Cell: The Building Block of LifeClass 9 Science NCERT Solutions
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Solution 1 of 16
Q1Revise, Reflect, Refine
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)
Solution
(i)
Cell membrane and cell wall (permeability):
The cell membrane is selectively permeable, meaning it allows only certain substances to pass through it. The cell wall, on the other hand, is fully permeable, allowing water and some dissolved minerals to pass through freely.
(ii)
RER and SER (structure):
The Rough Endoplasmic Reticulum (RER) has ribosomes attached to its surface, giving it a rough appearance under a microscope. The Smooth Endoplasmic Reticulum (SER) does not have ribosomes on its surface, hence it has a smooth appearance.
(iii)
Chloroplasts and chromoplasts (pigments):
Chloroplasts are plastids that contain the green pigment chlorophyll, which is essential for photosynthesis. Chromoplasts are plastids that contain pigments other than chlorophyll, such as yellow, orange, or red pigments, which give colour to flowers and fruits.
Q2Revise, Reflect, Refine
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.
Solution
The correct statement is (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.
Explanation:
The movement of water occurs due to osmosis, which is the diffusion of water across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration.
- Cell X in pure water: Pure water is a hypotonic solution compared to the cell's cytoplasm. Therefore, water moves from the pure water into Cell X, causing it to swell.
- Cell Y in concentrated salt solution: The salt solution is a hypertonic solution compared to the cell's cytoplasm. Therefore, water moves from inside Cell Y out into the salt solution, causing it to shrink.
Q3Revise, Reflect, Refine
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.
Solution
Based on the diagram of the plant cell in Fig. 2.20, the labeled parts are matched with their functions as follows:
- (a) Cell wall: (v) Provides structural rigidity to the cell.
- (b) Cell membrane: (iv) Separates the cell contents from surroundings.
- (c) Nucleus: (i) Controlling all the activities of a cell.
- (d) Vacuole: (iii) Storage organelle that also provides rigidity to the cell.
- (e) Chloroplast: (vii) Helps in manufacturing food.
- (f) Mitochondrion: (ii) Site of cellular respiration.
- (g) Golgi apparatus: (vi) Packs and stores materials received from ER.
Q4Revise, Reflect, Refine
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
Solution
The correct option is (i).
Explanation:
- Leucoplasts are a type of plastid present in plant cells for storage. They are absent in animal cells.
- Cell walls are present in plant cells but are absent in animal cells. Therefore, both placements in option (i) are correct. Let's analyze the other options:
- (ii) Mitochondria are present in both plant and animal cells. Ribosomes are present in both.
- (iii) Golgi apparatus is present in both plant and animal cells.
- (iv) Lysosomes are typically found in animal cells, and Endoplasmic Reticulum is present in both.
Q5Revise, Reflect, Refine
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.
Solution
Renu is correct.
Justification:
Plastids are not just limited to chloroplasts, which perform photosynthesis. There are other types of plastids as well. The text mentions leucoplasts, which are colorless plastids that store food materials such as starch, oils, or proteins. These are found in non-photosynthetic parts of the plant, including roots and storage organs like potato tubers. Therefore, while roots do not contain chloroplasts because they are not exposed to sunlight for photosynthesis, they do contain other plastids like leucoplasts for food storage. Rohit's reasoning is incomplete because he only considered one type of plastid (chloroplasts).
Q6Revise, Reflect, Refine
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.
Solution
Similarities between Mitochondria and Chloroplasts:
-
Structural Similarities:
- Double Membrane: Both are surrounded by a double membrane, an outer and an inner membrane.
- Own DNA and Ribosomes: Both mitochondria and chloroplasts have their own circular DNA and ribosomes, which allows them to synthesize some of their own proteins, independent of the nucleus.
-
Functional Similarities:
- Energy Conversion: Both are involved in energy conversion processes within the cell.
Differences between Mitochondria and Chloroplasts:
-
Structural Differences:
- Internal Structure: The inner membrane of a mitochondrion is folded into cristae, while the interior of a chloroplast contains disc-shaped structures that contain chlorophyll.
- Pigments: Chloroplasts contain the green pigment chlorophyll, while mitochondria do not.
-
Functional Differences:
- Process: Chloroplasts are the site of photosynthesis, where they convert light energy into chemical energy (glucose). Mitochondria are the site of cellular respiration, where they break down glucose to release chemical energy in the form of ATP.
- Energy Flow: Chloroplasts capture energy, while mitochondria release energy.
- Presence: In eukaryotes, chloroplasts are found only in plant cells and some protists, whereas mitochondria are found in almost all eukaryotic cells, including both plant and animal cells.
Q7Revise, Reflect, Refine
Which of the following pairs of cell organelles contains DNA?
(i)
Chloroplasts, Ribosomes
(ii)
Mitochondria, Nucleus
(iii)
Golgi bodies, Ribosomes
(iv)
Nucleus, Lysosomes
Solution
The correct option is (ii) Mitochondria, Nucleus.
Explanation:
The chapter states that the nucleus contains the cell's primary genetic material in the form of DNA within chromosomes. It also mentions that mitochondria and plastids (such as chloroplasts) have their own DNA. Ribosomes, Golgi bodies, and lysosomes do not contain DNA.
Q8Revise, Reflect, Refine
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?
Solution
(i)
Hypothesis: The researcher wants to test the hypothesis that plant cells gain or lose water through osmosis when placed in solutions of different concentrations. Specifically, she is testing how hypotonic (plain water) and hypertonic (salt solution) environments affect the turgidity of carrot cells.
(ii)
Suggestions for improvement:
- Quantify the results: The researcher should measure and record the initial and final weight of both carrots to get quantitative data on water loss or gain.
- Control variables: Use carrots of the exact same size, weight, and freshness. Ensure the volume of the liquid in both containers is the same.
- Replication: The experiment should be repeated multiple times to ensure the results are reliable and not due to chance.
(iii)
Explanation:
- Carrot in plain water: The plain water is a hypotonic solution relative to the carrot cells. Due to osmosis, water moves from the surrounding water into the carrot cells. This influx of water makes the cells turgid (firm), causing the carrot to stay stiff and crunchy.
- Carrot in salt solution: The concentrated salt solution is a hypertonic solution relative to the carrot cells. Due to osmosis, water moves out of the carrot cells and into the salt solution. This loss of water causes the cells to become flaccid (limp), making the carrot rubbery and limp.
Q9Revise, Reflect, Refine
Indicate the presence or absence of following structures in bacterial and animal cells:
Structures in a cell Bacterial cell Animal cell Chromosome Nucleus Mitochondria Golgi complex Chromoplasts
Solution
Here is the completed table indicating the presence or absence of the structures:
| Structures in a cell | Bacterial cell | Animal cell |
|---|---|---|
| Chromosome | Present (single, circular) | Present (multiple, linear) |
| Nucleus | Absent | Present |
| Mitochondria | Absent | Present |
| Golgi complex | Absent | Present |
| Chromoplasts | Absent | Absent |
Q10Revise, Reflect, Refine
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.
(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.
Solution
(i)
Water gathers in Cup B and Cup C because of osmosis. The sugar in Cup B and salt in Cup C create a hypertonic (concentrated) solution inside the potato cavity. The cells of the potato contain a less concentrated solution (hypotonic). The potato cells act as a selectively permeable membrane. Water moves from the potato cells (area of higher water concentration) into the hollowed portion (area of lower water concentration), causing water to accumulate in Cups B and C.
(ii)
Cup A is necessary because it serves as the control for the experiment. By keeping Cup A empty, we can confirm that water does not gather in the cavity on its own. This proves that the accumulation of water in Cups B and C is a direct result of adding sugar and salt, and not some other factor.
(iii)
Water does not gather in Cups A and D for the following reasons:
- Cup A: There is no solute (sugar or salt) in the cavity, so there is no concentration gradient to drive the movement of water via osmosis.
- Cup D: This cup is made from a boiled potato. Boiling destroys the cell membranes of the potato cells, making them no longer selectively permeable. Without a functional selectively permeable membrane, osmosis cannot occur. Therefore, even though there is a concentration gradient, water cannot move into the cavity.
Q11Revise, Reflect, Refine
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
Solution
The incorrectly matched pair is (ii) SER - Lipid and cellulose synthesis.
Explanation:
- (i) Ribosomes are correctly identified as the sites of protein synthesis.
- (iii) Lysosomes contain enzymes that break down waste materials and foreign agents, so their function is digestion.
- (ii) The Smooth Endoplasmic Reticulum (SER) is involved in the synthesis of lipids (fats) and hormones. However, the text states that the plant cell wall is made of cellulose, but it does not state that SER synthesizes cellulose. Therefore, this pairing is incorrect.
Q12Revise, Reflect, Refine
What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell?
Solution
If all the mitochondria were removed from a eukaryotic cell, the cell would not be able to perform aerobic cellular respiration. Mitochondria are known as the 'powerhouses of the cell' because they are the primary site for the production of ATP (Adenosine Triphosphate), the energy currency of the cell. Without mitochondria, the cell would have to rely on less efficient anaerobic processes for energy, which produce far less ATP. This lack of energy would prevent the cell from carrying out most of its essential life functions, such as synthesis of new molecules, transport, and cell division. Ultimately, the cell would die.
Q13Revise, Reflect, Refine
Which phenomenon inhibits the formation of tumors in the human body? Can plants also develop tumors? Explain.
Solution
The phenomenon that inhibits the formation of tumors in the human body is contact inhibition.
Explanation:
Contact inhibition is a regulatory mechanism where normal animal cells stop dividing once they come into contact with neighboring cells, forming a single layer. Cancer cells lose this ability and continue to divide uncontrollably, piling up on top of each other to form a mass or tumor.
Tumors in Plants:
The text states that plant cells do not show contact inhibition due to their rigid cell walls and they follow a different growth pattern. While they do not form tumors in the same way as animals (by losing contact inhibition), plants can develop abnormal growths or galls that are sometimes referred to as tumors. These are often caused by infections from bacteria, viruses, fungi, or insects, which disrupt the normal hormonal balance and lead to uncontrolled cell division and enlargement in a localized area.
Q14Revise, Reflect, Refine
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.
Solution
The cell organelles that help in the synthesis of the cell membrane are the Endoplasmic Reticulum (ER) and the Golgi apparatus.
- Lipids for the membrane are synthesized in the Smooth Endoplasmic Reticulum (SER).
- Proteins for the membrane are synthesized by ribosomes on the Rough Endoplasmic Reticulum (RER).
Path of synthesis and transport:
- Synthesis: Proteins are made on the ribosomes of the RER and enter the ER. Lipids are made in the SER.
- Transport to Golgi: Both proteins and lipids are transported from the ER to the Golgi apparatus, often in small membrane-bound sacs called transport vesicles.
- Processing in Golgi: The Golgi apparatus modifies, sorts, and packages these proteins and lipids.
- Transport to Cell Membrane: The finished proteins and lipids are packaged into new vesicles that bud off from the Golgi apparatus.
- Fusion: These vesicles move to the cell membrane and fuse with it, releasing their contents outside the cell (secretion) or embedding the proteins and lipids into the cell membrane itself, thus contributing to its growth and repair.
(Diagram)
A labelled diagram would show the nucleus connected to the Rough ER (with dots representing ribosomes) and Smooth ER. Arrows would point from the ER to the Golgi apparatus. Another arrow would point from the Golgi apparatus to a vesicle that is shown fusing with the cell membrane. This pathway is illustrated in Figure 2.13 in the textbook.
Q15Revise, Reflect, Refine
What would happen if gametes are formed by mitotic divisions?
Solution
If gametes (sperm and egg cells) were formed by mitosis instead of meiosis, the chromosome number would double with each generation.
Explanation:
- Mitosis is a type of cell division that produces two daughter cells that are genetically identical to the parent cell, having the same number of chromosomes. In humans, somatic (body) cells are diploid (2n), containing 46 chromosomes.
- If gametes were formed by mitosis, they would also be diploid (2n) with 46 chromosomes.
- During fertilization, when a diploid sperm (2n) fuses with a diploid egg (2n), the resulting zygote would be tetraploid (4n), containing 92 chromosomes.
- In the next generation, this 4n individual would produce 4n gametes, leading to an 8n zygote. This doubling of chromosomes in every generation would lead to severe genetic abnormalities and is not a viable process for sexual reproduction. Meiosis is essential because it is a reductional division that halves the chromosome number (to haploid, n) in gametes, ensuring that the normal diploid number is restored upon fertilization.
Q16Revise, Reflect, Refine
A farmer, Deepa, was very happy with the harvest of amla (Indian Gooseberry) and lemons on her farm. However, she could sell only onefourth 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?
Solution
(i)
The scientific concept applied by the farmer is osmosis.
(ii)
Adding high concentrations of salt or sugar creates a hypertonic environment around the food. When spoilage-causing microorganisms like bacteria and fungi come into contact with this environment, they lose water from their cells to the surroundings through osmosis. This process, called plasmolysis, causes the microbial cells to dehydrate and shrivel. Without sufficient water, their metabolic activities are inhibited, and they cannot grow or reproduce, thus preserving the food from spoilage.
(iii)
A healthy recipe for food preservation could be a spicy lemon pickle with jaggery. The recipe would involve cutting lemons, mixing them with salt, turmeric, and other spices. Instead of refined sugar, jaggery can be used to add sweetness and act as a preservative. The salt and the acidity of the lemon juice also act as preservatives. This reduces the use of refined sugar and oil, making it a healthier option.
(iv)
The scientific values addressed in this case include:
- Application of Scientific Knowledge: Using the principle of osmosis for a practical purpose like food preservation.
- Problem-Solving: Finding an effective solution to the problem of post-harvest wastage.
- Sustainability: Creating a model that reduces waste and is economically viable.
- Resourcefulness: Making the best use of available resources to create value-added products.
- Critical Thinking: Recognizing a problem and applying a known scientific method to solve it.