Chapter 5.3 Active Transport - Biology 2e

Key Concepts

• Active transport requires energy (ATP) for the movement of substances across a membrane against their concentration gradient.
• Distinction between primary active transport and secondary active transport.

Electrochemical Gradient

• Combination of concentration gradient and electrical gradient affecting ions.
• Interior of cells is electrically negative compared to the extracellular fluid.
• High concentrations of potassium (K+) inside cells and sodium (Na+) outside.
• Na+ moves into the cell due to both concentration and electrical gradients.
• K+ electrical gradient drives it into the cell, but concentration gradient drives it out.

Moving Against a Gradient

• Cells use ATP to move substances against gradients.
• Primary active transport: Direct use of ATP to move ions and create charge differences across membranes.
• Secondary active transport: Utilizes electrochemical gradients created by primary transport to move substances.

Carrier Proteins for Active Transport

• Uniporter: Transports one specific ion/molecule.
• Symporter: Transports two different ions/molecules in the same direction.
• Antiporter: Transports two different ions/molecules in opposite directions.
• Examples: Na+-K+ ATPase and H+-K+ ATPase (antiporters), Ca2+ ATPase, and H+ ATPase (uniporters).

Primary Active Transport

• Sodium-Potassium Pump (Na+-K+ ATPase):
  • Moves 3 Na+ ions out and 2 K+ ions into the cell.
  • Generates an electrochemical gradient crucial for cellular functions.
  • Operates through a six-step cycle:
    1. High affinity for Na+ inside the cell; binds 3 ions.
    2. ATP hydrolysis phosphorylates the carrier.
    3. Carrier changes shape, releases Na+ outside.
    4. High affinity for K+ outside the cell; binds 2 ions.
    5. Releases phosphate; carrier reorients to interior.
    6. Releases K+ inside; restarts cycle.

Secondary Active Transport (Co-transport)

• Utilizes kinetic energy of Na+ ions to bring other compounds into the cell against gradients.
• Electrochemical gradient of Na+ drives the co-transport of other substances like glucose.
• Important in cellular uptake of amino acids and glucose.

Visual Aids

• Figure 5.16: Depicts electrochemical gradients.
• Figure 5.17: Illustrates how proton gradients are used in secondary active transport.
• Figure 5.18: Shows different types of carrier proteins.
• Figure 5.19: Demonstrates the sodium-potassium pump mechanism.
• Figure 5.20: Shows co-transport process.

Study Questions

• Why is injecting potassium solution lethal?
• How do changes in pH outside the cell affect amino acid transport into the cell?

Learning Outcomes

• Understand the role of ATP in active transport.
• Explain how electrochemical gradients facilitate transport processes.
• Identify different carrier proteins and their functions in active transport.