DIGESTION AND ABSORPTION

• DIGESTION: process in which large molecules (in food are too big to cross cell membranes so can’t be absorbed from the gut into the blood) are hydrolysed by enzymes into small molecules which can be absorbed and move across cell membranes into the blood to be transported around the body for use by the body cells

  in humans, as with many organisms, digestion takes place in 2 stages:

  1. PHYSICAL BREAKDOWN
  2. CHEMICAL DIGESTION
• DIGESTION

  1, PHYSICAL BREAKDOWN

  • if the food is large, it’s broken down into smaller pieces by means of structures such as the teeth
  • this not only makes it possible to ingest the food but also provides a large surface area for chemical digestion
  • food is churned by the muscles in the stomach wall and this also physically breaks it up
• DIGESTION

  2. CHEMICAL DIGESTION

  • chemical digestion hydrolysed large, insoluble molecules into smaller soluble ones
  • it’s carried out by enzymes
  • all digestive enzymes function by hydrolysis. Hydrolysis is the splitting up of molecules by adding water to the bonds that hold them together
• WHAT ARE CARBOHYDRATES BROKEN DOWN INTO

  Carbohydrates are hydrolysed into disaccharides and then monosaccharides
• WHAT ARE FATS BROKEN DOWN INTO?

  Fats are broken down into monoglycerides and fatty acids
• WHAT ARE PROTEINS BROKEN DOWN INTO

  Proteins are broken down into amino acids
• DIGESTION

  • Carbohydrates are hydrolysed into disaccharides and then monosaccharides.
  • Fats are broken down into monoglycerides and fatty acids.
  • Proteins are broken down into amino acids.
• STRUCTURE OF DIGESTIVE SYSTEM

  The human digestive system made up of a long muscular tube (alimentary canal or gastrointestinal tract) + its associated glands

  • glands produce enzymes that hydrolyse large molecules into small ones ready for absorption
  • digestive system = an exchange surface through which food substances are absorbed
  • liver, pancreas and gallbladder = organs involved
  • enzymes, hormones, nerves + blood breakdown food, modulate digestive process + deliver products
  • mesentery = large stretch of tissue that supports + positions digestive organs in the abdomen, so they can function
• DIGESTIVE ORGANS

  1. Mouth salivary glands
  2. oesophagus
  3. stomach
  4. small intestine
  5. liver
  6. gallbaldder
  7. pancreas
  8. large intestine (colon)
  9. rectum and anus
• DIGESTIVE ORGANS AND FUNCTION

  1, MOUTH - SALIVARY GLANDS:

  • salivary glands produce saliva which contains amylase which hydrolyses starch into maltose.
  • They pass their secretions via a duct into the mouth
  • chewing and saliva breaks down food into bolus. This increases the surface area of the food.
• DIGESTIVE ORGANS AND FUNCTION

  2. OESOPHAGUS

  • a 25cm hollow tube carrying food from the mouth to the stomach
  • nerves in surrounding oesophageal tissue sense bonus trigger peristalsis (muscle contractions)
• DIGESTIVE ORGANS AND FUNCTION

  3. STOMACH

  • structure: muscular sac which has inner lining that produces enzymes
  • muscular tissues churn food into chunks and mixes it with enzymes
  • hormones trigger the release of acids and enzyme-rich juices
  • glandular tissues release enzymes (pepsin) and stomach acid
  • hormones also trigger pancreas, liver and gallbladder to produce justices and bile in preparation for next stage
  • bolus is now chyme and moves into small intestine
• DIGESTIVE ORGANS AND FUNCTION

  3. STOMACH

  • STOMACH ACID (HCl): helps to unravel proteins to enable enzyme activity and lower pH so pepsin (optimum pH) can work and damages microorganisms
  • PEPSIN and other proteases break down proteins into amino acids
• DIGESTIVE ORGANS AND FUNCTION

  4. SMALL INTESTINE (1)

  • structure: 3 sections - duodenum, jejunum and ileum. Smooth muscle tissue in the walls allows peristalsis to move food along. The lumen is the hole where food goes through.
  • DUODENUM: main site of carbohydrate, protein and lipid digestion. Bile from gallbladder and enzymes break fat molecules into fatty acid and glycerol, proteins into amino acids and carbohydrates into glucose
  • JEJUNUM: absorption of soluble products of digestion
• DIGESTIVE ORGANS AND FUNCTION

  4. SMALL INTESTINE (2)

  • ILEUM: final absorption of soluble products

  -by active transport, co-transport and facilitated diffusion

  -ileum is lined with villi and Microvilli to increase the surface area to maximise absorption into the blood

  -water absorption can also occur here

  • the blood takes the molecules into cells
• DIGESTIVE ORGANS AND FUNCTION

  5/6. LIVER AND GALLBLADDER

  • liver sends bile to gallbladder through the common bile duct which secretes it into the duodenum via the cystic duct which dissolves fats which can be easily digested
  • bile emulsifies fat into droplets which have a larger surface area so enzymes can break fat down.
  • bile also neutralises the acidity of chyme
• DIGESTIVE ORGANS AND FUNCTION

  7. PANCREAS

  • a large gland situated below the stomach
  • it produces a secretion called pancreatic juice which contains proteases, lipase and amylase to hydrolyse food
• DIGESTIVE ORGANS AND FUNCTION

  8. LARGE INTESTINE (COLON)

  • structure: transverse and digesting limb
  • undigested fibre, water and dead cells move into the colon
  • drains the water through intestinal wall
  • leaves a soft mass called stool
• DIGESTIVE ORGANS AND FUNCTION

  9. RECTUM AND ANUS

  • stool is squeezed into the rectum
  • nerves sense it expanding and tell body to expel the waste through the anus by egestion
• DIGESTIVE ENZYMES (CHEMICAL BREAKDOWN)

  • digestive enzymes produced by specialised cells in the digestive system are used to catalyse the breakdown of biological molecules in food
  • since enzymes are specific and only work with specific substrates, different enzymes are needed to catalyse the breakdown of different food molecules
  • all digestive enzymes function by hydrolysis
  • more than one enzyme is needed to hydrolyse a large molecule as one enzyme hydrolyses a large molecule into section and these are then hydrolysed into smaller molecules by one or more additional enzymes
• DIGESTIVE ENZYMES

  • TYPE OF ENZYME: CARBOHYDRASE - amylase
  • HYDROLYSES: carbohydrates / starch
  • PRODUCTS: maltose
  • PRODUCED WHERE: salivary glands and pancreas
  • RELEASED WHERE: mouth and small intestine
• DIGESTIVE ENZYMES

  • TYPE OF ENZYME: MEMBRANE-BOUND DISACCHARIDES - maltase
  • HYDROLYSES: maltose (starch)
  • PRODUCTS: alpha glucose
  • PRODUCED WHERE: small intestine, epithelial lining
  • RELEASED WHERE: attached to the cell membranes of epithelial Cells lining the ileum
• DIGESTIVE ENZYMES

  • TYPE OF ENZYME: MEMBRANE-BOUND DISACCHARIDES - sucrase
  • HYDROLYSES: sucrose (nat. Foods)
  • PRODUCTS: glucose and fructose
  • PRODUCED WHERE: small intestine, epithelial lining
  • RELEASED WHERE: attached to the cell membranes of epithelial cells lining the ileum

  
• DIGESTIVE ENZYMES

  • TYPE OF ENZYME: MEMBRANE-BOUND DISACCHARIDES - Lactase
  • HYDROLYSES: lactose (milk)
  • PRODUCTS: glucose and galactose
  • PRODUCED WHERE: small intestine, epithelial cells
  • RELEASED WHERE: attached to the cell membranes of epithelial cells lining the ileum
• DIGESTIVE ENZYMES

  • TYPE OF ENZYME: lipase
  • HYDROLYSES: lipids
  • PRODUCTS: glycerol and fatty acids
  • PRODUCED WHERE: pancreas and salivary glands
  • RELEASED WHERE: small intestine
• DIGESTIVE ENZYMES

  • TYPE OF ENZYME: protease
  • HYDROLYSES: proteins
  • PRODUCTS: amino acids
  • PRODUCED WHERE: stomach (pepsin) and pancreas
  • RELEASED WHERE: stomach and small intestine
• CARBOHYDRATES DIGESTION (STARCH): 1

  Amylase hydrolyses starch to maltose (polysaccharide to disaccharide)

  1. Amylase produced by salivary glands, released into mouth

  • saliva also contains mineral salts that help to maintain the pH at around neutral. This is the optimum pH for salivary amylase to work.

  2. The food is swallowed and enters the stomach, where the conditions are acidic. This acid denatures the amylase and prevents further hydrolysis of starch

  3. After time the food is passed into the small intestine, where it mixes with the secretion from the pancreas called pancreatic juice
• CARBOHYDRATES DIGESTION (STARCH): 2

  3.

  • the pancreatic juice contains pancreatic amylase. This continues the hydrolysis of any remaining starch to maltose
  • alkaline salts are produced by both the pancreas and intestinal wall to maintain the pH at around neutral so that amylase can function

  4. Maltase is a membrane-bound disaccharidase (attached to epithelial cells lining the ileum of the small intestine) which hydrolyses maltose to alpha glucose by hydrolysis of glycosidic bond
• CARBOHYDRATES DIGESTION (DISACCHARIDE)

  • Membrane-bound disaccharidases (attaches to epithelial cells lining the ileum of the small intestine).
  • they help to break down disaccharides into monosaccharides which involves the hydrolysis of glycosidic bonds

  DISACCHARIDES:

  • maltose
  • sucrose
  • lactose
• CARBOHYDRATE DIGESTION (DISACCHARIDES)

  • maltase - maltose —> alpha glucose + alpha glucose
  • sucrase - sucrose —> alpha glucose + fructose
  • lactase - lactose —> alpha glucose + galactose
  • maltose comes from hydrolysis of starch
  • sucrose and lactose are naturally found in foods
• WHAT DOES AMYLASE HYDROLYSE STARCH INTO?

  Amylase hydrolyses starch to maltose (polysaccharide to disaccharide)
• ABSORPTION OF MONOSACCHARIDES

  • fructose - facilitated diffusion
  • glucose and galactose - cotransport
• ABSORPTION OF MONOSACCHARIDES: 1

  1, SODIUM POTASSIUM PUMP (CARRIER PROTEIN BETWEEN EPITHELIAL CELL AND BLOODSTREAM)

  1. Na+ from epithelial cell —> bloodstream by active transport in the sodium potassium pump

  a . K+ from blood —> epithelial cells

  2. As ileum has a high concentration of Na+ ions following digestion, concentration of Na+ ions now lower in epithelial cells than ileum, creating a concentration gradient between lumen of ileum and epithelial cells
• ABSORPTION OF MONOSACCHARIDES: 2

  2. SODIUM GLUCOSE CO-TRANSPORTER (PROTEIN IN MEMBRANE OF EPITHELIAL CELL)

  3. Na+ ions into the epithelial cells from ileum via facilitated diffusion down concentration gradient. Attaches to complementary shape receptor

  4. In the same cotransporter (symport), glucose then attaches to it + is absorbed into epithelial cells from the ileum against its concentration gradient

  5. The Na+ is then released on the other side which enables glucose to be released. Transported together

  6. Glucose is absorbed by active transport which ensures its absorbed at fast rate
• ABSORPTION OF MONOSACCHARIDES: 3

  7. The energy for active transport of glucose comes indirectly from the concentration gradient of the Na+ ion

  8. Glucose then enters the bloodstream by facilitated diffusion. Glucose concentration is high in the epithelial cells and is carried straight away in blood as it always flows, so there’s always a low concentration in the bloodstream
• PROTEIN DIGESTION

  Proteins are large, complex molecules that are hydrolysed by a group of enzymes called peptidases (proteases) which hydrolyse the peptide bonds between amino acids. There are a number of different peptidases:

  • endopeptidases
  • exopeptidases
  • dipeptidases
• PROTEIN DIGESTION

  ENDOPEPTIDASES: hydrolyse the peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules (trypsin, pepsin)

  • create more ‘ends’ increasing surface area for exopeptidases
• PROTEIN DIGESTION

  EXOPEPTIDASES: hydrolyse the peptide bonds on the terminal (end) amino acids of the peptide molecules formed by endopeptidases. In this way they progressively remove single amino acids.
• PROTEIN DIGESTION

  DIPEPTIDASES: hydrolyse the bond between the two amino acids of a dipeptide. Dipeptidases are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum.
• ABSORPTION OF AMINO ACIDS: CO-TRANSPORT: 1

  1, SODIUM POTASSIUM PUMP (CARRIER PROTEIN BETWEEN EPITHELIAL CELL AND BLOODSTREAM)

  1. Na+ from epithelial cell→ bloodstream by active transport in the sodium potassium pump.
  2. K+ from blood → epithelial cells

  2. As ileum has a high concentration of Na+ ions following digestion, concentration of Na+ ions now lower in epithelial cells than ileum, creating a concentration gradient between lumen of ileum and epithelial cells.