Cell of Living organisms, Definition, Structure, Function of cell

Cell of Living organisms, Definition, Structure, Function of cell

Cell of Living organisms, Definition, Structure, Function of cell

Cell of Living organisms, Definition, Structure, Function of cell
What is cell in Biology?

In Biology, cell is define as a building block of all living creature. Because living bodies consist of cells. Almost 37.2 trillion cells are present in human body. It is a structural and functional unit of living bodies which can exist independently. Cell in Living organisms is a smallest unit in which all the functions of life performed.

What is Cell Theory?

Cell theory is a key theory that opened the door of every research in biological field can be related with this theory. In 1839, German scientists Theodor Schwann, Matthias Sheldon and Rudolph Virchow proposed this theory. the main principals are:

  • All living organisms are consist of cells. the may be unicellular or multicellular.
  • The cell is the basic unit life.
  • Cells arise from pre-existing cells.

In fact following ideas are also include in the modern Cell Theory now a days.

  • Energy flow occurs within cells.
  • Heredity information (DNA) is passes on from cell to cell.
  • All cells have the same basic chemical composition.

Who discovered the cell?

In 1665, invention of microscope opened the door of research for hidden secrets of the world. A British scientist ” Robert Hook” designed his light microscope to observe the living organisms. He saw tiny boxlike cavities while observing the slice of cork under microscope, he describes them as cell.

Robert Hook’s discovery of cell led to the understanding of cell as the unit of living organisms.

What are the functions of cell of living organisms?

It performs an important function in growth and development of the living organisms.

  • Structure and support: Cells makes the structural base of organism as like brick for house. It maintain the structure and shape of organisms. as well as it also gives support to the organisms.
  • Growth: Cells in living organisms play a vital role in organisms growth through the process of mitosis. Mitosis is a process in which a cell divides into two daughter cells.
  • Transport: It is responsible for the transport of essential molecules for the chemical processes.
  • Energy: Organisms require energy to perform their functions. This energy comes from the process of respiration.
  • Metabolism: Cells provide the site for the chemical reactions that are important to keep the organisms alive.
  • Reproductions: It take part in reproduction through the process of mitosis and meiosis.

What is difference in plant and animal cell:

Plant cell:   A rigid cell wall made of cellulose is present outside the membrane.Animal cell:   In animal cell, cell wall is absent.
These are square or rectangular in shape.These are irregular in shape.
Plant cells are larger in size.These are smaller in size.
A large vacuole is present in it.Many small vacuoles are present in animal cell.
Plastids are present in plant cell.Plastids are absent in animal cell.
Plant cell have no cilia.Most of animal cells have cilia e.g. sperm cells.
Centrosomes are absent in plant cells.Centrosomes are present in animal cells.
Centrioles are absent in plant cells.Centrioles are present in animal cells.
Lysosomes are rarely present in cells.Lysosomes are present in animal cells.
Few mitochondria are present in plant cell.Numerous mitochondria are present in animal cell.

Difference between plant and animal cell

Similarities in in plant cell and animal cell

As we know that plants and animals both are multicellular organisms. These are comprise of many cells. These are similar in some ways that are followings.

  • Nucleus:

Plant and animal both cells contain well defined nucleus. Well prominent nucleolus and DNA are present in nucleus. DNA contain all genetic information which control the activities of cell.


Membrane bound organelles such as Golgi apparatus, mitochondria, ribosomes are present in cell of living organisms. Membranes are present around these organelles.

  • Cell membrane:

A protective cell membrane around the cell is present in both plants and animals cells. It is semi permeable membrane because it act as barrier which allow particular chemical to pass through it.

What is Structure of cell of living organisms?

Some structures of cell are not actually organelles but are very important for it. These includes cell wall, cell membrane, cytoplasm, and cytoskeleton.

Cell wall:

Cell wall is a non-living, strong component of cell. It is located outside cell membrane. It provides shape, strength, protection and support to protoplasm of cell. Plant cells have a variety of chemicals in their cell walls.

Outer layer:

The outer layer of the plant cell wall is primary wall and cellulose is the most common chemical in it.

Secondary wall:

Some plant cells, for example xylem cells, also have secondary walls on the inner side of the primary wall. It is much thicker and lignin and other chemicals are fix in it.


                In the walls of neighboring cells there are cytoplasmic connections called Plasmodesmata. Through these connections, cells transfer chemicals among each other.

Cellwall of fungi:

                Fungi and many protists have cellwalls although they do not contain cellulose. Their cellwalls are made of a variety of chemicals. For example, chitin is present in the cellwall of fungi. Prokaryotes have a cellwall composed of peptidoglycan that is a single large polymer of amino acids and sugar.

Cell Membrane:

It is outer covering of animal cell. It is a thin and elastic membrane covering the cytoplasm. It is a semi permeable barrier that allow few molecules across it. Another lipid cholesterol is an important component of cell membranes embedded in the inner region of lipid bilayer. Most bacterial cell membranes do not contain cholesterol. In Eukaryotic cells most organelles e.g. mitochondria, chloroplast, Golgi Apparatus and Endoplasmic Reticulum are also bounded by cellmembranes.

Chemical analysis reveals that cell membrane Is mainly composed of proteins and lipids with small quantities of carbohydrates.

According to fluid- mosaic model of cell membrane:

  • Lipids are aligned in such a way that they make a bilayer. It gives fluidity and elasticity to the cell membrane.
  • Proteins may be fully submerged in the lipid bilayer or some of them may “stick out into the interior and outside of the cell. These proteins function as gateways that allow certain molecules to cross into and out of the cell.

 Small amounts of carbohydrates are also found in cell membranes. These are joined with proteins (in the form of glycoprotein) or with lipids in the form of glycolipids). Both these forms act as fingerprints of the cell

Function of Cell Membrane:

                All prokaryotic and eukaryotic cells have a thin and elastic cell membrane, covering the cytoplasm.

  • The cell membrane functions as a semi-permeable barrier, allowing a very few molecules across it while fencing the majority of chemicals inside the cell.
  • The membrane maintains the internal composition of the cell to a constant or nearly constant level.
  • It can also sense chemical messages and can identify materials and other cells.


Cytoplasm is defined as the material between the plasma membrane (cell membrane) and the nuclear envelope. It is a semi-viscous and semi-transparent substance. The chemical analysis of cytoplasm reveals that it contains water in which many organic (proteins, lipids and carbohydrates) and inorganic salts are completely or partially dissolved.

Function of cytoplasm:

The cytoplasm of cell provides space for the proper functioning of organelles and also acts as a site for various biochemical (metabolic) reactions for example Glycolysis (breakdown of glucose during cellular Respiration).


The Cytoskeleton is an important, Complex and dynamic cell component. It is invisible under light Microscope. Cytoskeleton makes the cell shapes, anchors organelles and moves parts of cells in growth and motility.

Types of Filaments:

There are many types of filaments that make up the cytoskeleton but the most important types are microtubules and microfilaments.


Microtubules are made up of tubulin subunits and are often used by cells to hold their shape. Microtubules are also the component of cilia and Flagella.


Microfilaments are made up of actin subunits. These microfilaments are approximately one-third of the diameter of the microtubule and are often used by cells to change their shapes to hold structures.

Cell organelles:

Organelle are small structures within cells that perform dedicated functions. There are about a dozen of organelles found in eukaryotic cells.


A prominent nucleus occurs in eukaryotic cells. In animal cells it is present in the center while in mature plant cells, due to the formation of large central vacuole, it is pushed to side.

Structure of Nucleus:

The nucleus is bounded by a double membrane known as nuclear envelope.

Nuclear Envelope:

The nuclear envelope contains many small pores that enable it to act as a differentially-permeable membrane.


Inside the nuclear envelope a granular matrix, the nucleoplasm, is present. The nucleoplasm contains one or two nucleoli (singular, nucleolus) and chromosomes.


The nucleolus Is usually visible as a dark spot and it is the site where ribosomal RNA is forme and assembled as ribosomes.


One of the rod shape bodies in the nucleus of cells that contain genetic information (DNA) Chromosomes are visible only during cell division while during interphase (non-dividing phase) of the cell they are in the form of fine thread-like structures are chromatin Structure of DNA. Chromosomes are compose of Deoxyribonucleic acid (DNA) and histone proteins Histone proteins provide structural support to DNA for making the structure of chromosome.

Function of DNA:

                 DNA contains the message for the synthesis of specific proteins. According to this message, a molecule of messenger Ribonucleic Acid (mRNA) is synthesized. In this way, message is hand over to mRNA, which carries it to ribosomes. Ribosomes manufacture specific protein according to the message present on mRNA.In this way DNA controls all the activities of the cell and is also responsible for thetransmission of characteristics to the next generation.


The prokaryotic cells do not contain prominent nucleus rather their chromosome is made of DNA only and is submerge in the cytoplasm.


 Mitochondria (singular: Mitochondrion are double membrane bound structures present only in Eukaryotes. They are the sites for aerobic respiration, and are the major energy production centers.

Structure of Mitochondria:

The outer membrane of mitochondria is smooth but the inner membrane forms many infoldings called cristae (singular: crista) in the inner mitochondrial matrix. They serve to increase the surface are of inner membrane on which membrane bound reactions can take place.

Functions of Mitochondria:

Mitochondria have their own DNA and ribosomes; and those ribosomes ore more similar to bacterial ribosomes then eukaryotic ribosomes.


Ribosomes are tiny granular structures that are either floating freely In the cytoplasm or are bounded to Endoplasmic reticulum.

Structure of Ribosomes:

Each ribosome is consist of almost equal amount of proteins and ribosomal RNA (rRNA). Membranes are not present in ribosomes that’s why it is also present in prokaryotic cell.

Function of Ribosomes:

Eukaryotic ribosomes ore slightly larger than Eukaryotic ones. Ribosomes are the sites for protein synthesis where the message of mRNA is translate into proteins. Protein synthesis is extremely Important to cells, and so large number of ribosomes are present throughout cells (often numbering in hundreds and thousands). Ribosomes disassemble in two subunits when not involved in protein synthesis.


Plastids are also membrane bound organelles that only occur in plants and photosynthetic protists. There are three types of Plastids:

1. Chloroplasts:

Chloroplasts are the sites of photosynthesis in eukaryotes. They contain chlorophyll, the green pigment necessary for photosynthesis, and associated accessory pigments. These pigments are present in thylakoids of the grana of chloroplasts. chloroplast is also surround by a double membrane. The outer membrane is smooth while the inner one gives rise to membranous sacs called thylakoids (the stack of thylakoids is known as a granum (plural = grana]] floating in a fluid is stroma.

2. Leucoplasts:

They are the third type of plastids. They are colorless and stores starch, proteins and lipids. They are present in those cell parts where food is stored.

3. Chromoplasts:

The second type of Plastids in Plants are chromoplasts. They contain pigments associated with bright colors and are present in the cells of Flower petals and Fruits, There function is to give color to the part and thus help in pollination and dispersal of the Fruit.

Endoplasmic reticulum:

It is a network of interconnected channels that extends from cell membrane to the nuclear envelope. This network exists in two forms;

Rough Endoplasmic Reticulum (RER):

Smooth Endoplasmic Reticulum (SER):

Rough Endoplasmic Reticulum (RER]:

              Rough endoplasmic reticulum appearance is rough due to presence of ribosomes. It connects to the nuclear envelope through which the messenger RNA [mRNA] travels to the ribosomes.

Smooth Endoplasmic Reticulum (SER):

                Smooth Endoplasmic Reticulum (SER) lacks ribosomes and is involve in lipid metabolism and in the transport of materials from one part of the cell to the other. It also detoxifies the harmful chemicals that have entered the cell.

Golgi Apparatus:

An Italian physician Camillo Golgi discovered a set of flattened sacs(cistemae) that are stacked over each other. Golgi named this set of cisternaeas Golgi apparatus. It is also called Golgi body or Golgi complex and found in both plant and animal cells.

Functions of Golgi Apparatus:

It modifies molecules coming from rough ER and packs them into small membrane bound sacs called Golgi vesicles. These sacs can be transported to various locations in the cell or to its exterior, in the form of secretions.


These are single-membrane bound organelles. Lysosomes contain strong digestive enzymes and work for the breakdown (digestion) of food and waste materials within the cell,


During its function, a lysosome fuses with the vacuole that contains the targeted material and its enzymes break down the material.


Animals and many unicellular organisms have hollow and cylindrical organelles known as centrioles. They are made up of nine triplets of microtubules which composed of an important protein known as tubulin. Animal cells have two centrioles located near the exterior surface of the nucleus. The two centrioles are collectively called centrosomes.

Function of centrioles:

Their function is to help in the formation of spindle fibers during cell division. In cells that contain cilia and flagella, centrioles are involve in the formation of cilia and flagella. They are made up of nine triplets of microtubules that are composed of an important protein known as tubulin. Animal cells have two centrioles located near the exterior surface of the nucleus. The two centrioles are collectively called centrosomes.

Function of centrioles:

Their function is to help in the formation of spindle fibers during cell division. In cells that contain cilia and flagella, centrioles are involved in the formation of cilia and flagella.


Vacuoles are the fluid filled single membrane bound organelles. Cells in living organisms have many vacuoles in their cytoplasm. When a plant cell matures its small vacuoles absorb water then fuse to form a single large vacuole in the center. The cell in this state becomes turgid. Many cells take in materials from the outside in the form of food vacuole. And it digest the material by the help of lysosomes. Some unicellular organisms use contractile vacuoles for the elimination of wastes from their bodies.

Similarities between Prokaryotic and Eukaryotic cells:

1. They both have DNA in their genetic material.

2. There are membranes bound organelles in both cells .

3. They both have ribosome.

4. They have similar metabolism.

5. They are both amazingly diverse in forms.

Difference Between Prokaryotic and Eukaryotic Cells:

Prokaryotic cellEukaryotic cell
Non prominent nucleusprominent nucleus
DNA floats in cytoplasmDNA present in nucleus
Membrane bound organelles are absentMembrane bound organelles are present
Smaller in sizeLarger in size
Cell wall is composed of peptidoglycanCell wall is composed of cellulose.

types of cell

The relationship between cell function and cell structure?

(a) Size and shape:

Nerve cells: Nerve cells are long for the transmission of nerve impulse.

Xylem cells: Xylem cells are tube-like and have thick walls for conduction of water and support.

Red blood cells: Red blood cells are round to accommodate globular hemoglobin.

(b) Surface area to the volume ratio:

Function of root hair cells: Root hair cells have large surface area for the volume ratio maximum absorption of water and salts.

(c) Presence or absence of organelles:

Cells involved in making secretions hove more complex ER and Golgi apparatus. Cells involved in photosynthesis have chloroplasts.


Define cell?

Cell is a building block of living organisms.

What is power house of cell?

Mitochondria is power house of cell.

How many types of cell?

There are two types of cells

Eukaryiotic cell ( cell with definite nucleus)

prokaryiotic cell ( cell with false nucleus)

What are somatic cell?

All cells other than germ cells are somatic cells.

Who discovered the cell?

In 1665 Robert Hook discovered the cell.

Why cell membrane is semi permeable membrane?

Cell membrane allow only some things to pass. It is a semi permeable membrane because it allow only some particles to move through it.

Where DNA is present?

DNA is a genetic material which is present in the nucleus of cell. DNA encodes all the instructions of cell activities.

What is smallest cell?

Mycoplasma gallicepticum is the smallest cell. Its size is 0.2-0.3µm. It is the smallest organisms on the earth.

What is largest cell?

Ostrich’s egg is largest cell. Its size is 15 to 18cm long and wide. It is single cell.

Where do cells come from?

Cells come from the division of existing cells by cell division.

Who proposed the cell theory?

Theodor Schwann proposed the cell theory.

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