Explain the following: (a) Passive transport (b) Active transport

a) Passive Transport

Definition: Movement of molecules across cell membrane without energy expenditure (no ATP)

Characteristics:

• Along concentration gradient (high → low)
• Spontaneous process (thermodynamically favorable)
• Continues until equilibrium reached
• Membrane plays passive role

Types:

1. Simple diffusion: Small, nonpolar molecules (O₂, CO₂, ethanol)
2. Osmosis: Water movement through aquaporins or lipid bilayer
3. Facilitated diffusion: Polar molecules via carrier/channel proteins (glucose, amino acids, ions)

Rate factors: Concentration gradient, temperature, molecular size, lipid solubility

(b) Active Transport

Definition: Uphill movement of molecules against concentration gradient (low → high)

Characteristics:

• Requires energy input (ATP hydrolysis or ion gradients)
• Uses carrier proteins (pumps, transporters)
• Highly selective and specific
• Shows saturation kinetics

Types:

1. Primary active transport: Direct ATP use
   • Example: Na⁺/K⁺-ATPase pump (3 Na⁺ out, 2 K⁺ in per ATP)
2. Secondary active transport: Uses ion gradient created by primary transport
   • Example: Glucose-Na⁺ cotransporter (intestinal absorption)

Significance: Maintains ion gradients, nutrient accumulation, neurotransmitter uptake