The cell is the basic structural unit of all living organisms, which can reproduce itself exactly. Cells contain cytoplasm; the fluid-like internal environment, in which is suspended a nucleus; the part containing DNA, and a collection of organelles; sub-units specialized to carry out particular activities within the cell.
Each cell is surrounded by a thin membrane composed of lipids, proteins and carbohydrates, which controls the passage of substances into and out of the cell. Molecules on the surface of cell membranes are responsible for various cellular functions, including adhesion, hormone reception, respiration and immune reactions.
Cholesterol is also a component of cell membranes, helping to stabilize the phospholipid molecules to prevent breakage. Phosphorus ‘heads’ are hydrophilic; attracted to body fluids. Lipid ‘tails’ are hydrophobic; repelled by body fluids. The tails gravitate towards each other forming the membrane of the cell.
The entire human body is made up of cells, each of which contains its own genetic material, or DNA; a long string of molecules that tell the cell what to do.
The membrane of a healthy cell separates the contents of that cell from the fluids that surround it. At the same time the membrane must permit certain substances to enter the cell and others to leave. Heavy molecular traffic moves continuously in both directions. Fragments of water, food, gases and waste pass in and out of each cell, facilitated by two transport processes:
- Passive transport: no cellular energy required.
- Active transport: cellular energy is required.
Small particles can diffuse easily through membranes; the rate of which depending on their solubility in body fluids and concentrations on either side of the membrane. Often the movement of a substance in an inward direction is coupled with movement of another substance in an outward direction. Lipid-soluble substances tend to penetrate membranes easily by virtue of their ability to dissolve in the lipid of membranes.
Larger molecules, or those travelling against the concentration gradient, are transported with the aid of carrier molecules e.g. a protein structure attached to the cell membrane. Such a process is energy consuming. The energy required for active transport is obtained from ATP (adenosine triphosphate) which is produced from the breakdown of ingested food sources.
Each cell is an environment in which numerous biochemical processes occur. Whilst amoeba are single-celled micro-organisms, complex beings are built of millions of cells that are specially adapted to carry out particular functions. The human body consists of many trillions of cells.
Cells come in various shapes and sizes, yet are fundamentally the same in terms of their basic anatomy and chemical composition. More than 96% of total body weight is formed from just four different elements; oxygen, carbon, hydrogen and nitrogen.
The functions of a typical cell can be generalized into eight processes:
- Ingestion and egestion (elimination).
- Communication and excitation (conduction of electricity).
- Energy production.
- Protein synthesis.
- Digestion and detoxification.
All cells carry out these basic functions; malfunction of any can lead to cellular disability or death.
Certain cells develop in such a way that one or more functions become their primary occupation; such cells are said to be specialized, or differentiated. At birth a human infant has about five trillion cells, differentiated into about 100 cell types. For example, red blood cells have the duty of transporting oxygen and carbon dioxide; whilst white blood cells are charged with destroying foreign organisms or scavenging and cleaning up damaged cells.
The process of cell differentiation begins early in the development of the embryo. Once fully formed, cells of a particular type (e.g. blood cells, skin cells) always give rise to cells of the same kind.
Each cell has a set number of chromosomes in its nucleus. A chromosome is a threadlike structure that carries genetic information in the form of genes. Chromosomes are composed of a long double filament of DNA coiled into a helix. Genes are arranged in a linear manner along their length.
Body cell divide for growth and repair. During mitosis a full set of chromosomes is first duplicated, then evenly distributed between two identical new cells i.e. one cell divides to form two, thus the number of cells multiply. During childhood the addition of cells helps body structures to grow in size. In the adult body mitosis is used to replace cells that have become less functional with age, or have been damaged by illness and injury.
Different cell types have varying capacity to undergo mitosis, for example:
- Labile cells: continue to multiply throughout life; including cells of the skin, mucous membranes, bone marrow and lymph nodes.
- Stable cells: have limited capacity in times of need; including cells of the liver, pancreas, kidneys and thyroid.
- Permanent cells: lose their ability to proliferate at around the time of birth; including cells of the central nervous system, heart and skeletal muscles.
Normally, the body forms new cells as they are needed, replacing old cells that die; all body cells are programmed to live, die and be replaced on a specific schedule. Every time a cell divides it loses part of itself; eventually it can no longer duplicate and then it dies; sometimes to be replaced, sometimes not. The process of aging is associated with more cells dying than are being replaced.
In a healthy body, cells divide at a controlled rate. This predetermined rate of cell division is what keeps the body healthy. However, if cells keep multiplying when new ones are not required, a mass of tissue called a growth, or tumour, is formed. A tumour can be either benign or malignant. Benign tumours can occur anywhere in the body and generally do not pose a threat to health; they do not metastasize (spread to other parts of the body) and do not grow back if removed. Malignant tumours grow uncontrollably, interfere with normal metabolic functioning, and have the ability to metastasize and invade other tissues.
Cells rarely function as individuals. Instead groups of cells, each similarly specialized to perform a limited number of functions, aggregate into tissues. The structure of a tissue group is always related to its function. For example, muscle tissues have a high degree of contractibility, whilst nervous tissue is electrically excitable.
There are four main types of tissue throughout the body:
- Epithelial: coverings and linings e.g. of the respiratory and gastrointestinal tracts.
- Connective: structure and framework e.g. cartilage, ligaments and tendons.
- Muscular: contracting fibers e.g. of the biceps and hamstrings.
- Nervous: impulse conduction e.g. neurons and sensory receptors.
Between adjacent tissue cells are contact and communication points, held together by protein strands and a basement membrane.
Specialized tissue cells have a limited range of functions, resulting in a relative loss of self-sufficiency and making them dependent of the continued operation of supporting cells around them; themselves each highly specialized.
Due to such co-dependent relationships, the death of a single cell can produce a ripple effect extending to others around it; a process termed necrosis. Cells may be injured by a variety of natural processes including genetics and family history; plus, numerous ‘unnatural’ factors such as excess weight, poor dietary habits, environmental pollutants and sedentary living.
Cellular reactions to injury vary depending on the type, duration and severity of the damage. The response can range from a minimal, reversible disturbance; to massive, irreversible trauma with accompanying loss of function and death. In the early stages of cell injury there is significant loss of phospholipids from the cell membrane. This leads to functional alterations, deterioration of cellular structures (e.g. organelles), and impaired capacity to generate ATP (energy) for new protein synthesis.
If an injury develops rapidly it is termed acute; the symptoms of which may be immediately apparent to the person concerned. Fortunately, cells can temporarily down-regulate themselves, decreasing their requirement for ATP to the extent that they can survive short periods of stress in a form of suspended animation.
Less intense, prolonged forms of injury are termed chronic. To maintain function as best as possible, cells then adapt to environmental abnormalities, being capable of augmented functions. For example, cells can respond to a loss of function (e.g. damaged liver cells following exposure to toxic compounds such as alcohol or drugs), or to the demand for more work (e.g. an athlete’s muscles as a result of intense training) by undergoing hypertrophy; increasing the size, rather than quantity, of cells already present.
However, if the stress is sufficiently prolonged, and the synthesis of vital new structures is compromised enough that renewal becomes critically impaired; cells will eventually die. As structure determines function; deterioration in the structure (de-struction) of cells leads to a decline in their function.
Between health, characterized by optimally functioning cells; and death, where cell structures come apart completely; there is a wide range of sub-optimal function and varying degrees of structural degeneration. Healing is to correct energy blockages and remove causative factors of cell degeneration, allowing the body’s innate healing abilities to balance out – homeostasis.