The human body is made up of over 10 million million cells. Most are smaller than a pinpoint, yet each is a living entity, receiving nourishment from, and releasing waste into, the fluid in which the cell is bathed. Hundreds of millions of cells die every minute, but many millions more are being born as cells divide and multiply.
Under a low-powered microscope, a typical cell may look like a blob of jelly. Higher magnification reveals that it includes some strange and complex structures.
Most cells contain a clearly demarcated control center called a nucleus, which is embedded in the jellylike cytoplasm that makes up most of the rest of the cell. The major constituent of the nucleus is the complex of protein and DNA called chromatin. When a cell divides, chromatin forms into threadlike chromosomes, each of which contains genes, the hereditary factors determining the characteristics of the new cells produced by cell division. The nucleus also includes at least one nucleolus a granular unit rich in ribonucleic acid (RNA) that helps to manufacture protein molecules according to “instructions” given by the DNA. The nuclear envelope around the nucleus and its nucleoli is a thin membrane serving as a sieve that regulates the flow of nutrients and wastes both in and out of the nucleus.
Extending from the nuclear membrane through the cytoplasm are paired membranes pleated like a half-closed concertina and called the endoplasmic reticulum. These membranes help to regulate the flow of chemicals. The outside surfaces of the endoplasmic reticulum in many cells are covered with tiny granular structures called ribosomes, where ribonucleic acid builds amino acids into protein, both for the cell itself and also for other uses.
Cytoplasm also contains a variety of tiny specialized structures, including Golgi bodies, mitochondria, lysosomes, and centrioles. Golgi bodies consist of groups of flattened parallel sacs derived from the endoplasmic reticulum. Their function seems to be to take in newly made protein and add carbohydrate to produce mucoprotein (the main constituent of mucus). Lysosomes are sacs containing enzymes that digest large molecules so that the products can be digested. Mitochondria resemble minute hollow sausages (some cells have thousands of them). These are the power plants of the cell, for here its respiration occurs, and its energy is stored as a chemical compound, adenosine triphosphate (ATP). Centrioles appear like cylindrical bundles of tiny rods or fibrils. They play a part in cell division.
Cytoplasm also contains many other structures, some with functions yet to be discovered.
The whole cell is surrounded by a plasma membrane made up of fat molecules sandwiched between two protein layers. Small molecules pass in and out of this cell wall by diffusion or by special membrane channels, which are regulated by cellular hormone pH or by electrolytes. Membrane channels in the eye are pressure sensitive. Larger molecules, such as those of glucose, may be brought inside by specialized receptor molecules that lie within the membrane.
Tissues and organs
By no means are all cells identical. Their form reflects their function, and cells may be specialized in many ways. For instance, neurons (nerve cells) act like minute cables, conveying messages. Long, slim muscle cells contract. And cells lining much of the respiratory system have projecting whiplike threads that help to move mucus.
Similar cells performing a similar task form a mass of tissue. Examples are muscular tissue; nerve tissue in the brain; the epithelial tissue of the skin and of internal body linings; connective tissue, which serves to support or pack other types of tissue or body organs (themselves made of specialized tissues); and skeletal tissue, which supplies the body’s framework.
Different tissues grouped together for a common purpose form an organ, such as the stomach, heart, or lung. Cooperating groups of organs, in turn, build up body systems.
The human body is built up of nearly a dozen major interrelated systems, each designed for a special function. The skeleton provides a strong framework. Muscles are the engines of the body. Bones (with tendons, ligaments, and fibrous sheaths linking joints and muscles) provide the system for translating muscular contractions into bodily movements. The fuel and oxygen that keep muscles working travel through thousands of miles of tubing in the circulatory system, which in turn depends upon the respiratory system to supply the blood with oxygen, and the digestive system, where food is broken down into nutrients that can be used as fuel. The urinary system gets rid of processed body wastes and acids, and helps to maintain the balance of salt and water in the body. The endocrine system and the nervous system control bodily activities. And the senses, extensions of the nervous system, keep us aware of our surroundings. All these interlocking systems are contained neatly by the skin.