9th science - chapter 5 - The Fundamental Unit of Life

 

chapter 5 - The Fundamental Unit of Life

 

NCERT Textbook for Class 9 Science – Page 59

Question 1. Who discovered cells, and how?

Ans : Cells were first discovered by Robert Hooke in 1665.

Hooke was using a compound microscope to examine the structure of a thin slice of cork (the bark of a tree). Under the microscope, he saw a pattern of tiny, box-like structures. These reminded him of the small rooms, or “cells,” that monks lived in at monasteries, so he called them cells.

Question 2. Why the cell is called the structural and Junctional unit of life?

Ans :  
Structural Unit of Life:
All living organisms are made up of cells. Whether it’s a single-celled organism like bacteria or a multicellular one like humans, cells form the basic building blocks of the body. Tissues, organs, and entire systems are all formed by the organization and cooperation of cells.

Functional Unit of Life:
All the life processes — like respiration, digestion, reproduction, and excretion — happen inside cells. Cells can perform all essential functions to sustain life independently in unicellular organisms. In multicellular organisms, different types of cells perform specialized functions, but they all work together to keep the organism alive.

 

Class 9 Science NCERT Textbook – Page 61

Question 1. How do substances like C02 and water move in and out of the cell? Discuss.

Ans: Substances like CO₂ (carbon dioxide) and water move in and out of the cell through processes that don’t require energy, called passive transport.

 1. Carbon Dioxide (CO₂): Moves by diffusion.
CO₂ is a gas produced during respiration in cells. Since the concentration of CO₂ is usually higher inside the cell, it diffuses out through the cell membrane into the surrounding environment where its concentration is lower. No energy required.

2. Water: Moves by a special kind of diffusion called osmosis.
Osmosis is the movement of water molecules through a selectively permeable membrane from an area of high water concentration to an area of low water concentration. Water can move into or out of the cell depending on the concentration of solutes inside vs. outside the cell. Also doesn’t need energy.

Question 2. Why is the plasma membrane called a selectively permeable membrane?

Ans: The plasma membrane is called a selectively permeable membrane because it allows only certain substances to pass through it, while blocking others.

 

Class 9 Science NCERT Textbook – Page 63

Question 1. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.

Prokaryotic Cell

1. Size: Generally small (1–10 µm)
   (1 µm = 10⁻⁶ m)
2. Nuclear region is poorly defined due to absence of a nuclear membrane and known as nucleoid.
3. There is a single chromosome.
4. Membrane-bound cell organelles absent.

Eukaryotic Cell

1. Size: Generally large (5–100 µm)
2. Nuclear region well defined and surrounded by a nuclear membrane.
3. There are more than one chromosomes.
4. Membrane-bound cell organelles present

NCERT Textbook for Class 9 Science – Page 65

Question 1. Can you name the two organelles we have studied that contain their own genetic material?

Ans: The two organelles which have their own genetic material are:

1. Mitochondria 2. Plastids

 

Question 2. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?

Ans: If the organization of a cell is destroyed due to some physical or chemical influence, the cell will not be able to function properly, and eventually, it may die and lysosomes will digest it.

Question 3. Why are lysosomes known as suicide hags?

Ans : Lysosomes are known as "suicide bags" of the cell because they contain powerful digestive enzymes that can break down almost all types of biological molecules — proteins, lipids, carbohydrates, and even worn-out organelles.

 

Question 4. Where are proteins synthesised inside the cell?

Ans : The proteins are synthesised in the ribosomes that are also known as protein factories.

 

Questions From NCERT Textbook for Class 9 Science

Question 1. Make a comparison and write down ways in which plant cells are also different from animal cells.

Ans: 

🌱 Plant Cells vs. 🐾 Animal Cells

Feature	Plant Cell 🌿	Animal Cell 🐾
Cell Wall	Present (made of cellulose)	Absent
Shape	Usually fixed, rectangular or box-like	Usually round or irregular
Chloroplasts	Present (for photosynthesis)	Absent
Vacuole	One large central vacuole	Small or absent
Centrioles	Usually absent	Present
Mode of Nutrition	Autotrophic (makes its own food)	Heterotrophic (depends on others)
Lysosomes	Rare	Common
Plasma Membrane	Present (inside the cell wall)	Present (outermost layer)

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🧠 Summary:

• Plant cells have chloroplasts, cell walls, and a large vacuole — all essential for photosynthesis and structure.
• Animal cells lack these but have centrioles and more lysosomes for their active lifestyle.

Question 2. How is prokaryotic cell different from a eukaryotic cell?

Ans: 

🧬 Prokaryotic Cell vs. Eukaryotic Cell

Feature	Prokaryotic Cell 🦠	Eukaryotic Cell 🧫
Nucleus	Absent (DNA in a region called nucleoid)	Present, well-defined with nuclear membrane
Size	Small (1–10 µm)	Larger (5–100 µm)
Organelles	No membrane-bound organelles	Has membrane-bound organelles (e.g. mitochondria, ER)
Cell Wall	Present (usually made of peptidoglycan)	Present in plant cells (cellulose), absent in animal cells
DNA Structure	Single circular chromosome	Multiple linear chromosomes
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists
Division	Binary fission (simple)	Mitosis or meiosis (complex)

Question 3. What would happen if the plasma membrane ruptures or breaks down?

Ans: 
If the plasma membrane ruptures or breaks down, the cell would not survive.

 What happens when the plasma membrane breaks?

1. Loss of Barrier:
• The plasma membrane acts as a protective barrier.
• If it breaks, harmful substances from outside can freely enter, and important substances like proteins, ions, and water can leak out.
2. Disruption of Balance:
• The membrane controls what enters and exits, maintaining homeostasis (internal balance).
• Without it, the cell can't control its internal environment, leading to imbalance and damage.
3. Cell Contents Spill Out:
• Cell organelles and enzymes may leak out, stopping all vital cell functions.
4. Cell Death:
• Eventually, the cell will die because it can’t carry out basic life processes without its membrane intact.

 

Question 4. What would happen to the life of a cell if there was no Golgi apparatus?

Ans: If there were no Golgi apparatus in a cell, the cell would face serious problems in handling, modifying, and delivering important substances.

If there were no Golgi apparatus:

1. Proteins and lipids wouldn’t be properly modified
   → Some enzymes and hormones might not work as needed.
2. No proper packaging and transport
   → Substances wouldn’t reach where they’re supposed to go (e.g., digestive enzymes wouldn’t reach lysosomes).
3. Cell communication would fail
   → Hormones and neurotransmitters might not be sent outside the cell.
4. Waste handling and recycling would suffer
   → This could cause toxic buildup inside the cell.

Without the Golgi apparatus, the cell’s "delivery system" would collapse, causing chaos inside the cell, and eventually the cell may die.

 

Question 5. Which organelle is known as the powerhouse of the cell? Why?

Ans: The mitochondrion (plural: mitochondria) is known as the powerhouse of the cell. 

Because mitochondria are the organelles that:

• Generate energy in the form of ATP (Adenosine Triphosphate),
• Through a process called cellular respiration,
• By breaking down glucose and oxygen into energy the cell can use.

Just like a power station produces electricity for a city, mitochondria produce energy for the cell to carry out all its activities — like movement, division, repair, and more.

 

Question 6. Where do the lipids and proteins constituting the cell membrane get synthesised?

Ans: The lipids and proteins that make up the cell membrane are synthesized in different parts of the cell:

1. Lipids:

• Synthesized in the Smooth Endoplasmic Reticulum (Smooth ER)
• These lipids are essential for forming the phospholipid bilayer of the plasma membrane.

2. Proteins:

• Synthesized in the Rough Endoplasmic Reticulum (Rough ER)
• Rough ER has ribosomes on its surface, which help make membrane proteins.
• These proteins are then sent to the Golgi apparatus for modification and packaging before being added to the membrane.

Question 7. How does Amoeba obtain it’s food?

Ans : Amoeba obtains its food through a process called phagocytosis, which is a type of endocytosis (taking in substances by engulfing them).

1. Sensing Food:
• Amoeba senses food particles (like bacteria or small algae) in its surroundings.
2. Forming Pseudopodia:
• It extends its pseudopodia (finger-like projections of its body) toward the food.
3. Engulfing the Food:
• The pseudopodia surround the food particle and enclose it, forming a small sac called a food vacuole.
4. Digestion:
• Digestive enzymes are secreted into the vacuole to break down the food into simpler substances.
5. Absorption:
• The digested nutrients are absorbed into the cytoplasm and used for energy, growth, and repair.
6. Excretion:
• Undigested waste is expelled out of the cell.

Amoeba eats by surrounding and swallowing food using pseudopodia, then digesting it internally — just like a tiny, shapeshifting blob with a built-in stomach!

 

Question 8. What is osmosis?

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

 

Question 9. Carry out the following osmosis 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. Put each potato cup in a trough containing water.
Now,
(a) Keep cup A empty
(b) Put one teaspoon sugar in cup B
(c) Put one teaspoon salt in cup C ‘
(d) Put one teaspoon sugar in the boiled potato cup D
Keep these for two hours. Then observe the four potato cups and answer the following:
(i) Explain why water gathers in the hollowed portion of B and C.
(ii) Why is potato A necessary for this experiment?
(iii) Explain why water does not gather in the hollowed out portions of A and D.

Ans: 

Observations & Explanations:

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(i) Why does water gather in the hollowed portion of B and C?

• Cup B (sugar) and Cup C (salt) create a concentrated solution inside the potato.
• The cells of the potato are still alive and selectively permeable.
• Due to osmosis, water from the trough (where water concentration is higher) moves into the cups (where the solution concentration is higher and water concentration is lower).
• Result: Water collects in the hollows of B and C.

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

• Potato A acts as a control.
• It shows what happens when there is no solute (sugar/salt) present — i.e., it helps prove that the movement of water is due to the presence of solutes.
• Since no water enters A, it confirms that osmosis happens only when there's a difference in concentration.

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(iii) Why does water not gather in the hollowed portions of A and D?

• Cup A has no solute to create a concentration difference → no osmosis.
• Cup D is made from boiled potato. Boiling kills the cells and destroys the selectively permeable membrane.
• So even though there’s sugar in D, osmosis cannot occur because the membrane is no longer functional.