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Cellular Life

Microscopic examination of living systems, beginning with Leeuwenhoek and Hooke in the seventeenth century, revealed that practically all living things except the non-cellular slime molds are composed of tiny units called cells. In 1839 Theodor Schwann suggested that all organisms are composed of cells, and Virchow in 1858 proposed the doctrine that living cells can come only from previously existing cells.

Cells vary greatly in size among different plants and animals, but generally they are quite small and can only be studied with the aid of a microscope. Units of measurement useful in microscopy are as follows:

1 centimeter (cm) = 0.4 inch

1 millimeter (mm) = 0.1 cm

1 micron (m) = 0.001 mm = 10^-4

1 nanometer (nm) = 10^-9 meter = 10^-7 cm

1 angstrom (Å) = 10^-8 cm

The average plant cell is about 0.005 cm in diameter, the average animal cell about 0.001 cm or 10 microns. Bacterial cells range in size from 5 microns down to the smallest types which cannot be viewed with a light microscope. Some cells are large enough to be seen with the naked eye - for example, insect or bird eggs, and the juice-filled cells which make up the sections of citrus fruits.

The cell is the smallest unit of life. In some animals, such as the amoeba or paramecium, one cell acts as a complete organism, taking in food and metabolizing, growing, and reproducing. In another more complex organism, such as man, there may be trillions of cells of many different types, each type forming a tissue with special tasks to perform. Using powerful electron microscopes, scientists have discovered that even one-celled organisms are highly complex systems. The one cell of an amoeba, for instance, can duplicate the primary functions of other living things composed of billions of cells. In contrast, the specialized cells of more complex organisms may each perform only one or a few functions.

The Structure and Composition of Cells²

The cells of plants, animals, fungi, and the one-celled organisms called protista share many similarities. The outer cell membrane encloses the protoplasm which composes most of the material of the cell. The protoplasm is made up of the nucleus, surrounded by the nuclear membrane, and the cytoplasm. The nucleus contains the information which controls the cell, and the cell organelles (meaning "little cells") contained in the cytoplasm carry out the functions which enable the cell to live and reproduce.

figure 4-1. Photomicrograph of surface layer cells of a lily leaf at 500X magnification. The lip-shaped structures are stomata (Greek stoma), mouth) composed of two guard cells with large, dark nuclei. The guard cells separate to form openings through which water, oxygen, and carbon dioxide diffuse. Evolutionary theory fails to explain the chance origin of any cell, let alone the highly specialized guard cells and stomata.

The common intestinal bacterium, Escherichia coli, illustrates the fantastic complexity of cells. Only 2 microns (slightly less than 1/10,000th inch) long, Escherichia contains about 30,000 tiny chemical factories called ribosomes which manufacture 2,000 to 3,000 different kinds of complicated protein molecules used by the cell. This tiny cell also contains about 2,000 other different kinds of molecules, large and small, which take part in the internal operations of the cell.

The average cell in the human body is roughly ten times as large and far more complex, linked with trillions of other cells in an amazing design to form the physical body of the being called man. These cells in man and the plants and animals contain many parts which make up their complex structure.

Some of the cell structures and organelles include:

1. Double-layered cell membrane (permits certain substances to enter or leave the cell, but blocks passage of other substances).

2. Endoplasmic reticulum, an intricately folded, double-layered membrane spread through much of the cytoplasm within the cell (apparently provides channels in its folds and convolutions for transport of substances between different parts of the cell).

3. Ribosomes, mostly attached to endoplasmic reticulum (assembly plants for the manufacture of new protein molecules).

4. Lysosomes (vesicles containing digestive enzymes).

5. Mitochondria (convert the chemical energy of sugar into other forms immediately usable by the cell).

6. Golgi bodies (package cell products in membrane sacs for transport to proper places in the cell).

7. Nucleus bounded by double-layered nuclear membrane (contains chromosomes and nucleolus).

8. Chromosomes (complex bodies composed of protein and the DNA molecules which carry the genetic information of the cell).

9. Nucleolus (manufactures ribosomes from RNA and protein molecules).

10. Centrioles formed of nine bundles of three microtubules each (organize formation of the spindle which appears when the chromosomes are duplicated in cell division).

11. Chloroplasts in plant cells (location of chlorophyll and other pigments which couple radiant energy in photosynthesis).

12. Cytoskeleton (a network of protein filaments and microtubules extending throughout the cytoplasm, involved in cellular shape and motion and the spacial organization of the structures in the cell).

All living cells possess protoplasm as their essential substance. Protoplasm is made up of many simple chemical substances such as water, inorganic ions, simple carbohydrates (sugars), amino acids, lipids (simple fat molecules) and nucleotides. Amino acid molecules, sugar molecules, and nucleotide molecules are called monomers. Most of the monomers are found in cells linked together in long chains called polymers. Since these polymer molecules are so large(containing up to hundreds of monomer molecules), they are called macromolecules. Two types of carbohydrate chains are called starch and cellulose. The amino acid polymers are called proteins. The nucleotides combine with sugar and phosphate molecules to form the long deoxyribose nucleic acid molecules (DNA) and ribose nucleic acids (RNA).

The functions of some of the components found in protoplasm may be familiar to the reader. Carbohydrates are consumed to provide energy for heat and movement. Fats (simple lipids) store energy and are combined with proteins in cell membranes. Proteins are used by living things for growth and repair of tissues and as enzymes (biological catalysts) to speed up and control chemical reactions in the cell. Proteins are also oxydized as a source of energy in the cells. Nucleic acids help to pass on an organism's characteristics to its offspring.

It is important to note that the molecules of nineteen of the twenty amino acids composing proteins exist in two forms which are mirror images of each other. These are the D and L optical isomers, termed enantiomers. Glycine, the smallest amino acid, does not have two enantiomers. All proteins in living organisms are composed of L enantiomers only. Insertion of a single D enantiomer of an amino acid in a protein chain produces a non-functional protein. As we shall see, this requirement for all living organisms poses a difficult problem for the theories of spontaneous origin of life.

The fact that all living things are made of certain basic, simple, building-block compounds, and that these substances work in the same general way in all organisms, could lead a biologist to two different conclusions. One who assumes a materialistic origin for life might say that this is evidence that all living things evolved from one common ancestral organism. A believer in the biblical God of creation would say that it provides evidence that the Creator inaugurated a careful plan which makes it possible for living things to work together with other creatures living in their environment. Intelligent planning would result in much similarity and parallelism between life systems because the most effective and efficient designs would be often used and shared as basic models.

It should be added, however, that while there are many chemical and structural features common to all forms of life, there are also many differences which are difficult to explain on the basis of common ancestry. For example, while the majority of organisms use the same system of enzymes to metabolize the sugar glucose, there are also numerous variations of this scheme. The many variations in biochemistry and structure of living things would suggest a separate creation of types, while the common features would suggest a common Creator.

The most fundamental difference between plants and animals is that plants have cells which contain chloroplasts, whereas animals do not. Chloroplasts are tiny structures containing the green pigment chlorophyll. This complicated molecules, in association with other pigments, absorbs the sun's energy and enables the plant to use the energy to transform carbon dioxide gas and water from the air and soil into simple carbohydrates (sugars) and oxygen gas. The sugar molecules produced in this photosynthesis are linked together in chains to make larger carbohydrate molecules, mainly starches and cellulose. The oxygen gas is released into the atmosphere.

Animals cannot make their food in this way, but must depend upon plants or other animals for food. Plants take the sun's energy and store it in high-energy food substances that animals can use.