BIOL 425/001CHAPTER 4: The Movement of Substances into and out of Cells Principles of water movement Bulk Flow The overall movement of a liquid The molecules of water move all together from one place to another because of differences in potential energy Potential energy of water = water potential Strictly speaking, water potential – chemical potential of water divided by the molal volume of water (the volume of 1 mole of water) Water moves from a region of high water potential to a region of low water potential Gravity is a source of water potential difference Pressure is another source Pressure-driven bulk flow is the predominant mechanism responsible for the long-distance transport of sap, which is an aqueous solution of sucrose and other solutes Sap moves in bulk flow from leaves to other parts of the plant body Water potential is measured in terms of the pressure required to stop the movement of water – thehydrostatic pressure (measured in bars or kPa) Water potential of pure water is zero, while the water potential of an aqueous solution of a substance will have a negative value, because a higher solute concentration lowers the water potential Diffusion Particles slowly spread from regions of high concentration to regions of low concentration Happens purely by chance (lots of particles = high chance that some will move farther away) Moving to lower concentration is moving down the concentration gradient Active transport is necessary to move up a concentration gradient Molecules diffuse independently of other types of molecules When molecules reach equilibrium, they continue to move, but now there is no net movement Defined as the dispersion of substances by a movement of their ions or molecules, which tends to equalize their concentrations throughout the system Cells and diffusion- Water, oxygen, carbon dioxide, and a few other simple molecules can diffuse freely across the plasma membrane- CO2 and O2 are nonpolar and are soluble in liquids, thus allowing them to move easily through the bilayer- Water molecules move without hindrance, apparently through momentary openings created by spontaneous movements of the bilayer- Other small, neutral, polar molecules can get through as well- Diffusion is how substances move within the cell- One of the major factors limiting cell size is dependence on diffusion – need a small distance for a slow process like diffusion- Need a steep concentration gradient Cells maintain this gradient by their metabolic activities Ex: nonphotosynthetic cells use oxygen as quickly as they get it, thereby making a steep gradient Osmosis A membrane that permits the passage of some substances but not others is selectively permeable Movement of water through a semi-permeable membrane – osmosis Involves a net flow of water from a solution with high water potential to a solution of low water potential Goes from low solute concentration to high solute concentration Not affected by what is dissolved, just by how much is dissolved Results in a buildup of pressure as water molecules continue to move The pressure that would have to be applied to stop water movement is called osmotic pressure Tendency of water to move across a membrane because of the effects of solutes on water potential is called the osmotic potential Osmosis in Living Organisms Organisms living in salt water typically have a solute concentration similar to the medium it inhabits Many types of cells live in environments with high water potentials (ex: Euglena) Water will move into the cell via osmosis Too much water could rupture the plasma membrane Prevented by an organelle called the contractile vacuole, which collects water from various parts ofthe cell body and pumps it out of the cell with a rhythmic contraction Turgor Pressure causes stiffness in plants If a plant cell is placed in a solution with a relatively high water potential, the protoplast expands and the plasma membrane stretches and exerts pressure against the cell wall The plant does not rupture because it is restrained by the wall Plant cells typically have strong solutions of salts in their vacuole as well as a store of accumulated sugars, organic acids, and amino acids Thus, plants absorb water through osmosis and build up their internal hydrostatic pressure This pressure against the cell wall is called turgor pressure Equal to and opposing the turgor pressure is the inwardly directed mechanical pressure of the cell wall (wall pressure) Turgor in the plant is especially important in the support of nonwoody plant parts If a turgid plant cell is placed in a solution with a relatively low water potential, the water will leave the cell The vacuole and the rest of the protoplast will shrink, thus causing the plasma membrane to pull away from the cell wall – this is called plasymolysis This can be reversed if the cell is transferred to pure water The loss of turgor pressure results in wilting of leaves Structure of cellular membranes Membranes are composed of a lipid bilayer with globular proteins embedded The portion of these transmembrane proteins embedded in the bilayer is hydrophobic, and the exposedportions are hydrophilic The two surfaces of a membrane differ considerable in their chemical composition The plant membrane is composed of phospholipids and sterols (particularly stigmasterol) The two layers of the bilayer have different concentrations of each The portions of the transmembrane proteins protruding from each side have different amino acid composition and tertiary structures Peripheral proteins (those lacking in discrete hydrophobic sequences) do not penetrate into the lipid bilayer Transmembrane proteins and other lipid-bound proteins are called integral proteins The lipid bilayer is quite fluid Some of the proteins float freely in the bilayer, and the lipid molecules can move laterally within it – giving the bilayer the term “fluid-mosaic” model Short chain carbs are attached to most of the proteins in the bilayer, forming glycoproteins Play an important role in molecule recognition This includes hormones, coat proteins of viruses, and molecules on the surface of bacteria Can also have glycolipids Arrangement of carb groups on the external surface of the membrane has been revealed largely by