Outline the role of condensation and hydrolysis in metabolic reactions involving carbohydrates.

Metabolic reactions are catalysed by enzymes. Explain how enzymes catalyse reactions and how a change in pH could affect this.

Describe the digestion of food in the human digestive system.

condensation is joining together molecules with the release of water;
(in general) two monosaccharides join to form a disaccharide / many mono-saccharides/disaccharides form polysaccharides;
example; (eg. two glucose from maltose)
hydrolysis is the breaking down of molecules with the addition of water;
(in general) disaccharides break into monosaccharides / polysaccharides break into disaccharides/monosaccharides;
example; (eg. maltose forms two glucose)

enzymes speed up the rate of chemical reactions;
lock and key model;
substrate fits into active site;
enzyme-substrate specificity;
enzymes work best at optimal pH/different enzymes have different optimal pHs;
increase/decrease from optimum pH decreases activity;
change in pH changes structure/charge of active site;
changing three-dimensional structure of enzyme/protein;
not allowing substrate to fit in active site;
enzymes can be denatured if change is extreme;
denaturing is loss of its biological properties;
sketch graph showing pH versus enzyme activity;

chewing food makes smaller particles/increases surface area of food;
starch digestion (begins) in the mouth/by saliva/(salivary) amylase/ptyalin;
digestion of proteins in stomach;
acid condition in stomach provides optimum pH for enzymes;
stomach muscle contraction causes mechanical digestion;
enzymes in small intestine complete digestion;
alkaline condition in small intestine provides optimum pH for enzymes;
bile salts help to emulsify fats;
example of amylase with source, substrate and products;
example of protease with source, substrate and products;
example of lipase with source, substrate and products;

In 5(a), most candidates clearly distinguished condensation and hydrolysis. A few candidates did not read the questions properly, giving examples of lipids or proteins instead of carbohydrates. Some marks were always scored.

Part 5(b) was an easy question for those who were well-prepared and most handled it well. Some candidates seemed to try to write everything they knew. They gave long explanations of factors beyond pH which can influence how enzymes catalyze reactions. In contrast, other candidates simply wrote that pH change can cause denaturation, without any further reference to change in active site or loss of biological function.

The weakest answers for question 5 appeared in 5 (c). The passage of food through various parts of the digestive system was frequently given rather than the breakdown of food.  Accurate detailed information was scarce. Although digestion in the mouth was accurately discussed, there was a lack of clarity on digestion in the stomach and intestine. Most candidates discussed mechanical digestion without any attention to chemical digestion.  Information on the conditions in each part of the digestion was sketchy. Very few candidates correctly included an example of enzyme source, substrate and product. The role of bile was not clear in most. Some made reference to absorption and egestion, instead of sticking to the question. Sadly, there were candidates who thought that as food progresses through the digestive tract, it stops at the pancreas.