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Transcript
The Cell Membrane AP Biology Overview The cell membrane separates a living cell from its nonliving surroundings thin barrier = 8nm thick (n=nano=10-9) Controls traffic in & out of the cell selectively permeable allows some substances to cross more easily than others- “choosy” Made of phospholipids, proteins , cholesterol, and carbohydrates. AP Biology Phospholipids Phosphate Fatty acid tails Hydrophobic “Water fearing” Phosphate group head Hydrophilic “Water loving” Arranged as a bilayer Fatty acid Inside cell Why do the phospholipids arrange themselves like this? AP Biology Outside cell More than lipids… In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer The composition of the cell membrane is called the fluid mosaic model because the phospholipid bilayer and the embedded proteins can move around like a “fluid” to let compounds into and out of the cell. AP Biology Cholesterol also makes up the cell membrane structure. It is between the tails of the phospholipids. Fluid outside the cell Phospholipids Cholesterol Cytoplasm AP Biology Membrane Proteins There are 2 types of membrane proteins: peripheral proteins loosely bound to the surface of the membrane AP Biology integral proteins Pass through the lipid bilayer Why are proteins the perfect molecule to build structures in the cell membrane? AP Biology 2007-2008 Protein Structure Proteins are molecules that are composed of amino acids linked together by peptide bonds. 20 different amino acids exist Proteins have complex shapes that include various folds, loops, and curves. Chemical bonding between portions of the protein chain aid in holding the protein together and giving it its 3dimensional shape. AP Biology AP Biology Some amino acids are hydrophobic and some are hydrophilic nonpolar & hydrophobic AP Biology AP Biology polar & hydrophilic Many Functions of Membrane Proteins Outside Plasma membrane Inside AP Biology Transporter Enzyme activity Cell surface receptor Cell surface identity marker Cell adhesion Attachment to the cytoskeleton Membrane carbohydrates Play a key role in cell-cell recognition ability of a cell to distinguish one cell from another basis for rejection of foreign cells by immune system AP Biology Sketch the structure of the cell membranelabel the phospholipid bilayer, cholesterol, integral protein, peripheral protein, and carbohydrate AP Biology Movement across the Cell Membrane AP Biology 2007-2008 What is the natural movement of molecules into and out of the cell? AP Biology Simple Diffusion Movement from HIGH to LOW concentration “passive transport” no energy needed AP Biology diffusion Diffusion across cell membrane The cell membrane is the boundary between the inside & outside cell needs food and materials in & products or waste out IN food sugars proteins lipids salts O2 H2O AP Biology OUT OUT IN waste salts CO2 H2O products Simple diffusion through phospholipid bilayer What molecules can get through directly? fats & other lipids inside cell NH3 What molecules can lipid salt NOT get through directly? outside cell AP Biology sugar aa H 2O H2O Salts Sugars How do the non-fat compounds get in/out? Membrane becomes selectivelypermeable with protein channels formed by integral proteins. This is called facilitated diffusion inside cell NH AP Biology 3 salt H 2O aa sugar outside cell Facilitated Diffusion no energy needed still moving molecules from high to low concentration high low AP Biology facilitate = to help “The Bodyguard” Osmosis Water is very important to cell function Diffusion of water from high concentration of water to low concentration of water is osmosis across a selectively-permeable membrane AP Biology Concentration of water The direction of osmosis is determined by comparing solute concentrations on each side of the membrane. What is a solute? Hypertonic - more solute, less water Hypotonic - less solute, more water Isotonic - equal solute, equal water water AP Biology hypotonic hypertonic net movement of water For each cell, label the solutions as hypertonic, hypotonic or isotonic and draw an arrow to show the direction of water movement. AP Biology Managing water balance Cell survival depends on balancing water uptake & loss AP Biology freshwater balanced saltwater Managing water balance A cell in fresh water example: Paramecium What type of environment? problem: gains water, swells & can burst ATP AP Biology solution: contractile vacuole pumps water out of cell which requires ATP (energy) freshwater Water regulation Contractile vacuole in Paramecium ATP AP Biology Managing water balance Another example: Plant cells When in a hypotonic (freshwater) environment they are constantly taking up water. Do they burst? No! Plant cells have a cell wall that AP Biology prevents bursting. They build up turgor pressure which makes the plant stand tall. Plants lose water out of their leaves= transpiration Managing water balance Another example: Plant cells How do they deal with hypertonic environment? AP Biology Getting through cell membrane Passive Transport diffusion with the concentration gradient high low Simple diffusion diffusion of hydrophobic molecules Lipids (fats) Facilitated transport diffusion of hydrophilic molecules through a protein channel high low concentration gradient Active transport diffusion against the concentration gradient low high AP Biology uses a protein pump requires ATP ATP Transport summary simple diffusion facilitated diffusion active transport AP Biology ATP Active Transport Why is active transport necessary? The Na+/K+ pump is the way that our nervous system works. By pumping ions up their concentration gradients, electricity is generated. conformational change ATP AP Biology Na+/K+ Pump The steps of the pump are as follows: 1. 3 Na+ ions inside the cell bind to the carrier 2. 3. 4. 5. 6. AP Biology protein (pump). A phosphate from ATP attaches to the pump and the pump changes shape. The 3 Na+ ions are dumped outside the cell. 2 K+ ions outside the cell bind to the pump. The phosphate detaches from the pump and the pump goes back to its original shape. The 2 K+ ions are dumped inside the cell. AP Biology How about large molecules and fluids? Endocytosis- Active Transport The taking in of large molecules (too big to pass through the membrane) or a large amount of a fluid by “engulfing”. To engulf means that the cell membrane pinches in and surrounds what is being ingested. The molecules are then digested in a pouch called a vesicle. phagocytosis = Ingesting large molecules pinocytosis = Ingesting large amounts of a fluid Draw endocytosis: AP Biology Exocytosis- Active Transport The opposite of endocytosis. The vesicle fuses with the cell membrane and it then opens up to release the contents. AP Biology