Introduction-All the living organisms are made of a small compartments or units.This compartments are the fundamental structural and functional unit of living organisms. They are called cells.

              Some organisms consist of single cell only, example amoeba. Many organisms are made up of many cells, example man.

Historical background of cell.

A.V. Leeuwenhook-Using microscopes of his own  construction, he was the first to observe sperm, bacteria and red blood cells.His observation laid the foundations for the science of bacteriology and microbiology.

Robert Hooke-Father of microscopy, discovered Cell in (1664-65).

Cell theory

Between 1838-39 Schleiden, German botanist and Schwann, a German zoologist proof that plants and animals are cellular in character and founded the cell theory.

       They postulated that the cell is the basic unit of life.

 Rudolf Virchow in 1855 proposed the idea that all cells arise from pre-existing cells.No cell  can originate spontaneously, but comes into being only by the division of already existing cells.

J.E.Purkinjee (in 1839) coined the term 'protoplasm' to living substance of the cell.

Study of cells is called cell biology (cytology).

Example of cells

1. Cells as organisms (single-celled or unicellular organism)-Bacterium,blue-green algae, Plasmodium (malarial parasite ),Amoeba, Chlamydomonas,Euglena,Paramecium etc.

2. Plant cells. Diatom, parenchyma,xylem,phloem,palisade cells etc.

3. Animal cells. Goblet cell,fat cell,liver cell,human sperm cell,ovum etc.

Types of cells:-There are mainly two types of cell on the basis of presence or absence of nucleus and complexity of cellular organisation in them.

1. Prokaryotic cells(Pro-means first, karyon-means nucleus)

The cells which do not have a true nucleus in them are called prokaryotic cells they are said to be primitive cells.

         The nucleus is called incipient nucleus. And don't have a nuclear membrane.

2. Eukaryotic cells (Eu-means true, karyon means nucleus).

The cells which have true nucleus in them are called eukaryotic cells they are advanced cells.

        The nucleus has a double nuclear membrane. It gives the nucleus a definite shape.

Difference between prokaryotic and eukaryotic cells:-

Prokaryotic cell

1. It is a primitive cell generally smaller in diameter.

2. The nucleus does not have a nuclear membrane.

3. The membrane bound organelles are not present.

4. Chromosome is single circular and in direct contact with cytoplasm.

5.Cell division by budding / fission.

6. Prokaryotes are bacteria and cyanobacteria.

Eukaryotic cell

1. It is an advanced cell generally big.

2. The nucleus has a nuclear membrane.

3. The membrane bound organelles are present and well developed.

4. Many linear chromosomes are present in the nucleus of cell.

5. Cell division by mitosis and meiosis.

6. Eukaryotes are cell of plant and animal body, amoeba etc.

Size of the cell:- Cells are from 0.1 micrometre to a few feet as well. The size also is influenced by the function of a cell has to do.

Smallest cell (0.1 micrometre or micron) are-Mycoplasma gallisepticum, Pleuropneumonia.

Largest cells are ostrich's egg (170 X 13.5 mm), nerve cell is the longest sized cell (about 1 metre) in human body.

Cell structure

The detailed structure of a cell has been studied under compound microscopes that use light and can magnify an object up to 2,000 times and electron microscopes that use beams of electrons and can magnified things as much as half a million times.

     Structurally the cell is formed of three parts:-

1. Plasma membrane

2. Cytoplasm

3. Nucleus

1.Plasma membrane or cell membrane or Plasmalemma

Each cell is bounded by an extremely delicate,thin,elastic, living membrane called the plasma membrane.

In plant cells it is present on the inner side of the cell wall.

It is made up of two layers of liquid fat molecules with protein molecules sandwiching it and embedded in it.

It is selectively permeable membrane which allows the flow of only some substances into the cell and out of the cell.

It also gives shape and rigidity to the cell.

      Viruses do not have any membranes

Functions

Gives a definite shape to the cell ,provide protection to the internal contents of the cell regulate entry and exit of the cell.

In animal cells it is involved  in adhesion , recognition and in the formation of vesicles,cilia and flagella,microvilli etc.

2.Cell wall

Plant cell have an additional protective wall outside the plasma membrane called the cell wall.

It is permeable and made up of cellulose a kind of carbohydrate.

Relatively thick non living and rigid envelope.

Functions

Determines the shape of the plant cells and prevent desiccation (drying up) of cells.

Protect plasma membrane and internal structures of the cell.

Does not allow too much of water in this way it prevents the cytoplasm from becoming too dilute.

Protoplasm

All the components of a cell including the cell membrane is known as protoplasm.

Colourless jelly like,viscous semifluid substance which can be distinguished in a centrally located nucleoplasm and the surroundings cytoplasm.

3.Cytoplasm

The protoplasm which surrounds the nucleus is called cytoplasm.

Formed by proteins,nucleic acids,carbohydrates,lipids and certain inorganic substances.

Many small organelles are found inside the cytoplasm this are known as cytoplasmic organelles.

It is the sites for various metabolic functions of the cell.

The important cytoplasmic organelles are described below:-

Endoplasmic reticulum

The endoplasmic reticulum is a network of tube like structure running through the cytoplasm and some parts are connected to the nuclear membrane while others are connected to the cell membrane.

          They are of two types are rough and smooth.

Ribosome attached to it the reticulum is rough endoplasmic reticulum otherwise it is smooth endoplasmic reticulum.

Functions

Increase the surface area of cytoplasm for various metabolic activities of the cell.

Gives internal support.

Associated with the synthesis,storage and transport of metabolic products.

Helps in the formation of the cell plate and nuclear membrane during cell division.

Rough endoplasmic reticulum is associated with the synthesis of protein.

Smooth endoplasmic reticulum separate lipids which along with protein considered cell membrane by a process called membrane biogenesis.

Smooth endoplasmic reticulum  detoxifying many poisonous and drugs.

Ribosome

Extremely small round bodies found either in the free state or attached to the surface of the endoplasmic reticulum.

Composed of ribonucleoprotein  (ribonucleic acid and protein)

Function

Protein synthesis

Mitochondria

Small rod shaped organelles found in large number.

Mitochondria is bounded by two membranes of outer and inner.

The inner membrane folded to form Cristae the finger like projections to give large surface area for ATP synthesizing.

The Cristae lie in a ground substance called matrix.

Outer membrane is very porous (has holes).

Function

Mitochondria provide energy for the vital activities of living cells.

The ATP (adenosine triphosphate) is stored in it. Hence mitochondria is called the power plant or powerhouse of the cell and ATP is called currency of the cell.

Golgi complex or Golgi bodies or apparatus

Golgi bodies was first discovered by Camillo Golgi.

Golgi bodies are formed by a stack of flattened membranes are flattened sacs called cisternae.

They are usually called dictyosomes in plants.

Functions

Form the cell plate during cell division.

The store modified package and contains the protein synthesized in the ribosome.

Plastids

They are double membrane and organelles which are found only in plant cells.

There is spherical or discoidal in shape,and their average sizes is 4 to 6 micron.

It show two distinct regions--grana and stroma.

Grana are stacks of membrane bound,flattened discoid sacs containing chlorophyll molecules.

Matrix in which grana are embedded is known as stroma.

Plastid are three types:-

1.Leucoplast are colourless plastids. They store food in the form of starch,protein and lipids.

2.Chloroplast are green plastids which contain chlorophyll. Green leaves have chloroplasts.

3.Chromoplast are variously coloured plastids present in flowers and fruit.

  Functions

By trapping solar energy , green plastids manufacture food through photosynthesis.

Chromoplast provide colour to various flowering plant parts.

Leucoplast help in the storage of protein,starch and liquid (oil).

Plastids are absent in animal cells.

Lysosomes

They are saclike,tiny,spherical,single membrane-bound vesicles contain enzymes,which are synthesized in Golgi complex.

They are formed by the Golgi complex.     

Functions

They help in breaking down (digesting) large molecules into small.

During starvation or damaged state they worn out organelles,bacteria etc and digest them.Hence they are called digestive bags,suicide bags,scavengers and demolition squads of cell.

Peroxisomes

Small membrane bound sac like structure mostly found in liver and kidney cell of the vertebrates and contain powerful oxidative enzymes.

Function

Detoxify the cells and help in photorespiration.

Centrosomes and centrioles

Centrosome is a organelle which usually contain two central granules called centrioles  which are lies perpendicular  to each other.

               The centrioles are a hollow cylindrical structure made of microtubules arranged in a specific manner.

Functions

They help in the formation of cilia and flagella then it is called basal body.

During cell division  centrioles form spindle fibres which help in the movement of chromatids( daughter chromosomes) in the daughter cell.

Cilia and flagella

Small hair like structure are called cilia they are many in number.

           Long Whip like structure of the cell is called flagellum they are usually few in number.

Function

Both help in lower portion of the single celled organisms example paramecium (cilia), Euglena (flagellum) chlamydomonas(flagella) etc.

Cytoskeleton

It is formed by microtubules and microfilaments.

Microtubules  are hollow tubules made of tubulin proteins. They form centrioles,flagella and cilia.

Microfilaments are thinner filaments made of actin protein.

Function

Cytoskeleton gives structural support and helps in cell movement. (Example amoeboid movement).

Vacuoles

Vacuoles are the structure without any definite shape and size .

It occupies about 50 - 90% of its space in plant cell and in animal cells vacuoles are smalls and many in number.

Functions

They work in osmoregulation,i.e,the maintenance of internal pressure.

Store various substances including waste products.

Nucleus (brain of the cell)

Nucleus  is usually a dense, spherical or oval organelle, generally located in the central position of the cell.

Nucleus has the following the structure given below

Nuclear membrane or envelope

Outer cover and selectively permeable, double layered which contain pores called nuclear Pores.

Nucleoplasm

Transparent semi solid granular matrix of the nucleus.

Nucleolus

Inside the nucleus there is one (or more) nucleolus which is dense spherical granule. Contents DNA which synthesizes ribosomal RNA.

Function

Formation and storing of ribonucleic acid which is helpful in protein synthesis in the cytoplasm of the cell.

Chromatins

Very fine thread like structures coiled and uniformly distributed in the nucleoplasm.

Chromosomes

During cell division chromatin fibre condensed to form chromosome structure.

Chromosomes have genes made up of DNA (deoxyribonucleic acid).

Functions

Chromosomes are carriers of the genetic trait from one generation to another.

Regulates the cell division (cellular reproduction) formation of cells from one cell and synthesis and store of proteins.

Plant cells and animal cells

Basic organisation of plant and animal cells is the same though there are some differences these are listed below:-

Plant cell

Plant cells are usually larger in size than animal cells.

Cell wall is present.

Plastid are present.

Vacuoles are present and larger in size.

Plant cell have many simpler unit of Golgi complex,called dictyosomes.

Centrioles are absent in plant cells  (except in a few lower plants).

Plant cells usually have a regular shape.

Animal cell

Animal cell are small in size.

Cell wall is absent.

Plastids are absent.

Vacuoles are less in number and smaller in size.

Animal cell have a single highly elaborate Golgi complex.

Centrioles are present in animal cell.

Animal cells are usually irregular in shape.

Movement of substance across the cell membrane.

Diffusion

Movement of molecules from higher concentration to lower concentration is called diffusion.

It takes place in solid, liquid and gas.

Osmosis

Diffusion of water molecules across a semipermeable membrane is called osmosis.

Significance of osmosis and diffusion

Diffusion helps in the exchange of gases between cells and their environment.

Single celled organisms that live in freshwater habitats gain water by osmosis.

Transport of water between cells and their surrounding

Hypotonic solution

The cell will gain water by osmosis because there is a higher concentration of water outside the cell. This process is called endosmosis.

Isotonic solution

Exactly the same water concentration as the cell there will be no net movement of water molecules across the cell membrane.

Hypertonic solution

Water will diffuse out of the cell by osmosis because there is a lower concentration of water outside the cell. This process is called exosmosis.

Plasmolysis

If exosmosis continues in a plant cell, the cytoplasm would appear from shrunken. This is called plasmolysis and the cell is said to be plasmolysed.

Significance of the cell membrane

Passive transport

Transport of substances occurs across the cell membrane from higher concentration region to lower concentration region and there is no expenditure of cellular energy then this type of transportation is called passive transport.

Cell membrane is selectively permeable and flexible which is made of lipids and proteins.

Active transport

The energy dependent transport of substances is called active transport.

Endocytosis

Process of cellular uptake of macromolecules and particulate substances by localised regions of the plasma membrane is called endocytosis.

Osmoregulation

Excess of water entered into the animal cell is regulated by removing it.It is called osmoregulation,as seen in amoeba.

Transpiration

Excess of water entered the plant cell is removed by the plant by transpiration as seen in all plants.

Some other facts of cell:-

Ribosome ---smallest organelle.

Plastid ---largest organelle (in plant cell).

Mitochondria ---largest organelle (in animal cell).

Membrane of vacuole is known as tonoplast.

Mitochondria and plastids can replicate themselves.

Eukaryotic double membrane cell organelles---Mitochondria ,plastid and nucleus.

Singular membrane in eukaryotic cell organelles---Golgi apparatus,endoplasmic reticulum , vacuoles and lysosomes.

Naked( without membrane) cell organelles ribosome and centriole.

Viruses are neither prokaryotic or eukaryotic.

DNA is not covered with nuclear membrane and so it is called nucleoid in prokaryotic cell where as ribosomes are present but are of smaller size.