CELL

The cell is the basic structural and 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] Organisms can be classified as unicellular (consisting of a single cell; including mostbacteria) or multicellular (including plants and animals). Humans contain about 10 trillion(10¹³) cells. Most plant and animal cells are between 1 and 100 µm and therefore are visible only under the microscope.

^[2]The cell was discovered by Robert Hooke in 1665. 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 Jacob  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

.^[3]The word cell comes from the Latin cella, meaning "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.

Anatomy

There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms.

Prokaryotic cells

Main article: Prokaryote

Diagram of a typical prokaryotic cell

The prokaryote cell is simpler, and therefore smaller, than a eukaryote cell, lacking a nucleus and most of the other organelles of eukaryotes. There are two kinds of prokaryotes: bacteria and archaea; these share a similar structure.

Nuclear material of prokaryotic cell consist of a single chromosome that is in direct contact with cytoplasm. Here, the undefined nuclear region in the cytoplasm is called nucleoid.

A prokaryotic cell has three architectural regions:

• On the outside, flagella and pili project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells;
• Enclosing the cell is the cell envelope – generally consisting of a cell wallcovering a plasma membrane though some bacteria also have a further covering layer called a capsule. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. Though most prokaryotes have a cell wall, there are exceptions such as Mycoplasma(bacteria) and Thermoplasma (archaea). The cell wall consists of peptidoglycanin bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and finally bursting (cytolysis) from osmotic pressure against a hypotonic environment. Some eukaryote cells (plant cellsand fungi cells) also have a cell wall;
• Inside the cell is the cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease). Though not forming a nucleus, theDNA is condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids enable additional functions, such as antibiotic resistance.

Eukaryotic cells

Main article: Eukaryote

Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about 15 times wider than a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is a cell nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. This nucleus gives the eukaryote its name, which means "true nucleus." Other differences include:

• The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
• The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria also contain some DNA.
• Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Cilia may thus be "viewed as sensory cellular antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."
• Eukaryotes can move using motile cilia or flagella. The flagella are more complex than those of prokaryotes.

Human nose

The visible part of the human nose is the protruding part of the face that bears the nostrils .
The shape of the nose is determined by the ethmoid bone and the nasal septum , which consists mostly of cartilage and which separates the nostrils. On average the nose of a male is larger than that of a female .

The nose has an area of specialised cells which are responsible for smelling (part of the olfactory system ). Another function of the nose is the conditioning of inhaled air, warming it and making it more humid. Hairs inside the nose prevent large particles from entering the lungs. Sneezing is usually caused by foreign particlesirritating the nasal mucosa, but can more rarely be caused by sudden exposure to bright light (called the photic sneeze reflex ) or touching the external auditory canal . Sneezing is a means of transmitting infections because it creates aerosols in which the droplets can harbour microbes.

Human ear

The ear is the organ that detects sound . It not only receives sound,but also aids in balance and body position. The ear is part of the auditory system .
The word "ear" may be used correctly to describe the entire organ or just the visible portion. In most mammals, the visible ear is a flap of tissue that is also calledthe pinna and is the first of many steps in hearing . In humans, the pinna is often called the auricle . Vertebrates have a pair of ears, placed somewhat symmetrically on opposite sides of the head. This arrangement aids in the ability to localize sound sources.

Our Teeth

Teeth (singular tooth ) are small, calcified , whitish structures found in the jaws ( Mouths ) of many vertebrates that are used to breakdown food .
Some animals, particularly carnivores , also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the maxilla (upper jaw) or the mandible (lower jaw) and are covered by gums . Teeth are made of multiple tissues of varying density and hardness.
Teeth are among the most distinctive (and long-lasting) features of mammal species. Paleontologists use teeth to identify fossil species and determine their relationships. The shape of an animal 's teeth is related to its diet.

For example, plant matter is hard to digest, so herbivores have many molars for chewing and grinding. Carnivores , on the other hand, need canines to kill prey and to tear meat.
Mammals are diphyodont , meaning that they develop two sets of teeth. In humans , the first set (also called the "baby", "milk","primary", and " deciduous " set) Normally starts to appear at about six months of age, although somebabies are born with one or more visible teeth, known as "Neonatal teeth."

Normal tooth eruption at about six months is known as Teething and can be painful.
Some animals develop only one set of teeth (Monophyodont) while others develop many sets (polyphyodont). Sharks ,
For example, Grow a new set of teeth every two weeks to replace worn teeth. Rodent incisors grow and wear away continually through gnawing, which helps maintain relatively constant length. The industry of the beaver is due in part to this qualification. Many rodents such as voles (but not mice ) and guinea pigs , as well as rabbits , have continuously growing molars in addition to incisors.
Dental trauma refers to trauma to the face, mouth, and especially the teeth lips and periodontium. The study of dental trauma is called "Dental traumatology".

A human tongue

The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates whichmanipulates food for mastication . It is the primary organ of taste ( gustation ), as much of the upper surface of the tongue is covered in papillae and taste buds . It is sensitive and kept moist by saliva , and is richly supplied with nerves and blood vessels . In humans a secondary function of the tongue is phonetic articulation.

[ 1 ] The tongue also serves as a natural means of cleaning one's teeth.

[ 2 ] The ability to perceive different tastes is not localised in different parts of the tongue, as is widely believed.

[ 3 ] This error arose because of misinterpretation of some 19th century research

Endocrine glands

Endocrine glands are glands of the endocrine system that secrete their products, hormones , directlyinto the blood rather than through a duct. The main endocrine glands include the pituitary gland , pancreas , ovaries , testes , thyroid gland , and adrenal glands . The hypothalamus is a neuroendocrine organ. Other organs which are not so well known for their endocrine activityinclude the stomach , which produces such hormones as ghrelin .
Local chemical messengers, not generally considered part of the endocrine system, include autocrines , which act on the cells that secrete them, and paracrines , which act on a different cell typenearby.
The ability of a target cell to respond to a hormone depends on the presence of receptors, within the cell or on its plasma membrane, to which the hormonecan bind.
Hormone receptors are dynamic structures. Changes in number and sensitivity of hormone receptors may occur in response to high or low levels of stimulating hormones.
Blood levels of hormones reflect a balance between secretion and degradation/ excretion . The liver and kidneys are the major organs that degrade hormones; breakdown products are excretedin urine and feces.
Hormone half-life and duration of activity are limited and vary from hormone to hormone.

Human skeleton »»

The human skeleton consists ofboth fused and individual bones supported and supplemented by ligaments , tendons , muscles and cartilage . It serves as a scaffold which supports organs, anchorsmuscles, and protects organs such as the brain , lungs and heart . [ citation needed ]
The biggest bone in the body is the femur in the thigh and the smallest is the stapes bone in the middle ear . Several factors contribute to the bone density and average mass of the human skeleton including; gender, race, hormonal factors, nutrition, physical activity and lifestyle behaviors.

[ 1 ] Because of these and other factors affecting an individual's weight the human skeleton may comprise between 12 and 20 percent of a person's total body weight with the average being 15 percent.

[ 2 ] Fused bones include those of the pelvis and the cranium . Not all bones are interconnected directly: there are three bones ineach middle ear called the ossicles that articulate only with each other. The hyoid bone , which is located in the neck and serves as the point of attachment for the tongue , does not articulate with any other bones in the body, being supported by muscles and ligaments.

Immune system »»

An immune system is a system of biological structures and processes within an organism that protects against disease . In order to function properly, an immune system must detect a wide variety of agents, from viruses to parasitic worms , and distinguish them from the organism's own healthy tissue .
Pathogens can rapidly evolve and adapt to avoid detection and destruction by the immune system. As a result, multiple defense mechanisms have also evolved to recognize and neutralize pathogens. Even simple unicellular organisms such as bacteria possess a rudimentary immune system, in the form of enzymes which protect against bacteriophage infections. Other basic immune mechanisms evolved in ancient eukaryotes and remain in their modern descendants, such as plants and insects. These mechanisms include phagocytosis , antimicrobial peptides called defensins , and the complement system . Jawed vertebrates , including humans, have even more sophisticated defense mechanisms, [ 1 ] including the ability to adapt over time to recognize specific pathogens more efficiently. Adaptive (or acquired) immunity creates immunological memory after an initial response to a specific pathogen, leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of vaccination .
Disorders of the immune system can result in autoimmune diseases , inflammatory diseases and cancer . [ 2 ] [ 3 ] Immunodeficiency occur when the immune system is less active than normal, resulting in recurring and life-threatening infections. In humans, immunodeficiency can either be the result of a genetic disease , such as severe combined immunodeficiency , or acquired conditions such as HIV / AIDS or the use of immunosuppressive medication. In contrast, autoimmunity result from a hyperactive immune system attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include Hashimoto's thyroiditis , rheumatoid arthritis , diabetes mellitus type 1 , and systemic lupus erythematosus . Immunology covers the study of all aspects of the immune system.

Bone Marrow

Bone marrow ( Latin : medulla ossium ) is the flexible tissue found in the interior of bones . In humans , bone marrow in large bones produces new blood cells . On average, bone marrow constitutes 4% of the total body mass of humans; in adults weighing 65 kg (143 lbs), bone marrow accounts for approximately 2.6 kg (5.7 lbs). The hematopoietic compartment of bone marrow produces approximately 500 billion blood cells per day, which use the bone marrow vasculature as a conduit to the body's systemic circulation.
[ 1 ] Bone marrow is also a key component of the lymphatic system , producing the lymphocytes that support the body's immune system .

Semen (disambiguation)

Semen is an organic fluid , also known as seminal fluid , that maycontain spermatozoa . It is secreted by the gonads (sexual glands) and other sexual organs of male or hermaphroditic animals and can fertilize female ova . In humans, seminal fluid contains several components besides spermatozoa: proteolytic and other enzymes as well as fructose are elements of seminal fluid which promote the survival of spermatozoa and provide a medium through which they can move or "swim".
Semen is produced and originates from the seminal vesicle , which is located in the pelvis. The process that results inthe discharge of semen is called "Ejaculation".