CBSE Notes Class 9 Science Chapter 5 - Fundamental Unit of Life 

Cells are the basic structural and functional unit of living organisms. Cell biology is a branch of Biology that deals with the study of cells in all respects of its structure and function.

1.0Discovery of Cell  

• Robert Hooke (1665) observed a dead cell which resembled honeycomb-like structures in cork (comes from bark of tree). He called these boxes ' cells'. He published what he observed under his self designed microscope in his book 'Micrographia'. 
• Anton Van Leeuwenhoek (1674) was the first to observe living cells in pond water. 
• Robert Brown (1831) discovered the nucleus. 
• Dujardin (1835) discovered a living semi fluid substance of cells and named Sarcode. 
• J. E. Purkinje (1839) used the term protoplasm for sarcode. It is living matter present inside the cell. (vi) Huxley (1868) called protoplasm "The physical basis of life". 
• Rudolf Virchow (1855) – German pathologist established that all cells arise from pre existing cells (“omnis cellula e cellula”). 

2.0Cell Theory 

• The "cell theory" was formulated by two biologists, M.J.Schleiden (1838), and T.Schwann (1839). 
• According to them, the cell is the structural and functional unit of all living beings. The cell theory was further expanded by Virchow. 

Cell theory states 

• Bodies of the living beings are made up of cells. The cell is the basic unit of life. 
• Activities of an organism are the sum total of activities of its cells. 
• Every new cell arises from pre-existing cells. 

3.0Basic Structure of the Cell

Plasma Membrane

• All living cells have a plasma membrane that encloses their contents. 
• It is flexible and made up of organic molecules called lipids, proteins and small fraction of carbohydrates. 
• Flexibility enables the cell to engulf in food and other materials (endocytosis). e.g. Amoeba. It serves as a semi permeable or selectively permeable barrier to the outside environment. 
• The plasma membrane is permeable to specific molecules, however, it allows nutrients and other essential elements to enter the cell and waste materials to leave the cell. 
• Small molecules such as oxygen, carbon dioxide, and water are able to pass freely across the membrane but the passage of larger molecules, such as amino acids and sugars, is carefully regulated.

Plasma membrane performs the following functions 

• It protects the internal components of the cell. 
• It provides shape to the cell. 
• It allows materials to enter and leave the cell through the tiny holes called pores.

Cell Wall

• It is the outermost rigid, freely permeable layer found outside the plasma membrane in all the plant cells, bacteria, blue-green algae, some protists and all fungi. 
• It is absent in animal cells. 
• Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (hypotonic) external media without bursting.

Structure and function 

• Cell wall of plant cells is formed of a fibrous polysaccharide called cellulose, while it is formed of peptidoglycan in bacteria and blue-green algae. 
• It is formed of chitin in most of the fungi. 
• Cell wall is a protective and supportive coat. It also provides a definite shape to the cell. 

Nucleus (Headquarter of the cell)

• Discovered by – Robert Brown (1831) "Nucleus is a double membrane bound dense protoplasmic body, which controls all cellular metabolism and encloses the genetic information of cells". Nucleus is considered as controller or director of the cell. 
• Structure : It is made up of following four components
• Nuclear membrane/ Nuclear envelope/Karyotheca 
• Nuclear sap/Nucleoplasm/Karyolymph. 
• Nucleolus. 
• Chromatin threads.

Functions of Nucleus 

• It controls all the metabolic activities of the cell. 
• It brings about growth of the cell by directing the synthesis of structural proteins. 
• It takes part in the formation of ribosomes. 
• It regulates cell cycle. 
• It contains genetic information and is concerned with the transmission of hereditary traits from one generation to another.

Cytoplasm                           

• The fluid content/protoplasmic mass of the cell inner to plasma membrane and excluding nucleus is called cytoplasm. 
• Cytoplasm has two parts: cytosol and cell organelles. 
• Cytosol forms the aqueous, nearly transparent, structureless ground substance inside the cell. 
• Cell organelles are subcellular structures which have characteristic form, structure and function where certain processes are localized in eukaryotes. 

4.0Cell Organelles

Endoplasmic Reticulum

• Endoplasmic reticulum is a complex network of membrane bound channels or sheets, tubules and vesicles. It does not work alone. 
• The ER works closely with the Golgi apparatus and ribosomes. It creates a network of membranes found through the whole cell. 
• The ER may also look different from cell to cell, depending on the cell's function. 
• Two types of Endoplasmic Reticulum ; 
• Smooth and Rough Smooth ER/Agranular ER : It has smooth membranes which do not bear ribosomes. It acts as a storage organelle. It is important in the synthesis and storage of steroids. It is mainly made up of vesicles and tubules. 
• Rough ER/Granular ER : It has a rough membrane because a number of ribosomes are attached to its outer surface. RER is engaged in protein synthesis. It is mainly made up of cisternae. Tubules are very few.           

Golgi Complex

Golgi Complex (Traffic police of cell)

• Discovered by Camillo Golgi (1898) in nerve cells of owls. Golgi carried out a revolutionary method of staining individual nerve and cell structures. 
• Position : It is located near the nucleus. Golgi bodies are pleomorphic structures, because components of the golgi body are different in structure & shape in different cells. 
• It is involved in cell secretion and acts as storage, modification and condensation or packaging membrane. 
• It forms the acrosome of sperm. 
• It forms the lysosomes and secretory vesicles. 
• It is the site of formation of glycolipids and glycoproteins.  
• Synthesis of cell wall material (polysaccharide synthesis). 
• Cell plate formation during plant cell division. 

Lysosomes (Little enzyme packages) 

• Lysosomes are spherical bag-like structures covered by a single membrane. 
• Lysosomes hold enzymes that were created by the cell. 
• The purpose of the lysosome is to keep the cell clean by digesting any foreign materials as well as worn-out cell organelles. 
• They might be used to digest food or break down the cell when it dies. 

Mitochondria

Mitochondria (Powerhouse of cell)

• Mitochondria are double membrane bound organelles of eukaryotic cells.
• Mitochondria are semi autonomous organelles bound by an envelope of two unit membranes and filled with a fluid matrix. 
• The outer membrane is smooth and has porous proteins which form channels for the passage of molecules through it. 
• The inner membrane is semipermeable. 
• It usually produces numerous infolds called cristae. 
• The cristae greatly increases the inner surface area of the mitochondria to hold a variety of enzymes. 
• Cristae bears minute, regularly spaced tennis racket shaped particles known as F1 particles or oxysomes. 
• Oxysomes are concerned with ATP synthesis. Matrix contain various respiratory enzymes. 

Plastids

• Plastids are major organelles found in the cells of plants and algae. 
• Plastids are the site of manufacture and storage of important chemical compounds used by the cell. 
• Plastids often contain pigments used in photosynthesis and the types of pigments present can change or determine the cell's color. 
• Plastids are responsible for photosynthesis, storage of products like starch.Chloroplasts are the sites of photosynthesis. 
• They contain enzymes and coenzymes necessary for the process of photosynthesis. 

Plastids are of three types- 

• Chromoplasts - For pigment synthesis and storage. Chromoplasts are red, yellow and orange in color and are found in petals of flowers and in fruits. Their color is due to two pigments, carotene and xanthophyll. 
• Leucoplasts - Leucoplasts are colorless or white plastid. They occur in plant cells not exposed to light, such as roots and seeds. Leucoplasts are the center of starch grain formation and they are also involved in the synthesis of oil and proteins. 
• Chloroplasts -  Chloroplasts are probably the most important among the plastids since they are directly involved in photosynthesis. They are usually situated near the surface of the cell and occur in those parts that receive sufficient light. e.g. the palisade cells of leaves. The green color of chloroplasts is caused by the green pigment chlorophyll.

Vacuoles

• Vacuoles are storage sacs found within cells, and in plant cells, they can take up a significant portion of the cell's volume. 
• They store nutrients and help maintain the cell's shape by providing rigidity and turgor.

Ribosomes 

• Ribosomes are the protein builders or the protein synthesizers of the cell. 
• These proteins might be used as enzymes or as support for other cell functions. Ribosomes are found in many places around the cell. 
• They are floating in the cytoplasm. Those floating ribosomes make proteins that will be used inside the cell. Other ribosomes are found on the endoplasmic reticulum (rough endoplasmic reticulum). 
• These attached ribosomes make proteins that will be used inside the cell and for export out of the cell. The 60-S and 40-S model works fine for eukaryotic cells. 
• Prokaryotic cells have ribosomes made of 50-S and 30-S subunits.

5.0Cell Division

• Cells are structural and functional units that carry out respiration, nutrition, and waste disposal. 
• Cell division, the process of replicating, is crucial for growth, tissue repair, and reproduction. 
• In most cells, division occurs through Mitosis, producing two identical daughter cells. In reproductive cells, division occurs through Meiosis, producing cells with half the genetic material necessary for reproduction.