Cell Transport Mechanisms
1. To define the following terms: differential permeability, passive and active processes of transport, diffusion (simple diffusion, facilitated diffusion, and osmosis), solute pump, pinocytosis, and phagocytosis. 2. To describe the processes that account for the movement of substances across the plasma membrane, and to indicate the driving force for each.
3. To determine which way substances will move passively through a differentially permeable membrane (given the appropriate information on concentration differences).
he molecular composition of the plasma membrane allows it to be selective about what passes through it. It allows nutrients to enter the cell but keeps out undesirable substances. By the same token, valuable cell proteins and other substances are kept within the cell, and excreta, or wastes, pass to the exterior. This property is known as differential, or selective, permeability. Transport through the plasma membrane occurs in two basic ways. In active processes, the cell provides energy (ATP) to power the transport. In the other, passive processes, the transport process is driven by concentration or pressure differences between the interior and exterior of the cell.
The two key passive processes of membrane transport are diffusion and filtration. Diffusion is an important transport process for every cell in the body. By contrast, filtration usually occurs only across capillary walls. Each of these will be considered in turn.
Recall that all molecules possess kinetic energy and are in constant motion. As molecules move about randomly at high speeds, they collide and ricochet off one another, changing direction with each collision. For a given temperature, all matter has about the same average kinetic energy. Because kinetic energy is directly related to both mass and velocity (KE 1⁄2 mv2), smaller molecules tend to move faster. When a concentration gradient (difference in concentration) exists, the net effect of this random molecular movement is that the molecules eventually become evenly distributed throughout the environment—in other words, the process called diffusion occurs. Hence, diffusion is the movement of molecules from a region of their higher concentration to a region of their lower concentration. Diffusion’s driving force is the kinetic energy of the molecules themselves. The diffusion of particles into and out of cells is modified by the plasma membrane, which constitutes a physical barrier. In general, molecules diffuse passively through the plasma membrane if they are small enough to pass through its pores (and are aided by an electrical and/or concentration gradient), or if they can dissolve in the lipid portion of the membrane as in the case of CO2 and O2.
The diffusion of solute particles dissolved in water through a differentially permeable membrane is called simple diffusion. The diffusion of water through a differentially permeable membrane is called osmosis. Both simple diffusion and osmosis involve movement of a substance from an area of its
higher concentration to one of its lower concentration, that is, down its concentration gradient.
Solute Transport Through
This computerized simulation provides information on the
passage of water and solutes through semipermeable membranes, which may be applied to the study of transport mechanisms in living membrane-bounded cells. ACTIVITY
Simulating Dialysis (Simple Diffusion)
Choose Exercise 5B: Cell Transport Mechanisms and Permeability from the drop-down menu and click GO. Watch the Cell Transport video to see an actual dialysis experiment performed. Then click Simple Diffusion. The opening screen
will appear in a...
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