1.5 Cell Wall and Plasma Membrane

The cell wall is a rigid, extracellular structure found outside the plasma membrane. It is not present in animal cells, likely due to their locomotor lifestyle.

• Presence: Plant cells, prokaryotes, fungi, and some protists.
• Functions:
  • Protects the cell.
  • Maintains the cell's shape.
  • Prevents excessive uptake of water (prevents lysis).
  • Provides structural support to the entire plant against gravity.
• Permeability: The cell wall is porous and fully permeable, allowing free passage of water and dissolved substances.

Chemical Composition

The chemical makeup of the cell wall varies between different types of organisms.

Structure of the Plant Cell Wall

The plant cell wall is secreted by the cell and consists of three main layers.

1. Primary Cell Wall
   • A true wall that develops in newly growing cells.
   • Location: Inner to the middle lamella.
   • Properties: Thin, flexible, and adapted for growth (stretches irreversibly).
   • Composition: Composed of cellulose microfibrils arranged in a crisscross pattern within a matrix of other polysaccharides like hemicelluloses and pectin. This arrangement provides great strength.

2. Secondary Cell Wall

   • Forms only after a cell has stopped growing.
   • Location: Between the primary cell wall and the plasma membrane.
   • Found in: Specialized cells like sclerenchyma, which are often dead at maturity.
   • Properties: Very thick and rigid; it does not allow for further growth.
   • Composition: Consists of cellulose, hemicelluloses, lignin, inorganic salts, and waxes. Lignin is a key component that cements cellulose fibers together, providing rigidity.
   • Function: Provides definite shape and strong mechanical support.

3. Middle Lamella

   • Location: A layer present between the primary walls of adjacent cells.
   • Function: Acts as a cement, holding the cells together to form tissues.
   • Composition: A sticky, gel-like substance made of pectin, calcium, and magnesium salts.

Plasma Membrane

Also called the cell membrane, it is the living boundary of the protoplasm and is found in all prokaryotic and eukaryotic cells.

• Function: It is selectively permeable, controlling the passage of materials into and out of the cell to maintain a stable internal environment (homeostasis).

Membrane Transport Mechanism→

Composition

The plasma membrane is chemically composed of:

• Proteins: 60-80%
• Lipids: 20-40% (mainly phospholipids and cholesterol)
• Carbohydrates: Small quantity (found on the outer surface)

Structure: The Fluid Mosaic Model

This model describes the plasma membrane as a dynamic and fluid structure.

• Phospholipid Bilayer: The core of the membrane is a double layer of phospholipids.
  • Hydrophilic (water-loving) heads face the watery environments inside and outside the cell.
  • Hydrophobic (water-fearing) tails face inward, away from water.
• Proteins: Protein molecules are embedded either partially or wholly within the bilayer, like "icebergs floating in a sea." These proteins can drift sideways.
• Cholesterol: These steroid molecules are wedged between phospholipids, helping to stabilize the membrane and regulate its fluidity.
• Carbohydrates: Chains of carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids) on the outer surface. They act as cell surface markers.
• Cytoskeleton: Filaments of the cytoskeleton are attached to the inner surface, providing support.

Functions of Membrane Components

• Proteins: Determine most of the membrane's functions.
  • Transport: Act as channels or carriers to move substances across the membrane.
  • Enzymatic Activity: Catalyze metabolic reactions on the membrane surface.
  • Signal Transduction (Receptors): Bind to specific molecules (like hormones) to transmit signals into the cell.
  • Cell Recognition (Antigens): Allow cells to recognize each other.
• Glycoproteins and Glycolipids:
  • Act as cell surface markers for cell-to-cell recognition (Glycocalyx).
  • Help cells stick together to form tissues.
• Lipids (Phospholipids and Cholesterol):
  • Control the fluidity of the membrane.
  • Restrict the passage of polar molecules and ions.