Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

3.3.5Exchange and Transport

Modelling Absorption — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 25 of 48 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

3.3.5 Modelling Absorption — AQA A Level Biology 7402 notes: Use models, calculations and data to explain and predict how surface area, concentration gradient and transport proteins affect rates of absorption.Absorption in the small intestine happens at a scale and in a place tha…

  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Visking tubing
  • Saturation
  • Peristalsis

Section 1 of 9Why model absorption?

Absorption in the small intestine happens at a scale and in a place that cannot easily be observed, so biologists use simple models to test ideas.

Why model absorption?
A model is a simplified representation that behaves in a similar way to the real system.
A good model shows which variables matter, makes predictions that can be tested, and has limitations that you should be able to state.

Section 2 of 9Visking tubing as a model gut

Visking (dialysis) tubing is a partially permeable membrane with tiny pores.

Visking tubing as a model gut
It allows small molecules such as glucose and amino acids to pass but not large molecules such as starch and protein.
A typical model: a bag of starch and amylase suspended in a beaker of water; after a period, the water outside is tested with Benedict's reagent for reducing sugar (maltose) and with iodine for starch.
Visking tubing as a model gut
The result shows that digestion produced small molecules which passed through the membrane, mimicking absorption, while starch remained inside.
Variables to test: temperature, enzyme concentration, tube diameter and length (surface area).
Limitations: the tubing has no active transport or co-transport, no blood flow to maintain a gradient, no villi or microvilli and no peristalsis.

Section 3 of 9Surface area calculations

Models help to show that a larger surface area gives faster absorption. The lining of the small intestine is about 6 m long, but with villi and microvilli the area is of the order of 250 m2, far larger than that of a simple tube.

For a cube-shaped model of side l: surface area = 6l2, volume = l3, SA:V ratio = 6/l.

Surface area calculations
Doubling the side length halves the ratio.
A flatter or folded shape keeps the ratio high.
If a tube of surface area 100 cm2 becomes 25 times larger through folding,
  • then its area is 2500 cm2, and, provided gradient and distance are unchanged, the rate of diffusion rises by the same factor.

Section 4 of 9Maintaining the gradient: modelling flow

If a model has a stationary solution on both sides of the membrane, diffusion slows as concentrations equalise and stops at equilibrium.

Maintaining the gradient: modelling flow
If the outside solution is changed frequently or flows past (as the blood does), the gradient is kept steep and the rate stays high.
In the gut, blood flow in capillaries and the contraction of muscle in the villi and the gut wall (peristalsis and segmentation) keep the concentrations different on each side.

The rate of absorption by facilitated diffusion or co-transport shows saturation: it increases with concentration and then levels off when all the carrier proteins are working at maximum speed.

Maintaining the gradient: modelling flow
Simple diffusion shows no plateau, and the rate increases in proportion to the gradient.
If an inhibitor of respiration is added, the rate of active uptake falls but the rate of passive diffusion is not affected.
These differences allow data to be used to deduce which mechanism is operating.

Section 5 of 9Interpreting graphs and data

1
A straight line through the origin for uptake against concentration indicates simple diffusion.
2
A curve that levels off indicates carrier proteins, so facilitated diffusion or active transport.
3
If uptake stops when oxygen or a metabolic inhibitor is removed, then ATP is needed and so active transport or co-transport is involved.
4
If the rate is higher at higher temperature, kinetic energy affects diffusion and protein action,
  • but a fall above the optimum suggests protein denaturation.

Section 6 of 9Extension: calculating the effect of folding

Suppose a model intestine is a smooth tube with an internal surface area of 40 cm2.

Extension: calculating the effect of folding
Adding folds multiplies the area by 3, adding villi multiplies it by 10 and adding microvilli multiplies it by 20.
  • The final area is 40 × 3 × 10 × 20 = 24 000 cm2, which is 2.4 m2.
If the rate of absorption per cm2 is constant, the rate rises 600-fold.
This shows how each level of folding makes a small additional adaptation which together produce a large effect.

Data questions may give uptake of a substance at several external concentrations, with and without an inhibitor.

1
Compare the curves at low and high concentrations.
2
At low concentration, active uptake is more effective than diffusion because it can proceed against a gradient.
3
At high concentration the passive component continues to rise while the carrier-mediated component has already reached its maximum.
4
Subtracting the curve with an inhibitor from the one without gives an estimate of the active component.

Section 7 of 9Exam-style questions

  1. A model gut contains starch and amylase in Visking tubing. After 20 minutes, the solution outside gives a positive Benedict's test and a negative iodine test. Explain these results. [3]3 marks

  2. Give two ways in which Visking tubing is a poor model of the small intestine. [2]2 marks

  3. The rate of glucose uptake by a tissue levels off as concentration increases, and falls when a respiratory inhibitor is added. Deduce the mechanism and explain. [3]3 marks

Section 8 of 9Key terms

TermMeaning
ModelSimplified representation of a system used to explain or predict.
Visking tubingPartially permeable membrane used to model the gut wall.
SaturationState where all carrier proteins are in use so rate cannot increase.
PeristalsisWaves of muscle contraction that move food along the gut.
Surface area to volume ratioSurface area divided by volume.
Partially permeableAllowing some molecules to pass but not others.

Section 9 of 9Exam technique

1
Saying Visking tubing has carrier proteins.
2
Forgetting that, once concentrations are equal, there is no net diffusion.
3
Using a larger tube without giving the effect on surface area to volume ratio.
4
Describing a plateau as showing that diffusion has stopped, instead of showing carrier saturation.
5
Failing to give limitations of a model.

Last updated: