Wize Grade 12 Biology Textbook > Transport Across Membranes

Secondary Active Transport: Cotransport

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Cotransport
The electrochemical gradients that form due to primary active transport can store energy. This energy can be released when the solute diffuses down it's concentration gradient, and simultaneously drive the active transport of another substance. This mechanism is known as secondary active transport.


Water is pumped uphill, and as it flows downhill it is able to drive a process, this is what's seen with secondary active transport! During this process,a cotransporter protein (a protein that can transport 2 substances) can couple the "downhill" diffusion of one solute with the "uphill"  transport of a second substance. 
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Cotransport: Sodium and Glucose
The concentration gradient for sodium ions, established by the ATP-powered sodium-potassium pump (primary active transport), helps with driving the active transport of glucose into the cell. 


photo by David Nascari and Alan Sved / CC BY

Mechanism:
Sodium ions will want to move down their concentration gradient; from the extracellular space into the intracellular space .
Through a carrier (cotransporter) protein, sodium ions can diffuse down their concentration gradient (into the cells) and simultaneously transport glucose molecules against their concentration gradient (into the cells). In this way the cotransporter protein uses the energy of the sodium gradient to drive the transport of glucose molecules.
In this case, since both the sodium ions and glucose are moving in the same direction (into the cell), this type of cotransporter is known as a symporter. 
If the molecules being cotransported were moving in opposite directions, the cotransporter protein would be called an anti-porter.

Practice: Cotransport
In plant cells, a protein that cotransports protons and sucrose into cells functions in the same way that the sodium/glucose cotransporter functions in animals. Which of the following is most likely to be true of for the proton/ sucrose cotransporter?

A) The protons are moving up their electrochemical gradient while sucrose is moving up.

B) The cotransporter can also transport chloride ions.

C) Sucrose entering the plant cell down its concentration gradient provides energy for the movement of protons up their electrochemical gradient.

D) A substance that blocks protons from binding to the cotransport protein will also block the transport of sucrose.

I don't know

Practice: Secondary Active Transport
Answer each question by writing the correct term beneath it:

A) What are cotransport proteins called if they transport two substances in the same direction?

B) What are cotransport proteins called if they transport two substances in the opposite direction?

I don't know