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The Invisible Living World: Beyond Our Naked Eye - What Is a Cell?

Grade 8CBSE

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

🔑Concepts

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The cell is the basic structural and functional unit of all living organisms. Just as bricks are the building blocks of a house, cells are the building blocks of life.

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Robert Hooke first discovered and named the 'cell' in 1665 while observing a thin slice of cork under a primitive microscope.

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Organisms are classified based on cell count: Unicellular (single-celled like Amoeba and Paramecium) and Multicellular (many-celled like plants and humans).

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Cell size varies significantly; the smallest cell is the bacteria Mycoplasma (0.1μm0.1 \mu m to 0.5μm0.5 \mu m) and the largest is the egg of an ostrich (170mm×130mm170 mm \times 130 mm).

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The main parts of a generalized cell include the Cell Membrane (Plasma Membrane), Cytoplasm, and the Nucleus.

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Prokaryotic cells lack a well-defined nucleus and membrane-bound organelles (e.g., Bacteria), whereas Eukaryotic cells have a nuclear membrane and organelles (e.g., Onion peel cells, human cheek cells).

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Plant cells differ from animal cells by the presence of a rigid Cell Wall made of cellulose and large central vacuoles.

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Organelles like Chloroplasts are found only in plant cells and are responsible for photosynthesis.

📐Formulae

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

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

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

Actual Size of Object=Observed Size under MicroscopeMagnification\text{Actual Size of Object} = \frac{\text{Observed Size under Microscope}}{\text{Magnification}}

💡Examples

Problem 1:

A student uses a microscope with a 10x10x eyepiece and a 45x45x objective lens to view a plant cell. Calculate the total magnification.

Solution:

Total Magnification=10×45=450x\text{Total Magnification} = 10 \times 45 = 450x

Explanation:

To find the total magnification of a compound microscope, you multiply the magnifying power of the eyepiece by the magnifying power of the objective lens.

Problem 2:

Convert the size of a bacterial cell measuring 0.2μm0.2 \mu m into meters.

Solution:

0.2μm=0.2×10−6m=2×10−7m0.2 \mu m = 0.2 \times 10^{-6} m = 2 \times 10^{-7} m

Explanation:

Since 1μm=10−6m1 \mu m = 10^{-6} m, we multiply the given value by 10−610^{-6} and then adjust the decimal to standard scientific notation.

Problem 3:

Identify the primary differences between a Prokaryotic cell and a Eukaryotic cell based on the nuclear region.

Solution:

Prokaryotic Cell: Nuclear region is poorly defined and known as a 'nucleoid'. Eukaryotic Cell: Nuclear region is well-defined and surrounded by a nuclear membrane.

Explanation:

The term 'Prokaryote' comes from 'pro' (primitive) and 'karyon' (nucleus), referring to organisms without a proper nucleus.