HomeCells & MoleculesDiseasesEcosystemHuman BiologyAQA BIOL1AQA HBIO1AQA HBIO2AQA HBIO4AQA HBIO5
BIOL1 > Digestive System
Welcome, Guest!
Login with Facebook | Login

The Digestive System

The digestive system is a tube through which food passes from the mouth where food is ingested to the anus where it is egested. It consists of a series of organs, each with a distinct structure and function. During the digestive transit food is broken down into substances suitable for absorption into the bloodstream.
The gut wall has the same basic structure along its length. There are three main layers:

  • An outer, muscular layer. Circular and longitudinal layers of smooth muscles are present. Alternate contraction of these muscles moves food along the digestive tract (peristalsis)
  • A middle layer of connective tissue - submucosa
  • An inner layer - mucosa

These three layers have different adaptations in different parts of the alimentary canal. The adaptations are underlined below.


  • Muscular tube carrying food from the mouth to the stomach


  • Elastic and muscular organ which can expand
  • Highly folded mucosa
  • Gastric pits secreting gastric juices containing digestive enzymes (proteases)
  • Contraction and relaxation of the muscular wall mix the food thoroughly

Small Intestine

  • The site of chemical digestion and absoption of the products of lipids, polysaccharides and proteins
  • Highly folded mucosa - arranged in villi (finger like projections to increase surface area for absorption)
  • Epithelial cells lining the small intestine have a folded cell membrane - microvilli to further increase the surface area for absorption

Large Intestine

  • The site of absorption of water
  • Undigested food matter forms faeces


  • Faecal matter is stored here before egestion


  • Large molecules (starch, proteins, TAG) are too big and insoluble to be absorbed
    • Polymers have to be broken down into monomers
    • With help of hydrolytic enzymes - reaction requires H2O
    • Note: TAGs are not polymers but also need to be broken down
  • Different enzymes break down different food
    • Work best at body temperature (37°)
    • Work in different conditions at different pH (stomach is acidic, intestine is alkaline)
  • Hydrolysis
    • Proteins → amino acids
      • Essential amino acids: cannot be synthesised and must be present in diet
      • Non-essential amino acids: synthesised from essential amino acids by transamination in the liver
    • TAG → glycerol and fatty acids
    • Polysaccharides → monosaccharides


  • Proteins are made up by different combinations of 20 amino acids
    • Common structure
      • -COOH group
      • -NH2 group
    • Amino acids differ in their R-group
  • Tertiary structure
    • Complex globular 3D shape
    • Folding and twisting of polypeptides (H-bond, ionic bonds, disulphide bridges)
    • Polypeptides contain many peptide bonds
  • Same amino acid sequenceALWAYS same shape
  • Bonds found in proteins
    • Hydrogen bonds
      • Between R-groups
      • Easily broken, but present in larger numbers
      • The more bonds, the stronger the structure
    • Ionic bonds
      • Between -COOH and -NH2 groups
    • Disulphide bridges
      • Between two sulphur-containing cysteine side chains
      • Strong bonds found in skin and hair
  • Denaturation
    • Destruction of tertiary structure, can be done by heat
    • Protein structure is lost and cannot reform → dysfunctional
  • Background Reading: Structure of Proteins http://www.chemguide.co.uk/organicprops/aminoacids/proteinstruct.html

What are enzymes?

  • All enzymes are globular proteins → spherical in shape
  • Control biochemical reactions in cells
  • They have the suffix "-ase"
  • Intracellular enzymes are found inside the cell
  • Extracellular enzymes act outside the cell (e.g. digestive enzymes)
  • Enzymes are catalysts → speed up chemical reactions
    • Reduce activation energy required to start a reaction between molecules
    • Substrates (reactants) are converted into products
    • Reaction may not take place in absence of enzymes (each enzyme has a specific catalytic action)
    • Enzymes catalyse a reaction at max. rate at an optimum state
  • Lock and key theory
    • Only one substrate (key) can fit into the enzyme's active site (lock)
    • Both structures have a unique shape
  • Induced fit theory
    • Substrate binds to the enzyme's active site
      • The shape of the active site changes and moves the substrate closer to the enzyme
      • Amino acids are moulded into a precise form
      • Enzyme wraps around substrate to distort it
    • This lowers the activation energy
    • An enzyme-substrate complex forms → fast reaction
    • E + S → ES → P + E
  • Enzyme is not used up in the reaction (unlike substrates)

Enzyme Activity

  • Changes in pH
    • Affect attraction between substrate and enzyme
    • Ionic bonds can break and change shape → enzyme is denatured
    • Charges on amino acids can change → ES complex cannot form
    • Optimum pH (enzymes work best)
      • pH 7 for intracellular enzymes
      • Acidic range (pH 1-6) in the stomach for digestive enzymes (pepsin)
      • Alkaline range (pH 8-14) in oral cavities (amylase)
    • pH measures the conc. of hydrogen ions → higher conc. will give a lower pH
  • Enzyme conc
    • Proportional to rate of reaction, provided other conditions are constant
    • Straight line
  • Substrate conc.
    • Proportional to rate of reaction until there are more substrates than enzymes present
    • Rate of reaction increases
      • Substrate binds to active site, but more enzymes are available
      • Rate increases if more substrate is added
    • Eventually, curve becomes constant (no increased rate)
      • Substrates occupy all active sites (all enzymes)
      • Adding more substrate won't yield more product, as no more active sites are available
  • Increased Temperature
    • Increases speed of molecular movement → chances of molecular collisions → more ES complexes
    • At 0-42°C rate of reaction is proportional to temp
    • Enzymes have optimum temp. for their action (usually 37°C in humans)
    • Above ≈42°C, enzyme is denatured due to heavy vibration that breaks -H bonds
    • Shape is changed → active site can't be used anymore
  • Decreased Temperature
    • Enzymes become less and less active, due to reductions in speed of molecular movement
    • Below freezing point
      • Inactivated, not denatured
      • Regain their function when returning to normal temperature
    • Thermophilic: heat-loving
    • Hyperthermophilic: organisms are not able to grow below +70°C
    • Psychrophiles: cold-loving
  • Monomer (-OH) + monomer (-H) polymer + H2O(l)
  • Condensation: monomers join to form polymers
    • Amino acids join to form a dipeptide (protein)
      • Two amino acids release -H and -OH groups (H2O)
      • Peptide bond forms between the alpha-carbon and nitrogen
    • Monosaccharides join to form disaccharides
      • Glycosidic bond forms between both monomers
  • Hydrolysis: break down of a polymer
    • Reverse of the condensation reaction
    • This is the process of digestion


  • Organic molecules which contain C, H and O
  • Bind together in the ratio Cx(H2O)y
  • Monosaccharides → single sugar (monomer)
    • Ribose found in RNA and DNA
    • Deoxyribose part of nucleic acids
    • Glucose is the main energy source in brain
    • Fructose is found in sweet-tasting fruits
  • Disaccharides → two sugar residues (2 monomers)
    • Sucrose (glucose + fructose) → transport carbohydrates in plants
    • Maltose (glucose + glucose) → formed from digestion of starch
    • Lactose (glucose + galactose) → found in milk
    • Lactose intolerance
  • Polysaccharides → many sugar residues (polymer)
    • Starch (alpha-glucose) → main storage of carbohydrates in plants
    • Glycogen (alpha-glucose) → main storage of carbohydrates in humans
    • Cellulose (beta-glucose) → component of plant cell wall, important for digestion


  • Consists of amylopectin and amylose (both are made of α-glucose)
    • Amylopectin is branched via 1,6-glycosidic bonds
    • Amylose forms a stiff helical structure via 1,4-glycosidic bonds
    • Both are compact molecules → starch can be stored in small space
  • The ends are easily broken down to glucose for respiration
  • Does not affect water potential as it is insoluble
  • Readily hydrolysed by the enzyme amylase produced by the pancreas and present in saliva
  • Found in corn (maize), wheat, potato, rice

Biochemical Tests

  • Reducing sugars (all monosaccharides and some disaccharides) can be tested for using Benedict’s reagent. After placing the sample and the reagent in a hot water bath a brick red precipitate will be produced if reducing sugars are present.
  • Non reducing sugars require a negative result using Benedict’s reagent. Add hydrochloric acid to the sample and heat. Neutralize the solution using sodium hydrogencarbonate and then test again with Benedict’s solution. A positive result will be found.
  • Starch can be tested for using iodine. In the presence of starch iodine will turn blue - black.