Final answer:
Primary active transport's affinity on the lower concentration side is increased due to conformational changes in the carrier protein, triggered by ATP hydrolysis. This alteration decreases protein's affinity for sodium ions, releasing them, and increases affinity for potassium ions, allowing these ions to attach at the cytoplasmic side. Such mechanisms set the stage for cycles of ion exchange necessary for maintaining cellular ionic gradients.
Step-by-step explanation:
To understand what increases the affinity at the lower concentration side for primary active transport, we should initially consider the process by which primary active transport operates. In primary active transport, ATP is utilized to induce a conformational change in a carrier protein, which in turn affects the protein's affinity for certain ions or molecules. For example, when the carrier hydrolyzes ATP, the resultant low-energy phosphate group attachment causes the protein to change its shape and re-orient towards the exterior of the membrane. This decreases the protein's affinity for sodium ions, allowing them to be released.
Subsequently, the shape change increases the carrier's affinity for potassium ions on the opposite side - the lower concentration side within the cytoplasm. The carrier protein's new configuration with a decreased affinity for potassium then allows for the release of potassium ions into the cytoplasm, and it regains a higher affinity for sodium ions, allowing the process to start anew. It's this cyclical change in affinity driven by the hydrolysis of ATP that enables primary active transport to move ions against their concentration gradients.
In an associated process known as secondary active transport or co-transport, the gradient created by primary active transport is exploited to move another substance against its gradient. For instance, in glucose reabsorption in the kidneys, the Na+/K+ ATPase maintains an electrochemical gradient that assists the movement of glucose against its concentration gradient through a Na+/glucose symport protein.
Finally, changes in external conditions, like pH, can also influence transport processes. A decreased pH outside the cell may change the conformational dynamics of transport proteins and could potentially affect the amount of amino acids transported into the cell.