Fundamental Unit of Life: Cell

Cell

Introduction

• A cell is the basic structural and functional unit of all life forms.
• Study of structure and composition of cell is called 'Cytology'.
• Cell was first discovered and observed by Robert Hooke in a thin dead slice of cork in the year 1665.
• First free living cell was discovered by A. V. Leeuwenhoek, in 1674.
• Protoplasm is an aggregate of various chemicals such as water, ions, salts and other organic molecules like proteins, carbohydrates, fats, nucleic acids, vitamins etc. Presents in cytoplasm along with cell organelles & nucleus that constitute a cell.
  

Cell Theory

Two biologists, Schleiden and Schwann (1838) gave the Cell theory which states that :

1. All plants and animals are composed of cells.
2. Cell is the basic unit of life.
3. All cell arise from pre-existing cells.

Note: Viruses are the exceptions of cell theory

Types of Cell

 On the Basis of Type of Organization Cells are of two kind 

Types of Organisms

On the basis of number of cell, Organisms are of two types

Cell Shape and Size

Cell Shape: Cells are of variable shapes and sizes. That varies according to their function and position. Generally cells are spherical but they may be elongated (nerve cell), branched
(pigmented), discoidal (RBC). Spindle- shaped (muscle cell) etc.

Cell Size: Size of cell is variable depending upon its position & function. Some are microscopic while some are visible with naked eyes. 

• The largest cell is ostrich egg (15 cm long 13 cm wide & weight 1.4 kg)
• The longest cell is nerve cell (upto 1m).
• Smallest cells so far known are PPLOs e.g., mycoplasma

Components of Cell

There is an occurrence of division of labour within a Eukaryotic cell as they all have membrane bound structure called ‘Cell organelles’. Each of them perform a specific function.
The three basic structure of all the cells are :
(i) Plasma membrane (ii) Nucleus (iii) Cytoplasm

Plasma Membrane

Plasma membrane is selectively permeable in nature, means it allows or permits the entry and exit of some materials in and out of the cell.

• It is the limiting boundary of each cell which separates cytoplasm from its surroundings. It is found in both plant as well as animal cells.
• It is the outermost covering of a cell in case of animals and lies below the cell wall in case of plants.
• It is made up of proteins and lipids.
• It is flexible and can be folded, broken and reunited.
  

Functions of Plasma Membrane:

• It regulates the movement of molecules inside and outside the cell.
• It helps in maintaining the distinct composition of the cell.

Transportation of molecules across the Plasma Membrane :
This can be done by following ways :

Diffusion: Movement of solutes or ions from a region of higher concentration to a region of lower concentration is called as diffusion. It does not require energy therefore, it is called as passive transport.
Osmosis : The movement of solvent or water from its higher concentration (solvent) to lower concentration (solvent) through a semipermeable membrane is called as osmosis.

Endomosis:Movement of solvent into the cell is called Endomosis.
Exosmosis:Movement of solvent outside the cell is called Exomosis.

Types of Solutions on the Basis of Concentration and its effect on cell : 

1. Isotonic Solution: When concentration of a solution outside the cell is equal to the concentration of cytoplasm of the cell, it is called as isotonic solution.
2. Hypertonic Solution: When concentration of a solution outside the cell is more than inside of the cell. Due to this, cell loses water and becomes plasmolysed (shrink).

Plasmolysis:- Shrinking of the protoplasm away from the cell wall due to excessive loss of water (exosmosis) is called Plasmolysis.

3. Hypotonic Solutions: When the concentration of the solutions outside the cell is lesser than that of cytoplasm of cell, due to excessive endosmosis cell swells up and animal cell may bursts.

Cell Wall 

• It is the outermost covering of the plant cells and cells of fungi.
• It is absent in animal cells.
• Cell wall is rigid, strong, thick porous and non-living structure.
• In plant it is made up of cellulose.
• In fungi it is primarily made up of Chitin.
• Cell walls of two adjacent cells are joined by a layer called middle lamellae and microscopic channels called plasmodesmata for transport.
  

 Functions of Cell Wall :

• It provides definite shape, structure, support and protection to the cell.
• It provides strength to the cell.
• It is permeable and allows entry of molecules of different sizes & thus control intercellular transport.

Nucleus

• Nucleus is the most important cell organelle which directs and controls all its cellular activities.
• It is called as 'controller of cell'.
• Nucleus was discovered by Robert Brown in 1831.
• In Eukaryotes, a well-defined nucleus is present while in Prokaryotes, a welldefined nucleus is absent. Prokaryotes contain a primitive nucleus called Nucleoid.
• Nucleus has double layered covering called as nuclear membrane.
• Besides nuclear membrane, nucleus also contains nucleolus and chromatin material. Chromatin is made up of DNA (Deoxy ribonucleic acid) and Protein, that ultimatly condense and forms chromosome.
• Chromosomes or chromatin material consists of DNA which stores and transmits hereditary information for the cell to function, grow and reproduce.
• The functional segment of DNA is called gene. 

 Functions of Nucleus:

• It direct and controls all the metabolic activities of the cell and regulates the cell cycle.
• It helps in transmission of hereditary characters from parents to their offsprings. 

Cytoplasm 

Cytoplasm is the fluid content enclosed by the plasma membrane. Cytoplasm was discovered by Kolliker in 1862.
It is the site of both biosynthetic and catabolic pathways (Metabolic activities)

It can be divided into two parts :
Cytosol: Aqueous soluble part contain various fibrous proteins forming cytoskeleton. It contain about 90% water, 7% Protein 2% carbohydrates & l% etc.
Cell organelles: Living part of the cells having definite shape, structure and function bounded by plasma membrane.

There are single membrane bound, double membrane bound and non membrane bound Cell organelles. 

Endoplasmic Reticulum 

It is the network of membrane bound tubules and sheet present in the cytoplasm.

Endoplasmic reticulum is of two type:

Function of ER:

• It serves as a channel for the transport of materials between various regions of cytoplasm and between cytoplasm and nucleus.
• It also functions as a cytoplasmic framework to provide surface for some of the biochemical activities.
• It forms endoskeleton of cell.
• It helps in synthesis of fats, protien, steroids, cholesterol etc.
• SER in liver cells plays a crucial role in detoxification of drugs and poisonous by products.
• Membrane biogenesis: Protein & Lipids produced by ER are used to produce cell membrane.

Golgi apparatus

Golgi apparatus consists of a system of membrane bounded fluid filled vesicles arranged parallel to each other in stacks called cisternae along with some large and spherical vesicles. It was discovered by Camillo Golgi. It is absent in prokaryotes, mammalian RBC's & sieve cells. 

Functions of Golgi apparatus :

• Its function include the storage, modification, Packaging & secretion of products in vesicles.
• It involved in the formation of lysosomes.
• It is secretary in nature. It helps in melanin synthesis.
• It also involved in the synthesis of cell wall & plasma membrane. 

Mitochondria 

•  It is a rod shaped structure found in cytoplasm of all eukaryotic cells except mammalian RBC's.
• These are also absent in prokaryotes.
• It is called 'Power House of the Cell'.
• It is double membranous structure where outer membrane has specific proteins while inner membrane is folded inside to form chambers called Cristae.
• Mitochondria has its own DNA & Ribosomes

Functions of Mitochondria :

• Its main function is to produce, store and release the energy in the form of ATP (Adenosine Triphosphate) The energy currency of the cell.
• It is the site for cellular respiration in which ATPs are produced. 

Ribosomes

• Ribosomes are the sites of protein synthesis.
• Synthesized proteins is transported by endoplasmic reticulum. 
• All structural and functional proteins (enzymes) coded by the nuclear DNA are synthesized upon cytoplasmic ribosomes. The DNA codes are transcripted into messenger RNA (mRNA) (Ribonucleic Acid) molecules, Which comes out of the Nucleolus
  and translated (Protein synthesis) by ribosomes attached to RER in the form of proteins.

Plastids

• It is double membranous, discoidal structure,found mainly in algae and plant cells.
• They also have their own DNA and ribosomes.

Depending upon the type of pigment present in them, they are of following three types :

1. Leucoplast: These are white or colourless and found in non-photosynthesis tissue of plant such as Root, bulb, seeds, etc. They can change into other type of plastids.
   The primary functions is storage of starch, oil, proteins.
2. Chromoplast: These are coloured plastids except green, these impart colour to fruits & flowers.
3. Chloroplast: It contain chlorophyll which impart green colour to leaves and, found in aerial parts of plants. It helps in the process of photosynthesis so it is called the "Kitchen of cell" in plant. 

Chloroplast: 

Chloroplast have following two parts:

• Grana: It constitutes the lamellar system. These are found layered on top of each other. These stacks are called Grana. Each granum of the chloroplast is formed by superimposed closed compartments called Thylakoids.
  Function: They are the sites of light reaction of photosynthesis as they contain photosynthetic pigment chlorophyll, photosynthetic units.
• Stroma: It is a granular transparent substance also called as matrix. Grana are embedded in it. Besides Grana they also contain lipid droplets, starch grains, ribosomes etc. 

Vacuoles

• These are membrane bounded regions in the cytoplasm containing water and other substances. They are bounded by a single membrane called Tonoplast.
• In animal cells vacuoles are absent or smaller in size.
• In plant cells a single large vacuole is found which occupies about 90% of the volume of cell. 

Functions: It helps in maintaining osmotic pressure in a cell & stores toxic metabolic products (Waste product) water, sugar, protein etc. 

Lysosome

• They are tiny single membrane bound cell organelle containing powerful digestive enzymes for intracellular digestion.
• Lysosome absent in RBC's
• Lysosomes are synthesised by golgi body & enzymes present in it are synthesised by RER.

Function:

• Their main function is phagy (digestion). Means they breakdown worn out cell parts.
• They are kind of waste disposal system of the cell.
• They help in digesting foreign materials like invading viruses and bacteria in the cell.

Suicidal Bag: During disturbances in cellular metabolism (i.e., in case of cell damage), lysosomes burst and their enzymes are released into the cytoplasm which digest their own cell. Therefore they are also called 'Suicidal Bags'. 

Plant cell and Animal cell

Cell Division

Cell Division: New cells are formed in organisms in order to grow, to replace old, dead and injured cells, and to form gametes required for reproduction. The process by which new cells are made is called cell division.

The two main types of cell division are:

1. Mitosis : The process of cell division by which most of the cells divide for growth is called mitosis. In this process, each cell called mother cell divides to form two identical daughter cells. The daughter cells have the same number of chromosomes as mother cell. It helps in growth and repair of tissues in organisms.
   
2. Meiosis: Specific cells of reproductive organs or tissues in animals and plants divide to form gametes, which after fertilisation give rise to offspring. They divide by a different process called meiosis which involves two consecutive divisions. When a cell divides by meiosis it produces four new cells instead of just two. The new cells have only half the number of chromosomes than that of the mother cells. These new cells are Transformed into gametes.