Digestive system — Combined Science 0653 - Biology Revision Notes & Topical Questions

Section 1 of 25The digestive system

The digestive system

Organ system

• Organ system a group of organs that work together.

Function

• Function the digestive system digests food and absorbs nutrients.

Section 2 of 25The alimentary canal

Alimentary canal

Alimentary canal the tube from mouth to anus through which food passes directly.

Organs in order:

• Mouth
• Oesophagus
• Stomach
• Small intestine (duodenum and ileum)
• Large intestine (colon, rectum and anus)

Section 3 of 25Associated (accessory) organs

• Associated organs
  • organs involved in digestion that food does not pass through.
  • Salivary glands
  • Pancreas
  • Liver
  • Gall bladder

Section 4 of 25Stages of digestion

1Ingestion

• Ingestion food and drink are taken in through the mouth.

2Mechanical digestion

• Mechanical digestion food is broken into smaller pieces without chemical change.

3Chemical digestion

• Chemical digestion large insoluble molecules are broken down into small soluble molecules.

4Absorption

• Absorption small food molecules and ions pass through the wall of the intestine into the blood.

5Assimilation

• Assimilation absorbed nutrients are taken up by the body's cells.

6Egestion

• Egestion undigested and unabsorbed food leaves the body as faeces.

Section 5 of 25Assimilation

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

Section 6 of 25Egestion

1Faeces

• Faeces the solid waste left in the colon leaves via the anus.

2Egestion is not excretion

• Egestion is not excretion excretion removes waste made by cells, whereas egestion removes undigested food.

Section 7 of 25More from the specification

More from the specification

Proteolytic digestion yielding amino acids

• Proteolytic digestion yielding amino acids gastric and pancreatic endopeptidases hydrolyse dietary polypeptides into free amino acids and carboxylic acids.

Functions of Gastric Hydrochloric Acid

• Parietal cells in the stomach wall secrete concentrated hydrochloric acid (pH 1.5–2.0) to destroy ingested bacterial pathogens and establish the optimum acidic pH for pepsin activation.

Role of Dentition in Mechanical Breakdown

• Specialized incisors cut, canines tear,
  • and premolars and molars crush food items, drastically increasing the accessible surface area for subsequent hydrolytic enzyme action.

Bile Synthesis and Duodenal Neutralisation

• Alkaline bile produced by liver hepatocytes neutralises acidic gastric chyme entering the duodenum, protecting intestinal mucosa and activating pancreatic digestive enzymes.

Scurvy

• A disease caused by severe vitamin C deficiency.

Adaptation

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

Physical digestion

• Breakdown of food into smaller pieces without chemical change.

Amino acid

• The unit from which proteins are made.

Dietary fibre

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

Enzyme

• A protein that acts as a biological catalyst.

Vitamin

• A nutrient needed in small amounts to keep healthy.

Starch

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

Protein

• Nutrient needed for growth and repair.

Emulsification

• Breaking large drops of fat into smaller ones.

Calcium

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

Section 8 of 25Organs and their functions

Structure	Function
Mouth	Food is ingested and the teeth break it into smaller pieces
Salivary glands	Secrete saliva, which lubricates food and contains amylase that begins to digest starch to maltose
Oesophagus	Tube from mouth to stomach, in which wall contractions force food downwards
Stomach	Muscular walls churn food, protease begins protein digestion, and hydrochloric acid gives the right pH and kills pathogens
Liver	Produces bile
Gall bladder	Stores bile until it is released into the duodenum
Pancreas	Makes amylase, protease and lipase, which are released into the duodenum
Duodenum	Receives food from the stomach, where bile neutralises the acid and enzymes complete chemical digestion
Ileum	Food and water are absorbed into the blood through the villi
Colon	Remaining water is absorbed, leaving solid waste
Rectum	Stores faeces
Anus	Faeces leave the body, which is egestion

Section 9 of 25Peristalsis

Peristalsis

Peristalsis contraction of the muscles in the wall of the oesophagus that forces food downwards.

Muscle action

• Muscle action circular muscles behind the food contract to squeeze it forward while longitudinal muscles ahead contract to widen the tube.

Position

• Position it works even when a person is upside down.

Section 10 of 25Swallowing

Bolus

• Bolus the tongue forms food into a ball.

Epiglottis

• Epiglottis it covers the windpipe so that food goes down the oesophagus.

Section 11 of 25The liver, gall bladder and pancreas

Liver

• Liver makes bile, which neutralises stomach acid and emulsifies fats.

Gall bladder

• Gall bladder stores bile before release along the bile duct.

Pancreas

• Pancreas makes the three types of digestive enzyme and releases them into the duodenum.

Section 12 of 25Small intestine

Duodenum

• Duodenum the first part, where the acidic stomach contents are neutralised and become slightly alkaline.

Ileum

• Ileum the second part, where the products of digestion are absorbed.

Section 13 of 25Large intestine

Colon

• Colon absorbs remaining water, so solid waste remains.

Rectum and anus

• Rectum and anus faeces are stored in the rectum and leave through the anus.

Section 14 of 25More from the specification

More from the specification

Macronutrient assimilation by somatic cells

• Macronutrient assimilation by somatic cells hydrolysed amino acids and monosaccharides cross capillary walls to fuel metabolic protein synthesis within living cells.

Emulsification

• Emulsification bile salts physically break large lipid droplets into smaller droplets, increasing the surface area for lipase action.

Lipid digestion

• Lipid digestion lipase breaks down emulsified fat molecules into glycerol and fatty acids within the small intestine.

Absorption in the intestine

• Absorption in the intestine microvilli vastly increase surface area for rapid absorption of amino acids, fatty acids, and water-soluble vitamins.

Mechanical Digestion and Bile Emulsification

• Bile produced by the liver and stored in the gall bladder performs emulsification, breaking large lipid globules into small droplets to increase surface area for lipase enzymes to digest fats into fatty acids and glycerol.

Absorption in the Small Intestine

• The lining of the small intestine (ileum) is adapted for efficient absorption of digested food molecules into the bloodstream via millions of microscopic villi and microvilli, providing an extensive surface area.

Dietary Energy Requirements and Nutrients

• The quantity of metabolic energy needed daily varies with age, biological sex,
• and physical activity level, balanced alongside essential micronutrients including vitamin C and vitamin D (preventing rickets).
  • (preventing scurvy)

Roles of Digestive System Organs

• Digestive organs coordinate chemical and mechanical breakdown: the stomach secretes hydrochloric acid and proteases, the pancreas produces digestive enzymes,
• and the large intestine absorbs remaining water.

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.

Amino acid

• The unit from which proteins are made.

Dietary fibre

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

Molecule

• Two or more atoms joined together chemically.

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.

Calcium

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

Section 15 of 25Physical digestion

Physical digestion

Physical (mechanical) digestion

Physical (mechanical) digestion the breakdown of food into smaller pieces without chemical change to the food molecules.

Purpose

• Purpose it increases the surface area of food so that enzymes can act on it faster.

Methods

• Methods the chewing action of the teeth, the churning action of the stomach and the emulsification of fats by bile in the duodenum.

Section 16 of 25Types of teeth

Tooth	Shape	Function
Incisor	Chisel-shaped	Biting and cutting
Canine	Pointed	Tearing, holding and biting
Premolar and molar	Large, flat with ridges	Chewing and grinding

Section 17 of 25Role of teeth

Position

• Position teeth are embedded in the gums and the bone of the jaw.

Chewing

• Chewing it increases the surface area of food and exposes it to saliva and enzymes.

Section 18 of 25Structure of a tooth

Part	Description
Enamel	Hard outer layer that protects the tooth
Dentine	Layer beneath the enamel that protects the nerves
Pulp	Soft inner tissue containing nerves and blood vessels
Cement	Hard layer around the root that anchors the tooth in the jaw

Section 19 of 25Tooth decay

Cause

• Cause bacteria feed on sugar in the mouth and make acid, which dissolves enamel.

Prevention

• Prevention brushing, eating less sugar and using fluoride toothpaste.

Section 20 of 25The stomach

Churning

• Churning muscles in the stomach wall contract to squeeze and mix food with the digestive juices.

Time

• Time food is digested in the stomach for several hours.

Tissues

• Tissues the stomach contains muscular, epithelial and glandular tissue.
• Mucus protects the stomach lining from acid and enzymes.

Section 21 of 25Chyme

• Chyme
  • churning makes a semi-liquid that is released a little at a time into the duodenum.

Section 22 of 25Emulsification by bile

• Bile
  • breaks large drops of fat into small droplets, which gives a larger surface area for lipase.
  • This is physical digestion because no chemical bonds are broken.

Section 23 of 25More from the specification

1Dietary nutrients supporting somatic cells

• Dietary nutrients supporting somatic cells balanced intake of amino acids, vitamins,
  • and minerals provides the biochemical building blocks required by human cells.

2Dietary energy requirements

• Dietary energy requirements daily energy intake depends on age and activity, increasing during pregnancy to sustain the developing foetus.

3Pancreas secretions

• Pancreas secretions the pancreas produces digestive enzymes that enter the small intestine to complete chemical breakdown of starch and lipids.

4Nutrient absorption

• Nutrient absorption soluble molecules pass through the lining of the small intestine into the bloodstream via specialized villi.

5Chemical digestion of starch

• Amylase enzymes produced by salivary glands and the pancreas hydrolyse dietary starch into maltose,
• which is then broken down into glucose by maltase enzymes for cellular energy release.

6Role of the pancreas and digestive enzymes

• The pancreas is a vital digestive organ that secretes pancreatic juice containing amylase, lipase,
• and trypsin proteases into the duodenum to complete nutrient digestion.

7Nutrient absorption in the small intestine

• The ileum of the small intestine possesses an extended inner surface area lined with villi and microvilli, facilitating rapid absorption of monosaccharides, amino acids,
• and water-soluble vitamins into capillary networks.

8Dietary vitamins and health

• Essential vitamins are micronutrients needed in small quantities to maintain metabolic functions, immune defenses,
  • (such as vitamin C and vitamin D)
• and bone mineralisation across bodily organs.

9Calcium

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

10Scurvy

• A disease caused by severe vitamin C deficiency.

11Adaptation

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

12Growth

• Permanent increase in size.

13Amino acid

• The unit from which proteins are made.

14Dietary fibre

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

15Vitamin

• A nutrient needed in small amounts to keep healthy.

16Absorption

• Movement of digested molecules into the blood.

17Starch

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

18Organ

• Different tissues working together to carry out a function.

19Pancreas

• Gland making insulin and glucagon

20Absorbed

• Taken in by a surface instead of being reflected.

Section 24 of 25Key terms

Term	Meaning
Alimentary canal	The tube from mouth to anus through which food passes.
Accessory organ	An organ that helps digestion but is not part of the food route.
Ingestion	Taking food into the body through the mouth.
Absorption	Movement of digested molecules into the blood.
Assimilation	Uptake of digested molecules by cells.
Egestion	Removal of undigested food as faeces.
Organ system	A group of organs working together.

Term	Meaning
Peristalsis	Waves of muscle contraction that push food along.
Bile	An alkaline liquid made in the liver that helps fat digestion.
Duodenum	The first part of the small intestine.
Ileum	The part of the small intestine where food is absorbed.
Colon	The part of the large intestine that absorbs water.
Saliva	A liquid from the salivary glands that contains amylase.
Bolus	A ball of chewed food.

Term	Meaning
Physical digestion	Breakdown of food into smaller pieces without chemical change.
Incisor	A chisel-shaped tooth for cutting.
Canine	A pointed tooth for tearing.
Molar	A large flat tooth for grinding.
Enamel	The hard outer layer of a tooth.
Dentine	The layer beneath the enamel.
Emulsification	Breaking large drops of fat into smaller ones.

Section 25 of 25Exam technique

1

Saying food passes through the liver and pancreas — they are accessory organs.

2

Confusing egestion with excretion — egestion removes undigested food.

3

Saying digestion and absorption are the same — digestion breaks food down and absorption moves it into the blood.

4

Saying the alimentary canal includes the liver — it includes only organs through which food passes.

1

Saying bile is made in the gall bladder — it is made in the liver and stored in the gall bladder.

2

Saying most digestion happens in the large intestine — it mainly absorbs water.

3

Saying the pancreas makes bile — the pancreas makes digestive enzymes.

4

Saying the stomach absorbs most nutrients — nutrients are absorbed in the ileum.

1

Saying emulsification is chemical digestion — it is physical digestion.

2

Saying the enamel is the soft inner part of a tooth — it is the hard outer layer.

3

Confusing incisors and molars — incisors cut and molars grind.

4

Saying churning changes the food molecules — it only breaks the food into smaller pieces.