Active transport

Active Transport

• Definition: Active transport is the process by which substances move against their concentration gradient, which requires energy.
  • Energy Source: The energy required is usually in the form of ATP.
  • Proteins Involved: Specific proteins, mainly carriers, are responsible for performing active transport.
  • Molecule Types: Active transport can move both uncharged molecules and charged ions.

Types of Pumps

• Overview: There are three primary types of pumps involved in active transport:
  • Uniporter: Carries one molecule or ion at a time.
  • Symporter: Carries two different molecules or ions in the same direction.
  • Antiporter: Carries two different molecules or ions in opposite directions. (Credit: modification of work by “Lupask”/Wikimedia Commons)

Types of Active Transport

• Categories:
  • Primary Active Transport: Directly uses ATP hydrolysis to transport solutes.
  • Secondary Active Transport (Co-transport): Utilizes energy supplied by an electrochemical gradient, which is established by primary active transport.

The Sodium-Potassium Pump (Na+/K+ ATPase)

• Function: This pump exemplifies primary active transport in animal cells.
  • Ion Concentration: In typical animal cells:
  • Lower concentrations of Na+ are found inside compared to outside the cell.
  • Higher concentrations of K+ are found inside the cell compared to outside.
  • Mechanism: The sodium-potassium pump uses the energy derived from one ATP molecule to:
  1. Pump three Na+ ions out of the cell.
  2. Pump two K+ ions into the cell.
  • Electrogenic Pump: It generates a charge or voltage imbalance, hence it's classified as a major electrogenic pump of animal cells.
  • An electrogenic pump is defined as a transport protein that generates a voltage difference across a membrane.
  • Energy Usage: Approximately 25% of all cellular ATP is used to power this pump.

Example of Sodium-Potassium Pump with ATP Hydrolysis

• Inputs and Outputs:
  • ATP is hydrolyzed to ADP and inorganic phosphate, thus providing energy.
  • General Process:
  1. Phosphorylation of the integral protein occurs when ATP is used.
  2. Active Ion Transport: This allows the sodium ions (Na+) to be pumped outside and potassium ions (K+) to be pumped inside the cell.

Example of Another Electrogenic Pump: Proton Pump

• Functionality: Another example of an electrogenic pump is the proton pump, found in bacteria, plants, and fungi.
  • Purpose: The proton pump generates a voltage differential across the plasma membrane by moving H+ ions.

Maintenance of Membrane Potential by Ion Pumps

• Membrane Potential: This is defined as the voltage difference across a cellular membrane.
  • Importance: It is critical for the maintenance and functioning of the nervous system.

Co-Transport (Secondary Active Transport)

• Definition: Co-transport utilizes the electrochemical gradient created by primary active transport to move a different solute against its own concentration gradient.
  • Examples of Molecules: Many amino acids and glucose enter the cell via this mechanism.
  • Electrochemical Gradients: They arise from the combined effects of concentration and electrical gradients.

Co-Transport Example

• Primary Active Transport: The Na+-K+ pump establishes a concentration gradient of Na+ using ATP hydrolysis.
  • Transport Mechanism:
  • The Na+-K+ pump functions as an antiporter.
• Secondary Active Transport: Na+ moves down its concentration gradient established by the Na+-K+ pump, driving the transport of glucose against its concentration gradient.
• Concentration Overview:
  • Outside the Cell: High concentration of Na+, low concentration of K+.
  • Inside the Cell: High concentration of K+, low concentration of Na+, and a lower concentration of glucose compared to the outside.

Another Co-Transport Example: Sucrose-H+ Co-Transporter

• Mechanism:
  • H+ ions diffuse down their concentration gradient, while the co-transporter brings sucrose into the cell against its concentration gradient.
• Inside the Cell: High concentration of sucrose is achieved.

Bulk Transport

• Definition: Bulk transport requires ATP and involves the packaging of larger molecules, such as proteins and polysaccharides.
  • Methods:
  • Exocytosis: The process of expelling molecules from the cell.
  • Endocytosis: The process of bringing molecules into the cell.
• Types of Endocytosis:
  • Phagocytosis (cellular eating): The cell membrane surrounds and engulfs particles.
  • Pinocytosis (cellular drinking): The cell membrane invaginates to surround a small volume of fluid.
  • Receptor-Mediated Endocytosis: Uptake is targeted by binding to specific receptors on the external surface of the membrane.

Endocytosis Types

• Phagocytosis: Engulfing of particles or food.
• Pinocytosis: Non-specific uptake of fluid.
• Receptor-Mediated Endocytosis: Specific uptake by ligand-receptor binding.

Zika Virus Entry Mechanism

• Overview: The Zika virus is a flavivirus that enters cells via endocytosis.
  • Process:
  1. The virus encounters the Golgi and enters an endosomal pathway.
  2. The pH in the endosomes drops, leading to viral fusion with the membrane.
  3. Results in the release of the virion into the cytoplasm.

SARS-CoV-2 Virus Mechanism

• Overview: SARS-CoV-2 is an enveloped, non-segmented, positive-sense single-stranded RNA virus.
  • Entry Mechanism: Spike glycoprotein of the virus binds to angiotensin-converting enzyme 2 (ACE2) and enters cells via either membrane fusion or endocytosis.
  • Viral Process:
  1. After entry, the viral genome is released into the cytoplasm.
  2. Translation occurs, leading to the replication of the RNA genome.
  3. Proteins are assembled to form a mature virion in the Golgi.
  4. Viral genome is completed through genomic and subgenomic RNA formation.
  • Components Involved:
  • Envelope (E) and Membrane (M) proteins combine with nucleocapsid proteins during the process.
  • Nucleocapsid structures form within the cytoplasm and complete the viral lifecycle in the endoplasmic reticulum (ER).