Biochemistry as a Biological Science

The major quality that distinguishes life is the constant renewal of a highly ordered structure, often accompanied by an increase in the complexity of that structure. The laws of thermodynamics require cellular creation of order and complexity in matter be paid for by the continual expenditure of energy. It is for this reason that living organisms must forever take energy from their surroundings - either from sunlight, as plants do, or from foodstuffs, as animals do.

Cells, the fundamental units of life, are of quite similar size. Most bacterial cells are about 1-2 um in diameter, and most cells of higher organisms are only about 5-10 times larger. In both plants and animals, the size of the cells bears no relationship to the size of the organism. An elephant and a flea have cells of about the same size. The elephant just has more of them.

The surface/volume ratio for an object of a given shape depends on its size (Figure 1.9). The complex chemical processes in a cell and the large molecules that participate in them require a significant volume. Yet the cell must also exchange substances with its surroundings to support the active metabolism within. Too large a cell will not have enough surface for this exchange to occur, unless it is highly elongated like a vertebrate nerve cell, increasing the surface/volume ratio. Bacterial cells are smaller than the cells of higher organisms because bacterial metabolism is simple. Viruses, which are even smaller than bacteria, do not have a metabolism of their own but exist as parasites in the cells they invade.

Major differences between cell structures define the two great classes of organisms - prokaryotic and eukaryotic. The prokaryotes, which are always unicellular, include the true bacteria (eubacteria) and an ancient class called archaebacteria. A typical prokaryotic organism is shown schematically in Figure 1.10. Prokaryotic cells are surrounded by a plasma membrane and usually by a rigid cell wall. Within the membrane is the cytoplasm, which contains the cytosol - a semiliquid concentrated solution or suspension - and the structures suspended within it. In prokaryotes the cytoplasm is not divided into compartments, and the genetic information is in the form of one or more DNA molecules that exist free in the cytosol. Also suspended in the cytosol are the ribosomes, which constitute the molecular machinery for protein synthesis. The surface of a prokaryotic cell may carry pili, which aid in attaching the organism to other cells or surfaces, and flagella, which enable it to swim.

All other organisms are called eukaryotes. These include the multicellular plants and animals, as well as the unicellular and simple multicellular organisms called protozoans, fungi, and algae. A few of the many differences between eukaryotes and prokaryotes are listed in Table 1.2. Most eukaryotic cells are larger (by about 10-fold) than prokaryotic cells, but they compensate for their large size by being compartmentalized. Their specialized functions are carried out in organelles - membrane-surrounded structures lying within the surrounding cytoplasm.

A schematic view of an idealized eukaryotic cell, showing organelles and molecular components is shown in Figure 1.13. Major organelles common to most eukaryotic cells are

> Mitochondria - specialize in oxidative metabolism

> Endoplasmic reticulum - a folded membrane structure rich in ribosomes

> Golgi complex - membrane-bound chambers that function in secretion and the intracellular shuttling of new proteins;

> Nucleus - contains the cell's genetic information, encoded in DNA that is packaged into chromosomes.

Some organelles are specific to plant or animal cells. Animal cells contain digestive bodies called lysosomes, which are lacking in plants. Plant cells have chloroplasts, the sites of photosynthesis; and usually a large, water-filled vacuole. Furthermore, whereas most animal cells are surrounded only by a plasma membrane, plant cells often have a tough cellulosic cell wall outside the membrane.

See also: Ribosomes, DNA