Biology IX | CELL- Structural organisation

CELL- Structural organisation


A single cell may constitute a whole organisms as in Amoeba, Chlamydomnas, Paramecium and bacteria these organisms are called unicellular organisms.

Many cells group together in a single body and perform different functions are called Multi cellular organism like fungi plants and animals.

Cells vary in number, shape and size. In man, the number of cells is estimated to be about 100 trillion (1014).

Cell is made of life giving substance called protoplasm. It is an aggregate of molecules of various chemicals mostly organic molecules like proteins, Carbohydrates fats, nucleic acid etc.

Cell were first discovered by Robert Hooke in 1665. He observed the cells in a cork slice of plant with the help of Primitive microscope.

Robert Brown discovered the Nucleus in cell in 1831.

Purkinje coined the term ‘Protoplasm’ for the fluid substance of the cell in 1839.

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Plasma membrane or cell membrane

Plasma membrane is the outermost covering of the cell that separate the contents of the cell from the surrounding medium.

Plasma membrane is made up of lipids and protein and provides a mechanical barrier for the protection of the inner cell content.

The plasma membrane allows or permits the entry and exit of same material in and out of the cell. Hence the cell membrane is called a selective permeable membrane.

Cell Wall

Plant cell in addition to the plasma membrane have another rigid outer covering called cell wall.

Plant cell wall is mainly composed of cellulose

Cellulose is a complex substance and provides structural strength to plants.

Plant cell loses water through the process of Osmosis due to which there is Shrink age or contraction of the of the cell away from the cell wall, this phenomena is known as plasmolysis.




Nucleus is prominent, spherical or oval structure, located near the center of the cell and it is the controlling center of all the cell activities.

Nucleus enclosed by a double-layered membrane, called nuclear membrane, which separates it from the cytoplasm.

The nuclear membrane has some pores, which allow the transfer of material between nucleoplasm present inside nucleus and the cytoplasm.

The chief components of the nucleus are as undermention.

(i) Chromatin material which is in the form of an intertwined mass of thread –like structures, the chromatin material, mainly consisting of DNA (Deoxyribonucleic acid) is responsible for storing and transmitting the heredity information from one generation to another. At the time of division chromatine material condenses into rod like bodies called Chromosomes. Genes are the segments of DNA. Gene is a functional unit of Chromosome.

(ii) Nucleolus, Containing RNA (Ribonucleic acid). RNA is helpful in protein Cytoplasm in the cytoplasm. A cell may be have one nucleus (uninucleated) as in many plants and animals, two nuclei (binucleate) as in a protozoan -e.g., paramecium, a many nucleic multinucleate as in certain fungi.




The portion of the Protoplasm with out nucleus is called cytoplasm.

Cytoplasm is clear, colorless, jelly-like, Viscose semisolid substance

Cytoplasm is the center for most of the metabolic activities of the cell and for this purpose it has a number of specialized structural called cytoplasmic organelles in it.


Cell organelles

The activities of the cell are performed by different cell organelles. These are-

Endoplasmic reticulum (ER) or Ergastoplasm


It is a membranous network, enclosing a fluid filled lumen.

ER are of two types

(i) Rough endoplasmic reticulum (RER) with ribosome’s attached on its surface for synthesizing proteins

(ii) Smooth endoplasmic reticulum (without ribosomes) for secreting lipids

ER occur in three morphological forms viz, cisternal vesicles and tubular.

ER is serve as channel for the transport of material (specially proteins) between various regions of the cytoplasm between the cytoplasm and the nucleus.

Golgi Apparatus

Golgi complex was described by Camillo Goligi in 1898 in the nerve cell of born owl.

Golgi complex show a membranous structure of the cell

Golgi apparatus occure in three morphological forms

(i) Cistern or lamellae (Fluid filled lumen)

(ii) Vacuoles

(iii) Vesicles

Lamellae occur as isolated structures and are not stacked as in animal cells, they are known as dictyosomes.

Golgi complex are concerned with the absorption and storage of lipids and also involved in the formation of lysosomes & persoisomes




Lysosomes was described by Christian de Duve in 1955.

Lysosomes are membrane bound vesicles and available in plenty in each cell and these contain powerful enzymes capable of digesting or breaking down all organic material.

Lysosomes serve as intracellular digestive system, hence called demolition squads scavengers and cellular house keeper.

Lysosomes destroy any foreign material inside the cell such as bacteria.

Lysosomes also remove the worn out and poorly working cell organelles by digesting them to make way for their new replacement.

Lysosomes digest the entire damaged or dead cell, that’s why lysosomes are also called suicide bags.


Mitochondria (Power House of the cell)

Mitochondria are thread-like or granular organelles present in the cytoplasm of the cells.

Each mitochondrion is bounded by a double membrane structure.

The outer membrane is smooth and continuous and the inner one folded into finger-like cristae which greatly increase the surface are of the inner membrane.

Mitochondria contain enzymes for cellular respiration in which energy is released.

The energy required for various chemical activities needed for life in the form of ATP (Adenosine triphosphate) molecules.

ATP is also known as the energy currency of the cell.

The body uses energy stored in ATP for making new chemical compounds and for mechanical work.

Mitochondria are able to make some of their own protein because of the presence of their own DNA and ribosomes.

It helps in breaking down of carbohydrates to release energy.


Plastids (kitchen of the cell)


Plastids are found in plants cell only.

They also have double membranes like mitochondria but no cristae.

They are grouped into two classes

(i) Leucoplasts:- These are colorless plastids and are responsible for forming and storing of starch grains and oil drops.

(ii) Chromoplasts:- These are present in different colours in the plant cell.

Chloroplast:- The chloroplasts have the green pigment called chlorophyll. Which is responsible for the synthesis of food by the process of photosynthesis.



Centrosome consists of two granules like centrioles and is found in animal cell only.

Centrosome helps in cell division.

In plant cells the polar caps perform the same function.




In animals it is found in smaller size than in plants.

In plants it occupies about 50 to 90% of the volume of a cell.

Vacuoles are full of liquid cell sap and thus provide turgidity and rigidity to the plant cell.

Many substances of importance in the life of the plant cell are stored in vacuoles are-amino acids, sugar, various organic acids and some protein.



When the living plant cell loses water through osmosis and there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.



Due to the flexibility of the cell membrane it also enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis.


Membrane Biogenesis

Some of the protein and lipids which are synthesis from E.R. help in building cell membrane. This process is known as membrane biogenesis.



The movement of water from a region of higher concentration to the lower water concentration through selectively permeable membrane is known as osmosis.



The movement of substance from a region of higher concentration to the lower concentration is known as diffusion.


Hypotonic Solution

If the medium surrounding the cell has higher water concentration than the cell, meaning that the outside solution is very dilute, the cell will gain water by osmosis. Such as solution is known as a hypotonic solution.


Isotonic Solution

If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution.


Hypertonic Solution

If the medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution, the cell will lose water by osmosis. Such a solution is known as a hypertonic solution.


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