Chemical digestion and absorption

A secondary active carrier uses the energy of an ion, usually sodium, flowing down its gradient. The sodium–potassium pump spends the ATP that keeps that gradient steep.

• ATP bound to the carrier:
• Correct: The gradient of another ion, usually sodium:
• Osmosis:

Brush border enzymes such as lactase, sucrase and maltase are fixed to the membrane of the microvilli, so they finish digestion right where absorption happens.

• In pancreatic juice:
• Correct: On the microvilli of the absorptive cells:
• In the gastric glands:

Bile salts coat fat droplets and keep them small, which multiplies the surface lipase can work on. Lipase does the splitting.

• Correct: Emulsify it into tiny droplets:
• Split it into fatty acids:
• Carry it into the blood:

Emulsification gives lipase more surface; lipase makes fatty acids and monoglycerides; micelles ferry them through the water next to the brush border; inside the cell they are rebuilt and packaged into chylomicrons, which are too large for blood capillaries and enter the lacteal instead.

• Correct order: 1. Bile salts emulsify fat into tiny droplets 2. Pancreatic lipase splits triglycerides into fatty acids and monoglycerides 3. Micelles carry fatty acids and monoglycerides to the brush border 4. Fatty acids and monoglycerides enter the absorptive cell and are rebuilt into triglycerides 5. Triglycerides are packaged into chylomicrons and leave the cell by exocytosis 6. Chylomicrons enter a lacteal and travel in lymph to the thoracic duct

Without enough lactase, lactose cannot be split or absorbed. It holds water in the small intestine by osmosis and is fermented by colon bacteria into gas and acids, giving bloating, hydrogen on the breath and osmotic diarrhea, while little of its sugar reaches the blood.

• Lactose reaching the colon: up. Too little lactase is left at the brush border to split the lactose, so much of it passes through the small intestine intact.
• Water in the lumen of the small intestine: up. Unabsorbed lactose is a solute that stays in the lumen and holds water there by osmosis.
• Hydrogen in the exhaled breath: up. Colon bacteria ferment the lactose and make hydrogen; some is absorbed into the blood and breathed out.
• Water content of the stool: up. The extra water, plus the solutes fermentation adds, is more than the colon can absorb, so the stool becomes loose.
• Rise in blood glucose after drinking: down. Undigested lactose yields no glucose or galactose for SGLT1 to absorb, so blood glucose rises less than it would after the milk in a person with high lactase.

SGLT1 uses the energy of sodium moving down its gradient into the cell. The pumps keep sodium inside the cell low. Once they stop, sodium inside rises, the gradient collapses, and SGLT1 can no longer pull glucose in against its own gradient.

• Correct: It stops once sodium inside the cell rises and the gradient collapses: Correct. With the pumps stopped, sodium leaks in and is not removed, the gradient collapses, and SGLT1 loses its energy source.
• It continues unchanged: SGLT1 uses no ATP directly, but it depends on the sodium gradient that the ATP-using pumps maintain.
• It speeds up: Sodium building up inside the cell shrinks the gradient, which slows SGLT1 rather than speeding it.
• It continues at the same rate by facilitated diffusion: Facilitated diffusion moves glucose only down its own gradient, so it cannot keep absorption going at the same rate once lumen glucose falls.

The toxin drives secretion but leaves SGLT1 working. Each glucose that enters through SGLT1 brings two sodium ions with it. The absorbed sodium and glucose raise the solute on the blood side, and water follows by osmosis, rehydrating the child even while secretion goes on.

• Glucose kills the cholera bacteria in the lumen: Glucose does not kill bacteria; it feeds most of them.
• Glucose blocks the chloride secretion caused by the toxin: The chloride secretion continues. The solution works by adding absorption, not by blocking secretion.
• Correct: Glucose pulls sodium in through SGLT1, and water follows: Correct. Sodium-glucose cotransport is the basis of oral rehydration.
• Glucose is the main fuel the child's colon needs to absorb water: The colon's epithelial cells burn mainly butyrate from bacteria, and fuel is not what limits absorption here.

Without bile salts, fat is poorly emulsified and no micelles form, so long-chain fatty acids, cholesterol and the vitamins that dissolve in fat, such as D and K, are poorly absorbed. Glucose and amino acids are water-soluble and absorbed by carriers that do not need bile.

• Correct: Long-chain fatty acids: Correct. Long-chain fatty acids need micelles to reach the brush border.
• Correct: Vitamin K: Correct. Vitamin K travels with fat in micelles; its loss is why obstructive jaundice can cause bleeding.
• Glucose: Glucose is absorbed through SGLT1, which does not need bile.
• Correct: Cholesterol: Correct. Cholesterol is absorbed from micelles.
• Amino acids: Amino acids use carriers powered by sodium, not micelles.
• Correct: Vitamin D: Correct. Vitamin D dissolves in fat and is absorbed with it.

Vitamin B12 bound to intrinsic factor is absorbed only in the terminal ileum, by receptor-mediated endocytosis. Removing it stops B12 absorption. The liver's store lasts three to five years, so the deficiency appears years after surgery.

• Correct: Loss of the site where B12 bound to intrinsic factor is absorbed: Correct. The terminal ileum is the only site where B12 bound to intrinsic factor is absorbed.
• Loss of intrinsic factor production: His stomach is healthy, so parietal cells still make intrinsic factor.
• Loss of pancreatic proteases that free B12 from haptocorrin: His pancreas is not affected by removing the ileum.
• Loss of the lactase needed to absorb B12: Lactase splits lactose; it has no role in B12 absorption.

Chylomicrons are too large for the villus blood capillaries. They enter the lacteals and travel in lymph through the thoracic duct into the veins where the left internal jugular and subclavian veins meet, bypassing the liver at first.

• Fatty acids and monoglycerides join bile salts in micelles: This step is right. Micelles carry the digested lipids through the water next to the brush border.
• At the brush border they leave the micelles and enter the absorptive cell: This step is right. The lipids cross the apical membrane; the bile salts stay in the lumen.
• The smooth endoplasmic reticulum rebuilds them into triglycerides: This step is right. Rebuilding keeps free fatty acids inside the cell low.
• Triglycerides are packaged with cholesterol and protein into chylomicrons: This step is right. Chylomicrons are triglyceride and cholesterol wrapped in phospholipid and protein.
• Correct: Chylomicrons enter the villus blood capillaries and go to the liver in the hepatic portal vein: This is the error. Chylomicrons enter the lacteals, not the blood capillaries.

Pancreatic amylase works in the lumen and still cuts starch into maltose, maltotriose and α-dextrins. The last step, by maltase and α-dextrinase, happens on the brush border, which is damaged. So the partly digested pieces build up, unabsorbed.

• Whole starch: Pancreatic amylase works in the lumen, not on the brush border, so starch is still cut into pieces.
• Glucose: Glucose is released by maltase and α-dextrinase on the brush border, which is damaged, so little is made.
• Correct: Maltose and α-dextrins: Correct. The brush border step fails, so its substrates accumulate.
• Galactose: Starch is made only of glucose; galactose comes from lactose.