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Cell Membrane and the Movement across it! AP Biology 2005-2006 Cell (plasma) membrane Cells need an inside & an outside… separate cell from its environment cell membrane is the boundary Can it be an impenetrable boundary? NO! OUT IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O AP Biology OUT IN waste ammonia salts CO2 H2O products 2005-2006 cell needs materials in & products or waste out Lipids of cell membrane Membrane is made of phospholipids phospholipid bilayer inside cell phosphate hydrophilic lipid hydrophobic outside cell AP Biology Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads Phospholipid bilayer What molecules can get through directly? inside cell NH3 outside cell AP Biology lipid salt sugar aa H 2O fats & other lipids can slip directly through the phospholipid cell membrane, but… what about other stuff? A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer AP Biology 2005-2006 Membrane Proteins Proteins determine most of membrane’s specific functions cell membrane & organelle membranes each have unique collections of proteins Membrane proteins: AP Biology peripheral proteins = loosely bound to surface of membrane integral proteins = penetrate into lipid bilayer, often completely spanning the membrane = transmembrane proteins 2005-2006 Many Functions of Membrane Proteins Outside Plasma membrane Inside Transporter Enzyme activity Cell surface receptor Cell surface identity marker Cell adhesion Attachment to the cytoskeleton The many functions of proteins…. Channel proteins – wide open passage Ion channels – gated Aquaporins – water only, kidney and plant root only Carrier proteins – change shape Transport proteins – require ATP Recognition proteins - glycoproteins Adhesion proteins – anchors Receptor proteins - hormones AP Biology 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 Cholesterol Provides stability in animal cells “temperature buffer” quality for membrane Replaced with sterols in plant cells AP 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 membrane Getting through cell membrane Passive transport No energy needed Movement down concentration gradient Active transport Movement against concentration gradient low high AP Biology requires ATP Diffusion 2nd Law of Thermodynamics - Universe tends towards disorder Diffusion AP Biology movement from high low concentration Simple diffusion across membrane Which way will lipid move? lipid inside cell low lipid lipid lipid lipid lipid high outside cell lipid lipid lipid lipid AP Biology lipid lipid lipid lipid 2005-2006 Diffusion of 2 solutes Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances AP Biology Facilitated diffusion Move from HIGH to LOW concentration through a protein channel passive transport no energy needed facilitated = with help AP Biology 2005-2006 Gated channels Proteins that open only in presence of stimulus (signal) stimulus usually different from transported molecule ex: ion-gated channels ex: voltage-gated channels AP Biology 2005-2006 Active transport Cells may need molecules to move against concentration situation need to pump against concentration protein pump requires energy ATP Na+/K+ pump in nerve cell membranes AP Biology Transport summary AP Biology 2005-2006 How about large molecules? Moving large molecules into & out of cell requires ATP(energy)! through vesicles & vacuoles endocytosis phagocytosis = “cellular eating” pinocytosis = “cellular drinking” receptor-mediated endocytosis AP Biology exocytosis exocytosis 2005-2006 Endocytosis phagocytosis pinocytosis receptor-mediated endocytosis AP Biology fuse with lysosome for digestion non-specific process triggered by ligand signal 2005-2006 The Special Case of Water Movement of water across the cell membrane AP Biology 2005-2006 Osmosis is diffusion of water Diffusion of water from high concentration of water to low concentration of water AP Biology across a semi-permeable membrane 2005-2006 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 hypertonic net movement of water AP Biology 2005-2006 Managing water balance Cell survival depends on balancing water uptake & loss AP Biology freshwater balanced saltwater 2005-2006 Hypotonicity animal cell in hypotonic solution will gain water, swell & possibly burst (cytolysis) Paramecium vs. pond water Paramecium is hypertonic H2O continually enters cell contractile vacuole - pumps H2O out of cell = ATP plant cell Turgid (turgor pressure) Cell wall AP Biology Hypertonicity animal cell in hypertonic solution will loose water, shrivel & probably die salt water organisms are hypotonic compared to their environment they have to take up water & pump out salt plant cells plasmolysis = wilt AP Biology 2005-2006 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 out2005-2006 of cell AP Biology Water Potential Water moves from a place of greater water potential to a place of lesser water potential (net). As the concentration of a solute increases in a solution, the water potential will decrease accordingly. Which has the greater water potential: Which has the greater water potential: AP Biology .2M or .8M? 20% or 80% water? Animal systems evolved to support multicellular life single cell aa O2 CH CHO CO2 aa NH3 CHO O2 O2 CH aa CO2 CO2 aa NH3 CO2 NH3 CO2 O2 NH3 CO2 CO2 aa NH3 CH NH3 NH3 CO2 AP Biology CO2 NH3 CO2 intracellular waste O2 NH3 but what if the cells are clustered? CHO CO2 aa Diffusion too slow! extracellular waste for nutrients in & waste out Overcoming limitations of diffusion Evolution of exchange systems for distributing nutrients circulatory system removing wastes aa excretory system gas exchange O2 respiratory system systems to support multicellular organisms AP Biology aa CO2 CO2 NH3 NH3 CO2 NH3 CO2 CO2 NH3 O2 NH3 NH3 CO2 NH3 NH3 CO2 CH CO2 CO2 CHO CO2 aa When is Diffusion Needed? Respiratory and Circulatory systems Oxygen/CO2 transport into and out of bloodstream Skins, gills, alveoli, capillaries Excretory systems Movement of wastes into or out of blood Skin, nephridia, nephrons, gills Q: What do these systems have in common for diffusion to occur? Large surface area AP Biology Gas exchange in many forms… one-celled amphibians echinoderms insects fish mammals cilia AP Biology • size water vs. land • endotherm vs. ectotherm Counter current exchange system Water carrying gas flows in one direction, blood flows in opposite direction AP Biology just keep swimming…. How counter current exchange works 70% front 40% 100% 15% water 60% 30% counter90% 5% current blood 50% 70% 100% 50% 30% concurrent water 5% blood Blood & water flow in opposite directions AP Biology maintains diffusion gradient over whole length of gill capillary maximizing O2 transfer from water to blood back Gas Exchange on Land Advantages of terrestrial life air has many advantages over water higher concentration of O2 O2 & CO2 diffuse much faster through air respiratory surfaces exposed to air do not have to be ventilated as thoroughly as gills air is much lighter than water & therefore much easier to pump expend less energy moving air in & out Disadvantages keeping large respiratory surface moist causes high water loss reduce water loss by keeping lungs internal AP Biology Intracellular Waste What waste products Animals poison themselves from the inside by digesting proteins! are made inside of cells? what do we digest our food into… carbohydrates = CHO CO2 + H2O lipids = CHO CO2 + H2O proteins = CHON CO2 + H2O + N nucleic acids = CHOPN CO2 + H2O + P + N cellular digestion… cellular waste NH2 = AP Biology ammonia H| O || H N –C– C–OH | H R CO2 + H2O Nitrogen waste Aquatic organisms can afford to lose water ammonia most toxic Terrestrial need to conserve water urea less toxic Terrestrial egg layers need to conserve water need to protect embryo in egg uric acid AP Biology least toxic H Land animals Nitrogen waste disposal on land H H H need to conserve water must process ammonia so less toxic N C O N urea = larger molecule = less soluble = less toxic 2NH2 + CO2 = urea Urea produced in liver costs energy kidney to synthesize, but it’s worth it! filter solutes out of blood reabsorb H2O (+ any useful solutes) excrete waste urine = urea, salts, excess sugar & H2O AP Biology urine is very concentrated concentrated NH3 would be too toxic mammals Animal Osmoregulation (blood solute levels) Kidneys – generate urine by filtering wastes from blood Nephrons – basic unit of kidney Water will move towards urine or blood as it becomes hypertonic via AQUAPORINS Anti-Diuretic Hormone increases water movement back to blood….why? Blood pressure is highly regulated by the kidney because without pressure, there is no filtration = toxic blood = death!!! AP Biology Osmotic control in nephron How is all this re-absorption achieved? tight osmotic control to reduce the energy cost of excretion use diffusion instead of active transport wherever possible the value of a counter current exchange system AP Biology Osmoregulation hypotonic Water balance vs. Habitat freshwater hypotonic to body fluids water flow into cells & salt loss saltwater hypertonic to body fluids water loss from cells hypertonic land dry environment need to conserve water may also need to conserve salt Why do all land animals have to conserve water? always lose water (breathing & waste) AP may lose life while searching for water Biology