(Lower Sec Science) The path of the hamburger through the digestive system

How do you tackle this which may appear in a free-response-questions section?

Question: Describe what happens to a regular hamburger in the digestive system. [10]

You can either draw a diagram labeled with descriptions or write out as follow:

The hamburger consists of bread, meat, cheese, lettuce and tomato. The nutrients present are:

• Bread – carbohydrates
  
• Meat and cheese – Proteins and fats
  
• Lettuce and tomato – Fibre, water, mineral salts, vitamins
  

In the digestive system, the three classes of enzymes are:

• carbohydrase which breaks down carbohydrates
  
• protease which breaks down proteins
  
• lipase which breaks down fats
  

Digestion begins in the mouth where the hamburger is bitten and chewed. Salivary gland secretes the carbohydrase enzyme amylase to digest starch to sugars.

The bolus of food from the mouth is then moved to the stomach through the gullet (esophagus) by a wave-like action called peristalsis. No digestion occurs in the gullet.

In the stomach, the food is squashed by peristalsis. The stomach produces a gastric juice which contains hydrochloric acid and a protease enzyme. The acid kills bacteria and provide an optimum low pH condition (about pH = 2) for the protease to act on the proteins in the meat and cheese. The proteins are digested into small molecules called amino acids.

After about 3 hours, the thick mixture of food and gastric juice called chyme leaves the stomach and enters the duodenum of the small intestine. Pancreatic juice is secreted by the pancreas into the duodenum. The pancreatic juice is slightly alkaline and helps to neutralize the acidic chyme. It contains amylase, a protease (not the same protease at that in the stomach) and a lipase. The amylase continues to digest starch to sugars while the protease continues to digest proteins to amino acids.

Bile, a green liquid, is also produced by the liver and stored in the gall bladder. It is secreted into the duodenum. The bile acts as an emulsifier to separate the fat globules into smaller fat droplets. This makes it efficient for the lipase to digest the fats. The fats are digested to smaller molecules called fatty acids and glycerol.

The glucose, amino acids, fatty acids and glycerol are absorbed by the walls of the intestine into the blood stream by diffusion. The water, minerals and vitamins are absorbed by the walls of the large intestine. The undigested food which includes the fibre is stored in the rectum before it is passed out of the body through the anus.

Try these questions before you refer to the answers that follow.

1. Complete the following table.

Location	Enzyme	Role
	Salivary amylase	-
Stomach		- Breaks down proteins into amino acids
Small intestine (from pancreas)	Amylase Protease Lipase	- - -

2. At pH = 7, does the protease in the stomach able to work? Why?

3. (a) Why is the chyme that just leaves the stomach acidic?
(b) Can the enzymes in the pancreatic juice function if it is not alkaline? Why?

4. Without bile, is the lipase able to efficiently digest the fats? Why?

5. The small food molecules diffuse through the walls of the small intestine into the blood stream as the blood has a __________ (higher / lower) concentration of food molecules.


Answers:
1.

Location	Enzyme	Role
Mouth	Salivary amylase	Breaks down starch into sugars
Stomach	Protease	Breaks down proteins into amino acids
Small intestine (from pancreas)	Amylase Protease Lipase	- Breaks down starch into sugars - Breaks down proteins into amino acids - Breaks down fats into fatty acids and glycerol

2. pH 7 is not the optimum condition for the protease in the stomach to function properly. The protease can only function in acidic condition.
3. (a) The chyme is acidic because of the hydrochloric acid in the gastric juice.
(b) The enzymes in the pancreatic juice would not function properly in the acidic chyme. They function properly in neutral condition.
4. The fat globules present a small surface area for the lipase to digest the fats. Without bile to separate the fat globules into smaller droplets thereby increasing the surface area, it is inefficient for the lipase to act on the fats.