The cell is the basic structural andfunctional unit of all knownliving organisms. It is the smallest unitof life that is classified as a living thing(except virus, which consists only fromDNA/RNA coveredby protein and lipids), and is oftencalled the building block oflife. Organisms can be classifiedas unicellular (consisting of a single cell;including most bacteria)or multicellular (including plantsand animals). Humans contain about10 trillion (1013) cells. Most plant andanimal cells are between 1 and100 µm and therefore are visible onlyunder the microscope.
The cell was discovered by Robert Hooke in 1665. The celltheory, first developed in 1839 by Matthias Jakob Schleiden andTheodor Schwann, states that all organisms are composed of oneormore cells, that all cells come from pre-existing cells, that vitalfunctions of an organism occur within cells, and that all cells containthe hereditary information necessary for regulating cell functions andfor transmitting information to the next generation of cells. Theword cell comes from the Latin cella, meaning "small room". Thedescriptive term for the smallest living biological structure wascoined by Robert Hooke in a book he published in 1665 when hecompared the cork cells he saw through his microscope to the smallrooms monks lived in.
1665: Robert Hooke discovers cells in cork, then in living plant tissue using an earlycompound microscope.1839: Theodor Schwann and Matthias Jakob Schleiden elucidate the principle that plantsand animals are made of cells, concluding that cells are a common unit of structure anddevelopment, and thus founding the cell theory.1855: Rudolf Virchow states that cells always emerge from cell divisions (omnis cellula excellula).1859: The belief that life forms can occur spontaneously (generatio spontanea) iscontradicted by Louis Pasteur (1822–1895) (although Francesco Redi had performed anexperiment in 1668 that suggested the same conclusion).1931: Ernst Ruska builds first transmission electron microscope (TEM) at the University ofBerlin. By 1935, he has built an EM with twice the resolution of a lightmicroscope, revealing previously unresolvable organelles.1953: Watson and Crick made their first announcement on the double-helix structure forDNA on February 28.1981: Lynn Margulis published Symbiosis in Cell Evolution detailing the endosymbiotictheory.
The prokaryote cell is simpler, and therefore smaller, than a eukaryotecell, lacking a nucleus and most of the other organelles of eukaryotes.There are two kinds of prokaryotes: bacteria and archaea; these share asimilar structure.The nuclear material of a prokaryotic cell consists of asingle chromosome that is in direct contact with the cytoplasm. Here, theundefined nuclear region in the cytoplasm is called the nucleoid.
Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. Thesecells are about 15 times wider than a typical prokaryote and can be as much as1000 times greater in volume. The major difference between prokaryotes andeukaryotes is that eukaryotic cells contain membrane-bound compartments inwhich specific metabolic activities take place. Most important among these isa cell nucleus, a membrane-delineated compartment that houses the eukaryoticcells DNA. This nucleus gives the eukaryote its name, which means "truenucleus."
MembraneThe cytoplasm of a cell is surrounded by a cell membrane or plasma membrane. Theplasma membrane in plants and prokaryotes is usually covered by a cell wall. Thismembrane serves to separate and protect a cell from its surrounding environmentand is made mostly from a double layer of lipids(hydrophobic fat-like molecules)and hydrophilic phosphorus molecules. Hence, the layer is called a phospholipidsbilayer, or sometimes a fluid mosaic membrane. Embedded within this membrane isa variety of protein molecules that act as channels and pumps that move differentmolecules into and out of the cell. The membrane is said to be semi-permeable, inthat it can either let a substance (molecule or ion) pass through freely, pass throughto a limited extent or not pass through at all. Cell surface membranes alsocontain receptor proteins that allow cells to detect external signalling moleculessuch as hormones.
The cytoplasm is the gel-like substance residing within the cell membrane holdingall the cells internal sub-structures (called organelles), outside the nucleus. All thecontents of the cells of prokaryote organisms (such as bacteria, which lack a cellnucleus) are contained within the cytoplasm. Within the cellsof eukaryote organisms the contents of the cell nucleus are separated from thecytoplasm, and are then called the nucleoplasm, The cytoplasm is about 70% to90% water and usually colourless.In cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed organelle found in most eukaryotic cells.These organelles range from 0.5to 1.0 micrometer (μm) in diameter. Mitochondria are sometimes described as"cellular power plants" because they generate most of the cells supplyof adenosine triphosphate (ATP), used as a source of chemical energy.
•Cell nucleus –A cells information center, the cell nucleus is the most conspicuousorganelle found in a eukaryotic cell. It houses the cells chromosomes, and is the placewhere almost all DNA replication and RNA synthesis (transcription) occur. The nucleus isspherical and separated from the cytoplasm by a double membrane called the nuclearenvelope. The nuclear envelope isolates and protects a cells DNA from variousmolecules that could accidentally damage its structure or interfere with its processing.During processing, DNA is transcribed, or copied into a special RNA, called messengerRNA (mRNA). This mRNA is then transported out of the nucleus, where it is translatedinto a specific protein molecule. The nucleolus is a specialized region within the nucleuswhere ribosome subunits are assembled. In prokaryotes, DNA processing takes place inthe cytoplasm.OrganellesThe human body contains many different organs, such as the heart, lung, andkidney, with each organ performing a different function. Cells also have a set of "littleorgans," called organelles, that are adapted and/or specialized for carrying out one ormore vital functions. Both eukaryotic and prokaryotic cells have organelles butorganelles in eukaryotes are generally more complex and may be membrane bound.There are several types of organelles in a cell. Some (such as the nucleus and golgiapparatus) are typically solitary, while others (suchas mitochondria, peroxisomes and lysosomes) can be numerous (hundreds tothousands). The cytosol is the gelatinous fluid that fills the cell and surrounds theorganelles.
Chloroplasts are organelles found in plant cells and someother eukaryotic organisms. Their functions include conducting photosynthesis, andin some algae, lipid synthesis. Photosynthesis is their main function, wherechloroplasts capture the suns light energy, and store it in the energy storagemolecules ATP and NADPH while breaking down water molecules.They then use the ATP and NADPH to make organic molecules from carbondioxide and free oxygen from water, in a process known as the Calvin cycle.
Golgi apparatus – eukaryotes only : The primary function of the Golgiapparatus is to process and package the macromolecules such as proteinsand lipids that are synthesized by the cell.Ribosomes: The ribosome is a large complex of RNA and protein molecules.They each consist of two subunits, and act as an assembly line where RNA fromthe nucleus is used to synthesise proteins from amino acids. Ribosomes can befound either floating freely or bound to a membrane (the rough endoplasmaticreticulum in eukaryotes, or the cell membrane in prokaryotes).
Vacuoles: Vacuoles store food and waste. Some vacuoles store extra water. They areoften described as liquid filled space and are surrounded by a membrane. Somecells, most notably Amoeba, have contractile vacuoles, which can pump water out ofthe cell if there is too much water. The vacuoles of eukaryotic cells are usually largerin those of plants than animals.Cell wallMany types of prokaryotic and eukaryotic cell have a cell wall. The cell wall acts toprotect the cell mechanically and chemically from its environment, and is anadditional layer of protection to the cell membrane. Different types of cell have cellwalls made up of different materials; plant cell walls are primarily made up ofpectin, fungi cell walls are made up of chitin and bacteria cell walls are made up ofpeptidoglycan.
1.Plant cells are larger than animal cells.2.Plant cells have chloroplasts unlike animal cells3.Plant cells have a cell wall unlike animal cells.4.Animal cells have a lot of lysosomes unlike plant cells.5.Animal cells have a centrosome unlike plant cells6.Plant cells have plasticids unlike animal cells7.Vacuoles are conspicuous in plant cells than animal cells i.e. large central vacuolein plant cells8. Animal cells can be phagocytic (engulf other cells) unlike plant cells9.Cells of Higher plants lack centrioles unlike animal cells.10.Plant cells have plasmodesmata which links pores in the cell wall allow andcommunication between adjacent cells unlike animal cells.The following are a differences between Plant cells and Animalcells:
Red blood cells, or erythrocytes, are the most common type of blood cell andthe vertebrate organisms principal means of delivering oxygen (O2) to the bodytissues via the blood flow through the circulatory system. They take up oxygen inthe lungs or gills and release it while squeezing through the bodys capillaries.These cells cytoplasm is rich in haemoglobin, an iron-containing biomolecule thatcan bind oxygen and is responsible for the bloods red colour.In humans, mature red blood cells are oval and flexible biconcave disks. They lacka cell nucleus and most organelles to accommodate maximum space forhaemoglobin. 2.4 million new erythrocytes are produced per second. The cellsdevelop in the bone marrow and circulate for about 100–120 days in the bodybefore their components are recycled by macrophages. Each circulation takesabout 20 seconds. Approximately a quarter of the cells in the human body are redblood cells.
White blood cells, or leukocytes (also spelled "leucocytes"; from the Greekword leuko- meaning "white"), are cells of the immune system involved indefending the body against both infectious disease and foreign materials.Five different and diverse types of leukocytes exist, but they are all producedand derived from a multipotent cell in the bone marrow known asa hematopoietic stem cell. They live for about three to four days in the averagehuman body. Leukocytes are found throughout the body, includingthe blood and lymphatic system.
Tissue is a cellular organizational level intermediate between cells and acomplete organism. A tissue is an ensemble of similar cells and from thesame origin, that together carry out a specific function. These are called tissuesbecause of their identical functioning. Organs are then formed by thefunctional grouping together of multiple tissues.In biology, an organ is a collection of tissues joined in a structural unit to serve acommon function.There is a "main" tissue, parenchyma, and "sporadic"tissues, stroma. The main tissue is the one that is unique for the specific organ. Forexample, the main tissue in the heart is the myocardium, while sporadic tissuesinclude the nerves, blood and connective tissues. Functionally related organs oftencooperate to form whole organ systems. Organs exist in all higher biologicalorganisms, in particular they are not restricted to animals, but can also be identifiedin plants.