Chapter 5: Digestion

• Digestion involves the breakdown of large, insoluble molecules into soluble substances that can be absorbed into the body
• Ingestion is the taking of substances into the body through the mouth
• Mechanical digestion is the breakdown of food into smaller pieces without changing the food molecules
• Chemical digestion is the breakdown of large molecules into smaller molecules
• Absorption is the movement of small food molecules/ions through the walls of the intestine into the blood
• Assimilation is the movement of digested food molecules into cells to be used
• Egestion is the passing out of food that has not been digested or absorbed through the anus
• Digestive enzymes:

  • Carbohydrase: enzymes that break down carbohydrates
  • Protease: enzymes that break down protein
  • Lipase: enzymes that break down lipids
• Saliva contains amylase to start the digestion of starch in the mouth
• Gastric juice in the stomach contains protease to digest protein and hydrochloric acid to maintain an optimal pH and kill bacteria
• The pancreas secretes pancreatic juice into the duodenum, containing protease, lipase, and amylase
• The liver makes bile, stored in the gall bladder, which contains salts to emulsify fats
• The rectum is where feces are stored before being egested through the anus
• The small intestine is the first part of the small intestine, receiving pancreatic juice (protease, lipase, amylase) for efficient absorption due to the increased surface area of villi
• Most biological molecules are organic molecules containing a carbon chain which may contain hydrogen, oxygen, nitrogen
• Polymer


  Long chain of monomers in a repeated pattern
• Carbohydrate


  Made from carbon, hydrogen, oxygen (CHO), molecules of repeating simple sugar called Saccharides
• Complex carbohydrates are made of long chains of units (Saccharide) e.g. starch & cellulose
• All amino acids have the same general structure except the R group, which defines the amino acid
• Lipids


  Made of carbon, hydrogen, oxygen (CHO), insoluble in water but soluble in organic solvents e.g. ethanol
• Enzymes


  • Biological catalysts involved in metabolic reactions
• Amino acid

  Monomer = amino acid, polymer = protein (polypeptide)
• Formation of monomers & polymers

  1. Hydrolysis - large molecule split into smaller ones - use Water

  2. Condensation - small molecules join to form larger ones - produce Water
• Level of protein structure

  • Primary - polypeptide chain
  • Secondary - polypeptide in coil or sheet
  • Tertiary - coil or sheet form tangle
  • Quaternary - more than one tangle - complex
• Monomer


  Small molecule
• Groups of organic molecules

  • Carbohydrate
  • Protein
  • Lipids
  • Nucleic acid
• Type of Saccharides

  • Disaccharide e.g. Sucrose
  • Polysaccharide e.g. starch
• Protein


  Long chain of molecules made of amino acids
• There are 20 types of amino acids, which are the building blocks of protein
• The most common type of lipid is triglycerides (neutral fat) formed when fatty acid bonds with glycerol (glycosidic bond)
• Biological molecules

  • Carbohydrate
  • Protein
  • Lipids
  • Nucleic acid
• Enzymes become denatured (lose shape) at extreme temperatures and pH
• Nucleic acid

  Made of monomers: nucleotides, polymers: DNA or RNA
• Enzymes


  • Speed up the rate of reaction by breaking down food
  • Are biological catalysts that increase the rate of reaction without being used up in the reaction
• Disadvantages of enzymes include the need to keep temperatures below 45°C and pH within controlled limits, which can be expensive to maintain and produce
• As temperature increases

  • Kinetic energy of molecules increases, speeding up movement of substrate molecules and increasing the rate of collision with enzymes, binding them together
  • Changing the acid-base conditions can denature enzymes as different enzymes work best at different pH levels
• How enzymes work

  1. Substrate molecules that enzymes act upon produce products molecules after the reaction

  2. Enzymes have a specific shape at their active site that complements the shape of the substrate

  3. When the substrate is in place, the enzyme and substrate bind together, weakening bonds and lowering activation energy to break down the substrate

  4. Enzymes can be reused as they are not used up during the reaction
• Advantages of enzymes include catalyzing reactions at relatively low temperatures and normal pressures, being cheap to run as they can be reused
• Enzymes are specific and only work on one substrate
• Denaturation occurs as temperature increases further, causing irreversible changes in enzyme molecules where the active site can no longer bind to the substrate