RBSE Class 9 Science Notes Chapter 5 The Fundamental Unit of Life

These comprehensive RBSE Class 9 Science Notes Chapter 5 The Fundamental Unit of Life will give a brief overview of all the concepts.

RBSE Class 9 Science Chapter 5 Notes The Fundamental Unit of Life

→ Cell is the basic structural and fundamental unit of life.

→ Cell was discovered by Robert Hooke in 1665, he observed the thin slice of cork (the bark of a tree) and found little rooms in the structure of honeycomb. Later on they were known as cells.

→ Cell theory was proposed by J.M. Schleiden and T. Schwann in 1838-39. The cell theory was further expanded by R. Virchow (1855) by stating, “Omnis Cellula e Callula” which means all cells arise from pre-existing cells.

→ Purkinje in 1839 coined the term for the fluid substance of the cell, “protoplasm”. The scientist Robert Brown discovered the nucleus of the cell.

→ Unicellular Organisms : A single cell may form a whole organism as in Amoeba, Paramecium, Chlamydomonas and Bacteria, etc. these are called unicellular organisms.

→ Multicellular Organisms : Fungi, plants and animals are multicellular organisms. In multicellular organisms, division of labour is seen.

→ Shape and size of cells are related to the specific function they perform. Some cells like Amoeba have changing shapes while in some cases, cell shape could be more or less fixed. For example : nerve cells.

→ Every cell has a certain specific component within it known as cell organelle. These organelles carry out some very important funcions in cells, e.g. making new cell organelle, clearing up waste material from cell and so on.

→ Structure of Cell : Each cell is divided into three functional regions – (i) Plasma/cell membrane (ii) Nucleus (ii) Cytoplasm. All activities inside and outside of the cell are possible due to these features.

→ Plasma Membrane : It is the outermost covering of the cell.

• It permits the entry and exit of some materials so that it is called selectively permeable membrane.
• It helps in osmosis and diffusion.
• It is made up of lipids and proteins.

→ Diffusion : It is the movement of substances from high concentration to low concentration e.g. exchange of CO₂ or O₂ with external environment.

→ Osmosis : It is the movement of water molecules from region of higher concentration to lower concentration through semipermeable membrane.

→ In addition to the plasma membrane, plants have another rigid outer covering called cell wall. It is made up of cellulose. In fungus, it is made up of chitin.

→ Nucleus : It has double layered neculear membrane having nuclear pores for transfer of material and chromosomes. The chromosomes contain information for inheritance of features from parents to next generation in the form of DNA and protein molecules. The functional segments of DNA are called genes.

→ On the basis of nuclear membrane, cells are of two types :

(i) Prokaryotic cells : In some organisms like bacteria nucleus is not surrounded by nuclear membrane. Hence they are called prokaryotic cells e.g. blue green algae, Eubacteria. Nucleolus and membrane bound organelles are absent in prokaryotic cells.

(ii) Eukaryotic cells : The organisms with cells having a nuclear membrane are called eukaryotic cells e.g plants and animals. Cytoplasm : It is a fluid content inside the plasma membrane. It contains many special cell organelles. The nucleus and cytoplasm is together called as protoplasm.

→ Various cell organelles are as follows :

(i) Endoplasmic Reticulum (ER) : There are two types of Endoplasmic Reticulum – Rough or Smooth. RER has ribosomes on its surface. It helps in protein synthesis. SER helps in manufacturing of fat molecules or lipids.

(ii) Golgi Apparatus : It was first designed by Camillo Golgi. It consists of membrane bound, fluid filled vesicles arranged parallel to each other in stacks called cisterns. It helps in storage, modification and packaging of products in vesicle. It is also involved in the formation of lysosomes.

(iii) Lysosomes : These are membrane bound sacs filled with digestive enzymes. It helps in removing any dead or old worn out cell organelles by digesting them and make space for new organelles. During the disturbance in cellular metabolism, e.g. when the cells get damaged, lysosomes may burst and release the powerful digestive enzymes which digest their own cell. So, lysosomes are called suicide bags of cell.

(iv) Mitochondria: It is known as the powerhouse of the cell. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP molecules. They have two membrane coverings. Mitochondria have their own DNA and ribosomes.

(v) Plastids : Plastids are found only in plant cells. These are of two types –
(a) Chromoplast (coloured) and (b) Leucoplast (colourless). The most important chromoplasts are chloroplast which contain green pigment called chlorophyll. It helps in synthesis of food in plants by photosynthesis. Plastids also have their own DNA and ribosomes.

(vi) Vacuoles : These are fluid or solid filled membrane bound sacs in cytoplasm. In animal cells they are small sized but in plant cells, a single, prominent large vacuole is present. Many important substances for the life of plants are stored in vacuoles. It is helpful in expelling excess water.

→ Cell Division : There are two types of cell division – Mitosis and Meiosis.

• The process in which most of the cells divide for growth is called mitosis.
• The process which produces four new cells instead of just two, it is called meiosis.