CBSE Class 9 Science Notes Chapter 5 The Fundamental Unit Of Life:

 

CBSE Class 9 Science Notes Chapter 5 The Fundamental Unit Of Life:

NOTES 

Chapter 5: The Fundamental Unit Of Life

Introduction: Cells are the basic structural and functional units of life. All living organisms are made up of one or more cells. The cell theory, proposed by Schleiden and Schwann, states that all living organisms are composed of cells, and new cells arise from pre-existing cells. The cell is the fundamental unit of life.

Cell: A cell is a microscopic, membrane-bound structure that contains cytoplasm, genetic material, and organelles. Cells can be classified into two types: prokaryotic and eukaryotic. Prokaryotic cells are primitive cells that lack a nucleus and membrane-bound organelles, whereas eukaryotic cells have a well-defined nucleus and organelles.

Cell Organelles: Organelles are specialized structures within cells that carry out specific functions. Some of the important organelles are:

• Nucleus: It is a spherical, membrane-bound organelle that contains the genetic material of the cell. It controls all the activities of the cell.

• Mitochondria: These are oval-shaped organelles that are responsible for generating energy in the cell. They are also known as the powerhouses of the cell.

• Endoplasmic Reticulum (ER): ER is a network of membrane-bound tubes and sacs that are involved in protein and lipid synthesis.

• Golgi Apparatus: It is a stack of flattened, membrane-bound sacs that modify, sort, and package proteins and lipids for transport to different parts of the cell.

• Lysosomes: These are membrane-bound organelles that contain digestive enzymes. They help in the breakdown of waste materials, old cell parts, and foreign substances.

• Ribosomes: These are small, non-membrane-bound structures that are involved in protein synthesis.

Cell Membrane: The cell membrane is a thin, flexible, and selectively permeable layer that surrounds the cell. It is made up of lipids, proteins, and carbohydrates. The cell membrane regulates the movement of materials into and out of the cell.

Cell Wall: The cell wall is a rigid layer that surrounds the cell membrane in plant cells, bacteria, and fungi. It is made up of cellulose in plant cells and peptidoglycan in bacterial cells. The cell wall provides shape, support, and protection to the cell.

Types of Cells: There are two main types of cells: prokaryotic and eukaryotic. Prokaryotic cells are found in bacteria and archaea and are smaller and simpler in structure. They do not have a nucleus or other membrane-bound organelles. Eukaryotic cells, on the other hand, are found in plants, animals, fungi, and protists. They are larger and more complex in structure, with a well-defined nucleus and membrane-bound organelles.

Plasma Membrane: The plasma membrane is a phospholipid bilayer that surrounds the cell. It is selectively permeable, meaning it regulates the movement of substances into and out of the cell. The membrane also contains proteins that play various roles in cell signaling, transport, and structure.

Cytoplasm: The cytoplasm is a gel-like substance that fills the cell and surrounds the organelles. It contains various molecules and ions that are important for cellular functions, such as metabolism and signaling.

Nucleus: The nucleus is a membrane-bound organelle that contains the genetic material of the cell in the form of DNA. It controls the activities of the cell by regulating gene expression and DNA replication.

Endoplasmic Reticulum (ER): The endoplasmic reticulum (ER) is a network of membrane-bound tubes and sacs that are involved in protein and lipid synthesis. There are two types of ER: rough ER, which is studded with ribosomes and involved in protein synthesis, and smooth ER, which lacks ribosomes and is involved in lipid synthesis and detoxification.

Golgi Apparatus: The Golgi apparatus is a stack of flattened, membrane-bound sacs that modify, sort, and package proteins and lipids for transport to different parts of the cell or outside the cell.

Mitochondria: Mitochondria are organelles responsible for generating energy in the cell. They carry out cellular respiration, a process that converts glucose and oxygen into ATP, the energy currency of the cell.

Lysosomes: Lysosomes are membrane-bound organelles that contain digestive enzymes. They help in the breakdown of waste materials, old cell parts, and foreign substances.

Chloroplasts: Chloroplasts are organelles found in plant cells that carry out photosynthesis, a process that converts light energy into chemical energy in the form of glucose.

Vacuoles: Vacuoles are membrane-bound sacs that are involved in storage, transport, and digestion of various substances in the cell. Plant cells have a large central vacuole that stores water and nutrients.