Class 9 New Science Book 2026 Ch 2 Cell : The Building Blocks Of Life (Notes, Solution)

Class 9 Science Ch 5 Fundamental unit of life-Cell (CONCEPT)

The discovery of the cell opened a new era in biological science. It helped scientists understand how living beings grow, repair, reproduce, and maintain themselves. The concept of the cell unites all living organisms under one common structure — showing that despite the diversity of life, all living things share a similar cellular foundation.

🔹 Discovery of the Cell

The first person to observe cells was Robert Hooke in 1665 when he examined thin slices of cork under a primitive microscope. He noticed small, box-like structures which he called “cells”, meaning “small rooms”. Later, Anton van Leeuwenhoek observed living cells for the first time using his handcrafted microscope.

The development of better microscopes over time allowed scientists to study cells in more detail, leading to the formation of the Cell Theory.

🔹 Cell Theory

The Cell Theory was proposed by Schleiden (a German botanist) and Schwann (a German zoologist) in 1838–1839. It was later modified by Rudolf Virchow in 1855. The main points of Cell Theory are:

1. All living organisms are made up of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. All new cells arise from pre-existing cells.

This theory forms the foundation of modern biology and explains the continuity of life.

🔹 Structure of the Cell

A typical cell consists of three main parts:

1. Cell Membrane (Plasma Membrane) – It is a thin, flexible boundary that separates the cell from its surroundings. It controls the movement of substances in and out of the cell, hence called selectively permeable.
2. Cytoplasm – It is a jelly-like substance that fills the cell and contains various organelles. The cytoplasm provides space for chemical reactions to occur and keeps the cell components in place.
3. Nucleus – It is the control center of the cell that contains genetic material (DNA). The nucleus directs all the activities of the cell such as growth, reproduction, and metabolism.

🔹 Cell Organelles and Their Functions

Each organelle within the cell performs a specific function necessary for the survival of the cell. Some important organelles are:

• Mitochondria – Known as the powerhouse of the cell, it releases energy from food during respiration.
• Endoplasmic Reticulum (ER) – It helps in the transport of materials; the rough ER is involved in protein synthesis, while the smooth ER synthesizes lipids.
• Golgi Apparatus – It modifies, sorts, and packages proteins and lipids for secretion.
• Lysosomes – Known as the suicidal bags of the cell, they contain digestive enzymes that break down waste materials and dead cell parts.
• Plastids (in plant cells) – They contain pigments; chloroplasts perform photosynthesis.
• Vacuoles – These store water, food, and waste materials. Plant cells have large central vacuoles.

🔹 Types of Cells

1. Prokaryotic Cells – These are simple, primitive cells without a well-defined nucleus or membrane-bound organelles (e.g., bacteria).
2. Eukaryotic Cells – These have a well-organized nucleus and membrane-bound organelles (e.g., plant and animal cells).

🔹 Differences Between Plant and Animal Cells

🔹 Concept Summary

The cell is the smallest structural and functional unit of life that performs all vital activities like respiration, growth, and reproduction. Cells form tissues, tissues form organs, and organs form complete organisms. Understanding the cell helps us understand the functioning of the entire living system.

The study of cells is known as Cytology, and it continues to be one of the most fascinating and essential branches of biology.

Class 9 Science Ch 5: Fundamental unit of life-Cell-New Book (CBSE UPDATED 2026)

1.Differentiate between the following pairs of terms based on the clues
given in parentheses:
(i) Cell membrane and cell wall (permeability)
(ii) RER and SER (structure)
(iii) Chloroplasts and chromoplasts (pigments)

2.Two similar animal cells are placed in two different solutions:
Cell X is placed in pure water.
Cell Y is placed in a concentrated salt solution

Cells are observed after some time. Cell X swells, and Cell Y shrinks.Which statement provides the correct explanation for the above observations?
(i) Salt molecules moved into Cell Y, causing it to shrink.
(ii) Water moved into Cell X and more water moved out of Cell Y than the salt solution entered in it.
(iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.
(iv) Solute movement caused osmosis in both cells.

Answer:

Cell X is placed in pure water.
Cell Y is placed in a concentrated salt solution.

Cells are observed after some time. Cell X swells, and Cell Y shrinks.

The correct statement is:

(iii) Water moved into Cell X and moved out of Cell Y.

Explanation:

• In pure water, water enters Cell X through osmosis, causing the cell to swell.
• In concentrated salt solution, water moves out of Cell Y through osmosis, causing the cell to shrink.

3. Look at the diagram of a cell in Fig. 2.20. Identify the parts labelled from
   (a) to (g) and correctly match them with their functions given below:
   (i) Controlling all the activities of a cell.
   (ii) Site of cellular respiration.
   (iii) Storage organelle that also provides rigidity to the cell.
   (iv) Separates the cell contents from surroundings.
   (v) Provides structural rigidity to the cell.
   (vi) Packs and stores materials received from ER.
   (vii) Helps in manufacturing food.

Answer:

Question 4

Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories?

Answer & Explanation

• Correct Option:(i) * Reasoning: * Leucoplasts are a type of plastid found specifically in plant cells (used for storing starch, lipids, or proteins).
  • Cell walls are present in plant cells but completely absent in animal cells.
  • The other options contain organelles like mitochondria, ribosomes, Golgi apparatus, and endoplasmic reticulum, which are present in both plant and animal cells.

Question 5

Two students, Renu and Rohit, were having a discussion on the plastids. Renu emphasised that all parts of the plants, even roots, contain plastids. However, Rohit did not agree with the statement and told her that plastids are absent in plant roots since the roots are underground and do not need to perform photosynthesis. Who is correct? Justify your answer.

Answer and Justification

Renu is correct.

Justification:

Storage Function: Leucoplasts specialize in storing nutrients. For example, amyloplasts (a type of leucoplast) store starch in root cells (like carrots or sweet potatoes), while others store oils and proteins necessary for the plant’s survival.

Plastids are not just for photosynthesis: While Rohit is right that underground roots do not perform photosynthesis, he is mistaken in assuming that photosynthesis is the only function of plastids.

Presence of Leucoplasts: Plant roots contain a specific type of non-pigmented plastid called leucoplasts. Since roots are underground and away from sunlight, they develop these colorless plastids instead of green chloroplasts.

6. Mitochondria and chloroplasts are two important organelles in a plant cell. Discuss how these two organelles are structurally and functionally similar to each other, and different from each other.

Mitochondria and chloroplasts are important cell organelles found in plant cells. They have some similarities as well as differences in their structure and functions.

Similarities between mitochondria and chloroplasts

1. Both are double membrane-bound organelles.
2. Both contain their own DNA and ribosomes.
3. Both are semi-autonomous organelles and can replicate independently.
4. Both are involved in energy conversion processes.
5. Both can synthesize some of their own proteins.

Differences between mitochondria and chloroplasts

7. Which of the following pairs of cell organelles contains DNA?

(i) Chloroplasts, Ribosomes
(ii) Mitochondria, Nucleus
(iii) Golgi bodies, Ribosomes
(iv) Nucleus, Lysosomes

Answer: (ii) Mitochondria, Nucleus

Explanation:

The nucleus contains the genetic material of the cell in the form of DNA. Mitochondria also contain their own DNA. Ribosomes, Golgi bodies, and lysosomes do not contain DNA.

Question 8

Experimental Setup: Two beakers are placed side-by-side:

• Beaker A: Labeled “A”, contains a carrot submerged in plain water.
• Beaker B: Labeled “B”, contains a carrot submerged in a concentrated salt solution. (Caption: Fig. 2.21: Experimental set-up having carrot (a) in plain water, and (b) in salt solution)

A researcher carried out an experiment in which she took two carrots of similar size. She placed one carrot in plain water and the other carrot in concentrated salt solution (Fig. 2.21). After 24 hours she recorded her observations.

• (i) What hypothesis does she want to test through this experiment?
• (ii) What would you suggest for the improvement of this experiment?
• (iii) Why does the carrot in plain water stay stiff and crunchy, but the carrot in concentrated salt solution become rubbery and limp?

Question 9

Indicate the presence or absence of following structures in bacterial and animal cells:

Ans:

10.Carry out the following experiment:
Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Place each of the potato cups in a beaker containing water (Fig. 2.22). Now, set up the experiment as follows: Fig. 2.22: Experimental set-up

(a) Keep Cup A empty.
(b) Add one teaspoon sugar in Cup B.
(c) Add one teaspoon salt in Cup C.
(d) Add one teaspoon sugar in the boiled potato in Cup D.
Observe the four potato cups at least two hours and answer the following questions:
(i) Explain why water gathers in the hollowed portion of Cup B and Cup C.
(ii) Why is Cup A necessary for this experiment?
(iii) Explain why water does not gather in the hollowed portions of Cups A and D.

(i) Explain why water gathers in the hollowed portion of Cup B and Cup C.

• Answer: Water gathers due to the process of osmosis (specifically, endosmosis).
• Explanation: The cells of the raw potato act as a semi-permeable membrane. Putting sugar in Cup B and salt in Cup C creates a highly concentrated solution (hypertonic solution) inside the hollowed cups compared to the plain water outside in the beaker. Water molecules naturally move from a region of higher water concentration (the beaker) to a region of lower water concentration (inside the potato cup) through the living potato cell walls.

(ii) Why is Cup A necessary for this experiment?

• Answer: Cup A serves as a control experiment.
• Explanation: Since Cup A is kept raw but completely empty, it provides a base for comparison. It demonstrates that water does not move into the potato cup purely on its own without a concentration gradient. It proves that the presence of a solute (like sugar or salt) is absolutely necessary for osmosis to happen.

(iii) Explain why water does not gather in the hollowed portions of Cups A and D.

• In Cup A: No water gathers because there is no solute (sugar or salt) placed inside the cavity. Without a concentration difference between the inside of the cup and the outside beaker, there is no driving force for osmosis to take place.
• In Cup D: No water gathers because the potato was boiled. Boiling kills the cells and destroys their cell membranes, completely stripping away their semi-permeable nature. Since a dead membrane cannot regulate or support active osmosis, no water can move into the cup even though sugar is present.

11.Identify the pair that incorrectly matches the cell organelle with its function.
(i) Ribosome — Protein synthesis
(ii) SER — Lipid and cellulose synthesis
(iii) Lysosome — Digestion of foreign agents

Answer

The incorrect pair is (ii) SER — Lipid and cellulose synthesis.

Why?

• SER (Smooth Endoplasmic Reticulum) synthesizes lipids and fats, but it is not involved in making cellulose.
• Cellulose is synthesized by enzymes at the cell’s plasma membrane.

Question 12

What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell?

Answer

If all mitochondria are removed, the cell will fail to generate sufficient energy and will eventually die.

Key Outcomes:

• No ATP Production: Mitochondria are the “powerhouses of the cell.” Without them, the cell cannot perform cellular respiration to produce ATP (the cell’s energy currency).
• Cellular Functions Stop: Lacking energy, crucial processes like protein synthesis, transport of materials, and cell division will completely shut down.
• Metabolic Crisis: The cell may briefly try to produce energy anaerobically (without oxygen) through glycolysis in the cytoplasm, but this produces too little energy to sustain a eukaryotic cell for long.

Question 13

Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain.

Answer

1. Phenomenon that inhibits tumors in humans:

The primary phenomenon is Apoptosis (programmed cell death).

When a cell becomes damaged, mutated, or old, the body triggers it to self-destruct so it cannot divide uncontrollably and form a tumor. Additionally, cell cycle checkpoints and the immune system help detect and destroy these abnormal cells.

2. Can plants develop tumors?

Yes, plants can also develop tumors. * How it happens: Plant tumors are commonly called galls. They are typically caused by bacteria (such as Agrobacterium tumefaciens, which causes Crown Gall disease), fungi, viruses, or insect bites. These pathogens insert their DNA into the plant cells, forcing them to produce excess hormones that lead to uncontrolled cell division.

• Why they are less dangerous than animal tumors: Unlike animals, plant cells have rigid cell walls that lock them in place. This prevents the tumor cells from breaking away and migrating to other parts of the body (metastasis). Therefore, plant tumors rarely kill the plant.

14.The cell membrane of a cell is made up of proteins and lipids. Which cell organelles help in the synthesis of cell membrane? Write the path of these compounds from their site of synthesis to the cell membrane and show this through a labelled diagram.

(a) Cell Organelles Involved:

The cell membrane is synthesized through a process called membrane biogenesis. The two primary organelles responsible are:

• Rough Endoplasmic Reticulum (RER): Synthesizes the proteins.
• Smooth Endoplasmic Reticulum (SER): Synthesizes the lipids.

(b) Path to the Cell Membrane:

1. Synthesis: Proteins are made by ribosomes on the RER, and lipids are made by the SER.
2. Transport to Golgi: These compounds are packed into transport vesicles and sent to the Golgi apparatus.
3. Modification & Packaging: The Golgi apparatus modifies, sorts, and packages these proteins and lipids into secretory vesicles.
4. Delivery: The vesicles move toward the cell surface and fuse with the Plasma Membrane, incorporating the new lipids and proteins into it.

15.What would happen if gametes are formed by mitotic divisions?

If gametes (sperm and egg cells) were formed by mitotic division (mitosis) instead of meiotic division (meiosis), the following would happen:

• Doubling of Chromosomes: Mitosis retains the exact same number of chromosomes. If normal diploid (2n) parents produced diploid (2n) gametes, the resulting offspring after fertilization would have a tetraploid (4n) number of chromosomes.
• Generation Escalation: With every successive generation, the chromosome number would continue to double (2n \rightarrow 4n \rightarrow 8n \rightarrow 16n).
• Genetic Chaos and Death: Such a massive inflation of DNA would disrupt the cell’s delicate genetic balance, leading to severe abnormalities, rendering reproduction impossible, and ultimately causing the extinction of the species.

16.A farmer, Deepa, was very happy with the harvest of amla (Indian Gooseberry) and lemons on her farm. However, she could sell only one fourth of the produce in the local market. Recognising that a significant amount of produce may be lost post-harvest, she employed a traditional yet scientifically sound method to extend the shelf life of amla and lemons. She turned perishable produce into profitable products, such as pickles and sharbat. She used the excess produce to prepare pickles, murabbas, and sharbat by adding appropriate amounts of salt, sugar, or jaggery to small pieces of fruit and their juices. These were then stored in small glass bottles for sale, helping her prevent the wastage of post-harvest produce. This shift from farming to agro-processing would strengthen food security and boost the local economy, creating a sustainable model that cuts waste while increasing her income. Based on the above passage answer the following questions:
(i) Which scientific concept has the farmer applied in the preservation of the farm produce?
(ii) How does the addition of high concentrations of salt and sugar create an environment that prevents the growth of spoilage-causing bacteria and fungi?
(iii) Suggest a healthy recipe of this kind for food preservation.
(iv) What are the scientific values addressed in this case?

Answer:

(i) Which scientific concept has the farmer applied?

The concept of osmosis (specifically exosmosis), which reduces moisture levels to preserve food.

(ii) How do high concentrations of salt and sugar prevent microbial growth?

• Hypertonic Environment: Salt and sugar create a highly concentrated environment outside the microbes.
• Plasmolysis: Water is drawn out of the bacterial or fungal cells via osmosis. This causes the microbial cells to shrink (plasmolyze), disabling their growth and killing them.

(iii) Suggest a healthy recipe of this kind.

Honey/Jaggery Amla Murabba:

• Steam fresh amla pieces and prick them.
• Submerge them completely in pure raw honey or a concentrated jaggery syrup in a clean glass jar.
• The high natural sugar concentration preserves the fruit beautifully without any chemicals.

(iv) What are the scientific values addressed?

• Resource Optimization: Smartly reducing post-harvest food waste.
• Practical Application: Using textbook science (osmosis) to solve a real-world economic problem.
• Sustainability: Creating a durable food model that benefits the local economy.

Math:

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• Class 10 Science – NCERT Solutions
• Class 9 Science – NCERT Solutions
• Class 8 Science – Oxford Solutions
• Class 7 Science – Oxford Solutions
• Class 6 Science – Oxford Solutions

NEET BIOLOGY

• Evolution
• Breathing and Exchange of Gases
• Anatomy of Flowering Plants
• Body Fluids and Circulation
• Human Health and Disease
• Microbes in Human Welfare
• Cell Cycle and Cell Division
• Biotechnology and Its Applications
• Biodiversity and Conservation
• Morphology of Flowering Plants

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