Digestion: The breakdown of large insolbule molecules into smaller soluble ones

  • Physical: Teeth break down large food pieces into small food pieces creating a larger surface area for enzymes to act on
  • Chemical: Digestive enzymes break down large insoluble food molecules into smaller soluble food molecules
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The functions of different body parts in digestive

  • Mouth: Chewing is an example of mechanical digestion
  • Salivary glands: Saliva moistens the food to make a bolus for swallowing. Salivary amylase begins the chemical digestion of starch
  • Oesphagus: A wave of muscle contractions [peristalsis] pushes the bolus into the stomach
  • Stomach: Muscular contractions continue mechanical digestion. Acid kills bacteria. Pepsin begins the digestion of proteins
  • Duodenum: Bile from the liver and gall bladder neutralizes acid and emulsifies facts. Pancreatic amylase and lipase digests carbs and fats. Tripsin digests polypeptides to amino acids
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The functions of different body parts in digestive

  • Ileum: Lower half of small intestine absorbs nutrients into the blood via villi
  • Large intestine: Water is reclaimed and returned to the blood, leaving semi-solid faeces. This is stored in the rectum
  • Egestion: Faeces (containing undigested food, dead cells and other waste) is forced out of the anus


One long tube with three accessory organs (alimentary cancal, pancreas and gall bladder)

Begins with the mouth and ends with the anus

Anything ingested, after digestion is absorbed into the bloodstream or elimnated as feaces

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Smooth muscles contract to move food through

The Alimentary Canal is controlled by the Automatic Nervous System

Peristalsis: Specific smooth muscles contract causing food to move

2 muscle layers working together:

  • Circular muscle layer: In rings around gut
  • Longitudinal muscle layer: Fibres running along the length of the gut

Circular muscles contract and longitudinal muscles relax the lumen is narrower

Longitudinal muscles contract and the circular muscles relax the lumen is wider

1. Contraction of longitudinal muscles expand the lumen in front of the food giving it more space to move into

2. Contraction of circular muscles behind the food propels it forwards

In small intestine the process of peristalsis mixes the food with enzymes and foces the products of digestion to come into contact with the wall of the intestine

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Enzymes in digestion

Enzymes: Globular proteins that are biological catalysts which increase the rate of a reaction by lowering the activation energy

Digestive enzymes are released into the gut from glands and are used in catabolic reactions that break down large molecules

Lowering the activation energy high temperatures are not needed because it would cause damage to cells and proteins

An enzyme allows for a reaction to occur more quickly at body temperature

Large food molecules must be digested before the nutrients can be absorbed

By digesting larger molecules the products are:

  • soluble
  • small enough for absorption into blood and assimilation into tissues

Large molecules are unable to diffuse across cell membranes into the blood

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Digestive Enzymes

Large molecules cannot pass the cell membrane

Only by

  • Digestion 
  • Absorption

can these molecules enter the cells and bloodsteam

One process of making large molecules smaller is chemical digestion

Proteins: Proteins - Amino acids done by Protease

Lipids: Trigylcerides - Glycerol and Fatty acids done by Lipase

Carbs: Mono/di/polysaccharides - Monosaccharides done by Carbohydrase example (Amylase)

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Main enzymes in Human Digestion

Amylases break down carbohydrates

Example: Salivary Amylase

Substrate: Starch    Product: Maltose

Source: Salivary glands in mouth

Optimum pH: 7 - 7.8

Proteases break down polypeptides

Example: Pepsin

Substrate: Polypeptides   Product: Amino acids

Source: Stomach

Optimum pH: 2

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Main enzymes in Human Digestion

Lipases break down fats and lipids

Example: Pancreatic lipase

Substrate: Triglycerides          Product: Fatty acids and glycerol

Source: Pancreas delivered into small intestine

Optimum: 7.2 -7.5

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Organs and digestive enzymes

Mouth: Amylase

Amylase is secreted by the salivary glands starts to break down starch into maltose

Stomach: Pepsin

Pepsin is a Protase enzyme that works in an optimum pH of 2

Hydrochloric acid kills bacteria and provides acidic conditions

Pancreas: Lipase

Pancreas secretes Pancreatic juice which has enzymes (carbohydrase, protase and lipase) which digest starch, proteins and lipids

Alkaline to neutralize stomach content


  • Allow digestive processes to occur at body temperature and at the necessary speed to meed organisms survival requirements
  • Specific for a given substrate this allows digestion of certain molecules
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Small Intestine

Final stages of digestion

  • Chyme [acidic fluid that contain gastric juices and partly digested food] enters the Duodenum 
  • Bile from the gall bladder and liver is emptied into the duodenum which neutralizes the acids and emulcifiying fats
  • Pancreatic enzymes are released
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The Pancreas

The Pancreas secretes pancreatic juice

Pancreatic juice has enzymes

  • Carbohydrase
  • Endopeptidase: Trypsin
  • Protase
  • Lipase

which digests starch, lipids and proteins

Alkaline to neutralize stomach content

Pancreatic juice is released through a duct into the Duodenum

The Pancreas also produces Insulin and Glucagon for Glucose Metabolism

Endopeptidase hydrolyses long polypeptides into smaller polypeptides. Other protein-digesting enzymes hydrolyze smaller polypeptides into amino acids

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Markscheme Answer

  • Chewing food makes smaller particles and increases surface area
  • Starch digestion begins in the mouth by saliva/ salivary Amylase
  • Digestion of proteins in the stomach
  • Acid condition in stomach provides optimum pH for enzymes
  • Stomach muscles contract through mechanical contration
  • Enzymes in Small Intestine complete digestion
  • Alkline codition in small intestine provides optimum pH for enzymes and bile to help emulsify fats
  • Example of amylase with source, substrate and product
  • Example of protase with source, substrate and product
  • Example of lipase with source, substrate and product
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Absorption happens using the Villi and Microvilli

Villi are:

  • finger-like projections
  • increase surface area for absorption
  • Epithelial cells with many mitochondria

Villi: Many folds of the inner cells lining the small intestine

Each villus contains:

  • Capilary bed for absorption and transportation
  • Lacteal (vessel of lymphatic system) to absorb larger monomers (fatty acids)

Adaptations for absorption:

Many villi potrude into the lumen greatly increasing the surface area for absorption

Single cell layer of epithelial cells - short path for diffusion

Micro villi of the surface of each cell increase surface area even more

Capiliries are close to epithelium - short path for diffusion, rich supply of blood

Rich blood supply maintains concentration gradient between lumen and blood

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Transport mechanisms for Absorption


Simple Diffusion: Nutrients pass down the concentration gradient between phospholipids in the membrane

Hydrophobic nutrients fatty acids and monoglycerides

Fructose and Vitamins

Facilitated Diffusion: Nutrients pass down the concentration gradient through specific channel proteins in the membrane

Hydrophillic nutrients fructose

Mineral ions

Active Transport: Nutrients are pumped through the membrane against the concentration gradient by specific pump proteins

Sodium, calcium and iron

Tryglycerides and cholesterol in lipoprotein particles

Endocytosis: In which small droplets of fluid are passed through the membrane by means of vesicles

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Digestion of Starch

There are two types of molecules in starch:

  • Amylose [unbranced] linked by 1,4 bonds
  • Amylopectin [branced] linked by 1,6 bonds

Amylase breaks the 1,4 bonds of Amylose into 4 or more glucose monomers. Amylose is broken down into maltose and NOT glucose

Amylase cannot break 1,6 bonds of Amylopectin because of the specificity of its active site

Dextrins: Fragments of the Amylopectin molecule containing 1,6 bonds that Amylase cannot digest

Enzymes in the membranes of microvilli namely maltase and dextrinase further digests maltose and dextrins into glucose

Protein pumps in the membranes of microvilli absorb the glucose products of the digested starch

Blood carrying glucose [and other products of digestion] flows through villus capilaries to venules in the submucosa of the wall of small intestine. Blood carried through hepatic portal vein to the liver where excess glucose absobed and converted to glycogen for storage

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Dialysis tubing to model Absorption

Dialysis tubing can be used to model absorption by the epithelial cells of the small intestine

The tubing is semi-permeable and contains pores 1 - 10 nm in diameter

Cola drinks contain a mixture of substances that models digested and undigested food in the intestine

The water surrounding the bag is tested at different intervals to see if substances in the cola have diffused through the dialysis tubing

Test solutions inside and outside dialysis tubing for starch and glucose before and after 15 minutes

Expected Result: Glucose and phosphoric acid [contain small-sized particles] diffuse through the tubing but caramel [contains larger polymers of sugar] does not

The model acts as a substitute or simplification of what is being studied and predicted

  • How is the function of dialysis tubing similar to the small intestine
  • What features of a real gut are missing from this model
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