3.3 Active Transport

Objective: State that protein carriers move molecules or ions across a membrane during active transport.

Active transport is a type of transport across the cell membrane that requires energy, usually in the form of ATP. Unlike passive transport, which moves substances down a concentration gradient, active transport moves substances against their concentration gradient. This requires the cell to expend energy to 'pump' molecules or ions across the membrane.

How Active Transport Works

Active transport relies on protein carriers embedded in the cell membrane. These protein carriers bind to specific molecules or ions and undergo a conformational change, which physically moves the substance across the membrane. This process requires energy, typically supplied by ATP.

Types of Protein Carriers

There are two main types of protein carriers involved in active transport:

• Pumps: These carriers actively transport ions across the membrane. They bind to an ion, change shape, and release the ion on the other side of the membrane.
• ATPases: These carriers use the energy from ATP hydrolysis to move molecules across the membrane.

Table: Comparison of Active and Passive Transport

Feature	Passive Transport	Active Transport
Energy Requirement	No energy required	Energy (ATP) required
Movement Direction	Down concentration gradient	Against concentration gradient
Carrier Proteins	Not required	Required (protein carriers/pumps)
Examples	Diffusion, Osmosis	Sodium-potassium pump, Proton pump

Examples of Active Transport

Several important examples of active transport occur in the body:

• Sodium-Potassium Pump: This pump is crucial for maintaining the electrochemical gradient across the neuron membrane. It pumps sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients.
• Proton Pump: Found in the stomach lining, this pump actively transports hydrogen ions into the stomach lumen, creating the acidic environment needed for digestion.
• Nutrient Uptake: Some cells actively transport nutrients like glucose and amino acids into the cell, even when the concentration of these substances is lower outside the cell.

In all cases of active transport, the protein carriers play a vital role in facilitating the movement of molecules or ions across the cell membrane by utilizing energy to overcome the concentration gradient.