Cell: The Building Block of Life

2.1 How to Study Cells?

• The human eye can distinguish two points as separate only if they are at least 0.1 mm apart when viewed from 25 cm — this is called the limit of resolution of the human eye.
• Most cells are too small to be seen by the unaided eye, so scientists use microscopes to study them.
• A convex lens or a combination of lenses (objective lens + eyepiece) is used for magnification — making objects appear larger.
• Robert Hooke was the first person to observe a cell in 1665 using a self-designed microscope (200–300X magnification). He observed thin slices of cork and named the box-like compartments ‘cells’.
• In school laboratories, light microscopes are used with different objective lenses (10X, 40X) to achieve better magnification and resolution under visible light.
• Scientists also use powerful electron microscopes that use a beam of electrons instead of light. They reveal cell structure at the nanometre scale (1 nanometre = one-billionth of a metre).
• Over the years, microscopes have been improved in three main features: resolution (clarity), contrast (difference in brightness), and magnification.
• The total magnification of a microscope = magnifying power of eyepiece × magnifying power of objective lens. (Example: 10X eyepiece × 10X objective = 100X total magnification)

Fig. 2.2: Structure of a light microscope

2.2 Structure of a Cell

• Cells are organised into specialised tissues, which form organs, which work together as organ systems.
• Even when organised into tissues and organs, the cell remains the fundamental unit of structure and function in all living organisms.
• Cells interact with one another and with their surroundings through the cell boundary (cell membrane), where substances move between cells and their external environment.
• Even single-celled organisms exchange materials and respond to their environment through the cell membrane.

2.2.1 Cell membrane — The universal feature of a cell

• The cell membrane (also called the plasma membrane) is a thin boundary that surrounds a cell and protects its contents.
• It defines the individuality of a cell.
• The cell membrane is selectively permeable — it allows some substances to pass through while blocking others.
• It is extremely thin — about 7 to 10 nanometres (nm) thick.
• It is made up of lipids (fats) and proteins.
• Its structure is explained by the fluid-mosaic model:
  • The membrane has a lipid bilayer — two layers of fat molecules with water-attracting (hydrophilic) heads facing outward and water-repelling (hydrophobic) tails facing inward.
  • Proteins are embedded in this lipid bilayer.
  • The molecules can move sideways, flip and rotate within the membrane — so it is called fluid.
  • Proteins act like gatekeepers, helping substances pass through.
  • Since molecules are arranged like tiles in a mosaic, it is called the ‘mosaic’ model.
• Osmosis: The movement of water through a selectively permeable membrane from a region of more water (dilute/hypotonic solution) to a region of less water (concentrated/hypertonic solution) until concentrations equalise.
• Diffusion: The net movement of particles from higher to lower concentration — occurs even without a membrane.
• Osmosis is the diffusion of water across a selectively permeable membrane. In plants, water from the soil enters root cells by osmosis.
• Isotonic solution: Solute concentration outside = inside the cell — no net movement of water.
• Hypotonic solution: Solute concentration outside < inside — water enters the cell, cell swells.
• Hypertonic solution: Solute concentration outside > inside — water leaves the cell, cell shrinks.

Fig. 2.7: Structure of a cell membrane (Fluid-mosaic model)

🔬 Activity 2.2: Let us experiment (Osmosis in potato)

1. Carefully cut a potato into two pieces of roughly equal size.
2. Measure and record the initial weight of both pieces using a weighing balance.
3. Put one piece in Beaker A with plain water.
4. Put the other piece in Beaker B with 20 per cent salt or sugar solution.
5. Leave them undisturbed for about an hour or until a visible change in size is observed.
6. Measure and record the final weight of each piece.
7. Calculate the difference between the initial and final weights.

Observation: Beaker A — potato piece swells (water enters by osmosis — hypotonic outside). Beaker B — potato piece shrinks (water leaves by osmosis — hypertonic outside).

Inference: The cell membrane allows water to move in and out but not the sugar or salt molecules.

🔢 Numerical Questions

1. A potato piece weighing 50 g is placed in plain water. After 1 hour its weight becomes 54 g. Calculate the percentage increase in weight.

► Increase = 54 − 50 = 4 g. Percentage increase = (4 ÷ 50) × 100 = 8%.

2. A potato piece of 50 g is placed in 20% salt solution. After 1 hour its weight is 46 g. Calculate the percentage decrease in weight.

Decrease = 50 − 46 = 4 g. Percentage decrease = (4 ÷ 50) × 100 = 8%.

3. The cell membrane is 8 nm thick. Express this thickness in mm. (1 nm = 0.000001 mm)

Thickness = 8 × 0.000001 = 0.000008 mm (8 × 10⁻⁶ mm).

📝 Questions

LOTS: What is the plasma membrane? Why is it called selectively permeable?

The plasma membrane (cell membrane) is a thin boundary that surrounds a cell and protects its contents. It is called selectively permeable because it allows only certain substances to pass through it while blocking others — for example, it allows water molecules to pass but blocks sugar or salt molecules.

Medium: Explain the fluid-mosaic model of the cell membrane.

The fluid-mosaic model explains the structure of the cell membrane. The membrane has a lipid bilayer — two layers of fat molecules with hydrophilic heads facing outward and hydrophobic tails facing inward. Proteins are embedded in this bilayer. The molecules can move sideways, flip and rotate — making it ‘fluid’. Since the arrangement looks like tiles in a mosaic, it is called a ‘mosaic’. Together, it is the fluid-mosaic model.

HOTS: If a cell is placed in a hypertonic solution, what will happen and why? What will happen if the same cell is placed in a hypotonic solution?

In a hypertonic solution, the solute concentration outside the cell is greater than inside. Water moves out of the cell by osmosis, causing the cell to shrink. In a hypotonic solution, the solute concentration outside is less than inside. Water moves into the cell by osmosis, causing the cell to swell. In both cases, the cell membrane acts as a selectively permeable barrier, allowing only water molecules to pass through.

HOTS: Why is it important to cut potato pieces to roughly equal size and measure their initial weight before placing them in different liquids?

Cutting potato pieces to roughly equal size ensures that the surface area exposed to the solution is similar in both pieces. Measuring the initial weight ensures a fair starting point for comparison. Without these controls, we cannot accurately determine whether any difference in final weight is due to osmosis or due to the difference in the size of the pieces — making the experiment scientifically invalid.

2.2.2 Cell wall — The outer covering of cells

• Cells of plants, fungi, and bacteria have an additional layer outside the cell membrane called the cell wall.
• Plants cannot move from place to place, so they need a rigid structure to withstand environmental stresses like wind and rain.
• The cell wall helps leaves and flowers remain firm, maintains their shapes, and helps plants stay upright.
• Although rigid, the cell wall is permeable — water and some dissolved minerals can pass through it.
• The combined permeability of the cell wall and selective permeability of the cell membrane help plant roots absorb water and nutrients from the soil.
• When plant cells lose water (osmosis in concentrated solution), the rigid cell wall maintains their shape — the inner content shrinks as the cell membrane pulls away from the cell wall.
• Animal cells do not have a cell wall. Therefore, when placed in a concentrated solution, they lose water and shrink.
• Without a rigid cell wall, animal cells can change shape easily — supporting movement and functioning of animal tissues.
• The plant cell wall is primarily made of cellulose — a carbohydrate formed by many glucose units linked together.
• Cellulose in our diet acts as roughage, helping in digestion.
• Some microorganisms like fungi and bacteria also have a cell wall for protection and structural support.

Fig: Comparison showing cell wall in plant cell vs animal cell

🔬 Activity 2.3: Let us investigate (Plant vs Animal cells under microscope)

1. Prepare temporary slides of a thin peel of an onion leaf or a Rhoeo (Cradle lily) leaf and mount it with safranin using a cover slip to observe plant cells under a microscope.
2. Similarly, prepare a temporary slide of cheek cells by gently scraping the inner side of your cheek with a cotton swab or the blunt end of a toothpick.
3. Spread the cheek cells on a clean glass slide.
4. Add a drop of water followed by a few drops of methylene blue stain and carefully place a coverslip.
5. Observe both the slides under a microscope.

Observation: Onion peel cells are box-shaped and regularly arranged (due to cell wall). Cheek cells are irregularly arranged (no cell wall).

Further observation: When 20% sugar solution is applied — plant cells maintain their outer boundary but inner content shrinks (plasmolysis). Cheek cells shrink considerably as they have no cell wall to maintain shape.

🔢 Numerical Questions

1. The cell wall is 0.2 µm thick and the cell membrane is 8 nm thick. Express both in nm and find their ratio.

► Cell wall = 0.2 µm = 0.2 × 1000 = 200 nm. Cell membrane = 8 nm. Ratio = 200 ÷ 8 = 25 : 1. The cell wall is 25 times thicker than the cell membrane.

2. A plant cell placed in 15% sugar solution loses 10% of its water content. If the cell initially had 200 units of water, how many units remain?

Water lost = 10% of 200 = 20 units. Water remaining = 200 − 20 = 180 units.

📝 Questions

LOTS: What is the main chemical composition of the plant cell wall?

The plant cell wall is primarily made of cellulose — a type of carbohydrate formed by many glucose units linked together. Cellulose in our diet acts as roughage, helping in digestion.

Medium: Why do plant cells not shrink when placed in a concentrated sugar solution, but animal cells do?

Plant cells have a rigid cell wall outside the cell membrane. When placed in a concentrated sugar solution, water leaves the cell by osmosis but the rigid cell wall maintains the outer shape of the cell — only the inner content (cytoplasm) shrinks and the cell membrane pulls away from the cell wall. Animal cells do not have a cell wall, so when water leaves by osmosis, the entire cell shrinks.

HOTS: What argument would you give for the necessity of a cell wall in plants but not in animals?

Plants are fixed in one place and are exposed to environmental stresses like wind, rain, and gravity. Without a rigid cell wall, plants would collapse and their cells would deform easily. Animals, on the other hand, move from place to place. Their cells need to be flexible to allow movement of tissues and organs. A rigid cell wall would prevent this flexibility. Therefore, animal cells have only a flexible cell membrane, while plant cells have a rigid cell wall for structural support.

HOTS: What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?

If the cell wall became as flexible as the cell membrane, the plant cell would lose its structural rigidity. The cell would swell or shrink depending on the surrounding solution — just like animal cells. Leaves, flowers, and stems would wilt and droop as they would no longer have structural support. The plant would be unable to withstand wind, rain, or gravity, and would collapse. Essentially, the plant would function more like an animal tissue — losing its shape and firmness.

2.3 The Cell Interior — A Coordinated Working System

• Most cells have three basic parts: (1) a selectively permeable plasma membrane, (2) a semi-fluid jelly-like substance called the cytoplasm, and (3) a prominent nucleus.
• The cytoplasm contains several sub-cellular components called organelles, along with other substances — most visible only under an electron microscope.
• A bacterial cell lacks a well-defined nucleus and membrane-bound organelles — such cells are called prokaryotic cells (pro = primitive, karyon = nucleus). Most cellular activities occur directly in the cytoplasm.
• Plant and animal cells have a well-defined nucleus and several membrane-bound organelles — such cells are called eukaryotic cells (eu = true, karyon = nucleus).
• In eukaryotic cells, a network of fine fibres forms the cytoskeleton — provides structural support, maintains cell shape, and enables cell movement and internal transport.
• The cytoplasm may also store cell inclusions — starch (in plant cells), or crystals of calcium oxalate or silica (in some plant cells).

Fig. 2.10: (a) A typical bacterial cell, (b) a typical plant cell, and (c) a typical animal cell

2.3.1 Why do eukaryotic cells need these organelles?

• Eukaryotic cells carry out various life processes in different cell organelles independently and simultaneously.
• Cell organelles help in building new materials, removing waste, and providing energy to the cell.
• A cell is like a tiny living factory — each part does a specific job.

Nucleus — House of coded instructions

• The nucleus has a double-layered covering called the nuclear membrane, which has pores that allow transfer of material between the nucleus and cytoplasm.
• The nucleolus is the dense round body inside the nucleus — site of synthesis of ribosomal subunits. These subunits exit the nucleus to the cytoplasm where they assemble to form ribosomes.
• The nucleus contains chromosomes — visible as rod-shaped structures only when the cell is about to divide.
• Chromosomes contain information for inheritance of characters in the form of DNA (Deoxyribonucleic acid) molecules.
• Chromosomes are composed of DNA and specific proteins.
• The functional segments of DNA are called genes.
• In a non-dividing cell, DNA is present as chromatin material — visible as an entangled mass of thread-like structures.
• When the cell is about to divide, chromatin material gets organised into chromosomes.
• In prokaryotic cells, DNA is present as a single circular molecule with specific proteins in a region called the nucleoid.
• Interesting fact: Mature Red Blood Cells (RBCs) in humans do not have a nucleus (enucleate). The absence of a nucleus provides more space for haemoglobin, allowing more oxygen transport. Since they lack a nucleus, they cannot repair or divide and survive only about 120 days.

Fig. 2.11 & 2.12: Structure of a nucleus and from cell to DNA

📝 Questions

LOTS: What is the function of the nucleolus?

The nucleolus is a dense round body inside the nucleus. It is the site of synthesis of ribosomal subunits. These subunits then exit the nucleus to the cytoplasm where they assemble to form ribosomes.

Medium: What is chromatin material and how does it relate to chromosomes?

In a non-dividing cell, DNA is present as chromatin material — visible as an entangled mass of thread-like structures. When the cell is about to divide, this chromatin material gets organised and condensed into distinct rod-shaped structures called chromosomes. So chromatin and chromosomes are the same material — just in different states of organisation.

HOTS: Why do mature Red Blood Cells (RBCs) not have a nucleus? What are the advantages and disadvantages of this?

Mature RBCs in humans lose their nucleus during development. The advantage is that the absence of a nucleus provides more space inside the cell for haemoglobin — allowing the cell to carry a larger amount of oxygen to all body cells. The disadvantage is that without a nucleus, the RBCs cannot repair themselves or divide. This limits their lifespan to approximately 120 days, after which they must be replaced by new RBCs produced in the bone marrow.

HOTS: How does the nucleus control the activities of a cell?

The nucleus contains DNA, which carries the genetic information in the form of genes. Genes provide instructions for making proteins — and proteins carry out virtually all the functions of a cell (as enzymes, structural components, hormones, etc.). The nuclear membrane has pores through which information (in the form of RNA) moves from the nucleus to the cytoplasm. This is how the nucleus directs all cellular activities — it is essentially the control centre or ‘house of coded instructions’ for the cell.

Ribosomes — The protein factories

• Ribosomes are tiny structures present either freely in the cytoplasm or attached to the endoplasmic reticulum.
• Ribosomes are the sites of protein synthesis — they are the protein factories of the cell.
• They are assembled in the cytoplasm from subunits produced in the nucleolus.
• Ribosomes are present in both prokaryotic and eukaryotic cells.

Endoplasmic Reticulum (ER) — Manufacturing factory

• The Endoplasmic Reticulum (ER) is a large organelle that spreads like a network within the cytoplasm.
• The ER is continuous with the outer membrane of the nuclear envelope.
• It plays a key role in the synthesis and transport of proteins, fats (lipids), and some hormones in specialised cells.
• There are two types of ER:
  • Rough Endoplasmic Reticulum (RER): Looks rough under an electron microscope because it has ribosomes attached to its surface. Mainly involved in protein synthesis and protein secretion (e.g., pancreatic cells).
  • Smooth Endoplasmic Reticulum (SER): Does not have ribosomes on its surface, so it looks smooth. Involved in the synthesis and storage of fats and hormones.

Golgi apparatus — The packaging and shipping centres

• The Golgi apparatus consists of stacks of flattened, sac-like structures (cisternae) — it acts like the cell’s post office.
• It is functionally linked to the ER, cell membrane, and other organelles.
• The Golgi apparatus modifies, sorts, and packages proteins and/or lipids into vesicles for transport, secretion, or lysosome formation.
• It was first observed in 1898 by Italian scientist Camillo Golgi in the nerve cells of a barn owl using special staining techniques. Its existence was confirmed by electron microscopy decades later.

Fig. 2.13: Endoplasmic reticulum and Golgi apparatus — pathway for protein processing and secretion

Lysosomes — The clean-up system

• Lysosomes are single membrane-bound sacs filled with digestive enzymes.
• They break down unwanted proteins, carbohydrates, fats, and even damaged parts of the cell — keeping it clean and healthy.
• Products formed by the breakdown are released into the cytoplasm where they may be reused in other cellular processes.
• Interesting fact: Human sperm cells contain lysosomal enzymes. When a sperm meets an egg, these enzymes help break down the outer layer of the egg, allowing fertilisation to take place.

📝 Questions — ER, Golgi, Ribosomes, Lysosomes

LOTS: What is the difference between RER and SER?

RER (Rough Endoplasmic Reticulum) has ribosomes attached to its surface, making it look rough under an electron microscope. It is mainly involved in protein synthesis and secretion. SER (Smooth Endoplasmic Reticulum) has no ribosomes on its surface and looks smooth. It is involved in the synthesis and storage of fats and hormones.

Medium: Why is the Golgi apparatus compared to a post office?

Just like a post office receives items, sorts them, packs them and dispatches them to different destinations — the Golgi apparatus receives proteins and lipids from the ER, modifies and sorts them, packages them into vesicles, and sends them to their destinations — either to the cell membrane for secretion, to other organelles, or for lysosome formation. This functional similarity makes it appropriate to compare the Golgi apparatus to a post office.

HOTS: Lysosomes are sometimes called ‘suicide bags’ of the cell. Why?

Lysosomes contain powerful digestive enzymes. Under normal conditions, these enzymes break down waste materials and damaged organelles, keeping the cell clean. However, if a cell is severely damaged or its functioning is no longer required (as in programmed cell death), lysosomes can burst open and release their enzymes into the cytoplasm — digesting the cell’s own contents and causing cell death. Because they have the ability to destroy the cell from within, they are nicknamed ‘suicide bags’ of the cell.

HOTS: Describe the complete path of a protein from its site of synthesis to secretion outside the cell.

Proteins are synthesised by ribosomes attached to the RER. The protein moves into the lumen (space) of the RER. From the RER, vesicles carrying the protein bud off and travel to the Golgi apparatus. In the Golgi apparatus, the protein is modified (e.g., sugar chains added), sorted, and packaged into vesicles. These vesicles then travel to the plasma membrane, fuse with it, and release the protein outside the cell by a process called exocytosis. This is the complete secretory pathway: Ribosomes (on RER) → ER lumen → Golgi apparatus → Secretory vesicles → Plasma membrane → Outside the cell.

Mitochondria — The powerhouse of the cell

• Mitochondria are called the ‘powerhouses of the cell’ because they supply the energy needed for most cellular activities.
• Each mitochondrion is surrounded by two membranes:
  • Outer membrane: smooth and porous.
  • Inner membrane: folded into finger-like projections called cristae, which increase the surface area for chemical reactions and facilitate energy production.
• In mitochondria, glucose and other molecules are broken down to release energy during cellular respiration.
• The energy released is stored in the form of ATP (Adenosine Triphosphate) — which acts as the energy currency of the cell and is used for most cellular activities.
• Mitochondria have their own DNA and ribosomes — they can make some of their own proteins. This suggests mitochondria share an evolutionary history with bacteria.

Fig. 2.14: Structure of a mitochondrion

🔢 Numerical Questions

1. A cell has 200 mitochondria and each produces 36 ATP per glucose molecule. How many total ATP molecules are produced when all mitochondria each process one glucose molecule?

► Total ATP = 200 × 36 = 7200 ATP molecules.

2. The inner membrane surface area increases from 2 cm² to 14 cm² due to cristae. By what factor has the surface area increased? Why is this important?

Factor = 14 ÷ 2 = 7 times. This increase in surface area provides more space for the enzymes and reactions of cellular respiration, making ATP production more efficient.

📝 Questions

LOTS: Why are mitochondria called the powerhouses of the cell?

Mitochondria are called the powerhouses of the cell because they are the site of cellular respiration — a process in which glucose and other molecules are broken down to release energy. This energy is stored in the form of ATP (Adenosine Triphosphate), which is used to power most cellular activities. Without mitochondria, the cell would not have the energy needed to carry out any of its functions.

Medium: What is the role of cristae in a mitochondrion?

Cristae are the finger-like inward folds of the inner membrane of the mitochondrion. By folding inward, the cristae greatly increase the surface area of the inner membrane. This increased surface area provides more space for the chemical reactions of cellular respiration to take place, making the energy production process more efficient.

HOTS: Instead of many small mitochondria, why does a cell not have a single giant mitochondrion? How does this relate to surface area?

A single giant mitochondrion would have a much smaller surface area-to-volume ratio compared to many small mitochondria of the same total volume. Chemical reactions in mitochondria (cellular respiration) occur on the membranes, especially the inner membrane. More mitochondria means more total membrane surface area available for reactions — leading to more efficient and faster energy production. Additionally, many small mitochondria can be distributed throughout the cell to deliver energy exactly where it is needed, whereas a single giant mitochondrion would be inefficient at transporting energy to distant parts of the cell.

HOTS: What would happen to a eukaryotic cell if all its mitochondria were removed?

If all mitochondria were removed, the cell would be unable to carry out aerobic cellular respiration. It would lose its primary source of ATP — the energy currency of the cell. Without ATP, the cell cannot carry out active transport across membranes, protein synthesis, cell division, movement, or any other energy-requiring process. The cell would rapidly run out of energy and die. Only anaerobic respiration (which occurs in the cytoplasm) could continue briefly, but it is far less efficient and cannot sustain the cell for long.

Plastids — Centre for food synthesis in the plant cells and beyond

• Plants use special organelles called plastids for food synthesis and storage — present only in plant cells.
• Chloroplasts — a type of plastid — contain the green pigment chlorophyll which absorbs sunlight for photosynthesis.
• Chloroplasts are double-membrane-bound organelles, like mitochondria.
• Inside the chloroplast is a semi-fluid substance called the stroma.
• Within the stroma are disc-shaped membrane structures (thylakoids) that contain chlorophyll — light energy is absorbed by them during photosynthesis.
• The sugars synthesised in photosynthesis are stored in the stroma along with starch granules.
• Like mitochondria, plastids also have their own DNA and ribosomes — suggesting a shared evolutionary history with bacteria.

Fig. 2.15: Structure of a chloroplast

How do flowers, fruits, and vegetables acquire varied colours?

• In flower petals and fruits, plastids contain pigments other than chlorophyll.
• These plastids are called chromoplasts (Greek: chroma = colour). Their pigments may be yellow, orange or red — source of bright colours in flowers and fruits.
• Bright colours of chromoplasts help in attracting pollinators for pollination and fruit-eating animals that help in seed dispersal.
• Some plastids lack pigments and are colourless — called leucoplasts (Greek: leukos = white).
• Leucoplasts store food material — starch, oils, or proteins — classified based on the type of food they store.
• Example: Leucoplasts in potato and taro (Colocasia) cells store starch.

📝 Questions — Plastids

LOTS: Name the three types of plastids and state the function of each.

(1) Chloroplasts — contain chlorophyll; site of photosynthesis; absorb sunlight and synthesise food (sugars) in plant cells. (2) Chromoplasts — contain coloured pigments (yellow, orange, red) other than chlorophyll; responsible for the bright colours in flowers, fruits, and vegetables; help attract pollinators and seed dispersers. (3) Leucoplasts — colourless plastids that store food materials such as starch, oils, or proteins; found in non-green parts of plants like roots and seeds.

Medium: Do white flowers contain any pigment? Give reasons.

White flowers may or may not contain pigment. Some white flowers contain pigments that reflect all wavelengths of visible light, making them appear white. Others contain pigments that are not visible to humans but may be visible to insects (such as UV-reflecting pigments). Some white flowers lack coloured plastids (chromoplasts) and appear white simply because they lack pigmentation. However, even white flowers contain chloroplasts in their green parts (sepals, leaves) for photosynthesis.

HOTS: Mitochondria and chloroplasts are believed to have evolved from ancient bacteria. What evidence supports this?

Both mitochondria and chloroplasts share several features with bacteria: (1) Both have their own DNA — a circular molecule like bacterial DNA. (2) Both have their own ribosomes that can synthesise some proteins independently. (3) Both are surrounded by double membranes — the inner membrane resembles the bacterial cell membrane. (4) Both can divide by a process similar to binary fission in bacteria. These features strongly suggest that both mitochondria and chloroplasts were once free-living bacteria that were engulfed by an ancestral eukaryotic cell and developed a mutually beneficial (symbiotic) relationship — a theory known as the endosymbiotic theory.

HOTS: A student says that plastids are only found in green parts of plants because only green parts perform photosynthesis. Is this correct?

The student is only partially correct. While chloroplasts (the green pigment-containing plastids) are indeed found in green parts of plants for photosynthesis, plastids as a group are not limited to green parts. Chromoplasts (coloured plastids) are found in petals, fruits, and roots of some plants. Leucoplasts (colourless plastids) are found in non-green parts like roots, seeds, and underground stems — where they store starch, oils, or proteins. So plastids are found throughout the plant — not just in green parts.

Vacuoles — The organelles for storage and support

• In a mature plant cell, there is usually one large central vacuole surrounded by a single selectively permeable membrane.
• The vacuole is filled with a watery fluid called cell sap.
• The vacuole stores water, minerals, sugars, and waste material.
• By storing large amounts of water, the vacuole helps maintain pressure (turgor pressure) inside the cell, which keeps a plant cell firm.
• When a plant does not get enough water, the vacuole loses water, the cells become less firm, and the plant wilts.
• In animal cells, vacuoles are sometimes present but are not as large as plant vacuoles. They help in the temporary storage of materials.

📝 Questions — Vacuoles

LOTS: Why do plants look wilted when they do not get enough water?

In a plant cell, the large central vacuole is filled with water (cell sap) and maintains turgor pressure — the pressure that keeps the cell firm. When a plant does not get enough water, the vacuole loses water. As a result, the cells become less firm and lose their rigidity. Without the turgor pressure, the plant cannot hold up its leaves and stems, so it droops and wilts.

Medium: Compare vacuoles in plant cells and animal cells.

Plant cells have one large central vacuole that occupies most of the cell’s volume. It is filled with cell sap and stores water, minerals, sugars, and waste. It maintains turgor pressure, keeping the plant firm. Animal cells may have vacuoles but they are much smaller and fewer. They are not permanently present and mainly help in temporary storage of materials. Unlike plant vacuoles, animal vacuoles do not maintain turgor pressure.

HOTS: How does the large central vacuole of a plant cell help it survive in a dilute soil solution without bursting?

When a plant cell absorbs water by osmosis from dilute soil solution, water enters the vacuole. The vacuole expands and pushes the cytoplasm against the cell wall, creating turgor pressure. The rigid cell wall resists this pressure and prevents the cell from bursting — unlike animal cells which can burst in hypotonic solutions because they lack a cell wall. The cell wall provides a counter-pressure (wall pressure) that balances the turgor pressure, maintaining the cell’s integrity while keeping it firm and hydrated.

HOTS: A plant cell and an animal cell are both placed in pure water. What will happen to each and why?

Both cells are in a hypotonic solution (pure water has no solutes). Water will enter both cells by osmosis. The animal cell will swell and may eventually burst (lyse) because it has no rigid cell wall to resist the increasing internal pressure. The plant cell will also absorb water — the vacuole will expand and push the cytoplasm against the cell wall (turgor pressure). However, the rigid cell wall will resist this pressure and prevent the cell from bursting. The plant cell will become turgid (firm) but will not burst.

2.4 How do Normal Cells Grow and Divide?

• When we get a cut on the skin, it heals because cells in our body can grow and divide to replace old, dead, or damaged cells.
• Growth happens not because cells get bigger — cells can only grow up to a certain size — but because cells divide to form new cells.
• Every day, an estimated hundreds of billions of cells in our body are replaced — almost 1% of the total number of cells.
• Both prokaryotic and eukaryotic cells divide, but eukaryotic cells divide in a more controlled and orderly manner by a process called the cell cycle.

2.4.1 Cell division

• Cell division is the process by which new cells are formed from pre-existing cells.
• It allows living organisms to grow, repair damaged tissues, and reproduce.
• Some cells (e.g., skin cells) divide continuously to replace cells that are lost regularly.
• There are two major types of cell division: mitosis and meiosis.
• Mitosis is important for normal growth, repair, maintenance, and asexual reproduction.
• Meiosis is important for sexual reproduction and creation of genetic diversity.

Mitosis

• Every human begins life as a single fertilised egg. This one cell divides repeatedly by mitosis to form trillions of cells.
• Mitosis is the most common type of cell division.
• Mitosis produces two genetically identical daughter cells from one parent cell.
• Each daughter cell gets the same DNA and the same number of chromosomes as the parent cell.
• This ensures that genetic information is largely maintained across body cells.

Meiosis

• Meiosis is a type of cell division that produces gametes (sex cells) and occurs only in the cells of reproductive organs.
• Gametes produced by meiosis create variations and diversity among living organisms — which is why children resemble parents but are not exactly the same.
• In animals (including humans): meiosis occurs in the testes (to produce sperm) and ovaries (to produce eggs).
• In plants: meiosis occurs in the anthers (male parts — to form pollen grains/sperm cells) and ovaries (female parts — to produce egg cells).
• In meiosis, the parent cell divides twice (one after the other) to form four daughter cells.
• During the first division: chromosomes are reduced to half in each daughter cell.
• The second division is similar to mitosis — each daughter cell divides to form two, giving four daughter cells with half the number of chromosomes.
• Each gamete has half the number of DNA compared to the parent cell.
• During fertilisation, gametes from two individuals combine — restoring the original chromosome number.
• Errors in mitosis: lead to uncontrolled cell divisions → formation of tumours and abnormal chromosome numbers in body cells.
• Errors in meiosis: may result in genetic disorders, developmental problems, distinctive physical features, early pregnancy loss, or reduced fertility.

Fig. 2.18 & 2.19: Mitosis produces 2 identical daughter cells; Meiosis is a two-step process producing 4 gametes

🔢 Numerical Questions

1. A human body cell has 46 chromosomes. How many chromosomes will be in (a) each cell after mitosis? (b) each gamete after meiosis? (c) the zygote after fertilisation?

► (a) After mitosis: 46 chromosomes (same as parent). (b) Each gamete after meiosis: 23 chromosomes (half of 46). (c) Zygote after fertilisation: 23 + 23 = 46 chromosomes (original number restored).

2. A single cell undergoes 5 rounds of mitosis. How many cells will be produced in total?

After each mitosis the number doubles. Cells = 2⁵ = 32 cells.

3. A plant cell with 24 chromosomes undergoes meiosis. How many chromosomes will each of the four daughter cells contain?

Meiosis halves the chromosome number. Each daughter cell = 24 ÷ 2 = 12 chromosomes.

📝 Questions — Cell Division

LOTS: What are the two major types of cell division and where does each occur?

The two major types of cell division are: (1) Mitosis — occurs in all body (somatic) cells; responsible for growth, repair, and maintenance. It produces two genetically identical daughter cells. (2) Meiosis — occurs only in cells of reproductive organs (testes and ovaries in animals; anthers and ovaries in plants); produces gametes (sex cells). It produces four daughter cells, each with half the number of chromosomes.

Medium: What would happen if gametes were formed by mitotic divisions instead of meiosis?

If gametes were formed by mitosis, each gamete would have the full (diploid) number of chromosomes, just like the parent cell. When two such gametes fused during fertilisation, the resulting zygote would have double the normal chromosome number. With each generation, the chromosome number would keep doubling, making it impossible to maintain the normal chromosome number of the species. This would cause genetic abnormalities and would be unsustainable.

HOTS: If the skin cells start dividing by meiosis instead of mitosis, what would happen to a cut on the skin?

Skin cells normally divide by mitosis to produce two identical daughter cells for repair. If they divided by meiosis instead, they would produce four daughter cells each with only half the number of chromosomes. These cells would have incomplete genetic information and would be genetically diverse (not identical to parent cells). As a result, they would not function properly as skin cells. The skin would be unable to heal the cut effectively. Additionally, the reduced chromosome number in each new cell would mean they lack the complete set of genes required for normal cell functioning, leading to defective or non-functional skin cells.

HOTS: How does meiosis contribute to genetic diversity in a population?

Meiosis contributes to genetic diversity in two key ways: (1) During meiosis, the first division involves the separation of chromosome pairs — but before separation, the chromosomes exchange segments with each other (crossing over), creating new combinations of genes. (2) The second division and the random combination of gametes during fertilisation further shuffle the genetic material. As a result, every gamete produced by meiosis is genetically unique. When gametes from two different individuals combine during fertilisation, the offspring have a unique genetic combination — different from both parents. This is why children resemble parents but are not identical to them, and why genetic diversity exists within a population.

2.5 Cell Theory — The Unifying Principle of Biology

• In 1838, German botanist Matthias Schleiden reported that all plants are made up of cells.
• In 1839, German zoologist Theodor Schwann found that all animals are also made up of cells.
• In 1855, German scientist Rudolf Virchow expanded the Cell Theory by stating that new cells are formed only from pre-existing cells.
• Together, their work led to the formulation of the Cell Theory.

🧪 Chapter Quiz — 25 MCQs

Test your understanding with this 25-question quiz on Chapter 2: Cell — The Building Block of Life. Answers are highlighted after each attempt.