Physiology · MU Sofia

3. Active transport

Covers active transport against a concentration gradient, split into primary pumps that use ATP directly, such as Na⁺/K⁺-ATPase, H⁺/K⁺-ATPase, calcium ATPases and ABC transporters, and secondary co-transport through symporters and antiporters. It then describes endocytosis and exocytosis, listing the endocytic types with their steps, and finishes with paracellular and transcellular transport across epithelial layers.

Inside: 2 sections · 5 labelled pictures · 33 flashcards · 8 min

Topic 3 of 104 in the MU Sofia syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 2 sections, about 1,219 words (8 min of reading), with 33 flashcards.

In this note

  1. Introduction
  2. Transport Across Epithelial Cell Layers

Introduction

Active Transport Across the Cellular Membrane

  • Active transport is a crucial cellular process where substances are moved across the cell membrane against their concentration gradient. This movement requires the expenditure of metabolic energy, typically in the form of ATP. Active transport is essential for maintaining concentration gradients of ions and other substances across the cell membrane, crucial for various cellular functions.
  • Types of Active Transport
  • 1. Primary Active Transport
  • Mechanism: Directly uses ATP to transport molecules against their concentration gradient.
  • Examples:
  • Na⁺/K⁺-ATPase: This pump expels three sodium ions from the cell and brings in two potassium ions against their respective gradients. This action is critical for maintaining the resting membrane potential and the overall ionic balance of the cell.
  • H⁺/K⁺-ATPase: Located primarily in the stomach, this pump helps in secreting hydrochloric acid by pumping hydrogen ions into the stomach, contributing to the gastric acid necessary for digestion.
  • Calcium ATPases: Found in various cellular compartments, these pumps help maintain low intracellular calcium levels by pumping calcium out of the cytosol into the extracellular matrix or into organelles like the sarcoplasmic reticulum or mitochondria.
  • ABC Transporters: These are involved in the transport of diverse molecules across the cell membrane using ATP, playing roles in drug resistance, lipid transport, and disease resistance.
  • 2. Secondary Active Transport (Co-transport)
  • Mechanism: Utilizes the energy stored in the form of an ionic gradient, established by primary active transport, to drive the transport of other substances.
  • Types:
  • Symporters: Transport two different molecules in the same direction. An example is the Na⁺-glucose transporter, which uses the sodium gradient created by the Na⁺/K⁺ -ATPase to facilitate glucose uptake against its gradient.
  • Antiporters: Transport two different molecules in opposite directions. For instance, the Na⁺/Ca²⁺ exchanger, which uses the energy of the sodium gradient to expel calcium from the cell.
  • Energy Requirement: The energy required for active transport is directly proportional to the concentration gradient against which transport occurs. The more a substance needs to be concentrated, the more energy is required, which can be substantial for maintaining essential cellular functions.
  • Physiological Importance of Active Transport
  • Cell Functionality: Active transport is fundamental for various physiological processes, including nerve signal transmission, muscle contraction, and the regulation of cellular pH.
  • Maintenance of Ionic Gradients: Essential for cellular homeostasis, as ionic gradients influence muscle contraction, neurotransmission, and overall cellular metabolism.

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