The Fundamental Unit of Life Class 9 Notes Science Chapter 5

Introduction

Cell is the structural and functional unit of life. It is the basic unit of life. It was first discovered by Robert Hooke in 1665 in cork slice with the help of primitive microscope. Robert Hooke saw  tiny honey comb like structures on cork. He named them 'cell'. Cell in latin means "Tiny room".

• Leeuwenhoek (1674), discovered the free living cells in pond water with the improved microscope.
• Robert Brown discovered the nucleus in the cell in 1831.
• Purkinje coined the term ‘protoplasm’ for the fluid substance of the cell in 1839.

Robert Hooke discovered Cell

Note: By peeling off a thin layer from an onion, placing it in water, and putting it on a glass slide with a special stain, we can use a microscope to see the onion's cells clearly.
The structures we see under the microscope are similar and together make up the onion bulb. All onion cells look the same, no matter the size of the onion. These small structures are called cells, and they are the building blocks of the onion. All living things are made of cells, and some cells can live on their own.

Cell and Cell Theory

 A cell is the structural and functional unit of all living organisms.

• Cell Theory: The theory that all the plants and animals are composed of cells and the cell is the basic unit of life, was presented by two biologists, Schleiden and Schwann.
• The cell theory was further expanded by Virchow by suggesting that all cells arise from pre-existing cells.

Types of organisms

(a) Unicellular Organism: These organisms are single celled which perform all the functions. Example: Amoeba, paramecium, bacteria.

(b) Multicellular Organism: Many cells grouped together to perform different function in the body  and also form various body parts. Example: fungi, plants, animals.

• The shape and size of cell are different according to the kind of function they perform. There is division of labour in cells.
• Each cell has certain kind of cell organelles to perform different type of function like mitochondria for respiration.

   Different types of Cells

What is the Structural Organisation of a Cell?

Cells,  are the basic building blocks of life, have specialized parts called organelles.
Four main parts: 

(a) The plasma membrane
(b) Cell Wall
(c) The nucleus 
(d) The cytoplasm

These parts help the cell carry out different activities and interact with its surroundings.

(a) Plasma Membrane or Cell Membrane 

This is the outermost covering of the cell that separates the contents of the cell from its external environment. The plasma membrane allows or permits the entry and exit of some materials in and out of the cell. It also prevents the movement of some other materials. The cell membrane, therefore, is called a selectively permeable membrane.

How does the movement of substances take place into the cell?

(i) Diffusion (For movement of gas)

The spontaneous movement of a substance from a region of high concentration to the region of low concentration is called diffusion.

• Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion. Cell also obtains nutrition from the environment.

(ii) Osmosis (For Movement of Water)

The movement of water molecules through selectively permeable membrane along the concentration gradient is called osmosis. Plant cell tend to obtain water through osmosis.

Do you know: Osmosis is special case of diffusion 

The movement of water across the plasma membrane is also affected by the amount of substance dissolved in water. Thus, osmosis is the net diffusion of water across a selectively permeable membrane toward a higher solute concentration.

What will happen if we put an animal cell or a plant cell into a solution of sugar or salt in water? One of the following three things could happen:

Hypotonic Solution

If the medium surrounding the cell has a higher water concentration than the cell, meaning that the outside solution is very dilute, the cell will gain water by osmosis. Such a solution is known as a hypotonic solution.

• Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The net (overall) result is that water enters the cell. The cell is likely to swell up.

Isotonic Solution

If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution.

• 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.

Hypertonic Solution

If the medium has a lower concentration of water than the cell, meaning that it is a very concentrated solution, the cell will lose water by osmosis. Such a solution is known as a hypertonic solution.

• Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink.
  
  Question for Chapter Notes: The Fundamental Unit of Life

  Try yourself:

  What is the function of the plasma membrane in a cell?

  • A.

    Allows the entry and exit of some materials
  • B.

    Prevents the movement of all materials
  • C.

    Only allows the entry of water
  • D.

    Only allows the exit of water

  Explanation

  - The plasma membrane acts as the outermost covering of the cell
  - It separates the contents of the cell from its external environment
  - Allows the entry and exit of some materials in and out of the cell
  - It is also known as a selectively permeable membrane, regulating the movement of substances

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(b) Cell Wall

Cell wall is another rigid outer covering in addition to the plasma membrane found in plant cell. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance which provides structural strength to plants.

Function of Cell Wall

• Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (hypotonic) external media without bursting.
• In such media the cells tend to take up water by osmosis. The cell swells, building up pressure against the cell wall. The wall exerts an equal pressure against the swollen cell.
• Because of cell wall, Plant cells can withstand much greater changes in the surrounding medium than animal cells.
• Plasmolysis- When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.

Animal Cell

(c) Nucleus

It controls all the activities of cell. The nucleus is enclosed by a double-layered membrane that has pores allowing material exchange between the nucleus and cytoplasm is called nuclear Membrane:

Nucleus of a Eukaryotic cell  

• Chromosomes and DNA: Inside the nucleus, chromosomes composed of DNA and proteins carry genetic information crucial for inheritance. DNA in the chromosomes organizes all cellular structures and functions.
• Chromatin: When the cell is not dividing, DNA is present as chromatin, a mass of thread-like structures. This chromatin organizes into distinct chromosomes during cell division.
• Cellular Reproduction: The nucleus is central to cellular reproduction, guiding the development of the cell by controlling its chemical activities and determining its final mature form.

Prokaryotes vs. Eukaryotes

• Prokaryotic Cells: Lack a defined nuclear membrane and contain a nucleoid, an area with nucleic acids but no surrounding membrane. These cells also lack other complex organelles found in eukaryotes and perform cellular functions through simpler structures.
• Eukaryotic Cells: Have a defined nuclear membrane and complex organelles. They perform specialized functions using these organelles.

A prokaryotic cell and it's Nucleiod 

Functions of Chromosomes

• Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. Chromosomes are composed of DNA and protein.
• DNA molecules contain the information necessary for constructing and organising cells.
• Functional segments of DNA are called genes.
• In non-dividing cell, this DNA is present as part of chromatin material.
• Chromatin material is visible as entangled mass of thread like structures. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes and perform cell division.

Functions of Nucleus

• The nucleus plays a central role in cellular reproduction. It is 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.

Nucleoid

• 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 nucleoid.

(d) 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.

Function of Cytoplasm

• It helps in exchange of material between cell organelles.
• It act as store of vital chemicals such as amino acid, glucose, vitamins and iron etc.
• It is the site of certain metabolic pathways such as glycolysis
  

Cell Organelles 

Organelles are specialized structures that perform various jobs inside cells. The term literally means “little organs.” In the same way organs, such as the heart, liver, stomach, and kidneys, serve specific functions to keep an organism alive, organelles serve specific functions to keep a cell alive.

1. Endoplasmic Reticulum (ER)

The endoplasmic reticulum (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. It is also made up of lipid and proteins.

Although the ER varies greatly in appearance in different cells, it always forms a network system.

Endoplasmic Reticulum

Types of Endoplasmic Reticulum

Functions of Rough / Smooth Endoplasmic Reticulum

•  The ribosomes on  RER  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.
• 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.
• One function of the ER is to serve as channels for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus
• The endoplasmic reticulum (ER) serves as a cellular framework in the cytoplasm, providing a foundation for specific cell activities.
• In the liver cells of the group of animals called vertebrates, SER plays a crucial role in detoxifying many poisons and drugs.                                                                          

2. Golgi Apparatus

The Golgi apparatus consists of 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.

GOLGI APPARATUS

Function of Golgi Body

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

3. Lysosomes

Lysosomes are a kind of waste disposal system of the cell. Lysosomes have membrane-bounded structure and contain digestive enzymes made by RER .

Functions of Lysosomes

• Lysosomes break foreign materials entering the cell, such as bacteria or food as well as old organelles into small pieces.
• They contain powerful digestive enzymes which break complex substances into simpler substances 
• During the disturbance in cellular metabolism such as 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.

4. Mitochondria

Mitochondria are known as the powerhouse of the cell.

Structure of mitochondria

• Mitochondria have two membrane coverings.
• 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.

Functions of mitochondria

• 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.
• Mitochondria have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins.

5. Plastids

Plastids are present only in plant cells.

There are two types of plastids:

• Chromoplasts (coloured plastids).
• Leucoplasts (white or colourless plastids)                                                                                                                                                                                                Chromoplasts that hold the green pigment chlorophyll are named chloroplasts   Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll.

A PLASTID Structure of Plastids

• The plastids' internal structure includes multiple layers of membranes surrounded by a substance known as the stroma.
• Plastids also have their own DNA and ribosomes like mitochondria and similar to its mitochondria in it's structure 

Function of Plastids

• Chloroplasts are important for photosynthesis in plants.
• Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored.                          

6. Vacuoles

• Vacuoles are storage sacs for solid or liquid contents.
• They 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.

Function of vacuoles

• In plant cells, vacuoles are filled with cell sap, providing turgidity and rigidity to the cell. 
• Many crucial substances in the life of plant cells, such as amino acids, sugars, various organic acids, and some proteins, are stored in vacuoles.
• In simple organisms like Amoeba, the food vacuole contains the food consumed by the Amoeba.
• Specialized vacuoles in some single-celled organisms also have significant roles in removing excess water and certain wastes from the cell.

Cell Division

The process by which new cells are made is called cell division. 

 Cell division is of two types:

• Mitosis
• Meiosis

Mitosis or Mitotic Cell Division 

Definition: It is the kind of cell division where cell divides to produce two genetically identical daughter cells from the dividing cells of the mother cell.

MITOSIS

• In this process, each cell called mother cell divides to form two identical daughter cells .
• The daughter cells have the same number of chromosomes as mother cell. 
• Most of the cells divide by mitosis for growth .
• It helps in growth and repair of tissues in organisms.

Meiosis or Meiotic Cell Division 

Definition: This cell division occurs in the sexually reproducing cells. Specific cells of reproductive organs or tissues in animals and plants divide to form gametes, which after fertilisation give rise to offspring. 

MEIOSIS

• The process of meiosis involves two consecutive divisions.
•  When a cell divides through meiosis, it produce  four new cells, not just two.
•  The new cells only have half the number of chromosomes than that of the mother cells.                                                                                                            

  Difference between Animal Cell and Plant Cell