CBSE Class 9 Science The Fundamental Unit of Life Notes

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Study Material for Class 9 Science Chapter 5 The Fundamental Unit of Life

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Class 9 Science Chapter 5 The Fundamental Unit of Life

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CELL

Cell is called the fundamental unit of life.

A cell is capable of independent existence and can carry out all the functions which are necessary for a living being. A cell carries out nutrition, respiration, excretion, transportation and reproduction; the way an individual organism does. Unicellular organisms are capable of independent existence which shows a cell’s capability to exist independently. Due to this, a cell is called the fundamental and structural unit of life. All living beings are composed of the basic unit of life, i.e. cell.

CELL THEORY (Schleiden, Schwann and Virchow):

• All living organisms are composed of one or more cells.

• The cell is the basic unit of structure, function, and organization in all organisms.

• All cells come from preexisting, living cells.

CELL SIZE, SHAPE AND NUMBER

There is much variation in size, shape and number of cells in different organisms, and also in various parts of the body. Most of the cells are only a few micrometres in diameter and are visible only with the help of a microscope. Cells may be spherical, spindle shaped, elongated, polyhedral or irregular in shape. The shape of the cells is determined by the specific function they perform. The number of cells is related to the size of the organ or body. Thus, small organisms have limited number of cells, while the larger ones such as elephant, whale or banyan tree have a countless number of cells. Some organisms can also have cells of different kinds. Look at the following picture. It depicts some cells from the human body.

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INTEXT QUESTIONS PAGE NO. 59
Q1. Who discovered cells, and how?
Answer: Cells were discovered in 1665 by an English Botanist, Robert Hooke. He used a primitive microscope to observe cells in a cork slice.
 
Q2. Why is the cell called the structural and functional unit of life?
 
Answer: Cells constitute various components of plants and animals. A cell is the smallest unit of life and is capable of all living functions. Cells are the building blocks of life. This is the reason why cells are referred to as the basic structural and functional units of life. All cells vary in their shape, size, and activity they perform. In fact, the shape and size of the cell is related to the specific functions they perform.
 
STRUCTURE OF CELL
PROTOPLASM
A cell is made of life giving substance called protoplasm. The protoplasm is a highly organised jelly like, viscous, semifluid, composed of molecules of various chemicals. Most of these are organic molecules such as proteins, carbohydrates, fats, nucleic acid etc. Protoplasm is commonly called the ‘physical basis of life’.
A plant cell consists of a cell wall and protoplast. Cell wall is absent in animal cells. Protoplast denotes the whole of protoplasm present in a cell. It is differentiated into plasma membrane, nucleus and cytoplasm.
 
PLASMA MEMBRANE
Plasma membrane is a semi-permeable membrane. It is composed of bilayer of lipid and protein. This is the outermost covering of the cell that separates the contents of the cell from its external environment. The plasma membrane allows or permits the entry and exit of some materials in and out of the cell. It also prevents movement of some other materials. The cell membrane, therefore, is called a selectively permeable membrane.
CBSE Class 9 Science The Fundamental Unit of Life Notes

Functions of Plasma Membrane
• Plasma membrane selectively regulates the entry and exit of the substances into and out of the cell. Therefore, it is called a selectively permeable membrane or semipermeable membrane.
• It provides an outer boundary to the cell and protects the cell from injury.
• It allows the flow of materials and information between different organelles of the same cell, as well as between the adjacent cells.
• It provides some organic connections between the adjacent cells.
 
INTEXT QUESTIONS PAGE NO. 61
Q1. How do substances like CO2 and water move in and out of the cell? Discuss.
Answer: The cell membrane is selectively permeable and regulates themovement of substances in and out of the cell.
Movement of CO2: CO2 is produced during cellular respiration. Therefore, it is present in high concentrations inside the cell. This CO2 must be excreted out of the cell. In the cell’s external environment, the concentration of CO2 is low as compared to that inside the cell.
Therefore, according to the principle of diffusion, CO2 moves from a region of higher concentration (inside the cell) towards a region of lower concentration (outside the cell).
Similarly, O2 enters the cell by the process of diffusion when the concentration of O2 inside the cell is low as compared to its surroundings.
Movement of water: Water moves from a region of high concentration to a region of low concentration through the plasma membrane. The plasma membrane acts as a semi-permeable membrane, and this movement of water is known as osmosis. However, the movement of water across the plasma membrane of the cell is affected by the amount of substance dissolved in water.
 
Q2. Why is the plasma membrane called a selectively permeable membrane?
Answer: The cell membraneor the plasma membraneis known as a selectively permeable membrane because it regulates the movement of substances in and out of the cell. This means that the plasma membrane allows the entry of only some substances and prevents the movement of some other materials.
 
CELL WALL
Cell wall is made of cellulose. Cell wall is present only in plant cells. It is a rigid protective covering outside the plasma membrane. Presence of cell wall in plant cells distinguishes them from animal cells. Most of the plant cell walls are made of cellulose.
CBSE Class 9 Science The Fundamental Unit of Life Notes
The cell wall consists of three layers namely, middle lamella, primary wall and secondary wall.
The middle lamella is a thin amorphous cement like layer between two adjacent cells. Primary wall is the first formed wall of the cell and is produced inner to the middle lamella. The secondary wall is a thick layer found inner to the primary wall.
 
Functions of Cell Wall:
• Cell wall gives a definite shape to the plant cells.
• It provides mechanical strength to the cell.
• It protects the protoplasm against injury.
• It gives rigidity to the cell.
 
CYTOPLASM
A cell is enclosed in a membranous casing and is filled with a liquid substance which is called the cytoplasm. There are many cell organelles in a typical cell. Some of the main structures of a cell are as follows: The cytoplasm is the fluid content inside the plasma membrane. It also contains many specialised cell organelles. Each of these organelles performs a specific function for the cell.
 
Functions of Cytoplasm
• Cytoplasm helps in intracellular distribution of enzymes, nutrients and other biomolecules within the cell.
• Synthesis of different types of biomolecules such as proteins, nucleotides, fatty acids etc.,takes place in the cytoplasm.
 CBSE Class 9 Science The Fundamental Unit of Life Notes
NUCLEUS
Nucleus is the major central structure in the cell. It is a dense spherical structure embedded in the cytoplasm. Nucleus has a double membraned envelope called nuclear envelope. Nuclear envelope encloses a ground substance called nucleoplasm or karyolymph. The nuclear envelope possesses many pores called nuclear pores.
The nucleoplasm has two types of nuclear structures i) the nucleolus and, ii) chromatin.
 CBSE Class 9 Science The Fundamental Unit of Life Notes
The nucleolus is a spherical body rich in protein and RNA. It is the site of ribosome formation.
There may be one or more nucleoli in the nucleoplasm. The chromatin is a network of fine threads composed of genetic material DNA (Deoxyribo nucleic acid) and proteins. During cell division chromatin is condensed into thick cord like structures called Chromosomes. The chromosomes contain genes and each gene is responsible for one hereditary character of the organism. Genes contain information for inheritance of features from parents to next generation in the form of DNA molecule.
Functions of Nucleus:
i) Nucleus controls all the metabolic activities of the cell.
ii) It controls the inheritance of characters from parents to off-springs.
iii) It controls cell division.
 
PROKARYOTES AND EUKARYOTES
Based on the complexity of organization, especially nuclear organization, the cells are classified into two types.
i) Prokaryotic cells.
ii) Eukaryotic cells.
 
PROKARYOTIC CELLS
The cells of Bacteria and Cyano Bacteria (blue green algae) lack a well organised nucleus and are called prokaryotic cells. Their DNA (Deoxyribo Nucleic Acid) is not enclosed by a nuclear membrane. They also lack membrane bound organelles. The organisms which possess prokaryotic cells are called prokaryotic organisms or prokaryotes. They are considered to be primitive organisms.
CBSE Class 9 Science The Fundamental Unit of Life Notes

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Chapter 01 Matter in Our Surroundings
CBSE Class 9 Science Matter in our sorroundings Notes
CBSE Class 9 Science Matter in our sorroundings Sure Shot Questions A
CBSE Class 9 Science Matter in our sorroundings Sure Shot Questions B
CBSE Class 9 Science Matter in our sorroundings Sure Shot Questions C
CBSE Class 9 Science Matter in our Surrounding Exam Notes
Chapter 02 Is Matter Around Us Pure
CBSE Class 9 Science Is Matter Around us Pure Exam Notes
CBSE Class 9 Science Is matter around us pure Notes
CBSE Class 9 Science Is matter around us pure Sure Shot Questions A
CBSE Class 9 Science Is matter around us pure Sure Shot Questions B
CBSE Class 9 Science Is matter around us pure Sure Shot Questions C
Chapter 03 Atoms and Molecules
CBSE Class 9 Science Atoms and Molecules Exam Notes
CBSE Class 9 Science Atoms and Molecules Notes
CBSE Class 9 Science Atoms and Molecules Sure Shot Questions A
CBSE Class 9 Science Atoms and Molecules Sure Shot Questions B
CBSE Class 9 Science Atoms and Molecules Sure Shot Questions C
Chapter 04 Structure of the Atom
CBSE Class 9 Science Structure of atom Sure Shot Questions A
CBSE Class 9 Science Structure of atom Sure Shot Questions B
CBSE Class 9 Science Structure of atom Sure Shot Questions C
CBSE Class 9 Science Structure of Atoms Exam Notes
CBSE Class 9 Science Structure of atoms Notes
Chapter 05 The Fundamental Unit of Life
CBSE Class 9 Science The Fundamental Unit of Life Notes
CBSE Class 9 Science The Fundamental Unit of Life Sure Shot Questions
CBSE Class 9 Science The Fundamental Unit of Life Sure Shot Questions B
CBSE Class 9 Science The Fundamental Unit of Life Sure Shot Questions C
Chapter 06 Tissues
CBSE Class 9 Science Tissue Exam Notes
CBSE Class 9 Science Tissues Notes
CBSE Class 9 Science Tissues Sure Shot Questions A
CBSE Class 9 Science Tissues Sure Shot Questions B
CBSE Class 9 Science Tissues Sure Shot Questions C
Chapter 07 Diversity In Living Organisms
CBSE Class 9 Science Diversity in living Organism Exam Notes
CBSE Class 9 Science Diversity in Living Organisms Notes
CBSE Class 9 Science Diversity in Organisms Sure Shot Questions A
CBSE Class 9 Science Diversity in Organisms Sure Shot Questions B
CBSE Class 9 Science Diversity in Organisms Sure Shot Questions C
Chapter 08 Motion
CBSE Class 9 Science Motion Exam Notes
CBSE Class 9 Science Motion Notes
CBSE Class 9 Science Motion Sure Shot Questions
Chapter 09 Force and Laws of Motion
CBSE Class 9 Science Force and laws of motion Exam Notes
CBSE Class 9 Science Force and laws of motion Notes
CBSE Class 9 Science Force and laws of motion Sure Shot Questions
Chapter 10 Gravitation
CBSE Class 9 Science Gravitation Exam Notes
CBSE Class 9 Science Gravitation Notes
CBSE Class 9 Science Gravitation Sure Shot Questions
Chapter 11 Work and Energy
CBSE Class 9 Science Work and Energy Exam Notes
CBSE Class 9 Science Work and energy Notes
Chapter 12 Sound
CBSE Class 9 Science Sound Exam Notes
CBSE Class 9 Science Sound Notes
Chapter 13 Why Do We Fall Ill
CBSE Class 9 Science Why do we fall ill Notes
CBSE Class 9 Science Why do we fall ill Sure Shot Questions A
CBSE Class 9 Science Why do we fall ill Sure Shot Questions B
CBSE Class 9 Science Why do we fall ill Sure Shot Questions C
Chapter 14 Natural Resources
CBSE Class 9 Science Natural Resources Exam Notes
CBSE Class 9 Science Natural Resources Notes
CBSE Class 9 Science Natural Resources Sure Shot Questions A
CBSE Class 9 Science Natural Resources Sure Shot Questions B
CBSE Class 9 Science Natural Resources Sure Shot Questions C
CBSE Class 9 Science Natural Resources Sure Shot Questions D
Chapter 15 Improvement in Food Resources
CBSE Class 9 Science Improvement in Food Resources Exam Notes
CBSE Class 9 Science Improvement in food resources Notes
CBSE Class 9 Science Improvement in food resources Sure Shot Questions A
CBSE Class 9 Science Improvement in food resources Sure Shot Questions B
CBSE Class 9 Science Improvement in food resources Sure Shot Questions C
More Useful Resources for Class 9 Science
CBSE Class 9 Science Human Disease Exam Notes
CBSE Class 9 Science Practicals
CBSE Class 9 Science The Cell Exam Notes
Guidelines on Answering Questions in Science Exams
Information on Science Practicals in Class IX issued by CBSE

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CBSE Class 9 Science Chapter 5 The Fundamental Unit of Life Study Material

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