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Subjects /Biology / Study of Cells - Cytology

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INTRODUCTION
10 Apr 2021

The word cell is a latin word that means ‘little room’. In science, cell also denotes such a room where many components work together to perform various functions and produce energy for cell itself and organs of the body.

Cell is the basic structural unit of an organ. Cells may be compared to bricks. Bricks are assembled to make a building. Similarly, cells are assembled to make the body of every organism. Cell is the basic structural and functional unit of all living organisms.cell by robert hooke

The discovery of microscope led to the discovery of cell. Robert Hooke in 1665 observed the slides of cork (part of the bark of tree) under microscope. He noticed partition boxes in the shape of honeycomb that were separated from each other. 

Leeuwenhoek in 1674 discovered the free living cells in pond water for the first time. It is interesting to note that cells of cork were dead cells and Leeuwenhoek discovered living cells in Mycoplasma (a kind of microorganism).

Robert Brown in 1831 discovered the nucleus in the cell.

Purkinje in 1839 coined the term protoplasm for the liquid outside nucleus and inside the cell wall.

Size of Cell

Most of the cells are very small in size and these cannot be seen with naked eyes. But some cells can be seen by the naked eyes, for example, an egg of a hen is a single cell.

Some cells are longer than others, for example, nerve cell is the longest cell in human body.

Number of Cells

Some microorganisms are made up of single cell. But, larger the animal, larger the number of cells in its body.

For example, human body has trillions of cells which vary in shapes and sizes. Different groups of cells perform a variety of functions.

Cell Theory

In 1839, Schilden and Schwan proposed cell theory. Later on, Rudolf Virchow added to this theory in 1858. The three parts of this theory are:

  • All organisms are made up of cells.
  • Cells are the basic unit of life.
  • Cells come from the preexisting cells.

It is interesting to note that virus does not follow cell theory.

Modern Cell Theory

After several technological innovations, new parts are added to the existing cell theory. It includes:

  • DNA is passed between cells during cell division.
  • Cells of all the organisms within the same species are all the same, both structurally and chemically.

Classification of Cells

On the basis of number of cells present in the organisms, cells can be classified as:

Unicellular: Uni means one, cellular means cell. Single celled organisms are called unicellular.

Multicellular: Multi means many; cellular means cell. Organisms made of more than one cell are called multicellular.

Difference between Unicellular and Multicellular organisms

S.No

Unicellular

Multicellular

1.

These are Single celled organisms.

These are made of more than one cell.

2.

All the functions in the body are performed by the single cell

Group of specialised cells forming tissue perform special functions.

3.

Single cell usually

Fertilised egg cell multiplies to form new cells.

4.

For eg: Amoeba, Paramecium

Examples: Fungi, Plants or animals

Types of Cell

There are two types of cells that are found in the living organisms. These are:

Prokaryotic Cell

  • Cells having nuclear material without a nuclear membrane are called prokaryotic cells

  • Examples: Bacteria, Virus, Mycoplasma, Blue-green algae (Cyanobacteria)

Eukaryotic Cell

  • Cells having well defined nucleus along with a nuclear membrane are called eukaryotic cells.

  • Examples: All plants and animals.

Prokaryote

Eukaryote

  • Very simple and primitive cell.
  • Cell organelles like – Mitochondria, Plastid, Golgi bodies, Endoplasmic Reticulum (ER), Lysosome, etc are absent.
  • Nucleus is absent or less developed.
  • Histone Protein is absent.
  • 70 S ribosomes (S – Svedberg Unit).

  • Complex and Advanced cells.
  • Cell organelles like – Mitochondria, Plastid, Golgi bodies, Endoplasmic Reticulum (ER), Lysosome, etc are present.
  • Nucleus is mature and fully developed.
  • Histone Protein is present.
  • 80 S ribosome.

Components of Cell

The basic components of cell includes:

  • Cell Membrane or Plasma Membrane: The porous boundary of a cell that separates its from other surroundings is called cell membrane.
  • Cytoplasm: The jelly like substance outside nucleus and inside cell membrane that contains other cell organelles is called cytoplasm.
  • Nucleus: The central dense round body in a cell is called Nucleus. Robert Brown discovered the nucleus in the cell.

Cell Membrane or Plasma Membrane

  • It is the outermost covering of the cell that separates it from the external environment.
  • Cell membrane gives shape to the cell.
  • It is made up of lipids (fats) and proteins.
  • It is a porous membrane that allows the movement of substances or materials inward and outward of the cell. It is very flexible.
  • Plasma membrane is a selectively permeable membrane.
  • The gaseous exchange between the cell and its external environment is an important function of the cell membrane.

What is Endocytosis?

 It is a process where the cell engulfs food and other material from the external environment. For example, Amoeba intakes food through cell membrane.

Cell Wall

Only Plant Cells have cell wall. Cell wall protects the cells from fluctuating temperature, harsh climate and wear and tear in external environment.

It provides structural rigidity to the cells and protects the plants or trees from outer environment.

It has 3 layers:

  • p-layer: The primary of inner layer. It is made up of cellulose.
  • s-layer: The secondary of middle layer. It is made up of lignin.
  • t-layer: The outer layer. It is made up of dead protoplasm.

Cell wall also prevents the bursting of cells in very hypotonic solution. For example, in plant or fungi, the cell swells building up pressure against the cell wall. The cell wall exerts equal pressure on the swollen cell. Thereby, preventing the bursting of the cell.


What is plasmolysis?

When a living plant cell loses water through osmosis, there is shrinkage or contraction of the contents of the cell away from the cell wall. It only occurs in living plant cells and not the dead cells.



Nucleus

  • It is the controlling centre of the cell.
  • It is spherical and located in the centre of the cell.
  • Nuclear membrane: It separates the nucleus from the cytoplasm. It is porous and allows the movement of the particles between cytoplasm and the inside of the nucleus.
  • The nucleus contains the following:
    • Nucleolus: It is spherical body in the body. It helps in transcription and processing of the ribosomal RNA (rRNA). It was founded by Fortoria.
    • Chromatin network: It is a wire type structure that are made up of DNA, RNA and Histone protein. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.
    • Chromosomes: Chromosomes are seen only when the cell is about to divide. These are thread like structures that carry genes.
  • Nucleus plays central role in cellular reproduction. It is a process by which a single cell divides and forms two new cells.
  • Nucleus plays a role in inheritance or transfer of the characters from the parent to the off springs.

What are Genes?

The functional segments of DNA are called genes. Gene is the unit of inheritance in the living organisms. Chromosomes contains information for inheritance of characters from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid).

For example, if your father has blue eyes, then it may be possible that you inherit blue eyes. This information is stored in the genes.

Cytoplasm and Protoplasm

Protoplasm

  • It is the physical base of life.
  • It is colorless, sticky fluid
  • It contains both organic and inorganic compounds.
  • It has protein, carbohydrates and DNA.
  • Protoplasm is made up of:
    • Cytoplasm
    • Nucleoplasm
  • Nucleoplasm is the fluid inside the nucleus.
  • Cytoplasm is the fluid contained inside the plasma membrane.
    • It contains many specialised cell organelles. Each of these organelles perform a specific function.

Cell organelles

  • Large and complex cells need a lot of chemical activities to support their complicated structure and functions.
  • To keep these activities of different kinds separate from each other, these cells use membrane bound little structures called organelles within themselves. Prokaryotic cells do not have cell organelles. Some organelles are:
    • Endoplasmic Reticulum
    • Golgi Bodies
    • Mitochondria

Endoplasmic Reticulum

  • Endoplasmic Reticulum (ER) is a large network of membrane-bound tubes and sheets.
  • It looks like long tubules or round or oblong bags (vesicles).
  • It functions as a cytoplasmic framework providing surface for various biochemical activities.
  • It serves as a channel for transfer of materials (especially proteins) between various regions of the cytoplasm and between cytoplasm and nucleus.

Ribosomes

Ribosomes are present on Rough Endoplasmic Reticulum. It is present in all living cells. The protein is manufactured by the ribosomes. It is then sent to various places in cell by ER depending on the needs.

  • There are two types of ER:
    • Rough Endoplasmic Reticulum (RER)
    • Smooth Endoplasmic Reticulum (SER)
  • RER is rough because of the ribosomes attached to its surface.
  • SER helps in the manufacture of the fat molecules or lipids required for the cell. In liver cells, it detoxifies various toxins.

What is Membrane Biogenesis?

The protein or lipids manufactured by the SER are used for building of the cell membrane. This process is called membrane biogenesis.

Golgi Apparatus

  • It was described by Camilo Golgi.
  • It consists of a system of membrane-bound vesicles arranged approximately parallel to each other in stacks called cisterns.
  • These are connected to ER.
  • Functions:
    • The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi Apparatus.
    • Storage, modification and packaging takes place in the vesicles.
    • In some cases, complex sugars are made from simple sugars in the Golgi Apparatus.
    • It is also involved in the formation of lysosomes.

Lysosomes

Lysosomes are membrane-bound sacs filled with digestive enzymes that are made by RER. These enzymes can break down any organic material like bacteria or food.

These are a kind of waste disposal system of the cell. These help to clean the cell by digesting any foreign material or any torn cell organelle.

Why Lysosomes are called suicide bags?

During the disturbance in cellular metabolism, for example, if a cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the suicide bags of cell.

Mitochondria

  • It is known as the powerhouse of the cell.
  • It has two membrane coverings. The outer porous while the inner membrane is deeply folded.
  • Apart from nucleus, mitochondria have their own DNA and Ribosomes. It can even synthesis its own protein.
  • These folds increase the area for ATP generating chemical reactions.

ATP means:

  • ATP is the energy currency of the cell.
  • The energy required for various chemical activities needed for life is released by mitochondria in form of ATP.
  • The body uses the energy stored in ATP for making new chemical compounds and for mechanical work.

Vacuoles

  • These are storage sacs for solid or liquid content.
  • These are of small size in animal cell whereas, in plant cell, the size of vacuole is very large. Because food has to be stored for long time in plant cells.
  • Vacuoles provide turgidity and rigidity in plant cells.
  • Substances like amino acid, sugar and even protein are stored in vacuoles.
  • In unicellular organisms, specialized vacuoles play an important role in expelling extra water.

Plastids

  • Plastids are present only in plant cells.
  • These have their own DNA and ribosomes.
  • Plastids are of two types:
    • Chromoplasts: colored plastids
    • Leucoplasts: white or colourless plastids.
  • Chlorophyll is present in chromoplasts that helps in photosynthesis and also gives green colour to plants.
  • Material like starch, oil or protein granules are stored in leucoplast.
  • Stroma: it is a numerous membrane layers in which plastids are embedded.

Did You Know

  • The first cell originated on Earth around 3.8 billion years ago.
  • There are 200 types of different cells in human body.