The Fundamental Unit of Life Cells Class 9 Free Notes and Mind Map (Free PDF Download)

This chapter explores the discovery of cells, their structure and organisation, the cell theory, different cell organelles and their functions, differences between prokaryotic and eukaryotic cells, and the processes of cell division (mitosis and meiosis). Understanding cells is crucial as they form the basic structural and functional units of all living organisms.

Introduction

While examining a thin slice of cork, Robert Hooke in 1665 saw that the cork resembled the structure of a honeycomb consisting of many little compartments. He called these boxes cells, a Latin word for ‘a little room’. This was the very first time that someone had observed that living things appear to consist of separate units. The use of the word ‘cell’ to describe these units is being used till this day in biology.

5.1 What are Living Organisms Made Up of?

When we peel the skin (epidermis) from the inner layer of an onion and observe it under a microscope after staining with safranin solution, we see small structures. These structures look similar to each other and together they form the onion bulb. The cells of the onion peel will all look the same, regardless of the size of the onion they came from.

These small structures that we see are the basic building units of the onion bulb. These structures are called cells. Not only onions, but all organisms that we observe around are made up of cells. However, there are also single cells that live on their own.

Cell Theory and Discovery

Important discoveries:

• Robert Hooke (1665): First discovered cells in a cork slice with a primitive microscope.
• Leeuwenhoek (1674): Discovered free living cells in pond water with an improved microscope.
• Robert Brown (1831): Discovered the nucleus in the cell.
• Purkinje (1839): Coined the term ‘protoplasm’ for the fluid substance of the cell.
• Schleiden (1838) and Schwann (1839): Presented the cell theory that all plants and animals are composed of cells and that the cell is the basic unit of life.
• Virchow (1855): Expanded the cell theory by suggesting that all cells arise from pre-existing cells.
• Electron microscope (1940): Made it possible to observe the complex structure of the cell and its organelles.

Unicellular and Multicellular Organisms

A single cell may constitute a whole organism as in Amoeba, Chlamydomonas, Paramoecium and bacteria. These organisms are called unicellular organisms (uni = single).

On the other hand, many cells group together in a single body and assume different functions to form various body parts in multicellular organisms (multi = many) such as some fungi, plants and animals.

Important: Every multi-cellular organism has come from a single cell. Cells divide to produce cells of their own kind. All cells thus come from pre-existing cells.

Shape and Size of Cells

Do all cells look alike?

• Cells vary in shape and size according to the specific function they perform.
• Some cells like Amoeba have changing shapes.
• Some cells have fixed and peculiar shapes for a particular type, for example, nerve cells have a typical shape.
• The human body has different types of cells: smooth muscle cell, ovum, blood cells, bone cell, sperm, nerve cell, fat cell, etc.

Cell Organelles and Division of Labour

Each living cell has the capacity to perform certain basic functions that are characteristic of all living forms. There is division of labour within a single cell. Each cell has certain specific components within it known as cell organelles. Each kind of cell organelle performs a special function, such as:

• Making new material in the cell
• Clearing up waste material
• Forming new proteins

A cell is able to live and perform all its functions because of these organelles. These organelles together constitute the basic unit called the cell.

Interesting fact: All cells are found to have the same organelles, no matter what their function is or what organism they are found in.

Questions (5.1) and Solutions

Q1. Who discovered cells, and how?

Solution:

• Robert Hooke discovered cells in 1665.
• He observed a thin slice of cork under a self-designed microscope.
• He saw that cork resembled the structure of a honeycomb with many little compartments.
• He called these boxes cells.

Q2. Why is the cell called the structural and functional unit of life?

Solution:

• Structural unit: All living organisms are made up of cells. Cells are the basic building blocks of all organisms.
• Functional unit: All life processes and functions (nutrition, respiration, excretion, reproduction) are carried out by cells.
• Each cell contains cell organelles that perform specific functions necessary for life.
• Therefore, the cell is both the structural and functional unit of life.

5.2 What is a Cell Made Up of? What is the Structural Organisation of a Cell?

If we study a cell under a microscope, we would come across three features in almost every cell:

1. Plasma membrane
2. Nucleus
3. Cytoplasm

All activities inside the cell and interactions of the cell with its environment are possible due to these features.

5.2.1 PLASMA MEMBRANE OR CELL MEMBRANE

The plasma membrane is the outermost covering of the cell that separates the contents of the cell from its external environment.

Functions:

• It allows or permits the entry and exit of some materials in and out of the cell.
• It prevents movement of some other materials.
• Therefore, the cell membrane is called a selectively permeable membrane.

Movement of Substances Across the Membrane

1. Diffusion:

Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion. Diffusion is the spontaneous movement of a substance from a region of high concentration to a region where its concentration is low.

Example:

• CO₂ (cellular waste) accumulates in high concentrations inside the cell.
• Outside the cell, CO₂ concentration is low.
• CO₂ moves out of the cell from high to low concentration by diffusion.
• Similarly, O₂ enters the cell by diffusion when its concentration inside decreases.

2. Osmosis:

Water also obeys the law of diffusion. The movement of water molecules through a selectively permeable membrane is called osmosis.

Definition: Osmosis is the net diffusion of water across a selectively permeable membrane toward a higher solute concentration.

Effect of Different Solutions on Cells

What happens if we put an animal cell or a plant cell into a solution of sugar or salt in water?

Three situations:

1. Hypotonic Solution:

• The medium surrounding the cell has a higher water concentration than the cell (very dilute solution).
• The cell will gain water by osmosis.
• More water will come into the cell than will leave.
• The cell is likely to swell up.

2. Isotonic Solution:

• The medium has exactly the same water concentration as the cell.
• There will be no net movement of water across the cell membrane.
• Water crosses the membrane in both directions equally.
• The cell will stay the same size.

3. Hypertonic Solution:

• The medium has a lower concentration of water than the cell (very concentrated solution).
• The cell will lose water by osmosis.
• More water leaves the cell than enters it.
• The cell will shrink.

Examples of Osmosis

Experiment 1: Egg in different solutions

• Remove the shell of an egg by dissolving it in dilute hydrochloric acid.
• Put the de-shelled egg in pure water → egg swells (water passes into it by osmosis).
• Place a similar egg in a concentrated salt solution → egg shrinks (water passes out by osmosis).

Experiment 2: Raisins or apricots

• Put dried raisins in plain water → they gain water and swell.
• Place them in a concentrated sugar or salt solution → they lose water and shrink.

Examples in nature:

• Unicellular freshwater organisms gain water through osmosis.
• Absorption of water by plant roots is also an example of osmosis.

Structure of Plasma Membrane

• The plasma membrane is flexible.
• It is made up of organic molecules called lipids and proteins.
• The structure can be observed only through an electron microscope.
• The flexibility enables the cell to engulf food and other material from its external environment.
• Such processes are known as endocytosis (example: Amoeba acquires food through endocytosis).

Questions (5.2.1) and Solutions

Q1. How do substances like CO₂ and water move in and out of the cell? Discuss.

Solution:

Movement of CO₂:

• CO₂ moves by the process of diffusion.
• Inside the cell, CO₂ accumulates in high concentration (as cellular waste).
• Outside the cell, CO₂ concentration is low.
• CO₂ moves out of the cell from high to low concentration.

Movement of Water:

• Water moves by the process of osmosis.
• Osmosis is the movement of water molecules through a selectively permeable membrane.
• Water moves from a region of higher water concentration to a region of lower water concentration.
• This helps in absorption of water by plant roots and maintains water balance in cells.

Movement of O₂:

• O₂ enters the cell by diffusion when its level inside the cell decreases.

Q2. Why is the plasma membrane called a selectively permeable membrane?

Solution:

• The plasma membrane allows or permits the entry and exit of some materials in and out of the cell.
• It prevents movement of some other materials.
• It selects which substances can pass through and which cannot.
• Therefore, it is called a selectively permeable membrane (also called semi-permeable membrane).

5.2.2 CELL WALL

Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane.

Composition:

• The plant cell wall is mainly composed of cellulose.
• Cellulose is a complex substance and provides structural strength to plants.

Plasmolysis

When a living plant cell loses water through osmosis, there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.

Experiment to observe plasmolysis:

• Mount the peel of a Rhoeo leaf in water on a slide.
• Observe under microscope – note small green granules called chloroplasts (contain chlorophyll).
• Put a strong solution of sugar or salt on the leaf.
• Wait for a minute and observe – plasmolysis occurs (cell contents shrink away from cell wall).

With dead cells:

• Place Rhoeo leaves in boiling water (kills cells).
• Mount on slide and add strong sugar/salt solution.
• Plasmolysis does NOT occur in dead cells.

Inference: Only living cells, not dead cells, are able to absorb water by osmosis.

Function of Cell Wall

• Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (hypotonic) external media without bursting.
• In such media, cells tend to take up water by osmosis.
• The cell swells, building up pressure against the cell wall.
• The wall exerts an equal pressure against the swollen cell.
• Because of their walls, such cells can withstand much greater changes in the surrounding medium than animal cells.

5.2.3 NUCLEUS

When we prepare a temporary mount of onion peel, we put iodine solution or safranin solution or methylene blue solution to stain the cells. According to their chemical composition, different regions of cells get coloured differently. Some regions appear darker than others.

Observing Human Cheek Cells

Procedure:

• Take a glass slide with a drop of water.
• Using an ice-cream spoon, gently scrape the inside surface of the cheek.
• Transfer the material to the slide and spread evenly.
• Put a drop of methylene blue solution to colour the material.
• Put a cover-slip and observe under microscope.

Observation:

• The cells are irregular or squamous in shape.
• There is a darkly coloured, spherical or oval, dot-like structure near the centre of each cell.
• This structure is called the nucleus.

Structure of Nucleus

Nuclear Membrane:

• The nucleus has a double-layered covering called nuclear membrane.
• The nuclear membrane has pores which allow the transfer of material from inside the nucleus to the cytoplasm.

Chromosomes:

• The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
• Chromosomes contain information for inheritance of characters from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules.
• Chromosomes are composed of DNA and protein.
• DNA molecules contain the information necessary for constructing and organising cells.
• Functional segments of DNA are called genes.

Chromatin Material:

• In a cell which is not dividing, DNA is present as part of chromatin material.
• Chromatin material is visible as an entangled mass of thread-like structures.
• Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.

Functions of Nucleus

• The nucleus plays a central role in cellular reproduction – the process by which a single cell divides and forms two new cells.
• It plays a crucial part in determining the way the cell will develop and what form it will exhibit at maturity.
• It directs the chemical activities of the cell.

Prokaryotes and Eukaryotes

Prokaryotes:

• In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane.
• Such an undefined nuclear region containing only nucleic acids is called a nucleoid.
• Organisms whose cells lack a nuclear membrane are called prokaryotes (Pro = primitive; karyote = nucleus).
• Example: Bacteria

Eukaryotes:

• Organisms with cells having a nuclear membrane are called eukaryotes.
• Example: Plants, animals, fungi

Differences:

• Prokaryotic cells also lack most of the other cytoplasmic organelles present in eukaryotic cells.
• The chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles but not with plastids.

5.2.4 CYTOPLASM

When we look at temporary mounts of onion peel or human cheek cells, we can see a large region of each cell enclosed by the cell membrane. This region takes up very little stain. It is called the cytoplasm.

Definition:

• The cytoplasm is the fluid content inside the plasma membrane.
• It contains many specialised cell organelles.
• Each organelle performs a specific function for the cell.

Membrane-bound Organelles

• Cell organelles are enclosed by membranes.
• In prokaryotes, membrane-bound cell organelles are absent.
• In eukaryotic cells, there are nuclear membrane and membrane-enclosed organelles.

Significance of membranes:

• Viruses lack any membranes and do not show characteristics of life until they enter a living body and use its cell machinery to multiply.

Question (5.2.4) and Solution

Q1. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.

Solution:

5.2.5 CELL ORGANELLES

Every cell has a membrane around it to keep its own contents separate from the external environment. Large and complex cells need a lot of chemical activities to support their complicated structure and function. To keep these activities separate from each other, cells use membrane-bound little structures (or ‘organelles’) within themselves. This is one of the features that distinguish eukaryotic cells from prokaryotic cells.

Some important cell organelles are:

• Endoplasmic Reticulum (ER)
• Golgi Apparatus
• Lysosomes
• Mitochondria
• Plastids
• Vacuoles

5.2.5 (i) ENDOPLASMIC RETICULUM (ER)

The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags (vesicles). The ER membrane is similar in structure to the plasma membrane.

Types of ER

1. Rough Endoplasmic Reticulum (RER):

• RER looks rough under a microscope because it has particles called ribosomes attached to its surface.
• Ribosomes are present in all active cells and are the sites of protein manufacture.
• The manufactured proteins are sent to various places in the cell using the ER.

2. Smooth Endoplasmic Reticulum (SER):

• SER helps in the manufacture of fat molecules, or lipids, important for cell function.
• Some proteins and lipids help in building the cell membrane – this process is known as membrane biogenesis.
• Some other proteins and lipids function as enzymes and hormones.

Functions of ER

• The ER serves as channels for transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and nucleus.
• The ER functions as a cytoplasmic framework providing a surface for biochemical activities.
• In liver cells of vertebrates, SER plays a crucial role in detoxifying many poisons and drugs.

5.2.5 (ii) GOLGI APPARATUS

The Golgi apparatus was first described by Camillo Golgi. It consists of a system of membrane-bound vesicles (flattened sacs) arranged approximately parallel to each other in stacks called cisterns. These membranes often have connections with the membranes of ER.

Functions of Golgi Apparatus

• The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus.
• Its functions include:
  • Storage
  • Modification
  • Packaging of products in vesicles
• In some cases, complex sugars may be made from simple sugars in the Golgi apparatus.
• The Golgi apparatus is also involved in the formation of lysosomes.

5.2.5 (iii) LYSOSOMES

Structure:

• Lysosomes are membrane-bound sacs filled with digestive enzymes.
• These enzymes are made by RER.

Functions of Lysosomes

• Lysosomes are a kind of waste disposal system of the cell.
• They help to keep the cell clean by digesting:
  • Any foreign material (such as bacteria or food)
  • Worn-out cell organelles
• Foreign materials and old organelles end up in the lysosomes, which break complex substances into simpler substances.
• Lysosomes contain powerful digestive enzymes capable of breaking down all organic material.

Lysosomes as ‘Suicide Bags’

• During disturbance in cellular metabolism (when the cell gets damaged), lysosomes may burst and the enzymes digest their own cell.
• Therefore, lysosomes are also known as the ‘suicide bags’ of a cell.

5.2.5 (iv) MITOCHONDRIA

Mitochondria are known as the powerhouses of the cell.

Structure of Mitochondria

• Mitochondria have two membrane coverings.
• The outer membrane is porous.
• The inner membrane is deeply folded.
• These folds increase surface area for ATP-generating chemical reactions.

Functions of Mitochondria

• The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphosphate) molecules.
• ATP is known as the energy currency of the cell.
• The body uses energy stored in ATP for:
  • Making new chemical compounds
  • Mechanical work

Special Feature

• Mitochondria are strange organelles because they have their own DNA and ribosomes.
• Therefore, mitochondria are able to make some of their own proteins.

5.2.5 (v) PLASTIDS

Plastids are present only in plant cells.

Types of Plastids

1. Chromoplasts (Coloured Plastids):

• Chromoplasts containing the pigment chlorophyll are known as chloroplasts.
• Chloroplasts are important for photosynthesis in plants.
• Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll.

2. Leucoplasts (White or Colourless Plastids):

• Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored.

Structure of Chloroplasts

• The internal organisation of the chloroplast consists of numerous membrane layers embedded in a material called the stroma.
• These are similar to mitochondria in external structure.
• Like mitochondria, plastids also have their own DNA and ribosomes.

5.2.5 (vi) VACUOLES

Vacuoles are storage sacs for solid or liquid contents.

Vacuoles in Different Cells

In Animal Cells:

• Vacuoles are small sized.

In Plant Cells:

• Plant cells have very large vacuoles.
• The central vacuole of some plant cells may occupy 50-90% of the cell volume.
• Vacuoles are full of cell sap and provide turgidity and rigidity to the cell.

Contents of Vacuoles

Many substances of importance in the life of the plant cell are stored in vacuoles:

• Amino acids
• Sugars
• Various organic acids
• Some proteins

In Unicellular Organisms:

• In Amoeba, the food vacuole contains the food items that the Amoeba has consumed.
• Specialised vacuoles play important roles in expelling excess water and some wastes from the cell.

Questions (5.2.5) and Solutions

Q1. Can you name the two organelles we have studied that contain their own genetic material?

Solution:
The two organelles that contain their own genetic material (DNA) are:

1. Mitochondria
2. Plastids (Chloroplasts)

Both have their own DNA and ribosomes, which enable them to make some of their own proteins.

Q2. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?

Solution:

• If the organisation of a cell is destroyed, the cell organelles will not be able to perform their functions.
• The cell will not be able to carry out basic life processes like:
  • Respiration
  • Nutrition
  • Waste removal
  • Protein synthesis
• The cell will lose its functional ability and will die.
• If lysosomes burst, their digestive enzymes will digest the cell’s own components, leading to cell death.

Q3. Why are lysosomes known as suicide bags?

Solution:

• Lysosomes contain powerful digestive enzymes capable of breaking down all organic material.
• During disturbance in cellular metabolism or when the cell gets damaged, lysosomes may burst.
• When lysosomes burst, the enzymes are released and they digest their own cell.
• This leads to cell death.
• Therefore, lysosomes are known as ‘suicide bags’ of a cell.

Q4. Where are proteins synthesised inside the cell?

Solution:

• Proteins are synthesised in the ribosomes.
• Ribosomes are present on the surface of Rough Endoplasmic Reticulum (RER).
• Ribosomes are also found free in the cytoplasm.
• Therefore, protein synthesis occurs on ribosomes attached to RER and on free ribosomes in the cytoplasm.

Cell Division

Each cell acquires its structure and ability to function because of the organisation of its membrane and organelles in specific ways. The cell has a basic structural organisation that helps it perform functions like respiration, obtaining nutrition, clearing waste material, and forming new proteins. Thus, the cell is the fundamental structural unit of living organisms. It is also the basic functional unit of life.

Why Does Cell Division Occur?

New cells are formed in organisms in order to:

• Grow
• Replace old, dead and injured cells
• Form gametes required for reproduction

The process by which new cells are made is called cell division.

Types of Cell Division

There are two main types of cell division:

1. Mitosis
2. Meiosis

Mitosis

The process of cell division by which most of the cells divide for growth is called mitosis.

Process:

• In mitosis, each cell called mother cell divides to form two identical daughter cells.
• The daughter cells have the same number of chromosomes as the mother cell.

Functions:

• It helps in growth of organisms.
• It helps in repair of tissues in organisms.

Meiosis

Specific cells of reproductive organs or tissues in animals and plants divide to form gametes, which after fertilisation give rise to offspring. They divide by a different process called meiosis.

Process:

• Meiosis involves two consecutive divisions.
• When a cell divides by meiosis, it produces four new cells instead of just two.
• The new cells have only half the number of chromosomes than that of the mother cell.

Reason for reduction:

• This reduction is necessary so that when two gametes (egg and sperm) fuse during fertilisation, the offspring has the correct number of chromosomes.
• If gametes had the full number of chromosomes, the offspring would have double the number, which would be harmful.

End-of-Chapter Exercises – Questions and Solutions

Q1. Make a comparison and write down ways in which plant cells are different from animal cells.

Solution:

Q2. How is a prokaryotic cell different from a eukaryotic cell?

Solution:

Q3. What would happen if the plasma membrane ruptures or breaks down?

Solution:

• If the plasma membrane ruptures or breaks down:
  • The cell will lose its shape and structure.
  • The cell contents will leak out into the external environment.
  • The cell will not be able to regulate the entry and exit of materials.
  • Essential substances will be lost and harmful substances may enter freely.
  • The cell will lose its identity and die.

Q4. What would happen to the life of a cell if there was no Golgi apparatus?

Solution:

• If there was no Golgi apparatus in the cell:
  • Proteins and lipids synthesised by ER could not be modified, packaged and dispatched to their correct destinations.
  • Secretion of materials out of the cell would not occur properly.
  • Lysosome formation would not take place (Golgi apparatus forms lysosomes).
  • Complex sugars could not be made from simple sugars.
  • The cell would not be able to perform its secretory functions and would eventually die.

Q5. Which organelle is known as the powerhouse of the cell? Why?

Solution:

• Mitochondria are known as the powerhouses of the cell.

Reason:

• Mitochondria release energy required for various chemical activities needed for life.
• This energy is released in the form of ATP (Adenosine triphosphate) molecules.
• ATP is the energy currency of the cell.
• The body uses energy stored in ATP for making new chemical compounds and for mechanical work.
• Therefore, mitochondria are called powerhouses because they generate energy (ATP) for the cell.

Q6. Where do the lipids and proteins constituting the cell membrane get synthesised?

Solution:

• Lipids are synthesised in the Smooth Endoplasmic Reticulum (SER).
• Proteins are synthesised in the Ribosomes present on the Rough Endoplasmic Reticulum (RER).
• These lipids and proteins are then assembled to form the cell membrane through a process called membrane biogenesis.

Q7. How does an Amoeba obtain its food?

Solution:

• Amoeba obtains its food through a process called endocytosis.
• The plasma membrane is flexible and enables the cell to engulf food and other material from its external environment.
• Amoeba extends its pseudopodia (false feet) around the food particle.
• The food particle is enclosed in a food vacuole.
• Digestive enzymes act on the food inside the vacuole and break it down.
• The digested food is then absorbed and used by the cell.

Q8. What is osmosis?

Solution:

• Osmosis is the movement of water molecules through a selectively permeable membrane.
• It is the net diffusion of water from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration).
• Osmosis is a special case of diffusion that involves only water molecules.
• It is important for:
  • Absorption of water by plant roots
  • Maintaining water balance in cells
  • Transport of water in living organisms

Q9. Carry out the following osmosis experiment:

Take four peeled potato halves and scoop each one out to make potato cups. One potato cup should be made from a boiled potato. Put each potato cup in a trough containing water. Now:

• (a) Keep cup A empty
• (b) Put one teaspoon sugar in cup B
• (c) Put one teaspoon salt in cup C
• (d) Put one teaspoon sugar in the boiled potato cup D

Keep these for two hours. Then observe and answer:

(i) Explain why water gathers in the hollowed portion of B and C.

Solution:

• In cups B and C, sugar and salt solutions have higher solute concentration than pure water.
• The potato cells have lower solute concentration (higher water concentration) than the sugar/salt solutions.
• Water moves from the potato cells (higher water concentration) into the sugar/salt solution (lower water concentration) by osmosis.
• This causes water to gather in the hollowed portions of B and C.

(ii) Why is potato A necessary for this experiment?

Solution:

• Potato A is kept empty (no sugar or salt added).
• It serves as a control or standard for comparison.
• It helps us understand that water gathering in B and C is due to the presence of solute (sugar/salt), not due to any other factor.
• By comparing A with B and C, we can see the effect of osmosis clearly.

(iii) Explain why water does not gather in the hollowed out portions of A and D.

Solution:

For A:

• Cup A is empty (no solute added).
• There is no concentration difference between the water in the trough and the hollow portion.
• Therefore, there is no net movement of water by osmosis.
• Water does not gather in A.

For D:

• Cup D is made from a boiled potato.
• Boiling kills the cells of the potato.
• Dead cells cannot perform osmosis (only living cells can).
• Even though sugar is present, the dead potato cells cannot allow selective movement of water.
• Therefore, water does not gather in D.

Q10. Which type of cell division is required for growth and repair of body and which type is involved in formation of gametes?

Solution:

For growth and repair of body:

• Mitosis is required.
• In mitosis, one mother cell divides to form two identical daughter cells.
• Daughter cells have the same number of chromosomes as the mother cell.
• This helps in growth of the organism and repair of damaged tissues.

For formation of gametes:

• Meiosis is involved.
• In meiosis, one cell divides to produce four new cells.
• These cells have half the number of chromosomes than the mother cell.
• This is necessary for sexual reproduction so that when two gametes fuse, the offspring has the correct number of chromosomes.

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