Co-transport and Coupled Movement Across Membranes
An ATP-driven pump maintains an ion gradient. A second carrier uses movement down that gradient to move another substance against its gradient. Sodium-dependent glucose entry into gut lining cells is the example.
What a learner can do afterwards
- Separates the step that spends ATP from the step that rides the gradient.
- Traces sodium export by the pump and coupled sodium and glucose entry through the symporter.
- Explains why blocking the pump reduces sodium-dependent glucose uptake as the sodium gradient weakens, although the symporter uses no ATP directly.
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You have met ATP-driven pumps. In a gut lining cell, the sodium potassium pump faces tissue fluid, not the gut lumen, the space holding food. It uses ATP to export sodium and import potassium, keeping sodium low inside. This direct ATP use is primary active transport. Ions respond to concentration and voltage differences together: an electrochemical gradient.
On the lumen-facing membrane, the sodium glucose symporter couples sodium entry down its electrochemical gradient to glucose entry, even against glucose's concentration gradient. Both substances enter together: this is symport. The symporter uses the gradient without splitting ATP. It is secondary active transport because its energy comes indirectly from the ATP-driven pump.
If a cell starts without a sodium gradient, the pump must establish one before it can support uptake. Blocking the pump later does not erase that gradient at once. The symporter may still use it briefly. As sodium enters without being pumped out, the gradient weakens and sodium-dependent glucose entry decreases. A gradient that supplies too little energy cannot sustain glucose entry against its concentration gradient.
A symporter couples movement of substances in the same direction across a membrane. An antiporter couples movement in opposite directions: one substance enters while another leaves. These names describe transport directions. They do not, by themselves, tell you whether a carrier uses ATP directly.
ATP powers the pump. Its sodium gradient drives coupled sodium and glucose entry through the symporter. Uptake decreases as that driving gradient weakens.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Linked videos by Khan Academy.
More to read
Following glucose into a gut cell
The gut lumen is the space containing food being digested. A gut lining cell has one surface facing this space and another facing tissue fluid.
The sodium potassium pump works on the tissue-facing surface. Using ATP, it moves sodium out and potassium in. This keeps sodium concentration low inside the cell.
On the lumen-facing surface, a sodium glucose symporter brings sodium and glucose into the cell together. Sodium's movement down its electrochemical gradient can supply enough energy to move glucose against its concentration gradient.
The two proteins do different jobs. The pump maintains the driving gradient; the symporter uses it. This explains how an ATP-driven protein on one surface can support glucose entry through another protein on the other surface.
Two ways to describe a transporter
A transporter's direction pattern and its energy source are separate questions.
A uniporter carries one substance. A symporter couples movement of substances in the same direction. An antiporter couples movement in opposite directions. These names describe what crosses the membrane and in which directions.
To identify the energy source, ask what drives the movement. The sodium potassium pump splits ATP directly. The sodium glucose symporter instead uses energy from sodium entering down its electrochemical gradient. The pump maintains that gradient using ATP.
For this reason, sodium-dependent glucose entry is secondary active transport. It still requires energy, even though the symporter does not split ATP. Saying that two proteins are involved is not enough to explain the classification; the link through the ion gradient matters.
A blocked pump and a remaining gradient
Consider a gut lining cell whose sodium potassium pump has just been blocked. Before the block, the pump maintained a sodium gradient. That existing difference does not disappear at the instant the pump stops.
The sodium glucose symporter may keep using the remaining driving gradient briefly. Sodium continues entering without being pumped out. As its inward electrochemical gradient weakens, less energy is available to support glucose entry against its concentration gradient.
This is a prediction about sodium-dependent entry through the symporter. It does not establish that every route of glucose movement stops immediately.
In an unblocked cell, the pump and symporter can work at the same time. Diagrams that place the pump before the symporter show an energy dependency, not a rule that the proteins must take turns.
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