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The digestive system - AQA SynergyEnzymes

The major nutrients required for a healthy diet are carbohydrates, proteins and lipids. The digestive system breaks down large molecules of food, which are then absorbed into the bloodstream.

Part of Combined ScienceTransport over larger distances

Enzymes

Most of the food we eat is complex , and . These must be broken down to be absorbed into the body.

The chemical reactions required to break them down would be too slow without .

Enzymes are biological 鈥 they speed up chemical reactions.

Enzymes are required for most of the chemical reactions that occur in . These reactions occur in the breakdown of chemical molecules, which we see in the .

Enzymes are also involved in the building up of chemical molecules elsewhere in the body.

Enzymes are proteins that have a complex 3D shape. Each enzyme has a region called an .

The 鈥 the molecule or molecules taking part in the chemical reaction 鈥 fits into the active site. Once bound to the active site, the chemical reaction takes place .

Diagram showing how enzymes work in the body

In an organism, the active site of each enzyme is a different shape. It is a perfect match to the shape of the substrate molecule, or molecules. This is essential to the enzyme being able to work. One enzyme is therefore specific to one substrate's chemical reaction, or type of chemical reaction.

This theory for the way in which enzymes work is called the .

Factors affecting enzyme action

Physical factors affect enzyme activity.

Temperature

At low temperatures, the number of successful collisions between the enzyme and substrate is reduced because their decreases. The reaction is slow.

The human body is maintained at 37掳C as this is the temperature at which the enzymes in our body work best. This is not true of the enzymes in all organisms.

Higher temperatures disrupt the shape of the active site, which will reduce its activity, or prevent it from working. The enzyme will have been .

Enzymes therefore work best at a particular temperature.

Enzyme action

Proteins are chains of joined end to end. This chain is not straight 鈥 it twists and folds as different amino acids in the chain are attracted to, or repel each other.

Each enzyme is made from proteins made of these twisting and folding amino acids, and therefore the enzyme has a unique shape. This structure is held together by weak forces between the amino acid molecules in the chain.

High temperatures will break these forces. The enzyme, including its active site, will change shape and the substrate will no longer fit. The rate of reaction will be affected, or the reaction will stop.

Diagram showing how high teperatures alter enzyme structures
Y axis: enzyme activity. X axis: temperature, centigrade.  Plotted line climbs slowly until about half way on x axis. Climbs steeply to optimum temperature then falls steeply to 0.
Figure caption,
A graph showing the effect of temperature on enzyme activity

The effect of pH

Enzymes are also sensitive to . Changing the pH of its surroundings will also change the shape of the active site of an enzyme.

Many amino acids in an enzyme molecule carry a . Within the enzyme molecule, positively and negatively charged amino acids will attract each other. This contributes to the folding of the enzyme molecule, its shape, and the shape of the active site.

Changing the pH will affect the charges on the amino acid molecules. Amino acids that attracted each other may no longer attract each other. Again, the shape of the enzyme, along with its active site, will change.

Extremes of pH also denature enzymes. The changes are usually permanent.

Enzymes work inside and outside cells, for instance in the digestive system where cell pH is kept at 7.0 to 7.4. Cellular enzymes will work best within this pH range.

Different parts of the digestive system produce different enzymes. These have different pHs.

The optimum pH in the stomach is produced by the secretion of hydrochloric acid.

The optimum pH in the duodenum is produced by the secretion of sodium hydrogencarbonate.

The following table gives examples of how some of the enzymes in the digestive system have different optimum pHs:

EnzymeOptimum pH
Salivary amylase6.8
Stomach protease (pepsin)1.5 - 2.0
Pancreatic protease (trypsin)7.5 - 8.0
EnzymeSalivary amylase
Optimum pH6.8
EnzymeStomach protease (pepsin)
Optimum pH1.5 - 2.0
EnzymePancreatic protease (trypsin)
Optimum pH7.5 - 8.0
Graph showing that as the pH increases so does the rate of enzyme activity
Figure caption,
A graph showing the effect of pH on enzyme activity

Question

Suggest an enzyme that would produce a trend as shown in the graph above.