Inside a villus — answer key

1. LQ1 · Practice

Options as numbered on the pupil sheet: 1 — capillary network; 2 — lacteal; 3 — microvilli; 4 — muscle layer; 5 — epithelium.

Model answer. P is the epithelium, one cell thick, covering the villus. Q, in the inset, is the microvilli on the epithelial cells. R points to one of the narrow vessels of the capillary network under the epithelium. S is the lacteal, closed at its tip and open at the base, in the middle.

A full answer includes

• I matched P to the epithelium.
• I matched Q to the microvilli.
• I matched R to the capillary network.
• I matched S to the lacteal.
• I did not match S to the capillary network.

Also accept

• P epithelium; Q microvilli; R capillary network; S lacteal

Watch for

• “S, in the middle, is a blood vessel.” — The tube in the middle, closed at its tip, is the lacteal: a lymph vessel. The blood vessels form the network just under the epithelium.
• “P is a muscle layer.” — The layer covering the villus is the epithelium, one cell thick. A villus has no muscle layer on its surface.

2. LQ2 · Practice

Model answer. The epithelium covering a villus is one cell thick, so absorbed molecules have only a short distance to travel to the blood.

A full answer includes

• I chose an epithelium one cell thick.
• I did not choose an epithelium many cells thick.

Also accept

• an epithelium one cell thick

Watch for

• “Many cells thick, to protect the villus.” — A thick lining would make the distance to the blood longer. The epithelium is one cell thick.
• “Two cells thick.” — The ×400 field shows a single row of tall cells: one cell thick.
• “A thin layer of muscle.” — Muscle does not cover a villus. Its surface is an epithelium, one cell thick, that absorbs the food.

3. LQ3 · Practice

Model answer. Glucose and amino acids are absorbed into the capillaries, which carry them away in the blood. Fatty acids and glycerol are absorbed into the lacteal, which carries them away in the lymph.

A full answer includes

• I wrote capillaries (or blood capillaries) in (1).
• I wrote lacteal in (2).
• I wrote lymph in (3).
• I did not write capillaries in (2).

Also accept

• (1) capillaries (or blood capillaries); (2) lacteal; (3) lymph

Watch for

• “Fatty acids and glycerol go into the capillaries.” — The products of fat digestion pass into the lacteal, and are carried in the lymph.

4. LQ4 · Practice

Options as numbered on the pupil sheet: 1 — the lacteal in the middle of the villus; 2 — the blood near the heart; 3 — an epithelial cell of the villus; 4 — larger lymph vessels outside the villus.

Model answer. A fatty acid crosses an epithelial cell of the villus, passes into the lacteal in the middle, and is carried out of the villus in the lymph. The lymph flows along larger lymph vessels and later empties into the blood near the heart.

A full answer includes

• I put the epithelial cell first.
• I put the lacteal second.
• I put the larger lymph vessels third.
• I put the blood near the heart last.
• I did not put the blood before the lymph vessels.

Also accept

• epithelial cell, lacteal, larger lymph vessels, blood near the heart

Watch for

• “The fatty acid goes straight into the blood in the villus.” — Fatty acids pass into the lacteal, not the capillaries. They reach the blood only later, when the lymph empties into it near the heart.
• “The lymph vessels come before the lacteal.” — The lacteal is the lymph vessel inside the villus. The larger lymph vessels collect its lymph after it leaves the villus.

5. LQ5 · Practice

Model answer. (a) A villus is a finger-like projection of the lining of the small intestine, sticking out into the space inside. (b) It is covered by an epithelium one cell thick; each epithelial cell has microvilli. (c) Inside, a network of capillaries lies just under the epithelium, and a lacteal, a lymph vessel closed at its tip, runs up the middle.

A full answer includes

• (a) I wrote that a villus is finger-like.
• (a) I wrote that it is a projection of the lining of the small intestine.
• (b) I wrote that the epithelium is one cell thick.
• (b) I wrote that the epithelial cells have microvilli.
• (c) I wrote a network of capillaries just under the epithelium.
• (c) I wrote a lacteal in the middle.
• I did not write that a villus is one cell.

Also accept

• finger-like; epithelium one cell thick with microvilli; capillary network; lacteal in the middle

Watch for

• “A villus is one cell.” — A villus is made of many cells: an epithelium over a core with blood and lymph vessels.

6. LQ6 · Practice

Model answer. The epithelium is one cell thick, so absorbed molecules have only a short distance to travel to the blood. The epithelial cells have microvilli, which increase the surface area, so more molecules are absorbed at once. A dense capillary network just under the epithelium carries absorbed molecules away.

A full answer includes

• I gave two features from the four lines below, each with how it helps.
• Epithelium one cell thick: I wrote that the distance to the blood is short.
• Microvilli: I wrote that they increase the surface area.
• Capillary network: I wrote that the blood carries absorbed molecules away.
• Lacteal: I wrote that it carries away the products of fat digestion.
• I did not write that microvilli move the food.

Also accept

• two of: one cell thick, short distance; microvilli, more surface; capillaries carry molecules away; lacteal takes fat products

Watch for

• “The villus moves food along with its microvilli.” — Microvilli increase the surface area for absorption; they do not move food.

7. LQ7 · Practice

Model answer. (a) R, the capillary network, carries blood; S, the lacteal, carries lymph. (b) S is closed at its tip and open only at the base, so the lymph flows down from the tip and out of the villus at its base. (c) Blood flowing through the capillaries carries the absorbed molecules away all the time, so they do not build up in the villus and absorption can go on.

A full answer includes

• (a) I wrote blood for R.
• (a) I wrote lymph for S.
• (b) I wrote that the lymph flows from the tip towards the base.
• (b) I wrote that S is open only at the base, so the lymph can leave only there.
• (c) I wrote that the blood carries absorbed molecules away as they arrive.
• (c) I wrote that this stops them building up, so absorption goes on.
• I did not write that S carries blood.

Also accept

• (a) R blood, S lymph; (b) tip to base, the only way out; (c) carries absorbed food away

Watch for

• “S carries blood, like R.” — S is the lacteal, a lymph vessel. It carries lymph, not blood.
• “Lymph flows into the villus at the base and out at the tip.” — The tip of S is closed, so nothing can leave there. The lymph leaves at the base.

8. LQ8 · Practice

Model answer. Normally the flowing blood carries glucose away as fast as it arrives. With the blood still, the absorbed glucose builds up in the capillaries of that villus and is not carried away, so absorption into that villus slows down.

A full answer includes

• I chose: absorption slows, because the glucose is not carried away.
• I did not choose the lacteal: it takes the products of fat digestion, not glucose.

Also accept

• absorption slows: the glucose builds up and is not carried away

Watch for

• “Absorption speeds up, because the glucose has more time to cross.” — Time is not the problem. Still blood fills up with glucose that nothing carries away, so less can be absorbed.
• “Nothing changes, because the epithelium and microvilli are still there.” — The surface is still there, but absorbed glucose must be carried away, or it builds up in the villus.
• “The glucose goes into the lacteal instead.” — The lacteal takes the products of fat digestion. Glucose is absorbed into the capillaries.

9. LT1 · Apply

Model answer. Lipase breaks fat into fatty acids and glycerol, which pass into the lacteals. With lipase blocked, less fat is digested, so fewer fatty acids and less glycerol are absorbed into the lacteals. Fat molecules are too large to be absorbed, so the undigested fat leaves in the faeces.

A full answer includes

• (a) I wrote that less fat is broken down into fatty acids and glycerol.
• (a) I wrote that fewer fatty acids and less glycerol pass into the lacteals.
• (b) I wrote that the undigested fat is not absorbed.
• (b) I wrote that it leaves in the faeces.
• I did not write that the fat goes into the capillaries instead.

Also accept

• fewer fatty acids and glycerol into the lacteals; undigested fat leaves in the faeces

Watch for

• “The fat goes into the capillaries instead.” — Undigested fat cannot be absorbed at all: neither into the capillaries nor into the lacteals.

10. LT2 · Apply

Model answer. Normally the epithelium is one cell thick, so absorbed molecules travel only a short distance to the capillaries and the lacteal. A lining several cells thick makes this distance longer, so absorption is slower and less food is absorbed in the time it takes to pass through.

A full answer includes

• (a) I wrote that absorption becomes slower.
• (b) I wrote that the distance to the capillaries is longer.
• I did not write that more cells absorb more.

Also accept

• slower absorption: longer distance to the blood

Watch for

• “A thicker lining absorbs more, because it has more cells.” — Absorbed molecules have to cross the lining to reach the blood. More layers of cells means a longer distance, so absorption is slower.

11. LT3 · Apply

Model answer. The products of fat digestion, fatty acids and glycerol, are absorbed into the lacteals and carried away in the lymph. (Inside the epithelial cells they are joined back into fat, which the lymph carries as tiny droplets.) After a fatty meal the lymph carries so much of this fat that it looks milky. Bread and egg white are digested to glucose and amino acids, which are absorbed into the blood capillaries instead, so the lymph stays almost clear.

A full answer includes

• (a) I wrote that the products of fat digestion pass into the lacteals.
• (a) I wrote that after a fatty meal the lymph carries a lot of these products, so it looks milky.
• (b) I wrote that bread and egg white give glucose and amino acids.
• (b) I wrote that these pass into the blood capillaries, not the lymph.
• I did not write that the lymph is milky because it carries blood.

Also accept

• fat products go into the lacteals and lymph; glucose and amino acids go into the blood

Watch for

• “The lymph is milky because it carries blood.” — Lymph carries no red blood cells. It looks milky when it carries a lot of the products of fat digestion from the lacteals.