Complete the sentences.
Choose answers from the list:
In the lungs, oxygen gas moves from a high concentration within the alveoli into the blood capillaries down a concentration gradient by the process of ________________________.
Marine kelp cells accumulate iodide ions from the surrounding seawater, where the iodide concentration is very low, into their cytoplasm where the concentration is high. This movement against the concentration gradient occurs by the process of ________________________.