Class 9 Science Ch5 The Fundamental Unit of Life Notes PDF

The cell is the smallest living unit that makes up the body of all plants and animals. In the chapter Cell: the fundamental unit of life notes, Class 9 students study how cells were discovered, the different types of cells, and the detailed structure of plant and animal cells. 

These important notes based on cbse syllabus class 9 science help students clearly understand how cells control life processes such as growth, respiration, and reproduction, making the chapter very important for exams and future biology studies.

Cell: Fundamental Unit Of Life Class 9 Notes

These important notes on Cell: A Basic Unit of Life are prepared to help Class 9 students understand the chapter in a clear and simple way. All key concepts, definitions, and diagrams are explained for easy learning and quick revision. For the complete and well-structured notes, download them from below.

<cta2>Download<cta2>

Cells are the basic building blocks of all living organisms. From tiny microorganisms to large plants and animals, every living being is made up of cells. This chapter helps you understand how cells are discovered, how they work and why they are essential for life.

1. What is a Cell?

• Cells are a structural and functional unit of all living organisms.
• All living organisms in this world are made up of cells.
• Cells are a very small structure in our body.
• Every living organism has a single cell or group of cells to function.

a) Discovery of cell

• 1665 → Robert Hooke discovered cells while observing cork.
• 1674 → Leeuwenhoek observed living cells in pond water.
• 1831 → Robert Brown discovered the nucleus.
• 1839 → Purkinje coined the term protoplasm.
• 1839 → Schleiden and Schwann proposed the cell theory.
• 1855 → Virchow stated that all cells arise from pre-existing cells.

b) Characteristics of Cells

• Cells are microscopic
• They perform all basic life activities.
• Cells contain DNA (genetic material).
• Each cell is surrounded by a cell membrane.

2. Unicellular and Multicellular organisms 

There are two types of organism:

a) Unicellular Organisms

• Made up of a single cell.
• Example: Amoeba, Paramecium

b) Multicellular Organisms

• Made up of many cells.
• Example: Humans, plants, fungi

‍

3. The Concept of Organisation of a Cell 

A cell contains three features:

1. The plasma Membrane 
2. Nucleus
3. Cytoplasm 

‍

a) Plasma Membrane 

• Outer covering of the cell.
• Separates the cell from the external environment.
• Also called selectively permeable membrane because it allows selective entry and exit of substances.

Structure of Plasma Membrane 

• Made of lipids, proteins and carbohydrates.
• Proteins help in transport of substances.
• Carbohydrates are present on the outer surface.

b) The Nucleus 

The nucleus is the most important structure in a cell. The nucleus is a prominent organelles present in the cell, which is the controlling centre of all activities around the cell 

Structure of the Nucleus 

• A nucleus has a nuclear membrane that covers the outside of the area of the nucleus.
• There are the pores present on the nuclear membrane that allow the movement of substances out of the nucleus.
• There are chromosomes, rod-shaped structures present in the nucleus that contain genetic material.

The chromosomes contain two types of things:

• DNA is responsible for organizing and constructing the new cells.
• Proteins help in the packaging and condensation of DNA.

Nucleolus

‍

• The nucleolus is also known as the brain of the nucleus.
• It comprises 25 percent of the volume of the nucleus. It consists of protein and RNA (ribonucleic acid). It helps in the formation of ribosomes, which help in the formation of protein.

c) Cytoplasm

The plasma membrane has a fluid-like substance in it, which is called cytoplasm.

Functions of Cytoplasm 

• It supports the cell organelles and molecules.
• Cytoplasm helps in the formation of proteins in the cells.
• It allows the transportation of hormones in cells.
• It dissolves cellular waste.

4. Movement of Water between the Cell and its External Environment 

The following are the processes that take place in the cell:

• Diffusion: The process where molecules move from an area of higher concentration to an area of lower concentration, naturally spreading out until they are evenly distributed.
• Osmosis: It is the movement of water molecules from a solution with a high concentration of water molecules to a solution with a lower concentration of water molecules, through a cell's partially permeable membrane.

a) Hypotonic Solutions 

If the concentration of water outside the cell is higher than the concentration of water inside the cell, it gains water by the process of osmosis. Water can move into the cell from the cell membrane.  

b) Isotonic Solutions

If the cells are put in an environment that has a similar concentration of water as present inside. This state allows for the free movement of water across the membrane without changing the concentration of solutes on either side.

c) Hypertonic Solutions

If the cells are kept in an environment that has a lower concentration of water than what is present inside the cells, then due to the process of osmosis.

5. Prokaryotic cells and Eukaryotic Cells

• Prokaryotic cells are organisms whose cells do not have a definite cell membrane.
• Eukaryotic Cells are organisms whose cells contain well-defined cell membranes.

a) Characteristics of Prokaryotic cells 

• There is no presence of the nucleus in this cell.
• In prokaryotic cells only a single chromosome is present.
• Prokaryotic cells Organisms mostly are unicellular organisms.
• There are no membrane-bound organelles in prokaryotic cells.
• Example: bacteria and blue-green algae.

b) Characteristics of Eukaryotic cells

• A nucleus is in eukaryotic cells.
• In eukaryotic cells multiple chromosomes are present.
• Eukaryotic cells Organisms are multicellular organisms.
• There is a member bound inside the cell.
• Example: fungi, plants and animals.

6. Cell Organelles 

In eukaryotic cells, the cells contain organelles that have their own apart from the overall cell membrane of the cell. The cells perform several functions in our body. The organelles play an important role in the function of the overall human body.

Organelles which carry out important activities in cells are:

• Endoplasmic Reticulum 
• Golgi Apparatus 
• Lysosomes 
• Mitochondria 
• Plastids 
• Vacuoles
• Ribosomes
• Peroxisomes
• Centrioles.

a) Endoplasmic Reticulum

The ER or endoplasmic reticulum, is a vast network of membrane-bound tubes and sheets. It acts as a conduit for the movement of materials, particularly proteins, between distinct cytoplasmic organs or between the cytoplasm and the nucleus. 

It also serves as a cytoplasmic scaffolding that provides a surface for certain of the cell's metabolic operations. Rough endoplasmic reticulum and smooth endoplasmic reticulum are the two forms of ER.

1. RER: These are rough on the outside and are linked to ribosomes. Protein synthesis is carried out by these cells.
2. SER: These are smooth on the outside and have nothing to do with ribosomes. It aids in the production of fat molecules, also known as lipids. It also aids in the detoxification of a variety of toxins and medications.

b) Golgi Apparatus 

The Golgi apparatus is named after Camillo Golgi, a scientist who was the first to describe it. A stack of membrane-bound cisternae makes up the Golgi.

These membranes are frequently connected to the membranes of the ER and so form part of a complex cellular membrane system. Its responsibilities include storing, modifying and packing items in vesicles. It has a role in the development of lysosomes as well.

c) Lysosomes

Lysosomes are enzyme-filled membranous sacs. RER produces these enzymes. They are a type of cell waste disposal device. 

They aid in the cleaning of the cell by digesting foreign substances as well as worn-out cell organelles. 

Hydrolytic enzymes in lysosomes are capable of digesting cellular macromolecules. When a cell is damaged, the lysosome may burst, allowing the cell's enzymes to digest it. As a result, lysosomes are referred to as ‘suicidal bags.'

d) Mitochondria

Mitochondria are cellular organelles that are known as the "powerhouses of the cells." A double membrane separates these from the rest of the body. The exterior membrane is smooth and the inner membrane is folded into cristae folds. 

This expands the cellular respiration area. Mitochondria produce ATP molecules, which are used to release energy. ATP is referred to as the cell's "energy currency." Mitochondria have their own DNA and ribosomes and are capable of producing some proteins. 

e) Plastids

Plastids are a type of bacterium found solely in plant cells. There are two varieties of these: chromoplasts (colored plastids) and leucoplasts (white plastids) (white or colorless plastids). Chloroplasts are plastids that contain the pigment chlorophyll. 

These are necessary for plant photosynthesis. Chromoplasts are organelles that contribute vibrant colors to plant structures such as buds, flowers and leaves.

Organelles that store starch, oils and protein granules are known as leucoplasts. Plastids are made up of several membrane layers that are encased in the stroma. Plastids have DNA and ribosomes of their own.

f) Vacuoles

Plant and animal cells both have vacuoles, which are membrane-bound compartments. These are solid or liquid-filled storage sacs. In animal cells, they are small, whereas in plant cells, they are larger. 

Plant cells have sap-filled vacuoles that give the cell turgidity and stiffness. Water, waste materials and compounds, including amino acids, carbohydrates and proteins, are all stored in these organelles. 

Specialized vacuoles serve a vital function in the expulsion of excess water and certain wastes from the cell in some unicellular organisms.

7. Cell Division

Cell division is the process by which new cells are formed in organisms for growth, replacing old, dead and injured cells and for reproduction. 

Cell wall

The cell wall is only found in plant cells. Cell walls are made of cellulose and are porous. It keeps the contents of the cell distinct from the rest of the world. It gives the cell its form and protects it.

There are two main types of cell division: mitosis and meiosis. 

1. Mitosis

It is the type of cell division responsible for growth and tissue repair. In mitosis, a mother cell divides into two identical daughter cells, each having the same number of chromosomes as the mother cell.

2. Meiosis

It occurs in the reproductive organs to form gametes, which are necessary for sexual reproduction. Meiosis results in four new cells, each with half the number of chromosomes compared to the mother cell. 

This reduction is important to maintain the chromosome number in offspring after fertilisation. The reduction in chromosome number during meiosis ensures that when gametes combine, the offspring have the correct number of chromosomes. 

In conclusion, Cell: the fundamental unit of life is one of the most important chapters in Class 9 Biology. Revising these notes will help you quickly recall key concepts before exams and class tests. Focus on understanding the diagrams and functions rather than memorizing facts. With regular revision and practice, this chapter can become easy, interesting and highly scoring.

FAQS

Q1. Who discovered the cell and when?

Ans. Robert Hooke discovered the cell in 1665 while observing a slice of cork under a microscope.

Q2. Why is the cell called the basic unit of life?

Ans. Because all life processes (growth, reproduction, metabolism) happen inside cells. Every living organism is made up of cells.

Q3. What is the function of mitochondria?

Ans. Mitochondria are called the powerhouse of the cell because they produce energy in the form of ATP.

Q4. How is a plant cell different from an animal cell?

Ans. Plant cells have a cell wall, large vacuole and plastids (like chloroplasts), which are not present in animal cells.

Q5. What does the nucleus do in a cell?

Ans. The nucleus controls all cell activities and contains the DNA, which carries hereditary information.