Frank Brothers Solutions for ICSE Class 9 Biology Chapter 3 The Cell A Unit Of Life

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Question 1:
Answer: All living organisms are made up of cells. Cells are the building blocks of all organisms and they give structural and functional stability to the organisms. All the metabolic processes needed to keep an organism alive occur at the cellular level. Hence cell is said to be the basic unit of life.
Cells are the tiny units that perform all the essential functions of life, such as taking in nutrients and converting them into energy. Every living thing, from a tiny bacterium to a giant whale, is built from these microscopic structures.
Teacher's Tip: Think of a cell as a miniature "living brick" that makes up the entire wall of an organism's body.
Exam Tip: To get full marks, always mention that cells are both the "structural" and "functional" unit of life.

Question 2:
Answer: In 1839, M.J. Schleiden and T. Schwann proposed the cell theory. This was further expanded by Virchow in 1858. The main features of the cell theory as known at present are:
1. All living organisms are composed of cell. Hence cell is the structural unit of living organisms.
2. All cells arise from pre-existing cells of the similar kind by cell division.
3. The chemical composition and metabolism of all cells is basically alike.
Cell theory is a fundamental principle in biology that explains how all living things are organized and how they reproduce. It highlights that life does not appear out of nowhere but continues through the process of cells splitting into new ones.
Teacher's Tip: Remember the three scientists' names: Schleiden, Schwann, and Virchow as the "fathers" of cell theory.
Exam Tip: If asked to list features, make sure to include the point about cells arising from "pre-existing cells" as it is very important.

Question 3:
Answer: Cell is the smallest structural and functional unit of all living organisms and is a membrane bound structure enclosing the protoplasm. The two types of cells are:
1. Prokaryotic cells - These cells lack a well-organised nucleus and membrane bound organelles. They are usually found in unicellular organisms, which are referred to as prokaryotes. Example of prokaryotes are bacteria and blue-green algae.
2. Eukaryotic cells - These cells possess a well-organised nucleus and membrane bound organelles. Organisms made up of eukaryotic cells are called eukaryotes. Examples of eukaryotes are plants and animals.
Prokaryotic cells are like a single-room house where everything happens in one space, while eukaryotic cells are like a mansion with many specialized rooms. The nucleus in a eukaryotic cell acts as the command center, holding the cell's genetic blueprints safely inside its own walls.
Teacher's Tip: "Pro" sounds like "Primitive" (older/simpler) and "Eu" means "True" (having a true nucleus).
Exam Tip: Always provide examples like "Bacteria" for prokaryotes and "Humans" or "Onions" for eukaryotes to illustrate the difference.

Question 4:
Answer: The organelles present in a typical cell are:
1. Mitochondria
2. Endoplasmic reticulum
3. Plastid
4. Golgi body
5. Vacuoles
6. Lysosomes
7. Centrosomes
(Write any 5)
Organelles are specialized "little organs" inside the cell that each have a specific job to do to keep the cell healthy. They work together like a team in a factory to produce energy, build proteins, and clean up waste.
Teacher's Tip: Imagine organelles as the internal organs of a single cell, just like you have a heart and lungs.
Exam Tip: If the question says "Write any 5," make sure you list exactly five clearly numbered points to keep your answer neat.

Question 5:
Answer: (a) Robert Hooke discovered and named cells.
(b) Virchow stated that new cells originate only from pre-existing cells (omnis cellula e cellula).
Robert Hooke first saw cells while looking at a thin slice of cork under a primitive microscope in 1665. Virchow's discovery was equally important because it proved that life is a continuous chain of cell divisions.
Teacher's Tip: Robert Hooke named them "cells" because they reminded him of the tiny rooms in a monastery.
Exam Tip: Memorize the Latin phrase "omnis cellula e cellula" to impress your examiner!

Question 6:
Answer: Cell wall:
It is the outermost covering of plant cell.
It is a non-living structure.
It is freely permeable.
It is composed of cellulose and pectin.

Cell membrane:
It is the outermost covering of animal cell.
It is a living structure.
It is semi-permeable.
It is majorly made up of phospholipids along with proteins and polysaccharides.
The cell wall acts like a rigid wooden fence that provides extra support to plants so they can grow tall without a skeleton. The cell membrane is like a soft, living skin that carefully decides which substances can enter or leave the cell.
Teacher's Tip: Remember: All cells have a membrane, but only plants (and some others) have a wall.
Exam Tip: In a comparison table, mention "Permeability" as it is a key functional difference between the two.

Question 7:
Answer: (a) Mitochondria - It is the powerhouse of the cell and provides energy for carrying out all the vital life activities of the cell.
(b) Ribosomes - It is the site of protein synthesis.
(c) Cell membrane - It gives definite shape to the cell, protects the internal cell contents and allows entry and exit of specific substances into and out of the cell.
Mitochondria are like tiny batteries that provide the electrical power needed for everything the cell does. Ribosomes are the busy workers that build the proteins used for growth and repair.
Teacher's Tip: Associate "Powerhouse" with Mitochondria and "Protein Factory" with Ribosomes.
Exam Tip: Describe the cell membrane as "selectively permeable" to explain its role in controlling entry and exit.

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Question 8:
Answer: Plastids are self-replicating organelles present only in plant cells and absent in animal cells. They are of three types:
1. Chloroplast - Green coloured plastids (due to presence of chlorophyll)
2. Chromoplast- Coloured plastids (except green colour)
3. Leucoplast - Colourless plastids
The main functions of plastids are:
1. Chloroplasts trap solar energy and use it to synthesize carbohydrates from carbon dioxide and water by the process of photosynthesis.
2. Chromoplasts attract insects and other animals to ensure pollination and dispersal of fruits.
3. Leucoplasts store starch, proteins or lipids in cell.
(Write any 2 functions)
Plastids give plants their vibrant colors and allow them to cook their own food using sunlight. Chloroplasts are specifically responsible for the green color of leaves and the miracle of photosynthesis.
Teacher's Tip: "Chloro" means green, "Chromo" means color, and "Leuco" means white or clear.
Exam Tip: Clearly state that plastids are *absent* in animal cells to show you know the difference between cell types.

Question 9:
Answer: During starvation, lysosomes burst and release their powerful enzymes, which act on their own cellular organelles and digest them. This results in cell death. Hence lysosomes are called suicidal bags.
Lysosomes usually act as the cell's garbage disposal system, breaking down waste and bacteria. However, if the cell is too damaged or starving, they turn their digestive power inward to recycle the cell itself.
Teacher's Tip: Think of lysosomes as the "cleanup crew" that can sometimes go on a "demolition mission."
Exam Tip: Use the phrase "powerful digestive enzymes" when explaining why they are called suicidal bags.

Question 10:
Answer: ER - Endoplasmic Reticulum
SER - Smooth Endoplasmic Reticulum
RER - Rough Endoplasmic Reticulum
These structures form a vast network of tunnels that transport materials throughout the cell. The "rough" version is covered in ribosomes, making it look bumpy under a microscope.
Teacher's Tip: Think of $ER$ as the cell's internal highway system for moving goods.
Exam Tip: If asked why $RER$ is "rough," mention that it has ribosomes attached to its surface.

Question 11:
Answer: Mitochondria contains enzymes necessary for the oxidation of carbohydrates and release energy needed for all vital life activities in the form of ATP. Hence they are called powerhouses of the cell.
Mitochondria take the food you eat and turn it into a chemical fuel called $ATP$. This $ATP$ is the "money" the cell uses to pay for all its activities, from moving to growing.
Teacher's Tip: $ATP$ stands for Adenosine Triphosphate, the cell's energy currency.
Exam Tip: Always link the term "powerhouse" to the production of $ATP$ energy.

Question 12:
Answer: Nucleus controls all the metabolic activities of the cell. It regulates cell division. Without nucleus, the cell dies and it cannot divide. Hence nucleus is the controller of cell.
The nucleus acts like the "brain" or "CEO" of the cell, holding all the important instructions in the form of $DNA$. It makes sure everything inside the cell is working correctly and decides when it's time for the cell to reproduce.
Teacher's Tip: The nucleus is the most important organelle because it holds the genetic "blueprints" for life.
Exam Tip: Mention that the nucleus regulates "cell division" to score full marks for its function.

Question 13:
Answer: Chromosomes are rod-shaped structures made of protein and DNA. A chromosome is composed of two identical chromatids attached by a centromere. Each chromatid is made up of tightly wound DNA wrapped around histones.
Chromosomes are only visible when the cell is preparing to divide into two new cells. They ensure that each new cell gets an exact copy of the parent's genetic information.
Teacher's Tip: Think of chromosomes as tightly packed "fenders" or "folders" containing the cell's $DNA$ instructions.
Exam Tip: Be able to label the "chromatids" and "centromere" on a diagram of a chromosome.

Question 14:
Answer: (a) Chloroplasts are the kitchens of the cell. They trap solar energy and use it to synthesize carbohydrates from carbon dioxide and water by the process of photosynthesis.
(b) Chromosomes are the hereditary material of the nucleus, concerned with the transmission of hereditary traits from parents to offspring.
(c) Endoplasmic Reticulum increases the surface area of the cytoplasm for different metabolic activities of the cell.
Chloroplasts allow plants to create food out of thin air and sunlight, which is why they are called "kitchens." Chromosomes are like biological couriers that carry traits like eye color from parents to their children.
Teacher's Tip: The Endoplasmic Reticulum provides extra "workspace" for the cell's chemistry to happen.
Exam Tip: Define "Heredity" as the passing of characteristics from one generation to the next.

Question 15:
Answer: (a) Acetabularia
(b) Deoxyribonucleic acid
(c) Theory
(d) Ostrich
(e) Nucleus
These facts help us understand the records and terminology of cell biology. For example, the Ostrich egg is famous for being the largest single cell in the world that we can see with the naked eye.
Teacher's Tip: Acetabularia is a giant single-celled alga that looks like a tiny mushroom!
Exam Tip: Practice spelling "Deoxyribonucleic acid" ($DNA$) correctly as it is a common test question.

Question 16:
Answer: Platform or work place for synthesis of proteins - Ribosomes
Power house of cells - Mitochondria
Help in formation of cell plate during cell division - Golgi complex
Site for photosynthesis - Chloroplast
Control tower of cell - Nucleus
Structure bearing genes - Chromosomes
These nicknames or descriptions help us remember the primary job of each part of the cell. For instance, the Golgi complex acts as a packaging and shipping department for the cell.
Teacher's Tip: Creating a "Job Title" for each organelle is a great way to memorize them.
Exam Tip: Associate "genes" specifically with "chromosomes" located inside the nucleus.

Question 17:
Answer: a - Cell membrane
b - Nucleus
c - Chromatin
d - Nuclear membrane
e - SER
f - Mitochondria
g - Golgi vesicles
h - Golgi complex
i - RER
j - Ribosomes
This list provides labels for a diagram showing the complex interior of a eukaryotic cell. Knowing these parts is essential for understanding how complex organisms function at their most basic level.
Teacher's Tip: Use these labels to practice your cell diagramming skills.
Exam Tip: Pay attention to the difference between "Golgi complex" (the stack) and "Golgi vesicles" (the small bubbles coming off).

Question 18:
Answer: (a) Robert Hooke
(b) Schleiden and Schwann
(c) Nerve cell
(d) Fibres of ramie
(e) ATP
(f) The cell of $Mycoplasma$ $laidlawii$
(g) Cell lacking a well-organised nucleus and membrane bound organelles is called prokaryotic cell.
The nerve cell is uniquely long and thin so it can carry messages quickly across your body like a telephone wire. $Mycoplasma$ is the record-holder for the smallest free-living cell, being much smaller than most other bacteria.
Teacher's Tip: $ATP$ is the "Universal Energy Currency" used by every living cell.
Exam Tip: Define a prokaryotic cell by its *lack* of a nuclear membrane.

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Question 19:
Answer: a - Plasmodesmata
b - Nucleolus
c - Mitochondria
d - Golgi apparatus
e - Chloroplast
f - cell membrane
g - cell membrane (Repeated in text)
h - ER
i - Microtubule
Plasmodesmata are tiny channels that act like tunnels through the stiff cell walls of plants so cells can share water and nutrients. Microtubules are like a scaffolding system that gives the cell its internal shape and structure.
Teacher's Tip: Think of Plasmodesmata as the "connecting doors" between neighboring plant cells.
Exam Tip: Don't get confused between the "Nucleus" (whole structure) and "Nucleolus" (dense spot inside the nucleus).

Question 20:
Answer: 1. (d) cells are functional and structural unit of plants and animals
2. (a) DNA
3. (c) plastid
4. (a) nucleus membrane
5. (b) Altman
6. (b) Robert Hooke
7. (a) Robert Hooke
These multiple-choice answers confirm the foundational facts about who discovered the cell and what makes it up. $DNA$ is the essential molecule found in chromosomes that holds all genetic information.
Teacher's Tip: When answering MCQs, read all options even if the first one looks correct!
Exam Tip: Remember that Robert Hooke's name is the most common answer for "Who discovered the cell?"