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Diffusion and Cell Membranes - I Objectives11. Define the following terms: solute, solvent, concentration gradient, osmotic pressure, and selectively permeable. 22. Define the following processes and identify the characteristics that distinguish them from one another: diffusion, osmosis, facilitated diffusion, and active transport33. Give three factors that affect the rate of diffusion and state whether they increase or decrease the rate. . IntroductionCells maintain a constant internal environment, a process called homeostasis. In a constant environment enzymes andother cellular components can operate at optimum efficiency. The ability to selectively exchange materials with the environment is one component of the cell’s homeostatic mechanism. Ions and molecules, such as sugars, amino acids and nucleotides, must enter the cell, and the waste products of cellular processes must leave the cell. Regardless of the direction of movement, the common mediator of these processes is the cell membrane, or plasma membrane. The plasma membrane is a fluid, or mobile, mosaic of lipids and proteins. Ions and molecules cross the plasma membrane by a number of processes. Large particles are engulfed by the membrane, forming a vesicle or vacuole thatcan pass into (endocytosis) or out (exocytosis) of the cell. Some small, electrically neutral molecules diffuse through the spaces between the lipid molecules of the plasma membrane. Others bind to transport proteins embedded in the plasma membrane and transported into or out of the cell. Atoms, ions and molecules in solution are in constant motion and continuously collide with each other because of their kinetic energy. As the temperature is raised, the speed of movement of the molecules increases and they collide more frequently and with greater force. An observable consequence of this motion is Brownian motion, an erratic, vibratory motion of small particles suspended in water, which is caused by the collisions of water molecules with the particles. Diffusion also results from the kinetic energy of molecules. If a small crystal of a soluble substance is added to water, molecules of the substance break away from the crystal surface and enter solution. As a consequence of the collisions with water molecules, molecules of the substance move in a random pattern in the solution, but always away from the crystal, with some moving to the farthest reaches of the solution. This process continues until the molecules of the substance are evenly distributed throughout the water (the solvent). In a general sense, in any localized region of high concentration, the movement of molecules is, on average, away from the region of highest concentration and towards the region of lowest concentration. The gradual difference in concentration over the distance between the regions of high and low concentration is called the concentration gradient. The steeper the concentration gradient, the greater
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Chemistry for Today: General, Organic, and Biochemistry
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Chapter 2 / Exercise 2.31
Chemistry for Today: General, Organic, and Biochemistry
Seager/Slabaugh
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