Class 9 Science Chapter 5 THE FUNDAMENTAL UNIT OF LIFE

by
Combustion and Flame

Here are the notes for Chapter 5 “THE FUNDAMENTAL UNIT OF LIFE,” drawing on the provided excerpts:

Chapter 5: THE FUNDAMENTAL UNIT OF LIFE

1. Introduction to Cells

  • Discovery of Cells:
    • In 1665, Robert Hooke observed a thin slice of cork through a self-designed microscope. He noticed a structure resembling a honeycomb with many “little compartments”.
    • He called these boxes “cells,” derived from the Latin word for ‘a little room’. This was the first time living things were observed to consist of separate units.
    • In 1674, Leeuwenhoek, using an improved microscope, discovered free-living cells in pond water for the first time.
    • Robert Brown discovered the nucleus in the cell in 1831.
    • Purkinje coined the term “protoplasm” for the fluid substance of the cell in 1839.
  • Cell Theory:
    • The cell theory was presented by two biologists: Schleiden (1838) and Schwann (1839).
    • It states that all plants and animals are composed of cells, and that the cell is the basic unit of life.
    • The theory was further expanded by Virchow (1855), who suggested that all cells arise from pre-existing cells.
  • The discovery of the electron microscope in 1940 made it possible to observe and understand the complex structure of the cell and its various organelles.
  • Cells are the basic building units of all organisms.
  • The cell is the fundamental structural unit of living organisms and the basic functional unit of life.

2. What are Living Organisms Made Up of?

  • Unicellular Organisms: Organisms consisting of a single cell. Examples include Amoeba, Chlamydomonas, Paramoecium, and bacteria. Some single cells can live on their own.
  • Multicellular Organisms: Organisms where many cells group together in a single body and assume different functions to form various body parts. Examples include some fungi, plants, and animals. Every multicellular organism originates from a single cell, which then divides to produce more cells.
  • Cell Shape and Size:
    • The shape and size of cells are related to the specific function they perform.
    • Some cells, like Amoeba, have changing shapes.
    • Other cells, such as nerve cells, have a more or less fixed and peculiar shape.
    • Cells from onion peels, regardless of the onion’s size, appear similar.

3. Structural Organisation of a Cell

  • Almost every cell has three main features: plasma membrane, nucleus, and cytoplasm.
  • These features enable all activities inside the cell and interactions with its environment.
  • Within a cell, there is a division of labour, with specific components called cell organelles performing special functions. These organelles together constitute the basic unit called the cell. All cells are generally found to have the same organelles.

4. Plasma Membrane or Cell Membrane

  • This is the outermost covering of the cell that separates its contents from the external environment.
  • It is called a selectively permeable membrane because it allows or permits the entry and exit of some materials while preventing the movement of others.
  • The plasma membrane is flexible and composed of organic molecules called lipids and proteins.
  • Movement of Substances across the Membrane:
    • Diffusion: Substances like carbon dioxide (CO2) and oxygen (O2) move across the cell membrane by diffusion. This is the spontaneous movement of a substance from a region of high concentration to a region of low concentration. Diffusion is crucial for gaseous exchange between cells and their environment.
    • Osmosis: The movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration. Osmosis is a special case of diffusion.
      • Hypotonic Solution: If the medium surrounding the cell has a higher water concentration (more dilute) than the cell, water will enter the cell by osmosis, causing it to swell up.
      • Isotonic Solution: If the medium has the same water concentration as the cell, there will be no net movement of water, and the cell will stay the same size.
      • Hypertonic Solution: If the medium has a lower water concentration (more concentrated) than the cell, water will leave the cell by osmosis, causing it to shrink.
      • Examples include the swelling of a de-shelled egg in pure water and its shrinking in concentrated salt solution, and raisins/apricots gaining water in plain water and shrinking in concentrated sugar/salt solution.
      • Absorption of water by plant roots is an example of osmosis.
  • Endocytosis: The flexibility of the cell membrane allows cells like Amoeba to engulf food and other materials from their external environment.

5. Cell Wall

  • Found in plant cells, fungi, and bacteria, it is a rigid outer covering located outside the plasma membrane.
  • It is mainly composed of a complex substance called cellulose, which provides structural strength to plants.
  • The cell wall permits plant cells to withstand very dilute (hypotonic) external media without bursting. When water enters the cell by osmosis, the cell swells and builds pressure against the cell wall, which in turn exerts equal pressure against the swollen cell.
  • Plasmolysis: The phenomenon where a living plant cell loses water through osmosis, leading to the shrinkage or contraction of the cell contents away from the cell wall. This occurs only in living cells.

6. Nucleus

  • A darkly coloured, spherical or oval, dot-like structure usually near the center of the cell.
  • It has a double-layered covering called the nuclear membrane, which has pores to allow material transfer between the nucleus and the cytoplasm.
  • The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
    • Chromosomes are composed of DNA (Deoxyribo Nucleic Acid) and protein.
    • DNA molecules contain information for the inheritance of characters from parents to the next generation, and for constructing and organising cells.
    • Functional segments of DNA are called genes.
    • In a non-dividing cell, DNA is present as chromatin material, an entangled mass of thread-like structures.
  • The nucleus plays a central role in cellular reproduction (cell division) and directs the chemical activities of the cell, determining its development and form at maturity.

7. Prokaryotic and Eukaryotic Cells

  • Prokaryotic Cells:
    • Organisms like bacteria have an undefined nuclear region called a nucleoid (containing only nucleic acids), due to the absence of a nuclear membrane.
    • They lack most other membrane-bound cytoplasmic organelles. Many organelle functions are performed by poorly organised parts of the cytoplasm.
    • Chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles, not plastids.
    • Generally small (1-10 µm).
    • Have a single chromosome.
  • Eukaryotic Cells:
    • Organisms with cells having a well-defined nuclear membrane.
    • Possess membrane-enclosed organelles.
    • Generally large (5-100 µm).
    • Have more than one chromosome.

8. Cytoplasm

  • The fluid content inside the plasma membrane.
  • It is a large region of the cell enclosed by the cell membrane, which takes up very little stain.
  • Contains many specialised cell organelles, each performing a specific function.
  • Cell organelles in eukaryotes are enclosed by membranes.

9. Cell Organelles (in Eukaryotic Cells)

  • Endoplasmic Reticulum (ER):
    • A large network of membrane-bound tubes and sheets, resembling long tubules, round, or oblong bags (vesicles). Its membrane structure is similar to the plasma membrane.
    • Rough Endoplasmic Reticulum (RER): Appears rough due to ribosomes attached to its surface. Ribosomes are the sites of protein manufacture. These proteins are then transported to various places in the cell.
    • Smooth Endoplasmic Reticulum (SER): Helps in the manufacture of fat molecules (lipids) important for cell function. In liver cells of vertebrates, SER plays a crucial role in detoxifying many poisons and drugs.
    • Functions include serving as channels for the transport of materials (especially proteins) between different regions of the cytoplasm or between cytoplasm and nucleus. It also acts as a cytoplasmic framework for biochemical activities. Some proteins and lipids help in membrane biogenesis (building the cell membrane).
  • Golgi Apparatus:
    • First described by Camillo Golgi, it consists of a system of membrane-bound vesicles (flattened sacs) called cisterns, arranged in stacks parallel to each other. Its membranes are often connected with ER membranes.
    • Functions: Packaging and dispatching materials synthesized near the ER to various targets inside and outside the cell. It is involved in storage, modification, and packaging of products in vesicles. Complex sugars may be made from simple sugars here. It is also involved in the formation of lysosomes.
  • Lysosomes:
    • Membrane-bound sacs filled with digestive enzymes (made by RER).
    • They are the waste disposal system of the cell, helping to keep it clean by digesting foreign materials (like bacteria or food) and worn-out cell organelles. They break down complex substances into simpler ones.
    • Known as the ‘suicide bags’ of a cell because their powerful digestive enzymes can break down all organic material, and if the cell is damaged, lysosomes may burst and digest their own cell.
  • Mitochondria:
    • Known as the ‘powerhouses of the cell’.
    • Have two membrane coverings: an outer porous membrane and an inner deeply folded membrane. The folds increase the surface area for ATP-generating chemical reactions.
    • They release energy for various chemical activities in the form of ATP (Adenosine triphosphate) molecules, which is known as the energy currency of the cell.
    • Unique among organelles, they have their own DNA and ribosomes, allowing them to make some of their own proteins.
  • Plastids:
    • Present only in plant cells.
    • Two types:
      • Chromoplasts: Coloured plastids.
        • Those containing the pigment chlorophyll are called chloroplasts. Chloroplasts are essential for photosynthesis in plants. They also contain other yellow or orange pigments.
      • Leucoplasts: White or colourless plastids. Their primary function is the storage of materials such as starch, oils, and protein granules.
    • Like mitochondria, plastids also have their own DNA and ribosomes.
  • Vacuoles:
    • Storage sacs for solid or liquid contents.
    • In animal cells, vacuoles are generally small.
    • In plant cells, they have very large central vacuoles, which can occupy 50-90% of the cell volume. These are full of cell sap and provide turgidity and rigidity to the cell. They store important substances like amino acids, sugars, organic acids, and proteins.
    • In single-celled organisms like Amoeba, food vacuoles contain the food items consumed. Some unicellular organisms also have specialised vacuoles for expelling excess water and wastes.

10. Cell Division

  • The process by which new cells are made.
  • New cells are formed for growth, to replace old/dead/injured cells, and to form gametes for reproduction.
  • There are two main types of cell division:
    • Mitosis:
      • Most cells divide by mitosis for growth and repair of tissues in organisms.
      • In this process, a mother cell divides to form two identical daughter cells.
      • The daughter cells have the same number of chromosomes as the mother cell.
    • Meiosis:
      • Specific cells of reproductive organs or tissues divide by meiosis to form gametes.
      • It involves two consecutive divisions.
      • When a cell divides by meiosis, it produces four new cells.
      • The new cells have half the number of chromosomes compared to the mother cell.
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