Nutrition and Digestion in Plants and Animals
This domain covers how green plants manufacture food and how animals break food down, absorb it and use it. The exam asks for the light and dark reactions with their sites, enzyme-substrate-pH pairs in human digestion, the role of bile, absorption in villi, and deficiency diseases.
Photosynthesis
- Site: chloroplast. Light reaction in the grana thylakoids - photolysis of water releases oxygen, and ATP and NADPH are formed. Dark reaction (Calvin cycle) in the stroma - carbon dioxide is fixed and sugar is built using ATP and NADPH.
- The oxygen released comes from water, not carbon dioxide.
- Limiting factors: light intensity, carbon dioxide concentration, temperature.
- Minerals: macronutrients nitrogen, phosphorus, potassium, calcium, magnesium, sulphur; micronutrients iron, manganese, zinc, copper, boron, molybdenum, chlorine. Nitrogen fixation by Rhizobium, Azotobacter and cyanobacteria; insectivorous plants supplement nitrogen.
Digestion in humans
- Mouth: salivary amylase, starch to maltose, slightly acidic to neutral pH.
- Stomach: hydrochloric acid gives an acidic pH; pepsin digests protein to peptones.
- Small intestine: bile from the liver emulsifies fat but is not an enzyme; pancreatic amylase, trypsin and lipase work in alkaline conditions; intestinal enzymes finish sugars and peptides.
- Absorption in the villi of the small intestine; water is absorbed in the large intestine.
Worked example
A piece of bread is chewed for two minutes and tastes sweet. Salivary amylase has converted some starch to maltose. If the same bolus reaches the stomach, amylase stops working because the acid pH denatures it, and pepsin begins protein digestion instead.
Traps that cost marks
- Bile is not an enzyme; it only emulsifies fat.
- Pepsin needs acid; trypsin needs alkaline conditions.
- Vitamins and minerals are not digested and provide no energy.