The Building block of Living Organisms: The Cell
1. Who discovered cells, and how?
Solution: In 1665, Robert Hooke discovered cells while examining a thin slice of cork through a self-designed microscope. He observed that the cork resembled the structure of a honeycomb consisting of numerous tiny compartments.
2. Why is the cell called the structural and functional unit of life?
Solution: Cells form the structure of an entity. They perform all the activities independently. Hence, cells are referred to as structural and fundamental units of life.
3. How do substances like CO2 and water move in and out of the cell? Discuss.
Solution: CO2 moves by diffusion. The cellular waste accumulates in high concentrations in the cell, whereas the concentration of CO2 in the external surroundings is comparatively lower. This difference in the concentration level inside and outside of the cell causes the CO2 to diffuse from a region of higher (within the cell) to a lower concentration.
H2O diffuses by osmosis through the cell membrane. It moves from a region of higher concentration to a lower concentrated region through a selectively permeable membrane until equilibrium is reached.
4. Why is the plasma membrane called a selectively permeable membrane?
Solution: The plasma membrane is called as a selectively permeable membrane as it permits the movement of only certain molecules in and out of the cells. Not all molecules are free to diffuse.
5. Fill in the gaps in the following table, illustrating the differences between prokaryotic and eukaryotic cells.
solution:
Prokaryotic Cell | Eukaryotic Cell |
1. Size: Generally small (1-10 μm) 1 μm = 10-6m 2. The nuclear region is poorly defined due to the absence of a nuclear membrane and is known as the nucleoid. 3. There is a single chromosome. 4. Membrane-bound cell organelles absent. | 1. Size: Generally large (5-100 μm) 2. Nuclear region: well-defined and surrounded by a nuclear membrane. 3. There is more than one chromosome. 4. Membrane-bound cell organelles present. |
6. Can you name the two organelles we have studied that contain their own genetic material?
Solution: The two organelles which have their own genetic material are
1. Mitochondria
2. Plastids
7. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?
Solution: In the event of any damage to cells and when the revival of cells is not possible, Lysosomes burst, and enzymes digest such cells. This is why lysosomes are often referred to as ‘suicide bags’.
8. Why are lysosomes known as suicide bags?
Solution: When there is damage to the cell and when revival is not possible, lysosomes may burst, and the enzymes digest their own cell. Consequently, lysosomes are known as suicide bags.
9. Where are proteins synthesised inside the cell?
Solution: Protein synthesis in cells takes place in ribosomes. Hence, ribosomes are also referred to as protein factories. Ribosomes are particles that are found attached to the rough endoplasmic reticulum.
1. Make a comparison and write down ways in which plant cells are different from animal cells.
Solution:
The following table depicts the differences between plant cells and animal cells.
Characteristic | Plant Cell | Animal Cell |
Cell Wall | Present | Absent |
Shape of Cell | With distinct edges, the shape is either rectangular or square-shaped. | Round and irregular shape |
Nucleus | Present. It lies on one side of the cell | Present. It lies in the centre of the cell |
Lysosomes | Rarely present | Always present |
Plastids | Present | Absent |
Structure of Vacuoles | Single or a few large vacuoles that are centrally located | Presence of numerous and small vacuoles |
2. How is a prokaryotic cell different from a eukaryotic cell?
Solution:
The following are the differences between prokaryotic and eukaryotic cells.
Feature | Prokaryotic Cell | Eukaryotic Cell |
Size | Generally small (1-10 μm) | Generally large (5-100 μm) |
Nuclear Region | Not well defined; nucleoid instead of nucleus | Well-defined; surrounded by a nuclear membrane |
Chromosomes | Single chromosome | More than one chromosome |
Membrane-bound Organelles | Absent | Present |
3. What would happen if the plasma membrane ruptures or breaks down?
Solution: If the plasma membrane ruptures or breaks down,
molecules of some substances will freely move in and
out of the cell.
As a result, essential protoplasmic material can leak
out or harmful substances can enter unchecked. This
disruption can lead to the death of the cell due to the
loss of its vital components and inability to function properly.
4. What would happen to the life of a cell if there
was no Golgi apparatus?
Solution: T
Without the Golgi apparatus, cells would be unable to
properly package and transport materials essential for
their functioning. The Golgi apparatus is involved in the
formation of lysosomes, which are essential for cellular
digestion and recycling. Therefore, the absence of a
functional Golgi apparatus would severely compromise
the cell’s ability to function and survive.
5. Which organelle is known as the powerhouse of
the cell? Why?
Solution: Mitochondria are referred to as the powerhouse
of the cell because they generate energy required for
various life activities. They achieve this by releasing
energy in the form of ATP (Adenosine triphosphate) molecules. ’.
6. Where do the lipids and proteins constituting the
cell membrane get synthesised?
Solution: Lipids and proteins are synthesised in the
ER (Endoplasmic Reticulum).
7. How does an Amoeba obtain its food?
Solution: Through the process of endocytosis, an Amoeba
obtains its food. The Amoeba’s flexible cell membrane
allows it to engulf food particles, forming a food vacuole
around them with the help of pseudopodia, which are
extensions of its cell membrane.
8. What is osmosis?
Solution: Osmosis is the process where water molecules
move across a semipermeable membrane from an area
of higher water concentration to an area of lower water
concentration.
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.
Solution: (i) Water accumulates in the hollowed
portions of B and C as a difference in the water
concentration is observed. Thereby, endosmosis
occurs as the cells act as a semipermeable membrane.
(ii) Potato A is essential in this experiment as it is
significant to compare different scenarios seen in
potato cups B, C and D. Potato A in this experiment
clearly shows that the potato cavity on its own cannot
bring about water movement.
(iii) Cup in A does not show any change in the water
flow concentration for osmosis to occur, which requires
concentration to be higher than the other. Cells in cup D
are dead; thus, there is no existence of a semipermeable
membrane for water flow. Consequently, osmosis
does not occur.
10. Which type of cell division is required for the growth and repair of the body, and which type is involved in the formation of gametes?
Solution: Mitosis: Mitosis is a type of cell division
involved in the growth, development, and repair
of the body.
Meiosis: Meiosis is a type of cell division that
occurs in specialized cells called germ cells,
which are involved in sexual reproduction.
Class 9 Science Chapter 5 Summary
Discovery of Cells:
Cells were first discovered by Robert Hooke in 1665 when he observed a thin slice of cork under a microscope.
Later, the development of better microscopes allowed scientists to explore cells more thoroughly, leading to the formulation of the Cell Theory.
Cell Theory:
Proposed by Schleiden and Schwann, this theory states:
All living organisms are composed of cells.
Cells are the basic structural and functional units of life.
New cells arise from pre-existing cells (added by Rudolf Virchow).
Structure of a Cell:
Cell Membrane (Plasma Membrane): A thin, flexible layer that surrounds the cell and controls the movement of substances in and out of the cell.
Cytoplasm: The jelly-like fluid inside the cell where various organelles are suspended and metabolic activities occur.
Nucleus: The control center of the cell, containing genetic material (DNA) that regulates cell growth, reproduction, and function.
Organelles and Their Functions:
Mitochondria: Known as the powerhouse of the cell, mitochondria are responsible for producing energy through cellular respiration.
Endoplasmic Reticulum (ER): A network of membranes involved in the synthesis of proteins (rough ER) and lipids (smooth ER).
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.
Lysosomes: Known as the cell’s waste disposal system, they contain enzymes that break down waste materials and cellular debris.
Differences Between Plant and Animal Cells:
Plant Cells:
Have a rigid cell wall made of cellulose, providing structural support.
Contain chloroplasts with chlorophyll, enabling photosynthesis.
Usually have a larger central vacuole that stores nutrients and waste products.
Animal Cells:
Lack a cell wall and chloroplasts.
Have smaller, more numerous vacuoles.
Often contain centrioles, involved in cell division.
Functions of Cells:
Cells perform various functions essential for life, such as metabolism, growth, reproduction, and response to environmental changes.
Different cells in multicellular organisms have specialized functions (e.g., nerve cells for transmitting signals, muscle cells for movement).
Cell Division:
Cells reproduce through processes like mitosis (for growth and repair) and meiosis (for the production of gametes).
Cell division is critical for growth, maintenance, and repair in organisms.
Importance of Studying Cells:
Understanding cells helps us grasp the fundamental processes of life, including how organisms grow, respond to their environment, and maintain homeostasis.
It also provides insights into health and disease, as many medical conditions result from abnormalities at the cellular level.
Class 9 Science Chapter 5 FAQs
Q1. What is the cell and why is it called the fundamental unit of life?
Ans. A cell is the smallest structural and functional unit of life. It is called the fundamental unit because all living organisms are made up of cells, and the cell is the basic unit that performs essential life processes.
Q2. Who discovered cells, and how were they discovered?
Ans. Cells were discovered by Robert Hooke in 1665 when he observed a thin slice of cork under a microscope and noted small, box-like structures, which he named "cells."
Q3. What is the cell theory, and who proposed it?
Ans. The cell theory, proposed by Schleiden and Schwann, states that all living organisms are made of cells, cells are the basic units of life, and all cells arise from pre-existing cells (Virchow added this later).
Q4. What are the main differences between plant cells and animal cells?
Ans. Plant cells have a cell wall, chloroplasts, and a large central vacuole, while animal cells lack a cell wall and chloroplasts but have smaller vacuoles and centrioles.
Q5. What is the function of the nucleus in a cell?
Ans. The nucleus acts as the control center of the cell, regulating growth, metabolism, and reproduction. It contains genetic material (DNA), which controls cell activities and passes hereditary information to offspring.
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Cell: The Building Block of Life – Questions & Answers: NEW NCERT
1. Differentiate between the following pairs of terms based on the clues given in parentheses:
(i) Cell membrane and cell wall (permeability)
| Basis | Cell Membrane | Cell Wall |
|---|---|---|
| Permeability | Selectively permeable | Freely permeable |
(ii) RER and SER (structure)
| Basis | RER | SER |
|---|---|---|
| Structure | Ribosomes present on surface | Ribosomes absent |
(iii) Chloroplasts and chromoplasts (pigments)
| Basis | Chloroplasts | Chromoplasts |
|---|---|---|
| Pigments | Contain chlorophyll | Contain carotene and xanthophyll pigments |
2. Two similar animal cells are placed
in different solutions.
Answer:
Correct option: (iii)
Water moved into Cell X and moved out of Cell Y through the cell membrane.
Cell X swells because water enters by osmosis.
Cell Y shrinks because water leaves the cell by osmosis.
3. Identify the labelled parts of a cell
and match them with functions.
| Label | Cell Part | Function |
|---|---|---|
| (a) | Nucleus | Controls all activities of the cell |
| (b) | Mitochondria | Site of cellular respiration |
| (c) | Vacuole | Storage and rigidity |
| (d) | Cell membrane | Separates cell contents from surroundings |
| (e) | Cell wall | Provides structural rigidity |
| (f) | Golgi apparatus | Packs and stores materials from ER |
| (g) | Chloroplast | Helps in manufacturing food |
4. Which pair of organelles is correctly placed under the categories?
Answer:
Correct option: (i) Leucoplast — Cell wall
5. Discussion on plastids by Renu and Rohit.
Answer:
Renu is correct.
Roots also contain plastids, mainly leucoplasts, which store food. Chloroplasts are absent because roots do not perform photosynthesis.
6. Similarities and differences
between mitochondria and chloroplasts.
| Basis | Mitochondria | Chloroplasts |
|---|---|---|
| Function | Cellular respiration | Photosynthesis |
| Pigment | Absent | Chlorophyll present |
| Occurrence | Plant and animal cells | Only green plant cells |
| Shape | Rod or oval | Disc-shaped or oval |
Similarities:
Double membrane-bound
Have their own DNA and ribosomes
Semi-autonomous organelles
Involved in energy conversion
7. Which pair of cell organelles
contains DNA?
Answer:
Correct option: (ii) Mitochondria, Nucleus
8. Experiment with carrots in
plain water and salt solution.
(i) Hypothesis
Water moves across cell membrane by osmosis.
(ii) Improvement
Measure mass/length before and after experiment and keep conditions constant.
(iii) Explanation
Carrot in plain water becomes stiff due to water absorption.
Carrot in salt solution becomes limp due to water loss.
9. Presence or absence of structures
in bacterial and animal cells.
| Structure | Bacterial Cell | Animal Cell |
|---|---|---|
| Chromosome | Present | Present |
| Nucleus | Absent | Present |
| Mitochondria | Absent | Present |
| Golgi complex | Absent | Present |
| Chromoplasts | Absent | Absent |
10. Identify the incorrectly matched pair.
Answer:
Correct option: (ii) SER — Lipid and cellulose synthesis
SER synthesises lipids but not cellulose.
11. What would happen if all
mitochondria are removed from
a eukaryotic cell?
Answer:
Energy production would stop.
Cellular activities would cease.
The cell would eventually die.
12. Which phenomenon inhibits
tumor formation? Can plants
also develop tumors?
Answer:
The phenomenon is contact inhibition.
Normal cells stop growing after touching neighboring cells, preventing tumor formation.
Yes, plants can also develop tumors due to infections like Agrobacterium tumefaciens.
13. Cell membrane synthesis
and transport pathway.
Answer:
SER synthesises lipids.
Ribosomes on RER synthesise proteins.
Golgi apparatus packages and transports them to the cell membrane.
Diagram:
Ribosomes on RER ──► Proteins
│
▼
Golgi Apparatus ──► Cell Membrane
▲
│
SER ──► Lipids ─────────────┘
14. What would happen if
gametes are formed by mitotic divisions?
Answer:
Gametes would contain the same chromosome number as body cells.
Chromosome number would double after fertilisation in each generation.
Genetic stability would be disturbed.
15. Case Study: Preservation of amla and lemons
(i) Scientific concept applied
Osmosis and food preservation using salt and sugar.
(ii) How do salt and sugar prevent spoilage?
They create a hypertonic medium, causing water loss from microbes and preventing their growth.
(iii) Healthy preserved food recipe
Amla murabba with jaggery
Lemon pickle with less oil
Homemade fruit squash
(iv) Scientific values addressed
Prevention of food wastage
Sustainable use of resources
Food security
Scientific thinking
Entrepreneurship
Hygiene awareness
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