Digestion — Combined Science 0653 - Biology Revision Notes & Topical Questions

Section 1 of 27Chemical digestion

Chemical digestion

Chemical digestion the breakdown of large insoluble molecules into small soluble molecules by enzymes.

Purpose

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

Result

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

Section 2 of 27Role of enzymes

Enzymes

• Enzymes biological catalysts that speed up the breakdown of food.

Specificity

• Specificity each enzyme acts on one substrate because its active site has a complementary shape.

Section 3 of 27Digestive enzymes

Enzyme	Made in	Substrate	Product	Acts in
Amylase	Salivary glands and pancreas	Starch	Maltose	Mouth and duodenum
Maltase	Wall of the small intestine	Maltose	Glucose	Small intestine
Protease (pepsin)	Stomach	Protein	Smaller polypeptides	Stomach
Protease (trypsin)	Pancreas	Protein	Amino acids	Duodenum
Lipase	Pancreas	Lipids	Fatty acids and glycerol	Duodenum

Section 4 of 27Digestion of starch

Amylase

• Amylase digests starch into maltose, a disaccharide.

Maltase

• Maltase digests maltose into glucose on the membranes of the epithelium of the small intestine.

Section 5 of 27Digestion of protein

Protease

• Protease a group of enzymes that break proteins into amino acids.

Pepsin

• Pepsin made in the stomach and works in acid conditions.

Trypsin

• Trypsin made in the pancreas and works in the alkaline duodenum.

Section 6 of 27Digestion of lipids

Lipase

• Lipase made in the pancreas and released into the duodenum.

Products

• Products lipids are broken into fatty acids and glycerol.

Section 7 of 27Optimum pH

Stomach

• Stomach pepsin has an optimum pH of about 2.

Small intestine

• Small intestine enzymes have a higher optimum pH than those in the stomach.

Amylase in the mouth

• Amylase in the mouth works best at about pH 7.

Section 8 of 27Fate of the products

Glucose

• Glucose carried in the blood to cells for respiration, with extra stored as glycogen.

Amino acids

• Amino acids used to make new proteins.

Fatty acids and glycerol

• Fatty acids and glycerol rebuilt into fats for energy storage and cell membranes.

Section 9 of 27More from the specification

More from the specification

Mammalian digestive tract organ system

• Mammalian digestive tract organ system coordinated peristaltic contractions propel luminal contents through specialized visceral organs comprising the alimentary system.

Chemical Hydrolysis of Dietary Macromolecules

• Amylase degrades starch into maltose, pepsin and trypsin hydrolyse proteins into peptides and amino acids,
  • and lipase cleaves lipids into fatty acids and glycerol.

Emulsification Mechanism by Bile Salts

• Bile salts mechanically divide large lipid globules into microscopic suspended emulsion droplets, multiplying the water-lipid interfacial area for rapid pancreatic lipase access.

Water Absorption in the Large Intestine

• Following terminal enzymatic nutrient absorption in the ileum, the colon reabsorbs the majority of remaining water and mineral salts, condensing indigestible residue into semisolid faeces.

Scurvy

• A disease caused by severe vitamin C deficiency.

Adaptation

• A feature that makes something well suited to what it does.

Growth

• Permanent increase in size.

Physical digestion

• Breakdown of food into smaller pieces without chemical change.

Dietary fibre

• Plant material that gives bulk to the food in the gut.

Vitamin

• A nutrient needed in small amounts to keep healthy.

Absorption

• Movement of digested molecules into the blood.

Emulsification

• Breaking large drops of fat into smaller ones.

Organ

• Different tissues working together to carry out a function.

Calcium

• A mineral needed for bones, teeth and blood clotting.

Section 10 of 27Gastric juice

Gastric juice

• Gastric juice the group of fluids made by the stomach.

Hydrochloric acid

• Hydrochloric acid one of these fluids.

Section 11 of 27Hydrochloric acid

Kills bacteria

• Kills bacteria the low pH denatures the enzymes in bacterial cells, so they cannot carry out the reactions that keep them alive.

Right pH for pepsin

• Right pH for pepsin pepsin has a very low optimum pH of about 2.
• The acid keeps the stomach in the optimum range so that pepsin works at its fastest rate.

Section 12 of 27Bile

Source

• Source liver cells make bile, which is stored in the gall bladder.

Alkaline

• Alkaline bile neutralises the hydrochloric acid that comes from the stomach.
• Enzymes in the small intestine have a higher, more alkaline optimum pH than those in the stomach.

Emulsification

• Emulsification bile breaks large drops of fat into smaller ones.
• The larger surface area lets lipase break the lipid into glycerol and fatty acids faster.

Not an enzyme

• Not an enzyme bile does not chemically digest fat.

Section 13 of 27Mechanical not chemical

• Comparison
  • emulsification is like tearing a large piece of paper into smaller pieces.
  • No bonds are broken, so it is mechanical digestion, not chemical digestion.

Section 14 of 27Digestion in different parts of the gut

Region	Enzyme	Conditions
Mouth	Amylase	Near neutral
Stomach	Pepsin	Acid, about pH 2
Duodenum	Amylase, trypsin, lipase	Alkaline after neutralisation by bile
Small intestine wall	Maltase	Alkaline

Section 15 of 27Investigating starch digestion

1

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

2

At timed intervals remove a drop and add it to iodine solution on a spotting tile.

3

Record the time at which the iodine no longer turns blue-black.

4

Repeat at other temperatures or pH values using buffer solutions.

Controls

• Controls keep the volume and concentration of enzyme and starch the same.

Meaning

• Meaning when iodine stays orange-brown all the starch has been digested.

Section 16 of 27More from the specification

1Nutrient transport to peripheral organs

• Nutrient transport to peripheral organs absorbed macronutrients and essential vitamins circulate throughout systemic vasculature to maintain vitality across all organ tissues.

2Mechanical Digestion by Teeth

• Mechanical breakdown of food by teeth increases surface area, facilitating subsequent chemical digestion by enzymes throughout the digestive system.

3Chemical Breakdown of Dietary Protein

• Protease enzymes break down large protein molecules into small soluble amino acids suitable for absorption into the blood.
  • (such as pepsin and trypsin)

4Nutrient Absorption in the Ileum

• The lining of the small intestine is adapted for nutrient absorption, transporting glucose and amino acids into blood capillaries to supply cellular energy.

5Essential Vitamins and Minerals

• A balanced diet requires small amounts of micronutrients to prevent nutritional deficiencies.
  • (such as vitamin C for tissue repair and vitamin D for calcium uptake)

6Calcium

• A mineral needed for bones, teeth and blood clotting.

7Scurvy

• A disease caused by severe vitamin C deficiency.

8Adaptation

• A feature that makes something well suited to what it does.

9Physical digestion

• Breakdown of food into smaller pieces without chemical change.

10Amino acid

• The unit from which proteins are made.

11Dietary fibre

• Plant material that gives bulk to the food in the gut.

12Molecule

• Two or more atoms joined together chemically.

13Vitamin

• A nutrient needed in small amounts to keep healthy.

14Absorption

• Movement of digested molecules into the blood.

15Protein

• Nutrient needed for growth and repair.

16Organ

• Different tissues working together to carry out a function.

17Pancreas

• Gland making insulin and glucagon

18Absorbed

• Taken in by a surface instead of being reflected.

Section 17 of 27Absorption

Absorption

Absorption the movement of digested food molecules from the digestive system into the blood or lymph.

Site

• Site nutrients are absorbed in the small intestine.

Section 18 of 27Where the products go

Product	Absorbed into
Glucose	Blood
Amino acids	Blood
Fatty acids	Lymph
Glycerol	Lymph

Section 19 of 27Absorption of water

Where

• Where water is absorbed in both the small intestine and the colon.

Amount

• Amount most absorption of water, about 80%, happens in the small intestine.

Section 20 of 27Assimilation

• Assimilation
  • digested food molecules are taken up by cells and used for growth, repair and energy production.

Section 21 of 27Adaptations of the ileum

Length

• Length the ileum is very long.

Folded surface

• Folded surface it is highly folded, with millions of villi, which are tiny finger-like projections.

Surface area

• Surface area these features greatly increase the surface area, so absorption is faster and more efficient.

Section 22 of 27Adaptations of a villus

Feature	Function
Wall one cell thick	Short distance for diffusion and active transport
Network of blood capillaries	Carries glucose and amino acids away in the blood
Lacteal in the centre	Carries fatty acids and glycerol away in the lymph
Microvilli on surface cells	Increase the surface area further

Section 23 of 27Steep concentration gradient

• Removal of products
  • blood flow carries absorbed molecules away.
  • This keeps a steep concentration gradient so more molecules are absorbed.

Section 24 of 27Mechanisms of absorption

Diffusion

• Diffusion some molecules cross the villus wall by diffusion.

Active transport

• Active transport glucose and other molecules can also be taken up by active transport.

Section 25 of 27More from the specification

1Nutrient distribution to vital organs

• Nutrient distribution to vital organs systemic arterial circulation transports dissolved glucose, amino acids,
• and oxygen to maintain cellular respiration throughout internal organs.

2Mechanical Digestion by Teeth and Stomach Churning

• Mechanical digestion physically breaks down food into smaller pieces without altering chemical bonds: teeth chew and grind food into a bolus,
• while muscular churning in the stomach increases surface area for digestive enzymes.

3Chemical Digestion in the Stomach and Duodenum

• In the stomach, hydrochloric acid creates an acidic pH (~2) to kill pathogens and activate pepsin for protein breakdown;
  • in the duodenum, alkaline bile and pancreatic enzymes digest starch, proteins,
• and lipids.

4Bile Emulsification of Dietary Lipids

• Bile salts secreted into the duodenum perform emulsification, converting large fat droplets into tiny lipid droplets to greatly expand the surface area available for pancreatic lipase action.

5Essential Nutrients and Dietary Vitamins

• In addition to carbohydrates and proteins, a balanced human diet requires essential micronutrients such as vitamin C and vitamin D / calcium (strengthening bones and teeth).
  • (collagen synthesis)

6Calcium

• A mineral needed for bones, teeth and blood clotting.

7Scurvy

• A disease caused by severe vitamin C deficiency.

8Chemical digestion

• Breakdown of large insoluble molecules into small soluble molecules.

9Physical digestion

• Breakdown of food into smaller pieces without chemical change.

10Dietary fibre

• Plant material that gives bulk to the food in the gut.

11Enzyme

• A protein that acts as a biological catalyst.

12Vitamin

• A nutrient needed in small amounts to keep healthy.

13Duodenum

• The first part of the small intestine.

14Starch

• The storage carbohydrate in plants, which turns iodine blue-black.

15Protein

• Nutrient needed for growth and repair.

16Emulsification

• Breaking large drops of fat into smaller ones.

17Organ

• Different tissues working together to carry out a function.

18Pancreas

• Gland making insulin and glucagon

Section 26 of 27Key terms

Term	Meaning
Chemical digestion	Breakdown of large insoluble molecules into small soluble molecules.
Amylase	An enzyme that digests starch.
Protease	An enzyme that digests protein.
Lipase	An enzyme that digests lipids.
Maltase	An enzyme that digests maltose to glucose.
Pepsin	A protease made in the stomach.
Substrate	The substance an enzyme acts on.
Optimum pH	The pH at which an enzyme works fastest.

Term	Meaning
Gastric juice	The mixture of fluids made by the stomach.
Hydrochloric acid	The acid in the stomach that kills bacteria and gives pepsin its pH.
Pepsin	A protease with an optimum pH of about 2.
Bile	An alkaline liquid that neutralises acid and emulsifies fat.
Emulsification	Breaking large fat drops into smaller droplets.
Buffer solution	A solution that keeps the pH constant.

Term	Meaning
Absorption	Movement of digested food molecules into the blood or lymph.
Villus	A finger-like projection of the ileum wall.
Lacteal	A lymph vessel in the centre of a villus.
Microvilli	Tiny projections on the surface of villus cells.
Lymph	A fluid carried in lymph vessels.
Assimilation	Uptake of digested molecules by cells.

Section 27 of 27Exam technique

1

Saying enzymes are used up — they are not changed in the reaction.

2

Saying all digestive enzymes work best in acid — only stomach enzymes do.

3

Saying bile is an enzyme — bile is not an enzyme.

4

Mixing up substrate and product — starch is the substrate and maltose the product.

1

Saying hydrochloric acid digests protein — pepsin digests protein and the acid provides the pH.

2

Saying bile contains enzymes — it does not.

3

Saying emulsification is chemical digestion — no bonds are broken.

4

Saying the whole gut has the same pH — the stomach is acid and the duodenum is alkaline.

1

Saying all products enter the blood — fatty acids and glycerol enter the lacteal.

2

Saying most water is absorbed in the colon — about 80% is absorbed in the small intestine.

3

Saying villi have thick walls — their walls are one cell thick.

4

Confusing absorption with assimilation — absorption is into the blood and assimilation is into the cells.