Active Transport and Co-transport (Ileum)
How carrier proteins and ATP hydrolysis drive active transport, and how the sodium-potassium pump sets up sodium-glucose co-transport in the ileum. Includes the 5-mark exam question, a worked model answer and the examiner trap on glucose using ATP directly.
Chapters (9)
Exam question [5 marks]
The concentration of glucose in the blood is higher than in the lumen of the ileum after a meal is finished. Explain how glucose is absorbed from the lumen of the ileum into the blood.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Sodium ions are actively transported out of the epithelial cell into the blood by the sodium-potassium pump / carrier protein, using ATP (hydrolysis) | "sodium-potassium pump", "Na+/K+ pump", "uses energy from ATP", "active transport of sodium out of the cell using ATP" alone | "glucose is actively transported using ATP"; "sodium diffuses out" |
| 2 | This lowers the concentration of sodium ions inside the cell (creating a sodium gradient between lumen and cell) | "low sodium inside", "concentration gradient for sodium" | "no sodium in the cell"; "sodium ions are used up" |
| 3 | Sodium ions enter the cell from the lumen down their concentration gradient by facilitated diffusion through a co-transporter protein, bringing glucose into the cell with them | "co-transport", "glucose and sodium enter together via the same protein", "indirect / secondary active transport using the sodium gradient", "channel protein", "sodium enters by diffusion" | "sodium enters by active transport"; "glucose enters by simple diffusion" |
| 4 | Glucose accumulates inside the cell, so its concentration is higher than in the blood | "glucose builds up", "concentration of glucose in the cell is higher than in the blood", "gradient for glucose from cell to blood" | "glucose is stored in the cell" |
| 5 | Glucose moves out into the blood by facilitated diffusion through a carrier / channel protein | "facilitated diffusion", "down its concentration gradient", "diffusion" (lenient) | "glucose leaves by active transport"; "glucose leaves by osmosis" |
The commonest error is saying glucose uses ATP directly; ATP is used by the pump, which maintains the sodium gradient. Sodium moves in by facilitated diffusion, not active transport. Always name the type of protein and state the direction relative to the gradient.
Model answer (5/5)
Sodium ions are actively transported out of the epithelial cell into the blood by the sodium-potassium pump, using ATP. This lowers the concentration of sodium ions inside the cell. Sodium ions then move into the cell from the lumen down their concentration gradient by facilitated diffusion through a co-transporter protein, which brings glucose into the cell with them. Glucose builds up inside the cell, so its concentration becomes higher than in the blood. Glucose then moves out into the blood by facilitated diffusion through a carrier protein.
Examiner trap
- Saying active transport moves substances 'down' a gradient
- Forgetting ATP hydrolysis and carrier proteins
- Saying glucose uses ATP directly in co-transport (sodium gradient maintained by ATP)
Key terms
- active transport
- carrier protein
- ATP hydrolysis
- co-transport
- sodium-potassium pump
- ileum epithelium
- concentration gradient
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:25Active transport against a gradient
First, active transport. It moves a substance against its concentration gradient, from a low concentration to a high concentration. That cannot happen by diffusion, so it needs a carrier protein, and it needs energy. Remember, active transport never goes down a gradient.
0:44Carrier proteins and ATP
The energy comes from the hydrolysis of ATP. ATP is hydrolysed to ADP and phosphate, and that releases energy. The energy makes the carrier protein change shape, and that shape change moves the substance across.
So the key words are carrier protein, and hydrolysis of ATP.
1:05The ileum epithelial cell
Now the ileum. The cells lining it have one side facing the lumen, the inside of the gut, and the other side facing the blood. After a meal, digestion releases glucose into the lumen. The cell has to move that glucose into the blood.
Glucose is polar, so it cannot slip through the bilayer. It needs a protein. Between meals the blood can hold more glucose than the gut, so diffusion alone would not be enough. The cell uses a clever three step trick.
1:36The sodium-potassium pump
Step one. On the blood side of the cell sit sodium potassium pumps. Each pump uses ATP to move sodium ions out of the cell, into the blood, and potassium ions into the cell. This is active transport, so it needs a carrier protein and the hydrolysis of ATP. The result is a very low concentration of sodium ions inside the cell.
2:01The sodium-glucose co-transporter
Step two. On the lumen side is a different protein, a co-transporter. There is now a high concentration of sodium ions in the lumen, and a low concentration inside the cell. So sodium ions move into the cell down their concentration gradient, by facilitated diffusion, through the co-transporter. And as they go, the same protein carries glucose in with them.
This is co-transport. Two substances, one protein. Glucose can move in even against its own gradient, because sodium is moving down its gradient.
Notice the glucose is not using ATP directly. The ATP is used by the pump, to keep the sodium gradient going.
2:46Glucose leaves into the blood
Step three. Glucose builds up inside the cell, so its concentration is now higher in the cell than in the blood. Glucose then leaves on the blood side, by facilitated diffusion, through a carrier protein. No ATP needed, because it is moving down its gradient. From the blood, it is carried away around the body to cells that need it for respiration.
Notice that three different proteins are involved, the pump, the co-transporter and the glucose carrier. Each one has its own job.
4:04Examiner trap and recap
The trap. Do not say glucose uses ATP directly. The pump uses ATP, to build the sodium gradient. The sodium gradient then drives the glucose in.
Recap. Pump out sodium, sodium in with glucose, glucose out into the blood.
Next time, why some cells move substances faster than others, with surface area, proteins and gradients.
Watch first
This lesson builds on:
- Simple vs Facilitated Diffusion · AQA 7402 · 3.2.3
Keep going
More from 3.2 Cells
- Fluid-Mosaic Model: Cell Membrane Structure · AQA 7402 · 3.2.3
- Simple vs Facilitated Diffusion · AQA 7402 · 3.2.3
- Osmosis and Water Potential Explained · AQA 7402 · 3.2.3
- Why Cells Transport Faster: Microvilli · AQA 7402 · 3.2.3
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-037.