The Fundamental Unit Of Life Class 9 CBSE Notes - Chapter 5

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

Cell:

All living beings are composed of the basic functional unit of life i.e., cell. So, a cell is the structural and functional unit of living organisms.

Discovery of Cell:

            In 1665, Robert Hooke first discovered and introduced the term cell. Later in 1831, Robert Brown discovered the nucleus of the cell. In 1885, Virchow expanded the cell theory by suggesting that all cells arise from pre-existing cells.

DO YOU KNOW?

The egg of an ostrich is the largest known cell of a living animal and an average egg is 15 cm long and 13 cm wide.

Types of Organisms:-

Ø  Unicellular Organisms.

Ø  Multicellular Organisms

Unicellular Organisms:-

Organisms made of only one cell are called unicellular organisms, e.g., Amoeba, Chlamydomonas, Paramoecium and bacteria.

Multicellular Organisms:-

 Organisms in which many cells group together to perform different functions of the body are called multicellular organisms, e.g., some fungi, plants and animals.

Shape and Size of a Cell

Ø          The shape and size of cells are related to the specific function they perform.

Ø          The shape of the cell may be variable (i.e., frequently changing its shape) or fixed and peculiar for a particular type of cell.

        Variable shape occurs in Amoeba while fixed shape of cell occurs in most plants and animals.

Ø  Cells can be spherical, spindle-shaped, elongated or branched in nature.

    Structure of Cell:

Some of the main structures of a cell are plasma membrane, nucleus and cytoplasm.

Plasma Membrane:

             It is the outermost layer of the cell that separates the contents of the cell from its external environment and is made up of lipids and proteins.

Functions of Plasma Membrane

   Ø   Its semipermeable nature allows or permits the entry and exit of some materials in and out of the cell.

   Ø  It acts as a mechanical barrier and protects the internal contents of the cell from leaking out.

Diffusion:

 It is the spontaneous movement of a substance from a region of higher concentration to a region of lower concentration. It plays an important role in gaseous exchange between the cells as well as the cell and its environment.

Osmosis:

 It is the flow of water molecules from higher to lower concentration through a selectively permeable membrane, e.g., absorption of water by plant roots.

DO YOU KNOW?

Osmosis is the net diffusion of water across a selectively permeable membrane towards a higher solute concentration.

Osmosis in Different Types of Solution

 Ø  The medium surrounding the cell is hypotonic if it has higher water concentration than the cell. The cell will gain water by osmosis. Water crosses the cell membrane in both directions, but more water will come into the cell than will leave. The cell is likely to swell up.

Ø  The medium surrounding the cell is isotonic if it has same water concentration as the cell. Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.

Ø  The medium surrounding the cell is hypertonic if it has lower water concentration than the cell. The cell will lose water by osmosis. Water crosses the cell membrane in both directions, but more water leaves. The cell than enters it. Therefore the cell will shrink.

Endocytosis:

            It is the property of cell that enables it to engulf in food and other material from its external environment. Amoeba acquires its food through endocytosis. 

DO YOU KNOW?

Diffusion helps in gaseous exchange between the cells and to obtain nutrition from the environment.

Cell Wall:

It is a tough, rigid, non-living covering outside the plasma membrane. It is made up of cellulose and provides structural strength to plants.

Functions of Cell Wall

It permits the cells of the plants, fungi and bacteria to withstand very dilute external media without bursting. It protects the plasma membrane and prevents desiccation of dryness in cells.

Nucleus:

The nucleus is a pivotal organelle responsible for regulating almost all forms of cellular activities. It has a double-layered covering called nuclear membrane. The nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside ie., to the cytoplasm. The nucleus contains chromosomes (composed of DNA and protein), which contains information for inheritance of characters from parents to next generation in the form of DNA (Deoxyribose Nucleic Acid) molecules. 

DO YOU KNOW? 

Functional segments of DNA are called genes. DNA molecules contain the information necessary for constructing and organising cells.

 Types of Cell:

Cells can be of two types:

    Ø  Prokaryotic Cell

   Ø  Eukaryotic Cell

 Prokaryotic Cell:

Such cells are primitive cells and they lack nucleus. Prokaryotes are unicellular organisms, e.g., blue-green algae, bacteria, etc.

Eukaryotic Cell:

Such cells are advanced and complete cells containing a nucleus. Eukaryotes include all living organisms whether they are unicellular or multicellular, e.g., fungi, plants and animals.

 Functions of Nucleus

Ø  It plays a crucial role in determining the way the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell.

Ø  It helps in the transmission of hereditary traits from parents to offsprings.

Ø  It plays a crucial role in cellular reproduction.

Ø  It controls all metabolic activities of cell.

 Cytoplasm:

It is the fluid content inside the plasma membrane. It contains many specialised cell organelles, which performs a specific function for the cell.

 DO YOU KNOW?

 Nucleoid is an undefined nuclear region in organisms Like bacteria that contains only nucleic acids.

 Functions of Cytoplasm

➤ It acts as a storehouse of molecules such as amino acid, glucose, vitamin, etc.

➤ It acts as the site for certain metabolic pathways such as glycolysis, etc.

 Cell Organelles:

These are membrane-bound structures that perform very crucial functions in cells. Some important cell organelles are endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids and vacuoles.

 Endoplasmic Reticulum (ER): It is a large network of membrane-bound tubes and sheets similar in structure to the plasma membrane.

It is of two types-

Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER).

 Rough Endoplasmic Reticulum (RER): RER has ribosomes (sites of protein manufacture) attached to its surface, hence, it appears to be rough.

 Functions of Endoplasmic Reticulum

Ø  SER helps in the manufacture of fat molecules or lipids.

Ø  ER helps in the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus.

Ø  ER also functions as a cytoplasmic framework providing a surface for some of the biological activities of the cell.

Ø  SER plays a major role in detoxification of poisons and drugs in liver cells of vertebrates.

 DO YOU KNOW?

 Membrane biogenesis is a process of building the cell membrane with the aid of proteins and lipids.

Golgi Apparatus:

            It is a network of membrane-bound vesicles called cisterns, arranged parallel to each other in stacks.

Functions of Golgi Apparatus

Ø  It carries out the storage, modification and packaging of substances in the vesicles.

Ø  It is helpful in the formation of lysosomes.

Ø  It also helps in the manufacture of complex sugar from simple sugars.

Lysosomes:

These are membrane-bound sacs filled with digestive enzymes (made by RER).

Functions of Lysosomes

Ø  They help in digestion of foreign substances and Disposal of worn out cell organelles.

Ø  ØThey provide protection against bacteria and virus and keep the cell clean.

Ø  ØDuring the disturbance in cellular metabolism, e.g. when the cell gets damaged, lysosomes burst and the enzymes digest their own cell so, they are called suicide bags of a cell.

Mitochondria:

             These are small rod-shaped organelles. It is a double membrane outer membrane is smooth whereas inner membrane is deeply folded, which structure where creates a large surface area for ATP-generating chemic reactions. It contains its own DNA and ribosomes.

Functions of Mitochondria:-

Ø      ØThey provide energy required for various chemic activities needed for life.

     Ø  The body uses energy stored in ATP (Adenosine Triphosphate) for making new chemical compounds and for mechanical work. Hence, ATP is known as energy currency of the cell and mitochondria as the powerhouse of the cell.

  

Plastids:

These are double membrane-bound organelles present in most of the plant cells and absent in animal cells.

Plastids are of three types:

·       Chloroplast

·       Leucoplast

·       Chloroplast

Chloroplasts:

Green-coloured plastids containing chlorophyll. They manufacture photosynthesis and are called kitchen of cells.

Chromoplasts:

They are colourful plastids that food provide colour to flowers and fruits.

Leucoplasts:

These are colourless plastids which help in the storage of substances like starch, proteins and oils.

DO YOU KNOW?

Plastids also have their own DNA and ribosomes.

 Vacuoles: These are the storage sacs for solid or liquid contents. They are large and permanent in plant cells whereas small-sized and temporary in animal cells. They are bounded by a membrane called as tonoplast.

 Functions of Vacuoles

 Ø  They are full of cell sap and provide turgidity and rigidity to the cell in plants.

Ø  They play an important role in expelling excess water and some wastes from the cell in some of the unicellular organisms.

Ø  Amino acids, sugars, various organic acids and some proteins are stored in vacuoles of plant cell.

Ø  In Amoeba, food vacuole contains the food items that the Amoeba has consumed.

Ø  Cell Division is the process of formation of new cells in organisms required for growth and reproduction. It helps to replace old, dead and injured cells as well as to form gametes.

 Types of Cell Division:-

    Ø  Mitosis

    Ø  Meiosis

 Mitosis is the process in which a mother cell splits into two daughter cells each having the same number of chromosomes as in mother cell. It helps in growth and repair of tissues in organisms.

 Meiosis is a two-step process in which a mother cell splits to form four daughter cells with half the number of chromosomes in comparison to mother cell. It is responsible for formation of gametes (in animals and plants), which after fertilisation give rise to off spring.