MYP Biology 4.2 Digestion Notes

What Is Digestion and Why Must Food Be Broken Down?

1. Food molecules such as proteins, fats, and starch are too large to pass into cells or enter the bloodstream.
2. Digestion reduces these large molecules into smaller ones like amino acids, glucose, and fatty acids that cells can absorb and use.

Why Does Digestion Begin Before Food Reaches the Stomach?

Digestion starts in the mouth because early processing makes later stages more efficient.

1. Mechanical digestion: Chewing breaks food into smaller pieces, increasing the surface area available to enzymes.
2. Chemical digestion: Saliva contains amylase, an enzyme that begins breaking starch into smaller sugars.
3. Bolus formation: Saliva moistens food so it can move smoothly through the digestive tract.

How Does the Stomach Contribute to Digestion?

The stomach performs both mechanical and chemical digestion.

1. Churning mixes food with digestive juices.
2. Hydrochloric acid unfolds proteins, making them easier for enzymes to work on.
3. Pepsin, a protease (protein enzyme), begins protein digestion by cutting long chains into shorter peptides.
4. Food becomes a semi-liquid mixture called chyme.

How Are Carbohydrates Broken Down?

Carbohydrate digestion aims to produce glucose, the body’s main energy source.

1. Mouth
   1. Amylase begins starch breakdown.
2. Small intestine
   1. Pancreatic amylase continues the process.
   2. Enzymes such as maltase convert small sugars into glucose.

How Are Proteins Broken Down?

Proteins must be reduced to amino acids before the body can use them.

1. Stomach
   1. Acid unfolds proteins.
   2. Pepsin cuts them into shorter chains.
2. Small intestine
   1. Proteases and peptidases complete digestion into amino acids.

How Are Fats Broken Down?

Fats require separate processing because they are hydrophobic.

1. Emulsification
   1. Bile breaks large fat droplets into smaller droplets.
   2. This increases the surface area for enzymes.
2. Chemical breakdown
   1. Lipase converts fats into fatty acids and glycerol.

How Does pH Support Digestion?

1. Different regions of the digestive system have different pH levels that optimize enzyme activity.
   1. Mouth: neutral pH for amylase
   2. Stomach: acidic pH for pepsin
   3. Small intestine: neutral pH for intestinal enzymes
2. Each environment ensures enzymes work at their highest efficiency.

How Is Digestion Coordinated as Food Moves Through the System?

Digestion relies on a sequence of specialised regions working together. Your:

1. Mouth begins mechanical processing and early starch breakdown.
2. Stomach mixes food and starts major protein digestion.
3. Small intestine receives enzymes and bile to finish chemical digestion of all macromolecules.
4. By this stage, carbohydrates, proteins, and fats have been reduced to small molecules ready for absorption.
5. This prepares your body for the next stage: nutrient absorption.