Explain the mechanism of transport of small and large molecules across plasma membrane.

Passive Transport

Passive transport does not require cellular energy expenditure and relies on the concentration gradient, electrical potential, or pressure gradient to move substances across the membrane. It can be further divided into:

Simple Diffusion

This involves the movement of small, nonpolar molecules like oxygen (O₂), carbon dioxide (CO₂), and nitrogen (N₂) directly across the phospholipid bilayer down their concentration gradient. The rate of diffusion is influenced by factors like temperature, pressure, and the solubility of the molecule.

Facilitated Diffusion

This process requires the assistance of membrane proteins – either channel proteins or carrier proteins – to transport molecules across the membrane. It’s still passive as it follows the concentration gradient.

• Channel Proteins: Form hydrophilic pores through which specific ions or small polar molecules can pass. Example: Aquaporins facilitating water transport.
• Carrier Proteins: Bind to the molecule and undergo a conformational change to transport it across the membrane. Example: Glucose transporters (GLUTs).

Osmosis

A special case of diffusion involving the movement of water across a semi-permeable membrane from a region of high water potential to a region of low water potential. Water potential is influenced by solute concentration and pressure.

Active Transport

Active transport requires cellular energy, typically in the form of ATP, to move substances against their concentration gradient. It is crucial for maintaining cellular gradients and accumulating essential nutrients.

Primary Active Transport

This directly utilizes ATP hydrolysis to drive the transport of molecules. A classic example is the Sodium-Potassium Pump (Na⁺/K⁺ ATPase), which maintains the electrochemical gradient by pumping 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell for each ATP molecule hydrolyzed. This pump is vital for nerve impulse transmission, muscle contraction, and maintaining cell volume.

Secondary Active Transport

This utilizes the electrochemical gradient established by primary active transport to move other molecules. It doesn't directly use ATP but relies on the potential energy stored in the gradient.

• Symport: Both molecules move in the same direction. Example: Sodium-glucose cotransporter (SGLT) in the kidney and intestine.
• Antiport: Molecules move in opposite directions. Example: Sodium-calcium exchanger.

Transport of Large Molecules

Large molecules like proteins and polysaccharides cannot cross the membrane via diffusion or carrier proteins. They utilize bulk transport mechanisms:

Endocytosis

The process by which cells internalize substances by engulfing them within vesicles formed from the plasma membrane.

• Phagocytosis ("cell eating"): Uptake of large particles, like bacteria or cellular debris.
• Pinocytosis ("cell drinking"): Uptake of extracellular fluid containing dissolved solutes.
• Receptor-mediated endocytosis: Highly specific uptake of molecules that bind to receptors on the cell surface. Example: Uptake of cholesterol via LDL receptors.

Exocytosis

The process by which cells release substances by fusing vesicles containing the molecules with the plasma membrane. This is crucial for secretion of hormones, neurotransmitters, and waste products.

Transport Mechanism	Energy Requirement	Concentration Gradient	Examples
Simple Diffusion	No	Down	O2, CO2
Facilitated Diffusion	No	Down	Glucose (GLUTs), Water (Aquaporins)
Primary Active Transport	Yes (ATP)	Against	Na+/K+ ATPase
Secondary Active Transport	No (uses gradient)	Against	SGLT, Na+/Ca2+ exchanger
Endocytosis	Yes (ATP)	Into cell	Phagocytosis, Pinocytosis, Receptor-mediated endocytosis
Exocytosis	Yes (ATP)	Out of cell	Hormone secretion, Neurotransmitter release