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

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NCERT Solutions PDF Class 9 PDF

This section provides detailed NCERT Solutions for Class 9 Science, Chapter 5, focusing on 'The Fundamental Unit of Life'. It addresses in-text questions that explore the historical discovery of cells by Robert Hooke and the fundamental reasons why cells are considered the basic structural and functional units of all living organisms. The solutions explain the selective permeability of the plasma membrane and how substances like carbon dioxide and water move across it via diffusion and osmosis, respectively. Understanding these concepts is crucial for grasping cell biology. These solutions offer clear explanations and step-by-step reasoning, aiding students in revising the chapter effectively for their exams.

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Quick info

BoardCBSE
ClassClass 9
SubjectScience
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 5

Chapter summary

Chapter 5, 'The Fundamental Unit of Life,' delves into the basic building blocks of living organisms. This NCERT Solutions set covers in-text questions that introduce the discovery of the cell and explain its significance as the fundamental structural and functional unit. It also details the mechanisms of substance transport across the selectively permeable plasma membrane, including diffusion for gases like CO2 and osmosis for water. These solutions are designed to clarify core concepts for Class 9 students.

Learning outcomes

  • Understand the discovery of cells and the contributions of Robert Hooke.
  • Explain why the cell is considered the basic structural and functional unit of life.
  • Describe the process of diffusion and its role in the movement of gases like CO2 in and out of cells.
  • Explain the process of osmosis and its role in the movement of water across the cell membrane.
  • Define and explain the concept of a selectively permeable membrane.

Topics covered

Paper topics

  • Cell Discovery
  • Robert Hooke
  • Structural Unit of Life
  • Functional Unit of Life
  • Plasma Membrane
  • Selectively Permeable Membrane
  • Diffusion
  • Osmosis
  • Movement of CO2
  • Movement of Water

Important topics

  • Role of the cell as structural and functional unit
  • Diffusion and its application in gas exchange
  • Osmosis and its role in water movement
  • Properties of the plasma membrane (selectively permeable)

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Questions and Solutions

Question 1

Who discovered cells and how?
Solution: Cells were first discovered in 1665 by Robert Hooke, an English botanist. He observed a thin slice of cork under a primitive microscope he had designed. In the cork slice, he noticed small, box-like compartments, which he named 'cells'.

Question 2

Why is the cell called the structural and functional unit of life?
Solution: The cell is referred to as the structural and functional unit of life for two main reasons:
  1. Structural Unit: All living organisms, whether unicellular or multicellular, are composed of cells. Cells are the fundamental building blocks that form the structure of any living being, much like bricks form a building.
  2. Functional Unit: Each cell is capable of carrying out all the essential processes of life, such as metabolism, respiration, reproduction, and response to stimuli. Even a single cell can perform all these functions independently. Therefore, it is also the basic functional unit. The specific shape and size of a cell are often related to the particular function it performs within an organism.

Question 1

How do substances like CO<sub>2</sub> and water move in and out of the cell? Discuss.
Solution: The movement of substances like carbon dioxide (CO<sub>2</sub>) and water into and out of the cell is regulated by the plasma membrane, which is selectively permeable. The processes involved are diffusion and osmosis.

Movement of CO<sub>2</sub>: Carbon dioxide is a waste product of cellular respiration and is usually present in a higher concentration inside the cell than outside. CO<sub>2</sub> moves from a region of higher concentration (inside the cell) to a region of lower concentration (outside the cell) through the process of diffusion. Similarly, oxygen (O<sub>2</sub>) enters the cell by diffusion when its concentration is lower inside the cell compared to the surroundings.

Movement of Water: Water moves across the plasma membrane from a region where its concentration is high to a region where its concentration is low. This movement of water through a semi-permeable membrane is called osmosis. The rate and direction of water movement depend on the concentration of solutes dissolved in the water both inside and outside the cell.

Question 2

Why is the plasma membrane called a selectively permeable membrane?
Solution: The plasma membrane is called selectively permeable because it does not allow all substances to pass through it indiscriminately. Instead, it regulates the entry and exit of materials into and out of the cell. It permits certain substances to cross its barrier while preventing others, thus controlling the cellular environment.

Common mistakes

  • Confusing diffusion and osmosis.
  • Not clearly articulating why the cell is both structurally and functionally fundamental.
  • Overlooking the role of concentration gradients in substance transport.

Revision tips

  • Review the historical context of cell discovery.
  • Focus on understanding the definitions and differences between diffusion and osmosis.
  • Visualize the movement of substances across the plasma membrane based on concentration.
  • Connect the structure of the plasma membrane to its function of selective permeability.

Practice MCQs

Q1. Who is credited with the discovery of cells in 1665?

Q2. Why are cells called the 'structural unit of life'?

Q3. The movement of CO2 out of the cell occurs primarily through:

Q4. What is the process by which water moves across the plasma membrane?

Q5. The plasma membrane is described as 'selectively permeable' because:

Frequently asked questions

Who discovered the cell and when?

Robert Hooke, an English botanist, discovered cells in the year 1665 by examining a slice of cork under a microscope.

Why is the cell considered the fundamental unit of life?

The cell is considered the fundamental unit because it is the smallest entity capable of performing all life functions and forms the basic structural component of all living organisms.

How does carbon dioxide (CO2) move in and out of a cell?

CO2 moves in and out of the cell through diffusion, moving from an area of higher concentration to an area of lower concentration across the cell membrane.

What is osmosis in the context of a cell?

Osmosis is the movement of water molecules across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.

What does it mean for the plasma membrane to be selectively permeable?

It means the plasma membrane allows certain substances to pass through it while restricting the passage of others, thereby controlling what enters and leaves the cell.

Important links

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