CBSE Class 9 Science The Cell Exam Notes

CBSE Class 9 Science The Cell Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

History

In 1665, Robert Hooke observed the cell in a cork slice with the help of a primitive microscope. He saw that cork resembled the structure of a honey comb consisting of many little compartments. Robert Hooke called these boxes cells. Cell is a Latin word for 'little room.'

Unicellular And Multicellular Organisms

All living organisms are made up of one or more cells.  On the basis of cell number, organism are grouped into 2 categories -

Unicellular organisms :

* Organism in which a single cell make the whole body. For example, Amoeba, Paramecium, bacteria & Chlamydomonas.           

Multicellular organism :

* Organism in which body contains more than one cell. For example - Plante, animals & fungi.

Cell Size :

* The size of different cells ranges between broad limits.

* Some plants and animals cells are visible to the  naked eye.

* Most cells are visible only with microscope

• The prokaryotic cells usually range between

1 to 10 mm.

• The eukaryotic cells usually range between 10 to 100 mm.

• Amoeba proteus may reach a diameter of 0.5 mm.

• The smallest cells are those of Mycoplasma laidlawiil (0.1µ in dimeter) or PPLO (pleuro pneumonia like organism).

• The largest cell is egg of an Ostrich.

- Cell Shape :

• The shape of cell may be variable or fixed.

• Variable shape occur in Amoeba, WBC etc.

• Fixed shape occur in most plant and animals.
• Cells may be diverse shapes such as polyhedral (8, 12 or 14 sides) spherical (e.g. eggs of mainly animals), spindle shaped (Smooth muscle fibres), elongated (e.g. Nerves cells) so on.

Ø ULTRA STRUCTURE OF CELLS

- Plasma Membrane :  

Introduction :  

Figure : A PLANT CELL AS SHOWN BY AN ELECTRON MICROSCOPE

• Cell surface in all the cells is enclosed by a living membrane which is called cell membrane by

1. Nageli and C. Kramer (1855).

Historical Account :

• J.Q. Plower (1931) coined the term Plasmalemma for cell membrane.

Ultrastructure :

• Plasma membrane forms outer covering of each cell.

• It is present in both plant and animal cells.

• Plasma membrane is a living, thin, delicate elastic, selectively permeable membrane.

• It separates contents of a cell from the surrounding medium.

- Fluid Mosaic Model :

• In 1972, Singer Nicolson proposed this model. According to this, cell membrane consists-two layers of phospholipid molecules, phospholipid & protein molecules are arranged as a mosaic.

• Phospholipid molecules have their polar heads directed outward non polar tail pointing inward.

• The proteins are of two types :

• Peripheral and integral. Peripheral proteins are located superficially while integral proteins are embeded in the phospholipid matrix. The protein monolayer have elasticity & mechanical support to the lipid matrix.
• 

Figure : STRUCTURAL DETAIL OF PLASMA MEMBRANE ACCORDING TO FLUID MOSAIC MODEL

- Functions of Plasma Membrane :

• The main function of plasma membrane is to regulate the movement of molecules inside and outside the cell.

• It allow the movement of gaseous substance from high concentration to low concentration which known as diffusion.

• Water also obeys the low of diffusion . The movement of water molecule through a selectively permeable membrane is called osmosis.

• The flexibility of cell membrane also enables the cell to engulf in food, which is also known as endocytosis. For example – in Amoeba

- Cell wall :

• In plants, one another rigid is called 'Cell wall'. It is made up of cellulose which provide structural strength to plant.

- Function of cell wall :

• It maintains the shape of cell.

• It protect the cells from mechanical injury & prevents their desication.

• It provide mechanical support against gravity. It is due to rigid cell walls that the aerial part of plant are able to keep erect & expose their leaves to sunlight.

• Cell walls permit the cells to with stand very dilute external media without bursting.

- Nucleus :

• It is the most important part of cell which control all the activities of cell.

- Structure of Nucleus :

• The nucleus has a double layered covering called nuclear membrane.

• The nuclear membrane has pores inside the nucleus to its outside, that is, to the cytoplasm.

• The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.

• Chromosomes are composed of DNA and protein.

• Functional segments of DNA are called genes.

• In a cell which is not dividing, this DNA is present as part of chromatin material.

- Function of nucleus :

• It play a important role in cellular reproduction.

• DNA contain the information necessary for constructing & organizing cells.

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