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Figure 3.19

(a) Microtubules help maintain the shape of a cell by forming an internal "scaffolding," or cytoskeleton, beneath the cell membrane and within the cytoplasm. (b) A falsely colored electron micrograph of cells showing the cytoskeleton (250x micrograph enlarged to 750x).

This is the condition of diffusional equilibrium (di-fuczhun-ul e3kwi-lib<ie-um). At diffusional equilibrium, although random movements continue, there is no further net movement, and the concentration of a substance will be uniform throughout the solution.

Random molecular movement that causes diffusion results from heat energy in the environment. The warmer the conditions and the smaller the molecules, the faster they move. At body temperature, small molecules like water move over a thousand miles per hour. However, the internal environment is a crowded place from a molecule's point of view. A single molecule may collide with other molecules a million times each second. So even at these high speeds, diffusion occurs relatively slowly. However, the small size of cells enables molecules and ions to diffuse in or out in a fraction of a second.

Consider sugar (a solute) put into a glass of water (a solvent), as illustrated in figure 3.21. The sugar at first remains in high concentration at the bottom of the glass. As the sugar molecules move about, they may collide with each other or miss each other completely. Since they are less likely to collide with each other in areas where there are fewer sugar molecules, sugar molecules gradually diffuse from areas of high concentration to areas of lower concentration (down the concentration gradient), and eventually the sugar molecules evenly distribute in the water.

To better understand how diffusion accounts for the movement of molecules through a cell membrane, imagine a container of water that is separated into two compartments by a completely permeable membrane (fig. 3.22). This membrane has many pores that are large enough for water and sugar molecules to pass through. The sugar molecules are placed in one compartment (A) but not in the other (B). Although the sugar molecules move in all directions, more move from compartment A (where they are in greater concentration) through the pores in the membrane and into compartment B (where they are in lesser concentration) than move in the other direction. Thus, sugar diffuses from compartment A to compartment B. At the same time, the water molecules diffuse from compartment B (where they are in greater concentration) through the pores into compartment A (where they are in lesser concentration). Eventually, equilibrium is achieved with equal concentrations of water and sugar in each compartment.

Diffusional equilibrium does not normally occur in living systems. Rather, the term physiological steady state, where concentrations of diffusing substances are unequal but stable, is more appropriate. For example, intracellular (in9trah-selcu-lar) oxygen is always low

Figure 3.20

(a) The pores in the nuclear envelope allow certain substances to pass between the nucleus and the cytoplasm. (b) A transmission electron micrograph of a cell nucleus (8,000x). It contains a nucleolus and masses of chromatin.

Figure 3.20

(a) The pores in the nuclear envelope allow certain substances to pass between the nucleus and the cytoplasm. (b) A transmission electron micrograph of a cell nucleus (8,000x). It contains a nucleolus and masses of chromatin.

(a)
(b)
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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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