The respiratory system is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals the respiratory surface is internalized as linings of the lungs. Gas exchange in the lungs occurs in millions of small air sacs called alveoli in mammals and reptiles, but atria in birds; these microscopic air sacs have a rich blood supply, thus bringing the air into close contact with the blood. These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the trachea, which branches in the middle of the chest into the two main bronchi; these enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles. In birds the bronchioles are termed parabronchi.
It is the bronchioles, or parabronchi that open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration. In most fish, a number of other aquatic animals the respiratory system consists of gills, which are either or external organs, bathed in the watery environment; this water flows over the gills by a variety of passive means. Gas exchange takes place in the gills which consist of thin or flat filaments and lammelae which expose a large surface area of vascularized tissue to the water. Other animals, such as insects, have respiratory systems with simple anatomical features, in amphibians the skin plays a vital role in gas exchange. Plants have respiratory systems but the directionality of gas exchange can be opposite to that in animals; the respiratory system in plants includes anatomical features such as stomata, that are found in various parts of the plant.
In humans and other mammals, the anatomy of a typical respiratory system is the respiratory tract. The tract is divided into a lower respiratory tract; the upper tract includes the nose, nasal cavities, sinuses and the part of the larynx above the vocal folds. The lower tract includes the lower part of the larynx, the trachea, bronchi and the alveoli; the branching airways of the lower tract are described as the respiratory tree or tracheobronchial tree. The intervals between successive branch points along the various branches of "tree" are referred to as branching "generations", of which there are, in the adult human about 23; the earlier generations, consisting of the trachea and the bronchi, as well as the larger bronchioles which act as air conduits, bringing air to the respiratory bronchioles, alveolar ducts and alveoli, where gas exchange takes place. Bronchioles are defined as the small airways lacking any cartilagenous support; the first bronchi to branch from the trachea are the right and left main bronchi.
Second only in diameter to the trachea, these bronchi enter the lungs at each hilum, where they branch into narrower secondary bronchi known as lobar bronchi, these branch into narrower tertiary bronchi known as segmental bronchi. Further divisions of the segmental bronchi are known as 4th order, 5th order, 6th order segmental bronchi, or grouped together as subsegmental bronchi. Compared to the, on average, 23 number of branchings of the respiratory tree in the adult human, the mouse has only about 13 such branchings; the alveoli are the dead end terminals of the "tree", meaning that any air that enters them has to exit via the same route. A system such as this creates dead space, a volume of air that fills the airways after exhalation and is breathed back into the alveoli before environmental air reaches them. At the end of inhalation the airways are filled with environmental air, exhaled without coming in contact with the gas exchanger; the lungs contract during the breathing cycle, drawing air in and out of the lungs.
The volume of air moved in or out of the lungs under normal resting circumstances, volumes moved during maximally forced inhalation and maximally forced exhalation are measured in humans by spirometry. A typical adult human spirogram with the names given to the various excursions in volume the lungs can undergo is illustrated below: Not all the air in the lungs can be expelled during maximally forced exhalation; this is the residual volume of about 1.0-1.5 liters. Volumes that include the residual volume can therefore not be measured by spirometry, their measurement requires special techniques. The rates at which air is breathed in or out, either through the mouth or nose, or into or out of the alveoli are tabulated below, together with how they are calculated; the number of breath cycles per minute is known as the respiratory rate. In mammals, inhalation at rest is due to the contraction of the diaphragm; this is an upwardly domed sheet of muscle that separates the thoracic cavity from the abdominal cavity.
When it contracts the sheet flattens. The contracting diaphragm pushes, but because the pelvic floo
Bronchoconstriction is the constriction of the airways in the lungs due to the tightening of surrounding smooth muscle, with consequent coughing and shortness of breath. The condition has a number of the most common being emphysema as well as asthma. Exercise and allergies can bring on the symptoms in an otherwise asymptomatic individual. With emphysema the shortness of breath due to effective bronchoconstriction from excessive thick mucous blockage can bring on panic attacks unless the individual expects this and has learned pursed lip breathing to more transfer oxygen to the blood via the damaged alveoli resulting from the disease; the most common cause of emphysema is smoking and smoking cessation is mandatory if this incurable disease is to be treated. Prevention of bronchoconstriction by this pathway is vital for emphysema sufferers and there are several anticholinergic medications that can improve the quality of life for these individuals. In combination with mucous thinning agents such as Guaifenesin significant improvement in breathing can be accomplished.
More termed exercise-induced asthma, the preferred and more accurate term exercise-induced bronchoconstriction better reflects underlying pathophysiology. It is preferred due to the former term giving the false impression that asthma is caused by exercise. In a patient with EIB, exercise follows the normal patterns of bronchodilation. However, by three minutes, the constriction sets in, which peaks at around 10–15 minutes, resolves itself by an hour. During an episode of this type of bronchoconstriction, the levels of inflammatory mediators leukotrienes and interleukin, increase. TH2-type lymphocytes are activated, with an increase in T cells expressing CD25, B cells expressing CD 23, causing increased production of IgE. After exercise, the conditions will fade within one to three minutes. In most sufferers of EIB, this is followed by a refractory period, of less than four hours, during which if exercise is repeated, the bronchoconstriction is less emphasised; this is caused by the release of prostaglandins.
The underlying cause of this type of bronchoconstriction appear to be the large volume of cool, dry air inhaled during strenuous exercise. The condition appears to improve when the air inhaled is more humidified and closer to body temperature; this specific condition, in the general population, can vary between 20 percent. This increases to around 80 percent in those with symptomatic asthma. In many cases, the constriction during or after strenuous exercise, is not clinically significant except in cases of severe to moderate emphysema. In May 2013, the American Thoracic Society issued the first treatment guidelines for EIB. While a different cause, this has similar symptoms, namely the immunological reaction involving release of inflammatory mediators. Inhalation of allergens in sensitized subjects develops into bronchoconstriction within 10 minutes, reaches a maximum within 30 minutes, resolves itself within one to three hours. In some subjects, the constriction does not return to normal, recurs after three to four hours, which may last up to a day or more.
The first is named the early asthmatic response, the latter the late asthmatic response. Bronchioconstriction can occur as a result of anaphylaxis when the allergen is not inhaled. Bronchoconstriction is defined as the narrowing of the airways in the lungs. Air flow in air passages can get restricted in three ways: a spasmodic state of the smooth muscles in bronchi and bronchioles an inflammation in the middle layers of the bronchi and bronchioles excessive production of mucus; the bronchial spasm is due to the activation of parasympathetic nervous system. Postganglionic parasympathetic fibers will release acetylcholine causing the constriction of the smooth muscle layer surrounding the bronchi; these smooth muscle cells have muscarinic M3 receptors on their membrane. The activation of these receptors by acetylcholine will activate an intracellular G protein, that in turn will activate the phospholipase C pathway, that will end in an increase of intracellular calcium concentrations and therefore contraction of the smooth muscle cell.
The muscle contraction will cause the diameter of the bronchus to decrease, therefore increasing its resistance to airflow. Bronchoconstriction is common in people with respiratory problems, such as asthma, COPD, cystic fibrosis. Medical management of transient bronchoconstriction or chronic bronchitis depends on the severity and etiology of the underlying disease and can be treated with combinations of the following medications: B-receptor agonists: Medications that stimulate the β2 receptor subtype on pulmonary smooth muscle will result in smooth muscle relaxation and increased airflow into the lungs during inhalation; these medications include short-acting beta agonists such as albuterol which last 4–6 hours, long-acting beta agonists such as salmeterol which lasts 12 hours. For example, during an acute asthma exacerbation where airway smooth muscle is constricted, inhalation of SABAs provide rapid relief of symptoms—within 5–15 minutes—and are called “rescue inhalers”. Due to their fast onset of action, they have been selected as first-line therapy for quick relief in persistent and intermittent asthma and bronchospasm.
Patients may experience dizziness, heart palpitations, hyperglycemia and muscle cramps when taking these medications. Medications that antagonize the β2 receptor may increase
Cocaine known as coke, is a strong stimulant used as a recreational drug. It is snorted, inhaled as smoke, or dissolved and injected into a vein. Mental effects may include loss of contact with reality, an intense feeling of happiness, or agitation. Physical symptoms may include a fast heart rate and large pupils. High doses can result in high blood pressure or body temperature. Effects begin within seconds to last between five and ninety minutes. Cocaine has a small number of accepted medical uses such as numbing and decreasing bleeding during nasal surgery. Cocaine is addictive due to its effect on the reward pathway in the brain. After a short period of use, there is a high risk, its use increases the risk of stroke, myocardial infarction, lung problems in those who smoke it, blood infections, sudden cardiac death. Cocaine sold on the street is mixed with local anesthetics, quinine, or sugar, which can result in additional toxicity. Following repeated doses a person may have decreased ability to feel pleasure and be physically tired.
Cocaine acts by inhibiting the reuptake of serotonin and dopamine. This results in greater concentrations of these three neurotransmitters in the brain, it can cross the blood–brain barrier and may lead to the breakdown of the barrier. Cocaine is a occurring substance found in the coca plant, grown in South America. In 2013, 419 kilograms were produced legally, it is estimated. With further processing crack cocaine can be produced from cocaine. Cocaine is the second most used illegal drug globally, after cannabis. Between 14 and 21 million people use the drug each year. Use is highest in North America followed by South America. Between one and three percent of people in the developed world have used cocaine at some point in their life. In 2013, cocaine use directly resulted in 4,300 deaths, up from 2,400 in 1990; the leaves of the coca plant have been used by Peruvians since ancient times. Cocaine was first isolated from the leaves in 1860. Since 1961, the international Single Convention on Narcotic Drugs has required countries to make recreational use of cocaine a crime.
Topical cocaine can be used as a local numbing agent to help with painful procedures in the mouth or nose. Cocaine is now predominantly used for lacrimal duct surgery; the major disadvantages of this use are cocaine's potential for cardiovascular toxicity and pupil dilation. Medicinal use of cocaine has decreased as other synthetic local anesthetics such as benzocaine, proparacaine and tetracaine are now used more often. If vasoconstriction is desired for a procedure, the anesthetic is combined with a vasoconstrictor such as phenylephrine or epinephrine; some ENT specialists use cocaine within the practice when performing procedures such as nasal cauterization. In this scenario dissolved cocaine is soaked into a ball of cotton wool, placed in the nostril for the 10–15 minutes before the procedure, thus performing the dual role of both numbing the area to be cauterized, vasoconstriction; when used this way, some of the used cocaine may be absorbed through oral or nasal mucosa and give systemic effects.
An alternative method of administration for ENT surgery is mixed with adrenaline and sodium bicarbonate, as Moffett's solution. Cocaine is a powerful nervous system stimulant, its effects can last from 30 minutes to an hour. The duration of cocaine's effects depends on the route of administration. Cocaine can be in the form of fine white powder, bitter to the taste; when inhaled or injected, it causes a numbing effect. Crack cocaine is a smokeable form of cocaine made into small "rocks" by processing cocaine with sodium bicarbonate and water. Crack cocaine is referred to. Cocaine use leads to increases in alertness, feelings of well-being and euphoria, increased energy and motor activity, increased feelings of competence and sexuality. Coca leaves are mixed with an alkaline substance and chewed into a wad, retained in the mouth between gum and cheek and sucked of its juices; the juices are absorbed by the mucous membrane of the inner cheek and by the gastrointestinal tract when swallowed. Alternatively, coca leaves can be consumed like tea.
Ingesting coca leaves is an inefficient means of administering cocaine. Because cocaine is hydrolyzed and rendered inactive in the acidic stomach, it is not absorbed when ingested alone. Only when mixed with a alkaline substance can it be absorbed into the bloodstream through the stomach; the efficiency of absorption of orally administered cocaine is limited by two additional factors. First, the drug is catabolized by the liver. Second, capillaries in the mouth and esophagus constrict after contact with the drug, reducing the surface area over which the drug can be absorbed. Cocaine metabolites can be detected in the urine of subjects that have sipped one cup of coca leaf infusion. Orally administered cocaine takes 30 minutes to enter the bloodstream. Only a third of an oral dose is absorbed, although absorption has been shown to reach 60% in controlled settings. Given the slow rate of absorption, maximum physiological and psychotropic effects are attained 60 minutes after cocaine is administered by ingestion.
While the onset of these effects is slow, the effects are sustained for approxima
Salmeterol is a long-acting β2 adrenergic receptor agonist used in the maintenance and prevention of asthma symptoms and maintenance of chronic obstructive pulmonary disease symptoms. Symptoms of bronchospasm include shortness of breath, wheezing and chest tightness, it is used to prevent breathing difficulties during exercise. It was patented in 1983 and came into medical use in 1990, it is marketed as Serevent in the US. It is available as a dry powder inhaler, it was available as a metered-dose inhaler but was discontinued in the US in 2002. It is still available as an MDI in the UK as of 2013. Salmeterol is used in moderate-to-severe persistent asthma following previous treatment with a short-acting β2 adrenoreceptor agonist such as salbutamol. LABAs should not be used as a monotherapy, they should be used concurrently with an inhaled corticosteroid, such as beclometasone dipropionate or fluticasone propionate in the treatment of asthma to minimize serious reactions such as asthma-related deaths.
In chronic obstructive pulmonary disease, LABAs may be used as monotherapy or in combination with corticosteroids. In exercise-induced bronchospasm monotherapy may be indicated in patients without persistent asthma. LABAs should not be used to treat acute symptoms; the primary noticeable difference of salmeterol from salbutamol, other short-acting β2 adrenoreceptor agonists, is its duration of action. Salmeterol lasts 12 hours in comparison with the salbutamol, which last about 4–6 hours; when used every day as prescribed, inhaled salmeterol decreases the number and severity of asthma attacks. However, like all LABA medications, it is not for use in relieving an asthma attack that has started. Inhaled salmeterol works like other β2 agonists, causing bronchodilation by relaxing the smooth muscle in the airway so as to treat the exacerbation of asthma; the long duration of action occurs by the molecules diffusing into the plasma membrane of the lung cells, slowly being released back outside the cell where they can come into contact with the β2 adrenoreceptors, with the long carbon chain forming an anchor in the membrane.
Salmeterol binding to the β2 adrenoreceptor does not induce desensitization or internalization of receptors which may contribute to its long therapeutic duration of action. Formoterol has been demonstrated to have a faster onset of action than salmeterol as a result of a lower lipophilicity, has been demonstrated to be more potent—a 12 µg dose of formoterol has been demonstrated to be equivalent to a 50 µg dose of salmeterol; the FDA assigns a Category C to salmeterol in pregnancy. Salmeterol use during pregnancy must be decided based on the risks versus benefits to the mother. There are no well-controlled studies with salmeterol in pregnant women; some animal studies showed developmental malformation when the mother was given several clinical doses orally. In rats, salmeterol xinafoate is excreted in the milk. However, since there are no data to show excretion of salmeterol in a mother's breast milk, a decision on whether to continue or discontinue therapy should be decided based on the important benefits it provides to the mother.
Pregnant and lactating women should consult their doctors before using salmeterol. Due to its vasodilation properties, the common side effects of salmeterol are dizziness, sinus infection, migraine headaches. In most cases, salmeterol side effects are minor and either don't require treatment or can be treated. Certain side effects, should be reported to a healthcare provider immediately; some of these more serious side effects include a fast heart rate, high blood pressure, worsening breathing problems. Salmeterol has an aryl alkyl group with a chain length of 11 atoms from the amine; this bulkiness makes the compound more lipophilic and it makes it selective to β2 adrenergic receptors. Salmeterol, first marketed and manufactured by Glaxo in the 1980s, was released as Serevent in 1990; the product is marketed by GSK under the Allen & Hanburys brand in the UK. In November 2005, the US Food and Drug Administration released a health advisory, alerting the public to findings that show the use of long-acting β2 agonists could lead to a worsening of symptoms, in some cases death.
While the use of inhaled LABAs are still recommended in asthma guidelines for the resulting improved symptom control, further concerns have been raised. A large meta-analysis of pooled results from 19 trials with 33,826 participants, suggests that salmeterol may increase the small risks of asthma-related deaths, this additional risk is not reduced with the additional use of inhaled steroids; this seems to occur because although LABAs relieve asthma symptoms, they promote bronchial inflammation and sensitivity without warning. Combinations of inhaled steroids and these long-acting bronchodilators are becoming more widespread. Vilanterol — an ultra-long-acting β2 adrenoreceptor agonist with a similar chemical structure
Formoterol known as eformoterol, is a long-acting β2 agonist used as a bronchodilator in the management of asthma and COPD. Formoterol has an extended duration of action compared to short-acting β2 agonists such as salbutamol, which are effective for 4 h to 6 h. LABAs such as formoterol are used as "symptom controllers" to supplement prophylactic corticosteroid therapy. A "reliever" short-acting β2 agonist is still required, since LABAs are not recommended for the treatment of acute asthma, it was patented in 1972 and came into medical use in 1998. It is marketed in the combination formulations budesonide/formoterol and mometasone/formoterol. In November 2005, the US FDA released a health advisory alerting the public to findings that show the use of long-acting β2 agonists could lead to a worsening of wheezing symptoms in some patients. At the current time, available long-acting β2 agonists include salmeterol, formoterol and sustained-release oral salbutamol. Combinations of inhaled steroids and long-acting bronchodilators are becoming more widespread – combination preparations include fluticasone/salmeterol and budesonide/formoterol.
Inhaled formoterol works like other β2 agonists, causing bronchodilation by relaxing the smooth muscle in the airway so as to treat the exacerbation of asthma. It is marketed in three forms: a dry-powder inhaler, a metered-dose inhaler and an inhalation solution, under various trade names including Atock, Atimos/Atimos Modulite, Foradil/Foradile, Oxeze/Oxis, Perforomist. Foradil/Foradile capsules for oral inhalation Oxeze/Oxis Turbuhaler DPI Atock Atimos/Atimos Modulite MDI Perforomist inhalation solution Arformoterol — an enantiopure compound used in the management of COPD Combination drugs: Aclidinium bromide/formoterol Budesonide/formoterol Mometasone furoate/formoterol
The bronchioles or bronchioli are the passageways by which air passes through the nose or mouth to the alveoli of the lungs, in which branches no longer contain cartilage or glands in their submucosa. They are branches of the bronchi, are part of the conducting zone of the respiratory system; the bronchioles divide further into smaller terminal bronchioles which are still in the conducting zone and these divide into the smaller respiratory bronchioles which mark the beginning of the respiratory region. A pulmonary lobule is the portion of the lung ventilated by one bronchiole. Bronchioles are 1 mm or less in diameter and their walls consist of ciliated cuboidal epithelium and a layer of smooth muscle. Bronchioles divide into smaller bronchioles, called terminal, which are 0.5 mm or less in diameter. Terminal bronchioles in turn divide into smaller respiratory bronchioles which divide into alveolar ducts. Terminal bronchioles mark the end of the conducting division of air flow in the respiratory system while respiratory bronchioles are the beginning of the respiratory division where gas exchange takes place.
The diameter of the bronchioles plays an important role in air flow. The bronchioles change diameter to either reduce air flow. An increase in diameter is called bronchodilation and is stimulated by either epinephrine or sympathetic nerves to increase air flow. A decrease in diameter is called bronchoconstriction and is stimulated by histamine, parasympathetic nerves, cold air, chemical irritants and other factors to decrease air flow; the primary bronchi, in each lung, which are the left and right bronchus, give rise to secondary bronchi. These in turn give rise to tertiary bronchi; the tertiary bronchi subdivide into the bronchioles. These are histologically distinct from the tertiary bronchi in that their walls do not have hyaline cartilage and they have club cells in their epithelial lining; the epithelium starts as a simple ciliated columnar epithelium and changes to simple ciliated cuboidal epithelium as the bronchioles decreases in size. The diameter of the bronchioles is said to be less than 1 mm, though this value can range from 5 mm to 0.3 mm.
As stated, these bronchioles do not have hyaline cartilage to maintain their patency. Instead, they rely on elastic fibers attached to the surrounding lung tissue for support; the inner lining of these bronchioles is thin with no glands present, is surrounded by a layer of smooth muscle. As the bronchioles get smaller they divide into terminal bronchioles; these bronchioles mark the end of the conducting zone, which covers the first division through the sixteenth division of the respiratory tract. Alveoli only become present when the conducting zone changes to the respiratory zone, from the sixteenth through the twenty-third division of the tract; the terminal bronchiole is the most distal segment of the conducting zone. It branches off the lesser bronchioles; each of the terminal bronchioles divides to form respiratory bronchioles which contain a small number of alveoli. Terminal bronchioles are lined with simple cuboidal epithelium containing club cells. Terminal bronchioles contain a limited number of no goblet cells.
Club cells are rounded protein-secreting cells. Their secretions are a non-sticky, proteinaceous compound to maintain the airway in the smallest bronchioles; the secretion, called surfactant, reduces surface tension, allowing for bronchioles to expand during inspiration and keeping the bronchioles from collapsing during expiration. Club cells, a stem cell of the respiratory system, produce enzymes that detoxify substances dissolved in the respiratory fluid; the respiratory bronchioles are the narrowest airways of one fiftieth of an inch across. The bronchi divide many times before evolving into the bronchioles; the bronchioles deliver air to the exchange surfaces of the lungs. They are interrupted by alveoli. Alveolar ducts are distal continuations of the respiratory bronchioles. Bronchospasm, a life-threatening situation, occurs when the smooth muscular tissue of the bronchioles constricts narrowing their diameter; the most common cause of this is asthma. Bronchospasm is treated by oxygen therapy and bronchodilators such as albuterol.
Diseases of the bronchioles include asthma, bronchiolitis obliterans, respiratory syncytial virus infections, influenza. The medical condition of inflammation of the bronchioles is termed bronchiolitis. Saladin, Kenneth S. Anatomy & Physiology: the Unity of Form and Function. New York, NY: McGraw-Hill, 2007. Dudek, Ronald W. High-Yield Histology, 3rd ed.. ISBN 0-7817-4763-5 Gartner, Leslie P. and James L. Hiatt. Color Atlas of Histology, 3rd ed.. ISBN 0-7817-3509-2 Gartner, Leslie P. and James L. Hiatt. Color Textbook of Histology. ISBN 0-7216-8806-3 Histology image: 13606loa – Histology Learning System at Boston University Histology image: 13607loa – Histology Learning System at Boston University Diagram at davidson.edu Histology at umdnj.edu
Metabolism is the set of life-sustaining chemical reactions in organisms. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes; these enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, respond to their environments.. Metabolic reactions may be categorized as catabolic - the breaking down of compounds. Catabolism releases energy, anabolism consumes energy; the chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by a specific enzyme. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by themselves, by coupling them to spontaneous reactions that release energy. Enzymes act as catalysts - they allow a reaction to proceed more - and they allow the regulation of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells.
The metabolic system of a particular organism determines which substances it will find nutritious and which poisonous. For example, some prokaryotes use hydrogen sulfide as a nutrient, yet this gas is poisonous to animals; the basal metabolic rate of an organism is the measure of the amount of energy consumed by all of these chemical reactions. A striking feature of metabolism is the similarity of the basic metabolic pathways among vastly different species. For example, the set of carboxylic acids that are best known as the intermediates in the citric acid cycle are present in all known organisms, being found in species as diverse as the unicellular bacterium Escherichia coli and huge multicellular organisms like elephants; these similarities in metabolic pathways are due to their early appearance in evolutionary history, their retention because of their efficacy. Most of the structures that make up animals and microbes are made from three basic classes of molecule: amino acids and lipids; as these molecules are vital for life, metabolic reactions either focus on making these molecules during the construction of cells and tissues, or by breaking them down and using them as a source of energy, by their digestion.
These biochemicals can be joined together to make polymers such as DNA and proteins, essential macromolecules of life. Proteins are made of amino acids arranged in a linear chain joined together by peptide bonds. Many proteins are enzymes. Other proteins have structural or mechanical functions, such as those that form the cytoskeleton, a system of scaffolding that maintains the cell shape. Proteins are important in cell signaling, immune responses, cell adhesion, active transport across membranes, the cell cycle. Amino acids contribute to cellular energy metabolism by providing a carbon source for entry into the citric acid cycle when a primary source of energy, such as glucose, is scarce, or when cells undergo metabolic stress. Lipids are the most diverse group of biochemicals, their main structural uses are as part of biological membranes both internal and external, such as the cell membrane, or as a source of energy. Lipids are defined as hydrophobic or amphipathic biological molecules but will dissolve in organic solvents such as benzene or chloroform.
The fats are a large group of compounds that contain fatty glycerol. Several variations on this basic structure exist, including alternate backbones such as sphingosine in the sphingolipids, hydrophilic groups such as phosphate as in phospholipids. Steroids such as cholesterol are another major class of lipids. Carbohydrates are aldehydes or ketones, with many hydroxyl groups attached, that can exist as straight chains or rings. Carbohydrates are the most abundant biological molecules, fill numerous roles, such as the storage and transport of energy and structural components; the basic carbohydrate units are called monosaccharides and include galactose and most glucose. Monosaccharides can be linked together to form polysaccharides in limitless ways; the two nucleic acids, DNA and RNA, are polymers of nucleotides. Each nucleotide is composed of a phosphate attached to a ribose or deoxyribose sugar group, attached to a nitrogenous base. Nucleic acids are critical for the storage and use of genetic information, its interpretation through the processes of transcription and protein biosynthesis.
This information is propagated through DNA replication. Many viruses have an RNA genome, such as HIV, which uses reverse transcription to create a DNA template from its viral RNA genome. RNA in ribozymes such as spliceosomes and ribosomes is similar to enzymes as it can catalyze chemical reactions. Individual nucleosides are made