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The Fundamental Unit of Life - Cell Theory

Grade 9CBSE

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 life, first discovered by Robert Hooke in 16651665 using a cork slice.

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The Cell Theory was proposed by Schleiden (18381838) and Schwann (18391839), stating that all plants and animals are composed of cells and the cell is the basic unit of life.

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Rudolf Virchow (18551855) expanded the cell theory by suggesting that all cells arise from pre-existing cells (Omnis cellula−e cellulaOmnis\ cellula-e\ cellula).

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Cells vary in size and shape depending on their function. For example, the Nerve cell is long to conduct impulses, while the RBCRBC (Red Blood Cell) is biconcave to increase surface area for O2O_2 transport.

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Diffusion is the spontaneous movement of a substance from a region of high concentration to a region of low concentration (e.g., CO2CO_2 and O2O_2 exchange).

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Osmosis is the passage of water (H2OH_2O) from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.

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In a HypotonicHypotonic solution (medium has higher water concentration), the cell gains water and swells. In an IsotonicIsotonic solution, there is no net movement. In a HypertonicHypertonic solution (medium has lower water concentration), the cell loses water and shrinks (PlasmolysisPlasmolysis).

📐Formulae

Magnification (M)=Height of ImageHeight of ObjectMagnification\ (M) = \frac{\text{Height of Image}}{\text{Height of Object}}

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

Surface Area to Volume Ratio (Cube)=6L2L3=6L\text{Surface Area to Volume Ratio (Cube)} = \frac{6L^2}{L^3} = \frac{6}{L}

1 μm=10−6 m1\ \mu m = 10^{-6}\ m

1 nm=10−9 m1\ nm = 10^{-9}\ m

💡Examples

Problem 1:

What will happen if a Rheo leaf is boiled in water first and then a drop of concentrated sugar solution is added to it?

Solution:

No plasmolysis will occur.

Explanation:

Boiling the Rheo leaf kills the cells. Since the cell membranes are no longer alive or selectively permeable, the process of osmosis (and thus plasmolysis) cannot take place, even when placed in a concentrated (hypertonic) solution.

Problem 2:

Calculate the total magnification of a compound microscope if the eyepiece is 10x10x and the objective lens is 45x45x.

Solution:

Total Magnification=10×45=450xTotal\ Magnification = 10 \times 45 = 450x

Explanation:

The total magnification is the product of the magnifying power of the individual lenses used in the light path.

Problem 3:

Why are lysosomes known as 'suicide bags' of the cell?

Solution:

They contain digestive enzymes like hydrolaseshydrolases.

Explanation:

When a cell gets damaged, lysosomes may burst and their powerful digestive enzymes (hydrolaseshydrolases) digest their own cell, leading to cell death.

Cell Theory - Revision Notes & Key Formulas | CBSE Class 9 Science