Cell the unit of life

So,what is cell?
The cell is the basic structural and functional unit of life. Focusing on the cell permits a detailed understanding of the tissues and organisms that cells compose. Some organisms have only one cell, while others are organized into cooperative groups with huge numbers of cells. On the whole, cell biology focuses on the structure and function of a cell, from the most general properties shared by all cells, to the unique, highly intricate functions particular to specialized cells.

CELL THEORY

The cell theory proposed by Schleiden and Schwann is important because

• It provides guidelines (rules) to define life i.e. an organism is believed to have life only if they follow the the three concepts of cell theory
• The theory gave a fundamental (basic) generalization of biology that cell is the smallest unit of all the living organisms (whether  unicellular or multicellular).
• The theory also proposed the origin of new cells (cells come pre-existing cells)

Exception to cell Theory

1.Viruses are biologists' puzzle. They are an exception to cell theory. They lack protoplasm, the essential part of the cell.

2. Bacteria and cyanobacteria (Blue Green algae) lack well organized nucleus.

 3.Some of the protozons are acellular

 4.The coenocytic hyphae of some fungi eg. Rhizopus have undivided mass of protoplasm, in which many nuclei remain scattered.

 5.Red Blood Corpuscles (RBC) and mature sieve tubes are without nuclei.

A cell may grow, secrete, divide or die while its adjacent cells may lie in a different physiological state. Many of the subsequent findings about the cell like this had necessitated modification in cell theory. The modified form of cell theory has been given the higher status as cell principle or cell Doctrine.

cell shape size and count

 Living organisms are made up of different types of cells, of different shapes and sizes. A unicellular organism differs in shape from another unicellular organism. Within a multicellular organism, there are a variety of cells. Some are long while others are short; some are circular while some are oval. Shape and size vary from cell to cell according to their functions and composition.

shape of cell

2. nucleus

3. cytoplasam

structure of cell 

the three  essential parts of cell :

1.cell membrane

2. nucleus

3. cytoplasm

cell organelle 

Plasma Membrane

The plasma membrane also termed as a Cell Membrane or Cytoplasmic Membrane. It is a selectively permeable membrane of the cell, which is composed of a lipid bilayer and proteins.
The plasma membrane is present both in plant and animal cell, which function as the selectively permeable membrane, by permitting the entry of selective materials in and out of the cell according to the requirement

Protoplasm

• Protoplasm is the living contents of a cell that is surrounded by a plasma membrane. 
• Protoplasm is composed of a mixture of small molecules such as ions, amino acids, monosaccharides and water, and macromolecules, such as nucleic acids, proteins, lipids and polysaccharides. 

Nucleus

The nucleus is a double-membraned organelle found in all eukaryotic cells. It is the largest organelle, which functions as the control centre of the cellular activities and is the storehouse of the cell’s DNA. By structure, the nucleus is dark, round, surrounded by a nuclear membrane. It is a porous membrane (like cell membrane) and forms a wall between cytoplasm and nucleus. Within the nucleus, there are tiny spherical bodies called nucleolus. It also carries another essential structure, called chromosomes.

Endoplasmic Reticulum

The Endoplasmic Reticulum is a network of membranous canals filled with fluid. They are the transport system of the cell, involved in transporting materials throughout the cell.
There are two different types of Endoplasmic Reticulum:

1. Rough Endoplasmic Reticulum – They are composed of cisternae, tubules, and vesicles, which are found throughout the cell and are involved with protein manufacture.
2. Smooth Endoplasmic Reticulum – They are the storage organelle, associated with the production of lipids, steroids, and also responsible for detoxifying the cell.

Mitochondria

Characteristics-

• Double walled, inner wall thrown into cristae.
• Have their own DNA and ribosome.

Function-

• Synthesises respiratory enzyme.
• Releases energy from pyruvic acid produced in cytoplasm in the form of ATP.

Plastids

Plastids are large membrane-bound organelles which contain pigments. Based on the type of pigments, plastids are of three types:

• Chloroplasts – Chloroplasts are double membrane-bound organelles, which usually vary in their shape – from disk shape to spherical, discoid, oval and ribbon. They are present in mesophyll cells of leaves, which store chloroplasts and other carotenoid pigments. These pigments are responsible for trapping light energy for photosynthesis. The inner membrane encloses a space called the stroma..
• Chromoplasts – The chromoplasts include fat-soluble, carotenoid pigments like xanthophylls, carotene, etc. which provide the plants with their characteristic color – yellow, orange, red, etc.
• Leucoplasts – Leucoplasts are colorless plastids which store nutrients. Amyloplasts store carbohydrates (like starch in potatoes), aleuroplasts store proteins, and elaioplasts store oils and fats.

Ribosomes

Ribosomes are nonmembrane-bound and important cytoplasmic organelles found in close association with the endoplasmic reticulum. Ribosomes are found in the form of tiny particles in a large number of cells and are mainly composed of 2/3rd of RNA and 1/3rd of protein.

Characteristics

• Small granules either scattered in the cytoplasm or attached to the outside of endoplasmic reticulum.
• Single walled dense, spherical bodies mainly of RNA.

Function

• Play important role in protein synthesis.

Golgi Apparatus

Golgi Apparatus also termed as Golgi Complex. It is a membrane-bound organelle, which is mainly composed of a series of flattened, stacked pouches called cisternae. This cell organelle is primarily responsible for transporting, modifying, and packaging proteins and lipid to targeted destinations.

centrosomes

Centrosomes are organelles which serve as the main microtubule organizing centers for animal cells. Centrosomes are made of from arrangement of two barrel-shaped clusters of microtubules, called “centrioles,” and a complex of proteins that help additional microtubules to form.

vacuole

Vacuoles are storage bubbles found in cells. They are found in both animal and plant cells but are much larger in plant cells. Vacuoles might store food or any variety of nutrients a cell might need to survive. They can even store waste products so the rest of the cell is protected from contamination

Cell Organelles	Structure	Functions
Cell membrane	A double membrane composed of lipids and proteins. Present both in plant and animal cell.	Provides shape, protects the inner organelle of the cell and acts as a selectively permeable membrane.
Centrosomes	Composed of Centrioles and found only in the animal cells.	It plays a major role in organizing the microtubule and Cell division.
Chloroplasts	Present only in plant cells and contains a green-coloured pigment known as chlorophyll.	Sites of photosynthesis.
Cytoplasm	A jelly-like substance, which consists of water, dissolved nutrients and waste products of the cell.	Responsible for the cell’s metabolic activities.
Endoplasmic Reticulum	A network of membranous tubules, present within the cytoplasm of a cell.	Forms the skeletal framework of the cell, involved in the Detoxification, production of Lipids and proteins.
Golgi apparatus	Membrane-bound, sac-like organelles, present within the cytoplasm of the eukaryotic cells.	It is mainly involved in secretion and intracellular transport.
Lysosomes	A tiny, circular-shaped, single membrane-bound organelles,  filled with digestive enzymes.	Helps in the digestion and removes wastes and digests dead and damaged cells. Therefore, it is also called as the “suicidal bags”.
Mitochondria	An oval-shaped, membrane-bound organelle, also called as the “Power House of The Cell”.	The main sites of cellular respiration and also involved in storge energy in the form of ATP molecules.
Nucleus	A largest, double membrane-bound organelles, which contains all the cell’s genetic information.	Controls the activity of the cell, helps in cell division and controls the hereditary characters.
Peroxisome	A membrane-bound cellular organelle present in the cytoplasm, which contains the reducing enzyme.	Involved in the metabolism of lipids and catabolism of long-chain fatty acids.
Plastids	Double membrane-bound organelles. There are 3 types of plastids: Leucoplast –Colourless plastids. Chromoplast–Blue, Red, and Yellow colour plastids. Chloroplast – Green coloured plastids.	Helps in the process of photosynthesis and pollination, Imparts colour for leaves, flowers and fruits and stores starch, proteins and fats.
Ribosomes	non-membrane organelles, found floating freely in the cell’s cytoplasm or embedded within the endoplasmic reticulum.	Involved in the Synthesis of Proteins.
Vacuoles	A membrane-bound, fluid-filled organelle found within the cytoplasm.	Provide shape and rigidity to the plant cell and helps in digestion, excretion, and storage of substances.