Final answer:
The movement of ions across plasma membranes is influenced by both the concentration gradient and the membrane's electric potential, which together form the electrochemical gradient. Active transport is required for ions moving against this gradient and involves energy expenditure by the cell.
Step-by-step explanation:
In addition to the concentration gradient, the movement of ions across plasma membranes is also affected by the membrane potential which introduces an electrical gradient. This electrical gradient, resulting from the negative charge inside the cell compared to the outside, influences the movement of ions. Positively charged ions (cations) are favored to move into the negatively charged cell, while negatively charged ions (anions) tend to move out.
The synergy of the chemical force due to concentration differences and the electrical force due to the charge differences constitutes the electrochemical gradient. This gradient is crucial for various cellular processes, particularly in nerve and muscle cells, where it plays a significant role in the generation and propagation of nerve impulses and muscle contraction.
When ions move against this electrochemical gradient, the cell must expend energy, often by using ATP, in a process known as active transport. This is carried out by carrier proteins or pumps within the cell membrane.