Nutrition and Digestion
What this domain tests
Expect questions on modes of plant nutrition, the human alimentary canal, enzyme–substrate–site matching, the role of bile, absorption in villi, and diet-deficiency conditions.
Core rules
- Plant modes: autotrophic, saprophytic (Rhizopus, mushroom), parasitic (Cuscuta), insectivorous (Nepenthes, Drosera), symbiotic (Rhizobium in root nodules, mycorrhiza, lichen).
- Digestion begins in the mouth: salivary amylase (ptyalin) at about pH 6.8 converts starch to maltose.
- Stomach: HCl gives pH about 2, kills microbes and activates pepsinogen to pepsin, which digests protein.
- Small intestine: bile from the liver, stored in the gall bladder, emulsifies fat but contains no enzyme; pancreatic amylase, trypsin and lipase act at about pH 8.
- Intestinal juice supplies maltase, sucrase, lactase, peptidases and lipase.
- Absorption: villi and microvilli give huge surface area; glucose and amino acids enter blood capillaries, fat products enter lymph vessels.
- Large intestine absorbs water and salts; gut bacteria synthesise vitamin K and some B vitamins.
Worked example
Trace a protein fragment through the tract. In the mouth there is no protease, so nothing happens. In the stomach, pepsin at pH 2 breaks protein into peptides. In the duodenum, trypsin at pH 8 continues the work, and peptidases on the intestinal brush border finish it into amino acids, which are absorbed into the blood capillaries inside a villus and carried to the liver by the hepatic portal vein.
Traps that cost marks
Saying bile digests fat; starting fat digestion in the mouth or stomach; forgetting that pepsin needs an acid medium; writing that amino acids are absorbed into lymph; claiming digestion is complete in the stomach.