may need to move molecules against concentration gradient 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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Transport Summary Passive Transport Active 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
active transport ATP
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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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The Special Case of Water Movement of water across the cell membrane
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Osmosis is diffusion of water
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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Passive Transport Lab
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