Generalized structure of a plant cell

Generalized structure of a plant cell

What is cell?

Cell is the structural, functional and biological unit of all organisms. A cell is the basic unit of biological activity.

A. G Loewy and P.siekevitz (1963) have defined a cell as a unit of biological activity be limited by a semipermeable membrane and capable of self- reproduction in a medium free of other living systems.Wilson and Morrison (1966) have defined the cell as- An integrated and continuously changing system.John Paul (1970) has defined the cell as- The simplest integrated organization in living systems, capable of independent survival.

The function of cell organs

a). Mitochondria

The organelle present in the cytoplasm that does all the functions, such as Krebs’s cycle, fatty acid cycle,electron transport system, etc. In order to generate the energy of a cell is called mitochondria. The mitochondria are generally known as the powerhouse of the cell. They are the site of the chemical events that supply the energy to the cell.

Discovery: In 1894 Altman discovered mitochondria (sing. mitochondrion).

Shape- It is usually round, filamentous or rod-like in shape.

Number: Each, on average, contains 300-500 mitochondria.


i) It is surrounded by a two-layered membrane envelope with an outer and inner membrane. Eachthe membrane is a typical unit membrane being about 50-70 A thick.ii]The inner membrane produces projections pointed towards the Centre, known as cristae.iii) There are plenty of round shaped organs on the outer surface of cristae, called oxysomes.iv). Besides, the liquid matrix is also present within the mitochondria. Outer membraneFunctions:i) Mitochondria supply energy for all kinds of physiological activities of a cellii) There are different kinds of enzymes, co-enzymes arc found in mitochondria for respirationiii) All reactions except glycolysis, such as Krebs’s cycle, electron transport, oxidative phosphorylationetc. take place within the mitochondriaiv) Mitochondria ensure the regulated flow of electron

v) ADP turns into ATP due to phosphorylation and is stored in the same organellevi) Mitochondria also take part in the formation of DNA and RNAvii) Mitochondria also take part in the metabolic activities of lipid.

b). Endoplasmic reticulum:

There are some sacs, cavities or tubules which form a network by themselves known as endoplasmicreticulum.

Discovery. In 1945 Porter along with his associates first discovered endoplasmic reticulum.Structure: They are surrounded by a single membrane.


(a) They are of two types according to the presence of ribosome:i) Smooth endoplasmic reticulum: Ribosomes are absent in this type of endoplasmic reticulum.ii) Rough endoplasmic reticulum: Ribosomes are present around the endoplasmic reticulum.(b) They are of three types on the basis of shape:i) Cisternae: They are long’ unbranched and are arranged parallelsii) Vesicle: They are round and oval-shaped and are distributed scatterediii) Tubules: Tubular and branched.


i) It helps in the formation of the cytoplasmic framework and Meckel activitiesii) It helps in the formationiii) Conduction of enzymes protein and Lipid and their proper distributioniv) Helps in protein and lipid synthesis cell organelles

C). Plastid:

The living organelle distributed in the cytoplasm pigmented or non-pigmented, which can only be foundin a plant, cell is called plastid.

Types of plastid:Plastid mainly is of two types, such as-1) Leucoplastid 2) Chromoplastid

1). Leucoplastid:

The cells of the particular area of the plant body that are deprived of sunlight contain Leucoplastid. Theyare colourless plastid (non-pigmented). Leucoplastid is of three types:

a) Amyloplast: It stores carbohydrateb) Elioplast: It stores lipidc) Alcuroplast or proteiunoplast: It stores protein.

2). Chromatopla​stid: 

The coloured plastids (pigmented) are said to be chroniatoplastids.On the basis of the presence of different pigments chromatoplastid is of two types:a)Chromoplast b) Chloroplast.

a) ChloroplastThe chromatoplastid that creates different colours other than green colour is called chromoplast, suchas- petals of flower, coloured seeds & fruits, modified root of carrot etc.

b) Chloroplast:The green pans of plants, such as- leaf, tender branches, green fruits etc. That contain colour- producingplastids are (red-carotin, yellow-xanthophyll) called chloroplast.The green coloured plastids are known as Chloroplast. They are green in colour becausc of the presenceof great deal of green pigments known as chlorophyll.

Position:It is present in the green parts of the plant especially in leaves.

Number.One to many per cell.Each higher plant cell: 20 to 40 in number. E.g. Spirogyra.Each lower plant cell: Less than that of a higher plant chloroplast. Chlamydomonas contains only a singlechloroplast.

Structure:It may be of the following structures:a) Spiral chloroplast:b) Belt shaped chloroplast: E.g. Ulothrix.c) Cup shaped chloroplast: E.g. Chlamydomonas.d) Star shaped chloroplast: E.g. Zygnema.

Structure of chloroplast:The structure of chloroplast is very complicated. In a chloroplast, three parts are seen clearly when it isobserved through electron microscope:

a) MembraneA chloroplast is covered with a two-layered membrane. It is composed of lipid and protein. Structurallyit is similar to plasma membrane.

b) Stroma:The hydrophilic matrix is covered with membrane. This matrix is called stroma.Ribosome, osmophilic granules etc.: are present in stroma.

c) Grana:i) Grana, which are sacs of coin shaped double membranes called thylakoids, are suspended in thestroma. The thylakoids contain chlorophyll and other pigments. In each chloroplast, there are usuallyabout 40 to 60 grana and each granum may contain from 2 or 3 to more than 100 stacked thylakoids

ii) Two grana remaining side by side are linked together by some minute tubulcs, called stroma lamellae(sing. lamella)

iii) In the inner wall of each granum coin, there are some crystalline substances, called quantasome.Each quantasome contains chlorophyll-a, chlorophyll-b. carotin, xanthophyll, phospholipids, quinon, sulpholipid and different enzymes.

Functions:i) To produce carbohydrate with the help of carbon dioxide and water by Sunlight photosynthesisii) To convert ADP to ATP with the help Chlorophyll of sunlight (Photophosphorylation)iii) To produce food elements, such as protein and lipid with the help of enzymesiv) To fix up C02 within the quantasomev) To help in Synthesizing protein by ribosome following a special method.

d). Nucleus

The most conspicuous organelle in a living cell, usually round, surrounded bylipoprotein madc membrane which contains chromosomes and regulates all the activities of a cell iscalled nucleus.

Discovery.In 1831, Robert Brown discovered nucleus in the cell.

Position:True nucleus is formed in all eukaryotic cells.

Number:Usually, a single nucleus is present in a typical cell. Prokaryotic cells do not have any nucleus. Feweukaryotic cells, in matured stage do not contain nucleus, such as- sieve cell, R,B.C. of humans etc. Thereare a few cells which may have more than one nucleus.

Shape:The shape of nucleus depends on the shape of cell. Usually they are round. Nucleus may also be ovaland disc shaped.

Structure:Nucleus is composed of the following organelles:

1) Nuclear membrane:The transparent, semi-permcable and flexible membrane around the nucleus is said to be nuclear membrane.

Structure:It is a porous and two layered membrane. It is made of lipoprotein. It disappears during the cell division.

Function:i) This membrane separates the nucleoplasm from cytoplasm and gives nucleus a particular shapeii) It regulates the movement of different substances in between cytoplasm and nucleusiii) It helps in conducting different substances synthesized in nucleus to cytoplasmiv) It keeps the nucleus attached to endoplasmic reticulumv) It protects the nucleus.

2) Nucleoplasm:

The granular, transparent and homogeneous liquid substance present in nucleus is known asnucleoplasm.

Structure:The nucleoplasm includes nuclcoprotein, nucleic acid, enzyme and mineral salts etc. Its chemicalstructure’ is complicated.

Function:i) It contains different organelles of the nucleusii) It regulates the physiological activities of nucleusiii) It helps in forming ribonucleic acid (R.N.A.) and De-oxyribo nucleic acid (D.N.A.)

3) Nucleolus:

The dense and round part present in nucleus is called nucleolus. Scientist Fontana in 1781 first observed it.Structure:It is made of protein, lipid, DNA and RNA; Besides, it also contains many enzymes.

Functions:i) Produces ribosomei1) Helps in cell divisioniii) Takes part in protein synthesisiv) Preserves RNA.

4)Chromatin fibres:

The coiled thread like structures suspended in the nucleoplasm is called chromatin fibres. A single coiledthread is called chromosome.

Structure:It is made of DNA, RNA, histone and non-histone protein. DNA and histone, n it remains at a rate of 35% and 55%.

Functions:i) It contains DNAii) Synthesizes proteinii) Helps in cell divisioniv) It’s a container and carrier of heredity.


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