Mammals are vertebrate animals constituting the class Mammalia, characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals; the largest orders are the rodents and Soricomorpha. The next three are the Primates, the Cetartiodactyla, the Carnivora. In cladistics, which reflect evolution, mammals are classified as endothermic amniotes, they are the only living Synapsida. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds; the line following the stem group Sphenacodontia split off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, have been among the dominant terrestrial animal groups from 66 million years ago to the present. The basic body type is quadruped, most mammals use their four extremities for terrestrial locomotion. Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals; the most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation. Most mammals are intelligent, with some possessing large brains, self-awareness, tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation.
Mammals can organize themselves into fission-fusion societies and hierarchies—but can be solitary and territorial. Most mammals are polygynous. Domestication of many types of mammals by humans played a major role in the Neolithic revolution, resulted in farming replacing hunting and gathering as the primary source of food for humans; this led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, the development of the first civilizations. Domesticated mammals provided, continue to provide, power for transport and agriculture, as well as food and leather. Mammals are hunted and raced for sport, are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, appear in literature, film and religion. Decline in numbers and extinction of many mammals is driven by human poaching and habitat destruction deforestation. Mammal classification has been through several iterations since Carl Linnaeus defined the class.
No classification system is universally accepted. George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself through the new concept of cladistics. Though field work made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Most mammals, including the six most species-rich orders, belong to the placental group; the three largest orders in numbers of species are Rodentia: mice, porcupines, beavers and other gnawing mammals. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes and lemurs. According to Mammal Species of the World, 5,416 species were identified in 2006.
These were grouped into 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 extinct; the word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therian m
In biology, a gene is a sequence of nucleotides in DNA or RNA that codes for a molecule that has a function. During gene expression, the DNA is first copied into RNA; the RNA can be directly functional or be the intermediate template for a protein that performs a function. The transmission of genes to an organism's offspring is the basis of the inheritance of phenotypic trait; these genes make up different DNA sequences called genotypes. Genotypes along with developmental factors determine what the phenotypes will be. Most biological traits are under the influence of polygenes as well as gene–environment interactions; some genetic traits are visible, such as eye color or number of limbs, some are not, such as blood type, risk for specific diseases, or the thousands of basic biochemical processes that constitute life. Genes can acquire mutations in their sequence, leading to different variants, known as alleles, in the population; these alleles encode different versions of a protein, which cause different phenotypical traits.
Usage of the term "having a gene" refers to containing a different allele of the same, shared gene. Genes evolve due to natural selection / survival of the fittest and genetic drift of the alleles; the concept of a gene continues to be refined. For example, regulatory regions of a gene can be far removed from its coding regions, coding regions can be split into several exons; some viruses store their genome in RNA instead of DNA and some gene products are functional non-coding RNAs. Therefore, a broad, modern working definition of a gene is any discrete locus of heritable, genomic sequence which affect an organism's traits by being expressed as a functional product or by regulation of gene expression; the term gene was introduced by Danish botanist, plant physiologist and geneticist Wilhelm Johannsen in 1909. It is inspired by the ancient Greek: γόνος, that means offspring and procreation; the existence of discrete inheritable units was first suggested by Gregor Mendel. From 1857 to 1864, in Brno, he studied inheritance patterns in 8000 common edible pea plants, tracking distinct traits from parent to offspring.
He described these mathematically as 2n combinations where n is the number of differing characteristics in the original peas. Although he did not use the term gene, he explained his results in terms of discrete inherited units that give rise to observable physical characteristics; this description prefigured Wilhelm Johannsen's distinction between phenotype. Mendel was the first to demonstrate independent assortment, the distinction between dominant and recessive traits, the distinction between a heterozygote and homozygote, the phenomenon of discontinuous inheritance. Prior to Mendel's work, the dominant theory of heredity was one of blending inheritance, which suggested that each parent contributed fluids to the fertilisation process and that the traits of the parents blended and mixed to produce the offspring. Charles Darwin developed a theory of inheritance he termed pangenesis, from Greek pan and genesis / genos. Darwin used the term gemmule to describe hypothetical particles. Mendel's work went unnoticed after its first publication in 1866, but was rediscovered in the late 19th century by Hugo de Vries, Carl Correns, Erich von Tschermak, who reached similar conclusions in their own research.
In 1889, Hugo de Vries published his book Intracellular Pangenesis, in which he postulated that different characters have individual hereditary carriers and that inheritance of specific traits in organisms comes in particles. De Vries called these units "pangenes", after Darwin's 1868 pangenesis theory. Sixteen years in 1905, Wilhelm Johannsen introduced the term'gene' and William Bateson that of'genetics' while Eduard Strasburger, amongst others, still used the term'pangene' for the fundamental physical and functional unit of heredity. Advances in understanding genes and inheritance continued throughout the 20th century. Deoxyribonucleic acid was shown to be the molecular repository of genetic information by experiments in the 1940s to 1950s; the structure of DNA was studied by Rosalind Franklin and Maurice Wilkins using X-ray crystallography, which led James D. Watson and Francis Crick to publish a model of the double-stranded DNA molecule whose paired nucleotide bases indicated a compelling hypothesis for the mechanism of genetic replication.
In the early 1950s the prevailing view was that the genes in a chromosome acted like discrete entities, indivisible by recombination and arranged like beads on a string. The experiments of Benzer using mutants defective in the rII region of bacteriophage T4 showed that individual genes have a simple linear structure and are to be equivalent to a linear section of DNA. Collectively, this body of research established the central dogma of molecular biology, which states that proteins are translated from RNA, transcribed from DNA; this dogma has since been shown to have exceptions, such as reverse transcription in retroviruses. The modern study of genetics at the level of DNA is known as molecular genetics. In 1972, Walter Fiers and his team were the first to determine the sequence of a gene: that of Bacteriophage MS2 coat protein; the subsequent development of chain-termination DNA sequencing in 1977 by Frederick Sanger improved the efficiency of sequencing and turned it into a routine laboratory tool.
An automated version of the Sanger method was used in early phases of the
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
Anatomical terminology is a form of scientific terminology used by anatomists and health professionals such as doctors. Anatomical terminology uses many unique terms and prefixes deriving from Ancient Greek and Latin; these terms can be confusing to those unfamiliar with them, but can be more precise, reducing ambiguity and errors. Since these anatomical terms are not used in everyday conversation, their meanings are less to change, less to be misinterpreted. To illustrate how inexact day-to-day language can be: a scar "above the wrist" could be located on the forearm two or three inches away from the hand or at the base of the hand. By using precise anatomical terminology such ambiguity is eliminated. An international standard for anatomical terminology, Terminologia Anatomica has been created. Anatomical terminology has quite regular morphology, the same prefixes and suffixes are used to add meanings to different roots; the root of a term refers to an organ or tissue. For example, the Latin names of structures such as musculus biceps brachii can be split up and refer to, musculus for muscle, biceps for "two-headed", brachii as in the brachial region of the arm.
The first word describes what is being spoken about, the second describes it, the third points to location. When describing the position of anatomical structures, structures may be described according to the anatomical landmark they are near; these landmarks may include structures, such as the umbilicus or sternum, or anatomical lines, such as the midclavicular line from the centre of the clavicle. The cephalon or cephalic region refers to the head; this area is further differentiated into the cranium, frons, auris, nasus and mentum. The neck area is called cervical region. Examples of structures named according to this include the frontalis muscle, submental lymph nodes, buccal membrane and orbicularis oculi muscle. Sometimes, unique terminology is used to reduce confusion in different parts of the body. For example, different terms are used when it comes to the skull in compliance with its embryonic origin and its tilted position compared to in other animals. Here, Rostral refers to proximity to the front of the nose, is used when describing the skull.
Different terminology is used in the arms, in part to reduce ambiguity as to what the "front", "back", "inner" and "outer" surfaces are. For this reason, the terms below are used: Radial referring to the radius bone, seen laterally in the standard anatomical position. Ulnar referring to the ulna bone, medially positioned when in the standard anatomical position. Other terms are used to describe the movement and actions of the hands and feet, other structures such as the eye. International morphological terminology is used by the colleges of medicine and dentistry and other areas of the health sciences, it facilitates communication and exchanges between scientists from different countries of the world and it is used daily in the fields of research and medical care. The international morphological terminology refers to morphological sciences as a biological sciences' branch. In this field, the form and structure are examined as well as the changes or developments in the organism, it is functional.
It covers the gross anatomy and the microscopic of living beings. It involves the anatomy of the adult, it includes comparative anatomy between different species. The vocabulary is extensive and complex, requires a systematic presentation. Within the international field, a group of experts reviews and discusses the morphological terms of the structures of the human body, forming today's Terminology Committee from the International Federation of Associations of Anatomists, it deals with the anatomical and embryologic terminology. In the Latin American field, there are meetings called Iberian Latin American Symposium Terminology, where a group of experts of the Pan American Association of Anatomy that speak Spanish and Portuguese and studies the international morphological terminology; the current international standard for human anatomical terminology is based on the Terminologia Anatomica. It was developed by the Federative Committee on Anatomical Terminology and the International Federation of Associations of Anatomists and was released in 1998.
It supersedes Nomina Anatomica. Terminologia Anatomica contains terminology for about 7500 human gross anatomical structures. For microanatomy, known as histology, a similar standard exists in Terminologia Histologica, for embryology, the study of development, a standard exists in Terminologia Embryologica; these standards specify accepted names that can be used to refer to histological and embryological structures in journal articles and other areas. As of September 2016, two sections of the Terminologia Anatomica, including central nervous system and peripheral nervous system, were merged to form the Terminologia Neuroanatomica; the Terminologia Anatomica has been perceived with a considerable criticism regarding its content including coverage and spelling mistakes and errors. Anatomical terminology is chosen to highlight the relative location of body structures. For instance, an anatomist might describe one band of tissue as "inferior to" another or a physician might describe a tumor as "superficial to" a deeper body structure.
Anatomical terms used to describe location
An alveolar macrophage is a type of macrophage found in the pulmonary alveolus, near the pneumocytes, but separated from the wall. Activity of the alveolar macrophage is high, because they are located at one of the major boundaries between the body and the outside world, they are responsible for removing particles such as dust or microorganisms from the respiratory surfaces. Alveolar macrophages are seen to contain granules of exogenous material such as particulate carbon that they have picked up from respiratory surfaces; such black granules may be common in smoker's lungs or long-term city dwellers. Inhaled air may contain organisms which would be pathogenic; the respiratory pathway is a prime site for exposure to toxic substances. The respiratory tree, comprising the larynx and bronchioles, is lined by ciliated epithelia cells that are continually exposed to harmful matter; when these offensive agents infiltrate the superficial barriers, the body's immune system responds in an orchestrated defense involving a litany of specialized cells which target the threat, neutralize it, clean up the remnants of the battle.
Deep within the lungs exists its constituent alveoli sacs, the sites responsible for the uptake of oxygen and excretion of carbon dioxide. There are three major alveolar cell types in the alveolar wall: Type I pneumocyte cells that form the structure of an alveolar wall. Type II pneumocyte cells that secrete pulmonary surfactant to lower the surface tension of water and allows the membrane to separate, thereby increasing the capability to exchange gases. Surfactant is continuously released by exocytosis, it forms an underlying aqueous protein-containing hypophase and an overlying phospholipid film composed of dipalmitoyl phosphatidylcholine. Macrophages that destroy foreign material, such as bacteria. Type 1 pneumocytes form the structure of the alveolus and are responsible for the gas exchange in the alveolus. Type 1 pneumocytes are squamous epithelial cells which are characterized by a superficial layer consisting of large, scale-like cells. Type 2 pneumocytes are important in that they can proliferate and differentiate into type 1 pneumocytes, which cannot replicate and are susceptible to a vast numbers of toxic insults.
Type 2 pneumocytes are important because they secrete pulmonary surfactant, which consists 80–90% of phospholipids and 5-10% of surfactant proteins. PS is synthesized as lamellar bodies, which are structures consisting of packed bilayers that are secreted and undergo transformation into a morphological form called tubular myelin. PS plays an important role in maintaining normal respiratory mechanics by reducing alveolar surface tension. By lowering alveolar surface tension, PS reduces the energy required to inflate the lungs, reduces the likelihood of alveolar collapse during expiration. Loosely attached to these alveoli sacs are the alveolar macrophages that protect the lungs from a broad array of microbes and aerosols by devouring and ingesting them through phagocytosis. Alveolar macrophages are phagocytes that play a critical role in homeostasis, host defense, the response to foreign substances, tissue remodeling. Since alveolar macrophages are pivotal regulators of local immunological homeostasis, their population density is decisive for the many processes of immunity in the lungs.
They are adaptive components of the innate immune system and can be modified to whatever functions needed depending on their state of differentiation and micro-environmental factors encountered. Alveolar macrophages release numerous secretory products and interact with other cells and molecules through the expression of several surface receptors. Alveolar macrophages are involved in the phagocytosis of apoptotic and necrotic cells that have undergone cell-death, they must be selective of the material, phagocytized because normal cells and structures of the body must not be compromised. To combat infection, the phagocytes of the innate immune system facilitates many pattern recognition receptors to help recognize pathogen-associated molecular patterns on the surface of pathogenic microorganisms. PAMPs all have the common features of being unique to a group of pathogens but invariant in their basic structure. Proteins involved in microbial pattern recognition include mannose receptor, complement receptors, DC-SIGN, Toll-like receptors, the scavenger receptor, CD14, Mac-1.
PRRs can be divided into three classes: signaling PRRs that activate gene transcriptional mechanisms that lead to cellular activation, endocytic PRRs that function in pathogen binding and phagocytosis, secreted PRRs that function as opsonins or activators of complement. The recognition and clearance of invading microorganisms occurs through both opsonin-dependent and opsonin–independent pathways; the molecular mechanisms facilitating opsonin-dependent phagocytosis are different for specific opsonin/receptor pairs. For example, phagocytosis of IgG-opsonized pathogens occurs through the Fcγ receptors, involves phagocyte extensions around the microbe, resulting in the production of pro-inflammatory mediators. Conversely, complement receptor-mediated pathogen ingestion occurs without observable membrane extensions and does not results in an inflammatory
The micrometre or micrometer commonly known by the previous name micron, is an SI derived unit of length equalling 1×10−6 metre. The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria, for grading wool by the diameter of the fibres; the width of a single human hair ranges from 10 to 200 μm. The longest human chromosome is 10 μm in length. Between 1 μm and 10 μm: 1–10 μm – length of a typical bacterium 10 μm – Size of fungal hyphae 5 μm – length of a typical human spermatozoon's head 3–8 μm – width of strand of spider web silk about 10 μm – size of a fog, mist, or cloud water droplet Between 10 μm and 100 μm about 10–12 μm – thickness of plastic wrap 10 to 55 μm – width of wool fibre 17 to 181 μm – diameter of human hair 70 to 180 μm – thickness of paper The term micron and the symbol μ were accepted for use in isolation to denote the micrometre in 1879, but revoked by the International System of Units in 1967; this became necessary because the older usage was incompatible with the official adoption of the unit prefix micro-, denoted μ, during the creation of the SI in 1960.
In the SI, the systematic name micrometre became the official name of the unit, μm became the official unit symbol. In practice, "micron" remains a used term in preference to "micrometre" in many English-speaking countries, both in academic science and in applied science and industry. Additionally, in American English, the use of "micron" helps differentiate the unit from the micrometer, a measuring device, because the unit's name in mainstream American spelling is a homograph of the device's name. In spoken English, they may be distinguished by pronunciation, as the name of the measuring device is invariably stressed on the second syllable, whereas the systematic pronunciation of the unit name, in accordance with the convention for pronouncing SI units in English, places the stress on the first syllable; the plural of micron is "microns", though "micra" was used before 1950. The official symbol for the SI prefix micro- is a Greek lowercase mu. In Unicode, there is a micro sign with the code point U+00B5, distinct from the code point U+03BC of the Greek letter lowercase mu.
According to the Unicode Consortium, the Greek letter character is preferred, but implementations must recognize the micro sign as well. Most fonts use the same glyph for the two characters. Metric prefix Metric system Orders of magnitude Wool measurement The dictionary definition of micrometre at Wiktionary
Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a biological membrane that forms the boundary between an organism and its extracellular environment. Gases are consumed and produced by cellular and metabolic reactions in most living things, so an efficient system for gas exchange between the interior of the cell and the external environment is required. Small unicellular organisms, such as bacteria and protozoa, have a high surface-area to volume ratio. In these creatures the gas exchange membrane is the cell membrane; some small multicellular organisms, such as flatworms, are able to perform sufficient gas exchange across the skin or cuticle that surrounds their bodies. However, in most larger organisms, which have a small surface-area to volume ratios, specialised structures with convoluted surfaces such as gills, pulmonary alveoli and spongy mesophyll provide the large area needed for effective gas exchange.
These convoluted surfaces may sometimes be internalised into the body of the organism. This is the case with the alveoli, which form the inner surface of the mammalian lung, the spongy mesophyll, found inside the leaves of some kinds of plant, or the gills of those molluscs that have them, which are found in the mantle cavity. In aerobic organisms, gas exchange is important for respiration, which involves the uptake of oxygen and release of carbon dioxide. Conversely, in oxygenic photosynthetic organisms such as most land plants, uptake of carbon dioxide and release of both oxygen and water vapour are the main gas-exchange processes occurring during the day. Other gas-exchange processes are important in less familiar organisms: e.g. carbon dioxide and hydrogen are exchanged across the cell membrane of methanogenic archaea. In nitrogen fixation by diazotrophic bacteria, denitrification by heterotrophic bacteria, nitrogen gas is exchanged with the environment, being taken up by the former and released into it by the latter, while giant tube worms rely on bacteria to oxidize hydrogen sulfide extracted from their deep sea environment, using dissolved oxygen in the water as an electron acceptor.
The exchange of gases occurs as a result of diffusion down a concentration gradient. Gas molecules move from a region in which they are at high concentration to one in which they are at low concentration. Diffusion is a passive process, meaning that no energy is required to power the transport, it follows Fick’s Law: J = − D d φ d x In relation to a typical biological system, where two compartments, are separated by a membrane barrier, where a gas is allowed to spontaneously diffuse down its concentration gradient: J is the flux, the amount of gas diffusing per unit area of membrane per unit time. Note that this is scaled for the area of the membrane. D is the diffusion coefficient, which will differ from gas to gas, from membrane to membrane, according to the size of the gas molecule in question, the nature of the membrane itself. Φ is the concentration of the gas. X is the position across the thickness of the membrane. Dφ/dx is therefore the concentration gradient across the membrane. If the two compartments are individually well-mixed this is simplifies to the difference in concentration of the gas between the inside and outside compartments divided by the thickness of the membrane.
The negative sign indicates that the diffusion is always in the direction that - over time - will destroy the concentration gradient, i.e. the gas moves from high concentration to low concentration until the inside and outside compartments reach equilibrium. Fig. 1. Fick's Law for gas-exchange surface Gases must first dissolve in a liquid in order to diffuse across a membrane, so all biological gas exchange systems require a moist environment. In general, the higher the concentration gradient across the gas-exchanging surface, the faster the rate of diffusion across it. Conversely, the thinner the gas-exchanging surface, the faster the gases will diffuse across it. In the equation above, J is the flux expressed per unit area, so increasing the area will make no difference to its value. However, an increase in the available surface area, will increase the amount of gas that can diffuse in a given time; this is because the amount of gas diffusing per unit time is the product of J and the area of the gas-exchanging surface, A: d q d t = J A Single-celled organisms such as bacteria and amoebae do not have specialised gas exchange surfaces, because they can take advantage of the high surface area they have relative to their volume.
The amount of gas an organism produces in a given time will be in rough proportion to the volume of its cytoplasm. The volume of a unicellular organism is small, therefore it produces a small amount of gas in a given time. In comparison to this small volume, the surface area of its cell membrane is large, adequate for its gas-exchange needs without further modification. However, as an organism increases in size, its surface area and volume do not scale in the same way. Consider an imaginary organism, a cube of side-length, L, its volume increases with the