9th Class Science The Fundamental Unit of Life Cells


Category : 9th Class

*       Cell


All the cell possess the following three basic characteristics. First all the cell contain genes in the form of DNA and RNA which have coded information for the regulation of cellular function. Secondly, they contain plasma membrane made up of protein and lipids. This allows the exchange of material between the cells. Thirdly, they have a metabolic machinery called cytoplasm which contain various cell organelles. An organism may be of single celled or multicellular. The unicellular organism includes bacteria, amoeba, Euglena, paramecium and yeast. All the cell is capable of carrying out certain basic function such as nutrition, respiration, growth and reproduction. Thus, all the living organisms are divided into two forms, Prokaryotic and Eukaryotic cells.


*            Prokaryotic Cells

The cell in which nuclear region is not surrounded by a nuclear membrane and nucleus and cytoplasm cannot be differentiated is called prokaryotic cell.

These organisms have primitive and incomplete cells. It consists of a single chromosome which is in direct contact with cytoplasm. However, ribosomes are present in such cell. For example, cyanobacteria, archaebacteria.



*             Eukaryotic cells

In this cell nuclear region is bounded by nuclear membrane. The nucleus and cytoplasm can easily be differentiated. They also contain membrane bound organelles and more than one chromosomes are presents in the nucleus. Cell division occurs by miotic or meiotic cell division. It may be unicellular or multicellular organisms.


*              Structure of Cell

All the cell basically consists of three parts such as plasma membrane, nucleus and cytoplasm. All the metabolic activities within the cell is possible due to these features in the cell.


*             Plasma Membrane

It is a thin layer of membrane which covers the outer most layer of the cell. It is present in microorganism, plant and animals. It is selectively permeable and can be seen only under the high power microscope. It is formed by phospholipids, proteins, cholesterol and polysaccharides. It has got fluid mosaic model.


According to this model plasma membrane is made up of a bilayer of phospholipids and two types of protein, that is intrinsic and extrinsic proteins. The protein presents in the membrane provides enzymes for catalyzing chemical reactions, transport protein for movement of water soluble ions, pumps for active transport and receptor proteins for endocytosis. Presence of lipids provides flexibility to the membrane.

The function of plasma membrane is that it regulates the entry and exit of selected material in the cell, hence it is also called the selectively permeable.

The movement of material takes place by diffusion and osmosis. Some substances like carbon dioxide, oxygen, water, etc. move by the process of diffusion. In this process the molecules move from the region of higher concentration to the region of lower concentration.

The spontaneous movement of water molecules through a selectively permeable membrane is called osmosis. If we put plant cell or animal cell into a solution of salt or sugar


The following three things can happen:

If the medium surrounding the cell has a higher water concentration than the cell, the cell will gain the water  by the process of osmosis. Such solution are called hypotonic solution.



If the medium surrounding the cell has same concentration as the cell, then there will be no movement of  water across the membrane. Such solution is called isotonic solution


If the medium has lower concentration of water than the cell, the cell will lose the water and water will move out of the cell by the process of osmosis. Such solution is called hypertonic solution.



*             Cell wall

It is the outer most layer of plant cell which is rigid in nature and protects the cell inside. It provides rigidity and shape to the cell. It is made up of fibrous polysaccharide called cellulose. It consists of thin cellulose fibers called microfibrils guled together. It provides mechanical strength to the plant which is increased by addition of lignin in tissue, such as xylem. It has a narrow pores through which fine particles are able to pass.

When there is shrinkage of the protoplasm away from the cell wall due to loosing of water by the cell, it is called plasmolysis. If we immersed a plant cell in a concentrated sugar solution, the concentration of water molecules inside the cell will be higher as a result of which water will move out of the cell by the osmosis and hence the cell will shrink away from the cell wall.



*            Nucleus

It is the central part of the cell which is bounded by two membrane called the nuclear membrane. Nuclear membrane separates nucleus from cytoplasm. The nucleus contains nucleolus and chromatin material. The nucleolus is not bounded by any membrane and is rich in protein and RNA molecules where ribosomes are formed. Ribosomes helps in protein synthesis in the cytoplasm. On the other hand the chromatin material is a thin thread like structure called chromosomes and is composed of the genetic material DNA and proteins.

DMA stores all the information necessary for the cell to function, grow and reproduce. The segments of DNA are called genes. During cell division it is condensed into two or more thick ribbon like structures. The chromosomes consists of two arms that extend out from a specialized region of DNA called centromere. Depending on the position of centromere chromosomes are of four types as metacentric, submetacentric, acrocentric and telocentric chromosomes.


The chromosomes in which the centromere is in the middle is called metacentric. In this case two arms of chromosomes have equal length and is in V shape.

The chromosomes in which the centromere is slightly above the middle point is called the submetacentric. Its one arm is slightly shorter and other arm is slightly longer. It is in the shape of during cell division.

The chromosomes in which the centromere is near the one end is called the

Acrocentric. Due to this position of centromere its one arm is very short and other is very long. It is in the shape of rod during cell division. The chromosomes in which the centromere is at the tip is called the telocentric chromosomes. Its arm are on the one side of the centromere and is also in rod shape during cell division.


*           Chromatid

The chromosomes in the living organisms duplicates itself before initiating the cell division. The two copies of the chromosomes remains attached itself at the centromere and are called the sister chromatids. During cell division the two sister chromatids separates itself and become independent chromatids. The number of chromosomes in human being is 46 of which 44 are autosomes and 2 are sex chromosomes in each body cell. The paired chromosomes is known as diploid and the cell containing diploid chromosomes are called diploid cell. The set of unpaired chromosomes is called haploid. The 46 chromosomesin each body cell represents diploid numbers and 23 chromosomes in sex cell represents haploid number.


*           Function of Nucleus

There are several functions of a nucleus. Some of the important function of nucleus are that it controls all the metabolic activities of the cell and regulate the cell cycle. It is also responsible for transmission of hereditary character from parents to their offspring.


*             Cytoplasm

The region between the nucleus and plasma membrane filled with the fluid is called cytoplasm. It consists of several cell organelles. The fluid material present in the cytoplasm is called cytosol. Cytosol contains protein fibres called cytoskeleton. The main function of cytoplasm is that it stores several vital materials such as amino acids, glucose, vitamins, minerals, etc. It is the site of several metabolic activities and is always in a state of movement,





         Which one of the following is prokaryotic?

(a) Blue green algae

(b) Fly

(c) Plasmodium

(d) Fungus

(e) None of these


Answer: (a)



       Which one of the following is absent in prokaryotic?

(a) Chromosomes

(b) Ribosomes

(c) Nucleolus

(d) Cytoplasm

(e) None of these


Answer: (c)



          Who discovered nucleus and in which year?

(a) Robert Hook (1831)

(b) Robert Hook in (1665)

(c) J. Purkinje (1839)

(d) Robert Brown (1831)

(e) None of these


Answer: (d)



         Which material is present in bulk in plasma membrane say 75%?

(a) Protein

(b) Phospholipids

(c) Cholesterol

(d) Polysaccharides

(e) None of these


Answer: (b)



          Which organism adopts the phagocytosis mode of feeding?

(a) Amoeba

(b) Fly

(c) Plasmodium

(d) Fungus

(e) None of these


Answer: (a)



*            Cell organelles

There are basically two types of cells, plant cell and animal cell that contain the cell organelles. There are many cell organelles which are present in plant cell and not in animal cells.



The various cell organelles are:


Endoplasmic Reticulum: It is a network of membrane bound tubules and sheets filled with lumen fluid. It is connected with outer membrane of nucleus and plasma membrane. It is found in three form cisternae, vesicles and tubles. Basically it is of two types rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum(SER). The ER is absent in RBC of mammals. The RER contains ribosomes on its surface which helps in protein synthesis and SER do not contains ribosomes.



  • It helps in packaging and distribution of material from one cell to another. The enzymes present in SER helps in synthesis of fats, steroids and cholesterol. SER also helps in detoxification of some of the poisonous substance in our body. It also synthesize steroid hormones such as estrogen, testosterone and cortisol. On the other hand RER produces digestive enzymes such as lysosomes. It also helps in the formation of Plasma membrane and other cellular membranes.
  • Ribosomes: These are the dense spherical and granular particles which occur freely in the cytosol. The major  constituents of ribosomes are RNA and proteins. It do not contains lipids. The main function of ribosomes is to synthesis protein.


Golgi Body: It is a sack of membrane bound vesicles usually stacked together in parallel rows. It is not founded in bacteria, blue green algae, mature sperm and RBC of humane. It arises from the membrane of the SER which in turn originates from the RER. The main function of Golgi body is to storing, packaging of various cellular materials and dispatch it from one cell to another. It is also involved in the synthesis of cell wall, plasma membrane and lysosomes.



Lysosomes: These are the tiny spherical sac like structure which is evenly distributed in the cytoplasm. It is bounded by a thin membrane that contains powerful enzymes, which are capable of digesting all organic materials. It is also called digestive bags as it helps in destroying the foreign material such as bacteria that enters into the cell and cause infection. It removes all the worn out cells from the body. During breakdown of cell structure lysosomes burst and release out the enzymes, which eats up their own cell and hence they are also called suicide bags.



Mitochondria: It is known as the power house of the cell. It is a small rod shaped structure found in the cell which is bound by a double membrane porous in nature. The inner layer is deeply folded creating a large surface area for ATP generating chemical reactions. It has its own DNA and ribosomes due to which it can make its own protein molecules. The inner part of mitochondria is filled with fluid matrix. It is not found in bacteria and RBC. It is also a site of cellular respiration and use atmospheric oxygen to oxidize carbohydrate and fat present in the cell into carbon dioxide and water vapour. Upon oxidation it releases energy in the form of ATP, which is used by the cell during cellular process.


  • Plastids: This organelles is found only in plant cell. It has its own DNA and ribosomes and can replicate itself.

It is of three types such as chloroplast, Chromoplast and Leucoplast. The chloroplast contains green colour pigments called chlorophyll, which helps in photosynthesis in plants. Each chloroplast is bound by double membrane which divides it into two distinct region Grana and Stroma. Grana is the region which contains the molecules of chlorophyll; whereas stroma is a homogenous matrix which contains grana. It also contains enzymes, starch, DNA and ribosomes. Grana is the site of light reaction and Stroma is the sister of dark reaction during photosynthesis. The main function of plastid is to trap solar energy and use it in manufacturing foods for the plants. The chromoplast imparts various colours to flowers to attract insect for pollination and leucoplast store food in the form of carbohydrates, fats and protein.


Vacuoles: It is a membrane bound structure which is filled with the fluid that helps in storage of water, glycogen and proteins. It is normally large and distinct in plant cell and small and temporary in animal cell. It occupies almost 90% of the volume of the cell. The membrane, which bounds the vacuoles is called tonoplast and fluid inside it is called cell sap. The main function of vacuoles is to maintain the somatic pressure in the cell and provide turgidity and rigidity to the cell.


Centrosomes: It is found only in animal cell and consists of two granules like centrioles, which are made up of microtubules. It helps in cell division in animals and forms spindles during cell division.


Difference between plant cell and Animal Cell  

Sl. No. Animal cell Plant cell
1. Cell wall is absent. Cell wall is present.
2. Vecuoles are small and temporary. Vacuoles are large and permanent.
3. Plastid is absent except for Euglena. Plastid is present.
4. Centrosomes is present Centrosomes is absent.
5. It is smaller is size. It is larger in size.





        Which one of the following is absent in plant cell?

(a) Nucleus

(b) Ribosomes

(c) Mitochondria

(d) Centrioles

(e) None of these


Answer: (d)



        Which one of the following is called the power house of the cell?

(a) Nucleus

(b) Ribosomes

(c) Mitochondria

(d) Centrioles

(e) None of these


Answer: (c)



        Which one of the following is called the suicide bag of the cell?

(a) Lysosomes

(b) Ribosomes

(c) Mitochondria

(d) Centrioies

(e) None of these


Answer: (a)



       Centriole is associated with which one of the following?

(a) Protein Synthesis

(b) DNA Formation

(c) Spindle Formation

(d) Fertilization

(e) None of these


Answer: (c)



       A plant cell placed in hypotonic solution will:

(a) Swell

(b) Shrink

(c) No change

(d) Exosmosis

(e) None of these


Answer: (a)

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