The Cell

Britannica Educational Publishing, Kara Rogers

• Publisher: Britannica Educational Publishing
• Year: 2010
• ISBN:
  9781615303861
• Topic: Scienze biologiche
• Subtopic: Biochimica

A single cell can be a self-sustaining organism or one of trillions in a larger life form. Though visible only with the help of a microscope, cells are highly structured entities that perform a myriad of functions in every living thing and store critical genetic information. This fascinating volume examines the organization of various types of cells and provides an in-depth look at how cells operate alone to generate new cells and act as part of a larger network with others.

CHAPTER 1
The Nature, Function, and Evolution of Cells

Cells are basic membrane-bound units that contain the fundamental molecules of life, the substances of which all living things are composed. A single cell is often a complete organism in itself, such as a bacterium or yeast. Other cells acquire specialized functions as they mature. These cells cooperate with other specialized cells and become the building blocks of large multicellular organisms—such as animals, including humans.

Although cells are much larger than atoms, they are still very small. The smallest known cells are a group of tiny bacteria called mycoplasmas. Some of these single-celled organisms are spheres about 0.3 micrometre in diameter, with a total mass of 10^-14 gram—equal to that of 8 billion hydrogen atoms. Cells of humans typically have a mass 400,000 times larger than the mass of a single mycoplasma bacterium, but even human cells are only about 20 micrometres across. It would require a sheet of about 10,000 human cells to cover the head of a pin, and each human organism is composed of more than 75 trillion cells.

A cell functions both as an individual unit and as a contributing part of a larger organism. As an individual unit, the cell is capable of metabolizing its own nutrients, synthesizing many types of molecules, providing its own energy, and replicating itself to produce succeeding generations. It can be viewed as an enclosed vessel, within which innumerable chemical reactions take place simultaneously. These reactions are under precise control so that they contribute to the life and procreation of the cell. In a multicellular organism, cells become specialized to perform different functions through the process of differentiation. To do this, each cell keeps in constant communication with its neighbours. As it receives nutrients from and expels wastes into its surroundings, it adheres to and cooperates with other cells. Cooperative assemblies of similar cells form tissues, and a cooperation between tissues in turn forms organs, which carry out the functions necessary to sustain the life of an organism.

EUKARYOTES AND PROKARYOTES

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Eukaryotes and prokaryotes are groups of organisms that are distinguished by fundamental differences in their cell plans. A eukaryote is any cell or organism that possesses a clearly defined nucleus. The eukaryotic cell has a nuclear membrane that surrounds the nucleus, in which the well-defined chromosomes (bodies containing the hereditary material) are located. Eukaryotic cells also contain organelles, including mitochondria (cellular energy exchangers), a Golgi apparatus (secretory device), an endoplasmic reticulum (a canal-like system of membranes within the cell), and lysosomes (digestive apparatus within many cell types).

A prokaryote is any organism that lacks a distinct nucleus and other organelles because of the absence of internal membranes. Bacteria are among the best-known prokaryotic organisms. The lack of internal membranes in prokaryotes distinguishes them from eukaryotes. The prokaryotic cell membrane is made up of phospholipids and constitutes the cell’s primary osmotic barrier. The cytoplasm contains ribosomes, which carry out protein synthesis, and a double-stranded deoxyribonucleic acid (DNA) chromosome, which is usually circular. Many prokaryotes also contain additional circular DNA molecules called plasmids, with additional dispensable cell functions, such as encoding proteins to inactivate antibiotics. Some prokaryotes have flagella. Prokaryotic flagella are distinct in design and movement from the flagella found on some eukaryotes.

THE ORGANIZATION OF CELLS

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A cell is enclosed by a plasma membrane, which forms a selective barrier that allows nutrients to enter and waste products to leave. The genetic material of cells contains the information necessary for cell growth and reproduction. Each eukaryotic cell contains only one nucleus, whereas other types of organelles are present in multiple copies in the cellular contents, or cytoplasm. These other organelles perform specific functions. For example, mitochondria are responsible for the energy transactions necessary for cell survival, lysosomes digest unwanted materials within the cell, and the endoplasmic reticulum and the Golgi apparatus play important roles in the internal organization of the cell by synthesizing selected molecules and then processing, sorting, and directing them to their proper locations. In addition, plant cells contain chloroplasts, which are responsible for photosynthesis, whereby the energy of sunlight is used to convert molecules of carbon dioxide (CO₂) and water (H₂O) into carbohydrates.

Animal cells and plant cells contain membrane-bound organelles, including a distinct nucleus. In contrast, bacterial cells do not contain organelles. Encyclopædia Britannica, Inc.

Between all these organelles is the space in the cytoplasm called the cytosol. The cytosol contains an organized framework of fibrous molecules that constitute the cytoskeleton, which gives a cell its shape, enables organelles to move within the cell, and provides a mechanism by which the cell itself can move. The cytosol also contains more than 10,000 different kinds of molecules that are involved in cellular biosynthesis, the process of making large biological molecules from small ones. Although specialized organelles are a characteristic of cells of eukaryotes and are absent from prokaryotes, all cells share strong similarities in biochemical function.

THE MOLECULES OF CELLS

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Cells contain a special collection of molecules that are enclosed by a membrane. These molecules give cells the ability to grow and reproduce. The overall process of cellular reproduction occurs in two steps: cell growth and cell division. During cell growth, the cell ingests certain molecules from its surroundings by selectively carrying them through its cell membrane. Once inside the cell, these molecules are subjected to the action of highly specialized, large, elaborately folded molecules called enzymes. Enzymes act as catalysts by binding to ingested molecules and regulating the rate at which they are chemically altered. These chemical alterations make the molecules more useful to the cell. Unlike the ingested molecules, catalysts are not chemically altered themselves during the reaction, allowing one catalyst to regulate a specific chemical reaction in many molecules.

Robert Hooke

(b. July 18, 1635, Freshwater, Isle of Wight, Eng.—d. March 3, 1703, London)

English physicist Robert Hooke is best known for his discovery of the law of elasticity, known as Hooke’s law. Hooke, however, performed research in a remarkable variety of fields, and in his studies of cork, he coined the term cell. He used this term to describe the microscopic structure of cork, which reminded him of the boxlike cells (rooms) found in monasteries.

In 1655 Hooke was employed by Robert Boyle to construct the Boylean air pump. Five years later, Hooke discovered his law of elasticity, which states that the stretching of a solid body (e.g., metal, wood) is proportional to the force applied to it. The law laid the basis for studies of stress and strain and for understanding of elastic materials. He applied these studies in his designs for the balance springs of watches. In 1662 he was appointed curator of experiments to the Royal Society of London and was elected a fellow the following year.

One of the first men to build a Gregorian reflecting telescope, Hooke discovered the fifth star in the Trapezium, an asterism in the constellation Orion, in 1664 and first suggested that Jupiter rotates on its axis. His detailed sketches of Mars were used in the 19th century to determine that planet’s rate of rotation. In 1665 he was appointed professor of geometry at Gresham College. In Micrographia (1665; “Small Drawings”) he included his studies and illustrations of the crystal structure of snowflakes and discussed the possibility of manufacturing artificial fibres by a process similar to the spinning of the silkworm. Micrographia also contained his descriptions of the cells he discovered in cork. His studies of microscopic fossils led him to become one of the first proponents of a theory of evolution.

He suggested that the force of gravity could be measured by using the motion of a pendulum (1666) and attempted to show that the Earth and Moon follow an elliptical path around the Sun. In 1672 he discovered the phenomenon of diffraction (the bending of light rays around corners), which he explained with the wave theory of light. He stated the inverse square law to describe planetary motions in 1678, a law that Newton later used in modified form. Hooke complained that he was not given sufficient credit for the law and became involved in bitter controversy with Newton. Hooke was the first man to state in general that all matter expands when heated and that air is made up of particles separated from each other by relatively large distances.

Biological catalysts create chains of reactions. In other words, a molecule chemically transformed by one catalyst serves as the starting material, or substrate, of a second catalyst and so on. In this way, catalysts use the small molecules brought into the cell from the outside environment to create increasingly complex reaction products. These products are used for cell growth and the replication of genetic material. Once the genetic material has been copied and there are sufficient molecules to support cell division, the cell divides to create two daughter cells. Through many such cycles of cell growth and division, each parent cell can give rise to millions of daughter cells, in the process converting large amounts of inanimate matter into biologically active molecules.

Albert Claude

(b. Aug. 24, 1898, Longlier, Belgium—d. May 22, 1983, Brussels)

Belgian-American cytologist Albert Claude developed the principal methods of separating and analyzing components of the living cell. For this work, on which modern cell biology is partly based, Claude, his student George Palade, and Christian de Duve shared the Nobel Prize for Physiology or Medicine in 1974.

Upon obtaining his M.D. at Liège University, Belgium, in 1928, Claude began research at the Rockefeller Institute for Medical Research (now Rockefeller University) in New York City. In attempting to isolate the Rous sarcoma virus from chicken tumours, he spun cell extracts containing the virus in centrifuges that concentrated heavier particles in the bottom of the test tube; lighter particles settled in layers above. For comparison, he began centrifuging normal cells. This centrifugal separation of the cell components made possible a biochemical analysis of them that confirmed that the separated particles consisted of distinct organelles. Such analysis enabled Claude to discover the endoplasmic reticulum (a membranous network within cells) and to clarify the function of the mitochondria as the centres of respiratory activity.

Claude turned in 1942 to the electron microscope—an instrument that had not been used in biological research—looking first at separated components, then at whole cells. His demonstration of the instrument’s usefulness in this regard eventually helped scientists to correlate the biological activity of each cellular component with its structure and its place in the cell.

Claude, who became a citizen of the United States in 1941, returned in 1949 to Belgium. Through a legal process, he held dual citizenship in the two countries from 1949. While holding professorships at Rockefeller University (to 1972) and the Université Libre in Brussels (to 1969), he served as director (1948–71) of the Jules Bordet Institute.

The Structure of Biological Molecules

Cells are largely composed of compounds that contain carbon. The study of how carbon atoms interact with other atoms in molecular compounds forms the basis of the field of organic chemistry and plays a large role in understanding the basic functions of cells. Because carbon atoms can form stable bonds with four other atoms, they are uniquely suited for the construction of complex molecules. These complex molecules are typically made up of chains and rings that contain hydrogen, oxygen, and nitrogen atoms, as well as carbon atoms. These molecules may consist of anywhere from 10 to millions of atoms linked together in specific arrays.

Most, but not all, carbon-containing molecules in cells are built up from members of one of four different families of small organic molecules: sugars, amino acids, nucleotides, and fatty acids. Each of these families contains a group of molecules that resemble one another in both structure and function. In addition to other important functions, these molecules are used to build large macromolecules. For example, the sugars can be linked to form polysaccharides such as starch and glycogen, the amino acids can be linked to form proteins, the nucleotides can be linked to form the DNA and RNA (ribonucleic acid) of chromosomes, and the fatty acids can be linked to form the lipids of all cell membranes.

APPROXIMATE CHEMICAL
COMPOSITION OF A TYPICAL
MAMMALIAN CELL

Most of the catalytic macromolecules in cells are enzymes, and most enzymes are proteins. Key to the catalytic property of an enzyme is its tendency to undergo a change in its shape when it binds to its substrate, thus bringing together reactive groups on substrate molecules. Some enzymes are macromolecules of RNA, called ribozymes. Ribozymes consist of linear chains of nucleotides that fold in specific ways to form unique surfaces, similar to the ways in which proteins fold. As with proteins, the specific sequence of nucleotide subunits in an RNA chain gives each macromolecule a unique character. RNA molecules are much less frequently used as catalysts in cells than are protein molecules, presumably because proteins, with the greater variety of amino acid side chains, are more diverse and capable of complex shape changes. However, RNA molecules are thought to have preceded protein molecules during evolution and to have catalyzed most of the chemical reactions required before cells could evolve.

Coupled Chemical Reactions

Cells must obey the laws of chemistry and thermodynamics. When two molecules react with each other inside a cell, their atoms are rearranged, forming different molecules as reaction products and releasing or consuming energy in the process. Overall, chemical reactions occur only in one direction. That is, the final reaction product molecules cannot spontaneously react, in a reversal of the original process, to reform the original molecules. This directionality of chemical reactions is explained by the fact that molecules only change from states of higher free energy to states of lower free energy. Free energy is the ability to perform work (in this case, the “work” is the rearrangement of atoms in the chemical reaction). When work is performed, some free energy is used and lost, with the result that the process ends at lower free energy. To use a familiar mechanical analogy, water at the top of a hill has the ability to perform the “work” of flowing downhill (i.e., it has high free energy), but, once it has flowed downhill, it cannot flow back up (i.e., it is in a state of low free energy). However, through another work process—that of a pump, for example—the water can be returned to the top of the hill, thereby recovering its ability to flow downhill. In thermodynamic terms, the free energy of the water has been increased by energy from an outside source (i.e., the pump). In the same way, the product molecules of a chemical reaction in a cell cannot reverse the reaction and return to their original state unless energy is supplied by coupling the process to another chemical reaction.

All catalysts, including enzymes, accelerate chemical reactions without affecting their direction. To return to the mechanical analogy, enzymes cannot make water flow uphill, although they can provide specific pathways for a downhill flow. Yet most of the chemical reactions that the cell needs to synthesize new molecules necessary for its growth require an uphill flow. In other words, the reactions require more energy than their starting molecules can provide.

Cells use a single strategy over and over again in order to get around the limitations of chemistry: they use the energy from an energy-releasing chemical reaction to drive an energy-absorbing reaction that would otherwise not occur. A useful mechanical analogy might be a mill wheel driven by the water in a stream. To flow downhill, the water is forced to flow past the blades of the wheel, causing the...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Introduction
• Chapter 1: The Nature, Function, and Evolution of Cells
• Chapter 2: Cell Membranes and Cell Walls
• Chapter 3: Cellular Organelles
• Chapter 4: The Cell Matrix and Mechanisms of Cell Communication
• Chapter 5: Cell Division, Growth, and Differentiation
• Chapter 6: The Study of Cells
• Glossary
• Bibliography
• Index