Cell: The Building Block of Life — Class 9 Science
Exploration · Chapter 2
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Chapter summary
Key ideas & terms
- Cell membrane (plasma membrane)
- The selectively permeable boundary of every cell, described by the fluid-mosaic model as a lipid bilayer with embedded proteins.
- Osmosis
- The movement of water across a selectively permeable membrane from a dilute (more water) to a concentrated (less water) region.
- Prokaryotic cell
- A cell without a membrane-bound nucleus or organelles; its genetic material lies in a region called the nucleoid (e.g. bacteria).
- Eukaryotic cell
- A cell with a true, membrane-bound nucleus and several membrane-bound organelles (e.g. plant and animal cells).
- Mitochondria
- Double-membraned organelles, the "powerhouse" of the cell, that release energy as ATP during cellular respiration.
- Cell theory
- The unifying idea that all organisms are made of cells, the cell is the basic unit of life, and all cells come from pre-existing cells.
Extra questions & answers
What is the limit of resolution of the human eye, and why does it matter for studying cells?
It is about 0.1 mm at the near point (25 cm) — two points closer than this look like one. Most cells are smaller than this, so microscopes are needed to see them.
Who first observed cells, and how?
Robert Hooke in 1665, using a self-made microscope, examined a thin slice of cork and saw tiny box-like compartments which he named "cells".
Differentiate between diffusion and osmosis.
Diffusion is the net movement of particles from higher to lower concentration and can occur without a membrane; osmosis is specifically the diffusion of water across a selectively permeable membrane.
What does the fluid-mosaic model say about the cell membrane?
The membrane is a lipid bilayer (water-attracting heads outward, water-repelling tails inward) with proteins embedded like tiles in a mosaic; the molecules can move, so the membrane is fluid, and proteins act as gatekeepers for what enters or leaves.
Why do plant cells have a cell wall while animal cells do not?
Plants are usually fixed in one place and need a rigid cellulose wall to withstand wind, rain and pressure changes and to stay upright; animal cells move and change shape, so a rigid wall would restrict them.
Distinguish prokaryotic from eukaryotic cells.
Prokaryotic cells (e.g. bacteria) lack a membrane-bound nucleus and organelles, keeping their DNA in a nucleoid; eukaryotic cells (plants, animals) have a true nucleus and membrane-bound organelles and are generally larger.
Why are mitochondria called the powerhouse of the cell?
They break down glucose and other molecules during cellular respiration and store the released energy as ATP, the energy currency used for most cellular activities.
Name the three types of plastids and their roles.
Chloroplasts (green, contain chlorophyll, carry out photosynthesis), chromoplasts (coloured pigments giving flowers and fruits their colours) and leucoplasts (colourless, store starch, oils or proteins).
Compare mitosis and meiosis.
Mitosis produces two genetically identical daughter cells with the full chromosome number, used for growth and repair; meiosis is a two-step division producing four cells with half the chromosomes, used to make gametes for reproduction.
State the cell theory and name the scientists who developed it.
All living organisms are made of one or more cells, the cell is the basic unit of structure and function, and all cells arise from pre-existing cells; it was built up by Schleiden (plants), Schwann (animals) and Virchow (new cells from existing cells).
Think It Over
Where does a cell come from?
A new cell comes only from a pre-existing cell by cell division. As per the Cell Theory proposed by Rudolf Virchow, omnis cellula-e-cellula, the existing cell grows and then divides to form daughter cells that receive genetic material from the parent cell.
How have technological interventions facilitated the creation of new knowledge in understanding the world beyond the naked eye?
The human eye can resolve only up to about
How is the cell structural and functional unit of life?
All living organisms, unicellular like bacteria and yeast or multicellular like plants and humans, are made of cells. Similar cells form tissues, tissues form organs and organ systems. Even in a complex body the cell carries out all life functions - respiration, protein synthesis, transport, storage and division - so it is the fundamental structural and functional unit.
How does a cell multiply?
A cell multiplies by cell division. In this process the genetic material is duplicated and the parent cell divides. Prokaryotic cells divide simply, while eukaryotic cells divide in a controlled cell cycle by mitosis for growth and repair and by meiosis for formation of gametes.
What if...
What will happen if mung bean seeds are kept in a concentrated solution after soaking in water for 12 hours? What will happen to them?
After 12 hours in water the seeds become swollen and turgid by endosmosis. If they are then placed in a concentrated salt or sugar solution, which is hypertonic, water will move out of the seed cells by osmosis through the selectively permeable membranes. The seeds will shrink, become flaccid and lose weight.
Pause and Ponder
1. What argument would you give for the necessity of a cell wall in plants usually fixed in one place versus in animals usually moving from one place to the other?
Plants are sessile and must withstand wind, rain, gravity and turgor pressure without moving away. A rigid, permeable cell wall made mainly of cellulose gives mechanical strength, maintains shape, keeps leaves and stems upright and prevents bursting when water enters by osmosis. Animals move to escape stress and need flexible bodies, so a rigid wall would hinder movement, muscle contraction and flexibility of tissues. Their flexible cell membrane is sufficient.
2. What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?
The cell would lose rigidity and support. On absorbing water it would swell too much like an animal cell and could burst, the plant would wilt, leaves would droop and the plant could not stay erect. Loss of protection against pathogens and mechanical injury would also occur.
3. Why is it important to cut the two potato pieces in roughly equal size and measure their initial weight before placing them in different liquids?
Equal size and initial weight act as a controlled variable. It ensures that any change in weight or size after one hour is due only to the type of solution (plain water versus
Pause and Ponder
4. Do white flowers contain any pigment? Give reasons.
White flowers appear white not because they lack plastids, but because they contain leucoplasts or chromoplasts with no coloured pigment and the petal cells contain air spaces that reflect all wavelengths of light. They have no chlorophyll and often no anthocyanin or carotenoid, so no visible colour is absorbed. Some white petals may have very dilute flavonoids that are not visible but still present.
5. Draw a well-labelled schematic diagram of a plant or an animal cell using these clues — (i) Nucleus appears as a dark and round body inside the cell.
The diagram should show a roughly rectangular plant cell with cell wall outside or a round animal cell with only cell membrane, and a prominent round dark nucleus near the centre with double membrane, nucleolus and chromatin inside. Label: nucleus. In a plant cell also show large central vacuole, chloroplasts and cell wall.
5. Draw a well-labelled schematic diagram of a plant or an animal cell using these clues — (ii) ER spreads like a network of extended nuclear envelope.
Extend from the outer nuclear membrane a network of flattened tubules and sheets throughout the cytoplasm. Label the rough portion with ribosomes on its surface as Rough Endoplasmic Reticulum and the smooth portion without ribosomes as Smooth Endoplasmic Reticulum, both continuous with the nuclear envelope.
5. Draw a well-labelled schematic diagram of a plant or an animal cell using these clues — (iii) Mitochondria and chloroplasts are rod shaped.
In the cytoplasm draw several rod or oval bodies for mitochondria with outer smooth membrane and inner folded cristae. In the plant cell also draw rod to oval chloroplasts with double membrane and internal stroma and discs containing chlorophyll. Label mitochondria as powerhouse and chloroplast as site of photosynthesis.
Pause and Ponder
6. Instead of many small ones, why does a cell not have a single giant mitochondrion? How does this relate to the concept of surface area?
Energy production depends on surface area of the inner membrane where cristae carry respiration reactions. Many small mitochondria provide a much larger total surface area to volume ratio than one giant mitochondrion of the same volume, so ATP synthesis is more efficient. Small, distributed mitochondria can also move to sites needing energy and if one is damaged others continue to function.
7. If the skin cells start dividing by meiosis instead of mitosis, what do you think will happen to a cut on the skin?
Mitosis produces two genetically identical diploid cells for growth and repair. Meiosis produces four haploid daughter cells with half the chromosomes and introduces variation, and involves two divisions. A cut would not heal properly because the new cells would have half the DNA, be genetically different, not replace lost cells accurately, and tissue continuity and function would be lost.
Revise, Reflect, Refine
1. Differentiate between the following pairs of terms based on the clues given in parentheses: (i) Cell membrane and cell wall (permeability)
Cell membrane is selectively permeable, composed of lipid bilayer and proteins, allows only certain substances to pass. Cell wall is permeable, composed mainly of cellulose, allows water and dissolved minerals to pass freely while providing strength. All cells have a membrane, only plants, fungi and bacteria have a wall outside it.
1. Differentiate between the following pairs of terms based on the clues given in parentheses: (ii) RER and SER (structure)
Rough Endoplasmic Reticulum has ribosomes attached to its outer surface, so it appears rough under the electron microscope and is involved in protein synthesis and secretion. Smooth Endoplasmic Reticulum lacks ribosomes, appears smooth, and is involved in synthesis and storage of lipids and hormones.
1. Differentiate between the following pairs of terms based on the clues given in parentheses: (iii) Chloroplasts and chromoplasts (pigments)
Chloroplasts contain green pigment chlorophyll for photosynthesis, found mainly in leaves. Chromoplasts contain non-green pigments like carotenoids (yellow, orange, red) that give colour to flowers and fruits and help attract pollinators, they do not perform photosynthesis.
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.
Correct explanation is (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane. Pure water is hypotonic, so water enters Cell X by osmosis and it swells. Concentrated salt solution is hypertonic, so water leaves Cell Y by osmosis and it shrinks. The membrane is selectively permeable to water, not to salt.
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.
(i) Controlling all the activities of a cell — (a) Nucleus. The nucleus contains DNA and chromosomes and regulates growth, division and metabolism.
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: (ii) Site of cellular respiration.
(ii) Site of cellular respiration — (c) Mitochondrion. Its cristae provide surface for respiration and ATP production.
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: (iii) Storage organelle that also provides rigidity to the cell.
(iii) Storage organelle that also provides rigidity — (f) Vacuole (large central vacuole in plant cell) filled with cell sap, stores water and maintains turgor.
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: (iv) Separates the cell contents from surroundings.
(iv) Separates the cell contents from surroundings — (d) Cell membrane / plasma membrane, selectively permeable boundary.
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: (v) Provides structural rigidity to the cell.
(v) Provides structural rigidity — (e) Cell wall, made of cellulose, outer covering of plant cell.
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: (vi) Packs and stores materials received from ER.
(vi) Packs and stores materials received from ER — (g) Golgi apparatus, stacks of flattened sacs that modify, sort and package proteins and lipids into vesicles.
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: (vii) Helps in manufacturing food.
(vii) Helps in manufacturing food — (b) Chloroplast, contains chlorophyll in stroma discs for photosynthesis.
4. Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories? Option Present in the plant cells Absent in the animal cells (i) Leucoplast Cell wall (ii) Mitochondria Ribosome (iii) Cell wall Golgi apparatus (iv) Lysosome Endoplasmic reticulum
Only option (i) Leucoplast — Cell wall is correctly placed. Leucoplasts for starch storage are present only in plants and cell wall is present in plants but absent in animal cells. Option (ii) is wrong because both have mitochondria and ribosomes. Option (iii) wrong because Golgi apparatus is present in both. Option (iv) wrong because both have lysosomes (though fewer in plants) and ER.
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.
Renu is correct. All plant cells have plastids, but the type differs. Roots contain leucoplasts which are colourless plastids that store starch, oils or proteins, e.g., in potato. They do not need chlorophyll because they are underground and do not photosynthesize, but they still need plastids for storage. Chloroplasts are absent or rare in roots, not plastids altogether.
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.
Similarities: Both are double-membrane bound, have their own circular DNA and ribosomes, can make some of their own proteins and are thought to have evolved from endosymbiotic bacteria. Both are involved in energy conversion and have an inner membrane system increasing surface area (cristae in mitochondria, thylakoid discs in chloroplast) and a semi-fluid matrix/stroma. Differences: Mitochondria are rod-shaped powerhouses that break down glucose by cellular respiration to produce ATP, found in both plant and animal cells. Chloroplasts are usually larger, contain chlorophyll, found only in plant cells and some protists, and synthesize food by photosynthesis trapping light energy and storing it as sugars in stroma.
7. Which of the following pairs of cell organelles contains DNA? (i) Chloroplasts, Ribosomes (ii) Mitochondria, Nucleus (iii) Golgi bodies, Ribosomes (iv) Nucleus, Lysosomes
Correct pair is (ii) Mitochondria, Nucleus. Mitochondria and chloroplasts have their own circular DNA; nucleus contains chromosomal DNA. Ribosomes, Golgi bodies and lysosomes lack DNA.
8. 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?
She wants to test that water moves by osmosis across the selectively permeable cell membranes, so a hypotonic medium (plain water) will cause water uptake and turgidity while a hypertonic medium (concentrated salt solution) will cause water loss and flaccidity in plant tissue.
8. 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. (ii) What would you suggest for the improvement of this experiment?
Use more replicates, keep carrots of equal weight, length and variety, measure initial and final weight with a balance, keep volume of solutions equal, maintain same temperature and time, include a control with isotonic solution, and peel carrots similarly to keep surface area comparable.
8. 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. (iii) Why does the carrot in plain water stay stiff and crunchy, but the carrot in concentrated salt solution become rubbery and limp?
In plain water, water enters the carrot cells by endosmosis, fills the central vacuole, increases turgor pressure against the cell wall and makes tissue firm and crunchy. In concentrated salt solution, water leaves the cells by exosmosis, vacuoles shrink, turgor is lost, cells become plasmolysed and the carrot becomes limp.
9. Indicate the presence or absence of following structures in bacterial and animal cells: Chromosome
Chromosome — Present in both. Bacterial cell has a single circular chromosome in the nucleoid region; animal cell has multiple linear chromosomes inside the nucleus.
9. Indicate the presence or absence of following structures in bacterial and animal cells: Nucleus
Nucleus — Absent in bacterial cell (prokaryotic, has nucleoid without membrane), Present in animal cell (eukaryotic, membrane-bound nucleus).
9. Indicate the presence or absence of following structures in bacterial and animal cells: Mitochondria
Mitochondria — Absent in bacterial cell (respiration at plasma membrane), Present in animal cell.
9. Indicate the presence or absence of following structures in bacterial and animal cells: Golgi complex
Golgi complex — Absent in bacterial cell, Present in animal cell for packaging and transport.
9. Indicate the presence or absence of following structures in bacterial and animal cells: Chromoplasts
Chromoplasts — Absent in both bacterial and animal cells; found only in plant cells for colour.
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: (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.
Cup B and Cup C contain living potato tissue acting as a selectively permeable membrane and a concentrated solute (sugar or salt) inside the cup. Water from the beaker (hypotonic) moves by osmosis through the potato cells into the hollow where concentration is higher, so water accumulates in the cups.
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: (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: (ii) Why is Cup A necessary for this experiment?
Cup A is the control. It is empty and shows that without a solute gradient no water gathers, proving that the water accumulation in B and C is due to osmosis caused by sugar/salt, not due to leakage or other factors.
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: (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: (iii) Explain why water does not gather in the hollowed portions of Cups A and D.
Cup A has no solute, so there is no concentration gradient, hence no net osmosis. Cup D is made of boiled potato where cell membranes are destroyed by heat and denatured proteins lose selective permeability, so osmosis cannot occur and water does not accumulate despite the presence of sugar.
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
Incorrect pair is (ii) SER — Lipid and cellulose synthesis. SER synthesizes lipids and hormones, but cellulose of the cell wall is synthesized by enzymes at the plasma membrane/Golgi, not by SER. The other two pairs are correct.
12. What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell?
The cell would lose its main ATP-producing powerhouse. Cellular respiration and oxidative energy release would stop, ATP levels would fall sharply, active transport, protein synthesis, division and other energy-demanding processes would cease and the cell would soon die.
13. Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain.
Contact inhibition inhibits tumour formation in animals. Normal cells stop dividing when they touch neighbouring cells, preventing uncontrolled growth. Cancer cells lose this control and form tumours. Plants can also develop tumour-like growths called galls or crown galls, often caused by Agrobacterium, but due to rigid cell walls they do not metastasize like animal cancers.
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.
Proteins are made by ribosomes on Rough Endoplasmic Reticulum (RER), lipids are made by Smooth Endoplasmic Reticulum (SER). Path:
15. What would happen if gametes are formed by mitotic divisions?
Mitotic gametes would remain diploid, like the parent cell. After fertilisation the zygote would have double the normal chromosome number (
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?
She applied osmosis and plasmolysis with osmotic preservation. High salt/sugar creates a hypertonic environment that draws water out of microbial cells, dehydrating them.
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: (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?
Salt and sugar lower water activity outside microbial cells. Water leaves microbes by exosmosis, causing plasmolysis, dehydration and inability to grow or reproduce. The concentrated solution also reduces available free water for enzymatic spoilage reactions.
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: (iii) Suggest a healthy recipe of this kind for food preservation.
A healthy recipe is amla murabba: wash amla, prick, blanch lightly, soak in
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: (iv) What are the scientific values addressed in this case?
Values include applying scientific knowledge to everyday life, reducing food waste, ensuring food security, promoting sustainable and eco-friendly preservation without harmful chemicals, encouraging entrepreneurship, and linking laboratory concepts like osmosis to community benefit.