Digestion & Absorption

• (PANCREAS:)

  • STRUCTURE: A large gland below the stomach which produces a secretion (pancreatic juice) 
  • PANCREATIC JUICE: contains enzymes protease (to hydrolyse proteins), lipase (to hydrolyse lipids), and amylase (to hydrolyse starch)
• (SALIVARY GLANDS:)

  • STRUCTURE: Near the mouth, pass their secretions via a duct in the mouth
  • SECRETION: contains the enzyme amylase which hydrolyses starch into maltose
• (OESOPHAGUS:)

  ROLE: Carries food from mouth to stomach
• (STOMACH:)

  • STRUCTURE: A muscular sac with an inner layer that produces enzymes
  • ROLE: Store & digest food (esp proteins)
  • GLANDS: Has glands that produce enzymes which digest proteins
• (ILEUM:)

  • STRUCTURE: A long muscular tube
  • ROLE: Digest food & absorb the products into the bloodstream
  • SECRETIONS & ENZYMES:(produced by the ileum walls & its glands) break food down further
  • LARGE SA: Inner walls are folded into villi -> villi have mills of microvilli (on the epithelial cells of each villus) -> large SA for absorption
• (ILEUM:)

  • VILLI: contain muscles so can move -> helps maintain diffusion gradients (movement mixes contents of the ileum ->digested products are replaced quickly with new products of digestion), are well supplied with blood vessels (can carry away absorbed materials quickly -> maintains a high diffusion gradient)
• (LARGE INTESTINE:)

  ROLE: Absorbs water. Most of the water absorbed= from the secretions of digestive glands
• (RECTUM:)

  • ROLE: Final section of the intestines. Faeces are stored in the rectum, before being removed by the anus during egestion (process of removal)
• BREAKDOWN (before digestion):

  • When an organism eats it ingests large biological molecules that allow the organism to survive. 
  • They could be used in cells for important reactions (e.g. carbohydrates providing glucose for respiration) but are too large to be transported into the cells, so they are broken down first
• DIGESTION (takes place in 2 stages):

  • PHYSICAL BREAKDOWN: Food is broken down by structures eg the teeth to ingest it & provide a larger SA for absorption. Food is also churned in the stomach wall
  • CHEMICAL DIGESTION: Hydrolyses large insoluble molecules into smaller soluble ones via enzymes that hydrolyse them (adds water to the chemical bonds that hold them together)
• HYDROLYSIS: 

  • DEFINITION: The reaction that breaks down large biological molecules. It requires water and splits larger molecules into their smaller components. 
  • EFFECT: These smaller molecules (eg glucose) diffuse into cells or be transported using protein channels.
• PRODUCTS OF HYDROLYSIS:  

  • PROTEINS: -> amino acids. 
  • CARBOHYDRATES: -> Disaccharides and monosaccharides. 
  • LIPIDS: -> fatty acids and monoglycerides.
• CARBOHYDRATES (digestion the mouth involving amylase):

  • AMYLASE: An enzyme produced in the salivary glands & pancreas. It catalyses the hydrolysis of starch (carbohydrate) into maltose (disaccharide). Hydrolysis of carbohydrates breaks glycosidic bonds
• CARBOHYDRATES (digestion the mouth involving amylase):

  • MEMBRANE-BOUND DISACCHARIDES: Are enzymes in the cell membranes of the epithelial cells in the ileum (small intestine). Membrane-bound disaccharides (eg maltose) into monosaccharides (eg glucose)
• CARBOHYDRATES (digestion the mouth involving amylase):

  • TRANSPORTER PROTEINS: Monosaccharides are transported into the epithelial cells in the ileum using transporter proteins (glucose & galactose are transported by co-transporter proteins, fructose is absorbed by facilitated diffusion). Once inside the cells monosaccharides can be used in respiration.
• LIPIDS (digestion takes place in the small intestine involving lipase):

  • LIPASE: An enzyme produced by the pancreas and released into the small intestine. It hydrolyses lipids into fatty acids and monoglycerides. Hydrolysis of lipids breaks ester bonds.
• LIPIDS (digestion takes place in the small intestine involving lipase):

  • BILE SALTS: Produced by the liver, helps digest lipids by forming small lipid droplets called micelles. Emulsification= process of forming micelles. The many micelles have a larger surface area than a single large micelle which allows lipids to be hydrolysed more quickly
• LIPIDS (digestion takes place in the small intestine involving lipase):

  • MICELLES: Allow the monoglycerides & fatty acids to be absorbed into the epithelial cells in the ileum. The monoglycerides and fatty acids diffuse through the cell surface membrane into the cell cytoplasm
• PROTEINS (digestion takes place in the small intestine, involves different enzymes called 

  proteases):

  • ENDOPEPTIDASES: Enzymes that catalyse the hydrolysis of bonds within a protein to amino acids. Hydrolysis of proteins breaks peptide bonds. Eg of endopeptidases: trypsin, chymotrypsin, pepsin. Trypsin and chymotrypsin are released by the pancreas into the small intestine. Pepsin is released into the stomach
• PROTEINS (digestion takes place in the small intestine, involves different enzymes called 

  proteases):

  • EXOPEPTIDASES: Enzymes that catalyse the hydrolysis of peptide bonds at the ends of proteins. Eg dipeptidases hydrolyse the peptide bond between 2 amino acids.
• PROTEINS (digestion takes place in the small intestine, involves different enzymes called 

  proteases):

  • CO-TRANSPORTER PROTEINS: Amino acids are transported into the epithelial cells in the ileum using co-transporter proteins
• CO-TRANSPORTERS:

  • FUNCTION: As the rate of facilitated diffusion depends on the size of the concentration gradient, and as the molecules moves into the epithelial cells the concentration gradient falls, facilitated diffusion cannot absorb all of the molecules so co transport is also used.
• CO-TRANSPORTERS:

  • ACTIVE TRANSPORT OF SODIUM IONS: Sodium-dependent co-transporter proteins are located in the epithelial cell membranes. The co-transporter proteins (which are each also specific to one type of molecule/ ion) actively transport sodium ions into the blood. This causes the concentration of sodium ions in the epithelial cells to decrease
• CO-TRANSPORTERS:

  • DIFFUSION OF SODIUM IONS: The decreased concentration of sodium ions in the epithelial cells causes sodium ions in the ileum to diffuse down the concentration gradient (from high to low concentration). The sodium ions diffuse through a co-transporter protein in the cell surface membrane
• CO-TRANSPORTERS:

  • BINDING OF AMINO ACIDS AND SUGARS: When sodium ions bind to a co-transporter protein, amino acids/ monosaccharides also bind to the protein. Binding of amino acids/ monosaccharides causes the transporter protein to undergo a conformational (shape/ structure) change. The amino acids/ monosaccharides along with the sodium ions are transported into the epithelial cell cytoplasm
• CO-TRANSPORTERS:

  • USE: Once inside the epithelial cells, the amino acids/ monosaccharides can be used inside the epithelial cells or absorbed into the bloodstream
• MICELLES (transport monoglycerides & fatty acids into epithelial cells):

  • FORMATION: After larger lipid molecules have been broken down into monoglycerides & fatty acids, bile salts are secreted by the liver. The bile salts associate with the monoglycerides & fatty acids to form micelles. Micelles are circular formations that are made up of phospholipid tails.
• MICELLES (transport monoglycerides & fatty acids into epithelial cells):

  • INCORPORATION: The micelles move into the ileum and come into contact with the epithelial cells. Once inside the epithelial cells, monoglycerides & fatty acids are transported to the endoplasmic reticulum where they combine to form triglycerides. The triglycerides associate with cholesterol & lipoproteins to form chylomicrons & move out by exocytosis to lymphatic capillaries (lacteals) in villuses
• ACTIVE TRANSPORT IN CARRIER PROTEINS:

  
• Active transport in carrier proteins

  1. Carrier protein transports chemical from lower to higher concentration (against concentration gradient)

  2. Requires metabolic energy (ATP)

  3. Molecule/ion attaches to receptor site on carrier protein on side of membrane with lower concentration

  4. ATP binds to carrier protein

  5. ATP undergoes hydrolysis, producing phosphate and ADP

  6. Phosphate attaches to carrier protein, causing shape change

  7. Shape change causes carrier protein to transport molecule/ion to other side of membrane (where it's released)

  8. Phosphate leaves carrier protein, causing it to return to previous shape
• Active transport requires metabolic energy, provided by the molecule ATP
• The phosphate and ADP will later reform ATP during respiration