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

What is cellular biology?

Cellular biology is a branch of biology that deals with the structure and function of cells. It also deals with processes such as cell division, signalling, 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 a 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 the outermost covering, the plasma membrane. They also observed that a plant cell, and an extra cellular structure is discovered only in plant cell, called the cell wall. It is the outermost covering of the cell.
According to cell theory, all living organisms consist of cells.
In 1855, a German physician, Roudolph Virchow hypothesised that cells are produced from preexisting living cells.
In 1862, Louis pasture proved Virchow's hypothesis of by showig that bacteria are only formed from pre-existing bacteria.
The most important features of cell theory are:

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

Structure of cell:

The structure of a cell can be studied under an electron microscope. A cell consists of the following basic parts:

Outermost covering of the cell: plasma membrane and cell wall.
Nucleus, containing genetic material.
Cytoplasm, containing cell organelles.

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

Plasma membrane:

The plasma membrane is also called the cell membrane, which is outer most covering of the cell. But in plants it is covered by the cell wall. Chemically, the 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 the cell membrane is a sandwich is not accurate.
A new model of the cell membrane is discovered called the fluid mosaic model. The protein layers are embedded in the lipid layer in a mosaic manner. Mosaic is a pattern which is produced by arranging stones.
This model is more 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 a very important function of the cell membrane.
It is also called a semipermeable membrane because only selective elements are passed through the cell membrane.
Neutral elements can easily enters than charged particles.
Lipid soluble easily pass through it more than others.
The cell membrane maintains the balance of materials in the internal and external environment of the cell.
This movement is through active transport or passive transport.
The energy is used in the form of ATP.

Cell wall:

The cell wall is the outermost boundary of a cell in some prokaryotes, plants, and fungi. Cell wall is different in structure and composition in different organisms.

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

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

The cell wall provides a definite shape and keep rigid.

Cytoplasm:

Cytoplasm and nucleus are the main contents of a 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 provides a ground for cell organelles.
All reactions take place in the cytoplasm.
It acts as a storehouse of vital chemicals.
The suspended organelles such as mitochondria move in the cytoplasm.

Endoplasmic reticulum:

A network of channels extends throughout the cytoplasm and plasma membrane. It is also in contact with the nuclear membrane. This network of membranes is called endoplasmic reticulum. In this network, a sac-like tubular structure is found through which material is separated from the ER, called cisternae. It is a single-membrane-bounded organelle. There are two types of ER.

Endoplasmic reticulum

Smooth endoplasmic reticulum(SER):

SER lacks ribosomes.
It is involved in metabolism, particularly the metabolism of lipids in liver cells.
It helps in detoxifying harmful drugs.
It is also involved in nerve transmission.
It plays an important role in transport of material from one cell to other.
It also provides mechanical support to the cell.

Rough endoplasmic reticulum(RER):

Due to the attachment of ribosomes, its surface is rough.
It plays a role in the synthesis of proteins.

Ribosomes:

There are free-floating granular structures in the cytoplasm called ribosomes. These are membrane-less organelles. It is also called ribonucleoprotein 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 subunit at 60S, and the smaller subunit at 40S sedimentation. If these sediments attach to 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 are known as polysome. Ribosomes are also present the in nucleolus because they synthesized protein.

Golgi apparatus:

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

Function:

Golgi complex produces secretions.
It also plays an important role in producing granules.
Transportation of substances in the cell through the Golgi apparatus.
It also converts proteins and lipids into glycoprotein and glycolipids.
Golgi complex synthesize polysaccharides in the plant cell wall.

Lysosomes:

Lysosomes are single-membrane-bound organelles. Its morphology is different from others. The word lysosome is derived from the Greek word lyso, meaning "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 cells and substances enter or found in cell. They have simple sacs which contain hydrolytic enzymes and acid phosphatase. These enzymes are produced in RER and further processed on the 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 framework in the cytosol formed of microtubules, microfilaments, 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 the cytoskeleton. It also maintains the structure of cell.

Microtubules:

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

Microfilaments:

Microfilaments more cylindrical than microtubules. It helps in the 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 cells. In an animal cell it is found in the centre of cell, while in the case of a plant, it is present at the corner of the cell due to a large vacuole. Hereditary material, chromosomes and DNA are present in the nucleus. Some cells have one nucleus, called mononucleate; with two nuclei, called binucleate and more than one is called multinucleate.

Function:

It controls all metabolic activities in the cell.
It gives instruction to make proteins.
It plays an important role in cell division.
It protects heredity material and control gene expression.
It is responsible for producing ribosomes.

Plastids:

There are three pigments in the 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 leaf's surface. It is green pigment in plant which give green colour to leaves. It has a heterogeneous structure. There are granular structures in the matrix of the chloroplast; these structures are called grana. This matrix is called stroma. These grana are formed by the arrangement of thylakoids on each other like coins. Thylakoids are flattend vesicles structure.

Function:

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

Leucoplast:

Leucoplasts are colourless pigments, found in the underground parts of plants such as roots. They have different or irregular shapes.

Function:

It is responsible for storeing food in the form of starch.
It is also involved in 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 pollination.
It produce fragmentan colour to attract insects.
It plays role in the dispersal of seeds.

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