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Cell

Human Cell

The Cell - The basic structural unit of all living organisms. The human body consists of billions of cells, structurally and functionally integrated to perform the complex tasks necessary for life. In spite of variation in size and function, most human cells have a similar basic structure.

Each cell is an invisibly small bag containing liquid cytoplasm, surrounded by a cell membrane that regulates the passage of useful substances (such as oxygen and nutrients) into the cell; and waste materials (such as carbon dioxide) and manufactured substances (such as hormones) out of the cell.

Some cells, such as those lining the small intestine, have microvilli, projections that increase the cells’ surface area to facilitate absorption. All cells, except red blood cells, have a nucleus, a control centre that governs all major cell activities by regulating the amount and types of proteins made in the cell.

Inside the nucleus are the chromosomes, which are made of the nucleic acid DNA. This contains the instructions for protein synthesis, which are carried into the cytoplasm by a type of RNA, another nucleic acid, and are decoded in particles called ribosomes. The nucleus also contains a spherical structure called
the nucleolus, which plays a role in the production of ribosomes.

The cell also contains various organelles, each with a specific role. Energy is generated from the breakdown of sugars and fatty acids by mitochondria. Substances that would damage the cell if they came into contact with the cytoplasm are contained in particles called lysosomes and peroxisomes. A system
of membranes in the cytoplasm called the endoplasmic reticulum transports materials through the cell.

Flattened sacs called the Golgi complex receive and process proteins dispatched by the endoplasmic reticulum. Products for export, such as enzymes and hormones, are secreted by vesicles at the cell surface. Other materials, water, and waste products are transported and stored in the cytoplasm by vacuoles. The cytoplasm itself has a network of fine tubes (microtubules) and filaments (microfilaments) known as the cytoskeleton, which gives the cell a definite shape.
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celecoxib

celecoxib - A COX-2 inhibitor drug (a type of nonsteroidal anti-inflammatory drug) used to relieve the pain and inflammation of rheumatoid arthritis and osteoarthritis. Side effects include nausea and diarrhoea. Gastrointestinal discomfort may also occur, but can be minimized by taking the drug with food.
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CD4 count

CD4 count - A blood test used to monitor HIV infection and AIDS. The procedure counts the number of CD4 lymphocytes (white blood cells responsible for fighting infection) in a blood sample. CD4 lymphocytes are destroyed by HIV, and reduced levels of these cells indicate the progression of HIV and the eventual development of AIDS.
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cavity, dental

cavity, dental - A hole in a tooth, commonly caused by dental caries (see caries, dental).
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cavernous sinus thrombosis

cavernous sinus thrombosis - Blockage of a venous sinus (a channel for venous blood deep in the skull behind an eye socket) by a thrombus (abnormal blood clot).

The condition is usually a complication of a bacterial infection in an area drained by the veins entering the sinus. Such infections include cellulitis of the face, infections of the mouth, eye, or middle ear, sinusitis, and septicaemia.

Symptoms include severe headache, high fever, pain and loss of sensation in and above the affected eye, and proptosis (protrusion of the eyeball). Vision may become blurred and eye movements paralysed due to pressure on the optic nerve and other cranial nerves.

Treatment with antibiotic drugs and anticoagulant drugs can save vision. Left untreated, blindness results, and the infection may prove fatal.
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cauterization

cauterization - The application of a heated instrument to destroy tissues, to stop bleeding, or to promote healing, used in conditions such as haemorrhoids and cervical erosion. Cauterization has been largely replaced by electrocoagulation.
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caustic

caustic - A term used for any substance that has a burning or corrosive action on body tissues or has a burning taste. Caustic agents such as silver nitrate are used to destroy warts.
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causalgia

causalgia - A persistent, burning pain, usually in an arm or leg, most often as a result of injury to a nerve by a deep cut, limb fracture, or gunshot wound. The skin overlying the painful area may be red and tender, or blue, cold, and clammy.

Causalgia may be aggravated by light sensations, such as touch, or emotional factors. In some cases, treatment with antidepressant drugs or anticonvulsant drugs may be effective. A few people benefit from sympathectomy, an operation in which nerves are severed.
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cauliflower ear

cauliflower ear - A painful, swollen distortion of the pinna (ear flap) resulting from blows or friction that have caused bleeding in the soft cartilage. Immediate treatment after an injury is with icepacks to reduce the swelling. In severe cases, a doctor may drain blood from the ear and apply a pressure bandage.
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caudal block

caudal block - A type of nerve block, in which a local anaesthetic is injected into the lower part of the spinal canal. Caudal block may be used for obstetric and gynaecological procedures.
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cauda equina

cauda equina - A “spray’’ of nerve roots resembling a horse’s tail that descends from the lower spinal cord and occupies the lower 3rd of the spinal canal.
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cats, diseases

cats, diseases - from Various parasites and infectious organisms can spread from cats to humans. The most serious disease is rabies. Cat-scratch fever is an uncommon illness caused by infection with the bacterium ROCHALIMAEA HENDELAE following a cat scratch or bite. Cats commonly carry the protozoan TOXOPLASMA GONDII, which causes toxoplasmosis.

Infection, usually from contact with cat’s faeces, is not generally serious but has serious consequences if a woman is infected during pregnancy. Cat faeces may also carry eggs of the cat roundworm, a possible cause of toxocariasis. Rarely, a larva from an ingested roundworm egg migrates to and lodges in an eye, causing deterioration of vision or even blindness.

Children who have been playing in sand or soil contaminated by cat faeces are most commonly affected. Other cat-related disorders in humans include tinea (ringworm), fungal infections of the skin, bites from cat fleas, and allergic reactions to dander that may cause asthma or urticaria.

Diseases from cats can be avoided by good hygiene, veterinary care for animals that are ill, and regular worming and flea treatment of cats.

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cat-scratch fever

cat-scratch fever - An uncommon disease that develops after a scratch or bite by a cat. Three quarters of cases occur in children. The fever is due to infection with a small bacterium called ROCHALIMAEA HENDELAE.

The main symptom, appearing after 3–10 days, is a swollen lymph node near the bite or scratch. The node may become painful and tender, and an infected blister may develop at the site of the injury.

A fever, rash, and headache may occur. Diagnosis is confirmed by biopsy of the swollen lymph node and a skin test. Analgesic drugs (painkillers) may be used to relieve the fever and headache.

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catheterization, urinary

catheterization, urinary - Insertion of a sterile catheter into the bladder to drain urine. The procedure is used when a person is unable to empty the bladder normally or is incontinent (see incontinence, urinary).

Urinary catheterization is also used during operations, in bladder function tests such as cystometry and cystourethrography, and to monitor urine production in the critically ill.
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catheterization, cardiac

catheterization, cardiac - A diagnostic test in which a fine, sterile catheter is introduced into the heart via a blood vessel.

It is used to diagnose and assess the extent of congenital heart disease (see heart disease, congenital) and coronary artery disease, and to diagnose and treat some disorders of the heart valves (see valvuloplasty).

During the procedure, the pressure within the heart’s chambers can be measured, samples of blood and tissue can be taken, and a radiopaque substance can be injected to allow the heart’s cavities to be X-rayed.

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catheter

catheter - A flexible tube inserted into the body to drain or introduce fluids or carry out other functions. Catheters are commonly used to drain urine from the bladder (see catheterization, urinary). Other types are used to investigate the condition of the heart (see catheterization, cardiac), to widen obstructed blood vessels, or to control bleeding. (See also balloon catheter.)
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cathartic

cathartic - A term that means having the power to purify or cleanse. A cathartic drug stimulates movement of the bowels (see laxative drugs).
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catharsis

catharsis - A term meaning purification or cleansing. Catharsis is used to refer to the process of cleaning out the bowels. Sigmund Freud used the term in psychoanalytic theory to describe the expression of repressed feelings and memories.
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catatonia

catatonia - A state in which a person becomes mute or adopts a bizarre, rigid pose. It is seen in a rare form of schizophrenia and some types of brain disease.
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catarrh

catarrh - Excessive secretion of mucus by the mucous membranes lining the nose (see rhinitis), sinuses (see sinusitus), or upper air passages, due to inflammation.
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cataract surgery

cataract surgery - Removal of the lens from the eye, performed to restore sight in people whose vision is impaired by a cataract. The lens is usually replaced with a plastic implant during the operation, although for young people and those with other eye disorders, a contact or spectacle lens fitted after the operation may be preferable.
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cataract

cataract - Loss of transparency of the crystalline lens of the eye, due to changes in its delicate protein fibres. At an advanced stage, the front part of the lens becomes densely opaque, but the cataract never causes total blindness.

Almost everyone over 65 has some degree of cataract. Regular exposure to ultraviolet light increases the risk. Other causes include injury to the eye, particularly if a foreign body enters the lens.

Cataract is common in people who have diabetes mellitus. Long-term use of corticosteroid drugs may contribute to cataract development. Congenital cataract may be due to an infection of the mother in early pregnancy, especially with rubella, to the toxic effects of certain drugs in pregnancy, or be associated with Down’s syndrome or galactosaemia.

Onset of symptoms is almost imperceptible, although night driving may be affected early on. There is slow, progressive loss of visual acuity. The person may become shortsighted and notice disturbances in colour perception. When vision has become seriously impaired, cataract surgery is performed to remove the lens.
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cataplexy

cataplexy - A sudden loss of muscle tone, causing an involuntary collapse without loss of consciousness. Cataplexy is triggered by intense emotion, particularly laughter, and occurs almost exclusively in those suffering from narcolepsy and other sleep disorders.

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catalepsy

catalepsy - A physical state in which the muscles of the face, body, and limbs are maintained in a semi-rigid, statue-like position for minutes, hours, or even days. Catalepsy occurs in people with schizophrenia or epilepsy, but may also be caused by brain disease or some drugs.
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catabolism

catabolism - A chemical process by which constituents of food stored in the body (for example, fats) are broken down, releasing energy into body cells (see biochemistry; metabolism).
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castration

castration - The removal of the testes (see orchidectomy). The term is sometimes used for removal of the ovaries (see oophorectomy). Castration is performed when organs are diseased or to reduce the level of testosterone or of oestrogen in people who have certain types of cancer that are stimlated by these hormones.
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castor oil

castor oil - A colourless or yellow-tinged oil obtained from the leaves of the castor oil plant. Given orally, castor oil has a laxative action. Zinc and castor oil are combined in a soothing ointment for conditions such as nappy rash.
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cast

cast - A rigid casing applied to a limb or other part of the body to hold a broken bone or dislocated joint in position as it heals. Most casts are made of bandages, impregnated with plaster of Paris, which are applied wet and harden as they dry.
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cartilage

cartilage
cartilage - A type of connective tissue made up of varying amounts of the gellike substance collagen.

Cartilage forms an important structural component of various parts of the skeletal system, including the joints. There are 3 main types. Hyaline cartilage is a tough, smooth tissue that lines the surfaces of joints.

Fibrocartilage is solid and strong and makes up the intervertebral discs that are situated between the bones of the spine and the shock-absorbing pads in joints. Elastic cartilage is soft and rubbery and found in structures such as the outer ear and the epiglottis.
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carpal tunnel syndrome

carpal tunnel syndrome - Numbness, tingling, and pain in the thumb, index finger, and middle fingers caused by compression of the median nerve at the wrist.  Symptoms may be worse at night.

The condition results from pressure on the nerve where it passes into the hand via a gap (the “carpal tunnel’’) under a ligament at the front of the wrist. It is common among keyboard users.

It also occurs without obvious cause in middleaged women, and is associated with pregnancy, initial use of oral contraceptives, premenstrual syndrome, rheumatoid arthritis, myxoedema, and acromegaly.

The condition often disappears without treatment. Persistent symptoms may be treated with a corticosteroid drug injected under the ligament, or the ligament may be cut to relieve pressure on the nerve.

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carotid artery

carotid artery
carotid artery - Any of the main arteries of the neck and head. There are 2 common carotid arteries (left and right), each of which divides into 2 main branches (internal and external).

The left carotid arises from the aorta and runs up the neck on the left side of the trachea (windpipe). The right carotid arises from the subclavian artery (which branches off the aorta) and follows a similar route on the right side of the neck.

Just above the level of the larynx (voice-box), each carotid artery divides to form an external carotid artery and an internal carotid artery. The external arteries have multiple branches that supply most tissues in the face, scalp, mouth, and jaws; the internal arteries enter the skull to supply the brain and eyes.

At the base of the brain, branches of the 2 internal carotids and the basilar artery join to form a ring of vessels called the circle of Willis. Narrowing of these vessels may be associated with transient ischaemic attack (TIA); obstruction of them causes a stroke.
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carotene

carotene - An orange pigment found in carrots, tomatoes, and leafy green vegetables. The most important form, called beta-carotene, is an antioxidant that is converted in the intestines into vitamin A, which is essential for vision and the health of the skin and other organs. Excessive intake of foods containing carotene may result in carotenaemia.
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carotenaemia

carotenaemia - A harmless condition in which the blood level of the orange pigment carotene, found in carrots and other vegetables, is excessively high. The condition may cause temporary yellowing of the skin.
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caries, dental

caries, dental - Tooth decay; the gradual erosion of enamel (the covering of the tooth) and dentine (the substance beneath the enamel). Initial decay usually occurs on the grinding surfaces of the back teeth and areas around the gum line.

The main cause is plaque, a sticky substance consisting of food deposits, saliva by-products, and bacteria that collects on the teeth. The breakdown of food deposits by bacteria creates an acid that eats into the enamel to form cavities. Unchecked decay spreads to the dentine, and as the cavity enlarges, bacteria may invade and destroy the pulp at the tooth's core.

Advanced decay causes toothache and bad breath. Treatment consists of drilling away the area of decay and filling the cavity (see filling, dental). In advanced decay, it may be necessary to remove the infected pulp (see extraction, dental).

Water fluoridation and the use of fluoride toothpaste helps prevent caries. The risk of caries is also reduced by cutting sugar consumption, practising good oral
hygiene, and visiting the dentist regularly.
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carditis

carditis - A general term for inflammation of any part of the heart or its linings. There are 3 types of carditis: myocarditis (inflammation of the heart muscle), which is usually caused by a viral infection; endocarditis (inflammation of the internal lining of the heart), which isn usually due to a bacterial infection; and
pericarditis (inflammation of the outer covering of the heart), which is usually due to a viral or bacterial infection but may be associated with a myocardial infarction or an autoimmune disorder, such as systemic lupus erythematosus.
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cardiovascular surgery

cardiovascular surgery - The branch of surgery concerned with the heart and blood vessels. Cardiovascular surgery includes operations to prevent or repair damage caused, for example, by congenital heart disease (see heart disease, congenital), atherosclerosis, or a myocardial infarction (heart attack). Procedures include heart valve surgery, coronary artery bypass, and heart transplant.
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cardiovascular disorders

cardiovascular disorders - Disorders of the heart, blood vessels, and blood circulation (see heart disorders; arteries, disorders of; veins, disorders of).
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cardiopulmonary resuscitation

cardiopulmonary resuscitation - The administration of life-saving measures to a person who has suffered a cardiac arrest. A person in cardiac arrest is not breathing and has no detectable pulse or heartbeat.

First, mouth-to-mouth resuscitation (see artificial respiration) is given; if this fails to restart breathing, repeated chest compressions, using the heel of the hand, are applied to the lower breastbone until trained help arrives. Both these measures are used to restore blood circulation to the brain. Brain damage is likely if the brain is starved of oxygen for more than 3–4 minutes.

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cardiopulmonary bypass

cardiopulmonary bypass - The method by which the circulation of blood around the body is maintained while the heart is stopped during heart surgery. A heart–lung machine is used to maintain the supply of oxygenated blood to the body.
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cardiomyopathy

cardiomyopathy - Any disease of the heart muscle that weakens the force of bcardiac contractions, thereby reducing the efficiency of blood circulation. Cardiomyopathies may have an infectious, metabolic, nutritional, toxic, autoimmune, or degenerative cause.

However, in many cases the cause is unknown. There are 3 main types. In hypertrophic cardiomyopathy, which is usually inherited, the heart muscle is abnormally thickened. In dilated cardiomyopathy, metabolism of the heart muscle cells is abnormal and the walls of the heart tend to balloon out under pressure.

Restrictive cardiomyopathy is caused by scarring of the endocardium (the inner lining of the heart) or by amyloidosis. Symptoms of cardiomyopathy include fatigue, chest pain, and palpitations.

The condition may lead to heart failure, symptoms of which include breathing difficulty and oedema. A chest X-ray may show enlargement of the heart, and echocardiography may show thickened heart muscle. A biopsy of heart muscle may reveal muscle cell abnormalities.

Symptoms may be treated with diuretic drugs to control heart failure and antiarrhythmic drugs to correct abnormal heart rhythm. In many cases, heart muscle function deteriorates, and the only remaining option is a heart transplant.
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cardiomegaly

cardiomegaly - Enlargement of the heart. Cardiomegaly may take the form of hypertrophy (thickening) of the heart muscle or of dilatation (increase in volume) of 1 or more of the heart chambers.

Hypertrophy occurs in conditions in which the heart has to work harder than normal to pump blood around the body. These include hypertension, pulmonary hypertension, and one type of cardiomyopathy. Dilatation of a heart chamber may be due to heart valve incompetence (failure of a valve to close properly after a contraction) such as occurs in aortic insufficiency.

Symptoms may not occur until the heart has enlarged to the point where it cannot cope with additional stress. Its nreduced pumping efficiency leads to heart failure, with symptoms of breathlessness and ankle swelling. Cardiomegaly is diagnosed by physical examination, chest X-ray, and ECG. Treatment is directed at the underlying cause.

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cardiology

cardiology - The study of the function of the heart and the investigation, diagnosis, and medical treatment of disorders of the heart and blood vessels.
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cardiac stress test

cardiac stress test - One of a group of tests used to assess the function of the heart in people who experience chest pain, breathlessness, or palpitations during exercise.

The test establishes whether the patient has coronary artery disease. An ECG machine records the patterns of the heart’s electrical activity while the heart is stressed.

This is usually achieved by the patient exercising on a treadmill or cycling. Specific changes in the electrical pattern as exercise levels increase indicate angina. Cardiac stress testing may be used in conjunction with radionuclide scanning to identify damaged areas of heart muscle.
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cardiac output

cardiac output - The measured volume of blood pumped by the heart each minute, used to assess how efficiently the heart is working. At rest, a healthy adult’s heart pumps 2.5–4.5 litres of blood per minute; during exercise this figure may be as much as 30 litres per minute. A low output during exercise indicates damage to the heart muscle or major blood loss.
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cardiac neurosis

cardiac neurosis - Excessive anxiety about the condition of the heart, usually following a myocardial infarction (heart attack) or heart surgery but sometimes occurring when there is no previous heart trouble.

The person experiences symptoms, such as breathlessness and chest pain, that are typical of heart disease, and may be reluctant to exercise or work for fear of an attack. Medical investigation reveals no physical cause. Psychotherapy may be of benefit.
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cardiac cycle

cardiac cycle - The sequence of events, lasting for less than a second, that make up each beat of the heart. A heartbeat has 3 phases. In diasystole, the heart relaxes.

During atrial systole, the atria contract, and in ventricular systole, the ventricles contract. The sinoatrial node (the heart’s pacemaker) regulates the timing of the phases by sending electrical impulses to the atria and ventricles.
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cardiac arrest

cardiac arrest - A halt in the pumping action of the heart that occurs when its rhythmic muscular activity ceases. The most common cause of cardiac arrest is a myocardial infarction (heart attack). Other causes include respiratory arrest, electrical injury, loss of blood, hypothermia, drug overdose, and anaphylactic
shock.

Cardiac arrest causes sudden collapse, loss of consciousness, and absence of pulse and breathing. The diagnosis is confirmed by monitoring the electrical activity of the heart by ECG. This distinguishes between ventricular fibrillation and asystole, the 2 abnormalities of heart rhythm that cause cardiac arrest.

Ventricular fibrillation may be corrected by defibrillation. Asystole, the complete absence of heart muscle activity, is more difficult to reverse but may respond to injection of adrenaline.

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carcinomatosis

carcinomatosis - The presence of cancerous tissue in different sites of the body due to the spread of cancer cells from a primary (original) cancerous tumour. Symptoms depend on the site of the metastases (secondary tumours).

Carcinomatosis may be confirmed by X-rays or by radionuclide scanning of the bones and lungs, by biochemical tests, or during an operation.

The condition is not improved by removing the primary tumour unless the tumour is producing a hormone that stimulates the growth of metastases. Anticancer drugs or radiotherapy may be given to treat metastases.

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carcinoma in situ

carcinoma in situ - The earliest, usually curable, stage of a cancer in which it has not yet spread from the surface layer of cells of an organ.
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carcinoma

carcinoma - Any cancerous tumour (see cancer) arising from cells in the covering surface layer or lining membrane of an organ. The most common cancers of the lungs, breast, stomach, skin, cervix, colon and rectum are carcinomas.

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carcinoid syndrome

carcinoid syndrome - A rare condition caused by an intestinal or lung tumour, called a carcinoid, which secretes excess amounts of the hormone serotonin.

Carcinoid syndrome is characterized by bouts of facial flushing, diarrhoea, and wheezing, but symptoms usually occur only if the tumour has spread to the liver or has arisen in a lung. Sometimes tumours in the intestine, lung, and, more rarely, the liver are removed surgically, but, in most cases, surgery is unlikely to be of benefit.

In these circustances, symptoms may be relieved by drugs that block the action of serotonin.
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carcinogenesis

carcinogenesis - The development of a cancer caused by the action of carcinogens (cancer-causing factors) on normal cells. Carcinogens are believed to alter the DNA in cells, particularly in oncogenes (genes that control the growth and division of cells).

An altered cell divides abnormally fast, passing on the genetic changes to all offspring cells. A group of cells is established that is not affected by the body’s normal restraints on growth.

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carcinogen

carcinogen - Any agent capable of causing cancer. Chemicals are the largest group of carcinogens. Major types include polycyclic aromatic hydrocarbons (PAHs), which occur in tobacco smoke, pitch, tar fumes, and soot.

Exposure to PAHs may lead to cancer of the respiratory system or skin. Certain aromatic amines used in the chemical and rubber industries may cause bladder cancer after prolonged exposure. The best-known physical carcinogen is high-energy radiation, such as nuclear radiation and X-rays. Exposure may cause cancerous changes in cells, especially in cells that divide quickly: for example, changes in the precursors of white blood cells in the bone marrow causes leukaemia.

The risk depends on the dosage and duration of exposure. Over many years, exposure to ultraviolet radiation in sunlight can cause skin cancer. Another known physical carcinogen is asbestos (see asbestos-related diseases).

Only a few biological agents are known to cause cancer in humans. SCHISTOSOMA HAEMATOBIUM, one of the blood flukes responsible for schistosomiasis, can cause cancer of the bladder; and ASPERGILLUS FLAVUS, a fungus that produces the poison aflatoxin in stored peanuts and grain, is believed to cause liver cancer.

Viruses vassociated with cancer include strains of the human papilloma virus, which are linked to cancer of the cervix; the hepatitis B virus, which is linked to liver cancer; and a type of herpes virus which is associated with Kaposi's sarcoma.

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carbuncle

carbuncle - A cluster of interconnected boils, usually caused by the bacterium STAPHYLOCOCCUS AUREUS. The back of the neck and the buttocks are common sites.

Carbuncles mainly affect people with reduced immunity, particularly those with diabetes mellitus. Treatment is usually with an antibiotic and hot compresses. Incision and drainage may be necessary if a carbuncle is persistent.
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carbon tetrachloride (CCl4)

carbon tetrachloride (CCl4) - A colourless, poisonous, volatile chemical with a characteristic odour that is present in some home dry-cleaning fluids and industrial solvents. It can cause dizziness, confusion, and liver and kidney damage if it is inhaled or swallowed.
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carbon monoxide (CO)

carbon monoxide (CO) - A colourless, odourless, poisonous gas present in motor exhaust fumes and produced by inefficient burning of coal, gas, or oil.

Carbon monoxide binds with haemoglobin nand prevents the transportation of oxygen to body tissues. The initial symptoms of acute high-level carbon monoxide poisoning are dizziness, headache, nausea, and faintness.

Continued inhalation of the gas may lead to loss of consciousness, permanent brain damage, and even death. Low-level exposure to carbon monoxide over a period of time may cause fatigue, nausea, diarrhoea, abdominal pain, and general malaise.
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carbon dioxide (CO2)

carbon dioxide (CO2) - A colourless, odourless gas. Carbon dioxide is present in small amounts in the air and is an important by-product of metabolism in cells.

It is produced by the breakdown of substances such as carbohydrates and fats to produce energy, and is carried in the blood to the lungs and exhaled. Carbon dioxide helps to control the rate of respiration: when a person exercises, CO2 levels in the blood rise, causing the person to breathe more rapidly in order to expel carbon dioxide and to take in more oxygen.

When it is compressed and cooled to -75ºC, carbon dioxide becomes solid dry ice, which is used in cryosurgery.

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Carbon

Carbon - A nonmetallic element present in all the fundamental molecules of living organisms, such as proteins, fats, and carbohydrates, and in some inorganic molecules such as carbon dioxide, carbon monoxide, and sodium bicarbonate. Pure carbon is the major constituent of diamond, coal, charcoal, and graphite.

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