Diffusion of Molecules free essay sample

Diffusion and Osmosis Laboratory Objectives After completing this lab topic, you should be able to: 1. Describe the mechanism of diffusion at the molecular level. 2. List several factors that influence the rate of diffusion. 3. Explain why diffusion is important to cells. 4. Describe a selectively permeable membrane, and explain its role in osmosis. 5. Define hypotonic, hypertonic, and isotonic in terms of relative concentrations of osmotically active substances. 6. Discuss the influence of the cell wall on osmotic behavior in cells. . Explain how incubation plant tissues in a series of dilutions of sucrose can give an approximate measurement of osmolarity of tissue cells. 8. Explain why osmosis is important to cells. Introduction Maintaining the steady state of a cell is achieved only through regulated movement of materials through cytoplasm, across organelle membranes, and across the plasma membrane. This regulated movement facilitates communication within the cell and between cyto plasm and the external environment. The cytoplasm and extracellular environment of the cell are aqueous solutions. We will write a custom essay sample on Diffusion of Molecules or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page They are composed of water, which is the solvent, or dissolving agent, and numerous organic and inorganic molecules, which are the solutes, or dissolved substances. Organelle membranes and the plasma membrane are selectively permeable, allowing water to freely pass through but regulating the movement of solutes. The cell actively moves some dissolved substances across membranes, expending adenosine triphosphate (ATP) (biological energy) to accomplish the movement. Other substances move passively, without expenditure of ATP from the cell, but only if the cell membrane is permeable to those substances. Water and selected solutes move passively through the cell and cell membranes by diffusion, a physical process in which molecules move from an area where they are in high concentration to one where their concentration is lower. The energy driving diffusion comes only from the intrinsic kinetic energy in all atoms and molecules. If nothing hinders the movement, a solute will diffuse until it reaches equilibrium. Osmosis is a type of diffusion, the diffusion of water through a selectively permeable membrane from a region where it is highly concentrated to a region where its concentration is lower. The difference in concentration of water occurs if there is an unequal distribution of at least one dissolved substance on either side of a membrane and the membrane is impermeable to that substance. In this situation, the substance is called an osmotically active substance (OAS). For example, if a membrane that is impermeable to sucrose separates a solution of sucrose from distilled water, water will move from the distilled water where it is in higher concentration, through the membrane into the sucrose solution, where it is in lower concentration. In this case, sucrose is the osmotically active substance. Three terms, hypertonic, hypotonic, and isotonic, are used when referring to two solutions separated by a selectively permeable membrane. The hypertonic solution has a greater concentration of OAS than the solution on the other side of the membrane. It is described, therefore, as having a greater osmolarity (solute concentration expressed as molarity). The hypotonic solution has a lower concentration of OAS, or a lower osmolarity, than the solution on the other side of the membrane. When the two solutions are in equilibrium, the concentration of OAS being equal on both sides of the membrane, the osmolarities are equal and the substances are said to be isotonic. The net flow of water is from the hypotonic to the hypertonic solution. When the solutions are isotonic, there is no net flow of water across the membrane. The concept of osmotic pressure must be understood when studying osmosis. The movement of water from a hypotonic solution through the membrane into a hypertonic solution can be prevented by applying force or pressure on the hypertonic side. The force that must be applied to prevent osmotic movement of water from hypotonic to hypertonic, measured in atmospheres, is referred to as osmotic pressure. Solutions with greater concentrations of OAS have greater osmotic pressures because greater force is required to prevent water movement into them. Distilled water has an osmotic pressure of zero. EXERCISE 1. Diffusion of Molecules Through a Selectively Permeable Membrane Materials string or rubber band500 – mL beaker one-third filled water sharpiehandheld test tube holder 30 % glucose solution3 standard test tubes starch solutiondisposable transfer pipettes I2KI solution2 400 – mL beakers to hold dialysis bag Benedict’s reagent30 – cm strip of moist dialysis tubing hot plate Introduction Dialysis tubing is a membrane made of regenerated cellulose fibers formed into a flat tube. If two solutions containing dissolved substances of different molecular weights are separated by this membrane, some substances may readily pass through the pores of the membrane, but others may be excluded. Working in groups of four students, you will investigate the selective permeability of dialysis tubing. You will test the permeability of the tubing to glucose, starch (a variable-length polymer of glucose), and iodine potassium iodide.