CELL

 CELL STRUCTURE

The cell is a fundamental structural and functional unit of living organisms.  Living organism are distinct in form, structure, behavior, heredity and development.  Cells within a single plant or animal vary widely in structure and function, they all represent units of living material and have important properties in common.  New cell arise from pre-existing cells and a zygote has ability to form the whole individual. Thus physiology and function of an organism is governed by the sum total of activities and interactions of its constituent cells.

Living being is formed by small units which are called as cells. Each cell is complete in itself. In unicellular organisms they function freely the different and various activities but in multicellular organisms they control the activities combinable. Each cell has a cell membrane which covers its protoplasm. This protoplasm.  However in plant cells fungi bacteria the cell is also surrounded by a cell wall. Protoplasm is the site of living activities of every cell.

• All living forms are composed of microscopic units called as “Cells”.
• A cell is the basic structural and functional unit of all life forms.
• Study of structure and composition of cell is called as “Cytology”.
• Cell was first observed by “Robert Hooke” in a dead cork slice in the year 1665. He described about this in his book “Micrographic”
• The term protoplasm was coined by Purkinje in 1839.
• Protoplasm was discovered by “Felix Dujardin” and named as sarcode.
• It’s consistency differs under different condition. It exists in sol-gel states.
• Protoplasm is an aggregate of various chemicals such as water, ions, salts and other organic molecules like proteins, carbohydrates, fats, nucleic acids, vitamins etc.
• The word cell was derived from a Greek word “Cellulae” which means small room.
• First living cell was discovered by A.V. Leeuwenhoek.

CELL DISCOVERY

The word cell is derived from latin word ‘cellula’ which means ‘a little room’. While examining thin slice of cork, Robert Hooke in 1665 discovered honey comb like structure called Cells. Cork is a substance which comes from bark of a tree. 

HISTORICAL BACKGROUND OF CELL

• 1665 – Robert Hooke discovered cell and coined the term cellula.
• 1673-1674 – Antonny Von Leeuwenhoek observed bacteria, spermatozoa, RBCs etc.
• 1682 – Malpighi and Grew laid the foundation of cell concept by writing anatomy of plants wherein they described that tissues contain minute elementary structure (or cells).
• 1772 – Alfonso Corti observed living matter in the cells.
• 1781 – F. Fontana discovered nucleolus in skin cell of eel. 
• 1802 – C.F. Mirbel observed that plants are formed by membranous cellular tissue.
• 1809 – J.B. Lamarck – No body can have life if its constituent parts are not formed of cells.
• 1824 – R.J.H. Dutrochet boiled Mimosa tissues in nitric acid and inferred that “tissues are really globular cells of an extreme smallness which are united by cohesion”.
• 1831 – Robert Brown discovered nucleus in cells of Orchid root.
• 1835 – Dujardin discovered living semi fluid substance of cells and named it sarcode.
• 1838-1839 – M.J. Schleiden and Theodore Schwann propounded cell theory. 
• 1839 – J.E. Purkinje coin the term protoplasm (supported by Von Mohl).

CELL THEORY

Two biologists, “Schleiden and Schwann” gave the “Cell theory” which was later on

expanded by “Rudolf Virchow ”. Cell theory states that

• All plants and animals are composed of cells.
• Cell is the basic unit of life.
• All cells arise from pre-existing cells.
• Viruses are the exceptions of cell theory.

CHARACTERISTICS OF CELLS

Cell shape 

Shape of cells varies not only in different organisms but also in different parts of same organism. 

Cells may be spherical, oval, elliptical, spindle shaped, cuboidal, polygonal, columnar or flat plate like.

Shape of the cell mainly depends upon the function it performs. 

Other factors which determine the cell shape are : surface tension, viscosity of the protoplasm; rigidity of cell covering ; mechanical pressure of adjoining cells, etc. 

CELL SIZE

The size of cells widely vary in different organisms. 

In prokaryotes, cell size range between 1 to 10 µm while in eukaryotes it range between 10 to 100 µm. Smallest cell (PPLO) – Pleuro pneumonia like organism. Mycoplasma gallisepticum (0.1 µm to 0.3 µm)Largest animal cell – Ostrich egg (170 × 135 mm).

Largest plant cell – Acetabularia (one celled organism - green alga) (6 – 10 cm).

Longest cell in plant – fibre of ramie 55 cm to 100 cm long.

Longest cell in animal – Nerve cell (1 metre).

CELL NUMBER

Some organisms are one-celled called acellular e.g. Amoeba, Paramecium (earlier were called unicellular) and are always microscopic, as can be seen only with the help of microscope but most of the organisms are multicellular and have indefinite number of cells e.g. a newly-born human infant has about 2 × 1012 cells, while at maturity the average adult male has about 100 trillions (1014).

Cell Volume

The volume of a cell :

•  Is fairly constant for a particular cell type.
•  Is independent of the size of an organism.

TYPES OF CELL & ORGANISM

On the Basis of Number of Cells Organisms can be categorized as:

(i) Unicellular organisms: These are organisms which made up of single cell only. This single cell .Performs all the vital body functions of an organism. e.g. Amoeba

(ii) Multicellular organisms: These are the organisms which made up of numerous cells. These cells then combine to form an organ and group of organs performing different functions forms an organ system which further forms an organism. e.g. plants and animals

On the basis of type of organization, cells are two types :

(i) Prokaryotic cells: these are primitive and incomplete cells. They have less developed nucleus without nuclear membrane & nucleolus. e.g.  Bacteria.

(ii)Eukaryotic cells: these are well developed cells. They have advanced nucleus with unclear membrane and nucleolus. e.g.  Plants & animals.

 

Differences between Prokaryotic cells and Eukaryotic cells

S.No.			Prokaryotic cell		Eukaryotic cell
1.	Size		0.1-5 μm		5-100 μm
2.	Intracellular compartmentalization		One envelope system		Two envelope system
3.	Cell wall		If Present, containing mucopeptide or peptidoglycan.		If present contains cellulose, no peptidoglycan
4.	Nucleus		Typical nucleus is absent, instead nucleoid or genophore is present		Typical nucleus made of nuclear envelope, chromatin, nucleoplasm, nuclear matrix and nucleoli.
5.	Occurrence of chromosome		Nucleoid is equal to a single chromosome called pro-chromosome		Nucleus contain chromatin material of two or more chromosomes
6.	DNA	1.	DNA lies freely in cytoplasm	1.	In eukaryotic cell DNA lies inside nucleus, mitochondria and plastids
		2.	DNA is generally circular	2.	DNA is commonly linear but circular in mitochondria and plastids
		3.	DNA is naked (without histone proteins)	3.	DNA is associated with histones
		4.	DNA content is low	4.	DNA content is comparatively high
7.	Plasmids		Plasmids may occur		Plasmids rare
8.	Cell membrane	1.	It is folded to form mesosomes	1.	Mesosome is absent
		2.	Cell membrane is involved in separating replication products.	2.	Such separating function of cell membrane is absent.
9.	Flagella	1.	Smaller, 4-5 μm × 12 nm	1.	Longer, 150 μm × 200 nm
		2.	Single stranded	2.	11 stranded
		3.	No distinction of Axoneme and sheath	3.	Clear distinction of Axoneme.
10.	Cyclosis		Cytoplasmic movement absent		Cytoplasmic movement present
11.	Sap vacuoles		Absent		Present
12.	Gas vacuoles		It may present to provide buoyancy and protection against intense radiation.		No gas vacuole
13.	Endocytosis and exocytosis		Absent		Present
14.	Endoplasmic reticulum		Absent		Present
15.	Ribosomes	1.	70S type	1.	80S type
		2.	Occur freely in cytoplasm as well as attach to plasma lemma	2.	In cytoplasm these occur freely as well attached to endoplasmic reticulum. Also found in mitochondria and plastids.
16.	Mitochondria		Absent		Present
			Respiratory enzymes are attached to plasma membrane		Respiratory enzymes are present in cytoplasm and mitochondria
17.	Golgi apparatus		Absent		Present
18.	Thylakoids		If present, lie freely in cytoplasm		These occur inside chloroplast
19.	Lysosomes, phaerosomes, glyoxysomes.		Absent		Present
20.	Centrosome		Absent		Present except in plants.
21.	Microtubules and microfilaments		Rarely present		Usually present
22.	Sexual reproduction		Usually absent		Commonly present

Example of Prokaryotic cells

An Eukaryotic Cell

Differences between Animal and Plant Cell

S.No.		Animal cells	Plant cells
1.	Presence of cell wall	Absent, thus cell shape is not firmed.	Present that’s why shape is firmed
2.	Presence of tissue fluid	Tissue fluid generally bathes the cells.	Absent
3.	Occurrence of protoplasm	Protoplasm fills the whole cell	Protoplasm is peripheral.
4.	Vacuoles	Contains many small vacuoles	Contains a large central vacuole
5.	Nucleus	Usually rounded, lies in centre	Usually oval, lies in peripheral cytoplasm.
6.	Cell anchorage	Cells are held together by various types of junctions.	In plants, cells held together by middle lamella.
7.	Plastids	Absent	Present
8.	Reserve food	Glycogen	Starch
9.	Mitochondria	Comparatively more in number, its cristae are plate like.	Comparatively fewer, cristae are tubular.
10.	Golgi apparatus	It is commonly a single complex.	It is made of a number of distinct units called Dictyosomes.
11.	Centrosome	Present	Absent except in some lower forms
12.	Lysosomes	Present	Absent
13.	Cell division method	Cleavage furrow method	Cell plate method
14.	Bio synthesis of materials	Animal cells are unable to synthesized amino acids, vitamins and coenzymes required by it.	Plant cells can synthesize all material required by them.
15.	If kept in kept in hypotonic solution	Animal cells burst when kept in water or hypotonic solution.	When placed in hypotonic solution or water these swell up (but do not burst).

Ultrastructure of a generalized plant cell 

Ultrastructure of generalized animal cell

DETAILED STRUCTURE  OF CELL

The detailed structure of the cell was studied under electron microscope which was invented by Knoll and Ruska. The flow chart of a cell showing its constituents is as follows:

All cells have three major functional regions :

• Plasma membrane (cell membrane) and cell wall

• The Nucleus

• The Cytoplasm

CELL MEMBRANE

It is a thin, transparent, elastic, regenerative and semipermeable membrane present in both prokaryotic and eukaryotic cells outside the protoplasm. It is also called cell membrane or plasma lemma. It lies inner to cell wall, actually representing the outermost layer of the protoplast.

Plasma membrane is living, dynamic and semipermeable. It is selectively permeable, controlling the movements of various dissolved substances in and out of cell.

STRUCTURE 

Plasma membrane is made up of Phospholipids, Proteins and carbohydrates.

Thickness of plasma membrane is - 75 Å.

Trilamellar model/Sandwich model of plasma membrane was proposed by - Danielli & Davson (1935).

In sandwich model of plasma membrane the bimolecular lipid zone (35 Å thick) is sandwiched between two protein layers (20 Å thick each).

FUNCTION OF PLASMA MEMBRANE:

Plasma membrane is a semipermeable or selectively permeable membrane which regulates the exchange of materials between the cytoplasm and extracellular fluid (ECF). 

It is a dynamic membrane whose permeability varies. Its permeability involves a number of processes like passive transport (of minerals and microbiomolecules) and endocytosis (intake of macro biomolecules).

PM also forms a mechanical barrier to protect various protoplasmic structures.

Plasma membrane performs the same function in plant cell and animal cell.

Transportation of molecules across the plasma membrane: This can be done by following  ways: 

• Diffusion: Movement of solutes or ions from higher concentration to 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 higher concentration (solvent) to lower concentration (solvent) through a semipermeable membrane is called as osmosis. Or The movement of solvent or water from lower concentration to higher concentration of solution through a semipermeable membrane is called as osmosis. Osmosis can also be called as diffusion of solvents”.
• Endosmosis: Movement of solvent into the cell is called as Endosmosis.

Importance of osmosis 

Unicellular fresh water organisms and most plant cells gain water through osmosis. Absorption of water by plant roots is an example of osmosis.

Differences between diffusion and osmosis

S. No.	Diffusion	Osmosis
1.	Diffusion takes place in all media.	Osmosis occurs only in liquid medium.
2.	It is the movement of atoms and molecules from the area of higher concentration to the area of its lower concentration.	It is the movement of water from the area of its higher concentration to the area of its lower concentration.
3.	It does not require a semi-permeable membrane.	It requires a semi-permeable membrane.
4.	In diffusion, molecules may diffuse in any direction.	The flow of molecules occurs in one direction only.
5.	Diffusion is faster than osmosis.	Osmosis is relatively slower than diffusion.
6.	Diffusion may disperse molecules over a very long distance.	Osmosis generally occurs over a short distance.

 

Types of solution on the basis of concentration:

• Isotonic solution: When the concentration of the solution outside the cytoplasm of the cell it is called as isotonic solution.
• Hypertonic solution: When the of concentration of the solution outside the cell is more   than that inside the cell. Due to this cell looses water and becomes plasmolysed.
• Hypotonic solution: When the of concentration of the solution outside the cell is lesser than that of cytoplasm of cell. Due to this cell swells up and bursts.

The flow-chart summarizing transport of materials across the plasma membrane is given below:-

 

They have great tensile strength, flexibility and elasticity which enable plant organs to withstand a variety of strains and tensions caused by the action of gravity, wind, etc.

 

•  Fibres are long, narrow, thick and lignified sclerenchyma cells, usually with pointed or blunt ends. In transverse section, they appear rounded or polygonal in outline with a well defined lumen.
• Pits are few and simple, except fibre tracheids.
• Fibres are always found in sheets.
• Fibres are perhaps the longest cells among plant kingdom.

Osmosis in Selectively Permeable Membrane

Osmosis is the movement of water across a semi-permeable membrane. Osmosis is a selective process since the membrane does not allow all molecules to pass through it. Water is usually the only free-flowing molecule across this membrane.

Isotonic, Hypotonic, and Hypertonic Solutions

• Isotonic solutions are those which have the same solute and pH concentration as the surrounding body fluid or the cytoplasm.
• Hypotonic solutions contain a lesser amount of solute concentration compared to the surrounding fluid and can force the cell to rupture due to excess input of water into the cell.
• Hypertonic solutions contain a higher concentration of solute compared to the surrounding fluid and thus push water out of the cell, shrinking it.

Cell Walls in Plants

Plant cells are different from animal cells due to the presence of a cell wall. The cell wall is made of cellulose and gives a rigid structure to the plant cell. It provides structural support to plants. Due to cell walls, cells of plants, fungi and bacteria can withstand greater changes in surrounding conditions than animal cells. E.g. Cell wall enables the cells to withstand hypotonic solution without bursting.

Cell Organelles

Eukaryotic cells contain various membrane-bound organelles that carry out various functions in the cell, e.g. Nucleus, Endoplasmic reticulum, Golgi apparatus, Lysosomes, etc.

Endocytosis

Endocytosis is the invagination of the cell membrane, followed by pinching off, forming a membrane-bound vesicle. This is commonly seen in Amoeba.

Nucleus in Cells

The nucleus is the processing unit of the cell. It is a double membrane-bound organelle which contains the genetic material for inheritance. The nuclear membrane contains pores that allow the transfer of materials from the nucleus to the cytoplasm. The nucleus is the brain of the cell as it controls various functions such as cell division, reproduction, inheritance, etc. The nucleus contains genetic material, i.e. chromosomes that contain DNA. DNA carries information for inheritance from parents to offspring. Prokaryotes lack a well-defined nucleus, and the genetic material is present in a region known as the nucleoid.

Chromosomes

Chromosomes are the genetic material present in the nucleus. It contains DNA with associated proteins. They are present as threads called chromatin. During the growth phase of the cell, the chromatin condenses into a much thicker structure called a chromosome. Human cells contain 23 pairs of chromosomes (46).

Chromatin

Chromatin is a thread-like structure which serves as the genetic material present inside the nucleus of the cell. It is made up of DNA and protein molecules. DNA contains the hereditary information needed for the structure and function of the organism.

Cytoplasm

The cytoplasm is the fluid found inside the cell. It gives the structure to the cell and houses different organelles of the cell.

Organelles

Organelles are structures present in the cytoplasm of the cell that helps in several functions of the cell.

Endoplasmic Reticulum

The endoplasmic reticulum is a membrane-bound cell organelle that plays an integral role in the interpretation of the genetic information present in the nucleus.

Rough ER

Rough ER is the one that has ribosomes on them. The ribosome is made up of nucleic acids and proteins. They are the site of protein synthesis. The Rough ER is also involved in the modification and folding of proteins.

Smooth ER

Smooth ER does not have ribosomes and thus is not involved in protein synthesis. They are, however, involved in lipid metabolism and detoxifying of poisonous molecules.

Golgi Apparatus

Golgi Apparatus is also called the post office of the cell. They package and transport the proteins across the cytoplasm.

Lysosomes

They are referred to as suicide bags of the cell as they contain potent enzymes that can digest a cell. Lysosomes also help in defence by attacking a foreign object.

Mitochondria

Mitochondria are also called the powerhouse of a cell. They generate ATP via the electron transport chain. They also have a DNA called mtDNA, which makes them a semi-autonomous organelle. Mitochondria can make their own proteins.

Plastids

There are various types of plastids in different cells based on the pigment they contain. The chloroplast is the plastid where photosynthesis occurs, they contain chlorophyll. Some of the other plastids are leucoplast and chromoplast. Leucoplasts store starch, oil and protein granules. Chloroplasts contain membranes known as thylakoids embedded in the stroma. Plastids also contain their own DNA and ribosomes.

Vacuoles

Vacuoles are large vesicles that hold water or air in them and give structural rigidity to the cell. Vacuoles are common in plant cells. In animals, the vacuoles are either very small or absent.