Chemical digestion — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

Section 1 of 25Why chemical digestion is needed

Chemical digestion

• The breakdown of large insoluble molecules into small soluble molecules.

Reason

• Large insoluble molecules cannot be absorbed through the wall of the small intestine.

Result

• Small soluble molecules are easily absorbed into the capillaries in the intestine wall.

Enzymes

• Enzymes are required for chemical digestion.

Section 2 of 25Digestive enzymes

Enzyme	Substrate	Product	Made in
Amylase	Starch	Smaller sugars (maltose)	Salivary glands and pancreas
Protease	Protein	Amino acids	Stomach and pancreas
Lipase	Lipids (fats and oils)	Fatty acids and glycerol	Pancreas, secreted into the duodenum

Section 3 of 25Hydrochloric acid in gastric juice

Gastric juice

• The stomach produces several fluids that together are called gastric juice, including hydrochloric acid.

Kills bacteria

• The low pH denatures the enzymes in bacterial cells so they cannot carry out cell reactions.

Optimum pH

• Pepsin has an optimum of about pH 2, and the acid keeps the stomach within this range.

Section 4 of 25Digestion of starch (Supplement)

Digestion of starch (Supplement)

Amylase is secreted in the mouth and, from the pancreas, into the duodenum.

Amylase digests starch to maltose, a disaccharide.

Maltase digests maltose to glucose on the membranes of the epithelium lining the small intestine.

Section 5 of 25Digestion of protein (Supplement)

Sites

• Protein digestion takes place in the stomach and the duodenum.

Pepsin

• Produced in the stomach, it breaks down protein in acidic conditions.

Trypsin

• Produced in the pancreas and secreted into the duodenum, it breaks down protein in alkaline conditions.

Section 6 of 25Role of bile (Supplement)

Made and stored

• Bile is made by liver cells and stored in the gall bladder.

Alkaline

• It neutralises the hydrochloric acid arriving from the stomach.
• Small intestine enzymes have a higher, more alkaline, optimum pH than stomach enzymes.

Emulsification

• It breaks large fat drops into smaller ones, giving lipase a larger surface area.

Section 7 of 25More from the specification

More from the specification

Nutrient assimilation by vital organs

• Nutrient assimilation by vital organs dietary iron, vitamins,
  • and amino acids are transported via systemic circulation to support the metabolism of internal organs.

Hydrolytic Enzymes of Chemical Digestion

• Specific biological catalysts cleave insoluble large polymers into soluble, permeable monomers capable of crossing intestinal epithelial membranes into the bloodstream.
  • (carbohydrases, proteases, and lipases)

Mechanical Preparations by Teeth and Stomach

• Physical mastication by specialized dentition and gastric muscular churning break food down physically, preventing digestive tract obstruction and maximizing substrate exposure to digestive juices.

Water and Mineral Absorption

• Water, mineral ions,
  • and water-soluble vitamins are absorbed across the intestinal epithelium down osmotic and electrochemical gradients without requiring prior chemical cleavage.

Scurvy

• A deficiency disease caused by lack of vitamin C.

Adaptation

• The process by which populations become better suited to their environment.

Growth

• A permanent increase in size and dry mass.

Digestive system

• Organ system that breaks down and absorbs food.

Physical digestion

• Breaking food into smaller pieces without chemical change.

Dietary fibre

• Indigestible plant material giving bulk to the intestine contents.

Vitamin

• A nutrient needed in small amounts to stay healthy.

Absorption

• Movement of small food molecules into the blood.

Organ

• A structure of different tissues working together for specific functions.

Section 8 of 25Why chemical digestion is needed

Why chemical digestion is needed

Large insoluble molecules

• Large insoluble molecules starch, protein and fat cannot pass through the gut wall.

Enzymes

• Enzymes hydrolyse them into small soluble molecules: glucose, amino acids, fatty acids and glycerol.

Absorption

• Absorption the small soluble molecules are easily absorbed into the capillaries in the wall of the small intestine.

Section 9 of 25Properties of enzymes

Enzymes

Enzymes biological catalysts made of protein that speed up reactions without being used up.

Specificity

• Specificity each enzyme has an active site that fits one substrate.

Optimum

• Optimum each works fastest at its optimum temperature and pH.

Denaturing

• Denaturing high temperature or extreme pH changes the shape of the active site, so the enzyme stops working.

Section 10 of 25Digestive enzymes and where they act

Enzyme	Made in	Acts in	Substrate to product	Optimum pH
Amylase	Salivary glands, pancreas	Mouth, small intestine	Starch to maltose	About 7
Maltase	Small intestine wall	Small intestine	Maltose to glucose	About 8
Protease (pepsin)	Stomach	Stomach	Protein to polypeptides	About 2
Protease (trypsin)	Pancreas	Small intestine	Protein to amino acids	About 8
Lipase	Pancreas	Small intestine	Fat to fatty acids and glycerol	About 8

Section 11 of 25Sequence of chemical digestion

1

Mouth

• Mouth salivary amylase digests starch to maltose, but food is there only briefly.

2

Stomach

• Stomach acid halts amylase and pepsin breaks proteins into shorter polypeptides.

3

Duodenum

• Duodenum pancreatic amylase, trypsin and lipase work in alkaline conditions produced by bile and sodium hydrogencarbonate.

4

Small intestine lining

• Small intestine lining maltase and peptidases on its surface complete digestion.

5

Carbohydrate

• Carbohydrate starch to maltose (amylase) to glucose (maltase).

6

Protein

• Protein protein to polypeptides (pepsin) to amino acids.
  • (trypsin and peptidases)

7

Fat

• Fat fat to fatty acids and glycerol (lipase), helped by bile emulsification.

Section 12 of 25Role of acid and bile

Hydrochloric acid

• Hydrochloric acid kills bacteria in food and gives the low pH (about 2) at which pepsin works best.
• Pepsin is denatured if it passes into the alkaline small intestine.

Bile

• Bile made in the liver and stored in the gall bladder; it is alkaline and contains no enzymes.
• It neutralises the acid so pancreatic enzymes have an alkaline pH, and emulsifies fat to increase the surface area for lipase.

Section 13 of 25Enzyme action and the lock and key model

Lock and key

• Lock and key only a substrate whose shape matches the active site binds, forming an enzyme-substrate complex.
• The reaction occurs, the products leave and the enzyme is unchanged and can be reused.

Temperature

• Temperature the rate rises from 0 °C as particles collide more, peaking at the optimum.
  • (about 37–40 °C for human enzymes)
• Above this the active site changes shape, so the graph falls sharply.

pH

• pH the rate against pH forms a bell shape with the peak at the optimum.
• This is about 2 for pepsin and 7–8 for amylase and trypsin.

Section 14 of 25Investigating amylase

1

Mix starch solution with amylase in a water bath at a set temperature.

2

At regular intervals take a sample and add it to iodine solution on a spotting tile.

3

Record the time taken for the iodine to stay brown, showing all starch has gone.

4

Repeat at different temperatures or pH values (using buffer solutions), keeping other variables constant.

Section 15 of 25Need for chemical digestion

1Large insoluble molecules

• Large insoluble molecules starch, protein and fat cannot pass through the gut wall.

2Enzymes

• Enzymes break them into small soluble molecules that can be absorbed.
  • (glucose, amino acids, fatty acids and glycerol)

Section 16 of 25Digestive enzymes

Enzyme	Site of action	Substrate to products
Amylase	Mouth, small intestine	Starch to maltose
Maltase	Small intestine	Maltose to glucose
Protease	Stomach (pepsin), small intestine	Protein to amino acids
Lipase	Small intestine	Fat to fatty acids and glycerol

Section 17 of 25How enzymes work

Enzymes

Enzymes protein catalysts that speed up reactions without being used up.

Active site

• Active site has a shape complementary to its specific substrate.
  • (lock and key model)

Enzyme-substrate complex

• Enzyme-substrate complex the substrate fits the site, the reaction occurs and the products leave.

Section 18 of 25Effect of temperature and pH

Temperature

• Temperature the rate increases as particles move faster and collide more often, up to the optimum.
  • (about 37 °C in humans)
• Above about 45 °C the active site changes shape, the substrate no longer fits and the enzyme is denatured permanently.

pH

• pH each enzyme has an optimum.
• Pepsin works best at about pH 2 in the stomach and amylase and lipase at pH 7–8;
  • extremes of pH denature the enzyme.

Low temperature

• Low temperature enzymes are inactive but not denatured, and they work again when warmed.

Section 19 of 25Effect of enzyme and substrate concentration

1Substrate concentration

• Substrate concentration more substrate raises the rate until all active sites are occupied, then the rate levels off.

2Enzyme concentration

• Enzyme concentration more enzyme raises the rate provided there is enough substrate.

3Graph of product against time

• Graph of product against time starts steep and flattens as substrate is used up;
  • the initial gradient gives the rate.

4In the gut

• In the gut enzymes are made in large quantities and substrate is supplied constantly, so digestion is efficient.

Section 20 of 25Investigating enzymes

1

Mix starch solution and amylase in a water bath.

2

Take a drop every minute and test it with iodine on a spotting tile.

3

Note the time when iodine stays yellow-brown, meaning all starch is digested.

4

Repeat with a changed temperature or pH, keeping other factors the same.

Section 21 of 25Interpreting enzyme data

Interpreting enzyme data

Times

• Times the shortest time for starch to disappear means the fastest rate; rate = 1 ÷ time.

Temperature curve

• Temperature curve rate rises to an optimum then drops sharply.

Zero rate at high temperature

• Zero rate at high temperature the enzyme has been denatured and cannot work even if cooled.

Section 22 of 25Digestive enzymes compared with respiratory enzymes

Location

• Location respiration enzymes work inside cells, whereas digestive enzymes are released into the gut (extracellular).

Inactive forms

• Inactive forms some are secreted inactive, such as pepsinogen activated by acid, to avoid digesting the cells that make them.

Duodenum pH

• Duodenum pH bile salts and pancreatic hydrogencarbonate raise it to around 8 so pancreatic and intestinal enzymes work at their optimum.

Section 23 of 25Uses of enzymes in industry

1Biological washing powders

• Biological washing powders contain proteases and lipases that break down stains.

2Fruit juice

• Fruit juice pectinase clears the juice.

3Lactose-free milk

• Lactose-free milk made using lactase.

4Food manufacture

• Food manufacture amylase converts starch to sugars.

Section 24 of 25Key terms

Term	Meaning
Chemical digestion	Breakdown of large insoluble molecules into small soluble molecules by enzymes.
Amylase	Enzyme that breaks starch into maltose.
Maltase	Enzyme that breaks maltose into glucose.
Protease	Enzyme that breaks protein into amino acids.
Pepsin	Protease made in the stomach with an optimum near pH 2.
Trypsin	Protease made by the pancreas that works in alkaline conditions.
Lipase	Enzyme that breaks fats and oils into fatty acids and glycerol.
Gastric juice	The fluids produced by the stomach, including hydrochloric acid.

Term	Meaning
Enzyme	Protein catalyst that speeds up a reaction without being used up.
Substrate	The molecule an enzyme acts on.
Active site	The part of the enzyme where the substrate binds.
Denatured	Permanently changed in shape so the enzyme no longer works.
Amylase	Enzyme that digests starch.
Protease	Enzyme that digests protein.
Lipase	Enzyme that digests fat.

Term	Meaning
Enzyme	A protein that acts as a biological catalyst.
Active site	Region of an enzyme where the substrate binds.
Denaturation	Permanent change in an enzyme's shape.
Optimum	The temperature or pH at which activity is highest.
Substrate	The substance acted on by an enzyme.
Protease	Enzyme that digests protein.

Section 25 of 25Exam technique

1

Saying salivary amylase works in the stomach — the acid there denatures it.

2

Saying bile digests fat chemically — bile only neutralises acid and emulsifies fat, and lipase digests it.

3

Giving every digestive enzyme pH 7 — pepsin works best at about pH 2 and the others in the small intestine are alkaline.

4

Thinking protease makes only one product — it breaks protein down into amino acids.

1

Saying enzymes are killed at high temperature — they are denatured; they were never alive.

2

Saying all enzymes work at pH 7 — pepsin works at about pH 2.

3

Writing that bile contains lipase — lipase is made in the pancreas.

4

Saying maltose is the final product of starch digestion — it is glucose.

1

Saying enzymes are living and are killed by heat — they are denatured.

2

Saying the enzyme is used up — it is unchanged and can be reused.

3

Saying cold denatures enzymes — cold only slows them.

4

Saying all digestive enzymes have the same optimum pH.

5

Saying one enzyme can digest all food types — each enzyme is specific.