Question

Discuss the active cell transport mechanisms responsible for movement of some materials through cell membranes.

Answer 1

As mentioned, active transport is the movement of a solute
across a membrane against its gradient—that is, from a region
of low concentration to higher concentration. Active transport
is energetically unfavorable and requires the input of energy. Primary active transport involves the functioning of a pump—
a type of transporter that directly uses energy to transport a
solute against a gradient. Figure 5.24a shows a pump that uses
ATP to transport H against a gradient. Such a pump can establish
a large H electrochemical gradient across a membrane.
Secondary active transport involves the use of a pre-existing
gradient to drive the active transport of another solute. For example,
a H/sucrose symporter can use a H electrochemical gradient,
established by an ion pump, to move sucrose against its
concentration gradient. In this regard, only sucrose
is actively transported. Hydrogen ions move down their electrochemical
gradient. H/solute symporters are more common in
bacteria, fungi, algae, and plant cells, because H pumps are
found in their plasma membranes. In animal cells, a pump that
exports Na maintains a Na gradient across the plasma membrane.
Na/solute symporters are prevalent in animal cells.
a type of transporter that directly uses energy to transport a
solute against a gradient. Figure 5.24a shows a pump that uses
ATP to transport H against a gradient. Such a pump can establish
a large H electrochemical gradient across a membrane.
Secondary active transport involves the use of a pre-existing
gradient to drive the active transport of another solute. For example,
a H/sucrose symporter can use a H electrochemical gradient,
established by an ion pump, to move sucrose against its
concentration gradient (Figure 5.24b). In this regard, only sucrose
is actively transported. Hydrogen ions move down their electrochemical
gradient. H/solute symporters are more common in
bacteria, fungi, algae, and plant cells, because H pumps are
found in their plasma membranes. In animal cells, a pump that
exports Na maintains a Na gradient across the plasma membrane.
Na/solute symporters are prevalent in animal cells.

Answer 2

So: there's primary active transport, where a pump is used to move molecules directly across a concentration gradient. And there's secondary active transport, which uses the potential energy from existing concentration gradients (which may be created by primary active transport) to move other molecules against their concentration gradient.