Class 9 Science: Cell: The Building Block of Life — Chapter Question Set

(d)

(d)

(b)

(a)

(b)

(a)

(a)

(b)

(d)

(b)

(d)

(b)

(b)

(d)

(d)

(a)

(b)

(a)

(c)

(a)

(c)

(a)

(c)

(d)

(d)

(c)

(c)

(d)

(b)

(d)

(a)

(d)

(a)

(b)

(c)

(b)

(a)

(b)

(a)

(a)

(b)

(a)

(d)

(a)

(c)

(c)

(a)

(b)

(c)

(d)

(d)

(b)

(c)

(a)

(d)

200–300X

selectively

osmosis

hypertonic

cellulose

nucleoid

genes

chromatin

Rough

vesicles

ATP (Adenosine Triphosphate)

cristae

leucoplasts

two

Rudolf Virchow

True

False (It is selectively permeable.)

True

False (Only plant, fungi and bacteria cells have a cell wall.)

False (They lack membrane-bound organelles.)

True

False (Rough ER with ribosomes is involved in protein synthesis; SER synthesises fats and hormones.)

True

True

False (Leucoplasts are colourless; chromoplasts contain pigments.)

True

True

False (Each daughter cell has half the number of chromosomes.)

True

False (Due to their rigid cell walls, plant cells do not show contact inhibition.)

A–2, B–4, C–3, D–1, E–5

A–2, B–3, C–4, D–1, E–5

(a)

(a)

(d)

(a)

(a)

(a)

(a)

(d)

The limit of resolution of the human eye is about 0.1 mm, meaning objects closer together than this cannot be seen as separate. Microscopes overcome this by using lenses — an objective lens and an eyepiece — that magnify objects so that details smaller than 0.1 mm become visible.

Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration, and it does not require a membrane. Osmosis is a special case of diffusion in which water molecules move across a selectively permeable membrane from a region of lower solute concentration to one of higher solute concentration.

In a hypertonic solution, both plant and animal cells lose water to the surrounding solution. In a plant cell, the inner content shrinks away from the rigid cell wall — a process called plasmolysis — while the wall maintains its shape. In an animal cell, which lacks a cell wall, the whole cell shrinks.

The cell membrane is made of a lipid bilayer in which water-attracting heads face outwards and water-repelling tails face inwards, with proteins embedded in it. The molecules can move within the layer, making it fluid, while the scattered arrangement of lipids and proteins gives it a mosaic appearance.

Plants are fixed in one place and need a rigid outer structure to withstand environmental stresses such as wind and rain. The cell wall provides mechanical support, helps the cell maintain its shape and keeps the plant upright even when turgor pressure changes.

Prokaryotic cells do not have a well-defined nucleus; their genetic material lies naked in a region called the nucleoid. Eukaryotic cells, by contrast, possess a well-defined nucleus enclosed by a nuclear membrane that separates the genetic material from the cytoplasm.

The nucleolus is a dense, round body inside the nucleus where the synthesis of ribosomal subunits takes place. These subunits move out into the cytoplasm, where they assemble into ribosomes that carry out protein synthesis.

Genes are functional segments of DNA that carry the information responsible for inheritance and the traits of an organism. They are located on chromosomes within the nucleus of the cell.

Rough endoplasmic reticulum (RER) has ribosomes attached to its surface and is mainly involved in protein synthesis. Smooth endoplasmic reticulum (SER) lacks ribosomes and is involved in the synthesis of fats (lipids) and hormones.

The Golgi apparatus modifies, sorts and packages proteins and lipids into vesicles for transport within the cell or secretion outside it. It is also involved in the formation of lysosomes.

Lysosomes are single membrane-bound sacs filled with digestive enzymes that break down unwanted proteins, carbohydrates, fats and damaged organelles. By digesting and removing this waste material, they keep the cell clean and healthy, hence their name.

Cristae are finger-like projections of the inner membrane of mitochondria. They greatly increase the surface area available for chemical reactions, which facilitates the production of energy (ATP) through respiration.

Chromoplasts contain pigments such as yellow, orange and red, which give colour to flowers and fruits and help attract pollinators. Leucoplasts are colourless plastids that store starch, oils or proteins, mainly serving as storage organelles.

The vacuole stores water, minerals, sugars and waste materials and maintains turgor pressure that keeps the cell firm and the plant upright. When water is lost from the vacuole, the cell loses turgidity and the plant wilts.

Mitosis produces two daughter cells that are identical to the parent cell and have the same chromosome number. Meiosis produces four daughter cells, each with half the chromosome number of the parent cell.

The classical Cell Theory was proposed by Schleiden (1838), Schwann (1839) and Virchow (1855). Its three main points are: all organisms are made of cells; the cell is the basic unit of structure and function in living things; and all cells arise from pre-existing cells.

Contact inhibition is the normal property of animal cells by which they stop dividing once they come into contact with neighbouring cells. Cancer cells lose this control and divide uncontrollably, forming masses called tumours.

Totipotency is the ability of any living plant cell to develop into a complete plant under suitable conditions. The concept was proposed by Gottlieb Haberlandt in 1902.

Mature human red blood cells lack a nucleus so that more space is available for haemoglobin, allowing them to carry more oxygen. Because they cannot repair or divide, they have a lifespan of about 120 days.

Errors in mitosis can lead to uncontrolled cell divisions, tumours and abnormal chromosome numbers. Errors in meiosis can cause genetic disorders, developmental problems and reduced fertility.

Introduction: The cell membrane is a thin boundary that surrounds every cell and protects its contents.
Key Points:
1. Selectively permeable boundary — It allows some substances to pass through while blocking others, defining the cell's individuality.
2. Fluid-mosaic structure — It has a lipid bilayer with proteins embedded; molecules move sideways, flip and rotate, making it fluid.
3. Proteins as gatekeepers — Embedded proteins help substances pass through and enable communication with neighbouring cells.
Conclusion: The cell membrane controls movement of materials and enables interaction between the cell and its surroundings.

Introduction: Osmosis is the movement of water through a selectively permeable membrane from a dilute to a concentrated solution.
Key Points:
1. Potato in plain water — Water moves into the potato cells, so the piece swells and gains weight.
2. Potato in salt solution — Water moves out of the potato cells, so the piece shrinks and loses weight.
3. Key principle — Water moves from more water (less solute) to less water (more solute) until concentrations equalise.
Conclusion: The cell membrane allows water but not solute molecules to pass, demonstrating osmosis.

Introduction: The nucleus is the house of coded instructions that controls all activities of the cell.
Key Points:
1. Double membrane with pores — The nuclear membrane has pores allowing material transfer between nucleus and cytoplasm.
2. Chromosomes and DNA — Contains chromosomes made of DNA and proteins; functional segments called genes carry hereditary information.
3. Nucleolus — The dense round body where ribosomal subunits are synthesised before assembly in the cytoplasm.
Conclusion: The nucleus stores genetic information and controls the structure and activities of the cell.

Introduction: Cells are classified as prokaryotic or eukaryotic based on the presence or absence of a well-defined nucleus.
Key Points:
1. Nucleus — Prokaryotic cells have a primitive nucleus (nucleoid); eukaryotic cells have a well-defined membrane-bound nucleus.
2. Organelles — Prokaryotic cells lack membrane-bound organelles; eukaryotic cells have several membrane-bound organelles.
3. Size and organisation — Prokaryotic cells are smaller (1–10 µm) and usually unicellular; eukaryotic cells are larger (10–100 µm) and can be uni- or multicellular.
Conclusion: Eukaryotic cells are more complex and carry out life processes in specialised organelles.

Introduction: Mitochondria are called the powerhouses of the cell because they supply energy for most cellular activities.
Key Points:
1. Double membrane — The outer membrane is smooth and porous; the inner membrane is folded into cristae increasing surface area.
2. Cellular respiration — Glucose and other molecules are broken down here to release energy during cellular respiration.
3. ATP as energy currency — Released energy is stored as ATP, used for most cellular activities; mitochondria have their own DNA and ribosomes.
Conclusion: Mitochondria are essential organelles that produce and store the energy currency of the cell.

Introduction: Plastids are special organelles in plant cells used for food synthesis and storage.
Key Points:
1. Chloroplasts — Double-membrane bound, contain chlorophyll; site of photosynthesis where sunlight is absorbed to synthesise sugars.
2. Chromoplasts — Contain pigments other than chlorophyll (yellow, orange, red); give bright colours to flowers and fruits, attracting pollinators.
3. Leucoplasts — Colourless plastids that store food materials such as starch, oils or proteins, e.g., in potato and taro cells.
Conclusion: Plastids enable plants to synthesise food, display colours and store nutrients efficiently.

Introduction: Mitosis is the most common type of cell division producing two genetically identical daughter cells from one parent cell.
Key Points:
1. Identical daughter cells — Each new cell gets the same DNA and the same number of chromosomes as the parent cell.
2. Growth and repair — It is important for normal growth, repair of damaged tissues, maintenance and asexual reproduction.
3. Genetic continuity — It ensures that genetic information is largely maintained across body cells throughout life.
Conclusion: Mitosis allows living organisms to grow, replace old or damaged cells and maintain genetic continuity.

Introduction: Meiosis is a two-step cell division that produces gametes for sexual reproduction.
Key Points:
1. Two successive divisions — The parent cell divides twice, producing four daughter cells from one parent cell.
2. Half the chromosomes — During the first division, chromosome number is reduced to half; each gamete has half the DNA of the parent cell.
3. Genetic diversity — Gametes produced create variations and diversity among organisms, so children resemble parents but are not identical.
Conclusion: Meiosis is essential for sexual reproduction and for creating genetic diversity in living organisms.

Introduction: The Cell Theory is a unifying principle of biology formulated through the work of three scientists.
Key Points:
1. Schleiden (1838) — German botanist who reported that all plants are made up of cells.
2. Schwann (1839) — German zoologist who found that all animals are also made up of cells.
3. Virchow (1855) — Expanded the theory by stating that new cells are formed only from pre-existing cells.
Conclusion: The Cell Theory unifies all biology from bacteria to humans and explains life's continuity through cell division.

Introduction: The Endoplasmic Reticulum is a large organelle that spreads like a network within the cytoplasm.
Key Points:
1. Continuous with nuclear envelope — ER is continuous with the outer membrane of the nuclear envelope, forming a network.
2. Rough ER (RER) — Has ribosomes on its surface, looks rough; mainly involved in protein synthesis and secretion.
3. Smooth ER (SER) — Lacks ribosomes, looks smooth; involved in synthesis and storage of fats and hormones.
Conclusion: The ER plays a key role in synthesis and transport of proteins, fats and hormones in the cell.

Introduction: Vacuoles are storage organelles that also provide structural support to cells.
Key Points:
1. Large central vacuole in plants — Mature plant cells have one large vacuole filled with cell sap storing water, minerals, sugars and waste.
2. Turgor and firmness — By storing water, vacuoles maintain pressure keeping the plant cell firm; water loss causes wilting.
3. Smaller in animal cells — Animal cells sometimes have small vacuoles for temporary storage of materials.
Conclusion: Vacuoles are essential for storage and for maintaining the firmness and structure of plant cells.

Introduction: The Golgi apparatus consists of stacks of flattened, sac-like structures that act as the cell's packaging centre.
Key Points:
1. Receives from ER — It is functionally linked to the ER, the cell membrane and other organelles.
2. Modifies and sorts — It modifies, sorts and packages proteins and lipids into vesicles for transport or secretion.
3. Lysosome formation — It also helps in lysosome formation and shipping materials within or outside the cell.
Conclusion: The Golgi apparatus ensures that cellular products are properly processed, packaged and delivered.

Introduction: Lysosomes are single membrane-bound sacs filled with digestive enzymes that keep the cell clean.
Key Points:
1. Waste breakdown — Enzymes break down unwanted proteins, carbohydrates, fats and damaged organelles.
2. Reuse of products — Breakdown products are released into the cytoplasm and may be reused in other cellular processes.
3. Defence — They help digest foreign materials and worn-out cell parts, keeping the cell clean and healthy.
Conclusion: Lysosomes are the cell's clean-up system, essential for maintaining cellular health and efficiency.

Introduction: Cell division is the process by which new cells are formed from pre-existing cells.
Key Points:
1. Purpose — It allows growth, repair of damaged tissues and reproduction in living organisms.
2. Mitosis — Produces two genetically identical daughter cells with the same chromosome number; important for growth and repair.
3. Meiosis — Produces four daughter cells each with half the chromosome number; important for sexual reproduction and genetic diversity.
Conclusion: Both types of cell division are essential for the continuity and diversity of life.

Introduction: Programmed Cell Death (PCD) is a genetically regulated and organised process of selective cell destruction.
Key Points:
1. Natural balance — Cells grow, divide, function and eventually die when no longer needed; PCD maintains this balance.
2. Development — During embryo development, PCD helps form fingers by eliminating cells between digits, preventing webbed hands.
3. Quality control — It is essential for normal development, cellular quality control and immune function.
Conclusion: PCD is a natural, essential process that ensures proper development and cellular quality in organisms.

(a) The process responsible is osmosis.
(b) In Beaker A, water moved into the potato cells; in Beaker B, water moved out of the potato cells.
(c) The concentrated salt solution drew water out of the cells, causing them to lose turgidity and shrink.
(d) Boiling kills the cells and destroys the selectively permeable membrane, so osmosis would not occur.

(a) Cell X is in a hypotonic solution and Cell Y is in a hypertonic solution.
(b) Water moves into Cell X and out of Cell Y.
(c) The first statement is correct — water moved into Cell X and out of Cell Y through the cell membrane.
(d) A plant cell would swell but its rigid cell wall would prevent it from bursting, providing rigidity.

(a) The rigid cell wall of the plant cell maintained its outer shape even as the inner contents shrank.
(b) This phenomenon is called plasmolysis.
(c) Animal (cheek) cells have no cell wall, so they shrink completely when they lose water.
(d) Plant cells have a cell wall that provides rigidity, while animal cells lack it and are therefore more flexible and prone to complete shrinkage.

(a) It demonstrated that DNA controls the structure and activities of a cell.
(b) No — only the DNA was synthetic; the rest of the cell was already living.
(c) The cytoplasm and the cell membrane were taken from an existing living cell.
(d) Such experiments raise ethical questions about creating synthetic life, safety and the responsible use of technology.

(a) The farmer has applied the concept of osmosis, where high concentration draws water out of microbial cells, killing or inhibiting them.
(b) Salt and sugar create a hypertonic environment that prevents bacterial and fungal growth by drawing water out of the microbes.
(c) Lemon pickle made with salt and jaggery, stored in sterilised glass bottles, is a healthy example.
(d) The case addresses food security, reducing waste, sustainable practices and the scientific application of knowledge.

(a) It suggests that these organelles share an evolutionary history with single-celled organisms.
(b) Mitochondria and the nucleus contain DNA; plastids also contain DNA.
(c) These organelles can make some of their own proteins using their own DNA and ribosomes.
(d) Mitochondria produce energy in the form of ATP, while chloroplasts carry out photosynthesis to synthesise food.

(a) The absence of a nucleus provides more space inside the cell for haemoglobin.
(b) More haemoglobin means more oxygen can be transported by each red blood cell.
(c) Without a nucleus, RBCs cannot divide or produce the proteins needed for repair.
(d) New RBCs are produced regularly in the bone marrow to replace the dead ones.

(a) Errors in mitosis can lead to tumours and abnormal chromosome numbers.
(b) Errors in meiosis can cause genetic disorders, developmental problems, early pregnancy loss or reduced fertility.
(c) Contact inhibition normally stops animal cells from dividing uncontrollably.
(d) No — plant cells have rigid cell walls and do not show contact inhibition; they follow a different pattern of growth.

(a) Root tips contain actively dividing cells in the meristematic zone, making them ideal for studying division.
(b) The process is called cell division.
(c) The cells show different structures because they are at different stages of cell division.
(d) Aceto-carmine stain is used to observe these cells.

(a) The cell wall (or plastids) is present in plant cells but absent in animal cells.
(b) Mitochondria, the nucleus, endoplasmic reticulum, Golgi apparatus, ribosomes, lysosomes and vacuoles are present in both.
(c) Vacuoles — they are small in animal cells but large in plant cells.
(d) Mitochondria and the nucleus contain DNA; plastids also contain DNA.

Introduction: The cell membrane is a universal feature of all cells, while the cell wall is an additional covering found in plant, fungi and bacteria cells.
Key Points:
1. Cell membrane structure — Thin (7–10 nm), selectively permeable, made of lipids and proteins described by the fluid-mosaic model.
2. Cell membrane function — Protects contents, controls movement of substances, and enables communication with surroundings and neighbouring cells.
3. Cell wall structure — Rigid outer covering outside the cell membrane, primarily made of cellulose in plants; permeable to water and dissolved minerals.
4. Cell wall function — Provides rigidity and strength, helps plants stay upright and withstand environmental stresses like wind and rain.
5. Key difference — When placed in concentrated solution, plant cells maintain shape due to the wall while animal cells shrink completely; the membrane is selectively permeable, the wall is permeable.
Conclusion: While the cell membrane is essential in all cells for regulating exchange, the cell wall provides additional structural support needed by plants.

Introduction: Eukaryotic cells carry out various life processes in specialised organelles that work together like a tiny living factory.
Key Points:
1. Nucleus — Double-membraned with pores; contains chromosomes, DNA and genes; controls all activities and stores hereditary information.
2. Endoplasmic Reticulum — Network continuous with nuclear envelope; RER synthesises proteins, SER synthesises fats and hormones.
3. Golgi apparatus and lysosomes — Golgi modifies, sorts and packages materials; lysosomes are enzyme-filled sacs that break down waste and damaged organelles.
4. Mitochondria — Double-membred with cristae; powerhouse producing ATP through cellular respiration; has its own DNA and ribosomes.
5. Plastids and vacuoles — Chloroplasts synthesise food, chromoplasts provide colour, leucoplasts store food; vacuoles store water, minerals and waste, maintaining cell firmness.
Conclusion: Each organelle performs a specific function, and together they keep the cell functioning as a coordinated living system.

Introduction: Cell division is the process by which new cells are formed from pre-existing cells, allowing organisms to grow, repair and reproduce.
Key Points:
1. Mitosis — identical daughter cells — One parent cell divides to produce two genetically identical daughter cells with the same chromosome number.
2. Mitosis — significance — Important for normal growth, repair of damaged tissues, maintenance and asexual reproduction throughout life.
3. Meiosis — four gametes — Parent cell divides twice to produce four daughter cells, each with half the chromosome number of the parent cell.
4. Meiosis — significance — Essential for sexual reproduction; creates genetic diversity so children resemble parents but are not identical.
5. Errors and consequences — Errors in mitosis cause tumours; errors in meiosis cause genetic disorders, developmental problems or reduced fertility.
Conclusion: Mitosis ensures growth and repair while meiosis ensures reproduction and diversity, both under controlled regulation.

Introduction: The Cell Theory, developed by Schleiden, Schwann and Virchow, is a unifying principle stating that all organisms are made of cells.
Key Points:
1. Three principles — All organisms are made of cells; the cell is the basic unit of structure and function; all cells arise from pre-existing cells.
2. Controlled growth — Cells grow and divide in a controlled way, performing functions and eventually dying when no longer needed.
3. Contact inhibition — In animal cells, division stops when cells touch neighbours; cancer cells lose this control and form tumours.
4. Cell life span — Every cell has a definite life span; dead cells are replaced by new cells performing the same function.
5. Programmed cell death — PCD is a genetically regulated process essential for development, quality control and immune function, like forming fingers in embryos.
Conclusion: The Cell Theory explains life's continuity through cell division, while controlled growth and death maintain healthy organisms.

Introduction: Plant and animal cells are both eukaryotic but differ in several structures that reflect their different modes of life.
Key Points:
1. Cell wall — Plant cells have a rigid cellulose cell wall outside the membrane; animal cells lack it, allowing flexibility and shape changes.
2. Plastids — Plant cells have chloroplasts, chromoplasts and leucoplasts for food synthesis and storage; animal cells lack plastids.
3. Vacuoles — Mature plant cells have one large central vacuole for storage and firmness; animal cells have small or temporary vacuoles.
4. Shape and arrangement — Plant cells are box-shaped and regularly arranged; animal cells are irregularly shaped and arranged.
5. Functional significance — These differences suit plants for stationary, self-feeding life and animals for movement and food-seeking.
Conclusion: The structural differences between plant and animal cells reflect their distinct lifestyles and modes of nutrition.

Introduction: The microscope is an essential tool for studying cells, which are usually too small to be seen by the unaided eye.
Key Points:
1. Robert Hooke's discovery — In 1665, Hooke observed box-like compartments in cork using a self-designed microscope and named them cells.
2. Light microscope — Uses objective and eyepiece lenses for magnification; total magnification = eyepiece × objective (e.g., 10X × 10X = 100X).
3. Electron microscope — Uses a beam of electrons instead of light to produce highly magnified images at the nanometre scale.
4. Three key features — Scientists improved microscopes by improving resolution (clarity), contrast (brightness difference) and magnification.
5. Estimating cell size — Diameter of visible field ÷ number of cells along the diameter gives the estimated size of one cell.
Conclusion: The microscope has been fundamental to cell biology, revealing structures invisible to the naked eye and enabling our understanding of cells.

Introduction: Plastids are special organelles in plant cells used for food synthesis and storage, playing a vital role in plant life.
Key Points:
1. Chloroplasts — Double-membrane bound organelles containing chlorophyll; site of photosynthesis where sunlight is absorbed to synthesise sugars.
2. Internal structure — Contains a semi-fluid stroma with disc-shaped membrane structures holding chlorophyll; sugars and starch granules are stored in the stroma.
3. Chromoplasts — Contain pigments other than chlorophyll (yellow, orange, red); give bright colours to flowers and fruits, attracting pollinators.
4. Leucoplasts — Colourless plastids that store food materials such as starch, oils or proteins, e.g., in potato and taro cells.
5. Own DNA and ribosomes — Like mitochondria, plastids have their own DNA and ribosomes, suggesting an evolutionary link with bacteria.
Conclusion: Plastids enable plants to synthesise food, attract pollinators through colour, and store nutrients for future use.

Introduction: The nucleus is the house of coded instructions that controls the structure and activities of a eukaryotic cell.
Key Points:
1. Nuclear membrane — Double-layered covering with pores that allow transfer of material between nucleus and cytoplasm.
2. Nucleolus — Dense round body where ribosomal subunits are synthesised before exiting to the cytoplasm for assembly into ribosomes.
3. Chromosomes — Rod-shaped structures visible only during cell division; composed of DNA and specific proteins carrying genetic information.
4. Genes and chromatin — Functional segments of DNA are called genes; in non-dividing cells, DNA exists as chromatin, an entangled mass of threads.
5. Prokaryotic contrast — Prokaryotic cells lack a well-defined nucleus; their DNA is a single circular molecule in a region called the nucleoid.
Conclusion: The nucleus stores and transmits genetic information, controlling all cellular activities and ensuring hereditary continuity.

Introduction: All living cells communicate with their surroundings and neighbouring cells, primarily through the cell membrane.
Key Points:
1. Selective exchange — The selectively permeable cell membrane allows exchange of materials like oxygen, carbon dioxide and water with the environment.
2. Osmosis and diffusion — Water moves by osmosis through the membrane; particles move by diffusion along concentration gradients.
3. Cell wall role — In plants, fungi and bacteria, the permeable cell wall allows water and dissolved minerals to pass through alongside the membrane.
4. Communication between cells — The membrane enables cells to interact with neighbouring cells, coordinating tissue and organ functions.
5. Root hair example — In roots, epidermal hair-like projections increase surface area for absorption of water and minerals from the soil.
Conclusion: The cell membrane and cell wall together enable cells to exchange materials, respond to their environment and communicate with each other.

Introduction: Normal cells grow, divide and die in a controlled manner, but when this system breaks down, serious problems arise.
Key Points:
1. Normal control — Cells divide in a controlled way, stay in the right place and die when no longer needed, maintaining balance.
2. Contact inhibition — In animal cells, division usually stops when cells come in contact with neighbouring cells, preventing overgrowth.
3. Cancer cells — They lose contact inhibition and keep dividing uncontrollably, leading to the formation of tumours.
4. Types of tumours — Tumours may be benign or malignant; cancerous tumours can invade nearby tissues and spread to other parts of the body.
5. Plant cell difference — Plant cells do not show contact inhibition due to their rigid cell walls and follow a different pattern of growth.
Conclusion: Understanding contact inhibition and its loss in cancer helps explain the importance of controlled cell division for healthy life.