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Cell: The Unit of Life - Structure of Prokaryotic and Eukaryotic cells

Grade 11CBSEBiology

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

🔑Concepts

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Cell Theory: Proposed by Matthias Schleiden and Theodore Schwann, later modified by Rudolf Virchow with the phrase 'Omnis cellula e cellulaOmnis \ cellula \ e \ cellula', meaning all cells arise from pre-existing cells.

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Prokaryotic Cells: Generally smaller (0.1μm0.1 \mu m to 5.0μm5.0 \mu m) and lack a membrane-bound nucleus and organelles. Genetic material is naked (Genophore/Nucleoid).

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Cell Envelope (Prokaryotes): Consists of a three-layered structure: the outermost Glycocalyx (slime layer or capsule), the Cell Wall, and the Plasma Membrane.

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Mesosomes: Specialized membranous structures in prokaryotes formed by the extension of the plasma membrane into the cell, helping in cell wall formation and DNA replication.

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Eukaryotic Cells: Characterized by an organized nucleus with a nuclear envelope and membrane-bound organelles like Mitochondria, Chloroplasts, and Golgi bodies.

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Fluid Mosaic Model: Proposed by Singer and Nicolson (19721972), stating that the plasma membrane is a 'mosaic' of proteins embedded in a fluid lipid bilayer composed of phosphoglycerides.

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Ribosomes: Sites of protein synthesis. Prokaryotes have 70S70S (composed of 50S50S and 30S30S subunits) while Eukaryotes have 80S80S (composed of 60S60S and 40S40S subunits) in the cytoplasm.

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Endomembrane System: A coordinated group of organelles including the Endoplasmic Reticulum (ER), Golgi complex, Lysosomes, and Vacuoles.

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Mitochondria: The 'powerhouse' of the cell, possessing a double membrane. The inner membrane forms infoldings called cristae to increase surface area for ATPATP production.

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Cytoskeleton: An elaborate network of filamentous proteinaceous structures (microtubules, microfilaments) that provide mechanical support and motility.

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Cilia and Flagella: Hair-like outgrowths of the cell membrane. Eukaryotic flagella show a '9+29 + 2' arrangement of microtubules covered by a plasma membrane.

📐Formulae

S=vaS = \frac{v}{a} (Where SS is the Svedberg unit/Sedimentation coefficient, vv is sedimentation velocity, and aa is acceleration)

Surface Area to Volume Ratio=6d\text{Surface Area to Volume Ratio} = \frac{6}{d} (For a cubical cell of side dd, showing why smaller cells are more efficient)

Total Magnification=Magobjective×Magocular\text{Total Magnification} = \text{Mag}_{objective} \times \text{Mag}_{ocular}

💡Examples

Problem 1:

Calculate the sedimentation coefficient of a ribosome if it is described as 70S70S. What do the subunits 50S50S and 30S30S signify in terms of mathematics?

Solution:

The 'SS' stands for Svedberg unit (10−1310^{-13} seconds). The subunits 50S50S and 30S30S do not add up mathematically to 80S80S because SS is a measure of density and shape (sedimentation rate) rather than additive mass.

Explanation:

Sedimentation units are determined by how fast a particle settles in a centrifuge; the shape of the combined 70S70S complex results in a different sedimentation rate than the sum of its individual parts.

Problem 2:

A cell has a diameter of 20μm20 \mu m. If a molecule travels at a rate of 2μm/s2 \mu m/s, how long does it take to cross the cell?

Solution:

Time=DistanceSpeed=20μm2μm/s=10 secondsTime = \frac{Distance}{Speed} = \frac{20 \mu m}{2 \mu m/s} = 10 \text{ seconds}

Explanation:

This demonstrates the importance of the small size of cells in ensuring rapid diffusion and transport of materials like O2O_2 and nutrients.

Structure of Prokaryotic and Eukaryotic cells Revision - Class 11 Biology CBSE