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Cell Biology - Structure and Function

February 10, 2025 • Biology • Class 9

The Cell — Basic Unit of Life

“All living organisms are composed of cells.” — Cell Theory

The cell is the structural and functional unit of all living organisms.

Types of Cells

Prokaryotic vs Eukaryotic

Feature Prokaryotic Eukaryotic
Nucleus Absent Present
Size 1–10 μm 10–100 μm
Organelles Few Many
Example Bacteria Animal/Plant cells

Cell Organelles

1. Nucleus

  • Function: Controls cell activities, contains DNA
  • Contains chromosomes made of DNA + Histone proteins
  • Nuclear membrane has nuclear pores

2. Mitochondria

  • Function: Cellular respiration (energy production)
  • Known as the “Powerhouse of the Cell”
  • Has its own DNA (maternal inheritance)
$$ C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP} $$

3. Endoplasmic Reticulum (ER)

  • Rough ER: Has ribosomes → Protein synthesis
  • Smooth ER: No ribosomes → Lipid synthesis

4. Chloroplast (Plant cells only)

  • Function: Photosynthesis
  • Contains chlorophyll (green pigment)
$$ 6CO_2 + 6H_2O \xrightarrow{\text{sunlight}} C_6H_{12}O_6 + 6O_2 $$

5. Cell Membrane

  • Phospholipid bilayer with embedded proteins
  • Selectively permeable
  • Controls entry and exit of substances

Plant Cell vs Animal Cell

Feature Plant Cell Animal Cell
Cell Wall Present Absent
Chloroplast Present Absent
Vacuole Large, central Small, multiple
Shape Fixed (rectangular) Irregular
Centrioles Absent Present

Key Terms to Remember

  • Osmosis: Movement of water from low to high solute concentration
  • Diffusion: Movement of molecules from high to low concentration
  • Plasmolysis: Shrinking of cell membrane in hypertonic solution
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