Active transport

1. Electrochemical Gradient

An electrochemical gradient is the combined effect of a concentration gradient and an electrical gradient across a plasma membrane.

• Concentration Gradient: The difference in the concentration of a substance across a space or membrane.

• Electrical Gradient: The difference in electrical charge across the plasma membrane.

• Membrane Potential and Ion Dynamics:

  • The interior of living cells has a net negative electrical charge relative to extracellular fluid due to negatively charged proteins and organic molecules.

  • Sodium Ions ($Na^+$): Concentrated outside the cell. Both the concentration gradient and the negative internal charge drive $Na^+$ into the cell.

  • Potassium Ions ($K^+$): Concentrated inside the cell. The electrical gradient drives $K^+$ into the cell, while the concentration gradient drives $K^+$ out of the cell.

1a. Moving Against a Gradient

• Active Transport: Transport mechanism requiring cellular energy ($ATP$) to move ions or molecules against an electrochemical or concentration gradient.

• Pumps: Active transport mechanisms maintaining vital internal ion concentrations.

• Mechanisms:

  • Primary Active Transport: Directly uses $ATP$ to move ions across a membrane and create charge differences.

  • Secondary Active Transport: Indirectly relies on primary active transport by utilizing the established electrochemical gradient to move materials.

1b. Carrier Proteins for Active Transport

Transporters (carrier proteins or pumps) facilitate active transport across membranes:

• Uniporter: Carries one specific ion or molecule in one direction.

• Symporter: Carries two different ions or molecules in the same direction.

• Antiporter: Carries two different ions or molecules in opposite directions.

2. Primary Active Transport

Primary active transport directly consumes $ATP$ to move ions across the plasma membrane.

• Sodium-Potassium Pump ($Na^+/K^+$ Pump):

  • An electrogenic pump that maintains membrane potential by generating a charge imbalance across the membrane.

  • Uses $ATP$ hydrolysis to transport three $Na^+$ ions out of the cell and two $K^+$ ions into the cell.

  • Maintains a higher $Na^+$ concentration outside and a higher $K^+$ concentration inside, keeping the cell interior slightly negative relative to the exterior.

3. Secondary Active Transport (Co-Transport)

Secondary active transport relies on the kinetic energy of ions moving down their established electrochemical gradient to drive the transport of other molecules against their gradients.

• Co-Transport Mechanism: As $Na^+$ ions flow into the cell down their electrochemical gradient created by primary active transport, other substances (such as glucose and amino acids) attach to the transport protein and enter against their concentration gradients.

• Energy Storage: Mitochondria accumulate high-energy hydrogen ions ($H^+$). The surge of $H^+$ back across the membrane through $ATP$ synthase provides kinetic energy to convert $ADP$ into $ATP$.

4. Bulk Transport

Bulk transport moves large particles, macromolecules, or whole cells across the plasma membrane using energy and membrane vesicles.

4a. Endocytosis

Endocytosis imports materials into the cell by invagination of the plasma membrane, which pinches off to form an intracellular vesicle.

• Phagocytosis ("Cell Eating"):

  • Plasma membrane engulfs large particles or microorganisms.

  • Example: Neutrophils engulfing invading bacteria.

• Pinocytosis ("Cell Drinking"):

  • Plasma membrane invaginates to import extracellular fluid containing required solutes.

  • Forms smaller vesicles that do not necessarily fuse with lysosomes.

• Potocytosis:

  • Specialized form of pinocytosis utilizing caveolin-coated pits (caveolae).

  • Transports small molecules across the cell via transcytosis or delivers cargo to organelles like the endoplasmic reticulum.

• Receptor-Mediated Endocytosis:

  • Targeted uptake relying on specific cell surface receptor proteins binding specific ligands.

  • Example: Uptake of low-density lipoprotein ($LDL$). Defective or missing receptors lead to familial hypercholesterolemia.

  • Pathogens (e.g., influenza virus, diphtheria, cholera toxin) can exploit these receptors to enter cells.

4b. Exocytosis

Exocytosis expels materials from the cell into the extracellular fluid.

• Secretory or waste-containing vesicles fuse with the inner surface of the plasma membrane, opening the envelope to release contents outside the cell.