The Fundamental Unit of Life – Class 9 Science Chapter 5 – Summary & Revision Notes
This article provides a complete summary and revision notes for the latest NCERT books. This chapter helps you understand cells — the building blocks of life, and introduces you to important cell structures like the cell membrane, cytoplasm, nucleus, Endoplasmic Reticulum, Golgi Apparatus, Lysosomes, Mitochondria, Vacuoles, Ribosomes, and more. In these notes, we’ll break down complex concepts like cell structure, organelles, and their functions into simple, easy-to-understand points. These notes are written in simple language, cover all important points, and follow the NCERT Class 9 Science syllabus. For Chapter Questions & Answers of this chapter, you should also see |
Cell
A cell is the basic structural and functional unit of all living organisms.
Discovery:
Robert Hook discovered cells in 1665 while observing a cork under a microscope.
Leeuwenhoek discovered free-living cells in pond water in 1674 using an improved microscope.
Robert Brown discovered the nucleus in the cells in 1831.
- Purkinje coined the term ‘protoplasm’ in 1839.
The cell theory
Schleiden (1838) and Schwann (1839) proposed the cell theory. The cell theory states that all plants and animals are composed of cells, and the cell is the basic unit of life.
Virchow (1855) further expanded the cell theory by suggesting that all cells arise from preexisting cells.
Types of Organisms
Unicellular Organisms: Unicellular organisms are made of a single cell that performs all life functions.(e.g., Amoeba, Chlamydomonas, Paramoecium, bacteria).
Multicellular Organisms: Multicellular organisms are made of more than one cell and are complex organisms. (e.g., fungi, plants, animals, humans).
All multicellular organisms start from a single cell through cell division.
Cell Structure and Function
Organelles
Organelles are specialized structures within a cell that perform specific functions to support the cell’s life processes. They act like “organs” of the cell, each with a distinct role in maintaining the cell’s structure, function, and survival.
Organelles are found in the cytoplasm of eukaryotic cells.
A cell has three main parts: plasma membrane, nucleus and cytoplasm.
Cell Membrane / Plasma Membrane
It’s the outermost covering of the cell that separates the contents of the cell from its external environment.
It controls what goes in and out (like a gatekeeper).
It is also called a selectively permeable membrane, as it allows certain substances to enter or exit the cell while restricting others.
- The plasma membrane is flexible and is made up of organic molecules called lipids and proteins.
Functions of Plasma Membrane:
It allows only certain substances (like oxygen, water, glucose) to enter or leave the cell and blocks harmful substances. This process is called selective permeability.
- It provides shape to the cell.
- It acts as a protective barrier that separates the inside of the cell from the outside environment.
- It helps in diffusion and osmosis. Gases like oxygen and carbon dioxide pass through by diffusion. Water moves in and out through osmosis.
- It maintains the internal environment.
- It also enables the cell to engulf food and other material from its external environment.
Diffusion
Diffusion is the spontaneous movement of a substance (gases, liquids, or solutes) from a region of higher concentration to a region of lower concentration until equilibrium is reached. It occurs without the need for energy.
Example :
Oxygen enters the cell because its concentration is higher outside the cell.
Carbon dioxide leaves the cell because its concentration is higher inside the cell.
Osmosis
Osmosis is the process of movement of water molecules from a region of higher water concentration to a region of lower water concentration through a selectively permeable membrane.
It continues until equilibrium is reached.
Types of Osmosis:
Endosmosis – Water moves into the cell (e.g., raisins swell in water).
Exosmosis – Water moves out of the cell (e.g., raisins shrink in salt solution).
Difference Between Diffusion and Osmosis
| Diffusion | Osmosis |
|---|---|
| Movement of particles from high to low concentration | Movement of water molecules from high to low concentration |
| Particles can be solids, liquids, or gases | Only in water molecules |
| No membrane required | Requires a semipermeable membrane |
| Occurs in both living and non-living systems | Occurs only in living cells |
Hypotonic Solution:
If the medium surrounding the cell has a higher water concentration than the cell, meaning that the outside solution is very dilute, the cell will gain water by osmosis. Such a solution is known as a hypotonic solution. The cell is likely to swell up.
A hypotonic solution has a lower concentration of solutes (like sugars) compared to the surrounding fluid.
In a hypotonic solution:
Water enters the cell (moves from outside to inside).
The cell swells up and may even burst (in case of animal cells) due to endosmosis.
Isotonic Solution:
If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution. The cell will stay the same size.
A 0.9% salt (NaCl) solution is isotonic to human red blood cells.
If red blood cells are placed in this solution, they remain normal in shape and size.
Hypertonic Solution:
If the medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution, the cell will lose water by osmosis. Such a solution is known as a hypertonic solution.
Therefore, the cell will shrink. This results in a decrease in the size of the cells (they shrink) as more water comes out of the cell.
Endocytosis
Endocytosis is the process by which a cell engulfs and takes in food or other materials from its external environment.
Cell Wall
The cell wall is a rigid outer layer present outside the plasma membrane in plant cells, bacteria, and fungi. It gives the cell shape, strength, and protection.
In plant cells, the cell wall is primarily composed of cellulose, a complex substance that provides plants with structural strength.
Functions of Cell Wall:
It supports the plant by providing mechanical strength.
It gives shape and rigidity to the cell.
It prevents the bursting of cells when water enters (especially in hypotonic solutions).
It allows free movement of substances in and out of the cell.
Plasmolysis
Plasmolysis is the process in which the contents of a living plant cell shrink away from the cell wall due to the loss of water through osmosis when placed in a hypertonic solution.
In Plasmolysis is the process in which the cell membrane shrinks away from the cell wall when a plant cell loses water by osmosis.
Nucleus
The nucleus is a double membrane-bound organelle in eukaryotic cells that acts as the control center. It contains DNA and is responsible for controlling cell activities and passing traits to the next generation.
Main Parts of the Nucleus:
Nuclear Membrane
A double-layered membrane that surrounds the nucleus.
It has pores which allow the transfer of material from inside the nucleus to the cytoplasm.
Chromatin
Chromatin is thread-like material present in a cell. The chromatin organises itself into chromosomes whenever the cell is about to divide.
Chromosomes
- The nucleus contains chromosomes, which are rod-shaped structures.
- Chromosomes are composed of DNA and protein.
- During cell division, chromatin condenses into chromosomes.
- Functional segments of DNA are called genes.
The chromosomes contain two types of things –
1. DNA – This is responsible for organising and constructing new cells
2. Proteins – These help in the packaging and condensation of DNA.
Nucleolus
It is called the Brain of the Nucleus. It consists of proteins and ribonucleic acids (RNA). It helps in the formation of ribosomes which help in the formation of proteins inside the cell.
Functions of the Nucleus:
It controls all the metabolic activities of the cell.
It takes part in the formation of ribosomes.
It helps in cell division and reproduction.
It regulates the production of proteins through genes.
- It contains genetic information and is concerned with the transmission of hereditary traits from one generation to another.
What is a nucleoid?
Sometimes cells do not have a well-defined nucleus because they lack a nuclear membrane. Such a nucleus with no definite nuclear boundaries is called a Nucleoid.
What are the prokaryotes?
Organisms whose cells do not have a definite cell membrane are called Prokaryotes.
What are eukaryotes?
Organisms whose cells contain a well-defined nuclear membrane are called Eukaryotes.
Distinguish between prokaryotes and eukaryotes.
| Prokaryotes | Eukaryotes |
| There is no presence of a nucleus. | The nucleus exists in the cells. |
| A single chromosome is present. | There are multiple chromosomes. |
| They are generally unicellular organisms. | They are generally multicellular organisms. |
| There are no membrane-bound cell organelles. | There are membrane-bound cell organelles present inside the cells. |
| They are generally small in size. (1–10 µm) | They are generally large (5–100 µm). |
| The nuclear region (nucleoid) is not surrounded by a nuclear membrane. | The nuclear region is well defined, surrounded by a Nuclear membrane. |
| Example – Bacteria, Blue-green algae (Cyanobacteria) | Example – Fungi, Plants and Animals. |
Cytoplasm
- Cytoplasm is a jelly-like fluid present between the plasma membrane and the nucleus in a cell.
- It contains all the cell organelles and provides a medium for their movement and function.
Functions of Cytoplasm
- It supports and suspends the cell organelles and molecules.
- It dissolves cellular wastes.
Many important life reactions take place in the cytoplasm.
- It helps in the movement of materials between different organelles.
- It maintains the shape and internal pressure of the cell.
Differtent Organelles in a Cell –
1. Endoplasmic Reticulum
2. Golgi Apparatus
3. Lysosomes
4. Mitochondria
5. Plastids
6. Vacuoles
7. Centrioles
8. Ribosomes
What is Endoplasmic Reticulum (ER)?
The Endoplasmic Reticulum (ER) is a large network of membrane-bound tubes and sheets found inside the cytoplasm of a cell.
It helps in the transport of materials and the synthesis of proteins and fats.
Types of Endoplasmic Reticulum:
1. Rough Endoplasmic Reticulum (RER)
It looks rough under a microscope because it has ribosomes attached to its surface.
Function: It helps in protein synthesis and transport.
2. Smooth Endoplasmic Reticulum (SER)
It has no ribosomes on its surface, so it looks smooth.
Function: It helps in the synthesis of lipids (fats), hormones, and the detoxification of poisonous molecules. (especially in liver cells).
Functions of Endoplasmic Reticulum:
It helps in the transport of materials inside the cell.
It helps in protein synthesis (by RER).
It helps in the synthesis of lipids (fats), hormones, and the detoxification of poisonous molecules. (by SER).
It helps in making the cell membrane.
In liver cells, SER helps in detoxifying many poisons and drugs.
What is Membrane Biogenesis?
- Membrane Biogenesis is the process of formation of new cell membranes using proteins and lipids made by the ER and processed by the Golgi apparatus.
RER vs SER (Rough ER vs Smooth ER)
| Rough Endoplasmic Reticulum (RER) | Smooth Endoplasmic Reticulum (SER) |
|---|---|
| It looks rough due to ribosomes attached to its surface. | It looks smooth because it lacks ribosomes on its surface. |
| It helps in protein synthesis | It helps in lipid (fat) synthesis |
| It sends proteins to the Golgi apparatus. It synthesizes enzymes and hormones. | It detoxifies drugs and poisons (especially in the liver).It synthesizes lipids |
| Found in the pancreas cells (which secrete proteins) | Found in liver cells (which detoxify) |
What is the Golgi Apparatus?
- The Golgi apparatus is a membrane-bound organelle found in cells, consisting of a series of flattened, stacked sacs called cisternae.
- It helps in the storage, modification, and packaging of materials made in the cell.
- It was first discovered by Camillo Golgi in 1898, and that’s why it is named after him.
Functions of the Golgi Apparatus:
Its functions include the storage, modification and packaging of products in vesicles.
It transports materials synthesised near the ER to within and outside of the cell.
- It is involved in the formation of lysosomes.
In some cases, the Golgi apparatus converts simple sugars into complex sugars.
- It helps in the secretion of hormones, enzymes, and other substances.
Lysosome
Lysosomes are membrane-bound sacs filled with digestive enzymes.
These enzymes are made by the Rough Endoplasmic Reticulum (RER).
The Golgi apparatus processes them and forms lysosomes.
Functions of Lysosomes (as per NCERT):
They digest foreign material as well as worn-out cell organelles and clear waste from the cell.
Lysosomes destroy harmful bacteria or viruses that enter the cell.
Suicide Bags of the Cell
If a cell is damaged, lysosomes may burst and release enzymes that digest the entire cell. That’s why they are called “suicide bags.”
- Lysosomes break down complex substances into simpler ones that can be reused by the cell.
Mitochondria
Mitochondria are double-membrane-bound cell organelles found in most eukaryotic cells.
- They are known as the “powerhouse of the cell” because they produce energy in the form of ATP.
- Mitochondria have two membranes: an outer porous membrane and an inner folded membrane that increases surface area for energy production.
Functions of Mitochondria
- The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphopshate) molecules.
- The folded inner membrane (cristae) provides space for chemical reactions that create ATP.
- Mitochondria have their own DNA and ribosomes. Therefore, it can make some of their own proteins.
Plastids
Plastids are membrane-bound organelles found only in plant cells.
There are two main types:
Chromoplasts – these are colored plastids.
Chloroplasts are a type of chromoplast that contain chlorophyll and help plants make food through photosynthesis. They also have yellow or orange pigments.
- Chloroplasts have many layers inside them, surrounded by a substance called stroma.
Leucoplasts – these are colorless and store things like starch, oil, and proteins.
Functions of Plastids:
1. Chloroplasts contain chlorophyll. They absorb sunlight and use it to make food through photosynthesis.
2. Chromoplasts contain pigments like red, orange, yellow. They give color to fruits, flowers, and some plant parts.
3. Leucoplasts are colorless plastids. They store starch, oils, and proteins.
4. Plastids have their own DNA and ribosomes. This allows them to make some of their own proteins.
Vacuoles
Vacuoles are membrane-bound storage sacs for solid or liquid
contents present in the cytoplasm of cells.
Vacuoles in Plant Cells:
It is very large in plant cells and centrally located.
It is filled with a fluid called cell sap and provides turgidity and rigidity to the cell.
Cell sap contains sugars, amino acids, various organic acids and some proteins.
It can occupy up to 50- 90% of the cell’s volume.
Vacuoles in Animal Cells:
Vacuoles are usually small or even absent in animal cells.
In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba has consumed.
- In some unicellular organisms, vacuoles expel excess water and some wastes from the cell.
Functions of Vacuoles:
It stores useful substances like sugars, amino acids, organic acids and some proteins.
It stores waste products to keep the cell clean.
The large central vacuole in plant cells is filled with cell sap, which provides turgidity and rigidity to the cell.
In some unicellular organisms, vacuoles expel excess water and some wastes from the cell.
In organisms like Amoeba, food vacuoles contain the food items that the Amoeba has
consumed.
Difference Between Plant Cell and Animal Cell
| Plant Cell | Animal Cell |
|---|---|
| Plant cells possess a cell wall. | Animal cells do not have a cell wall. |
| Plant cells are box-like or rectangular in shape. | Animal cells are round or irregular in shape. |
| Plant cells contain plastids (like chloroplasts). | Animal cells do not have plastids. |
| Plant cells have one large central vacuole. | Animal cells have small or no vacuoles. |
| Plant cells do not have centrioles. | Animal cells have centrioles for cell division. |
| Lysosomes are rare in plant cells. | Lysosomes are present in animal cells. |
| Plant cells store energy as starch. | Animal cells store energy as glycogen. |
| The nucleus is usually at the side due to the vacuole. | The nucleus is usually in the center. |