Answers · Absorption

Villi under the microscope — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. VQ1 · Practice

Model answer. A. Villi are finger-like projections of the whole lining: each villus is made of many cells, and villi can be seen with a light microscope at ×100. Microvilli are part of a single lining cell, seen clearly only with an electron microscope.

A full answer includes

I chose row A.
I did not choose row D: that row describes microvilli.

Also accept

Watch for

“Row B: a villus can be seen only with an electron microscope.” — Villi are about half a millimetre long: a light microscope at ×100 shows them clearly. It is the microvilli that need an electron microscope.
“Row C: a villus is part of one cell.” — A villus is a fold of the whole lining, covered by many epithelial cells. The projections of a single cell are the microvilli.
“Row D: a villus is part of one cell, seen only with an electron microscope.” — That row describes microvilli. A villus is made of many cells and is large enough to see at ×100.

2. VQ2 · Practice

Model answer. Times longer = 5.2 mm ÷ 1.3 mm = 4.0. The folded edge is 4 times longer than a flat lining of the same straight length.

Checked automatically

Times longer: 4.0 (within ±0.05)
- Common wrong answer 0.25 : You divided the straight length by the string. Work out 5.2 ÷ 1.3.
- Common wrong answer 3.9 : You subtracted. "Times longer" needs a division: 5.2 ÷ 1.3.

A full answer includes

I divided 5.2 mm by 1.3 mm.
I wrote 4.0.
I did not divide 1.3 by 5.2.

Also accept

Watch for

“1.3 ÷ 5.2 = 0.25.” — Divide the longer, folded length by the straight length: the answer is more than 1.
“5.2 − 1.3 = 3.9.” — "Times longer" is a ratio: divide, do not subtract.

3. VQ3 · Practice

Model answer. Villi and microvilli increase the internal surface area of the small intestine, so more nutrient molecules can be absorbed in the same time.

A full answer includes

I wrote surface area in (1).
I wrote more in (2).
I wrote time in (3).
I did not write that villi make the small intestine longer.

Also accept

(1) surface area; (2) more; (3) time

Watch for

“Villi make the small intestine longer.” — The villi do not change the length of the tube: they fold its lining, so its inner surface is larger.

4. VQ4 · Practice

Model answer. (a) A villus is a finger-like fold of the lining, made of many cells and visible at ×100. Microvilli are tiny projections of the membrane of one epithelial cell, too small to see one by one with a light microscope. (b) Microvilli add surface to every epithelial cell, so the internal surface area is larger again and more molecules can cross the lining in the same time.

A full answer includes

(a) Any one of: I wrote that a villus is many cells and a microvillus is part of one cell; I wrote that villi are seen with a light microscope and microvilli only with an electron microscope.
(b) I wrote that microvilli increase the surface area of each epithelial cell.
(b) I wrote that more molecules are absorbed in the same time.
I did not write that microvilli move the food along.

Also accept

(a) villus: many cells, light microscope; microvilli: part of one cell, electron microscope; (b) more surface, more absorbed per second

Watch for

“Microvilli speed up absorption by pushing food along.” — Microvilli do not move food. They increase the surface across which molecules are absorbed.

5. VQ5 · Practice

Model answer. Stretch A: 3.6 ÷ 1.2 = 3.0. Stretch B: 10.0 ÷ 2.0 = 5.0. Mean = (3.0 + 5.0) ÷ 2 = 4.0. Two stretches give a more reliable estimate than one.

Checked automatically

Stretch A: times longer: 3.0 (within ±0.1)
Stretch B: times longer: 5.0 (within ±0.1)
Mean: 4.0 (within ±0.1)
- Common wrong answer 4.25 : You divided the total string by the total straight length. Find the two ratios first, then their mean.

A full answer includes

I divided each string length by its straight length.
I wrote 3.0 for stretch A.
I wrote 5.0 for stretch B.
I added the two ratios and divided by 2.
I wrote a mean of 4.0.
I did not divide the total string length by the total straight length.

Also accept

A 3.0, B 5.0, mean 4.0

Watch for

“The mean is (3.6 + 10.0) ÷ (1.2 + 2.0) = 4.25.” — That adds the lengths of two different stretches. The mean of the results comes from the two ratios: work out each one first, then add them and divide by 2.

6. VQ6 · Practice

Model answer. (a) Stretch 3: its ratio, 1.9, is far below the other three. Where the string was lifted across the tear it missed the folds of the edge there, so the string was too short and the ratio too small. (b) Mean = (4.6 + 4.2 + 4.8) ÷ 3 = 13.6 ÷ 3 = 4.53, about 4.5.

Checked automatically

(a) Number of the anomalous stretch: 3 (exact value)
- Common wrong answer 4 : Stretch 4 is close to stretches 1 and 2. Look for the result far from all the others.
(b) Mean of the other three: 4.53 (within ±0.04)
- Common wrong answer 3.9 : You included stretch 3. Leave the anomalous result out: (4.6 + 4.2 + 4.8) ÷ 3.
- Common wrong answer 3.4 : You divided by 4. Three results are left, so divide their total by 3.

A full answer includes

(a) I chose stretch 3.
(a) I wrote that the lifted string skipped the folds, so it was too short.
(b) I added 4.6, 4.2 and 4.8 and divided by 3.
(b) I wrote 4.5 (or 4.53).
I did not include 1.9 in the mean.

Also accept

(a) stretch 3; (b) 4.5

Watch for

“Every result goes into the mean: (4.6 + 4.2 + 1.9 + 4.8) ÷ 4 = 3.9.” — Stretch 3 is anomalous: the torn edge made its string too short. Leave it out and find the mean of the other three.
“Stretch 4 is anomalous, because it is the largest.” — Stretch 4 is close to stretches 1 and 2. The result that does not fit the pattern is the 1.9 of stretch 3.

7. VT1 · Apply

Model answer. Taller villi increase the internal surface area of each length of the small intestine that is left. More nutrient molecules can then be absorbed in the time the food takes to pass through, which makes up for some of the length that was removed.

A full answer includes

I wrote that taller villi give a larger internal surface area.
I wrote that more nutrient molecules are absorbed in the same time.
I wrote that this makes up for some of the length removed.
I did not write that taller villi slow the food down.

Also accept

taller villi: larger surface area per length; more absorbed; makes up for the lost length

Watch for

“Taller villi make the food move more slowly.” — The villi increase the surface for absorption; the point is how much surface the food meets.

8. VT2 · Apply

Model answer. Any two of these. The two sections were not cut the same way: the angle of the cut is a second variable, so the difference may come from the cut, not from the villi. In the rabbit section, cut at a slant, many villi were cut away from the edge, so the string did not run round them and the ratio is too small. And only one section of each animal was measured: more sections, from more animals, are needed before rats and rabbits can be compared.

A full answer includes

I gave two different reasons from the three lines below.
I wrote that the two sections were not cut the same way, so the comparison is not fair.
I wrote that villi cut away from the edge were not on the string, so the rabbit ratio is too small.
I wrote that only one section of each animal was measured.
I did not give only the reason that a ratio is a length, not an area: both ratios were measured the same way.

Also accept

two of: the sections were cut at different angles, not a fair test; villi cut away from the edge were missed in the rabbit; only one section of each animal

Watch for

“No comparison can be made, because the string measures a length, not an area.” — Both ratios are lengths measured in the same way, so they could be compared if the sections were alike. The problem is that the two sections were not cut the same way.
“The rabbit ratio is smaller, so rabbits must have shorter villi.” — They might, but this test cannot show it: cutting at an angle alone makes the ratio smaller, because villi that do not touch the edge are missed.