Why Cells Transport Faster: Microvilli
How microvilli, extra channel and carrier proteins and steeper gradients increase the rate of transport across cell membranes. Includes the 4-mark exam question, a worked model answer and the examiner trap on describing an adaptation without its effect on rate.
Chapters (8)
Exam question [4 marks]
Cells lining the ileum absorb glucose very quickly. Explain how the features of these cells and their surroundings allow rapid absorption of glucose from the gut.
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 | Microvilli give a large surface area, so more glucose is absorbed / transported at once (faster rate) | "folded membrane", "finger-like projections", "more area for transport", "thin epithelium / short diffusion distance" (with effect on rate) | "microvilli" alone; "large cell" |
| 2 | Many carrier proteins (or channel proteins) in the membrane, so more glucose is transported per second | "more transport proteins", "large number of co-transporters", "facilitated diffusion proteins", "co-transport with sodium ions" | "a large number of proteins" with no effect on rate; "enzymes" |
| 3 | Blood carries glucose away, maintaining a steep concentration gradient, so faster movement in | "glucose removed by blood flow", "keeps concentration low in the blood", "Na+/K+ pump maintains the sodium ion gradient" | "high concentration in the cell"; "gradient" with no maintenance and no effect |
| 4 | Many mitochondria make ATP for active transport / co-transport, so the rate is maintained | "more ATP", "respiration supplies energy for active transport" | "mitochondria produce energy" with no link to transport; "make ATP for diffusion" |
Each mark needs a feature and its effect on rate. A feature named with no effect scores nothing. The three factors in the specification are surface area, number of channel or carrier proteins, and the gradient; point 4 is standard textbook content that the mark scheme accepts.
Model answer (4/4)
The cells have microvilli, which give a large surface area, so more glucose can be absorbed at once. They have many carrier proteins in the membrane, so more glucose is transported across each second. The blood carries glucose away, which maintains a steep concentration gradient, so glucose moves in faster. Many mitochondria make ATP for active transport and co-transport, so glucose absorption continues at a high rate.
Examiner trap
- Describing an adaptation without stating its effect on rate
- Not linking rate to each of the three factors in the question
Key terms
- microvilli
- surface area
- channel proteins
- carrier proteins
- concentration gradient
- water potential gradient
- transport rate
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:29Smooth cell versus microvilli
First factor, surface area. Transport happens across the membrane, so a larger surface area of membrane means more carrier and channel proteins can be fitted in, and more molecules cross at the same time. Cells lining the ileum fold their surface membrane into tiny finger-like projections called microvilli. The membrane is the same cell, but there is much more of it.
Think of a shop with many more doors. More people can get through in the same time. Larger surface area, more molecules crossing each second, faster rate.
1:05More channel and carrier proteins
Second factor, the number of channel and carrier proteins. Facilitated diffusion, active transport and co-transport all depend on these proteins. Each protein can only move so many molecules at once, so more proteins means more molecules moved each second. So ileum cells have large numbers of carrier proteins and channel proteins in their microvilli membranes.
Channel proteins let specific ions or small polar molecules diffuse through. Carrier proteins change shape to carry a molecule across. Either way, the membrane is a lipid bilayer, and without proteins these molecules cannot readily cross. Fewer proteins, slower rate. More proteins, faster rate.
1:50Steeper gradients
Third factor, the gradient. For diffusion, the bigger the difference in concentration between the two sides, the faster the net movement. For osmosis it is the same idea, but with water potential. The bigger the difference in water potential, the faster the water moves. In the ileum, there is a high concentration of glucose in the lumen, and a low concentration in the blood.
The blood keeps flowing and carries glucose away, so the gradient stays steep.
2:20Rate against concentration difference
One extra detail. Simple diffusion keeps speeding up as the gradient gets steeper. But facilitated diffusion levels off once all the carrier or channel proteins are in use. The rate plateaus, and the number of proteins is the limiting factor. The only way to raise the ceiling is to add more proteins. So the gradient and the number of proteins work together, and the exam wants you to name both.
2:46Mitochondria and ATP supply
Active transport and co-transport also need ATP. In the ileum, ATP is hydrolysed by the sodium-potassium pump, which keeps the sodium ion gradient that drives glucose in. So cells that do a lot of active transport have many mitochondria, because aerobic respiration provides the ATP. That keeps active transport going, and the gradients steep. Without enough ATP, active transport slows down, however many carriers there are.
3:58Examiner trap and recap
The trap. Naming an adaptation is not enough. Say what it does, and say that the rate goes up.
Recap. Surface area, number of proteins and steepness of the gradient each change the rate. Next time, Required Practical 3, where you use potato cylinders and a calibration curve to find the water potential of plant tissue.
Watch first
This lesson builds on:
- Simple vs Facilitated Diffusion · AQA 7402 · 3.2.3
- Osmosis and Water Potential Explained · AQA 7402 · 3.2.3
- Active Transport and Co-transport (Ileum) · 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
- Active Transport and Co-transport (Ileum) · 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-038.