What is a cell? Definition, Functions & types | FAQ

What is a cell? Definition, Functions & types | FAQ

Cell is the Structural, functional and biological units of all living things. It is made up of biomolecules such ascarbohydrates,  lipids, Proteins etc. All cells contain cytoplasm, which is surrounded by a cell membrane. The size of cells ranges from 1–100 micrometers. The cells can't  see by naked eye they can seen under a microscope. According to the number of cells in an organism, they are classified as unicellular (e.g., bacteria) and multicellular (e.g., plants, animals, etc.). The number of cells varies according to the species of plants and animals. Such as in the human body. There are 1 trillion cells are present.

All cells show some similar properties. For example, cells breathe, cells need food, cells excrete wastes, cells grow, and cells die. Nutrients, oxygen and carbon dioxide are exchanged from the cell surface. Although there are differences in the size of living things in the same creation, the size of their cells is the same. For example, elephants and rats differ in body size. This is because the number of cells in an elephant's body is higher than the number of cells in a rat's body. Also, the liver of an elephant is larger than the liver of a rat because the number of cells in the liver of an elephant is higher than the number of cells in the liver of a rat. 

Cell size does not increase beyond a certain limit. This is because as the size of the cell increases, so does the ratio of cell surface to volume. A high ratio of cell surface to volume is essential for cell function. This is because the cells receive nutrients through the surface, oxygen and carbon dioxide are exchanged, and waste is expelled.

What is a cell? Definition, Functions & types | FAQ

What is Cell ?

  1. The cell is the anatomical and functional unit of living organisms and often has the ability to spontaneously regenerate. It is that small organized form of various substances in which all those actions are occurs collectively called as life.
  2. The English word for cell is derived from the Latin word 'cellula' which means 'a small room'. Some living organisms like bacteria are made up of a single cell, they are called unicellular organisms whereas some living organisms like human body are made up of many cells, they are called multicellular organisms. Robert Hooke discovered the cell in 185 AD.
  3. In 1939, Schleiden and Schwann introduced cell theory according to which the body of all living beings is made up of one or more cells and all cells originate from a pre-existing cell.
  4. All biological functions of living organisms occur within cells. Within the cells, there are essential genetic information that controls the functions of the cell and the information is transferred to the next generation of cells.
  5. The proper study of cells is called Cytology or Cell Biology.

History of discovery of  Cell 
Most of cells cannot be seen without the use of instruments. Its discovery is directly associated with the development of microscopes, instruments formed by lenses that allow the enlargement of images. The microscope, invented in the 16th century, was used for the first time to observe living beings in the 15th century, by the Dutchman Antony van Leeuwenhoek (1632-1723).

In his analyzes, he observed, among other organisms, beings in the form of small rods. That's how he discovered bacteria . In Greek, the word  bakteria  means "small stick", hence the name of these microscopic beings.

The Biologist Robert Hooke (1635-1703) made improvements to the Leeuwenhoek microscope, adding another lens, and with it he observed different materials, including pieces of cork. Hooke realized that cork was formed by innumerable empty compartments, as if they were holes, which he called cells (from the Latin  cella =  closed room).

Cell Theory
Among the countless advances in the area, it is essential to consider the work of researchers Matthias Schleiden and Theodor Schwann who discovered the cell theory. Cell theory is one of the most important and central postulates in the field of modern biology. This theory also describes the role of cells in the evolutionary history of life on the planet . From this he explains the main characteristics of living beings .

The cell theory postulates:
1. All living things are made up of cells :  Therefore, these are the minimum functional unit of life, in its different levels of structural complexity. One cell is enough to constitute an organism (unicellular), but many cells can organize themselves in colonies or in a single organism (multicellular), diversifying their functions and reaching a very high margin of interdependence.

2. Metabolism and functions of organisms take place within cells : Through biochemical processes, and are controlled by substances that cells secrete. Each cell operates as a unique open system exchanging matter and energy with its environment in a controlled manner. Furthermore, in each cell of an organism the same vital functions occur as in the entire organism: birth, growth, reproduction, death.

3. All the cells that exist come from pre-existing cells by Cell division : By cell division or by formation from stem cells. The oldest and most primitive cells in the world are prokaryotes (without a cell nucleus)

4. The cell is the Basic Structural and functional unit of living thingsThe cell is the anatomical and functional unit of living organisms and often has the ability to spontaneously regenerate. It is that small organized form of various substances in which all those actions are occurs collectively called as life.

Types of Cell :

There are two types of cells.
  1. Prokaryotic Cell and 
  2. Eukaryotic Cell 

1. Prokaryotic Cell
Prokaryotic cells are often independent. The prokaryotic cell has no clear nucleus. Organisms whose cells are prokaryotic are known as prokaryotes and are usually primitive, single-celled and smaller organisms. The plasma membrane of prokaryotic cells is made up of lipids, complex proteins, enzyme. Inside prokaryotic cells, DNA is packed into a structure called a nucleoid. The nucleoid DNA is connected to the plasma membrane through the mesosomes. 

Prokaryotic organisms, having developed in anaerobic environments, have very complex metabolic processes, such as glycolysis (or glycolysis). This consists of the degradation of glucose under anaerobic conditions, and it is present in all cells where ATP synthesis occurs, an important source of chemical energy. Their shape is minimal and the shape is spherical, oval, columnar, porous, flagellate, polygonal etc.  

Prokaryotes were the first life forms on earth, so it is logical to assume that the prokaryotic cell was the first type of organized cell to exists. Prokaryotic cells present an enormous diversity of nutrition methods, depending on the type of organism and the habitat. Examples of prokaryotic cells are those of bacteria (both gram-negative and gram-positive) or archaebacteria or archaea. Some are non-pathogenic and live outside our bodies. Having no cell nucleus, prokaryotic cells divide in only two ways:

Asexual : By binary fission or bipartition, a quick and easy way in which the cell grows until it divides into two new ones of identical genetic material . 

Parasexual : A method of DNA exchange that allows obtaining genetic variability between prokaryotic cells and that can occur in three ways Transformation, conjugation and transduction.

The eukaryotic cell that has a defined nucleus, in which the genetic material (DNA) of the organism is present and protected by the cytoplasm and a membrane that constitutes the cell envelope. The eukaryotic cell and the prokaryotic cell differ because the prokaryotic cell is more primitive and lacks a defined cell nucleus, so the genetic material is scattered in the cytoplasm of prokaryoticcell. Organisms composed of eukaryotic cells are known as eukaryotes, and after the appearance of these more specific cells they originated other types of multicellular organisms classified as protozoa, fungi, plants and animals.

The word eukaryote derives from the Greek eukayron, Meaning : eu- (true), and karyon (nucleus). Therefore its meaning is 'true nucleus'. Eukaryotic cells fulfill different functions that are important for eukaryotic organisms, such as protein synthesis, Photosynthesis, glycolysis through which they obtain energy to perform other functions.

Eukaryotic cell types
There are many types of eukaryotic cells.  three mainly stand out, which are the ones that we are going to specify below.

Animal Cells: Name indicates that cells found animals  called animal cell. They do not have a cell wall so their shape varies according to their needs for food and movement within the body.

Plant Cells: They are those found in plants, as their name says and one of their main characteristics is that they are equipped with everything necessary to carry out photosynthesis such as chlorophyll. They have a cell wall so their shape is a little more defined than the previous one in addition to providing firmness.

Fungi Cells: They are very similar to animal cells, although their biggest difference is cell wall of fungi cell is made up of chitin. In addition to everything, it has the presence of the septa that are the ones that allow the passage of what is the cytoplasm and the organelles.

Except for some special differences, the structure of all types of cells, plant and animal cells is almost the same. They are made up of both living and non-living units. There are three main parts of a normal cell or typical cell, cytoplasm, cytoplasm and nucleus. The cytoplasm is the outermost covering or enclosure of the cell. In the plant cell, the cell wall and cell membrane combine to form cytoplasm.

Read : Animal Cell Structure and Functions

Cell shape and size
  1. The cell structure is generally made up of three basic elements: plasma membrane, cytoplasm, and genetic material (DNA). 
  2. Cells have the ability to perform the many important functions like Metabolism, excretion and reproduction, Photosynthesis cell division.
  3. The shape of cells is basically determined by their function. 
  4. The shape  of can vary from cell to cell depending on the absence of a rigid cell wall, adjoining cell binding stresses, cytosol viscosity, osmotic phenomena, and the type of internal cytoskeleton.
  5. The size of the cells is also extremely variable from cell to cell. 
  6. The factors that limit their size are the nutrient uptake capacity of the environment that surrounds them and the functional capacity of the nucleus.

Structure of Cell :

The Structure of Prokaryotic and eukaryotic  cell carry different cell organelles like Cytoplasm, nucleus,  plasma membrane, ribosomes, endoplasmic reticulum, Nucleoid, Lysosomes, Golgi apparatus etc.

Intracellular components of Prokaryotic cell and eukaryotic cells : Overall, the internal and extracellular components of Prokaryotic cell and eukaryotic cells are considered here. The differences between these two cells in terms of their constituents are recorded when informing each component.

Cytoplasm:  The cytoplasm of eukaryotic cells consists of cytoplasm and cell organs. The cell nucleus is surrounded by a double sheath that contains DNA molecules in the form of chromosomes. In cytoplasm, Endoplasmic reticulum, ribosomes, Golgi complex, fibroblasts, vacuoles, lymphocytes, peroxisomes, stellar centers, microtubules, chloroplasts Mitochondria etc are present.

Functions of Cytoplasm
  1. Cytoplasm provides  support to the cell and suspend cell organelles and cellular molecules within it.
  2. Cytoplasm is the site for most of the metabolic reactions of the cells.
  3. Many metabolic processes occur in the cytoplasm like  protein synthesis, the first stage of cellular respiration (known as glycolysis), mitosis, and meiosis etc.
  4. One of the important functions of cytoplasm is that it provides proper shape to cell. It fills the cells thus allowing the organelles to remain in place.
  5. The cytoplasm acts as a regulator and protects the cell's genetic material as well as cellular organisms from damage by motion and collision with other cells.
  6. Cytoplasm contains many enzymes that are crucial in the break down of the waste products and to perform enzymatic reactions

Cell membrane: Cell membrane is also called as biological membrane or Biomembrane. Cell membrane thin, film like structure encloses to the cytoplasm in the cell. Cell membrane is visible only under the electron microscope. Normally the cell membrane is made up of a double layer of phospholipids, thereby protecting the cell from the surrounding environment and protecting it. Cell membrane consist  of Lipids (20-40%), Proteins (60-80%), Carbohydrates (1-5%). Nageli and Cramer in 1885 used the term cell membrane for outermost thin covering. In 1926 Gorter and Grendel proposed that cell membrane contains lipid bilayers.  In 1972 Singer and Nicholson proposed the universal accepted model of plasma membrane called Fluid Mosaic Model. 

The cytoplasm contains different molecules of proteins, and they act as conductors and pumps for different molecules to enter or exit the cells. The cell membrane contains a number of receptor proteins, which identify molecules that carry out signals, such as hormones. 

Functions of Cell Membrane:
  1. Main function of cell membrane is transport. It controls the entry of selected substances. They are impermeable to other substances.
  2. Cell membrane also controls cell metabolism through selective permeability of cell membrane.
  3. Plasma membrane protects the cell from Machanical injuries.
  4. Different enzymes are located in the Biomembrane for performing different metabolic processes like Synthesis of ATP.
  5. Cell membrane contains different receptors for harmones, Antibodies, Antigens, Neurotransmitters etc.
  6. Cell membrane separates the cell from its external environment and cell organelles from cytosol.
  7. Cell membrane used in endocytosis, exocytosis, electron transport, impulse transmission, Cell junction, cell movements, Osmosis, Cell recognition etc.

Cytoskeleton: In both prokaryotic and eukaryotic cells the cytoskeleton is useful for maintaining the size of cells, keeping cell membranes in place, helping in cell proliferation when extracellular material enters, cell separation after cell division, and cell growth and movement. The cytoplasm of optical cells is made up of microscopic or microscopic, mesenteric and microtubules. A large number of proteins are attached to these components and proteins perform the functions of holding the components of the cell together, keeping the fibers in a line as well as in one direction. The microfiber contains a protein called actin, while the microtubule contains a protein called tubulin, the mesentery contains more than one protein. The cytoskeleton is made up of three different types of filaments: actin, intermediate filaments, and microtubules.

Functions of cytoskeleton :
  1. Function of cytoskeleton is mainly supportive and maintain the cell structure and shape of cell.
  2. The cytoskeleton mediates the intracellular movements of substances.
  3. The cytoskeleton connect cell with its external environment, physically and biochemically.
  4. Microtubules functions in the mobilization of organelles.
  5. Cytoskeleton participates in the process of phagocytosis.
  6. Cytoskeleton participate in the processes of cell division, forming the mitotic spindle.
  7. The functions of the cytoskeleton in bacteria are quite similar to the functions they fulfill in eukaryotes: support, cell division, maintenance of cell shape, among others.

Read : Plant Cell Structure and Functions

Nucleus: It regulates all the activities in the cell like a control room. The membrane around the nucleus separates the nucleus from the cytoplasm. The fluid in the nucleus is called nucleus fluid. Its components are different from cytoplasm. The nucleus contains chromosomes and nuclei. Chromosomes are thread-like, microscopic structures made up of chromatin. Each chromosome contains a DNA molecule and a histone protein. Genetic traits are transmitted from one generation to the next through chromosomes. Fragments of specific lengths of DNA in a chromosome are called genes. Since the DNA in the cell is in the form of chromatin, the chromosomes do not appear under a microscope; Chromosomes can be seen under a microscope if pigment is used during cell division. The nucleus contains ribosomes, RNA and proteins. Ribosomes are formed in the nucleus. In proinflammatory cells, genetic processes or DNA processes occur in the cytoplasm.

Genetic material: Visual cells contain two types of genetic material, DNA and RNA. Biological information about living things is stored symbolically in the form of DNA sequences. RNA is used to carry information (m-RNA), for the function of receptors (r-RNA), as well as for the synthesis of amino acids in protein production (t-RNA).

In stem cells, the genetic material is in one place in the cytoplasm. It is called the nucleus. The nucleus accumbens contains the genetic material divided into chromosomes in the nucleus and a small number of fibers and chloroplasts.

In human cells, the genetic material is between the nucleus and the fibroblast. The human genome is divided into 46 chromosomes. Of these, 22 pairs are of cognate chromosomes, while 1 pair is of sex chromosome. The chromosomes in the fibers are coarse in size and differ from the chromosomes in the nucleus. They are mainly involved in energy production and the function of t-RNA.

It was discovered in 1890 AD by a scientist named Altman. It is a very important cell organs found in cytoplasm of eukaryotic cell which is scattered in cytoplasm. Altman named it Bioblast and Benda named mitochondria. Its shape and size are vary according types of cells. It is scattered in the cytoplasm in the form of particles (chondriomits), filaments, chondriconts and balls. Each mitochondria is surrounded by an outer membrane and an innermost membrane, and in the middle of it is a fluidized cavity, called the mitochondrial cavity or perymitichondrial space. From the inner membrane of the mitochondria, many amplitudes emerge and hang in the mitochondrial cavity matrix (Cristae). F1 particles or oxysomes are found on Christie's surface. The mitochondria are called the power house of the cell. This is called the energy house of the cell because of the 36 ATP molecules that are formed by the breakdown of one glucose molecule, 34 ATP (during the Krebs cycle) are formed in the mitochondria.

Function of mitochondria
  1. The most important function of mitochondria to produce the energy currency of the cell, ATP through respiration.
  2. Mitochondria generally used in breakdown of nutrients to get energy through oxidative phosphorylation.
  3. Mitochondria generate ATP from glycolysis-derived pyruvate, fatty acids, and amino acids through oxidative phosphorylation.
  4. Another important function of mitochondria is it Maintenance of proper concentration of calcium ions in the cell.
  5. The mitochondria of Liver cells have certain enzymes that help in the detoxification of Ammonia.
  6. It helps in apoptosis or programmed cell death.

Read : Mitochondria structure & Functions

Chloroplasts and chlorophylls
Chloroplasts are the cellular organelles of plants and green algae that are responsible for carrying out photosynthesis. Chloroplasts are the energy-producing centers of plant cells. Chloroplasts are found in eukaryotic organisms. Chloroplasts found in large numbers and their sizes and shape changed from cell to cell. They are usually oval or spherical in shape. The number of Chloroplasts in a cell can range from 100 to 10,000. There are made up of  two layers of membrane. The outer layer is round and the inner layer is rounded. The DNA in a Chloroplasts is different from the DNA in a nucleus. Chlorophylls are found only in plant cells and absent in Animal cell and are the main centers of photosynthesis.

Functions of Chloroplasts:
  1. Photosynthesis : Chloroplasts are responsible for photosynthesis . Two types of reactions occur in photosynthesis:
  2. Light-dependent reactions : such as those that produce ATP and NADPH.
  3. Light-independent reactions : which use the energy obtained by those who depend on light, to fix CO2 andmainlyform carbohydrates.
  4. Fatty acid biosynthesis : They use carbohydrates, NADPH and ATP synthesized in the light phase of photosynthesis.
  5. Reduction of nitrates to nitrites : Nitrites are reduced to ammonia , which is the nitrogen source for the synthesis of amino acids and nucleotides .

Endoplasmic reticulum: This reticulum formed by a single membrane carries the substance produced in the cell to the specific place. There are two types of mesh, granular and smooth; There are ribosomes on the granular lattice. The proteins formed in the ribosome are released into the cytoplasm from the endocrine glands. Calcium ions are transported through a smooth intracellular lattice.

Common functions of SER and RER
  1. Mechanical support : The memoranous network of endoplasmic reticulum provides mechanical support to otherwise colloidal complex of cytoplasmic matrix.
  2. Localisation of enzymes: E.R. membranes are sites for a number of enzymes and cytochromes to carry out specific reactions.
  3. Large surface area: The large surface area is useful for rapid synthesis of biomolecules.
  4. Desmotubules: With the help of desmotubules, ER of one cell communicates with ER of adjacent cells.
  5. Conduction of Information: It conducts information from outside to inside of cell and between different organelles of the same cell.
  6. Intracellular Transport: ER functions as circulating system of the cell for quick transport of materials.
  7. Vacuoles: It forms vacuoles.
  8. Nuclear Membranes: During telophase, part of the nuclear envelope is formed by endoplasmic reticulum.
  9. Membranes to Golgi Apparatus: ER provides membranes to Golgi apparatus for production of vesicles and Golgian vacuoles.
  10. Storage: Glycosomes or glycogen storing particles seem to be formed by ER.

Golgi Complex: This golgi bodies located near the center and is made up of a single layer panel. This information was given by the scientist Golji. The system is like a moving bush stacked on top of one another. The functions of the gallbladder system are to collect the proteins formed in the cell, to cover them and to expel them from the cell in the form of vesicles.

Functions of Golgi Bodies  :
  1. Cell wall matrix: Carbohydrates of cell wall matrix are synthesized in Golgi complex, e.g. pectic compounds, mucopolysaccharides.
  2. Special carbohydrates: Golgi apparatus takes part in synthesis of special carbohydrates like galactose and sialic acid.
  3. Animal tissue matrix: Chondroitin sulphate and hyaluronic acid are formed by Golgi complex.
  4. Acrosome: It is a special structure present near the tip of sperm that contains hydrolytic enzymes for digesting the protective coverings of the egg.
  5. Hormones: Hormone synthesis in endocrine glands is mediated through Golgi complex.
  6. Vitellogenesis: In oocytes, yolk is deposited around Golgi apparatus.

Lysosome: Lysosomes are single membrane bound small vehicular organelles rich in acid hydrolytic enzymes. Lysosomes are round. They break down excess or worn-out cell membranes through their digestive tract, as well as feeding on bacteria and viruses. It was discovered by Christian de Duve in 1958 AD. Lysosomes are formed by activity of endoplasmic reticulum and Golgi bodies.  The pathway is RER ( lysosomal enzymes ) ---> SER ---> Golgi apparatus ---> Lysosomes.

It is very microscopic cells that are found in the form of small vesicles. There is a thin membrane around it. It is very small and pouch-like in shape. It contains enzymes that have the ability to dissolve or destroy organisms. When the cell is damaged due to interruption of cellular metabolism, the lysosomes burst and the enzymes present in it digest their own cells. This results in cell death. Hence, it is also called suicide bag. It digests large particles and foreign substances entering the cell. It cleans the cell by securing the broken parts of endocytic materials and organs. It protects against bacteria and viruses.

Functions of Lysosomes
  1. Lysosones are takes part in lntracellular digestion. The ingested food digested by the activity of lysosomes
  2. The key role of lysosomes in Fertilization. Acrosome present sperm is actually a special lysosome which contains hydrolytic enzymes for clearing the path of sperm into egg.
  3. Another important function of lysosomes is it performs Phagocytosis and Autophagocgtosis. 
  4. Lysosomes provides Nourishment to the eggs, lysosomes take part in digestion of stored food.
  5. During metamorphosis, the larval or embryonic organs are resorbed through the agency of lysosomes.
  6. The aged and dead cells are removed by means of lysosomes by the process of Autophagocytosis .
  7. Leucocytes and macrophages of the body are specialized to ingest bacteria and other microorganisms, foreign proteins, etc. Such harmful foreign matter is destroyed by lysosome.
  8. Lysosomal proteases produce Specific harmones like thyroid hormones, thyroxin and triiodothyronine.
  9. Some common storage diseases due to lysosomal inetficiency are Hunter's syndrome, Tay-Sach's disease, Gaucher's disease.

Peroxisomes: The peroxisomes are few organelles spherical, small, present in animal cells and plant surrounded by a unitary membrane 75a, containing a oxidase enzymes among which the peroxidase and catalase. The peroxisomes are organelles very similar to the lysosomes but differ from them that do not contain acid hydrolases, but enzymes Oxidative. Peroxisomes are abundant in cells that synthesize, store, or break down lipids .

Functions of peroxisomes :
  1. Main function of Peroxisomes is Detoxification of some toxic substances for the body, such as uric acid, ethanol, methanol.
  2. Peroxisomes play key role in Degradation of fatty acids and amino acids that will not generate ATP, as occurs in the mitochondria but energy in the form of heat.

Vacuoles: Vacuoles are the non cytoplasmic spaces present inside the cell which are separated from the cytoplasm by selectively permeable membrane..Vacuoles is paneled and separates waste. Unicellular organisms, such as amoebae, have a vacuole for contraction, which acts like a pump and expels water from the cell if the water is too high. The vacuoles of plant cells and fungal cells are larger in size than the vacuoles of animal cells. Vacuoles in plants cell store water. On the basis of composition and functions vacuoles are of FOUR types : Sap Vacuoles, Contractile vacuoles, Food vacuoles and Air vacuoles (pseudo vacuoles).

Functions of  vacuoles:
  1. Main function of Vacuoles is storage, it store mineral salt, sugar,  insulin, amino acids,  esters and waste products.
  2. Contractile vacuoles is useful in osmoregulation and excretion of waste products.
  3. Vacuoles play role in Endosmosis, 
  4. Vacuoles functions in cell growth by enlargement of cell during there growth and by providing stored material.
  5. Food vacuoles formed by the phagosomes and lysosomes for digestion of food.
  6. Air vacuoles use to store Gases, provide Machanical protection and protection from harmful radiations.

Ribosomes: Ribosomes float freely in the nucleus of found in both prokaryotic and eukaryotic cells and in the form of microscopic particles in the cytoplasm or are attached to the endothelial network. Ribosomes play an important role in protein production. These are popularly known as protein factories. It was discovered by Palade in 1955 AD. Ribosomes are the small particles that are visible only through electron microscopes. On the basis of  sedimentation rate ribosomes are two types 80S ribosomes and 70S ribosomes. 80S found in eukaryotic cells while 70S ribosomes are found in prokaryotic cells and some eukaryotic cell organelles like Mitochondria and chloroplasts. These endoplasmic membranes are adjacent to the surface of the membranes or are scattered singly or in clusters in cytoplasm. Such ribosomes, which are found in clusters, are called polyribosomes or polysome. These compositions are protein and RNA. (RNA). 

Functions of ribosomes
  1. Fist important function of ribosomes is that they are sites for protein synthesis or polypeptide synthesis.
  2. Ribosomes are  generally two types free ribosomes are attached ribosomes. 
  3. Free ribosomes are used to synthesize structural and enzymatic proteins which is  use inside the cell for different functions. The attached ribosomes are used to synthesise proteins for transport.
  4. Another key role of Ribosomes is it provide enzymes (e.g. Peptidyl transferase) and factors for condensation of amino acids for formation of polypeptide chain.
  5. Ribosome carries rRNA to providing attachment points for mRNA and tRNAs.
  6. Ribosomes has a tunnel for mRNA so that it can be translated properiy.
  7. Newly synthesised polypeptide chain provides protection from cytoplasmic proteolytic enzymes by enclosing it in tne groove of larger subunit of ribosome till it gains Secondary structure.

Extracellular components of cell :
Many cells have certain structures outside the cell membrane. They are not protected from the external environment as they do not have a semicircle. Also, their constituents are embedded inside the cells to form them.

Cell wall: There are many types of protoplasm as well as visual center cells.  Cell wall Is the outer rigid, semi-elastic protecve, supportive and semi-transparent covering of plant cells, fungi, bacteria and some protists. It was first seen by Robert Hooke in 1665. when he saw dead cork cells under thne microscope.The Chemical Composition of cell wall is Water (60%) Hemi cellulose (5-15%), Pectic substances (2-8%) Lipids (0.5-3.0%), Proteins (1-2%). The cell wall has three parts middle lamella, primary wall, and secondary wall. The function of the cell wall is to protect the cell from its environment, from mechanical shock, from chemical elements, and to surround the cell membrane. In different types of cells, the cell wall is made up of different substances; The cell wall of plant cells is composed of cellulose, the cell wall of fungi is composed of chitin, and the cell wall of bacteria is composed of peptidoglycone.

Function of cell wall
  1. Cell wall Protects the protoplasm against mechanical injury, from attack of pathogens, Provides rigidity to the cell, Provides shape to the cell and helps in cell expansion.
  2. Cell wall may be permeable or impermeable. Means they are use in transport of substances into tje cell and out of the cell.
  3. In cell wall Pits are present and they are use to produce a protoplasmic continuum or symplast amongst cells.
  4. One of the important function of cell wall is it prevents bursting of plant cells by inhibiting excessive endosmosis.
  5. The components of cell wall Cutin and suberin reduce the loss of water through transpiration.
  6. Cell Walls contain of sieve tubes, tracheids and vessels and they are specialized for long distance transport

Flagella : The movement of cells by these senses. Some are long and thread-like and are made up of proteins. Some of the bacteria originate from the cytoplasm and protrude beyond the mesentery and cell wall. The cells of the focal cells are of different types.

Functions of Flagella : 
  1. Flagella help in locomotion of flagellate organisms.
  2. Certain organisms have ciliated larvae. The latter bring about dispersal of species.
  3. Flagella are able to perceive a number of sensations.
  4.  Flagella helps in capturing of food in some protists and animals.
  5. Flagella creats current in water for quick dispersal of co2 and replenishment of water.

Cilia: Cilia are fine hair like, membrane covered protoplasmic Outgrowth that occur on the free surface of the cells and take part in producing a current in fluid medium for passage of materials and locomotion. Flagellum are longer but finer; cilia are shorter but numerous. Cilia occur in ciliate protozoans, flame cells of Worms of many invertebrates, epithelium of respiratory tract, renal tubules, oviducal funnel etc. Cilia Also known as 'Rome'. They are thin, thin, and hairy, with bacteria on the surface. Through them the bacteria take refuge.

Functions of Cilia : 
  1. Cilia help in locomotion of ciliated organisms.
  2. Certain organisms have ciliated larvae. The latter bring about dispersal of species.
  3. Cilia are able to perceive a number of sensations.
  4.  Cilia helps in capturing of food in some protists and animals.
  5. Cilia creats current in water for quick dispersal of co2 and replenishment of water.
  6. The ovem released from ovary passes into fallopian tube due to ciliary action.
  7. In land animals cilia of nasal cavity, trachea and bronchitis move the mucus trapped dust particle towards pharynx for elimination.

Functions Of cells :

Immunization: When cell division occurs, two daughtercells are formed. Cell division leads to the proliferation of multicellular organisms, while the proliferation of unicellular organisms occurs. The progenitor cells divide in a bifurcated manner, while the visceral cells divide in a molecular manner, eventually dividing into a cytoplasm and dividing into cells. The meiosis of a dicotyledonous cell divides into four monocotyledonous cells. In multicellular organisms, such monocotyledonous cells are called gamete cells, and when they combine, new diploid cells are formed. DNA replication or DNA replication is formed only when cell division occurs by point division or by bifurcation method.

Cell division occurs twice during meiosis, DNA replication occurs only once. This occurs before DNA antimicrobial division () occurs. During meiosis (ⅠⅠ), DNA replication does not occur when cell division occurs a second time. Proteins are also needed for DNA replication.

Growth and metabolism:
Metabolic functions take place in cells. In this process, nutrients are processed in each cell. In catabolism, complex molecules are broken down into simple molecules by the cell to generate energy, and in the process of metabolism, complex molecules are formed by the cell and other biological functions take place. For example, complex sugars consumed by living organisms can be converted into glucose. After entering the cell, glucose decomposes to form ATP molecules, which store energy and can be used for physiological functions.

Protein production:
Cells have the ability to make new proteins, which are needed to regulate cellular activity. In this process, proteins are formed by the synthesis of amino acids according to the signaling information stored in DNA or RNA molecules (see: Proteins → Protein synthesis).

Algae move to find food or to get rid of predators. For this, they use Kashabhika. In multicellular organisms, cells move from one place to another to heal wounds, to fight off pathogens, and to prevent the spread of diseases such as cancer. For example, white blood cells accumulate in the wound area to prevent infection by microorganisms and to heal the wound. Cell movements require many proteins such as receptors, clusters, binding, adhesions, stimuli, and so on.

In multicellular organisms, the cells are adapted to their function and the cells are named accordingly. For example, mammals have different types of skin cells, muscle cells, nerve cells, blood cells, stem cells, fibroblasts, etc. Although different types of cells have different forms and functions, they are genetically identical or similar. Most different types of cells originate from a single complete cell, the zygote (the fertilized egg before differentiation), which develops into hundreds of different types of cells.

Reproduction : Another important function of the cell is the reproduction.

Read : Reproduction definition, types-and importance

Frequently Asked Questions 

1. Do cells also commit suicide?
Answer: Yes! Cells also commit suicide. When cells are damaged due to external or internal causes, they protect the whole body by destroying themselves. The process of self-destructing of cells in this way is called apoptosis.

The cells made to do any specific work in the body become unnecessary after the work is finished, so their destruction is good for the body.
Cells whose DNA is damaged for some reason die on their own. That means she commits suicide. If such bad cells start living and start dividing cells, they can also turn into cancer cells.

Similarly, when virus divides from our cell and prepares to spread the disease, then suddenly our virus sick cells die. Together, the viruses also end. In this way, the body is protected from disease.

What kinds of Cell organs are present in Cell ?
Answer: Cells contain many types of organs; Some cell organs, such as cell nuclei, Golgi apparatus, chloroplasts, peroxisomes, lymphocytes,  Mitochondria etc.

2. Robert Hook discovered a dead cell, but do you know who is credited with the discovery of a living cell?
Answer: The living cell was discovered in 1674 by Dutch scientist Antoine van Leuwenhoek. They are also credited with the discovery of protozoa, bacteria and sperm cells.

3. Who discovered the mitochondria that produce energy inside the cell?
Answer: Albert von Kolliker was discovered in 1857 by mitochondria.

4. Who gave the cell theory?
Answer: Cell theory was formulated in 1838 by Theodore Schwann and Matthias Jacob Schleiden.

5. What is a cell study called?
Answer: The study of a cell is called cytology. The structure and function of the cell is studied under this science.

6. What is the name of world's smallest cell?
Answer: The world's smallest cell is considered 'Mycoplasma'. Its size is about 10 micrometers. The sperm cell is considered to be the smallest cell in the human body.

7. Who is considered the world's largest cell?
Answer: The largest cell in the world is considered the egg of an ostrich bird. The largest cell in the human body is considered the ovum.

8. What are the main types of cells?
Answer: There are mainly two types of cells
Prokaryotic Cell: This is a cell in which the nucleus is not found in the middle. Organisms with this type of cell are very small in size. Like - different types of bacteria

Eukaryotic Cell: The nucleus is found in the middle of this cell. These types of cells are larger in size than an acentric cell and their structure is also complex. This type of cell is found in humans and plants.

9. What is a vesicle?
Answer : A vesicle is a membrane-enclosed small organ inside the cell, which contains various types of fluid. The vesicles form during exocytosis and endocytosis. On the other hand, liposomes are formed artificially. The membrane that surrounds the vesicle is a phospholipid bilayer. Unilamellar liposomes have a single phospholipid boiler around the vesicle. Multimalar liposomes are enclosed by two phospholipid bilayers. Vesicles can fuse with the plasma membrane as well as organelles in the cell to release their contents.

10. What are lysosomes
Answer : Lysosomes are an important type of vesicles involved in digestion. Food vacuoles are associated with lysosomes, which contain enzymes to digest food. Lysosomes are also involved in phagocytosis. On the other hand, lysosomes destroy damaged organs in the process known as autophagy.

11. What is a vacuole
Answer : A vacuole is a type of vesicle that contains mostly water. Vacuole is a distinctive feature of plant cells, but it can also be found in animal cells, bacterial cells, protists, and fungal cells. It contains organic compounds including enzymes and inorganic compounds in addition to water. The vacuole is formed by the fusion of several vesicles. The size and shape of a vacuole varies depending on the requirements of the cell.

12. What is cell wall?
Answer : A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible.

Cell Wall Function
  1. Gives the cell a definite shape and structure.
  2. Provides structural support.
  3. Protection against infection and mechanical stress.
  4. Separates interior of the cell from the outer environment.

13. What is a cell in biology
Answer : What is Cell: Cell Biology Definition - Cell is the anatomical and functional unit of the body of organisms. It is a small organized form of various forms. Which has all those actions, which we collectively call 'life'.