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The Invisible Living World: Beyond Our Naked Eye - Why Is Cell Considered to Be a Basic Unit of Life?

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The cell was first discovered by Robert Hooke in 16651665 while observing a thin slice of cork under a primitive microscope.

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The cell is known as the basic structural unit because all living organisms are built of cells, similar to how bricks are the building blocks of a house.

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It is called the functional unit because all the vital physiological and chemical functions (like respiration, excretion, and nutrition) are carried out inside cells.

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The Cell Theory, proposed by Schleiden, Schwann, and Virchow, states: 1) All living things are made of cells. 2) Cells are the basic unit of life. 3) All cells come from pre-existing cells.

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Organisms are classified as Unicellular (single-celled like AmoebaAmoeba and ParameciumParamecium) or Multicellular (composed of many cells like plants and humans).

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Cells exhibit various shapes (spherical, spindle-shaped, elongated) and sizes. The size of the smallest cell (MycoplasmaMycoplasma) is about 0.10.1 to 0.5μm0.5 \mu m.

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Prokaryotic cells (e.g., bacteria) lack a well-defined nuclear membrane, while Eukaryotic cells (e.g., onion peel cells, cheek cells) have a membrane-bound nucleus.

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The basic components of a cell include the Cell Membrane (plasma membrane), Cytoplasm, and the Nucleus.

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Plant cells differ from animal cells by the presence of a Cell Wall and Plastids (like Chloroplasts).

📐Formulae

1μm (micrometer)=10−6m1 \mu m \text{ (micrometer)} = 10^{-6} m

1nm (nanometer)=10−9m1 nm \text{ (nanometer)} = 10^{-9} m

Total Magnification=Magnification of Objective Lens×Magnification of Eyepiece Lens\text{Total Magnification} = \text{Magnification of Objective Lens} \times \text{Magnification of Eyepiece Lens}

Cell→Tissue→Organ→Organ System→Organism\text{Cell} \rightarrow \text{Tissue} \rightarrow \text{Organ} \rightarrow \text{Organ System} \rightarrow \text{Organism}

💡Examples

Problem 1:

If a microscope has an eyepiece lens of 10x10x magnification and an objective lens of 40x40x magnification, calculate the total magnification of the cell being observed.

Solution:

The total magnification is calculated by multiplying the magnification of the lenses: Total Magnification=10×40=400x\text{Total Magnification} = 10 \times 40 = 400x

Explanation:

Magnification is a product of the power of the two lenses used in a compound microscope to enlarge the image of the cell.

Problem 2:

Explain why the size of the cell is not related to the size of the body of the animal or plant.

Solution:

The size of a cell is related to its function rather than the size of the organism. For example, a nerve cell in both an elephant and a rat performs the same function of transferring messages; therefore, both possess long and branched nerve cells.

Explanation:

Biological efficiency depends on the surface area to volume ratio and the specific task of the cell, not the total mass of the organism.

Problem 3:

A scientist observes a cell that lacks a nuclear membrane and membrane-bound organelles. Categorize this cell and provide an example.

Solution:

The cell is categorized as a Prokaryotic cell. An example is a bacterial cell or Blue-green algae.

Explanation:

Cells without a well-organized nucleus (lacking a nuclear envelope) are called prokaryotic cells (propro: primitive; karyonkaryon: nucleus).