Cotransport systems utilize indirect active transport to move solutes across membranes, where the movement of one solute along its concentration gradient provides energy for another solute to move against its gradient. Sodium-potassium pumps create the necessary electrochemical gradients for this process, allowing glucose to be transported into cells by utilizing the energy from sodium moving inwards. The carrier proteins involved in glucose cotransport harness this energy to facilitate the uptake of glucose, demonstrating a vital mechanism of nutrient absorption in cells.

1. Cotransport systems indirectly provide energy for active transport
 A cotransport system moves solutes across a membrane by
indirect active transport.
 Two solutes are transported at the same time.
 The movement of one solute down its concentration gradient
provides energy for transport of some other solute up its
concentration gradient.
 However, an energy source such as ATP is required to power
the pump that produces the concentration gradient.
 Sodium–potassium pumps (and other pumps) generate
electrochemical concentration gradients.
 Sodium is pumped out of the cell and then diffuses back in
by moving down its concentration gradient.
 This process generates sufficient energy to power the active
transport of other essential substances.
 In these systems, a carrier protein cotransports a solute
against its concentration gradient, while sodium, potassium,
or hydrogen ions move down their gradient.
 Energy from ATP produces the ion gradient.
 Then the energy of this gradient drives the active transport
of a required substance, such as glucose, against its gradient.
 We have seen how glucose can be moved into the cell by
facilitated diffusion.
 Glucose can also be cotransported into the cell. The sodium
concentration inside the cell is kept low by the ATP-
requiring sodium–potassium pumps that actively transport
sodium ions out of the cell.
 In glucose cotransport, a carrier protein transports both
sodium and glucose. As sodium moves into the cell along its

2. concentration gradient, the carrier protein captures the
energy released and uses it to transport glucose into the cell.
 Thus, this indirect active transport system for glucose is
“driven” by the cotransport of sodium.