Class 9 Science
Chapter 2: Cell: The Building Block of Life
Easy Notes and NCERT Book Solutions
Simple explanations • Important concepts • Complete end-of-chapter solutions
PART 1 — SHORT NOTES
1. Origin of Life and the Cell
Scientists believe that life originated in water. Some researchers suggest that the earliest life may have developed in small water pools with changing environmental conditions.
- Hot springs, such as those in Puga Valley, Ladakh, have very high temperatures.
- They contain heat-loving, unicellular bacteria called thermophiles.
- Scientists from the Birbal Sahni Institute of Palaeosciences studied these hot springs and found rapid formation of calcium carbonate deposits.
- Such deposits may have protected early organic molecules and helped in the formation of the first protective cell membrane.
What is a cell?
A cell is the basic structural and functional unit of life. All living organisms are made up of one or more cells.
| Type | Meaning | Examples |
|---|---|---|
| Unicellular | Made up of only one cell. | Bacteria, yeast |
| Multicellular | Made up of many cells that work together. | Plants, fish, birds, humans |
Similar cells → Tissues → Organs → Organ systems → Organism
Example: The nasal cavity, trachea and lungs are parts of the respiratory system.
2. How to Study Cells?
Limit of resolution of the human eye
The limit of resolution is the minimum distance between two points at which they can still be seen as separate objects.
Most cells are too small to be seen clearly with the unaided eye. Scientists use microscopes to study them.
Robert Hooke
In 1665, Robert Hooke observed a thin slice of cork using a microscope he designed. He saw small box-like compartments and named them cells.
Types of microscopes
| Microscope | Important features |
|---|---|
| Light microscope | Uses visible light and lenses to magnify objects. |
| Electron microscope | Uses electrons to show much finer details of cell structures. |
| Scanning electron microscope (SEM) | Shows detailed surface features of objects. |
The three important features improved in microscopes are:
- Magnification: How many times larger an object appears.
- Resolution: How clearly two close points can be seen separately.
- Contrast: Difference in brightness between different parts of an object.
Estimating the size of a cell
Unit conversion:
1 millimetre (mm) = 1000 micrometres (µm)
Example: If the field of view is 5 mm and 25 cells fit across its diameter:
5 mm = 5000 µm
Size of one cell = 5000 ÷ 25 = 200 µm
Total magnification
Example: Eyepiece = 10X and objective lens = 10X.
Total magnification = 10 × 10 = 100X.
3. Structure of a Cell
Most cells have three basic parts:
- Cell membrane: Surrounds the cell and controls the movement of substances.
- Cytoplasm: Jelly-like material containing cell organelles.
- Nucleus: Contains genetic information and helps control cell activities.
4. Cell Membrane — The Boundary of a Cell
The cell membrane, also called the plasma membrane, is a thin covering around the cell. It protects the contents and separates the cell from its surroundings.
It allows some substances to pass through but restricts others.
Osmosis
Osmosis is the movement of water through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
- Potato in plain water: Water enters the potato cells, so the potato becomes swollen and its mass increases.
- Potato in concentrated salt or sugar solution: Water leaves the cells, so the potato shrinks and its mass decreases.
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. It can occur without a membrane.
Example: The spreading of perfume fragrance through a room.
Difference between diffusion and osmosis
| Diffusion | Osmosis |
|---|---|
| Particles move from higher to lower concentration. | Water moves across a selectively permeable membrane. |
| Does not always require a membrane. | Requires a selectively permeable membrane. |
| Can involve different types of particles. | Involves the movement of water. |
Isotonic, hypotonic and hypertonic solutions
| Solution | Meaning | Effect on a cell |
|---|---|---|
| Isotonic | Solute concentration is equal inside and outside the cell. | No net movement of water. |
| Hypotonic | Solute concentration outside is lower than inside. | Water tends to enter the cell. |
| Hypertonic | Solute concentration outside is higher than inside. | Water tends to leave the cell. |
Fluid-mosaic model
The cell membrane is approximately 7–10 nanometres thick and is mainly made of lipids and proteins.
- Lipid bilayer: Two layers of lipid molecules form the basic structure.
- Hydrophilic heads: Face the watery surroundings.
- Hydrophobic tails: Face inwards, away from water.
- Proteins: Help transport substances and perform other functions.
- Fluid: Many molecules can move sideways within the membrane.
- Mosaic: Different proteins are arranged within the lipid bilayer.
5. Cell Wall — The Outer Covering of Some Cells
The cell wall is a firm outer covering present outside the cell membrane in plants, fungi and bacteria.
- It provides support, protection and shape.
- It helps plant cells remain firm and upright.
- It is generally permeable to water and certain dissolved substances.
- The plant cell wall is mainly made of cellulose.
- Cellulose in our diet acts as roughage and helps digestion.
When a plant cell loses water in a concentrated solution, its inner contents shrink and the cell membrane may pull away from the cell wall. This is called plasmolysis.
The rigid cell wall helps maintain the plant cell’s outer shape even when its inner contents shrink.
Plant cells and animal cells
| Plant cell | Animal cell |
|---|---|
| Usually has a cell wall. | Does not have a cell wall. |
| Often has a large central vacuole. | Vacuoles, when present, are usually smaller. |
| May contain plastids such as chloroplasts. | Does not contain plastids. |
| Often has a more regular shape. | Can have different shapes. |
6. The Cell Interior — A Coordinated Working System
The cytoplasm is a semi-fluid, jelly-like material inside the cell membrane. It contains cell organelles and supports many cellular activities.
Cell organelles are specialised structures that perform particular functions inside a cell.
Prokaryotic and eukaryotic cells
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No well-defined, membrane-bound nucleus. | Has a well-defined nucleus. |
| Genetic material | Usually present in a nucleoid region. | Enclosed within the nucleus. |
| Membrane-bound organelles | Absent. | Present. |
| Typical size | About 1–10 µm. | About 10–100 µm. |
| Examples | Bacteria. | Plant and animal cells. |
Prokaryotic cells are usually unicellular. Eukaryotic organisms may be unicellular or multicellular.
The cytoskeleton is a network of fine fibres that supports cell shape, helps movement and assists internal transport. Cell inclusions are stored materials such as starch grains or crystals.
7. Important Cell Organelles and Their Functions
7.1 Nucleus — House of coded instructions
- Contains genetic material and helps control cell activities.
- Has a double-layered nuclear membrane with pores.
- The nucleolus helps produce ribosomal subunits.
- Chromosomes contain DNA and associated proteins.
- DNA carries genetic information.
- Genes are functional segments of DNA.
- Chromatin is the thread-like DNA-protein material that becomes organised into visible chromosomes before cell division.
In bacteria, the genetic material lies in a region called the nucleoid, which is not surrounded by a nuclear membrane.
Mature human red blood cells do not have a nucleus. This leaves more room for haemoglobin, which carries oxygen. Their average lifespan is about 120 days.
7.2 Ribosomes — Protein factories
Ribosomes are the sites of protein synthesis. They may float freely in the cytoplasm or attach to the rough endoplasmic reticulum.
7.3 Endoplasmic reticulum (ER) — Manufacturing and transport network
The ER is a network of membranes inside the cell. It is continuous with the outer membrane of the nuclear envelope.
| Rough ER (RER) | Smooth ER (SER) |
|---|---|
| Has ribosomes attached to its surface. | Does not have ribosomes on its surface. |
| Mainly helps in protein synthesis and processing. | Helps in lipid synthesis and other specialised functions. |
7.4 Golgi apparatus — Packaging and shipping centre
- Made of stacks of flattened, sac-like structures.
- Modifies, sorts and packages proteins and lipids.
- Forms vesicles for transport and secretion.
- Helps in the formation of lysosomes.
It was named after the Italian scientist Camillo Golgi, who observed it in 1898.
7.5 Lysosomes — Clean-up system
Lysosomes are membrane-bound sacs containing digestive enzymes.
- Break down unwanted substances and damaged cell parts.
- Help digest certain foreign materials.
- Allow useful products to be reused by the cell.
Lysosomal enzymes in sperm cells help break down the outer layers surrounding an egg during fertilisation.
7.6 Mitochondria — Powerhouses of the cell
- Supply energy for most cellular activities.
- Have an outer membrane and a folded inner membrane.
- The folds are called cristae.
- Cellular respiration releases energy from food molecules.
- Much of the usable energy is transferred to ATP (adenosine triphosphate).
- They have their own DNA and ribosomes.
7.7 Plastids — Food synthesis, colour and storage
Plastids are organelles found in plant cells. They include chloroplasts, chromoplasts and leucoplasts.
| Plastid | Function | Example |
|---|---|---|
| Chloroplast | Contains chlorophyll and carries out photosynthesis. | Green leaves. |
| Chromoplast | Contains pigments that give yellow, orange or red colours. | Many flowers and fruits. |
| Leucoplast | Colourless plastid that stores food. | Starch stored in potato and taro. |
Chloroplasts have an outer and an inner membrane. The internal fluid is called the stroma. Their chlorophyll-containing membranes absorb light energy for photosynthesis.
Mitochondria and plastids contain their own DNA and ribosomes, which suggests an evolutionary connection with bacteria.
7.8 Vacuoles — Storage and support
- A mature plant cell usually has one large central vacuole.
- It is surrounded by a membrane and contains watery cell sap.
- It stores water, minerals, sugars and waste materials.
- Water in the vacuole helps maintain pressure and keeps plant cells firm.
- When plant cells lose water, the plant may wilt.
7.9 Cell wall and cell membrane are different
| Cell membrane | Cell wall |
|---|---|
| Present in all cells. | Present in plants, fungi and bacteria, but absent in animal cells. |
| Selectively permeable. | Generally permeable to water and certain dissolved substances. |
| Made mainly of lipids and proteins. | Plant cell walls are mainly made of cellulose. |
| Controls movement of substances into and out of the cell. | Provides support, protection and shape. |
8. Synthetic Biology and Cell Culture
Synthetic DNA experiment
In 2010, J. Craig Venter and his team synthesised a copy of the DNA of the bacterium Mycoplasma mycoides in the laboratory. They placed the synthetic DNA into a related bacterial cell from which the original DNA had been removed.
The cell began growing and dividing according to the instructions in the synthetic DNA. The experiment showed the important role of DNA in controlling cell structure and activities. However, the entire cell was not created from scratch; other cell components came from an existing living cell.
Cell culture
Cell culture is the growth of cells outside an organism under carefully controlled conditions.
It is useful for studying cells and producing biochemicals, food products, medicines and vaccines. Suitable nutrients, temperature, moisture and sterile conditions are required.
9. How Do Cells Grow and Divide?
Cells grow and divide to help organisms grow, repair damaged tissues, replace old cells and reproduce.
Cell division
Cell division is the process by which new cells are formed from pre-existing cells.
| Mitosis | Meiosis |
|---|---|
| Usually produces two daughter cells. | Produces four daughter cells after two divisions. |
| Daughter cells generally have the same chromosome number as the parent cell. | Daughter cells have half the chromosome number of the parent cell. |
| Important for growth, repair, maintenance and asexual reproduction. | Important for sexual reproduction and genetic variation. |
| Produces genetically identical daughter cells in normal circumstances. | Produces genetically varied cells in normal circumstances. |
Mitosis
In mitosis, one parent cell divides to form two genetically identical daughter cells with the same chromosome number as the parent cell.
Example: Skin cells divide by mitosis to replace damaged or lost cells.
Meiosis
In meiosis, one parent cell undergoes two successive divisions to produce four cells, each with half the chromosome number of the parent cell.
- It helps produce reproductive cells or their precursors.
- It contributes to genetic variation.
- In humans, meiosis occurs in cells in the testes and ovaries that give rise to sperm and eggs.
- In flowering plants, meiosis occurs in reproductive structures and produces spores that contribute to pollen and embryo-sac formation.
During fertilisation, the joining of two gametes restores the usual chromosome number for the species.
Errors in cell division
- Errors in mitosis can contribute to uncontrolled cell division, tumours or abnormal chromosome numbers.
- Errors in meiosis may cause genetic disorders, reduced fertility or pregnancy loss.
10. Cell Theory — The Unifying Principle of Biology
Cell theory developed through the work of three scientists:
- Matthias Schleiden (1838): Reported that all plants are made up of cells.
- Theodor Schwann (1839): Reported that all animals are made up of cells.
- Rudolf Virchow (1855): Stated that new cells arise from pre-existing cells.
- All living organisms are made up of one or more cells.
- The cell is the basic unit of structure and function in living beings.
- All cells arise from pre-existing cells.
Do cells grow and reproduce forever?
No. Cells have limited lifespans and are regulated by processes that control their growth, division and death.
Contact inhibition: Many animal cells stop dividing when they come into contact with neighbouring cells. Cancer cells can lose this control and divide uncontrollably, forming tumours.
Plant cells have rigid cell walls and show different patterns of growth; they do not generally exhibit contact inhibition in the same way as many animal cells.
Programmed cell death
Programmed Cell Death (PCD) is a genetically regulated process in which selected cells die in an organised way. It supports normal development, removes unwanted cells and helps maintain healthy tissues.
Example: During embryo development, the removal of cells between developing fingers helps separate the digits.
Totipotency
Totipotency is the ability of a plant cell to develop into a complete plant when provided with suitable conditions. Gottlieb Haberlandt proposed this idea in 1902. It helped lay the foundation for plant tissue culture technology.
Viruses, viroids and prions
| Agent | Basic nature |
|---|---|
| Virus | Genetic material enclosed in a protein coat. |
| Viroid | Infectious agent made of RNA without a protein coat. |
| Prion | Infectious misfolded protein without genetic material. |
These are acellular infectious agents; they are not made up of cells.
11. Quick Revision Table
| Cell part | Main function |
|---|---|
| Cell membrane | Controls movement of substances. |
| Cell wall | Support, protection and shape. |
| Cytoplasm | Contains organelles and supports cell activities. |
| Nucleus | Contains DNA and helps control cell activities. |
| Ribosomes | Protein synthesis. |
| Rough ER | Protein synthesis and processing. |
| Smooth ER | Lipid synthesis and other specialised functions. |
| Golgi apparatus | Modifies, sorts and packages substances. |
| Lysosomes | Digestion of unwanted materials. |
| Mitochondria | Energy release during cellular respiration. |
| Chloroplasts | Photosynthesis. |
| Chromoplasts | Colour due to pigments. |
| Leucoplasts | Food storage. |
| Vacuole | Storage and support in plant cells. |
PART 2 — NCERT BOOK SOLUTIONS
Revise, Reflect, Refine and The Journey Beyond
The questions below follow the supplied chapter text and retain the original numbering and wording. Answers are kept short and easy to understand.
Revise, Reflect, Refine
(i) Cell membrane and cell wall (permeability)
(ii) RER and SER (structure)
(iii) Chloroplasts and chromoplasts (pigments)
Cell X is placed in pure water. Cell Y is placed in a concentrated salt solution. Cells are observed after some time. Cell X swells, and Cell Y shrinks. Which statement provides the correct explanation for the above observations?
(i) Salt molecules moved into Cell Y, causing it to shrink.
(ii) Water moved into Cell X and more water moved out of Cell Y than the salt solution entered in it.
(iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.
(iv) Solute movement caused osmosis in both cells.
Water enters Cell X by osmosis, making it swell. Water leaves Cell Y by osmosis, making it shrink.
(i) Controlling all the activities of a cell.
(ii) Site of cellular respiration.
(iii) Storage organelle that also provides rigidity to the cell.
(iv) Separates the cell contents from surroundings.
(v) Provides structural rigidity to the cell.
(vi) Packs and stores materials received from ER.
(vii) Helps in manufacturing food.
- Controlling cell activities — Nucleus.
- Site of cellular respiration — Mitochondrion.
- Storage organelle that provides rigidity — Vacuole.
- Separates cell contents from surroundings — Cell membrane.
- Provides structural rigidity — Cell wall.
- Packs and stores materials received from ER — Golgi apparatus.
- Helps in manufacturing food — Chloroplast.
| Option | Present in the plant cells | Absent in the animal cells |
|---|---|---|
| (i) | Leucoplast | Cell wall |
| (ii) | Mitochondria | Ribosome |
| (iii) | Cell wall | Golgi apparatus |
| (iv) | Lysosome | Endoplasmic reticulum |
Leucoplasts occur in plant cells, and cell walls are absent in animal cells.
- Similarities: Both have two membranes, their own DNA and ribosomes.
- Mitochondria: Release usable energy from food during cellular respiration and help produce ATP.
- Chloroplasts: Contain chlorophyll and use light energy to make food by photosynthesis.
(i) Chloroplasts, Ribosomes
(ii) Mitochondria, Nucleus
(iii) Golgi bodies, Ribosomes
(iv) Nucleus, Lysosomes
Both contain DNA. Chloroplasts also contain their own DNA, but ribosomes, Golgi bodies and lysosomes do not.
(i) What hypothesis does she want to test through this experiment?
(ii) What would you suggest for the improvement of this experiment?
(iii) Why does the carrot in plain water stay stiff and crunchy, but the carrot in concentrated salt solution become rubbery and limp?
| Structures in a cell | Bacterial cell | Animal cell |
|---|---|---|
| Chromosome | Present | Present |
| Nucleus | Absent (no well-defined nucleus) | Present |
| Mitochondria | Absent | Present |
| Golgi complex | Absent | Present |
| Chromoplasts | Absent | Absent |
Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Place each of the potato cups in a beaker containing water (Fig. 2.22). Now, set up the experiment as follows:
(a) Keep Cup A empty.
(b) Add one teaspoon sugar in Cup B.
(c) Add one teaspoon salt in Cup C.
(d) Add one teaspoon sugar in the boiled potato in Cup D.
Observe the four potato cups at least two hours and answer the following questions:
(i) Explain why water gathers in the hollowed portion of Cup B and Cup C.
(ii) Why is Cup A necessary for this experiment?
(iii) Explain why water does not gather in the hollowed portions of Cups A and D.
(i) Ribosome — Protein synthesis
(ii) SER — Lipid and cellulose synthesis
(iii) Lysosome — Digestion of foreign agents
SER helps synthesise lipids, but cellulose synthesis is not its function. Ribosomes make proteins, and lysosomes help digest unwanted materials.
The Journey Beyond
- Proteins: Ribosomes attached to the rough ER help make membrane proteins.
- Lipids: Smooth ER helps synthesise membrane lipids.
- The ER transports these materials in vesicles to the Golgi apparatus, where they may be modified and sorted. Transport vesicles then carry them to the cell membrane.
Flow of materials used in cell membrane formation.
(i) Which scientific concept has the farmer applied in the preservation of the farm produce?
(ii) How does the addition of high concentrations of salt and sugar create an environment that prevents the growth of spoilage-causing bacteria and fungi?
(iii) Suggest a healthy recipe of this kind for food preservation.
(iv) What are the scientific values addressed in this case?
Important Concepts to Remember
- The cell is the basic structural and functional unit of life.
- All cells have a cell membrane and cytoplasm.
- Prokaryotic cells lack a well-defined nucleus; eukaryotic cells have one.
- Osmosis is the movement of water through a selectively permeable membrane.
- The cell wall supports and protects plant cells.
- The nucleus contains DNA, which carries genetic information.
- Mitochondria help release energy from food.
- Chloroplasts carry out photosynthesis.
- Mitosis produces two daughter cells; meiosis produces four cells with half the chromosome number.
- Cell theory states that all living organisms are made of cells and new cells arise from existing cells.
Prepared for studycbse.online | Class 9 Science | Chapter 2: Cell: The Building Block of Life