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IGCSE Biology 0610 · Chapter 6 notes · ~10 min read

Human nutrition

A balanced diet, scurvy and rickets, the digestive system organ by organ, teeth, enzymes and bile, and how the villi absorb food — with the exact definitions examiners want.

Balanced dietDigestive systemDigestion & enzymesAbsorptionTeeth
ASby Aiza Ma'am
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • A balanced diet has carbohydrate, protein, fat, vitamins, minerals, fibre and water in the right amounts. Scurvy = lack of vitamin C; rickets = lack of vitamin D (or calcium).
  • Digestion breaks large insoluble molecules into small soluble ones. Physical digestion: teeth, stomach churning, bile. Chemical digestion: enzymes amylase, protease, lipase.
  • Absorption happens mostly in the small intestine, which has villi. Assimilation uses the food in the cells. Egestion removes undigested food.

By the end you can

  • Name the components of a balanced diet, their uses and sources.
  • Describe the causes and effects of scurvy and rickets.
  • Identify the organs of the alimentary canal and state their functions.
  • Describe the structure and functions of the types of tooth.
  • State where amylase, protease and lipase are made and act, and what they produce.
  • Explain how bile helps digestion. EXT
  • Describe how the small intestine and villi are adapted for absorption. EXT

1A balanced diet

What the body needs

CARBOHYDRATEUSEenergySOURCESrice, bread,potatoesPROTEINUSEgrowth andrepair of cellsSOURCESfish, eggs,beans, meatFAT (LIPID)USEenergy store,insulationSOURCESbutter, oils,nutsVITAMINSUSEC: skin, gumsD: bonesSOURCESfruit, veg,dairy, sunlightMINERALSUSECa: bones, teethFe: haemoglobinSOURCESmilk, cheese,red meatFIBREUSEkeeps food moving(peristalsis)SOURCESwhole grains,vegetablesWATERUSEsolvent, transport,reactions in cellsSOURCESdrinks, fruit,vegetablesA balanced diet has the right amount of all seven
Seven components, their main use and some sources.
  • Carbohydrates — energy. Fats — energy store and thermal insulation. Proteins — growth and repair of cells.
  • Vitamins and minerals are needed in tiny amounts: vitamin C keeps skin and gums healthy; vitamin D helps the body absorb calcium; calcium is needed for bones and teeth; iron is needed to make haemoglobin.
  • Fibre (mostly cellulose) is not digested, but it keeps food moving through the gut by peristalsis.
  • Water is the solvent for chemical reactions, the main part of blood, and is needed to remove wastes.
  • Energy needs depend on age, activity level and pregnancy. A teenager playing sport needs more than an adult with an office job.

Deficiency diseases

SCURVYlack of VITAMIN Cbleeding gums, slow-healingwounds, loose teethprevent:citrus fruit, tomatoes, peppersRICKETSlack of VITAMIN D (or calcium)normalricketssoft, weak bones in children(vitamin D helps absorb calcium)prevent:oily fish, eggs, dairy, sunlight on skinBoth are DEFICIENCY diseases: caused by missing nutrients
Scurvy and rickets are caused by lacking particular nutrients.
  • Scurvy — lack of vitamin C. Gums bleed, wounds heal slowly. Prevent it with citrus fruit and fresh vegetables.
  • Rickets — lack of vitamin D (or calcium). Bones are soft and bend, causing bowed legs in children. Prevent it with oily fish, eggs, dairy and sunlight.
Common mistake"Scurvy is lack of vitamin D." Scurvy goes with C: scurvy — vitamin C. Rickets goes with vitamin D (and calcium) for bones.

2The digestive system

The alimentary canal

salivaryglandsoesophagusstomachlarge intestinerectummouthlivergall bladderpancreassmallintestineanus(not to scale)
The alimentary canal is a long tube from mouth to anus. The liver, gall bladder and pancreas are associated organs.
OrganFunction
mouthingestion; teeth chew food (physical digestion); saliva contains amylase
oesophaguscarries food to the stomach by peristalsis
stomachmuscular wall churns food; hydrochloric acid kills bacteria; protease (pepsin) digests protein
livermakes bile
gall bladderstores bile and releases it into the small intestine
pancreasmakes amylase, protease and lipase, released into the small intestine
small intestinedigestion is finished here; absorption of digested food and most water
large intestine (colon)absorbs water; the remaining waste becomes faeces
rectum and anusrectum stores faeces; egestion through the anus
INGESTIONtaking foodand drinkinto themouthDIGESTIONbreaking biginsoluble foodinto smallsoluble piecesABSORPTIONsmall moleculespass into theblood (mostlysmall intestine)ASSIMILATIONcells use themolecules forenergy, growthand repairEGESTIONremovingundigestedfood (faeces)via the anusDigestion = physical (teeth, churning) + chemical (enzymes)Egestion is NOT excretion: faeces were never part of body cells
The five stages of nutrition in animals.

Ingestion — taking substances (food and drink) into the body through the mouth.

Digestion — the breakdown of large, insoluble food molecules into small, soluble molecules. It can be physical or chemical.

Absorption — the movement of digested food molecules through the wall of the intestine into the blood or lymph.

Assimilation — the movement of digested food molecules into the cells of the body, where they become part of the cells.

Egestion — the passing out of food that has not been digested or absorbed (faeces).

Common mistakeConfusing egestion (undigested food leaving the gut) with excretion (waste made by cells, like urea and CO₂).
squeezerelaxedtime 1squeezerelaxedtime 2squeezerelaxedtime 3Peristalsis: waves of muscle contraction push food along the gutcircular muscles squeeze behind the food (bolus) · longitudinal muscles shorten the gut
Waves of muscle contraction push food along the gut.
  • Peristalsis moves food along the oesophagus and intestines. Circular muscles squeeze behind the food and relax in front of it.

3Physical digestion

Teeth

INCISORcut and bitefront, chisel-shapedCANINEtear and grippointedPREMOLARcrush and grind2 bumps (cusps)MOLARcrush and grindbroad, 4+ cuspsFour types of tooth: different shapes, different jobsTeeth chew food: this is PHYSICAL digestion (bigger surface area for enzymes)
Incisors cut, canines tear, premolars and molars crush and grind.
  • Physical digestion breaks food into smaller pieces, giving a larger surface area for enzymes to work on.
  • Incisors — cut and bite. Canines — tear and grip. Premolars and molars — crush and grind.
enamel(hardest tissue)dentine(bone-like)pulp cavity(nerves, bloodvessels)gumcement + jawbone(anchor the root)Section through a toothcrownrootSugar → bacteria make acid → acid dissolves enamel: decay
A tooth has a hard enamel coat, softer dentine, and a pulp cavity with nerves and blood vessels.
  • Enamel — the hardest substance in the body: protects the tooth. Dentine — bone-like, softer. Pulp cavity — nerves and blood vessels. Cement — fixes the root into the jawbone.
  • Tooth decay: bacteria in the mouth feed on sugar and make acid, which dissolves the enamel. Brushing, less sugar and fluoride reduce decay.

The stomach and bile

  • The stomach's muscular walls churn food with gastric juice (acid and enzyme): more physical digestion.
1 fat globule2 bile added3 emulsifiedone big droplet:small surface areabile salts (green)break it upmany tiny droplets:large surface areaBile (made in the liver, stored in the gall bladder) does NOT contain enzymesIt emulsifies fat and is alkaline: it neutralises stomach acid in the small intestine
EXT Bile breaks large fat droplets into tiny ones. This is emulsification.
  • EXT Bile is made in the liver, stored in the gall bladder and released into the small intestine. It has no enzymes.
  • EXT Bile emulsifies fats: it breaks large globules into tiny droplets, giving a larger surface area for lipase to work on.
  • EXT Bile is alkaline: it neutralises the hydrochloric acid from the stomach so that the pancreatic enzymes (optimum pH about 8) work properly.
Common mistake"Bile digests fats." Bile does not digest (no enzymes). It only emulsifies. Lipase digests fats.

4Chemical digestion

Enzymes and their products

enzyme (above the arrow) · where it works (below)STARCHamylasemouth + small int.maltosemaltasesmall intestineGLUCOSEPROTEINprotease (pepsin, trypsin)stomach (pH 2) + small intestineAMINO ACIDSFATlipase (after bile has emulsified the fat)small intestineFATTY ACIDS+ GLYCEROLLarge insoluble molecules → small soluble molecules that can be absorbedEach enzyme is specific: amylase only starch, protease only protein, lipase only fatStomach acid (HCl) kills bacteria and gives pepsin its optimum pH of about 2
Three types of digestive enzyme and their end products. Each works on only one type of food.
EnzymeMade inWorks inSubstrate → products
amylasesalivary glands, pancreasmouth, small intestinestarch → maltose
maltasewall of the small intestinesmall intestinemaltose → glucose
protease (pepsin in the stomach)stomach, pancreasstomach, small intestineprotein → amino acids
lipasepancreassmall intestinefats → fatty acids and glycerol
  • The small intestine receives juices from the pancreas and bile from the liver, so most chemical digestion is finished there.
  • Hydrochloric acid in the stomach kills bacteria in food and gives the right acidity (about pH 2) for pepsin.
  • EXT Digestion of starch happens in two steps: amylase makes maltose, then maltase makes glucose.
Worked example

Follow a cheese sandwich (bread contains starch; cheese contains protein and fat). Name the enzymes and the products at each stage.

  1. mouth: salivary amylase starts digesting starch into maltose
  2. stomach: pepsin (protease) digests protein into smaller chains; acid kills bacteria
  3. small intestine: bile emulsifies fat, lipase makes fatty acids and glycerol, protease finishes protein to amino acids, amylase and maltase finish starch to glucose
Exam tipQuick recall: Amylase — starch; Protease — protein; Lipase — lipid. Amylase → starch, Protease → protein, Lipase → lipid (fat).

5Absorption

The small intestine and villi

glucose +amino acidsfatty acids +glycerolepithelium(one cell thick)microvilli(huge surface area)capillary network(blood)lacteal(lymph vessel)One villus (finger-like projection of the small intestine wall)glucose + amino acids → capillary → hepatic portal vein → liverfatty acids + glycerol → lacteal → lymph → blood
A villus. The wall is one cell thick and there is a rich blood supply.
  • The small intestine is about 6 m long. Its inner surface is covered in finger-like villi, and each villus cell has microvilli. This gives a huge surface area.
  • Each villus has a network of capillaries (blood) and a lacteal (a lymph vessel).
Feature of the villusHow it helps absorption
huge surface area (villi + microvilli)more room for absorption
wall one cell thickshort diffusion distance
rich blood supply (capillaries)carries away absorbed food, keeping a steep concentration gradient
lactealabsorbs fatty acids and glycerol
  • Glucose and amino acids pass into the capillaries by diffusion and active transport, then go to the liver in the hepatic portal vein.
  • Fatty acids and glycerol pass into the lacteal.
  • Water is absorbed in the small intestine and the colon (large intestine).

6Test yourself

Test yourself — cover the answers first

Quick check

  1. Name the nutrient needed for growth and repair.
  2. What deficiency disease is caused by lack of vitamin C?
  3. Which organ stores bile?
  4. Name the enzyme that digests starch and the product.
  5. Name the end products of protein digestion.
  6. What is the function of hydrochloric acid in the stomach?
  7. Define absorption.
  8. EXT What is the function of bile?
  9. EXT Name the vessel in a villus that absorbs fatty acids and glycerol.

Exam-style

  1. Explain why chewing food helps digestion. [2]
  2. Describe how the small intestine is adapted for absorption. [4]
  3. EXT Explain why bile is needed for fat digestion, although it contains no enzymes. [3]
  4. Name two food groups, a source and a function of each. [4]
Show answers
  1. protein
  2. scurvy
  3. gall bladder
  4. amylase; maltose
  5. amino acids
  6. kills bacteria and gives the right pH for pepsin
  7. movement of digested food molecules through the wall of the intestine into the blood or lymph
  8. emulsifies fats and neutralises stomach acid
  9. lacteal
  10. breaks food into smaller pieces
    larger surface area for enzymes
  11. very long, folded with villi and microvilli
    large surface area
    wall one cell thick: short diffusion distance
    good blood supply to carry food away
  12. bile emulsifies fat into small droplets
    larger surface area for lipase
    bile is alkaline so it provides the right pH for lipase
  13. e.g. protein: fish — growth and repair
    carbohydrate: rice — energy
Practise this chapter
DietDigestive system & digestionAbsorption
Next chapterTransport in plants →
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