CBSE Class 9 Science Chapter 5 Revision Notes

Chapter 5:The Fundamental Unit of Life Revision Notes

THE COMPOSITION OF A LIVING ORGANISM

• All living creatures are built up of a basic unit of life known as a “cell.”
• A cell is a Latin word that means** “a little room.”**
• The scientist Robert Hooke saw a little compartment in the cork (tree bark) that resembled a honeycomb structure. The term “Cell” was coined to characterize these units, and it is still used in Biology as “Cell Biology.”
• For the first time, scientist Leeuwenhoek observed free-living cells in pond water.
• The cell’s nucleus was found by scientist Robert Brown.
• Schleiden and Schwann proposed the cell hypothesis, which argues that all plants and animals are made up of cells.
• Virchow elaborated on the cell hypothesis by arguing that “all cells develop from pre-existing cells.”
• Cells vary in size, form, and structure.
• Onion cells, Smooth muscle cells, Blood cells, Bone cells, Fat cells, Nerve cells, Ovum, Sperm, and so on are examples of cell types. Each type of cell has a distinct purpose.
• In unicellular creatures, such as Amoeba, Chlamydomonas, Paramecium, and Bacteria, a single cell can become a whole organism. Division of work is evident in multicellular organisms (human beings).

THE COMPOSITION OF A CELL

• The plasma membrane, nucleus, and cytoplasm are present in practically every cell.

• Plasma membrane: The plasma membrane is the cell’s outer covering.

• The membrane is termed as a selectively permeable membrane (because it prevents the movement of some materials).

• Diffusion and osmosis are aided by it.

• Diffusion is the transfer of a chemical from a high to a low concentration.

• For example, carbon dioxide or oxygen exchange with the outside world.

• Osmosis is the movement of water across a selected permeable membrane from a location of high water concentration to a region of low water concentration.

  a) If the medium around the cell has a greater water concentration (Hypotonic solution) than the cell, the cell accumulates water.

  b) The cell’s water concentration remains constant (isotonic solution), and water passes through the cell membrane in both directions.

  c) If the medium has a lower water concentration (hypertonic solution) than the cell, the cell loses water.

  Note: Endosmosis is the process of a cell ingesting water; ex-osmosis is the process of a cell omitting water.

• Endocytosis is the process by which a cell consumes food and exocytosis is the process by which it ejects solids. Endocytosis is the process by which an amoeba obtains nourishment, whereas exocytosis is the process by which a solid is excreted.

• The cell wall is a cellulose-based stiff outer coating. Plant cells rely on it for structural support. When a live cell loses water, the contents of the cell shrink and move away from the cell wall. Plasmolysis is the term for this phenomenon. Plants, fungi, and bacteria have cell walls that allow them to survive very dilute (Hypotonic) external fluids without bursting.

The Nucleus

• Nucleus is a dark-colored, round or oval, dot-like structure found at the cell’s core. The nucleus is an essential component of cellular activity and reproduction.
• Chromosomes are formed from the chromatin material. In the form of DNA (Deoxyribonucleic Acid) and protein molecules, chromosomes provide information for the inheritance of traits from parents to subsequent generations. Genes are the functional portions of DNA.
• The nucleus of certain species, such as Bacteria, is not covered by the nuclear membrane. As a result, it is known as a prokaryote. (Karyote = karyon = nucleus; pro= primal.) Eukaryotes are creatures that have a nuclear membrane in their cells.

CELL ORGANELLES

The fluid matrix that surrounds every cell (apart from the nucleus) is known as cytoplasm. Protoplasm is the combination of the nucleus and the cytoplasm. Purkinje invented the word “protoplasm.” Endoplasmic reticulum (ER), Golgi apparatus, Lysosomes, Mitochondria, Plastids, and vacuoles are all significant cell organelles.

The Endoplasmic Reticulum (ER)

• It is a vast network of tubules and vesicles that are membrane-bound.
• Endoplasmic Reticulum is divided into two categories.
• Rough Endoplasmic Reticulum (RER) (It has a rough appearance due to the presence of ribosomes on its surface.) They are the places where proteins are made).
• Smooth Endoplasmic Reticulum (SER) (It has a smooth appearance due to the absence of ribosomes on its surface.) They are the places where fat molecules are made).

1. SER; aids in the proper functioning of enzymes and hormones in the biochemical process.

2. SER helps the cell detoxify a variety of toxins and medications.

3. The ER is a route for transporting material between different parts of the cytoplasm and the nucleus.

4. Membrane biogenesis is aided by the proteins and lipid molecules generated by the ER.

Lysosomes

• Lysosomes are membrane-bound sacs that contain potent digestive enzymes (enzymes are produced by RER) that degrade worn-out cell organelles.
• When a cell is injured, lysosomes may rupture, allowing enzymes to devour their own cell, giving rise to the term “suicidal sacks of a cell.” It is the cell’s waste disposal system.

Mitochondria

• Mitochondria are surrounded by a double membrane.
• The inner membrane is extensively folded and the outside membrane is highly porous. These folds provide a wide surface area for the creation of the ATP (Adenosine Triphosphate) molecule.
• Because ATP is a cell’s energy currency, Mitochondria are known as the** “Power House of the Cell.”** Mitochondria have their own DNA and ribosomes, so they can produce proteins on their own.

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Plastids

• They are a kind of bacterium found solely in plant cells. There are two kinds of them.
• **Chromoplasts **(Colored Plastids: Chloroplasts – Green pigmented and important in Photosynthesis, as well as other pigments such as yellow or orange)
• Leucoplasts are a kind of leucoplast that is found in the (White or colourless plastids; stores materials such as oils, proteins, fats etc. ) A double membrane also protects plastids. Stroma is the name of the matrix, which serves as a hub for enzymatic activity. Plastids have their own DNA and ribosomes, thus they can produce proteins on their own.

Vacuoles

• They are solid or liquid-filled storage sacs. In mammals, they are tiny, but in plants, they are huge, occupying 50-90 percent of the cell space.
• It contributes to the cell’s turgidity and stiffness. Many chemicals are kept in vacuoles, including amino acids, carbohydrates, organic acids, and proteins.
• The food vacuole in Amoeba is specialized to serve a critical role.

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