Lesson 3 Water and Life slide 1 30 Water is the biological medium on Earth All living organisms require water more than any other substance most cells surrounded by water cell composed of 70 95 water The water molecule is a polar molecule the opposite ends have opposite charges o Polarity allows water molecules to form hydrogen bonds with each other Four of water s properties that facilitate an environment for life are o 1 Cohesive behavior o 2 Ability to moderate temperature o 3 Expansion upon freezing o 4 Versatility as a solvent Hydrogen bonds hold water molecules together called cohesion o Cohesion helps the transport of water against gravity in plants Adhesion is an attraction between different substances o Ex water and plant cell walls Surface tension is a measure of how hard it is to break the surface of a liquid Water can absorb or release a large amount of heat with only a slight change in its own temperature o Takes lots of heat energy to heat up water o Conversely water molecules have to lose lots of heart energy to cool down Kinetic energy is the energy of motion Heat is a measure of the total amount of kinetic energy due to molecular motion Temperature measures the intensity of heat due to the average kinetic energy of molecules o A calorie cal is the amount of heat required to raise the temperature of 1 g of water by 1 C o Kilocalories kcal where 1 kcal 1 000 cal o The joule J is another unit of energy where 1 J 0 239 cal or 1 cal 4 184 J Specific heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1 C o Water resists changings its temperature because of its high specific heat Heat is absorbed when hydrogen bonds break Heat is released when hydrogen bonds form Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to gas o Consumer application alcohol distillation As liquid evaporates its remaining surface cools a process called evaporative cooling helps stabilize temperatures in organisms and bodies of water o Ex Sweating cools the underlying skin Ice floats in liquid water because hydrogen bonds in ice are more ordered making ice less dense o Water molecules are spaced wider apart from each other in ice form o Water has greatest density at 4 C Water The Solvent of Life o A solution is a liquid that is a homogenous mixture of substances A solvent is the dissolving agent of a solution Solute is the substance that is dissolved o An aqueous solution is one in which water is the solvent o When an ionic compound is dissolved in water each ion is surrounded by a sphere of water molecules called a hydration shell Water can dissolved ionic compounds and nonionic polar molecules Hydrophilic substance is one that has an affinity for water generally polar o Some hydrophilic substances are so large that they do not dissolve in water and become suspended in it Colloid is a stable suspension of fine particles in a liquid Hydrophobic substance is one that does not have an affinity for water non polar o Ex Oils and Hexane Lesson 11 Diffusion Active Transport Osmosis Diffusion is the tendency for molecules to spread out evenly into the available space o Exhibit a net movement in one direction o Substance diffuse down their concentration gradient the region along which the density of a chemical substance increases or decreases Movement from higher lower o No work must be down to move substance down the concentration gradient o Considered passive transport because no energy is expended by the cell to make it happen Spontaneous exergonic o Simple diffusion is the movement of gases and lipids through the phospholipid bilayer o Facilitated diffusion use transport proteins that speed the passive movement of molecules across the plasma membrane o Transport proteins include 1 Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Ex Aquaporins for water and ion channels gated 2 Carrier proteins undergo a subtle change in shape that translocates the solute binding site across the membrane Both channels and proteins carriers are specific for specific ions molecules o No energy is requires since the molecule is still moving down its concentration gradient Active Transport moves substances against their concentration gradients o Requires energy usually ATP non spontaneous o Performed by specific proteins that are all carrier proteins o Sodium Potassium pump is an example Terminal phosphate group from ATP is transferred to carrier protein to provide energy for the pump o Two combined forces called electrochemical gradient drive the diffusion of ions across a membrane 1 A chemical force ion s concentration gradient 2 An electrical force the effect of the membrane potential on the ion s movement o An electrogenic pump is a transport protein that generates voltage across a membrane The main electrogenic pump of plants fungi and bacteria is a proton pump o Cotransport occurs when active transport of a solute indirectly drives transport of other solutes Ex Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell o Bulk Transport Large molecules such as polysaccharides and proteins cross the membrane in bulk via vesicles Requires ATP Ex hormones and enzymes types o In exocytosis transport vesicles migrate to the membrane fuse with it and release their contents o In endocytosis the cell takes in macromolecules by forming vesicles from the plasma membrane 3 1 Phagocytosis a cell engulfs a particle in a vacuole fuses with a lysosome to digest the particle 2 Pinocytosis molecules are taken up when extracellular fluid is gulped into tiny vesicles 3 Receptor mediated endocytosis binding of ligands to receptors triggers vesicle formation Ligand any molecules that binds specifically to a receptor site of another molecule Osmosis is the diffusion of water across a selectively permeable membrane o Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides o Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water DNA o Isotonic solution Solute concentration is the same as that inside the cell no net water movement across the plasma membrane o Hypertonic solution Solute concentration is greater than that inside the cell cell loses water o Hypotonic
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