2.2.1 The Human Digestive System

What is the digestive system?

The digestive system is an organ system made up of organs working together to digest and absorb food.

Name the main organs of the digestive system.

Salivary glands, pancreas, stomach, small intestine, liver, gall bladder, large intestine, rectum, anus.

What is the role of the salivary glands and pancreas?

They produce digestive juices containing enzymes that break down food.

What is the role of the stomach?

The stomach produces hydrochloric acid to kill bacteria and provide the optimum pH for protease enzymes.

What is the role of the small intestine?

The small intestine is where soluble molecules are absorbed into the blood.

What is the role of the liver and gall bladder?

The liver produces bile, which is stored in the gall bladder and helps digest lipids.

What is the role of the large intestine?

The large intestine absorbs water from undigested food, forming faeces.

What is an enzyme?

An enzyme is a biological catalyst that increases the rate of a reaction without being used up.

What are the properties of enzymes?

They are proteins, they are specific to certain reactions, and each has a uniquely shaped active site.

What is the lock and key hypothesis?

The lock and key hypothesis states that the substrate fits into the enzyme's active site like a key in a lock to form an enzyme-substrate complex.

What is the role of bile in digestion?

Bile neutralises stomach acid (as it is alkaline) to provide the optimum pH for enzymes in the small intestine and emulsifies fats to increase surface area for lipase action.

What does emulsification mean?

Emulsification is the breakdown of large fat droplets into smaller droplets, increasing surface area for enzyme action.

What are the three main groups of digestive enzymes?

Carbohydrases, proteases, and lipases.

What do carbohydrases do?

Carbohydrases convert carbohydrates into simple sugars. Amylase breaks down starch into maltose.

Where is amylase produced?

Amylase is produced in the salivary glands, pancreas, and small intestine.

What do proteases do?

Proteases convert proteins into amino acids.

Where is pepsin (a protease) produced?

Pepsin is produced in the stomach. Other proteases are produced in the pancreas and small intestine.

What do lipases do?

Lipases convert lipids into fatty acids and glycerol.

Where are lipases produced?

They are produced in the pancreas and small intestine.

What test is used for sugars?

Benedict's test turns brick red in the presence of sugars.

What test is used for starch?

Iodine test turns blue-black in the presence of starch.

What test is used for protein?

Biuret test turns purple in the presence of protein.

What tests are used for lipids?

Emulsion test forms a cloudy layer if lipids are present, or Sudan III test forms a red layer on top.

What factors affect enzyme activity?

Temperature and pH.

What happens when enzymes are exposed to high temperatures?

The enzyme denatures, bonds break, and the active site changes shape so the substrate no longer fits.

What happens when the pH is too high or too low for an enzyme?

The active site changes shape, the enzyme denatures, and it no longer works.

What is the optimum temperature for most enzymes in the human body?

About 37 °C (body temperature).

What is the optimum pH for most enzymes?

Most enzymes work best at pH 7, but some, like those in the stomach, work in acidic conditions.

What is the required practical for enzyme activity?

Investigate the effect of pH on the rate of amylase activity by measuring the time it takes for starch to be broken down.

How is the enzyme practical carried out?

Amylase, starch, and buffer solution are warmed in a water bath. Samples are taken at intervals and tested with iodine. The time until iodine stays orange (no starch) is recorded.

How is the rate of enzyme reaction calculated?

The rate is calculated as 1000 ÷ time.

Why should the enzyme practical be repeated at different pH values?

To compare results and identify the optimum pH for enzyme activity.