How does cutting down only mature trees in a forest extend the life of the forest?
a. it maintains forest habitats.
b. it start a new generation of trees.
c. it increases demand for new lumber.
d. it increases the supply of new lumber.

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explanation:

many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.