Chemical digestion Revision Notes

Chemical digestion uses enzymes to break down large, complex molecules that cannot dissolve in water into smaller, simpler molecules that can dissolve.

For example, starch is broken down into glucose, proteins into amino acids, and fats into fatty acids and glycerol.

This breakdown changes the chemical structure of food molecules, making them soluble.

• Large insoluble molecules are broken down.
• Smaller soluble molecules are produced.
• Enzymes catalyse this breakdown.

Only small soluble molecules can pass through the walls of the digestive system into the bloodstream.

Large insoluble molecules are too big and cannot dissolve, so they cannot be absorbed directly.

Chemical digestion ensures nutrients are in a form the body can absorb and use.

• Soluble molecules can be absorbed.
• Insoluble molecules cannot be absorbed.
• Chemical digestion enables absorption.

The table below shows examples of large molecules and their smaller soluble products after chemical digestion.

This table summarizes key examples of chemical digestion products.

Large food molecules like proteins, carbohydrates, and fats are too big and insoluble to be absorbed directly.

Chemical digestion breaks these into smaller, soluble molecules such as amino acids, simple sugars, and fatty acids.

These small soluble molecules can pass through the walls of the small intestine into the bloodstream.

• Large molecules are insoluble and too big to absorb.
• Chemical digestion produces small soluble molecules.
• Small molecules can be absorbed into the blood.

Once absorbed, these molecules are transported by the blood to body cells.

Cells use these nutrients for energy, growth, and repair.

Without chemical digestion, nutrient absorption would not be possible, leading to malnutrition.

• Small molecules enter bloodstream via small intestine.
• Nutrients are transported to cells for use.
• Chemical digestion is vital for nutrient uptake.

The table below links chemical digestion products to their absorption and use.

This table summarizes the importance of chemical digestion in nutrient absorption.

Amylase is an enzyme that targets starch, a complex carbohydrate found in many foods.

It breaks starch molecules down into simple sugars, such as maltose, which are easier for the body to absorb and use for energy.

Amylase begins its action in the mouth and continues in the small intestine.

• Converts starch into simple sugars.
• Speeds up starch digestion.

Proteases are enzymes that break down proteins, which are large molecules made of amino acids.

They convert proteins into amino acids, the building blocks that the body uses to build new proteins and repair tissues.

Proteases act mainly in the stomach and small intestine.

• Convert proteins into amino acids.
• Essential for protein absorption.

Lipase is the enzyme responsible for breaking down fats and oils, which are large molecules that cannot be absorbed directly.

It converts fats and oils into fatty acids and glycerol, which the body can absorb and use for energy or storage.

Lipase acts primarily in the small intestine where fats are emulsified for digestion.

• Breaks down fats and oils.
• Produces fatty acids and glycerol for absorption.

Amylase is produced in the salivary glands and pancreas.

It begins starch digestion in the mouth and continues to act in the small intestine where pancreatic amylase is released.

This two-stage action ensures starch is efficiently broken down.

• Produced in salivary glands and pancreas.
• Acts in mouth and small intestine.

Protease enzymes are produced in the stomach lining and pancreas.

Protease starts protein digestion in the acidic stomach and continues in the small intestine with pancreatic proteases.

This allows proteins to be broken down completely into amino acids.

• Produced in stomach and pancreas.
• Acts in stomach and small intestine.

Lipase is produced only in the pancreas and secreted into the small intestine.

It acts in the small intestine to break down fats and oils into fatty acids and glycerol.

Lipase requires the small intestine environment to function effectively.

• Produced in pancreas.
• Acts in small intestine.

Hydrochloric acid creates a highly acidic environment in the stomach.

This acidity kills many bacteria and pathogens that enter the body with food, reducing the risk of infection.

This protective role is vital for maintaining health.

• Kills harmful microorganisms in food.
• Protects the body from infection.

Certain digestive enzymes, such as pepsin, require an acidic environment to work effectively.

Hydrochloric acid lowers the pH in the stomach to around 1.5 to 3.5, which activates these enzymes.

Without this acid, protein digestion would be inefficient.

• Creates acidic environment (low pH).
• Enables enzymes like pepsin to function optimally.

Hydrochloric acid is secreted by cells in the stomach lining.

It both protects the body by killing microbes and aids digestion by activating enzymes.

This dual function is essential for safe and effective digestion.

• Secreted by stomach lining cells.
• Kills microbes and activates enzymes.

Amylase is an enzyme that breaks down starch, a large carbohydrate molecule, into maltose, a smaller sugar made of two glucose units.

This process starts in the mouth and continues in the small intestine where pancreatic amylase is secreted.

Breaking starch into maltose makes it easier to digest further.

• Amylase breaks starch into maltose.
• Occurs in mouth and small intestine.

Maltase is an enzyme located on the surface of the epithelial lining of the small intestine.

It converts maltose into glucose, a simple sugar that can be absorbed into the bloodstream.

This final step completes carbohydrate digestion, allowing glucose to be used as energy.

• Maltase converts maltose into glucose.
• Located on small intestine lining.
• Glucose is absorbed for energy.

The two-step process ensures starch is fully broken down into glucose.

Glucose is the main carbohydrate absorbed and used by the body for energy.

This efficient digestion supports healthy metabolism.

• Sequential enzyme action ensures complete digestion.
• Glucose is the absorbable form of carbohydrate.

Pepsin is a protease enzyme that starts protein digestion by breaking large protein molecules into smaller peptides.

It functions best in the acidic environment of the stomach, where hydrochloric acid lowers the pH to around 2.

This acidic condition activates pepsin and allows it to efficiently cleave protein bonds, preparing proteins for further digestion downstream.

• Pepsin breaks proteins into smaller peptides.
• Works optimally in acidic pH of the stomach.
• Activated by stomach acid (hydrochloric acid).

After leaving the stomach, the partly digested proteins enter the small intestine, where the environment is alkaline due to bicarbonate secretions.

Trypsin is a protease enzyme that continues protein digestion by breaking peptides into even smaller peptides and amino acids.

This alkaline pH (around 8) is ideal for trypsin activity, ensuring proteins are fully broken down into absorbable units.

• Trypsin breaks peptides into smaller peptides and amino acids.
• Functions best in alkaline pH of the small intestine.
• Completes protein digestion for absorption.

Different protease enzymes require specific pH conditions to work effectively.

The stomach’s acidic pH activates pepsin, while the small intestine’s alkaline pH activates trypsin.

This change in pH along the digestive tract ensures proteins are digested completely by different enzymes suited to each environment.

• Pepsin is active in acidic conditions.
• Trypsin is active in alkaline conditions.
• Sequential pH changes enable efficient protein digestion.

Bile is an alkaline (basic) substance produced by the liver and stored in the gallbladder.

It has a higher pH than the acidic gastric juices from the stomach.

• Bile is alkaline, meaning it has a pH above 7.
• Produced by the liver, stored in the gallbladder.

When food mixed with acidic gastric juices leaves the stomach, it enters the duodenum with a low pH (acidic).

Bile is secreted into the duodenum to neutralize this acidity.

This neutralization raises the pH to a level suitable for enzymes in the small intestine to function.

• Bile neutralizes the acid from gastric juices.
• Neutralization prevents damage to the intestinal lining.
• Creates a less acidic environment for enzymes.

Digestive enzymes in the small intestine work best at a neutral or slightly alkaline pH.

If the environment remains too acidic, enzymes become less effective or inactive.

By neutralizing acid, bile ensures digestion proceeds efficiently.

• Enzymes require optimal pH to function.
• Bile's neutralizing action supports enzyme activity.
• Efficient digestion depends on correct pH.