WHAT IS CELL BY AHMAD YAKUBU ALCHEMY

The cell (from Latin cella, meaning "small room"^[1]) is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life that can replicate independently, and cells are often called the "building blocks of life". The study of cells is called cell biology.
Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids.^[2] Organisms can be classified as unicellular (consisting of a single cell; including bacteria) or multicellular (including plants and animals). While the number of cells in plants and animals varies from species to species, humans contain more than 10 trillion (10¹³) cells.^[3] Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometres.^[4]
The cell was discovered by Robert Hooke in 1665, who named the biological unit for its resemblance to cells inhabited by Christian monks in a monastery.^[5][6] 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 cells are the fundamental unit of structure and function in all living organisms, that all cells come from preexisting cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.^[7] Cells emerged on Earth at least 3.5 billion years ago.^[8][9][10]

Comparison of features of prokaryotic and eukaryotic cells
	Prokaryotes	Eukaryotes
Typical organisms	bacteria, archaea	protists, fungi, plants, animals
Typical size	~ 1–5 µm[11]	~ 10–100 µm[11]
Type of nucleus	nucleoid region; no true nucleus	true nucleus with double membrane
DNA	circular (usually)	linear molecules (chromosomes) with histone proteins
RNA/protein synthesis	coupled in the cytoplasm	RNA synthesis in the nucleus protein synthesis in the cytoplasm
Ribosomes	50S and 30S	60S and 40S
Cytoplasmic structure	very few structures	highly structured by endomembranes and a cytoskeleton
Cell movement	flagella made of flagellin	flagella and cilia containing microtubules; lamellipodia and filopodia containing actin
Mitochondria	none	one to several thousand
Chloroplasts	none	in algae and plants
Organization	usually single cells	single cells, colonies, higher multicellular organisms with specialized cells
Cell division	binary fission (simple division)	mitosis (fission or budding) meiosis
Chromosomes	single chromosome	more than one chromosome
Membranes	cell membrane	Cell membrane and membrane-bound organelles

Cells are of two types, eukaryotic, which contain a nucleus, and prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular.

Prokaryotic cells

Main article: Prokaryote

Structure of a typical prokaryotic cell

Prokaryotic cells were the first form of life on Earth, characterised by having vital biological processes including cell signaling and being self-sustaining. They are simpler and smaller than eukaryotic cells, and lack membrane-bound organelles such as the nucleus. Prokaryotes include two of the domains of life, bacteria and archaea. The DNA of a prokaryotic cell consists of a single chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter.^[12]

A prokaryotic cell has three architectural regions:

• Enclosing the cell is the cell envelope – generally consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsule. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea) which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment. Some eukaryotic cells (plant cells and fungal cells) also have a cell wall.
• Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of inclusions. The genetic material is freely found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease.^[13] Though not forming a nucleus, the DNA is condensed in a nucleoid. Plasmids encode additional genes, such as antibiotic resistance genes.
• 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.

Eukaryotic cells

Main article: Eukaryote

Structure of a typical animal cell

Structure of a typical plant cell

Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles (compartments) in which specific metabolic activities take place. Most important among these is a cell nucleus, an organelle that houses the cell's DNA. This nucleus gives the eukaryote its name, which means "true kernel (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 a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."^[14]
• Motile cells of eukaryotes can move using motile cilia or flagella. Motile cells are absent in conifers and flowering plants.^[15] Eukaryotic flagella are less complex than those of prokaryotes.

Subcellular components

Illustration depicting major structures inside a eukaryotic animal cell

All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, the cytoplasm takes up most of the cell's volume. All cells (except red blood cells which lack a cell nucleus and most organelles to accommodate maximum space for hemoglobin) possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article lists these primary components of the cell, then briefly describes their function.

Membrane

Main article: Cell membrane

The cell membrane, or plasma membrane, is a biological membrane that surrounds the cytoplasm of a cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of phospholipids, which are amphiphilic (partly hydrophobic and partly hydrophilic). Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaic membrane. Embedded within this membrane is a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is said to be 'semi-permeable', in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones.

Cytoskeleton

Main article: Cytoskeleton

A fluorescent image of an endothelial cell. Nuclei are stained blue, mitochondria are stained red, and microfilaments are stained green.

The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of microfilaments, intermediate filaments and microtubules. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis.^[16] The subunit protein of microfilaments is a small, monomeric protein called actin. The subunit of microtubules is a dimeric molecule called tubulin. Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. But some of the subunit protein of intermediate filaments include vimentin, desmin, lamin (lamins A, B and C), keratin (multiple acidic and basic keratins), neurofilament proteins (NF - L, NF - M).

Genetic material

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is encoded in its DNA sequence. RNA is used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA). Transfer RNA (tRNA) molecules are used to add amino acids during protein translation.

Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory).

A human cell has genetic material contained in the cell nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). In humans the nuclear genome is divided into 46 linear DNA molecules called chromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomes. The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes, it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.

Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Certain viruses also insert their genetic material into the genome.

Organelles

Main article: Organelle

Organelles are parts of the cell which are adapted and/or specialized for carrying out one or more vital functions, analogous to the organs of the human body (such as the heart, lung, and kidney, with each organ performing a different function). Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound.

There are several types of organelles in a cell. Some (such as the nucleus and golgi apparatus) are typically solitary, while others (such as mitochondria, chloroplasts, peroxisomes and lysosomes) can be numerous (hundreds to thousands). The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.

Eukaryotic

Human cancer cells with nuclei (specifically the DNA) stained blue. The central and rightmost cell are in interphase, so the entire nuclei are labeled. The cell on the left is going through mitosis and its DNA has condensed.

• Cell nucleus: A cell's information center, the cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called messenger RNA (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The nucleolus is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the cytoplasm.
• Mitochondria and Chloroplasts: generate energy for the cell. Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Respiration occurs in the cell mitochondria, which generate the cell's energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. Mitochondria multiply by binary fission, like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through photosynthesis.

Diagram of an endomembrane system

• Endoplasmic reticulum: The endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes. The smooth ER plays a role in calcium sequestration and release.
• Golgi apparatus: The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids that are synthesized by the cell.
• Lysosomes and Peroxisomes: Lysosomes contain digestive enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.
• Centrosome: the cytoskeleton organiser: The centrosome produces the microtubules of a cell – a key component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.
• Vacuoles: Vacuoles sequester waste products and in plant cells store water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells.

Eukaryotic and prokaryotic

• 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 from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes).^[17]

Structures outside the cell membrane

Life timeline

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Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the semipermeable cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.

Cell wall

Many types of prokaryotic and eukaryotic cells have a cell wall. The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Different types of cell have cell walls made up of different materials; plant cell walls are primarily made up of cellulose, fungi cell walls are made up of chitin and bacteria cell walls are made up of peptidoglycan.

Prokaryotic

Capsule

A gelatinous capsule is present in some bacteria outside the cell membrane and cell wall. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococci. Capsules are not marked by normal staining protocols and can be detected by India ink or methyl blue; which allows for higher contrast between the cells for observation.^[18]:87

Flagella

Flagella are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through the cell membrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature. A different type of flagellum is found in archaea and a different type is found in eukaryotes.

Fimbria

A fimbria also known as a pilus is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae, or pili are formed of a protein called pilin (antigenic) and are responsible for attachment of bacteria to specific receptors of human cell (cell adhesion). There are special types of specific pili involved in bacterial conjugation.

Cellular processes

Growth and metabolism

Main articles: Cell growth and Metabolism

Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into simpler sugar molecules called monosaccharides such as glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), a molecule that possesses readily available energy, through two different pathways.

Replication

Bacteria divide by binary fission, while eukaryotes divide by mitosis or meiosis.

Main article: Cell division

Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms. Prokaryotic cells divide by binary fission, while eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells.

DNA replication, or the process of duplicating a cell's genome, always happens when a cell divides through mitosis or binary fission. This occurs during the S phase of the cell cycle.

In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before meiosis I. DNA replication does not occur when the cells divide the second time, in meiosis II.^[19] Replication, like all cellular activities, requires specialized proteins for carrying out the job.

Protein synthesis

An overview of protein synthesis.
Within the nucleus of the cell (light blue), genes (DNA, dark blue) are transcribed into RNA. This RNA is then subject to post-transcriptional modification and control, resulting in a mature mRNA (red) that is then transported out of the nucleus and into the cytoplasm (peach), where it undergoes translation into a protein. mRNA is translated by ribosomes (purple) that match the three-base codons of the mRNA to the three-base anti-codons of the appropriate tRNA. Newly synthesized proteins (black) are often further modified, such as by binding to an effector molecule (orange), to become fully active.

Main article: Protein biosynthesis

Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from amino acid building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription and translation.

Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give messenger RNA (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called ribosomes located in the cytosol, where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to transfer RNA (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.

Movement or motility

Main article: Motility

Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include flagella and cilia.

In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis. For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins.^[20] The process is divided into three steps – protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton.^[21][22]

Multicellularity

Main article: Multicellular organism

Cell specialization

Staining of a Caenorhabditis elegans which highlights the nuclei of its cells.

Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms.^[23]

In complex multicellular organisms, cells specialize into different cell types that are adapted to particular functions. In mammals, major cell types include skin cells, muscle cells, neurons, blood cells, fibroblasts, stem cells, and others. Cell types differ both in appearance and function, yet are genetically identical. Cells are able to be of the same genotype but of different cell type due to the differential expression of the genes they contain.

Most distinct cell types arise from a single totipotent cell, called a zygote, that differentiates into hundreds of different cell types during the course of development. Differentiation of cells is driven by different environmental cues (such as cell–cell interaction) and intrinsic differences (such as those caused by the uneven distribution of molecules during division).

Origin of multicellularity

Multicellularity has evolved independently at least 25 times,^[24] including in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, Magnetoglobus multicellularis or Methanosarcina. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants.^[25] It evolved repeatedly for plants (Chloroplastida), once or twice for animals, once for brown algae, and perhaps several times for fungi, slime molds, and red algae.^[26] Multicellularity may have evolved from colonies of interdependent organisms, from cellularization, or from organisms in symbiotic relationships.

The first evidence of multicellularity is from cyanobacteria-like organisms that lived between 3 and 3.5 billion years ago.^[24] Other early fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon.^[27]

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure.^[24]

Origins

Main article: Evolutionary history of life

The origin of cells has to do with the origin of life, which began the history of life on Earth.

Origin of the first cell

Stromatolites are left behind by cyanobacteria, also called blue-green algae. They are the oldest known fossils of life on Earth. This one-billion-year-old fossil is from Glacier National Park in the United States.

Further information: Abiogenesis and Evolution of cells

There are several theories about the origin of small molecules that led to life on the early Earth. They may have been carried to Earth on meteorites (see Murchison meteorite), created at deep-sea vents, or synthesized by lightning in a reducing atmosphere (see Miller–Urey experiment). There is little experimental data defining what the first self-replicating forms were. RNA is thought to be the earliest self-replicating molecule, as it is capable of both storing genetic information and catalyzing chemical reactions (see RNA world hypothesis), but some other entity with the potential to self-replicate could have preceded RNA, such as clay or peptide nucleic acid.^[28]

Cells emerged at least 3.5 billion years ago.^[8][9][10] The current belief is that these cells were heterotrophs. The early cell membranes were probably more simple and permeable than modern ones, with only a single fatty acid chain per lipid. Lipids are known to spontaneously form bilayered vesicles in water, and could have preceded RNA, but the first cell membranes could also have been produced by catalytic RNA, or even have required structural proteins before they could form.^[29]

Origin of eukaryotic cells

Further information: Evolution of sexual reproduction

The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. DNA-bearing organelles like the mitochondria and the chloroplasts are descended from ancient symbiotic oxygen-breathing proteobacteria and cyanobacteria, respectively, which were endosymbiosed by an ancestral archaean prokaryote.

There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or vice versa: see the hydrogen hypothesis for the origin of eukaryotic cells.

History of research

• 1632–1723: Antonie van Leeuwenhoek teaches himself to make lenses, constructs basic optical microscopes and draws protozoa, such as Vorticella from rain water, and bacteria from his own mouth.
• 1665: Robert Hooke discovers cells in cork, then in living plant tissue using an early compound microscope. He coins the term cell (from Latin cella, meaning "small room"^[1]) in his book Micrographia (1665).^[30]
• 1839: Theodor Schwann and Matthias Jakob Schleiden elucidate the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, and thus founding the cell theory.
• 1855: Rudolf Virchow states that new cells come from pre-existing cells by cell division (omnis cellula ex cellula).
• 1859: The belief that life forms can occur spontaneously (generatio spontanea) is contradicted by Louis Pasteur (1822–1895) (although Francesco Redi had performed an experiment in 1668 that suggested the same conclusion).
• 1931: Ernst Ruska builds the first transmission electron microscope (TEM) at the University of Berlin. By 1935, he has built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles.
• 1953: Watson and Crick made their first announcement on the double helix structure of DNA on February 28.
• 1981: Lynn Margulis published Symbiosis in Cell Evolution detailing the endosymbiotic theory.

Cell Structure

A cell is the smallest working unit of all living organisms on our planet earth, which is capable of performing life functioning. Hence it can also be defined as a fundamental unit of life. The term cell was first observed and identified by an English physicist Robert Hook in the year 1665.There were many theories developed for cell. Later in the year 1839 a two German scientist – Schwann and Schleiden provided few basic principles of cell.

Cell Structures

There are many cells in an individual, which performs several functions throughout the life. The different types of cell include- prokaryotic cell, plant and animal cell.  The size and the shape of the cell range from millimeter to microns, which are generally based on the type of function that it performs. A cell generally varies in their shapes. A few cells are in spherical, rod, flat, concave, curved, rectangular, oval and etc. These cells can only be seen under microscope.

Cell Theory

• Every living organism is made up of a single cell (unicellular) (or) many cells (multicellular) and all types of cells have certain structures in common like: genetic material and plasma membrane.
• Cell is the smallest living thing.
• Each cell arises only from pre-existing cells.

Cell Structure and Function

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Cell wall: It helps in protecting the plasma membrane and plays a vital role in supporting and protecting the cells.    It is a thick outer layer made of cellulose. 

Cell membrane: It is a double layered, thin barrier, surrounding the cell to control the entry and exit of certain substances.

Cytoplasm: It is a membrane, which protects the cell by keeping the cell organelles separate from each other. This helps to keep a cell in stable. Cytoplasm is the site, where many vital biochemical reactions take place.

Nucleus: They are the membrane bound organelles, which are found in all eukaryotic cells. It is the very important organelle of a cell as it controls the complete activity of a cell and also plays a vital role in reproduction.

Nuclear membrane: The bilayer membrane, which protects the nucleus by surrounding around it and acts as a barrier between the cell nucleus and other organs of a cell.

Nucleolus: It is an important membrane found inside the nucleus. It plays a vital role in the production of cell's ribosome.

Chromosomes: It is made up of DNA and stored in the nucleus, which contains the instructions for traits and characteristics. 

Endoplasmic reticulum: It helps in the movement of materials around the cell. It contains an enzyme that helps in building molecules and in manufacturing of proteins. The main function of this organelle is storage and secretion. 

Ribosome: It plays a vital role in protein synthesis.

Mitochondria: They are double membrane, filamentous organelles, which play a vital role in generating and transforming the energy. Mitochondria play a vital role in various functions of the cell metabolisms including oxidative phosphorylation.

Golgi Bodies: It helps in the movement of materials within the cell.

Lysosomes: It is also called as suicidal bags as it helps in cell renewal and break down old cell parts.

Vacuoles: It helps plants in maintaining its shape and it also stores water, food, wastes, etc.

Chloroplast: They are the site of photosynthesis, which are present in chlorophyll bacteria, blue-green algae, etc.

Prokaryotic Cell Structure

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They are the first organisms to be present on our planet earth. Organisms, with this cell type are known by the term prokaryotic organisms (or) prokaryotes. Bacteria, blue green algae and E.coli are few examples of this category. Prokaryotic cells are single-celled organisms, with the absence of nucleus and comprises of capsule, cell wall, cell membrane, cytoplasm, nucleiod, ribosome, plasmids, pili and flagella.

Prokaryotic Cell General Features

• The size of a cell ranges from 1-10 microns. Few prokaryotic cells vary in their size.
• They are single-celled (unicellular), which forms a colony or filamentous.
• The shape of the cell includes spherical, rod and flat shaped organisms.
• Mode of nutrients-- few organisms are photosynthetic (performing food with the help of sunlight), feed on living things and dead things.
• They reproduce asexually by the process called binary fission, transformation, conjugation, transduction.

Structure and Functions of a Prokaryotic Cell

Capsule: It is the slimy outer coating of the cell wall. It is composed of the polypeptide. The main function of the capsule is to protect the cell from getting dry and also helps in protecting cells from external pressures.

Cell wall: It is the tougher and a rigid structure, which provides the shape and protects the internal organelles of a cell. It is the middle layer, which is present in between the capsule and cell membrane.

Cell membrane: It is the inner delicate structure, which plays a vital role in regulating the entry and exits of materials in the cell. It acts a permeable membrane and separates the cell from its environment. It is of about 5-10nm in thickness, which helps in the secretion of proteins and elimination of waste products. It is also called by a name plasma membrane.

Cytoplasm: It is the liquid membrane, which is present in between the cell membrane and nucleiod. It plays a vital role in storing all types of materials, which are required for an organism to sustain the life.

Nucleiod: It is the cytoplasm region containing genetic material. The DNA of a prokaryotic organism is one big loop or a circular, which is located inside the nucleiod. It plays a vital role in cell division.

Ribosome: It  comprises of both RNA and proteins. It helps in protein synthesis in the cell. They are smallest membrane present inside the cytoplasm.

Plasmids: They are smallest membrane of a cell with double stranded DNA. Plasmids are rarely present in prokaryotic organisms. The main role of plasmids is it helps in DNA exchanging between the bacterial cells.

Pilli: It is the thinnest membrane of a prokaryotic cell. They are composed of protein complex called pilin and are mainly involved in sticking to the objects especially during sexual reproduction.

Flagella: It is the helical shaped membrane, whose sizes ranges from 19-20nm in diameter and plays a vital role in motility of an organism from one place to another place. It also helps in swimming, gliding, spinning and rotating both in clockwise and anti clockwise directions.

Eukaryotic Cell Structure

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They are the cells with the presence of true nucleus. Organisms, with this cell type are known by the term eukaryotic organisms (or) eukaryotes. Animals, plants and other organisms excluding bacteria, blue green algae and E.coli have been grouped into this category. Eukaryotic cells are more complex than prokaryotic cells. These organisms have membrane bound nucleus with many cell organelles to perform several cellular functions within the system.

Eukaryotic Cell General Features

• The size of a eukaryotic cell ranges from 10-100 microns. Few eukaryotic cells vary in their size.
• They are large, advanced, multicellular and have membrane bound organelles.
• They reproduce both by sexually and by asexually.
• Mode of nutrients - Autotrophic and heterotrophic.
• Kingdom protozoa, algae, fungi, Plantae and Animalia are organisms with eukaryotic cell.

Structure and Functions of a Eukaryotic Cell

Plasma membrane: They are semi permeable membrane that acts as a boundary of a cell, which protects and separates the cell from the external environment.

Nucleus: It is surrounded by double layered of Phospho lipid bilayer called as nuclear envelope.  

They are the storehouse for the cell’s genetic materials in the form of DNA and store all the necessary information, which are required for a cell to control all types of activities. 

Nuclear membrane: It is the double membrane layer that surrounds the nucleus and it plays a role of entry and exits of materials within the nucleus.

Nucleolus: It is the non membrane bound organelles, which is present within the nucleus and is mainly involved in controlling all types of cellular activities including cellular reproduction.

Mitochondria: They are the double smooth membrane, which are present in all eukaryotic cells. They are the powerhouse of the cell. It plays a vital role in the synthesis of ATP and converts glucose to ATP.

Endoplasmic reticulum: They are the double membrane organelle, which divides the cell into compartments. It is connected to the nuclear membrane of the cell. It plays a vital role in protein synthesis, biosynthesis of lipids and steroids, stores and regulates calcium and metabolism of carbohydrates.  Endoplasmic reticulum is of two types – rough and smooth Endoplasmic reticulum.

Ribosome: It is present in the cytoplasm. They are the site for cell’s protein synthesis, which are composed of ribosomal RNA and proteins.

Golgi Bodies: It is the flattened membrane, which are mainly used to store the substances made by the cell. This membrane also helps in preserving, transporting materials within the cell. Hence it is also called as the post office of a cell.

Lysosomes: They are the membrane bound organelles, which contains digestive enzymes to break down macromolecules. Lysosome plays a vital role in protecting cell by engulfing or destroying foreign bodies entering the cell.

Cytoplasm: They are the jelly types of organelles, which are present in the inner region of a cell. It plays a vital role in keeping a cell in a stable and keeps the cell organelles separate from each other.

Chromosomes: The rod shaped structures, which are composed of proteins and DNA. Chromosomes also play a vital role in determining a sex of an individual. All human cells contain 46 numbers of chromosomes.

Plant Cell Structure

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A plant cells are eukaryotic cells, with the presence of true nucleus, multicellular large and advanced membrane bound organelles. These plant cells are quite different from animal cells like in shape and other few organelles which are only found in animal cells but are absent in plant cells. Based on structure and functions, plant cells comprise of:

Cell wall: It is the outer layer of a plant cell, which helps a providing the shape and strength to the entire plant. A cell wall is composed of cellulose that protects and supports the plant to grow.

Cell membrane: It is a biological membrane that separates living cell organelles from non living structures. This membrane plays a vital role in helping a cell to communicate with the external environment and also in transporting proteins and other molecules throughout the cell.

Chloroplasts: They are green coloured oval shaped double membrane organelles, which are the sites of photosynthesis. The green colour pigment (chlorophyll) present in the leaves helps plants in absorbing solar energy to prepare food.

Cytoplasm: A jelly types double membrane organelles, which are present in the inner region of a cell. It helps by keeping a cell in stable and protects the cell organelles by separating them from each other.

Lysosomes: They are single membrane round organelles, which helps in digesting proteins, fats and carbohydrates. It also helps in excretion of undigested materials from the plant cells. It also helps in cell renewal and breakdown of large cells into smaller cells, old and dead cells.

Golgi Bodies (or) Gogi complex: The sac like structures, which are present in a cell to manufacture, store, packing and shipping the materials throughout the cell.

 Endoplasmic reticulum: The network of membrane, which helps in transporting materials around the cell. It forms a connection between nuclear envelope and the cell membrane of a cell.

Mitochondria: They are rod shaped organelles, plays an important role in releasing energy and they are the powerhouse of a cell. In plant cell, they are the sites of cellular respiration.

Nucleus: They are large, oval shaped organelle that contains one or more nucleoli with DNA. The main role of nucleus in plant cell is it controls all types of cellular activities.

Nucleolus: They are the spherical membranes, present inside the nucleus. This membrane contains RNA, which is used to build proteins. They are the sites of RNA synthesis.

Vacuole: They are the fluid sacs, which are present in large numbers in plant cells. The main function is this membrane is to store food and other waste materials. Vacuoles are the largest organelles present in the plant cells.

Animal Cell Structure

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Animal cells are eukaryotic cells, with the presence of true nucleus; multicellular large and advanced membrane bound organelles. Like plant cells, animal cells have same organelles except the cell wall, chloroplasts, number of vacuoles and many more. Due to the absence of cell wall the shape of an animal cell is irregular. Based on structure and functions, animal cells comprise of:

Cell membrane: They are semi-permeable membrane surrounding the cell. It helps in holding the cell together and allows entry and exits of nutrients into the cell.

Nucleus: The largest organelle in the cell, which contains DNA and other cell's hereditary information. The main role of nucleus in animal cell is it controls all cellular activities.

Vacuole: They are the fluid sacs, which are present in less numbers in animal cells compared to plant cells. The main function is this membrane is to store food and other waste materials.

Golgi Bodies (or) Gogi complex: The sac like structures, which are present in a cell to manufacture, store, packing and shipping the selected particles throughout the cell.

Ribosome: It is present in the cytoplasm. They are the site of protein synthesis, which are composed of ribosomal RNA and proteins.

Endoplasmic reticulum: The network of membrane, which helps in transporting materials around the cell and also helps in the synthesis of lipids and proteins. It forms a connection between nuclear envelope and the cell membrane of a cell.

Mitochondria: They are rod shaped organelles, plays an important role in releasing energy and they are the powerhouse of a cell.

Cytoplasm: A jelly types double membrane organelles, which are present in the inner region of a cell. It helps by keeping a cell in stable and protects the cell organelles by separating them from each other.

Lysosomes: They are round single membrane round organelles, which helps in recycling of cell organelles. Lysosome of an animal cell contains some digestive enzymes that help in the digesting out the cell debris.

All types of cells – eukaryotic and prokaryotic cell, animal and plant cells have many similarities and also differences in them, which all depends on its structure and functions of each cell organelles.

Cell Theory

Cell Metabolism

• Cell Structure
• Cell Structure and Function
• Prokaryotic Cell Structure
• Eukaryotic Cell Structure
• Plant Cell Structure
• Animal Cell Structure

Related Concepts

Cell and Cell Structure

Animal Cell Structure and Function

B Cell Structure

Bacterial Cell Structure

Bacterial Cell Wall Structure

Cell Nucleus Structure

cell organelle structure and function

Cell Plasma Membrane Structure

Related Formulas

Structural Equation

Ethanol Structural Formula

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