1.0 Introduction to Cells
Every living thing is made up of tiny units called cells. Plants, animals, humans, birds, insects and even very tiny organisms are made of cells. A cell is so small that most cells cannot be seen with our naked eyes. We need a microscope to observe them clearly.
A cell is called the basic unit of life because it is the smallest part of a living organism that can perform life activities. Just as a house is made of bricks, the body of a living organism is made of cells.
Cell: A cell is the smallest structural and functional unit of a living organism.
A wall is made of bricks. Similarly, our body is made of cells. Each cell has a job to do, and many cells work together to keep the body alive and healthy.
The human body has trillions of cells. These cells are not all the same. Blood cells, nerve cells, muscle cells and skin cells have different shapes and functions.
For ICSE exams, remember this exact idea: Cell is the basic structural and functional unit of life.
2.0 Discovery of Cells
Cells are very small, so they were discovered only after microscopes were made. A microscope is an instrument that makes tiny objects look bigger, so that we can study them clearly.
The word cell was first used by Robert Hooke. He observed a thin slice of cork under a microscope. Cork is a part of the bark of a tree. Hooke saw many tiny box-like spaces in the cork. These small spaces reminded him of small rooms, so he called them cells.
Microscope: A microscope is an instrument used to see very small objects that cannot be seen clearly with the naked eye.
| Scientist | What He Observed | Importance |
|---|---|---|
| Robert Hooke | He observed a thin slice of cork under a microscope. | He used the word cell for the tiny box-like spaces he saw. |
Remember Hooke saw Cork Cells. H-C-C: Hooke, Cork, Cells.
3.0 Living Organisms Are Made of Cells
All living organisms are made of cells. Some organisms are made of only one cell, while others are made of many cells. In large organisms like humans, plants and animals, cells work together in an organized way.
Many similar cells join together to form a tissue. Different tissues work together to form an organ. Different organs work together to form an organ system. Organ systems together form a complete organism.
Tissue: A group of similar cells that work together to perform a particular function is called a tissue.
| Level | Meaning | Simple Example |
|---|---|---|
| Cell | Smallest unit of life. | Muscle cell |
| Tissue | Group of similar cells doing the same job. | Muscle tissue |
| Organ | Group of tissues working together. | Heart |
| Organ System | Group of organs working together. | Circulatory system |
| Organism | Complete living being. | Human being |
In a school, students form a class, classes form sections, and sections form the whole school. Similarly, cells form tissues, tissues form organs, organs form organ systems, and organ systems form a complete organism.
Remember the order using CTOOO: Cell → Tissue → Organ → Organ System → Organism.
4.0 Unicellular and Multicellular Organisms
Living organisms may be made of one cell or many cells. Organisms made of only one cell are called unicellular organisms. Organisms made of many cells are called multicellular organisms.
Unicellular Organism: An organism made up of only one cell is called a unicellular organism.
Multicellular Organism: An organism made up of many cells is called a multicellular organism.
In a unicellular organism, one cell performs all life activities such as movement, feeding, respiration and reproduction. In a multicellular organism, different cells perform different functions.
| Point of Difference | Unicellular Organisms | Multicellular Organisms |
|---|---|---|
| Number of cells | Made up of only one cell. | Made up of many cells. |
| Work done by cells | One cell performs all life activities. | Different cells perform different functions. |
| Examples | Amoeba, bacteria | Humans, dogs, mango tree |
Remember: Uni = One and Multi = Many. Unicellular organisms have one cell; multicellular organisms have many cells.
3 Key Takeaways
- A cell is the smallest structural and functional unit of life.
- Robert Hooke observed cork cells and used the word cell.
- Unicellular organisms have one cell, while multicellular organisms have many cells.
Why is a cell called the basic unit of life?
5.0 Shape and Size of Cells
Cells are not all alike. Different cells have different shapes and sizes because they perform different functions. The shape of a cell usually helps it do its work properly.
Some cells are round, some are long, some are flat and some have branches. Most cells are very small and can be seen only with a microscope. However, some cells are large enough to be seen without a microscope, such as the egg of a bird.
Think of different vehicles. A bus, bicycle and ambulance have different shapes because they do different jobs. Similarly, cells have different shapes according to their functions.
| Type of Cell | Shape | How Shape Helps |
|---|---|---|
| Nerve cell | Long and branched | Helps carry messages quickly in the body. |
| Red blood cell | Round and disc-like | Helps carry oxygen through blood. |
| Muscle cell | Long and fibre-like | Helps in movement of body parts. |
| Plant cell | Usually fixed and box-like | Cell wall gives support and shape. |
The longest cells in the human body are nerve cells. Some nerve cells can be very long because they carry messages from one part of the body to another.
Remember: Shape Supports Function. The shape of a cell helps it perform its special work.
6.0 Basic Structure of a Cell
A cell has different parts inside it. These parts work together to keep the cell alive. At Class 6 level, the three basic parts of a typical cell are cell membrane, cytoplasm and nucleus.
| Cell Part | Where It Is Found | Main Function |
|---|---|---|
| Cell membrane | Outer boundary of the cell. | Protects the cell and controls movement of substances. |
| Cytoplasm | Jelly-like substance inside the cell membrane. | Holds cell parts and is the place for many cell activities. |
| Nucleus | Usually present inside the cytoplasm. | Controls the activities of the cell. |
A cell is like a small factory. The cell membrane is like the factory gate, the cytoplasm is like the working area, and the nucleus is like the manager that controls the activities.
Remember the three basic parts using MCN: Membrane, Cytoplasm and Nucleus.
For a simple animal cell diagram, practise labelling cell membrane, cytoplasm and nucleus.
7.0 Cell Membrane
The cell membrane is a thin outer covering of the cell. In an animal cell, it forms the outer boundary. In a plant cell, the cell membrane is present just inside the cell wall.
The cell membrane protects the cell and gives it shape. It also controls what enters and leaves the cell. Useful substances such as water and nutrients can enter the cell, while waste materials can move out.
Cell Membrane: The thin outer covering of a cell that protects the cell and controls the movement of substances into and out of the cell is called the cell membrane.
The cell membrane is like a security gate. It allows useful things to enter and helps unwanted or waste materials move out.
Remember: Membrane = Manager of Movement. It controls what enters and leaves the cell.
8.0 Cytoplasm
The cytoplasm is a jelly-like substance present between the cell membrane and the nucleus. It fills most of the inside of the cell. Many important cell parts are present in the cytoplasm.
The cytoplasm is important because many activities of the cell take place in it. It also holds the nucleus and other cell organelles in position.
Cytoplasm: The jelly-like substance present inside the cell membrane in which the nucleus and other cell parts are found is called cytoplasm.
The cytoplasm is not empty. It contains many tiny cell parts that help the cell perform different activities.
Remember: Cyto = Cell, Plasm = Jelly-like material. Cytoplasm is the jelly-like living material inside the cell.
3 Key Takeaways
- Cells differ in shape and size according to their functions.
- The three basic parts of a typical cell are cell membrane, cytoplasm and nucleus.
- The cell membrane controls movement, while cytoplasm holds the cell parts.
Why is the cell membrane compared to a security gate?
9.0 Nucleus
The nucleus is one of the most important parts of a cell. It is usually round or oval in shape and is present inside the cytoplasm. The nucleus controls the activities of the cell, so it is often called the control centre of the cell.
The nucleus helps the cell grow, repair itself and carry out different life activities. It also contains information that helps in passing characters from parents to offspring. At Class 6 level, we can understand this simply as the nucleus carrying instructions for the cell.
Nucleus: The nucleus is the part of the cell that controls all cell activities.
A nucleus is like the principal of a school. The principal guides and controls the activities of the school. In the same way, the nucleus controls the activities of the cell.
Remember: Nucleus = Notice board of instructions. It gives instructions and controls the cell.
A very common ICSE question is: Which part is called the control centre of the cell? Answer: Nucleus.
10.0 Important Cell Organelles
Inside the cytoplasm, there are tiny structures that perform special functions. These tiny structures are called cell organelles. Each organelle has a particular job, just like different workers in a factory do different types of work.
Cell Organelles: Cell organelles are tiny structures present inside the cell that perform special functions.
Mitochondria
Mitochondria are known as the powerhouses of the cell. They help release energy from food. This energy is used by the cell to perform different activities.
Vacuoles
Vacuoles are storage spaces inside the cell. They store water, food and waste materials. Plant cells usually have a large vacuole, while animal cells may have small vacuoles.
Chloroplasts
Chloroplasts are present in green plant cells. They contain chlorophyll, which helps plants prepare food by photosynthesis. Animal cells do not have chloroplasts.
| Cell Organelle | Simple Meaning | Main Function |
|---|---|---|
| Mitochondria | Powerhouses of the cell. | Release energy from food. |
| Vacuoles | Storage spaces of the cell. | Store water, food and waste materials. |
| Chloroplasts | Green organelles in plant cells. | Help in photosynthesis. |
Remember important organelles using MVC: Mitochondria gives energy, Vacuole stores, Chloroplast makes food in plant cells.
Plant cells can prepare food because they have chloroplasts. Animal cells cannot prepare food because they do not have chloroplasts.
11.0 Plant Cell Structure
A plant cell has many parts that help the plant live, grow and prepare food. Plant cells have some special structures that are not found in animal cells. These special parts are cell wall, chloroplasts and a large vacuole.
Cell Wall
The cell wall is a thick and rigid outer covering present outside the cell membrane in plant cells. It gives shape, strength and support to the plant cell.
Cell Wall: The rigid outer covering present outside the cell membrane in plant cells is called the cell wall.
Large Vacuole
Plant cells usually have a large central vacuole. It stores water and helps keep the plant cell firm. This is one reason why fresh plants look upright and healthy.
Chloroplasts
Chloroplasts are green structures found in plant cells. They contain chlorophyll and help the plant prepare food by photosynthesis.
When a plant does not get enough water, its cells lose water and the plant may wilt. The large vacuole in plant cells helps store water and keep the plant firm.
12.0 Animal Cell Structure
Animal cells are different from plant cells in some important ways. An animal cell does not have a cell wall. It has only a cell membrane as its outer boundary. Animal cells also do not have chloroplasts, so they cannot prepare their own food.
Animal cells usually have small vacuoles or fewer vacuoles. They may have different shapes depending on their function. For example, nerve cells are long and branched, while red blood cells are round and disc-like.
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Outer covering | Has cell wall and cell membrane. | Has only cell membrane. |
| Chloroplasts | Present in green plant cells. | Absent. |
| Vacuoles | Usually one large central vacuole. | Small or fewer vacuoles. |
Remember plant cell special parts using WVC: Wall, large Vacuole and Chloroplasts.
3 Key Takeaways
- The nucleus controls the activities of the cell.
- Mitochondria release energy, vacuoles store materials and chloroplasts help in photosynthesis.
- Plant cells have a cell wall, chloroplasts and a large vacuole, while animal cells do not have cell walls or chloroplasts.
Why can plant cells prepare food but animal cells cannot?
13.0 Plant Cell and Animal Cell Differences
Plant cells and animal cells have many common parts such as cell membrane, cytoplasm, nucleus, mitochondria and vacuoles. But they are not exactly the same. Plant cells have some special parts that animal cells do not have.
The most important differences are related to cell wall, chloroplasts, vacuoles and shape. These differences help plant and animal cells perform their own functions properly.
| Point of Difference | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present outside the cell membrane. | Absent. |
| Chloroplasts | Present in green plant cells. | Absent. |
| Vacuoles | Usually one large central vacuole. | Small vacuoles or fewer vacuoles. |
| Shape | Usually fixed and box-like because of the cell wall. | Usually irregular or rounded because there is no cell wall. |
| Food preparation | Can prepare food if chloroplasts are present. | Cannot prepare food. |
Remember plant cell special features using WCV: Wall, Chloroplasts and large Vacuole.
The most common ICSE comparison question from this chapter is: Differentiate between plant cell and animal cell. Learn at least three points: cell wall, chloroplasts and vacuole.
14.0 Functions of Different Cell Parts
A cell is like a small living unit with many working parts. Each part has a special function. When all cell parts work together, the cell stays alive and performs its activities properly.
| Cell Part | Main Function | Easy Comparison |
|---|---|---|
| Cell membrane | Protects the cell and controls movement of substances in and out. | Security gate |
| Cell wall | Gives shape, strength and support to plant cells. | Strong outer wall |
| Cytoplasm | Holds cell parts and allows many cell activities to occur. | Working area |
| Nucleus | Controls all activities of the cell. | Control centre |
| Mitochondria | Release energy from food. | Powerhouse |
| Vacuoles | Store water, food and waste materials. | Storage tank |
| Chloroplasts | Help green plant cells prepare food by photosynthesis. | Food factory |
Remember cell functions like a factory: Gate = membrane, Manager = nucleus, Power = mitochondria, Storage = vacuole, Food maker = chloroplast.
15.0 Levels of Organization in Living Things
In multicellular organisms, cells are arranged in an organized way. One cell alone cannot do all the work in a large body. So similar cells work together to form tissues, tissues form organs, organs form organ systems and organ systems form the whole organism.
This arrangement helps different body parts perform special jobs. For example, muscle cells form muscle tissue. Muscle tissues help form muscles, and muscles help in movement.
Organ: An organ is a body part made up of different tissues working together to perform a particular function.
| Level | Meaning | Example |
|---|---|---|
| Cell | Smallest unit of life. | Muscle cell |
| Tissue | Group of similar cells doing the same job. | Muscle tissue |
| Organ | Group of tissues working together. | Heart |
| Organ system | Group of organs working together. | Digestive system |
| Organism | Complete living being. | Human being |
Remember the order using CTOOO: Cell → Tissue → Organ → Organ system → Organism.
16.0 Importance of Cells in Daily Life
Cells are important because every life activity happens through cells. Our body grows because cells grow and divide. Wounds heal because new cells are formed. We move, breathe, digest food and think because different cells perform different functions.
Cells Help in Growth
A baby grows into a child and then into an adult because the number and size of cells increase. Plants also grow because new cells are formed in their roots, stems and leaves.
Cells Help in Repair and Healing
When we get a small cut, new cells are formed to repair the damaged skin. This is why wounds slowly heal.
Cells Help in Body Functions
Different cells help in different body functions. Blood cells carry oxygen, nerve cells carry messages, muscle cells help in movement and stomach cells help in digestion.
Plant Cells Help Plants Prepare Food
Green plant cells have chloroplasts. Chloroplasts help plants prepare food by photosynthesis. This food is used by the plant for growth and energy.
When a small wound on your skin heals after a few days, it is because your body forms new cells to replace damaged cells.
Your skin is constantly replacing old cells with new cells. This is one reason why small scratches can heal naturally.
17.0 Chapter-End Exam Revision
The chapter "Cell Structure and Function" teaches us that cells are the basic units of life. Living organisms may be made of one cell or many cells. Cells have different parts that perform different functions. Plant cells and animal cells have some similarities and some important differences.
Practise neat diagrams of a plant cell and an animal cell. Important labels include cell membrane, cytoplasm, nucleus, vacuole, cell wall and chloroplast.
Important Points to Remember
- A cell is the smallest structural and functional unit of life.
- Robert Hooke first used the word cell after observing cork.
- Organisms may be unicellular or multicellular.
- The three basic parts of a cell are cell membrane, cytoplasm and nucleus.
- The cell membrane controls movement of substances in and out of the cell.
- The cytoplasm is a jelly-like substance where many cell activities occur.
- The nucleus is the control centre of the cell.
- Mitochondria are called the powerhouses of the cell.
- Plant cells have a cell wall, chloroplasts and a large vacuole.
- Animal cells do not have a cell wall or chloroplasts.
Quick Revision Table
| Topic | Quick Revision Point | Example or Function |
|---|---|---|
| Cell | Smallest structural and functional unit of life. | Basic unit of living organisms |
| Robert Hooke | First used the word cell. | Observed cork cells |
| Unicellular organism | Organism made up of one cell. | Amoeba, bacteria |
| Multicellular organism | Organism made up of many cells. | Humans, plants, animals |
| Cell membrane | Thin covering of the cell. | Controls entry and exit of substances |
| Cytoplasm | Jelly-like substance inside the cell. | Holds cell parts |
| Nucleus | Control centre of the cell. | Controls cell activities |
| Mitochondria | Powerhouses of the cell. | Release energy from food |
| Vacuoles | Storage spaces of the cell. | Store water, food and waste |
| Cell wall | Rigid outer covering of plant cells. | Gives shape and support |
| Chloroplasts | Green structures in plant cells. | Help in photosynthesis |
| Tissue | Group of similar cells working together. | Muscle tissue |
| Plant cell | Has cell wall, chloroplasts and large vacuole. | Usually fixed shape |
| Animal cell | Does not have cell wall or chloroplasts. | Usually irregular or rounded shape |
3 Key Takeaways
- Plant cells and animal cells differ mainly in cell wall, chloroplasts and vacuoles.
- Each cell part has a specific function that helps the cell stay alive.
- Cells form tissues, tissues form organs, organs form organ systems and organ systems form organisms.
If the nucleus controls the cell, what may happen if the nucleus is damaged?