the fundamental unit of life class 9

Lets read the chapter 5 of  biology ncert – the fundamental unit of life class 9.

Chapter – 5

The fundamental unit of life

• Cell is the fundamental unit of life.
• Cells were first discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a primitive microscope.
• Leeuwenhoek discovered free living cells in pond water for the first time .
• Robert brown in 1831 who discovered the nucleus in the cell.
• The cell theory that all the plants and animals are composed of cells and that the cell is the basic unit of life was presented by 2 biologists Schleiden and Schwann in 1839.
• The cell theory was further expanded by Virchow (1855) by suggesting that all cells arise from pre existing cells.

What are living organisms are made up of ?

All organisms that we observe around are made up of cells. There are two types of organisms that is unicellular and multicellular.

1. Unicellular organisms – A single cell may constitute a whole organism as in Amoeba , Chlaymydomonas , Paramoecium and bacteria . These organisms are called unicellular organisms. uni=single
2. Multicellular organisms – Many cells group together in a single body and assume different function in it to form where is body parts in multicellular organism. multi=many

            Examples – some fungi, plants and animals. Every multi-cellular organism has come from a single cell. Cells                    divide to produce cells of their own kind all cells that come from pre existing cells.

What is a cell made up of ? What is the structural organisation of a cell ?

Every cell has three features-

• plasma membrane
• nucleus
• cytoplasm

All activities inside the cell interactions of the cell with its environment are possible due to these features.

1. Plasma membrane or Cell membrane

• Outermost covering of cell.
• separates the contents of the cell from its external environment.
• allows or permits the entry and exit of some materials in and out of the cell. E.g. exchange of CO₂ and O₂ ,the movement of water molecules.
• prevents movement of some other materials.
• The plasma membrane is flexible and is made up of organic molecules called lipids and proteins.
• The flexibility of the cell membrane also enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis. Amoeba acquires its food through such processes.
• The cell membrane, therefore, is called a selectively permeable membrane.

2. Cell wall

• Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall.
• The cell wall lies outside the plasma membrane.
• The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance and provides structural strength to plants.
• Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (hypotonic) external media without bursting. Because of their walls, such cells can withstand much greater changes in the surrounding medium than animal cells.

3. Nucleus

• Spherical or oval, dot-like structure near the centre of each cell. This structure is called nucleus.
• The nucleus 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, that is, to the cytoplasm .
• The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide. Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules.
• The nucleus plays a central role in cellular reproduction, the process by which a single cell divides and forms two new cells.
• It also plays a crucial part, along with the environment, in determining the way the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell.
• In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane. Such an undefined nuclear region containing only nucleic acids is called a nucleold. Such organisms, whose cells lack a nuclear membrane, are called                                                                                              (Pro = primitive or primary; karyote = karyon = nucleus).
• Organisms with cells having a nuclear membrane are called eukaryotes.

4. Cytoplasm

• a large region of each cell enclosed by the cell membrane. This very little stain. It is called the cytoplasm.
• The cytoplasm is the fluid content inside the plasma membrane.
• It also contains many specialised cell organelles. Each of these organelles performs a specific function for the cell. Cell organelles are enclosed by membranes.

5. Cell organelles

• Organelles are small structures that are specialized to perform specific function like nucleus stores genetic information, mitochondria produce energy.
• Every cell has a membrane around it to keep its own contents separate from the external environment. Large and complex cells, including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function. To keep these activities of different kinds separate from each other, these cells use membrane-bound little structures (or ‘organelles’) within themselves. This is one of the features of the eukaryotic cells .
• Some important examples of cell organelles are: Endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids  and vacuoles. They are important because they carry out some very crucial functions in cells.

1. Endoplasmic Reticulum (ER)

• 
• The ER is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags (vesicles).
• The ER membrane is similar in structure to the plasma membrane.
• There are two types of ER–
  1. 1. Rough endoplasmic reticulum (RER)
     2. Smooth endoplasmic reticulum (SER)

1. 1. RER looks rough under a microscope because it has particles called ribosomes attached to its surface. The ribosomes, which are present in all active cells, are the sites of protein manufacture. The manufactured proteins are then sent to various places in the cell depending on need, using the ER.
   2. The SER helps in the manufacture of fat molecules, or lipids , important for cell function. Some of these proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis. Some other proteins and lipids function as enzymes and hormones. Although the ER varies greatly in appearance in different cells, it always forms a network system.

ER serves as channels for the transport of materials especially proteins between various regions of the cytoplasm or between the cytoplasm and the nucleus. The ER also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell.

SER plays a crucial role in detoxifying many poisons and drugs.

        B. Golgi apparatus

• The Golgi apparatus, first described by Camillo Golgi.
• Golgi apparatus is a system of membrane-bound vesicles arranged approximately parallel to each other in stacks called cisterns.
• These membranes often have connections with the membranes of ER and therefore constitute another portion of a complex cellular membrane system.
• The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus.
• Its functions include

1. 1. the storage
   2. modification and
   3. packaging of products in vesicles.
   4. In some cases, complex sugars may be made from simple sugars in the Golgi apparatus.
   5. The Golgi apparatus is also involved in the formation of lysosomes.

       C. Lysosomes

• Lysosomes are a kind of waste disposal system of the cell.
• Lysosomes help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles.
• Foreign materials entering the cell, such as bacteria or food, as well as old organelles end up in the lysosomes, which break them up into small pieces.
• Lysosomes are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material.
• During the disturbance in cellular metabolism, for example, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the ‘suicide bags’ of a cell.
• Structurally, lysosomes are membrane-bound sacs filled with digestive enzymes .These enzymes are made by RER.

      D. Mitochondria                          

• Mitochondria are known as the powerhouses of the cell.
• Mitochondria are double membrane bound organelle.
• The outer membrane is’ very porous while the inner membrane is deeply folded. These folds create a large surface area for ATP-generating chemical reactions.
• Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins.
• The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphopshate) molecules. ATP is known as the energy currency of the cell. The body uses energy stored in ATP for making new chemical compounds and for mechanical work.

      E. Plastids 

• Plastids are present only in plant cells.
• There are two types of plastids – chromoplasts (coloured plastids) and leucoplasts (white or colourless plastids).
• Chromoplasts containing the pigment chlorophyll are known as chloroplasts.
• Chloroplasts are important for photosynthesis in plants.
• Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll.
• Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored.
• The internal organisation of the plastids consists of numerous membrane layers embedded in a material called the stroma.
• Plastids are similar to mitochondria in external structure. Like the mitochondria. Plastids also have their own DNA and ribosomes.

     F. Vacuoles 

• Vacuoles are storage sacs for solid or liquid contents.
• Vacuoles are small sized in animal cells while plant cells have very large vacuoles.
• The central vacuole of some plant cells may occupy 50-90% of the cell volume.
• In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the cell.
• Many substances of importance in the life of the plant cell are stored in vacuoles. These include amino acids, sugars, various organic acids and some proteins.
• In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba has consumed. In some unicellular organisms, specialised vacuoles also play important roles in expelling excess water and some wastes from the cell.

Conclusion-

Each cell thus acquires its structure and ability to function because of the organisation of its membrane and organelles in specific ways. The cell thus has a basic structural organisation. This helps the cells to perform functions like respiration, obtaining nutrition, and clearing of waste material, or forming new proteins.

Thus, the cell is the fundamental structural unit of living organisms. It is also the basic functional unit of life.

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