3.3.3 Digestion and absorption

• Exchange features in the ileum

  • Villi and microvilli increase surface area for large SA:V
  • Villi lining and capillary endothelium are one cell thick for shorter diffusion gradient
  • Ileum contains muscle which contracts to mix contents and maintain concentration gradients
  • Well supplied with blood vessels for circulation and maintaining concentration gradient
  • Epithelial cells dense with carrier proteins for amino acids and glucose making them selectively permeable
• When lipids are added to water, an emulsion is formed because fatty acids are hydrophobic and insoluble in water
• Triglyceride Digestion

  Emulsification must occur to aid digestion
• Maltose hydrolysis

  Membrane-bound maltase hydrolyses maltose into glucose
• During emulsification, large lipid droplets are broken down into smaller droplets
• Peptidases are found in the stomach and are also secreted by the pancreas in pancreatic juice
• Micelles carry monoglycerides/fatty acids to the epithelial cell membrane
• Lipases in pancreatic juice

  Hydrolyse triglycerides into monoglycerides and fatty acids by breaking ester bonds
• Dipeptidases

  Break the peptide bond between the 2 amino acids of a dipeptide
• Protein digestion involves endopeptidases, exopeptidases, and dipeptidases
• Bile salts are produced by the liver and stored in the gall bladder
• Lipid digestion can occur at an increased rate after emulsification
• When lipids are added to water, an emulsion is formed because fatty acids are hydrophobic so insoluble in water.
• Micelles are an association of bile salts, monoglycerides, and fatty acids
• Micelles increase the digestion and absorption of lipids by increasing the lipid surface area and maintaining higher concentrations at the lining for diffusion
• Proteases are also called peptidases
• Proteases break peptide bonds

  • Hydrolyses polypeptides to dipeptides
  • Hydrolyses dipeptides to amino acids
• Carbohydrases and disaccharidases break glycosidic bonds.

  • Hydrolyse polysaccharides to disaccharides
  • Hydrolyse disaccharides to monosaccharides 
• pH decreases during digestion of triglycerides and dipeptides because when they are hydrolysed they produce acids.
• Describe how a molecule of starch is digested into glucose (5 marks)

  • Salivary amylase hydrolyses starch into maltose
  • Pancreas makes pancreatic amylase which further hydrolyses starch into maltose
  • Membrane-bound maltase hydrolyses maltose into glucose
  • Glycosidic bonds are broken
• Explain the advantage of lipid droplet and micelle formation (3)

  • Droplets increase surface area
  • So faster hydrolysis of lipids
  • Micelles carry fatty acids and glycerol to epithelial cell membrane
• During triglyceride digestion large lipid droplets are split up into tiny droplets called micelles by bile salts.

  This increases the lipids surface area, increasing the activity of lipases
• The action of endopeptidases and exopeptidases can increase the rate of protein digestion. Describe how (2)

  • exopeptidases hydrolyse peptide bonds at the ends of a protein and endopeptidases hydrolyse internal peptide bonds within a polypeptide.
  • this creates more ‘ends’
• endopeptidases

  hydrolyse internal peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules
• Exopeptidases

  hydrolyse peptide bonds at the ends (terminal amino acids) of a protein
• Digestion is the process of hydrolysing large molecules into smaller molecules that can be absorbed into the bloodstream
• Physical (mechanical) digestion is the process of breaking down large molecules into smaller ones using structures such as teeth or churning by muscles in the stomach wall. This is important because it provides a large surface area for chemical digestion.
• Chemical digestion hydrolyse large, insoluble molecules into smaller, soluble molecules. It is carried out by enzymes.
• Major parts of the digestive system

  • oesophagus: carries food from mouth to the stomach
  • stomach: stores and digests food, especially proteins using enzymes and churning
  • ileum: digests food further using enzymes
  • large intestine: absorbs water
  • rectum: stores faeces before it is removed via the anus (egestion)
  • salivary glands: secrets the enzyme amylase
  • pancrease: produces a pancreatic juice that contains all 3 enzymes - lipase, proteases and amylase
• Ileum contain muscle which contracts. This movement mixes the contents of the ileum and so maintains concentration gradients.
• The epithelial cells in the ileum are dense with carrier proteins for amino acids and glucose = selectively permeable.
• Glucose and amino acids are absorbed into the blood by co-transport.
• Co-transport of glucose

  • Sodium ions are actively transported out of ileum epithelial cell into blood. There is now lower concentration of sodium ions inside the cell.
  • This forms a diffusion gradient for sodium to enter epithelial cell from the ileum lumen
  • Glucose now enters the epithelial cell along with sodium ions via the co-transporter protein.
  • Glucose then moves into the blood via facilitated diffusion by a carrier protein.
• Epithelial cells make chylomicrons. These lipoproteins are adapted for transporting dietary lipids from intestines to other parts of the body.
• How are Golgi apparatus involved in absorption of lipids?

  • modifies triglycerides
  • combines triglycerides with proteins
  • packages for exocytosis
• Lipid absorption

  • Micelles come into contact with epithelial cell membrane and break down, releasing the monoglycerides and fatty acids
  • monoglycerides and fatty acids diffuse across the phospholipid bilayer into the epithelial cells.
  • monoglycerides and fatty acids move to endoplasmic reticulum where they are recombined to form triglycerides.
  • Triglycerides move to golgi apparatus, where they associate with cholesterol and lipoproteins to form chylomicrons.
  • Chylomicrons move out of epithelial cells by exocytosis into a lacteal
• Explain how digestion of starch in the gut leads to an increase in the concentration of glucose in the blood. (3)

  • hydrolysed by amylase
  • produces glucose
  • small enough to diffuse across the gut wall into the blood
• Describe the transport of carbohydrates in plants (5)

  • sucrose is co-transported with hydrogen ions into phloem
  • by companion cells
  • lowers water potential in phloem and water enters from xylem by osmosis
  • produces higher hydrostatic pressure
  • mass flow to respiring cells
  • carbohydrates unloaded from phloem by active transport
• Describe the complete digestion of starch by a mammal (4)

  • hydrolysis
  • of glycosidic bonds
  • starch is hydrolysed into maltose by amylase
  • maltose is hydrolysed into glucose by maltase (membrane-bound)
• Lacteal

  Tiny lymphatic vessels 

  Transport fat and slowly release them into the bloodstream