Cell: The Fundamental Unit of Life – Class 9 Notes (New NCERT)

eSaral ›Foundation courses Cell: The Fundamental Unit of Life – Class 9 Notes (New NCERT)

🚀 Checkout eSaral Courses

Introduction

Biology can feel overwhelming when you first open the new NCERT textbook, but almost every topic you will study eventually connects back to one idea: the cell. Without understanding the cell, no other biology chapter — genetics, human systems, or reproduction — makes complete sense.

In this lecture-based guide, faculty Nikk Tanjum walks through the new NCERT chapter Cell – The Building Block of Life line by line, explaining not just definitions but the reasoning behind them, using everyday analogies like houses, bricks, and companies to make abstract ideas concrete.

This article follows that explanation topic by topic, so you can revise the entire chapter in one place before your exam.

What Is a Cell? The Building Block of Life 

A cell is called the building block of life because it is the basic unit that makes up every living organism, the same way a brick is the basic unit that makes up a house. Individual bricks are joined to form walls, walls form rooms, and rooms together form a complete house. In exactly the same way, cells join together to form the human body.

A simple test separates living things from non-living things: an object like a pen is not made of cells and is not alive, while your body — made of cells — is alive. This is the core reason a cell is described as the unit that gives an organism its "life."

How Did the First Cell Originate? 

Scientists believe that life, and therefore the first cell, originated in water. Some research suggests that life began in small water pools or hot springs, where environmental conditions changed frequently. Such conditions were present on early Earth roughly 3.5 billion years ago. The hot springs of Puga Valley in Ladakh are given as a present-day example of this kind of environment.

These hot springs contain thermophiles — heat-loving, unicellular bacteria that survive and multiply in hot-water environments. Thermus aquaticus is named as an example of a heat-loving unicellular bacterium found in hot springs.

Scientists also observed that calcium carbonate deposits formed around these hot springs. These deposits are believed to have protected early organic molecules from harmful radiation and helped in the formation of the first cell membrane. In simple terms: small organic compounds formed first, and once a membrane formed around them, that structure became the first cell.

💡 Expert Tip : Remember this one line for exams — cells are believed to have originated in water, under conditions similar to those found in hot springs like Puga Valley, Ladakh, roughly 3.5 billion years ago.

Unicellular and Multicellular Organisms 

Because every living organism is made of cells, the cell is called the basic unit of life or fundamental unit of life. Based on the number of cells present, organisms are classified into two types:

In a unicellular organism, the single cell performs every function on its own — respiration, taking in food, digestion, and excretion are all carried out by that one cell. In a multicellular organism like a human being, there are many cells, and because of this, the body shows division of labour: different groups of cells handle different jobs instead of one cell doing everything.

From Cells to Organ Systems: Division of Labour 

The idea of division of labour is explained using the analogy of a growing business: a single person initially does every task alone, but as the business grows, different people are hired for different jobs — one for procurement, one for cooking, one for selling. The human body works the same way.

Cells that are similar in structure, function and origin group together to form a tissue. Tissues combine to form organs, and organs combine to form organ systems — such as the respiratory system, digestive system, circulatory system, and excretory system. Each organ system performs a specific job:

• Respiratory system — carries oxygen in and carbon dioxide out (structures include nasal pores, nasal cavity, trachea, lungs)
• Digestive system — breaks down food into a simpler, absorbable form

This is exactly why the cell is called the structural and functional unit of life: it forms the entire body structure (structural unit), and all life activities — respiration, nutrition, growth, excretion and reproduction — actually take place inside cells (functional unit). Even though cells are tiny, they carry out all life processes.

How Do We Study Cells? Microscopes and Resolution

Cells cannot be seen with the naked eye because the human eye cannot clearly see very tiny objects. This is explained through the concept of the limit of resolution — the minimum distance at which two points can still be seen separately. For the human eye, this limit of resolution is 0.1 mm. Since most cells are smaller than this, a microscope is needed to study them.

Who Discovered the Cell?

Robert Hooke discovered the cell while examining a thin section of cork under a microscope. He observed box-like structures and named them "cells." Because cork is dry, dead plant tissue, the structures Hooke observed were dead cells. Later, Leeuwenhoek observed living cells using a microscope.

Types of Microscopes

• Light microscope — uses visible light and convex lenses, and can magnify objects up to about 1000 times. This is the type of microscope commonly used in school and college laboratories. Its parts include the eyepiece, objective lens, stage, coarse and fine adjustment knobs, and a mirror that reflects light up through the specimen.
• Electron microscope — uses electrons instead of light, is far more advanced, and can reveal structures at the nanometre level with much greater clarity — useful for seeing structures too small for a light microscope, such as ribosomes, proteins, and lipids.

Key features of a microscope include:

• Magnification — how much larger the microscope makes the object appear
• Resolution — the ability to distinguish two close points clearly
• Contrast — the difference in brightness between different parts of the specimen

As per the NCERT chart referenced in the lecture: objects like a chicken egg or human height can be seen with the unaided eye; smaller items such as a fish egg still need no aid; structures like Amoeba or plant/animal cells (around 100 micrometres or less) require a light microscope; and extremely small structures such as bacteria, viruses, ribosomes, proteins and lipids require an electron microscope.

How to Estimate the Size of a Cell 

Since an individual cell cannot be isolated and measured directly, scientists estimate cell size by comparing the microscope's field diameter with the number of cells visible across it. Using an onion peel cell as an example:

1. Note the diameter of the visible field under the microscope (in micrometres)
2. Count the number of cells visible along that diameter
3. Divide the field diameter by the number of cells to get the approximate size of one cell

Worked example from the lecture: If the field diameter is 5000 micrometres and 25 cells are visible across it, then one cell's size is 5000 ÷ 25 = 200 micrometres.

Structure of a Cell: The Three Main Parts 

Every cell has three main parts:

1. Cell membrane — the outermost boundary of the cell
2. Cytoplasm — the substance filling the inside of the cell
3. Nucleus — present roughly at the centre of the cell

In addition, eukaryotic cells contain organelles. A quick distinction introduced here: prokaryotic cells are primitive-type cells that do not have a well-developed nucleus (their DNA lies loosely in the cytoplasm) and lack most membrane-bound organelles. Eukaryotic cells have a true, membrane-bound nucleus along with organelles such as mitochondria, chloroplast (in plants), endoplasmic reticulum and Golgi body.

Cell Membrane, Selective Permeability and Osmosis

The cell membrane (also called the plasma membrane) is present in both plant and animal cells. It is a thin, protective boundary around the cell that separates the cell from its surroundings and controls the movement of substances in and out.

Its defining property is that it is selectively permeable — it allows only certain useful substances to pass through, while blocking others, much like a gatekeeper controlling who enters a house.

What Is Osmosis?

Osmosis is the movement of water through a selectively permeable membrane, from a dilute solution to a concentrated solution.

• A dilute solution contains more water relative to solute (e.g., a mild sugar solution)
• A concentrated solution contains less water relative to solute (e.g., a very sugary syrup)

Water always moves from the side where it is in higher amount (dilute) to the side where it is in lower amount (concentrated), through the selectively permeable membrane.

Everyday examples from the lecture:

• A potato placed in plain water swells up, because the outside is dilute and the inside of the potato cell is comparatively concentrated — water moves into the cell.
• A potato placed in a salt solution shrinks, because the outside becomes concentrated — water moves out of the cell.
• Raisins (kishmish) in kheer or soaked in water swell up as water enters their cells — this is called endosmosis.
• Salt sprinkled on cut vegetables (like onion or cucumber) causes them to release water, because the outer environment becomes concentrated and draws water out of the cells.

The movement of particles from higher concentration to lower concentration is called diffusion.

Types of Solutions

The Fluid Mosaic Model 

The structure of the cell membrane is explained by the Fluid Mosaic Model, the most widely accepted model after earlier models proposed by other scientists. According to this model, the plasma membrane is made of a lipid bilayer (two layers of fat molecules) with proteins arranged within it.

• Integral proteins are embedded within the lipid layer
• Peripheral proteins sit on the surface

These proteins act as gatekeepers, forming channels that open and close to control what enters or exits the cell. Because the lipid layer behaves like a fluid and the proteins are arranged within it like a mosaic pattern (similar to tiles), this arrangement is called the Fluid Mosaic Model. The membrane's thickness is 7 to 10 nanometres.

Cell Wall: Structure and Functions 

The cell wall is an extra rigid (hard) layer found outside the cell membrane, but only in plant cells — animal cells do not have a cell wall, so in animal cells, the cell membrane is the outermost layer.

Functions of the cell wall:

• Provides shape and support to the cell
• Provides protection against environmental stress
• Keeps the plant in an upright position
• Allows water and minerals to pass through

The cell wall is mainly made of cellulose, a carbohydrate made of glucose units. In food, cellulose acts as roughage. Because animal cells lack a cell wall, they can easily change shape, while plant cells, protected by the rigid cell wall, generally have a fixed shape.

Plant Cell vs Animal Cell: Key Differences 

Prokaryotic Cell vs Eukaryotic Cell 

Cells are of two types based on nucleus organisation:

A bacterial cell is a type of prokaryotic cell. Inside such a cell, a jelly-like substance called cytoplasm holds all the cell's organelles.

Cell Organelles and Their Functions 

Nucleus — The Control Centre

The nucleus is the control centre of the cell, controlling all cellular activities because it contains the cell's DNA — the store of hereditary/genetic information. Its structure includes:

• A double-layered nuclear membrane with pores (allowing movement between the nucleus and cytoplasm)
• Nucleolus — makes ribosomes
• Chromatin — thread-like, loosely spread DNA
• Chromosomes — the same DNA in a condensed form, visible during cell division

The functional segment of DNA responsible for a particular trait is called a gene — genes are the functional parts of DNA, and a single DNA molecule contains many genes.

Ribosomes — The Protein Factory

Ribosomes are present in both prokaryotic cells (70S type) and eukaryotic cells (80S type). Ribosomes are the protein factory of the cell — they synthesise protein, and they may float freely in the cytoplasm or remain attached to the endoplasmic reticulum (ER).

Endoplasmic Reticulum (ER)

The ER is a network-like organelle connected to the nucleus, and comes in two types:

• Rough ER (RER) — has ribosomes attached, and therefore makes proteins
• Smooth ER (SER) — has no ribosomes attached, and makes fats/lipids

Golgi Apparatus — The Packaging and Transport Centre

The Golgi apparatus is described as the cell's "post office" — it modifies, packages, and transports proteins and lipids produced by the ER to their correct destination, whether inside or outside the cell.

Lysosomes — The Cleanup Crew

Lysosomes contain digestive enzymes. They break down waste material and destroy damaged organelles, keeping the cell clean — much like household cleaning staff.

Mitochondria — The Powerhouse of the Cell

Mitochondria are called the "powerhouse of the cell" because they produce energy through cellular respiration — breaking down food (glucose) in the presence of oxygen to release energy in the form of ATP (adenosine triphosphate).

Structural highlights:

• Double membrane-bound organelle — outer membrane and inner membrane
• The inner membrane folds are called cristae
• Mitochondria contain their own 70S-type ribosomes
• Mitochondria contain their own DNA — a fact specifically flagged as an important, exam-relevant point

Plastids — Only in Plant Cells

Plastids occur in three types:

• Chloroplast — contains the green pigment chlorophyll; performs photosynthesis
• Chromoplast — contains coloured pigments (red, yellow, orange) that give colour to flowers and fruits
• Leucoplast — a colourless plastid that stores oil and starch

Vacuoles — The Storage Room

Vacuoles store water, minerals, sugar and waste. In plant cells, there is typically one large central vacuole that also helps maintain turgidity — loss of water causes wilting. In animal cells, vacuoles are smaller and more numerous.

Cell Division: Mitosis and Meiosis 

Cell division is the process by which new cells are formed from existing cells. It is essential for growth, repair, replacement, and reproduction — body growth mainly occurs through cell division, since a cell can only grow up to a certain limit before it must divide.

Mitosis

In mitosis, the daughter cells produced are identical to the mother cell, including having the same chromosome number. For example, if a human cell has 46 chromosomes, cells produced through mitosis also have 46 chromosomes. Mitosis produces two identical daughter cells, and is used for:

• Growth
• Tissue repair
• Asexual reproduction

Meiosis

In meiosis, the chromosome number becomes half. This occurs specifically during the formation of gametes (sperm and ovum) during reproduction. If a parent cell has 46 chromosomes, the gametes formed have 23 chromosomes each. When sperm (23) and ovum (23) fuse during fertilisation, the resulting zygote has 46 chromosomes again — restoring the normal number. This is essential to keep the chromosome number constant across generations; if gametes were formed by mitosis instead, the chromosome number would double every generation, which is not biologically possible.

Meiosis produces four daughter cells with half the chromosome number, and occurs in two steps (the first step producing two cells, the second producing four). Meiosis is also linked to genetic variation, since crossing over occurs, causing exchanges in DNA segments.

Cell Culture 

Cell culture refers to growing cells outside the body, in the laboratory, under controlled/artificial conditions. Bacterial cells are often used, with a particular DNA inserted so the cells produce a desired protein. Cell culture is used for medicine production, vaccine production, and scientific research.

Cell Theory: Schleiden, Schwann and Virchow 

The Cell Theory was developed through the work of three scientists:

1. Schleiden (a botanist) — proposed that all plants are made of cells
2. Schwann (a zoologist) — proposed that all animals are made of cells
3. Rudolf Virchow — added that "Omnis cellula e cellula", meaning new cells arise from pre-existing cells through division

Combined, these three points form the Cell Theory: all living organisms are made up of cells, the cell is the basic structural and functional unit of life, and all cells arise from pre-existing cells.

Cell Life Span, Contact Inhibition and Programmed Cell Death 

Not all cells live forever. Every cell has a fixed lifespan — old cells die and are replaced by new ones.

Contact Inhibition

Normal cells divide (mitosis) until they come into contact with neighbouring cells — once cells touch their neighbours, they stop dividing. This property is called contact inhibition.

Cancer and Tumours

Cancer cells lose the property of contact inhibition and continue to divide uncontrollably, forming tumours. This connects back to the earlier point that errors in mitosis can cause tumours.

Programmed Cell Death (Apoptosis)

Cells that age naturally die through a controlled process called Programmed Cell Death (PCD), also known as apoptosis. During this process, changes occur within the cell (such as in lipid arrangement) that signal immune cells to break the dying cell down through phagocytosis, after which lysosomes digest the remaining organelles. A specific example given is during embryo development, where cells between the developing fingers die through programmed cell death so that the fingers separate from what starts out as a single, paddle-like structure.