Mechanical vs Chemical Digestion and Digestive Enzymes

Mechanical digestion physically breaks food into smaller pieces without changing its chemistry, while chemical digestion uses enzymes to split large food molecules into small soluble ones. In brief: chewing and churning are mechanical, while the action of amylase on starch, proteases on proteins and lipases on fats is chemical. Amylase makes sugars from starch, proteases make amino acids from proteins, and lipases make fatty acids and glycerol from fats.

Both types of digestion are needed, because food molecules are too large to be absorbed and must be broken down. This guide describes each type, explains how digestive enzymes work and the conditions they need, lists the main enzymes in a table, works through a starch-digestion investigation and corrects the usual mistakes.

What is mechanical digestion?

Mechanical digestion is the physical breakdown of food into smaller pieces. The substances are not changed chemically, but the surface area becomes larger, so enzymes can reach more of the food.

• Chewing in the mouth: teeth cut, tear and grind food.
• Churning in the stomach: muscular walls squeeze and mix food with gastric juice.
• Segmentation and peristalsis in the intestines: muscle movements mix the contents and push them along.
• Emulsification of fats by bile: large droplets are broken into tiny droplets. This is physical, because bile contains no enzymes.

What is chemical digestion?

Chemical digestion is the breakdown of large molecules into their small building blocks by enzymes. The reactions are hydrolysis reactions, which means that water is used to break the bonds. Food molecules such as starch, proteins and triglycerides are polymers or large molecules, and they cannot cross the wall of the small intestine. Their small products can.

What are digestive enzymes?

Enzymes are biological catalysts made of protein. They speed up reactions without being used up. Each enzyme has an active site into which its specific substrate fits, so one enzyme usually acts on one type of molecule. Enzyme names often end in “-ase”, and the first part shows the substrate.

Enzymes work best under certain conditions. Most human digestive enzymes have an optimum temperature near 37 °C. At higher temperatures, the protein structure is damaged and the enzyme is denatured and stops working. Each also has an optimum pH. Pepsin in the stomach works best in strong acid (about pH 2), while enzymes in the small intestine work best in slightly alkaline conditions (about pH 7 to 8). This is the reason that stomach acid is neutralized when it enters the intestine.

What does amylase do?

Amylase breaks down starch into the sugar maltose. It is made by the salivary glands and the pancreas, so the digestion of starch starts in the mouth and continues in the small intestine. An enzyme called maltase, on the surface of the intestinal cells, then splits maltose into glucose, which can be absorbed. Amylase from saliva is inactivated by the acid in the stomach.

What do proteases do?

Proteases break the bonds between amino acids in proteins. Pepsin, from the stomach, cuts proteins into shorter chains. In the small intestine, trypsin and other proteases from the pancreas, together with enzymes from the intestinal wall, continue the process until single amino acids are released. Proteases are made in an inactive form to protect the cells that produce them from damage.

What do lipases do?

Lipases split fats into three fatty acids and one glycerol molecule. Most fat digestion is carried out by pancreatic lipase in the small intestine. Bile first emulsifies the fat to give the enzyme a bigger surface to work on. Fatty acids lower the pH slightly as they are formed.

Digestive enzymes at a glance

Worked example: investigating amylase

A student wants to test how temperature affects the digestion of starch by amylase. She mixes starch solution with amylase solution in several test tubes at different temperatures. Every 30 seconds she takes a drop from the mixture and adds it to iodine solution on a spotting tile. Iodine is blue-black with starch and stays orange-brown when starch is gone.

1. The rate is calculated as 1000 ÷ time: for 10 °C, 1000 ÷ 600 = 1.7; for 25 °C, 1000 ÷ 240 = 4.2; for 37 °C, 1000 ÷ 120 = 8.3.
2. The rate increases between 10 and 37 °C because the molecules move faster and collide more often.
3. At 60 °C the starch is never digested, because the enzyme has been denatured and its active site has changed shape.
4. Controlled variables are pH (use a buffer), volumes and concentrations of the solutions and the mixing method.

The conclusion is that the enzyme works fastest near body temperature and is destroyed by high heat. Testing temperatures between 37 and 60 °C would show the exact optimum.

How were enzymes discovered?

In 1833 Anselme Payen and Jean-François Persoz isolated a substance from malt that converted starch to sugar and called it diastase, the first enzyme found. Theodor Schwann isolated pepsin in 1836. In 1926 James Sumner showed that an enzyme, urease, is a protein, and he received the Nobel Prize in Chemistry in 1946. Modern X-ray methods now reveal the exact structure of active sites.

Which mistakes are common?

Mistake 1: Saying that heat “kills” enzymes

Enzymes are not alive. High temperature denatures them by changing their shape.

Mistake 2: Thinking that chewing is chemical digestion

Chewing is mechanical. The chemical part in the mouth is the action of amylase in saliva.

Mistake 3: Calling bile an enzyme

Bile is not an enzyme. It emulsifies fats physically.

Mistake 4: Assuming all enzymes work at the same pH

Pepsin prefers acid, whereas pancreatic enzymes work best in slightly alkaline conditions.