NCERT Class 9 Science Chapter 5 Notes - The Fundamental Unit of Life PDF Notes

NCERT Class 9 Science Chapter 5 Notes The Fundamental Unit of Life: The cell study is crucial in the understanding of the basic function and structure of living organisms. This chapter of NCERT discusses the theory that the cell is the basic unit of life. The chapter gives a thorough explanation of cell structure, types, functions, and how they play a role in the life processes of living organisms.

This Story also Contains

1. Cell
2. Cell Organelles and Their Functions
3. Cell Division
4. Important Topics of Class 9 Science Chapter 5: The Fundamental Unit of Life
5. Most Important Questions of Class 9 Science Chapter 5
6. Approach to solve questions of Class 9 Biology Chapter 5 The Fundamental Unit of Life
7. Chapter-Wise NCERT Class 9 Notes Science

CBSE exam aspirants can refer to these notes for rapid revision. They contain definitions, examples, and descriptions of important concepts like plasma membrane, nucleus, cytoplasm, and cell organelles. NCERT Notes for Class 9 Science and NCERT Solutions for Class 9 Science Chapter 5 prove to be beneficial in systematic study by providing concept clarity and increased retention.

Also, students can refer to:

• NCERT Solutions for Class 9 Science Chapter 5 The Fundamental Unit of Life

• NCERT exemplar class 9 Science chapter 5 solutions The Fundamental Unit of Life

Cell

A cell is the smallest functional and structural unit of life. The history of cells started when Robert Hooke examined a thin piece of cork using a microscope in 1665 and found tiny compartments that he referred to as 'cells.' He likened these compartments to a honeycomb and set the stage for future research in cell biology.

Cell Theory

The cell theory was developed through the work of various scientists:

• Matthias Schleiden (1838) asserted that all plants consist of cells.
• Theodor Schwann (1839) noted that all animals are made up of cells and suggested that cells constitute the fundamental unit of life.
• Rudolf Virchow (1855) established that new cells develop from pre-existing cells as a result of cell division.

This resulted in the development of the Cell Theory, which argues:

• All living organisms consist of cells.
• Cells are the structural and functional unit of living organisms.
• All cells come from existing cells.

The establishment of the cell theory was a significant breakthrough in the concept of the continuity of life.

Cell Shape, Size, and Number

The form of cells is different based on their function. For instance, nerve cells are elongated to carry signals, whereas Amoeba is irregularly shaped and constantly changing.

The cells vary in size from 0.5 to 20 micrometers in most organisms. RBCs are the smallest cells in the human body, whereas nerve cells are the longest.

Unicellular organisms are made up of one cell, while multicellular organisms contain many cells specialized for various functions.

Structure of a Cell

A cell consists of three major parts:

• Plasma Membrane (Cell Membrane)
• Cytoplasm
• Nucleus
• Plasma Membrane (Cell Membrane)

The outermost portion of the cell that isolates the interior of the cell from the outside world.

It is selectively permeable since it lets through some substances but not others.

Movement of molecules across the plasma membrane is by:

Diffusion – Movement of gases such as oxygen and carbon dioxide.

Osmosis – Movement of water across a selectively permeable membrane.

Cell Wall (Only in Plant Cells)

• An inelastic outer sheath present in plant cells, fungi, and bacteria.
• Mainly made of cellulose that gives strength to the structure as well as shields it.
• Puts a block on excess absorption of water and prevents the explosion of the cell when placed under hypotonic conditions.

Nucleus

• The central organizing point of the cell, with a nuclear envelope covering it.
• Has chromosomes consisting of DNA along with proteins.
• It is responsible for heredity and cell reproduction.
• The nucleolus inside the nucleus functions to synthesize ribosomes.

Cytoplasm

• A jelly-like substance that holds cell organelles and allows them to move.
• It is the location of most biochemical reactions that sustain cells.
• Organelles within the cytoplasm are specialized in function and are vital for cell metabolism.

Cell Organelles and Their Functions

The cell organelles and their functions are described as:

Endoplasmic Reticulum (ER)

Rough ER (RER): Ribosomes are attached, and it is involved in protein synthesis.

Smooth ER (SER): Does not have ribosomes, synthesizes lipids and detoxifies toxic substances.

Golgi Apparatus

• Alters, sorts, and packages proteins and lipids.
• Aids in the creation of lysosomes and movement of substances across the cell.

Lysosomes

• Have enzymes for digestion to destroy waste products, diseased organelles, and foreign materials.
• Also referred to as the suicidal bags of the cell because they can lead to self-destruction when the cell is harmed beyond repair.

Mitochondria

The energy-generating part of the cell, producing energy from cellular respiration.

Manufactures ATP (Adenosine Triphosphate), the cell's energy currency.

Has its own DNA and ribosomes, so it can reproduce itself.

Plastids (Only in Plant Cells)

Chloroplasts: Have chlorophyll, which is needed for photosynthesis.

Leucoplasts: Store starch, oils, and proteins.

Chromoplasts: Give color to flowers and fruits.

Vacuoles

• Sacs filled with fluid that store nutrients, waste products, and other materials.
• Larger in plant cells, assist in maintaining turgidity and cell shape.

Cell Division

Cells divide to provide for growth, repair, and reproduction. There are two primary types of cell division:

Mitosis

• Happens in body (somatic) cells.
• Creates two identical daughter cells.
• Has the same amount of chromosomes as the parent cell.
• Is vital for growth, repair, and asexual reproduction.

Meiosis

• Takes place in reproductive cells (gametes).
• Forms four daughter cells with half the amount of chromosomes.
• Results in genetic variation, which is critical for evolution.

Subject wise NCERT Solutions

• NCERT solutions for class 9 Maths

• NCERT solutions for class 9 Science

Important Topics of Class 9 Science Chapter 5: The Fundamental Unit of Life

Important quick revision points from this chapter are listed below:

1. Cell Theory

2. Types of Cells

3. Cell Organelles

4. Cell Membrane

5. Cell Division

Most Important Questions of Class 9 Science Chapter 5

To help you prepare effectively, here’s a list of the most important questions from this chapter. Revise these carefully for a thorough grasp of the topic.

Question 1: Mitochondria is used to produce food molecules. True or False

Answer:

Mitochondria are used by the cell to break down food molecules and release energy. Hence, the given statement is false.

Question 2: What are the structures performing different functions in the cell called?

Answer:

All the functions of the cell are assigned to different structures of the cell, such as mitochondria, endoplasmic reticulum, Golgi apparatus, etc. These are called organelles of the cell.

Question 3: What is the role of chromoplasts?

Answer:

The role of chromoplasts is to absorb sunlight and use it to trap the energy in the form of food molecules by the process of photosynthesis.

Question 4: Who first described the Golgi apparatus in the cell?

Answer:

The Golgi apparatus was first described in the cell by Camillo Golgi.

Question 5: What is the function of the vacuoles?

Answer:

Function of the vacuole:

• The primary function of the vacuole is to store excess food and secondary metabolites in plants.
• It is also used as an excretory organelle in some of the unicellular organisms.
• It is used to store food material in the Amoeba.

Subject Wise NCERT Exemplar Solutions

• NCERT Exemplar Class 9 Maths solutions

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Approach to solve questions of Class 9 Biology Chapter 5 The Fundamental Unit of Life

To solve questions from this chapter effectively, it's important for students to first understand the structure and function of various cell components. They can start by going through the Cell Theory, writing down all the important points of this can help to retain this theory better, and for a long time. A table needs to be prepared for all the differences between prokaryotic and eukaryotic cells, as they often come up in the exam. Repeated revision of the functions of different organelles like the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus is required. Go through the previous years' questions to get a hint of the question pattern, and it also increases the understanding of concepts.

Chapter-Wise NCERT Class 9 Notes Science

Below are the links to detailed notes for each chapter, designed to help you quickly revise and understand key concepts.