Selina Concise Solutions for ICSE Class 9 Biology Chapter 2 Cell The Unit Of Life

Exercise 1

Question A:
Answer: (a) Ribosomes - Synthesis of proteins
(b) a single cell
(d) cell membrane
(a) absence of centrosome
(c) Cellulose
(a) Mitochondria
Each part of a cell has a specific job, like a factory with different departments. For example, ribosomes act as the assembly line for proteins while mitochondria provide the electricity.
Teacher's Tip: Remember "Mito" sounds like "Mighty" to remind you it is the powerhouse of the cell.
Exam Tip: Always read every option in a multiple-choice question before picking your answer to ensure it is the best fit.

Question B.1:
Answer: (a) Mitochondria
(b) Ribosomes
(c) Chromosomes
(d) Centrosome
(e) Lysosomes
(f) Cell membrane
These scientific names refer to the microscopic "organs" or organelles that keep a cell alive. Learning these names is the first step in understanding how life works at its most basic level.
Teacher's Tip: Use flashcards with the organelle name on one side and its function on the other for quick study sessions.
Exam Tip: Pay close attention to the spelling of "Mitochondria" and "Chromosomes" as they are commonly misspelled in exams.

Question B.2:
Answer: (a) F (False). Animal cells do not contain a cell wall.
(b) F (False). A cell wall is made up of cellulose.
(c) T (True)
(d) T (True)
(e) F (False). In eukaryotes, cytoplasm is the part of the cell which surrounds the nucleus.
(f) T (True)
(g) T (True)
True and false statements help clarify the differences between plant and animal cells. For instance, the presence of a rigid cell wall is a key feature that defines plant structures.
Teacher's Tip: Remember that "Protoplasm" is the "Total" living matter, while "Cytoplasm" is just the liquid part outside the nucleus.
Exam Tip: If a question asks you to correct false statements, always provide the full corrected sentence to get full marks.

Question B.3:
Answer: 23 pairs of chromosomes are found in human cells.
Chromosomes are thread-like structures that carry our genetic information. These 23 pairs (making 46 total) determine everything about us, from our hair color to our height.
Teacher's Tip: Think of chromosomes as the "blueprints" for building a human being.
Exam Tip: Don't forget to specify "pairs" or "total number" as 23 and 46 are different answers.

Question B.4:
Answer: DNA (Deoxyribonucleic acid)
DNA is the molecule that contains the instructions for every living thing. It is shaped like a twisted ladder and is tucked safely inside the nucleus of the cell.
Teacher's Tip: Say it slowly: "De-oxy-ribo-nucleic acid" to master the long name.
Exam Tip: Always provide the full expansion in brackets after writing the abbreviation DNA.

Question B.5:
Answer:
Column A - Column B
(a) Vacuoles - (iii) Covered by tonoplast
(b) Nucleolus - (v) Forms RNA
(c) Lysosomes - (i) Intracellular digestion
(d) Anthocyanin - (iv) Dissolved in the cytoplasm
(e) Cristae - (ii) Respiratory enzymes
Matching these components helps us link the structure of a cell part to its specific biological function. For example, the "tonoplast" is specifically the protective skin around a cell's storage tank (vacuole).
Teacher's Tip: Associate "Cristae" with "Respiration" to remember they belong to the mitochondria.
Exam Tip: When matching, rewrite the correct pairs clearly side-by-side instead of drawing messy lines.

Question B.6:
Answer: (a) Lysosome
(b) Centriole
(c) Plasma membrane
(d) Insects
(e) Genes
(f) Leucoplast
Each of these terms represents a unique part or category within biology. "Genes" are particularly interesting because they are the individual units of heredity found on our chromosomes.
Teacher's Tip: Think of "Leuco" in Leucoplast as "Light-colored" or "White" because these store food and have no pigment.
Exam Tip: Be ready to define each of these terms individually as they often appear as short-answer questions.

Question C.1:
Answer: Protoplasm is the living matter of the cell. Protoplasm cannot be analysed chemically because the chemical composition of protoplasm is very complex. It varies slightly from one cell to another, although the common elements included in the composition of protoplasm such as carbon, hydrogen, oxygen, nitrogen, sulphur, iron and phosphorus are still the same in all the cells.
Protoplasm is essentially the "soul" of the cell, containing everything that makes it alive. Even though it is made of common elements like oxygen and carbon, the way they are mixed is so special that scientists still find it difficult to replicate.
Teacher's Tip: Think of Protoplasm as the "Soup of Life" inside every cell.
Exam Tip: List at least four of the common elements (C, H, O, N) when describing protoplasm's composition.

Question C.2:
Answer: Organs of an organism are the parts of the body which have a definite shape and structure and perform specific functions. Cell organelles are also parts of the cell which have a definite shape and structure and perform specific functions. Organelles have the same status in a cell as the organs have in the entire body of an animal or a plant performing specific functions.
Just as your heart and lungs work together to keep you alive, organelles like the nucleus and mitochondria work together to keep a cell alive. The word "organelle" actually means "little organ."
Teacher's Tip: Compare a cell to a city: the nucleus is City Hall and organelles are the different departments.
Exam Tip: Use the analogy of "Body organs" to explain the importance of "Cell organelles."

Question C.3:
Answer: The cells of an elephant would be of the same size as the cells of a rat. The size of cells does not vary within the organisms, however, the number of cells varies from one organism to another. A larger animal like an elephant will have more number of cells as compared to a smaller animal like a rat. However, the size of the cell will be the same.
This is a common misconception; big animals don't have giant cells, they just have trillions more of them. Cells stay small so they can quickly move nutrients in and waste out.
Teacher's Tip: Think of a big wall; it isn't made of bigger bricks than a small wall, it just has many more bricks.
Exam Tip: Emphasize that "number of cells" is what changes, not the "cell size."

Question C.4:
Answer: (a) Protoplasm and cytoplasm
Protoplasm: It is the living matter, the total substance of a living cell, i.e. the cytoplasm and the nucleus.
Cytoplasm: It is a mixture of water and soluble organic and inorganic compounds, in which various cell organelles are embedded.

(b) Nucleolus and nucleus
Nucleolus: It is a round-shaped nucleoli present inside the nucleus.
Nucleus: It is a dense spherical structure present in the cell that contains a network of thread-like structures called chromatin fibres.

(c) Centrosome and chromosome
Centrosome: (i) It is a clear area of cytoplasm close to the nucleus, from which spindle fibres develop during cell division. (ii) Centrosome is found only in an animal cell.
Chromosome: (i) Chromosomes carry hereditary information or genes which transmit genetic characters from parents to offspring. (ii) Chromosomes are found in the nucleus of both, animal and plant cells.

(d) Cell wall and cell membrane
Cell wall: (i) It is a non-living rigid layer. (ii) It is made of cellulose. (iii) It is freely permeable.
Cell membrane: (i) It is a living, thin, flexible membrane. (ii) It is made of lipoproteins. (iii) It is semi-permeable.

(e) Plant cell and animal cell
Plant cell: (i) Cell wall is present. (ii) Centrosome is absent. (iii) Vacuoles are large and prominent. (iv) Plastids are present.
Animal cell: (i) Cell wall is absent. (ii) Centrosome is present. (iii) Vacuoles are small and temporary. (iv) Plastids are absent.

(f) Prokaryotes and eukaryotes
Prokaryotes: (i) Organisms with cells containing a primitive, undefined nucleus are called prokaryotes. (ii) They contain small ribosomes. (iii) They lack other cell organelles. (iv) Examples: Bacteria, blue-green algae.
Eukaryotes: (i) Organisms with cells containing a well-defined nucleus with a nuclear membrane are called eukaryotes. (ii) They contain larger ribosomes. (iii) They contain other cell organelles. (iv) Examples: $Euglena$, Human beings.
Understanding these differences is crucial for identifying different types of life under a microscope. Each structure has a specific chemical makeup, like the cellulose in plant walls versus the lipoproteins in membranes.
Teacher's Tip: "Pro" means before/primitive (no nucleus) and "Eu" means true (has a nucleus).
Exam Tip: When distinguishing between two things, always use a table or bullet points for clarity.

Question C.5:
Answer: Features found only in plant cells:
1. Presence of cell wall
2. Presence of large vacuoles. The liquid contained in vacuoles is called cell sap
3. Presence of plastids
Features found only in animal cells:
1. Presence of centrosome
These unique features explain why plants can stand tall without a skeleton (cell wall) and how they make their own food (plastids). Animal cells lack these but have centrosomes to help them divide and grow.
Teacher's Tip: Remember: Plants have "Plastids" to "Photosynthesize."
Exam Tip: Be sure to mention the "cell sap" in plant vacuoles as it is a specific technical detail.

Question C.6:
Answer: Cells generally remain small in size because:
1. To enable different regions of the cell to communicate with each other rapidly for the cell to function effectively
2. To have a large surface area is to volume ratio for greater diffusion of substances, in and out of the cell
If a cell were too large, it would take too long for signals and nutrients to travel from the edge to the center. Staying small ensures that the cell can react quickly to its environment and stay healthy.
Teacher's Tip: Think of a small room versus a huge stadium; it's much easier to hear a message in the small room.
Exam Tip: Mention the "surface area to volume ratio" as this is the scientific reason for small cell size.

Question D.1:
Answer: Postulates of cell theory:
1. Cell is the smallest unit of structure of all living things.
2. Cell is the unit of function of all living things.
3. All cells arise from pre-existing cells.
Cell theory was propounded by Theodor Schwann and Matthias Schleiden in the year 1839 and was modified by Rudolf Virchow in 1858.
These three rules are the foundation of all modern biology, proving that everything from a blade of grass to a whale is built from cells. It also tells us that life can only come from life that already exists.
Teacher's Tip: Remember the three "S-S-V" scientists: Schleiden, Schwann, and Virchow.
Exam Tip: Memorize the three postulates verbatim; they are very likely to appear on your test.

Question D.2:
Answer:
Living cell: 1. Non-rigid living structure. 2. Mainly composed of cellulose. 3. Freely permeable.
Brick in a wall: 1. Rigid non-living structure. 2. Mainly composed of soil. 3. Impermeable.
While both "cells" and "bricks" are basic building blocks, a living cell is dynamic and constantly exchanging materials with its environment. Bricks are dead matter designed only for strength and stability.
Teacher's Tip: A brick is just for support, but a cell is for support AND life activities.
Exam Tip: Focus on the "living" versus "non-living" aspect to score full marks here.

Question D.3:
Answer:
Cells - Plastid - Pigment
(a) petals of sunflower - Chromoplasts - Xanthophyll
(b) ripe tomato - Chromoplasts - Carotene
(c) skin of green mango - Chloroplasts - Chlorophyll
(d) cells of potato - Leucoplasts - No pigment
Plastids are like tiny paint pots for plants, giving flowers their bright colors and leaves their green hue. Chromoplasts provide the "warm" colors like red and yellow, while leucoplasts are used for storage.
Teacher's Tip: "Chromo" means color, "Chloro" means green, and "Leuco" means colorless.
Exam Tip: If asked about potatoes, remember they store starch in colorless "Leucoplasts."

Question D.4:
Answer: (a) Plasma membrane: 1. Separates contents of the cell from its surroundings. 2. Regulates the entry of certain solutes and ions. 3. Maintains the shape of animal cell.
(b) Ribosome: 1. Protein synthesis.
(c) Lysosomes: 1. Intracellular digestion. 2. Destroy foreign substances. 3. When the cell is old or injured, lysosomes rapidly destroy cell organelles and hence, are called suicide bags.
(d) Mitochondria: 1. Synthesis of respiratory enzymes. 2. Release of energy from pyruvic acid produced in cytoplasm in the form of $ATP$.
(e) Golgi apparatus: 1. Synthesis and secretion of enzymes, hormones, etc. 2. Formation of acrosome of sperm.
(f) Cytoplasm: 1. Different organelles contained in it perform different functions. 2. All metabolic activities occur in it.
(g) Asters of centrosome: 1. Initiates and regulates cell division. 2. Forms spindle fibres.
(h) Chromosomes: 1. Carry genetic characters from parents to offspring.
(i) Glycogen granule: 1. Serves as food for the cell.
(j) Vacuoles: 1. Gives turgidity to the cells. 2. Storage of water and other substances, food, pigments and waste products.
Every part of the cell has a job to do, ensuring that the cell can breathe, eat, and reproduce. Lysosomes are particularly interesting because they act as both a defense system and a "suicide bag" if the cell becomes too sick to survive.
Teacher's Tip: Think of the Golgi apparatus as the "Post Office" that packages and sends out cell products.
Exam Tip: Be sure to use the term $ATP$ when discussing energy release in the mitochondria.

Question D.5:
Answer: Common features found in both plant and animal cells:
1. Presence of cell membrane
2. Presence of liquid matrix called cytoplasm in the cell
3. Presence of mitochondria which produces energy
4. Presence of ribosomes that synthesize proteins
5. Presence of Golgi body
6. Presence of a prominent nucleus
Despite their differences, both plant and animal cells share these fundamental components that allow them to process energy and replicate. These shared traits prove that all complex life shares a common evolutionary beginning.
Teacher's Tip: Most of the "work" in a cell happens in these shared parts, like the nucleus and mitochondria.
Exam Tip: If asked for common features, don't mention the cell wall or plastids as those are plant-only.

Question E.1:
Answer: (a) Fig. B is a plant cell. It has a cell wall and a large vacuole which pushes the nucleus towards the periphery.
(b) Cell membrane, ribosomes, nucleus, endoplasmic reticulum, lysosomes, Golgi body and mitochondria are common to both the types.
(c) Plastids and cell wall are found only in plant cell. Centrosome is found only in animal cell.
Looking at a diagram helps us visualize the actual layout of a cell's interior. In plants, the giant central vacuole is so big that it forces the nucleus to sit off to the side instead of in the center.
Teacher's Tip: Look for the "outer box" (cell wall) to instantly identify a plant cell in a picture.
Exam Tip: When identifying a cell type from a figure, always list the specific structures (like cell wall) that led to your conclusion.