Cell The Building Block of Life Class 9 Notes Science Chapter 2

Reviewing Class 9 Science Notes and Exploration Chapter 2 Cell The Building Block of Life Class 9 Notes regularly helps in retaining important facts.

Class 9 Science Chapter 2 Cell The Building Block of Life Notes

Class 9 Science Exploration Chapter 2 Notes

Class 9 Science Chapter 2 Notes – Class 9 Cell The Building Block of Life

→ Human eye and Microscopy: The human eye can only see objects larger than a certain size. Very small objects (like cells and microorganisms) require a microscope to be observed.

→ Thermophiles: These are heat-loving unicellular microorganisms(bacteria) that live in extreme hot conditions such as hot springs. They give clues about early life on Earth.

→ Scientists have found that calcium carbonate deposits around hot springs may have protected early organic molecules from harmful conditions and aided in the formation of the first cell membrane.

→ Cell: The cell is the basic structural and functional unit of life. All living organisms are made up of cells. There are two types of organisms based on the number of cells.

• Unicellular: These organisms are made up of only one cell, and a single cell performs all life processes. Example, Amoeba, yeast, bacteria.
• Multicellular: These organisms are made up of many cells, and different cells perform different functions in the body. Example, humans, birds, plants, fish, etc.

→ Organisation in multicellular organisms: Cells combine to form tissues, tissues form organs, and organs work together to form organ systems (e.g., respiratory system includes nose, trachea, and lungs).

→ Limit of Resolution: It is the ability of the eye or an instrument to distinguish two close points as separate. For the human eye, it is about 0.1 mm.

→ Magnification: It is the process of enlarging the appearance of an object using lenses so that small details become visible. Robert Hook was the first person to observe a cell in 1665. He observed small box-like compartments in a thin slice of cork using a self-designed microscope and named them cells.

→ Light Microscope: This is an instrument that uses lenses to magnify tiny objects so that details not visible to the naked eye can be seen.

→ Electron Microscope: This is a powerful microscope that uses beam of electrons instead of light to produce highly magnified images of cell structures at the nanometre scale (a nanometre is one-billionth of a metre).

→ Estimating Cell Size:
Estimated size of the onion peel cell = \(\frac{\text { Diameter of the visible field in micrometre }}{\text { Number of cells along the diameter }} \)

→ Cell interaction: Cells interact with each other and their environment through the cell membrane, which controls the exchange of substances and helps them function as a unit.

→ Cell membrane (Plasma membrane): This is a thin, selectively permeable boundary that controls the entry and exit of substances. It also helps in processes like the exchange of gases in alveoli.

→ Movement of Substances:

• Osmosis: It is the movement of water across a selectively permeable membrane from a higher water concentration to a lower water concentration.
• Diffusion: It is the movement of particles from a region of higher concentration to a region of lower concentration.

→ Types of Solutions of Different Concentrations:

• Isotonic solution: A solution in which the concentration of solutes (and water) is the same inside and outside the cell, so there is no net movement of water and the cell remains unchanged in size.
• Hypotonic solution: A solution in which the surrounding medium has a higher water concentration (lower solute concentration) than the cell, causing water to enter the cell and the cell swells.
• Hypertonic solution: A solution in which the surrounding medium has a lower water concentration (higher solute concentration) than the cell, causing water to leave the cell, and the cell shrinks.

→ Structure of Cell Membrane: The cell membrane is very thin (about 7 – 10 nm thick). It is made up of lipids (fats) and proteins.

• Fluid-mosaic model: This model describes the structure of the cell membrane as a flexible layer made up of a double layer of lipids (lipid bilayer) with proteins embedded within it. The membrane has a lipid bilayer with:
• Water-attracting heads facing outward
• Water-repelling tails facing inward

→ The lipids and proteins can move sideways. Making the membrane fluid in nature. The proteins act as channels, carriers, and receptors, helping in transport of substances and communication. It is called “mosaic” because the molecules are arranged like tiles.

→ Cell wall: Plant, fungal, and bacterial cells have an additional outer layer called the cell wall. It provides rigidity, strength, protection and maintains the shape of the cell. The plant cell wall is mainly composed of cellulose. It is a complex substance and provides structural strength to plant cells. It provides protection and structural support, while cellulose in our diet acts as roughage, aiding digestion.

→ The Cell Interior: Most cells have three basic parts:

• The outer boundary is the plasma membrane, which is selectively permeable.
• The cell contains cytoplasm, a semi-fluid, jelly-like substance.
• A prominent nucleus is present inside the cell.

→ Cytoplasm: The cytoplasm is the fluid content inside the plasma membrane. It also contains many specialised cell organelles (subcellular components). Each of these organelles performs a specific function for the cell. Most organelles are visible only under an electron microscope.

→ Prokaryotic cells: These are simple, small cells without a true nucleus and membrane-bound organelles; mostly unicellular (e.g., bacteria).

→ Eukaryotic cells: These are complex, larger cells with a true nucleus and membrane-bound organelles; they can be unicellular or multicellular (e.g., plant and animal cells).

→ Viruses are non-cellular particles that require a host to reproduce, while viroids are even simpler infectious agents made only of RNA without a protein coat.

→ Comparison between bacterial, plant and animal cells:

• Bacterial cells lack a true nucleus and membrane-bound organelles, while plant and animal cells have them.
• Cell wall is present in bacteria and plant cells but absent in animal cells.
• All cells have a cell membrane and cytoplasm, but structure and complexity differ.

→ Importance of organelles in eukaryotic cells:
Cell organelles are specialised structures that perform specific functions, making the cell work efficiently like a “mini factory.”

→ Nucleus: The nucleus controls all cell activities and stores genetic material (DNA), while the nucleoid is a primitive, non-membrane-bound region containing DNA in prokaryotic cells. The nucleus is also known as the house of coded instructions.

→ Ribosomes: These are the sites of protein synthesis, where proteins essential for cell functions are made.

→ Endoplasmic Reticulum (ER) (Channels, Network for transport): The ER is a large network of membrane-bound tubes and sheets. It looks like long tubules or round sacs.

→ There are two types of ER:

• Rough endoplasmic reticulum [RER]: RER has particles called ribosomes attached to its surface, and is mainly involved in protein synthesis and secretions (e.g., pancreatic cells)
• Smooth endoplasmic reticulum [SER]: It lacks ribosomes, hence appears smooth. The SER helps in the synthesis of fat molecules, or lipids, and some hormones.

→ Golgi apparatus: It is made of stacks of flattened sacs and is connected to the ER and other organelles. It modifies, sorts, and packages proteins and lipids for transport, secretion, or lysosome formation.

→ Lysosomes: These act as the cell’s cleanup system by removing waste and damaged organelles. They are single membrane-bound sacs containing enzymes that break down proteins, fats, and carbohydrates. The broken down materials are released into the cytoplasm and reused by the cell. Human sperm cells contain lysosomal enzymes.

→ Mitochondria: These are called the powerhouses of the cell as they produce energy in the form of ATP. Each mitochondrion has a double membrane-the outer membrane is smooth, while the inner membrane is folded into cristae, increasing the surface area for ATP production.

→ Plastids: In plant cells, special organelles called plastids perform functions such as food synthesis and storage.

→ Similarities between mitochondria and plastids:

• Both are double membrane-bound organelles.
• Both have their own DNA and ribosomes.
• Both can synthesise some of their own proteins.
• Both are considered semi-autonomous organelles.
• Both show similarity to bacteria, suggesting an evolutionary origin.

→ Chloroplasts (a type of plastid) contain chlorophyll, which absorbs sunlight for photosynthesis. They are double membrane-bound and have a semi fluid stroma with disc-shaped structures inside. Plastids are present in the stroma along with starch granules.

→ Chromoplasts: Plastids which contain pigments that give colour to flowers and fruits and help attract pollinators and animals for seed dispersal.

→ Leucoplasts: Colourless plastids that store food materials like starch, oils, and proteins. For example, leucoplasts in potato and taro (Colocassia) store starch.

→ Vacuoles: These are large, single membrane-bound sacs in cells filled with cell sap. It stores water, minerals, sugars, and waste, and maintains cell firmness. Loss of water from the vacuole causes cells to shrink, leading to wilting in plants. In animal cells, vacuoles are smaller and help in temporary storage.

→ Cell Division and Growth: Cell division is the process by which new cells are formed from pre-existing cells, enabling growth, repair, and maintenance of the body. The series of events leading to cell division is called the cell cycle.

→ There are two main types of cell division: mitosis and meiosis.

• Mitosis produces two genetically identical daughter cells and plays an important role in growth and tissue repair.
• Errors In mitosis can lead to uncontrolled cell division, which may result in the formation of tumours and abnormal chromosome numbers in body cells.

→ Meiosis occurs in the reproductive organs of animals, such as testes and ovaries. In plants, meiosis occurs in anthers (male parts) to form pollen grains and in ovaries (female parts) to form egg cells. Meiosis produces four daughter cells with half the number of chromosomes in two-step division process, which is essential for sexual reproduction and maintaining genetic continuity.

→ Errors in meiosis may result in genetic disorders or developmental problems. They can also cause distinct physical features in an individual. Faulty meiosis may further lead to early pregnancy loss or reduced fertility.

→ Cell Theory: It was proposed by scientists such as Matthias Schleiden, Theodor Schwann and Rudolf Virchow.

→ Cell theory states that:

• All living organisms are made of cells.
• The cell is the basic unit of life.
• All cells arise from pre-existing cells.

→ Cell Growth and Division: Cells grow and divide in a controlled way, perform specific functions, and die when they are no longer needed. Dead cells are replaced by new cells that perform the same function. Thus, every cell has a definite life span. If cells do not die at the right time or die too early, it can cause problems in the body.

→ Contact Inhibition: Normal cells stop dividing when they come in contact with neighbouring cells.

→ Tumours: Cancer cells lose contact inhibition and continue to divide uncontrollably, leads to the tumours (abnormal growth of cells).

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