Cell, in biology, the basic membrane-bound unit that contains the fundamental molecules of life and of which all living things are composed. A single cell is often a complete organism in itself, such as a bacterium or yeast. Other cells acquire specialized functions as they mature. These cells cooperate with other specialized cells and become the building blocks of large multicellular organisms, such as humans and other animals. Although cells are much larger than atoms, they are still very small. The smallest known cells are a group of tiny bacteria called mycoplasmas; some of these single-celled organisms are spheres as small as 0.2 μm in diameter (1μm = about 0.000039 inch), with a total mass of 10−14 gram—equal to that of 8,000,000,000 hydrogen atoms. Cells of humans typically have a mass 400,000 times larger than the mass of a single mycoplasma bacterium, but even human cells are only about 20 μm across. It would require a sheet of about 10,000 human cells to cover the head of a pin, and each human organism is composed of more than 75,000,000,000,000 cells. The cell (from Latin cella, meaning "small room") is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are often called the "building blocks of life". The study of cells is called cell biology or cellular biology.

Smallest Cell In Bidy = Sperm.
Biggest Cell In Body =  Ovum.

Types Of Cells:

1: Stem Cells: Before a cell becomes specialised, it first starts out as a stem cell. The unique feature of stem cells is that they are pluripotent - they have the potential to become any type of cell in the body. These incredible cells are the ancestors of all cells in the body, from simple skin cells to complex neurons. Without these cells, we wouldn't be as complex or functional as human beings. Not only this, these “magic” cells even have the power to replicate into healthy cells in order to speed up regeneration after certain pathological conditions. The process that allows stem cells to transform into any kind of cell is known as cell differentiation and is controlled by a combination of internal genetics and external factors such as chemicals and physical contact with other cells. Stem cells have the ability to divide and replicate themselves for long periods of time. There are two types of stem cells, embryonic stem cells and adult stem cells. Embryonic stem cells are from embryos. Generally used in a research setting, embryonic stem cells are harvested from fertilised eggs. Adult (or somatic) stem cells are present throughout the human body [amongst other specialised tissue cells]. They exist in order to repair and maintain surrounding specialised tissues. As these cells are unspecialised, stem cell anatomy is that of a simple cell. Stem cells have a cell membrane, surrounding the cytoplasm. The cytoplasm contains a nucleus, mitochondria, ribosomes, endoplasmic reticulum, golgi apparatus, lysosomes and centrioles. The nucleus contains DNA and RNA, which are expressed when differentiation occurs in the cell.

2: Red Blood Cells: Red blood cells are known as erythrocytes, and are the most common type of blood cell. They are shaped like a biconcave disc (I.e. donut shaped). They have a diameter of around 6 to 8 µm and have an average thickness of 2 µm, being 2.5 µm thick at their thickest point and 1 µm thick at the center. Red blood cells are fairly flexible, allowing them to squeeze through thin blood capillaries. The main role of red blood cells is to transport oxygen around the body using haemoglobin. However, they also help to control pH of the blood by forming an acid-base buffer maintaining the blood at a neutral pH of 7.35 to 7.45. They also release an enzyme called carbonic anhydrase, which causes water in the blood to carry carbon dioxide to the lungs, so that it can be expelled from the body.

3: White Blood Cells: White blood cells, also known as leukocytes, are a vital component of the immune system. There are five different types, which fall under two main categories; granulocytes and agranulocytes. As suggested by their names, granulocytes contain granules in the cytoplasm as agranulocytes do not. Granulocytes include neutrophils, eosinophils and basophils. Agranulocytes include lymphocytes and monocytes.

Neutrophils: Neutrophils are the most common type of leukocyte, making up around 65% of all white blood cells. They are 12 to 14 µm in diameter, and contain a single nucleus. They contain few cell organelles and protein synthesis does not take place within them. Neutrophils originate in the bone marrow and circulate in the bloodstream for 6 to 10 hours, before entering the surrounding tissues. Once in the tissues, they destroy damaged cells and bacteria through phagocytosis, before self-destructing.

Eosinophils: Eosinophils are rare in the bloodstream. They are 12 to 17 µm in diameter and contain toxic proteins. Like neutrophils, they originate in the bone marrow and move into the bloodstream before entering loose connective tissue in the respiratory tract and intestines. Here they destroy antigen-antibody complexes using phagocytosis. The cells release the specialised enzymes histaminase and arylsulfatase B which are involved in the inflammatory response. Eosinophils also play a role in destroying bacteria, viruses and parasites that invade  the body.

Basophils: Basophils are the rarest form of white blood cell and are involved in the body’s defense against parasites. They are 14 to 16 µm in diameter. They accumulate at infected areas, releasing histaminesserotonin and prostaglandins to increase blood flow which causes an inflammatory response.

Lymphocytes: Lymphocytes can be divided into two different types, B-cells and T-cells. Lymphocytes vary in size, with most being around 6 to 9 µm in diameter, and a tenth of them being 10 to 14 µm in diameter. The largest lymphocytes tend to be favored, and contain more cytoplasm, mitochondria and ribosomes than their smaller counterparts.

4: Platelets: Just like the white and red blood cells, platelets also form an important component of the blood. Technically platelets are fragments of cells rather than true cells, but are vital in the control of bleeding. They are fragments of large cells called megakaryocytes which are produced in the bone marrow. They have surface proteins which allow them to bind to one another, and to bind to damaged blood vessel walls. Platelets are recruited when bleeding occurs, initiating a process known as hemostasis. They plug the source of the bleeding, coagulating and sticking together to form a blood clot, together with a fibrous protein known as fibrin.

5: Nerve Cells: Nerve cells, commonly known as neurons, transmit information throughout the body in the form of electrical signals or nerve impulses. Structurally, neurons have four specific regions; the cell body, dendrites, the axon and axon terminals. The cell body contains a nucleus and is responsible for synthesising neural proteins. The axon is long and thin, and protrudes from the cell body like a tail and can be myelinated or unmyelinated. Axons are responsible for conducting electrical impulses in the form of action potentials, away from the cell body. Neurons can have multiple, two or one dendrite(s) which makes them multipolarbipolar or unipolar respectively. They convert chemical signals from the synapse into small electrical impulses, and transmit them towards the cell body. Electrical disturbance in the dendrites is transmitted to a structure called the axon hillock at the base of the axon, and with enough voltage, generates an action potential which moves down the axon and continues its course. 

6: Neuroglial Cells: Neuroglial cells, more commonly known as glial cells or glia, are cells of the nervous systemthat are not involved in the conduction of nervous impulses. Glia are very common in the brain, outnumbering neurons at a ratio of 3 to 1. Glia are smaller than neurons, and do not have axons or dendrites. They have a variety of roles in the nervous system, they modulate synaptic action and rate of impulse propagation, they provide a scaffold for neural development, and aid recovery from neural injuries. There are four types of glial cells in the central nervous system; astrocytes, oligodendrocytes,microglial cells, and ependymal cells.

7: Muscle Cells: There are 3 types of muscle cells, known as myocytes, in the human body. These types are skeletal, cardiac and smooth muscle. Skeletal and cardiac muscle cells are known as striated, due to the aligned arrangement of myosin and actin proteins within them. Actin and myosin allow muscle contraction by sliding past one another, as described by sliding filament theory. Actin and myosin are arranged more randomly in smooth muscle cells, creating a smooth rather than striated appearance.

Skeletal Muscle Cells: Skeletal muscle cells are attached to bones and tendons and can reach up to 30 cm in length, although they are usually 2 to 3 cm long. Skeletal muscle cells are responsible for voluntary movements. They are multinucleated and comprise a sarcolemma (cell membrane), sarcoplasm (cytoplasm), myofibrils (actin and myosin), sarcosomes (mitochondria) and a sarcoplasmic reticulum, which is like the smooth endoplasmic reticulum of other cells. They also contain two proteins called troponin and tropomyosin which regulate the interaction between actin and myosin during contraction. The basic units of striated muscle cells comprising actin and myosin are known as sarcomeres.

Cardiac Muscle Cells: Cardiac muscle cells are also called cardiomyocytes which together make up the most important muscular tissue in the entire body, the tissue of the heart. Individually, they are about 0.02 mm wide and 0.1 mm long and linked together via gap junctions. The cells contract in unison creating the contractions of the heart. This is coordinated by nervous impulses which depolarises the cell membrane, spreading from cell to cell relatively quickly as the cells are very closely anchored  via intercalated discs. Cardiomyocytes contain many sarcosomes to provide sufficient energy for contraction.

Smooth Muscle Cells: S​mooth muscle cells are responsible for involuntary contractions in hollow and visceral organs like the bladder and lungs, and the walls of blood vessels. They are responsible for peristalsis, whereby food is propelled through the digestive system via wavelike contractions. They are 10 to 600 µm long spindle-shaped cells and have a central nucleus. Smooth muscle cells are arranged in sheets allowing them to contract simultaneously. As they are smaller than cardiomyocytes and skeletal myocytes, they contain fewer cell organelles, and do not contain sarcomeres.
Cartillage Cells: Cartillage cells, also known as chondrocytes, make up cartilage, a firm tissue that is vital to the body’s structure. Cartilage is found in joints between bones, in the ears and nose, in the airways as well as other locations. For example, cartilage can be found between the vertebrae in the spinal column. Chondrocytes produce and maintain the extracellular matrix of cartilage, comprising collagen, proteoglycan and elastin fibers. They lack blood vessels meaning that cartilage is repaired slower than other tissues, and nutrients have to be absorbed by diffusion from the tissue surrounding the cartilage, known as the perichondrium. Articular cartilage (cartilage found in synovial joints) differs from other cartilages since it does not contain perichondrium.
8: Bone Cells: There are four types of bone cells in the body; osteoblasts, osteoclasts, osteocytes and lining cells.
Osteoclasts: Osteoclasts are large multinucleated cells that are involved in bone resorption. This is where the bone is broken down during the process of renewal. Osteoclasts break down bone by forming sealed compartments on its surface, and releasing enzymes and acids. After they complete the process, they die by apoptosis (programmed cell death).

Osteoblasts: Osteoblasts have the opposite function, they are involved in the generation of new bone. They are cuboidal in shape and have one central nucleus. They work by synthesising protein which forms the organic matrix of the bone. They are triggered to create new bone by hormones such as vitamin D and estrogen, and have specialised receptors on their surfaces  which detect them.

Osteocytes: Osteocytes are cells that are found inside the bone. They have long branched structures protruding from them allowing cell to cell contact and access to the bone’s surface. Osteocytes can sense mechanical strain being placed on the bone, and secrete growth factors which activate bone growth in response.

Lining Cells: The final type of bone cells are lining cells. These originate as osteoblasts before becoming flat in structure. As their name suggests, they line the surface of the bone and are responsible for the release of calcium from the bone into the bloodstream when it falls too low. Lining cells have receptors on their surfaces which are receptive to hormones and other chemicals that signify a need for bone growth and remodeling. They also work to protect the bone from chemicals in the blood which might be damaging to the bone’s structure.

9: Skin Cells: There are many different types of cells in the epidermis (top layer) of the skin. The epidermis contains the following cell types:
Keratinocytes: These cells make up 95% of the epidermis and are sometimes known as basal cells, as they are found in the basal layer of the epidermis. Keratinocytes generate the protein keratin, but are also important in protecting the body by blocking toxins and pathogens, and preventing loss of heat and moisture. They also stimulate inflammation and secrete inhibitory cytokines. The outermost layer of epidermis is formed bykeratinized epithelial cells which are responsible for forming the protective barrier. Hair and nails are examples of fully keratinized epithelial cells.

Melanocytes: The role of melanocytes in the skin is to produce the pigment melanin, which determines skin coloration.
Langerhans cells: These are dendritic cells involved in antigen processing when the skin becomes infected, they act as antigen-processing cells. They contain large organelles known as Birbeck granules, but the exact function of these is still unknown.
Merkel cells: These act as mechanosensory cells and are involved in touch reception (the ability to feel). Other types of sensory cells are present within the skin, however are found in the deeper layers and known as cutaneous receptors.

10: Endothelial Cells: Endothelial cells are the cells that form the lining of blood vessels. They are flat in structure, and are between 1 and 2 µm thick. They have a central nucleus, and are connected to one another via intercellular junctions. Endothelial cells are highly adaptable, being able to migrate and adjust their numbers and arrangements to accommodate the body’s needs. This allows growth and repair of body tissues, as new blood vessel networks can easily form. As well as healthy body tissues, cancer cellsalso rely on endothelial cells and blood vessels to survive As a result, a lot of research is focused on preventing the formation of blood vessels in cancerous tissues. Endothelial cells express different surface proteins, depending on whether they are forming veins or arteries.

11: Epithelial Cells: Epithelial cells make up the linings of cavities in the body such as the lungs, small intestine and stomach. They are joined to one another forming sheets called epithelia, and are connected by tight junctions, adherens, desmosomes and gap junctions. Tight junctionsare unique to epithelial cells and form the closest type of junction between any cell type in the body. They are supported by a basement membrane known as a basal lamina, which covers a capillary bed. The nucleus of an epithelial cell is found close to the basal lamina, towards the bottom of the cell. Epithelial cells are innervated with nerve endings, and can become sensory cells, detecting stimuli such as scent. Epithelial cells can also specialise to become secretory cells, that release mucous, hormones and enzymes into the body. These cells contain vesicles of hormones or enzymes ready to be released. Specialised secretory epithelial cells include goblet cells and paneth cells in the intestines, which secrete mucous and antibacterial proteins respectively.

12: Fat Cells: Fat cells, also referred to as adipocytes and lipocytes are the cells of the body that are specialised to store energy in the form of adipose tissue, or fat. There are two types of fat cell, white fat cells and brown fat cells. White fat cells, or unilocular cells, are vacuolar cells that contain a lipid droplet and cytoplasm. They have a nucleus which is flat and at the edge of the cell, rather than the centre. White fat cells vary in size, but on average they are around 0.1 mm in diameter. The fat inside white fat cells is mainly made up of triglycerides and cholesteryl ester, and is stored in semi-liquid form. B​rown fat cells​, or multilocular cells, have multiple vacuoles and are shaped like polygons. They contain more cytoplasm that white fat cells, and fat droplets are scattered throughout them. The nucleus is not flattened but round, and is found randomly positioned towards the centre of the cell. The key role of brown fat is to generate heat energy, and therefore the cells contain many mitochondria, which give them their brownish coloration.

13: Sex Cells: Sexual reproduction is the result of the fusion of two different types of sex cells called gametes. Male sex cells are commonly known as sperm cells, or spermatozoa, and female gametes are known as eggs or ova. When they fuse together, fertilization occurs and a zygote is formed.

Spermatozoa: Spermatozoa and ova are structurally very different from one another. Spermatozoa are smaller, being about 50 µm long, and have a head, a midpieceregion and a long tail (flagellum) for propulsion and motility. The head contains an acrosome, which is a type of covering filled with enzymes that enable penetration of the female ovum during fertilisation. The head of the cell contains a nucleus that is densely packed with DNA, with little cytoplasm present. The midpiece region of the cell contains mitochondria which provide the energy required for locomotion.

Ova: Ova are very large compared to other cell bodies, being as large as 0.2 mm in diameter. They are round in shape and are produced in the ovaries during embryological development. The cell itself comprises a nucleus, cytoplasm, the zona pellucida and the corona radiata. The zona pellucida is a membrane that surrounds the cell membrane of the cell, and the corona radiata forms protective layers which surround the zona pellucida. During the process of fertilization, the spermatozoa binds with the ovum at the zona pellucida. After, the penetration of the spermatozoa and the release of its contents into the ovum can then occur (acrosome reaction).

14: Cancer cells: Cancer cells are cells that divide relentlessly, forming solid tumors or flooding the blood with abnormal cells. Cell division is a normal process used by the body for growth and repair. A parent cell divides to form two daughter cells, and these daughter cells are used to build new tissue, or to replace cells that have died because of aging or damage. Healthy cells stop dividing when there is no longer a need for more daughter cells, but cancer cells continue to produce copies. They are also able to spread from one part of the body to another in a process known as metastasis.


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