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Cell - Structure of a Cell

Grade 9CBSE

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

🔑Concepts

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Cell as the Basic Unit: All living organisms are composed of cells. Robert Hooke first observed cells in 1665 using a cork slice.

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Cell Theory: Proposed by Schleiden and Schwann, and later expanded by Virchow, stating all cells arise from pre-existing cells (Omnis cellula e cellulaOmnis \: cellula \: e \: cellula).

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Plasma Membrane: A selectively permeable membrane made of lipids and proteins. It regulates the movement of substances via Diffusion (CO2CO_{2} and O2O_{2}) and Osmosis (H2OH_{2}O).

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Osmosis: The movement of water from a region of high water concentration to low water concentration through a semi-permeable membrane. Effects include: Hypotonic (cell swells), Hypertonic (cell shrinks), and Isotonic (no change).

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Nucleus: The control center containing chromosomes, which are made of DNADNA (Deoxyribonucleic Acid) and proteins. Functional segments of DNADNA are called genes.

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Cytoplasm: The fluid content inside the plasma membrane containing specialized cell organelles.

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Endoplasmic Reticulum (ER): A network of membrane-bound tubes. RER (Rough ER) has ribosomes for protein synthesis; SER (Smooth ER) helps in lipid manufacture and detoxification.

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Golgi Apparatus: Consists of stacks of membrane-bound vesicles (cisterns) involved in storage, modification, and packaging of products.

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Mitochondria: Known as the 'Powerhouse of the cell'. They produce energy in the form of ATPATP (Adenosine Triphosphate) molecules through cellular respiration. They have their own DNADNA and ribosomes.

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Plastids: Present only in plant cells. Chromoplasts (colored) and Leucoplasts (white/colorless). Chloroplasts contain chlorophyll for photosynthesis: 6CO2+6H2O→lightC6H12O6+6O26CO_{2} + 6H_{2}O \xrightarrow{light} C_{6}H_{12}O_{6} + 6O_{2}.

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Lysosomes: Known as 'suicide bags' because they contain digestive enzymes that can digest the cell itself if it becomes damaged.

📐Formulae

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

Surface Area to Volume Ratio=6a2a3=6a\text{Surface Area to Volume Ratio} = \frac{6a^{2}}{a^{3}} = \frac{6}{a}

Net Movement=Inward Flux−Outward Flux\text{Net Movement} = \text{Inward Flux} - \text{Outward Flux}

💡Examples

Problem 1:

A plant cell has a diameter of 50μm50\mu m. If a microscope has an eyepiece of 10x10x and an objective lens of 40x40x, what will be the apparent size of the cell in μm\mu m?

Solution:

First, calculate total magnification: 10×40=400x10 \times 40 = 400x. Then, Apparent Size = 50μm×400=20,000μm50\mu m \times 400 = 20,000\mu m.

Explanation:

The total magnification is the product of the ocular and objective lenses. Multiplying the actual size by this factor gives the perceived size under the microscope.

Problem 2:

Calculate the difference in size between a large plant cell (100μm100\mu m) and a small animal cell (15μm15\mu m) using vertical subtraction.

Solution:

100−1585\begin{array}{r} 100 \\ -15 \\ \hline 85 \end{array}

Explanation:

The difference in size between the two types of cells is 85μm85\mu m.

Problem 3:

What happens to a Red Blood Cell (RBCRBC) when placed in a solution where the external concentration of solutes is higher than the internal concentration?

Solution:

The solution is Hypertonic. Water moves out of the cell via osmosis, causing the cell to shrink (crenation).

Explanation:

In a hypertonic environment, the water potential outside is lower than inside, leading to exosmosis.

Structure of a Cell Class 9 Notes & Examples | CBSE Science