Cellular Biology: Cell Theory, Structure of Cell, Cell Organelles and its Function

What is cellular biology?

Cellular biology is a branch of biology which deal with structure and function of cell. It also deal with processes such as cell division, signaling, and energy production.

Cell theory:

The first scientist to observe and name dead cells was Robert Hook. In 1665 he examined a piece of cork under a compound microscope. He observed that there are empty spaces resembling a honeybee hive. Today we know these structures as cells.
In 1831 Robert Brown discovered the nucleus. This discovery proved that cells are not empty spaces.
A German botanist Mtthias Schleiden and German zoologist Theodor Schwann work independently and presented a theory called the cell theory. They observed that there are three basic pats of a cell: Cytoplasm, nucleus, and outermost covering, plasma membrane. They also observe plant cell and an extra cellular structure is discovered only in plant cell, called cell wall. It is outermost covering of cell.
According to cell theory all living organisms are consist of cells.
In 1855, a German physician Roudolph Virchow hypothesized that cells are produce from preexisting living cells.
In 1862, Louis pasture prove the hypothesis of Virchow by telling that bacteria is only formed from pre-existing bacteria.
The most important features of cell theory is:

All organisms are made of cell.
All cells are originate from pre-existing cells.
Cell is basic structural and functional unit of life.

Structure of cell:

The structure of cell can be studied under an electron microscope. A cell is consist of following basic parts:

Outermost covering of cell, plasma membrane and cell wall.
Nucleus, containing genetic material.
Cytoplasm, containing cell organelle.

However there are other parts of cell which are called cell organelles. These are described below.

Plasma membrane:

Plasma membrane is also called cell membrane which is outer most covering of cell. But in plants it is covered by cell wall. Chemically cell membrane is composed of 20-40% lipids(phospholipids, glycolipids and cholesterol) and 60-80% proteins(Integral proteins, peripheral proteins). A small quantity of carbohydrates is also present.
Mosaic pattern

Basic structure of cell membrane is called unit membrane. According to this structure cell membrane is composed of lipid bilayer sandwiched between inner and outer layers of proteins. The modern technology that cell membrane is sandwich is not accurate.
A new model of cell membrane is discovered called fluid mosaic model. The protein layers are embedded in lipid layer in mosaic manner. Mosaic is a pattern which is produced by arranging stones.
This model is most acceptable than others. There are pores in membrane for the movement of material.

Fluid mosaic model of cell membrane

Function of cell membrane:

Transport of material is very important function of cell membrane.
It is is also called semipermeable membrane because only selective elements are passes through cell membrane.
Neutral elements can easily enters than charged particles.
Lipid soluble easily passes through it than others.
Cell membrane maintain balance of material in internal and external environment of cell.
This movement is through via active transport or passive transport.
The energy is used in the form of ATP.

Cell wall:

Cell wall is outermost boundary of cell in some prokaryote plants and fungi. Cell wall is different in structure and composition in different organisms.

There are three layers of cell wall: Primary wall secondary wall and middle lamella.

The primary wall is composed of cellulose which is present in the form of criss cross arrangement and some deposition of pectin and hemicellulose. The primary wall is develop in newly growing cell.
The middle lamella is formed in between primary wall and other neighbouring cell. It s composed of polysaccharides and pectin salts of calcium and magnesium.
The secondary wall is formed on the inner surface of cell wall. It is thick and rigid than other layers. It is chemically composed of inorganic salts, silica, waxes, cutin etc. Fungal cell wall contain chitin. Prokaryotes cell wall lacks cellulose.

Cell wall provides definite shape and keep rigid.

Cytoplasm:

Cytoplasm and nucleus is main content of eukaryotic cell. These are collectively called protoplasm. The aqueous soluble part of cytoplasm form ground substance and it contains cell organelles, insoluble waste and storage products. It is called cytosol. It is a solution of fundamental biomolecules. In which 90% is water. Some large molecules in cytoplasm form a colloidal solution. It may be viscous(gel) or non-viscous(sol).

Function:

Cytoplasm provide ground to cell organelle.
All reaction take place in cytoplasm.
It acts as store house of vital chemicals.
The free floating organelles such as mitochondria move in cytoplasm.

Endoplasmic reticulum:

A network of channels is extending throughout cytoplasm and plasma membrane. It is also in contact with nuclear membrane. These network of membrane is called endoplasmic reticulum. In these network a sac like tubular structure is found through which material is separate from ER is called cisternae. It is single membrane bounded organelle. There are two types of ER.

Endoplasmic reticulum

Smooth endoplasmic reticulum(SER):

SER lacks ribosomes.
It involved in metabolism particularly metabolism of lipid in liver cells.
It help in detoxifying harmful drugs.
It also involves in nerve transmission.
It play an important role in transport of material from one to other cells.
It also provide mechanical support to cell.

Rough endoplasmic reticulum(RER):

Due to attachment of ribosomes it s surface is rough.
It play role in synthesis of protein.

Ribosomes:

There are free floating granular structures in cytoplasm called ribosomes. These are membrane less organelles. It is also called ribonucleo-protein particles because it is composed of RNA and protein. RNA which are present in ribosomes are called ribosomal RNA. There are two subunits of ribosomes the larger sediments at 60S and smaller sediments at 40S sediments. If these sediments attached with each other form 80S. Some ribosomes are free in cytoplasm, some are attached to ER and some ribosomal subunits are attached to mRNA. Ribosomes attached to mRNA known as polysome. Ribosomes are also present in nucleolus because they synthesized protein.

Golgi apparatus:

In 1898 golgi apparatus was discovered by Golgi. There are membrane bounded flattened sacs called cisternae. This apparatus is found in all eukaryotic cells. The cisternae together with associated vesicles are called Golgi complex or Golgi apparatus. It has two faces outer face is convex called forming face and inner face is concave called maturing face.

Function:

Golgi complex produce secretions.
It also play an important role in producing granules.
Transportation of substances in cell through Golgi apparatus.
It also convert proteins and lipids in glycoprotein and glycolipid.
Golgi complex synthesize polysaccharides in plant cell wall.

Lysosomes:

Lysosomes are single membrane bounded organelles. Its morphology is different from others. Word lysosome is derived from Greek word lyso means "splitting" and soma means "body". It was first discovered by De Duve in 1949. It is found in all eukaryotic cells.

Function:

It helps in phagocytosis. Phagocytosis means eating and digestion of cell and substances enter or found in cell. They have simple sacs which contain hydrolytic enzymes and acid phosphate. These enzymes produced in RER and further processed on Golgi apparatus.
These enzymes digest phagocytosed food particles.
Phagocytosis(eating process of cell) and autophagy(self eating) are very important function lysosome.

Cytoskeleton:

There are cytoskeletal fabric in cytosol formed of microtubules, microfilament and intermediate filaments. These tubules are made of several protein which are actin, tropomyosin, myosin, tubulin etc. Some organelles such as cilia, centrioles, basal bodies and flagella are derived from microtubules. Moveme

nt of microorganisms and cell organelles are due to cytoskeleton. It also maintain the structure of cell.

Microtubules:

Microtubules are unbranched, long and cylindrical tubules. It play an important role in assembly and disassembly of spindles during mitosis.

Microfilaments:

Microfilaments more cylindrical than microtubules. It helps in movement of organelles and microorganisms.

Intermediate filaments:

Intermediate filaments play important role in determining the shape of cell. Its size is

Vacuoles:

Vacuoles are single membrane bounded organelles which are present in both animal and plant cell. There is one large vacuole in plant but in animal cell these are small in size and large in numbers. During the growth and development of plants by coalescence of small vacuoles form a large vacuole.

Function:

These are responsible for shape of cell and storage of water in cell.
It provide rigidity to leaf and younger plants.
It help in movement of ion and regulate ph.
It maintain osmotic balance and regulate turgor pressure.
It store nutrients, water and other waste products.

Centriole:

Centriole is is composed of nine microtubules and these tubules are composed of three further tubules. It only present in animal and lower plant cell. They are absent in higher plant.

Function:

They help during cell division in the location of furrowing.
It play role in the formation of cilia.
It also help in sperm development.

Mitochondria:

Mitochonria is also nown as power house of cell because it produce energy in the form of ATP. It is foun in all eukaryotic cells. It is double membrane bounded organelles. Outer membrane is smooth while inner membrane is infolding in matrix, these folding are called cristae. Inner membrane is seprate from outer membrane by intermembrane space. Mitochondria contain their own DNA and ribosoms.

Function:

It produce energy for cell.
It also poduce protein due to presence of ribosoms
Mostly metabolic activities are take place in Mitochondiral matrix.

Nucleus:

Nucleus is known as the brain of the cell becaue it control all the activity and life of cell. It was firstly dicovered by Robert Brown in 1831. Nucleus is double membrane bounded organelle.
It is present in both prokaryotic and eukaryotic cell. In animal cell it is found in the centre of cel while in the case of plant it is present at the corner of cell due to a large vacuole. Heredity material chromosomes and DNA is present in nucleus. Some cells have one nucleus called mononucleate, with two nucleus called binucleate and more than one is called multinucleate cell.

Function:

It controls all metabolc activities in cell.
It gives instruction to make protein.
It play an important role in cell division.
It protects heredity material and control gene expression.
It is responsible for producing ribosomes.

Plastids:

There are three pigmens in plant body, called plastids. There are three main types of plastids which are chloroplast, leucoplast and chromoplast.

Chloroplast:

Chloroplast

Chloroplast is bounded by three membrane an outer, inner and thylakoid membrane. It is found on the surface of leaf. It is green pigment in plant which give green colour to leaves. It has heterogeneous structure. There are granular structure in the matrix of chloroplast, these structures are called grana. This matrix is called stroma. These grana are are formed by the arrangment of thylakoids on each other like coins. Thylakoids are flattend vesicles structur
e.

Function:

It is responsible for prodcing ATP because photosynthesis is take place on it.
It is the haemoglobin of plant because it transport oxygen.

Leucoplast:

Leucoplast are clourless pigment, found in the underground parts of plants such as roots.They have different or irregular shapes.

Fnction:

It is responsible fo store food in the form of starch.
It also ivolved inn the synthesis of amino acids and fatty acids.

Chromoplast:

Chromoplasts are coloured pigment. It gives colour to some parts of plants such as flower and plants.

Function:

They help in polination.
It produce fragmentan colour to attract insects.
It play role in dispersal of seed.