Cell Transport Mechanisms

Topics: Osmosis, Diffusion, Molecular diffusion Pages: 31 (8945 words) Published: August 27, 2012


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Cell Transport Mechanisms and Permeability
P R E - L A B Q U I Z 1. Circle the correct term. A passive process, diffusion / osmosis is the movement of solute molecules from an area of greater concentration to an area of lesser concentration. 2. A solution surrounding a cell is hypertonic if: a. it contains fewer nonpenetrating solute particles than the interior of the cell. b. it contains more nonpenetrating solute particles than the interior of the cell. c. it contains the same amount of nonpenetrating solute particles as the interior of the cell. 3. Which of the following would require an input of energy? a. diffusion b. filtration c. osmosis d. vesicular transport 4. Circle the correct term. In pinocytosis / phagocytosis, parts of the plasma membrane and cytoplasm expand and flow around a relatively large or solid material and engulf it. 5. Circle the correct term. In active / passive processes, the cell provides energy in the form of ATP to power the transport process.

Exercise Overview
The molecular composition of the plasma membrane allows it to be selective about what passes through it. It allows nutrients and appropriate amounts of ions to enter the cell and keeps out undesirable substances. For that reason, we say the plasma membrane is selectively permeable. Valuable cell proteins and other substances are kept within the cell, and metabolic wastes pass to the exterior. Transport through the plasma membrane occurs in two basic ways: either passively or actively. In passive processes, the transport process is driven by concentration or pressure differences (gradients) between the interior and exterior of the cell. In active processes, the cell provides energy (ATP) to power the transport. Two key passive processes of membrane transport are diffusion and filtration. Diffusion is an important transport process for every cell in the body. Simple diffusion occurs without the assistance of membrane proteins, and facilitated diffusion requires a membrane-bound carrier protein that assists in the transport. In both simple and facilitated diffusion, the substance being transported moves with (or along or down) the concentration gradient of the solute (from a region of its higher concentration to a region of its lower concentration). The process does not require energy from the cell. Instead, energy in the form of kinetic energy comes from the constant motion of the molecules. The movement of solutes continues until the solutes are evenly dispersed throughout the solution. At this point, the solution has reached equilibrium. A special type of diffusion across a membrane is osmosis. In osmosis, water moves with its concentration gradient, from a higher concentration of water to a lower concentration of water. It moves in response to a higher concentration of solutes on the other side of a membrane. In the body, the other key passive process, filtration, usually occurs only across capillary walls. Filtration depends upon a pressure gradient as its driving 1



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Exercise 1

force. It is not a selective process. It is dependent upon the size of the pores in the filter. The two key active processes (recall that active processes require energy) are active transport and vesicular transport. Like facilitated diffusion, active transport uses a membranebound carrier protein. Active transport differs from facilitated diffusion because the solutes move against their concentration gradient and because ATP is used to power the transport. Vesicular transport includes phagocytosis, endocytosis, pinocytosis, and exocytosis. These processes are not covered in this exercise. The activities in this exercise will explore the cell transport mechanisms individually. A C T I V I T Y 1

This activity provides information on the passage of water and solutes through...
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