THE CELL

The cell is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life.^[1] Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. Humans have about 100 trillion or 10¹⁴ cells; a typical cell size is 10 µm and a typical cell mass is 1 nanogram. The longest human cells are about 135 µm in the anterior horn in the spinal cord while granule cells in the cerebellum, the smallest, can be some 4 µm and the longest cell can reach from the toe to the lower brain stem (Pseudounipolar cells).^[2] The largest known cells are unfertilised ostrich egg cells, which weigh 3.3 pounds.^[3][4]
In 1835, before the final cell theory was developed, Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.^[5]

The word cell comes from the Latin cellula, meaning, a small room. The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in.^[6]

Reproductive system

The reproductive system or genital system is a system of organs within an organism which work together for the purpose of reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system.^[1] Unlike most organ systems, the sexes of differentiated species often have significant differences. These differences allow for a combination of genetic material between two individuals, which allows for the possibility of greater genetic fitness of the offspring.^[2]
The major organs of the human reproductive system include the external genitalia (penis and vulva) as well as a number of internal organs including the gamete producing gonads (testicles and ovaries). Diseases of the human reproductive system are very common and widespread, particularly communicable sexually transmitted diseases.^[3]
Most other vertebrate animals have generally similar reproductive systems consisting of gonads, ducts, and openings. However, there is a great diversity of physical adaptations as well as reproductive strategies in every group of vertebrates.

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Common octopus

TIGER

The tiger (Panthera tigris), a member of the Felidae family, is the largest of the four "big cats" in the genus Panthera.^[4] The tiger is native to much of eastern and southern Asia, and is an apex predator and an obligate carnivore. The larger tiger subspecies are comparable in size to the biggest extinct felids,^[5][6] reaching up to 3.3 metres (11 ft) in total length, weighing up to 300 kilograms (660 pounds), and having canines up to 4 inches (100 mm) long.^[7] Aside from their great bulk and power, their most recognisable feature is a pattern of dark vertical stripes that overlays near-white to reddish-orange fur, with lighter underparts. The most numerous tiger subspecies is the Bengal tiger, while the largest is the Siberian tiger.
Tigers have a lifespan of 10–15 years in the wild, but can live longer than 20 years in captivity.^[8] They are highly adaptable and range from the Siberian taiga to open grasslands and tropical mangrove swamps.
They are territorial and generally solitary animals, often requiring large contiguous areas of habitat that support their prey demands. This, coupled with the fact that they are indigenous to some of the more densely populated places on earth, has caused significant conflicts with humans. Three of the nine subspecies of modern tiger have gone extinct, and the remaining six are classified as endangered, some critically so. The primary direct causes are habitat destruction, fragmentation, and hunting.
Historically, tigers have existed from Mesopotamia and the Caucasus throughout most of South and East Asia. Today, the range of the species is radically reduced. All surviving species are under formal protection, yet poaching, habitat destruction, and inbreeding depression continue to threaten the tigers.

Tigers are among the most recognisable and popular of the world's charismatic megafauna. They have featured prominently in ancient mythology and folklore, and continue to be depicted in modern films and literature. Tigers appear on many flags and coats of arms, as mascots for sporting teams, and as the national animal of several Asian nations, including India.^[9]

THE TURTLE

The painted turtle is the only species of Chrysemys, a genus of pond turtles. It lives in slow-moving freshwaters, from southern Canada to the Louisiana Gulf Coast and northern Mexico, and from the Atlantic to the Pacific. Four U.S. states name the painted turtle as their official reptile. Fossils show that the painted turtle existed 15 million years ago, but four regionally based subspecies (the eastern, midland, southern, and western) evolved during the last ice age.

The turtle's skin is olive to black with distinctive red, orange, or yellow stripes. Reliant on warmth from its surroundings, the painted turtle can frequently be seen basking on logs. Crayfish and dragonflies are among the turtle's preferred prey. Many predators eat the turtle eggs or hatchlings, but the adult's shell protects it from most enemies except for raccoons, alligators and humans. Turtles in the wild can live for more than 55 years. (more...)

ADRENOCORTICOTROPIC HORMONE

Adrenocorticotropic Hormone

An adrenocorticotropic hormone test measures the level of adrenocorticotropic hormone (ACTH) in the blood to check for problems with the pituitary gland or the adrenal glands. See pictures of the pituitary gland and adrenal glands .

ACTH is made in the pituitary gland in response to the release of another hormone, called corticotropin-releasing hormone (CRH), by the hypothalamus. In turn, the adrenal glands then make a hormone called cortisol, which helps your body manage stress. Cortisol is needed for life, so its levels in the blood are closely controlled. When cortisol levels rise, ACTH levels normally fall. When cortisol levels fall, ACTH levels normally rise.

Both ACTH and cortisol levels change throughout the day. ACTH is normally highest in the early morning (between 6 a.m. and 8 a.m.) and lowest in the evening (between 6 p.m. and 11 p.m.). ACTH levels may be tested in the morning or evening if your doctor thinks that they are abnormal. Cortisol levels are often measured at the same time as ACTH.
ACTH is released in bursts, so its levels in the blood can vary from minute to minute. Interpretation of the test results is difficult and often requires the skill of an endocrinologist.

Why It Is Done

A test to measure ACTH is done to check for:

• A problem with the adrenal glands or pituitary gland. A high level of ACTH and a low level of cortisol (or low ACTH and high cortisol levels) could be caused by a problem with the adrenal glands. Low levels of ACTH and cortisol could be caused by a problem with the pituitary gland.
• Overproduction of ACTH. This may be caused by an overactive pituitary gland. In response, the adrenal glands release too much cortisol (one form of Cushing's syndrome).
• The correct dose of corticosteroid medicine.

How To Prepare

You may not be able to eat or drink for 10 to 12 hours before an ACTH test. Your doctor may ask you to eat low-carbohydrate foods for 48 hours before the test. Be sure to ask your doctor if there are any foods that you should not eat.
Many medicines can change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take. If you take a medicine, such as a corticosteroid, that could change the test results, you will need to stop taking it for up to 48 hours before the test. Your doctor will tell you exactly how long depending on what medicine you take.
Do not exercise for 12 hours before this test.
Try to avoid emotional stress for 12 hours before the test.
Collecting the blood sample at the right time is often important. Your blood will be drawn in the morning if your doctor wants a peak ACTH level. Your blood will be drawn in the evening if your doctor wants a low (trough) ACTH level.
Talk to your doctor about any concerns you have about the need for the test, its risks, how it will be done, or what the results will mean. To help you learn about this test and how important it is, fill out the medical test information form(What is a PDF document?).

Hormone

A hormone (from Greek ὁρμή "impetus") is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one cell to another. All multicellular organisms produce hormones; plant hormones are also called phytohormones. Hormones in animals are often transported in the blood. Cells respond to a hormone when they express a specific receptor for that hormone. The hormone binds to the receptor protein, resulting in the activation of a signal transduction mechanism that ultimately leads to cell type-specific responses.
Endocrine hormone molecules are secreted (released) directly into the bloodstream, whereas exocrine hormones (or ectohormones) are secreted directly into a duct, and, from the duct, they flow either into the bloodstream or from cell to cell by diffusion in a process known as paracrine signalling.
Recently it has been found that a variety of exogenous modern chemical compounds have hormone-like effects on both humans and wildlife. Their interference with the synthesis, secretion, transport, binding, action, or elimination of natural hormones in the body can change the homeostasis, reproduction, development, and/or behavior the same as endogenous produced hormones."^[1]

Human evolution

Human evolution is the phenotypic history of the genus Homo, including the emergence of Homo sapiens as a distinct species and as a unique category of hominids ("great apes") and mammals. The study of human evolution uses many scientific disciplines, including physical anthropology, primatology, archaeology, linguistics and genetics.^[1]
The term "human" in the context of human evolution refers to the genus Homo, but studies of human evolution usually include other hominids, such as the Australopithecines, from which the genus Homo had diverged by about 2.3 to 2.4 million years ago in Africa.^[2][3] Scientists have estimated that humans branched off from their common ancestor with chimpanzees about 5–7 million years ago. Several species and subspecies of Homo evolved and are now extinct, introgressed or extant. Examples include Homo erectus (which inhabited Asia, Africa, and Europe) and Neanderthals (either Homo neanderthalensis or Homo sapiens neanderthalensis) (which inhabited Europe and Asia). Archaic Homo sapiens evolved between 400,000 and 250,000 years ago.
The dominant view among scientists concerning the origin of anatomically modern humans is the hypothesis known as "Out of Africa", recent African origin of modern humans, ROAM, or recent African origin hypothesis,^[4][5][6] which argues that Homo sapiens arose in Africa and migrated out of the continent around 50,000 to 100,000 years ago, replacing populations of Homo erectus in Asia and Neanderthals in Europe.
Scientists supporting an alternative multiregional hypothesis argue that Homo sapiens evolved as geographically separate but interbreeding populations stemming from a worldwide migration of Homo erectus out of Africa nearly 2.5 million years ago. Evidence suggests that an X-linked haplotype of the Neanderthal origin is present among all non-African populations, and Neaderthals and other hominids, such as Denisova hominin may have contributed up to 6% of their genome to modern humans.^[7][8] Archaic genetic contribution contradicts

science

Science (from Latin: scientia meaning "knowledge") is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the world.^[1][2][3][4] An older and closely related meaning still in use today is that of Aristotle, for whom scientific knowledge was a body of reliable knowledge that can be logically and rationally explained (see "History and etymology" section below).^[5]
Since classical antiquity science as a type of knowledge was closely linked to philosophy. In the early modern era the two words, "science" and "philosophy", were sometimes used interchangeably in the English language. By the 17th century, "natural philosophy" (which is today called "natural science") had begun to be considered separately from "philosophy" in general.^[6][7] However, "science" continued to be used in a broad sense denoting reliable knowledge about a topic, in the same way it is still used in modern terms such as library science or political science.
In modern use, science is "often treated as synonymous with ‘natural and physical science’, and thus restricted to those branches of study that relate to the phenomena of the material universe and their laws, sometimes with implied exclusion of pure mathematics. This is now the dominant sense in ordinary use."^[8] This narrower sense of "science" developed as a part of science became a distinct enterprise of defining "laws of nature", based on early examples such as Kepler's laws, Galileo's laws, and Newton's laws of motion. In this period it became more common to refer to natural philosophy as "natural science". Over the course of the 19th century, the word "science" became increasingly associated with the disciplined study of the natural world including physics, chemistry, geology and biology. This sometimes left the study of human thought and society in a linguistic limbo, which was resolved by classifying these areas of academic study as social science. Similarly, several other major areas of disciplined study and knowledge exist today under the general rubric of "science", such as formal science and applied science.^[

The skin

Skin is a soft outer covering of an animal, in particular a vertebrate. Other animal coverings such as the arthropod exoskeleton or the seashell have different developmental origin, structure and chemical composition. The adjective cutaneous means "of the skin" (from Latin cutis, skin). In mammals, the skin is the largest organ of the integumentary system made up of multiple layers of ectodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs.^[1] Skin of a different nature exists in amphibians, reptiles, and birds.^[2] All mammals have some hair on their skin, even marine mammals which appear to be hairless. Because it interfaces with the environment, skin plays a key role in protecting (the body) against pathogens^[3] and excessive water loss.^[4] Its other functions are insulation, temperature regulation, sensation, and the protection of vitamin D folates. Severely damaged skin will try to heal by forming scar tissue. This is often discoloured and depigmented.
Hair with sufficient density is called fur. The fur mainly serves to augment the insulation the skin provides, but can also serve as a secondary sexual characteristic or as camouflage. On some animals, the skin is very hard and thick, and can be processed to create leather. Reptiles and fish have hard protective scales on their skin for protection, and birds have hard feathers, all made of tough β-keratins. Amphibian skin is not a strong barrier to passage of chemicals and is often subject to osmosis. For example, a frog sitting in an anesthetic solution could quickly go to sleep.