CELL ORGANELLES – DETAILED NOTES
1. Cell Wall (Plant, Fungi, Bacteria)
Definition
A rigid, non-living outer covering present outside the plasma membrane that provides
shape, protection, and mechanical support.
Composition
Plants: Cellulose, hemicellulose, pectin
Fungi: Chitin
Bacteria: Peptidoglycan (murein)
Algae: Cellulose, silica (diatoms)
Structure
Primary wall – thin, flexible
Secondary wall – thick, lignified
Middle lamella – calcium pectate (cements adjacent cells)
Functions
Protection from mechanical injury
Maintains cell shape
Prevents cell bursting (turgidity)
Allows plasmodesmata for cell-to-cell communication
Exam Notes
Absent in animal cells
Lignin provides rigidity (important for vascular plants)
Middle lamella is rich in pectin
, 2. Plasma Membrane (Cell Membrane)
Definition
A selectively permeable living membrane enclosing the cytoplasm.
Structure
Fluid Mosaic Model (Singer & Nicolson, 1972)
Phospholipid bilayer:
Hydrophilic heads outward
Hydrophobic tails inward
Embedded proteins:
Integral (transmembrane)
Peripheral
Components
Lipids (phospholipids, cholesterol)
Proteins
Carbohydrates (glycoproteins, glycolipids)
Functions
Selective transport
Cell recognition (antigens)
Signal transduction
Endocytosis and exocytosis
Exam Notes
Cholesterol maintains membrane fluidity
Glycocalyx important for cell recognition
More fluid at higher temperature
1. Cell Wall (Plant, Fungi, Bacteria)
Definition
A rigid, non-living outer covering present outside the plasma membrane that provides
shape, protection, and mechanical support.
Composition
Plants: Cellulose, hemicellulose, pectin
Fungi: Chitin
Bacteria: Peptidoglycan (murein)
Algae: Cellulose, silica (diatoms)
Structure
Primary wall – thin, flexible
Secondary wall – thick, lignified
Middle lamella – calcium pectate (cements adjacent cells)
Functions
Protection from mechanical injury
Maintains cell shape
Prevents cell bursting (turgidity)
Allows plasmodesmata for cell-to-cell communication
Exam Notes
Absent in animal cells
Lignin provides rigidity (important for vascular plants)
Middle lamella is rich in pectin
, 2. Plasma Membrane (Cell Membrane)
Definition
A selectively permeable living membrane enclosing the cytoplasm.
Structure
Fluid Mosaic Model (Singer & Nicolson, 1972)
Phospholipid bilayer:
Hydrophilic heads outward
Hydrophobic tails inward
Embedded proteins:
Integral (transmembrane)
Peripheral
Components
Lipids (phospholipids, cholesterol)
Proteins
Carbohydrates (glycoproteins, glycolipids)
Functions
Selective transport
Cell recognition (antigens)
Signal transduction
Endocytosis and exocytosis
Exam Notes
Cholesterol maintains membrane fluidity
Glycocalyx important for cell recognition
More fluid at higher temperature
