5.3

Active transport mechanisms require energy, usually as adenosine triphosphate ( $\text{ATP}$ ).
Moves substances against concentration gradients (from lower to higher concentration).
Transports small-molecular weight materials (e.g., ions) and larger molecules.

Moving Against a Gradient

Requires energy derived from cell metabolism ( $\text{ATP}$ ) to work against concentration or electrochemical gradients.
Active transport mechanisms are known as pumps.
Maintains required concentrations of ions and other substances despite passive movement.
Example: Red blood cells utilize most metabolic energy to maintain sodium and potassium concentration imbalances.
Sensitive to metabolic poisons that disrupt the $\text{ATP}$ supply.
Two Primary Mechanisms:
- Primary Active Transport: Directly depends on $\text{ATP}$ to move ions across a membrane, creating a charge difference.
- Secondary Active Transport: Does not directly require $\text{ATP}$ ; uses the electrochemical gradient generated by primary active transport.

Carrier Proteins for Active Transport

Active transport relies on specific carrier proteins or pumps:
- Uniporter: Carries one specific ion or molecule.
- Symporter: Carries two different ions or molecules in the same direction.
- Antiporter: Carries two different ions or molecules in opposite directions.
All three transporter types can also carry small, uncharged organic molecules (e.g., glucose).
These transporter types are also used in facilitated diffusion, where they do not require $\text{ATP}$ .
Examples of Active Transport Pumps:
- $\text{Na}^+/\text{K}^+$ ATPase: Antiporter carrying sodium and potassium ions.
- $\text{H}^+/\text{K}^+$ ATPase: Antiporter carrying hydrogen and potassium ions.
- $\text{Ca}^{2+}$ ATPase: Pump carrying only calcium ions.
- $\text{H}^+$ ATPase: Pump carrying only hydrogen ions.