Pre-Test

THE WORK OF CELLS

1   Living is work, and all forms of life require an ongoing supply of energy to perform the work of life. Energy is the capacity to do work, where work is defined as the ability to move matter or to rearrange a collection of matter. The cell is the basic unit of life. The work of life depends on the ability of cells to use energy to perform their many tasks. Energy enables cells to cause specific changes that are necessary for life. Many cells move or change their shapes. They grow and reproduce. Cells organize small organic molecules into proteins and DNA. Cells pump substances across membranes. They export products that are used in other parts of the organism. Cells must work just to maintain their own complex structure. The changes caused by cellular activities involve the transformation of energy from one type to another. Different kinds of change define the kinds of work performed by cells, which fall into three main categories: mechanical, synthetic, and concentration.

2   Mechanical work involves a physical change in the position of a cell or some part of the cell. One example is the movement of a cell in relation to its environment. Such movement requires the presence of some sort of appendage, such as the long, thread–like structure called a flagellum . Many bacteria have such appendages, and they wag these tail–like structures to push themselves through the environment. Sometimes, however, the environment is moved past a cell. This occurs with the beating of cilia, hair–like cellular structures that move rhythmically. The human trachea is lined with cilia that beat upward to sweep inhaled particles back to the mouth or nose, thus protecting the lungs. An example of mechanical work that involves not just a single cell but a large number of cells is muscle contraction. Muscle tissue is specialized for mechanical work. It consists of bundles of muscle fibers, each of which is an individual muscle cell containing numerous myofibrils, the contractile elements of the cell. Still another example of mechanical work occurs within a cell, and this is the movement of chromosomes during cell division. Matching pairs of chromosomes move apart and are propelled along the surface of the nucleus by the lengthening spindle fibers between them. The chromosomes eventually travel to opposite poles of the cell.

3   Synthetic work involves changes in chemical bonds. A cell is a miniature chemical industry, where thousands of reactions occur within a microscopic space. Almost every cell is continuously engaged in the important work of biosynthesis, which causes the formation of new bonds and the generation of new molecules. This activity can be observed in a population of growing cells, where the cells increase in size, number, or both, as additional molecules are being synthesized. Most structural components of a cell are in a state of constant turnover. The molecules that compose the structure are continuously being degraded and replaced. Almost all of the energy that cells need for biosynthetic work goes toward making energy–rich organic molecules from simpler starting materials, such as proteins from amino acids, and into activating these molecules for incorporation into larger molecules.

4   Concentration work is the transport of substances across a membrane or boundary when the substances are pumped against the direction of spontaneous movement. The purpose of concentration work is either to accumulate substances within a cell or to remove by–products of cellular activity that are not needed or might be toxic if allowed to remain. The transport of substances enables the cell to maintain internal concentrations of small molecules that differ from concentrations in the environment. For example, compared to its surroundings, an animal cell has a much higher concentration of potassium molecules and a much lower concentration of sodium molecules. The cell’s plasma membrane helps maintain these differences by pumping sodium out of the cell and potassium into the cell. A specialized case of concentration work is electrical work, which involves the movement of ions across a membrane against an electrical gradient. Every membrane has some electrical potential that is generated through concentration. Electrical work is important in the mechanism whereby impulses are conducted in nerve and muscle cells.

Glossary:
appendage: a part attached to an organism, such as an arm, leg, or tail
trachea: the tube that carries air from the throat to the lungs; windpipe