H2 Digestion

1. H2 Digestion

2. Assessment Statements • H.2.1 State that digestive juices are secreted into the alimentary canal by glands, including salivary glands, gastric glands in the stomach wall, the pancreas and the wall of the small intestine. • H.2.2 Explain the structural features of exocrine gland cells. • H.2.3 Compare the composition of saliva, gastric juice and pancreatic juice. • H.2.4 Outline the control of digestive juice secretion by nerves and hormones, using the example of secretion of gastric juice. • H.2.5 Outline the role of membrane-bound enzymes on the surface of epithelial cells in the small intestine in digestion. • H.2.6 Outline the reasons for cellulose not being digested in the alimentary canal. • H.2.7 Explain why pepsin and trypsin are initially synthesized as inactive precursors and how they are subsequently activated. • H.2.8 Discuss the roles of gastric acid and Helicobacter pylori in the development of stomach ulcers and stomach cancers. • H.2.9 Explain the problem of lipid digestion in a hydrophilic medium and the role of bile in overcoming this.

3. Secretion of digestive juices • digestive juices are secreted into the alimentary canal by glands • These glands includes; • salivary glands,- secrets saliva • gastric glands in the stomach wall,- secret gastric juice • pancreas, - secrets pancreatic juice • wall of the small intestine, - secrets intestinal juice

4. Structural features of exocrine gland cells • Exocrine glands secrets into a space, lumen or duct • The clustered secretary cells of an exocrine gland, arranged around the space into which secretion takes place, are called acini (sing. acinus) • Secretory cells have a distinctive structure, the cyrtoplasmis packed with: • rough endoplasmic reticulum (rER) which synthesis of proteins to be packaged for export • many mitochondria – the source of the ATP necessary for the protein synthesis • several Golgi apparatus – processing & packaging proteins from rER • nucleoli in the nucleus – synthesis ribosomes for rER

5. Group of acini with their ducts connecting together

6. TEM of secretory cells of the exocrine glands of the pancreas

7. By use of a table, compare the composition of saliva, gastric juice & pancreatic juice.

9. Control of gastric juice secretion by nerves & hormones.

10. Control of gastric juice secretion by nerves & hormones • sight & smell of food initiate release of gastric juice before food is taken into the mouth • i.e. before food reaches stomach, gastric juice is already secreted by reflex action • chemoreceptors & stretch receptors in gastric mucosa detect presence of food in the stomach • impulses from these receptors are sent to brain, which sends impulses to gastric glands • causing the release more gastric juice • impulses are also sent to endocrine glands in gastric mucosa to release gastrin into the blood stream • gastrin stimulates gastric glands to increase secretions of gastric juice • gastric juices contains hydrochloric acid & pepsinogen • low pH of stomach & hormones inhibits gastrin production through negative feedback mechanism

11. The control of gastric secretion

12. Role of membrane-bound enzymes • some digestive enzymes such as maltase are immobilised in the plasma membrane of epithelial cells on the surface of intestinal villi • enzyme immobilisation is when the enzyme molecule is attached to a fixed surface • being fixed to the membrane of the gut epithelium is more efficient since the enzyme is not removed (reused) & can be linked to secondary functions such as membrane transport • maltose binds into the active site of maltase on the cell membrane enzyme. • maltose is hydrolysed into glucose moleculeswhich are immediately absorbed into epithelial cells & pass into blood capillary

13. Reasons for cellulose not being digested in the alimentary canal • cellulose exists in plant cell walls • it is insoluble in water, causing a problem with it’s digestion • humans do not have enzyme cellulasethus they are not able to digest cellulose • cellulose makes up the roughage or dietary fibre that is an essential component of our diet • eventually, cellulose is egested along with other undigested materials as faeces

14. Synthesis & activation of Pepsin & Trypsin • pepsin and trypsin are protease enzymes • if they were produced in an active form they would digest the exocrine cells that make them • thus they are produced in an inactive form (precursors), pepsinogen & trypsinogen • pepsinogen is activated by HClwhile trypsinogen is activated by enterokinase

15. Helicobacter pylori (H. pylori) • Helicobacter pylori is a bacterium which enters the stomach & survives by attaching to receptors on the plasma membrane of the cells of the stomach mucosa, below the mucus lining • in stomach, the bacteria are protected from strong acidity by: • the mucus layer that lines the inner surface; • secreting the enzyme urease which neutralized the acidic environment of the stomach by converting urea into the basic ammonia and buffer bicarbonate • bacteria cells can't be destroyed by the body’s immune system because the mucus layer prevents antibodies from reaching the bacteria

16. Roles of gastric acid &Helicobacter pylori in the development of stomach ulcers &stomach cancers. • about 80 % of gastric ulcers are caused by Helicobacter pylori (H. pylori) • H. pylori is a bacterium which causes gastritisi.e. inflammation of stomach lining • stomach ulcers are open sores in the stomach wall lining • prolonged presence of ulcers may lead to the formation of tumours • cancer of the stomach is a malignant tumour in the stomach wall • H. pylori survives in the stomach mucosa by producing urease which neutralizes gastric acid • colonization by H. pylori opens up & weakens the protective mucus liningfor digestive attack by gastric acid (HCl), causing ulcers • linking H. pylori to stomach ulcers was a paradigm shift in medicine • cause was previously thought to be stress, lifestyle, or diet • H. pylori isnow thought to be primary cause of gastric ulcers & is now treated as infectious disease

17. Problem of lipid digestion in a hydrophilic medium & the role of bile in overcoming this • lipids are strongly hydrophobic & resist break-up into small droplets in a hydrophilic (aqueous) medium • lipid molecules tend to coalesce forming droplets of fat • lipase is water soluble but has an active site to which its hydrophobic substrate binds • bileis a strongly alkaline yellow green, mucous fluid containing the bile salts, bile pigments & cholesterol • bile’s main role is to emulsify fats i.e. break fats into tiny droplets which enormously increases their surface area therefore increasing the rate at which it digest lipid • bile also neutralisesthe acidity of the chyme

18. Emulsification of fats • bile salts are molecules with both hydrophilic & hydrophobic properties • tiny spheres of lipid are formed, with the hydrophobic part of bile salts embedded in their surfaces, and the hydrophilic parts exposedto interact with water & prevent lipid molecules from coalescing with each other • the droplets in this condition, known as micelles, remain suspended in an aqueous medium. • this process is called emulsification

19. Revision Questions • Explain the structural features of exocrine gland cells. [4] • Compare gastric juice and pancreatic juice. [3] • Outline the control of the secretion of gastric juice by nerves and hormones. [4] • Outline the reason for one named substance found in food not being digested and absorbed by humans. [2] • Discuss the roles of gastric acid and Helicobacter pylori in the development of stomach ulcers and cancers. [5] • Explain why trypsin is initially synthesized as an inactive precursor and how it is activated. [3] • State two components of bile. [2] • Explain the role of bile in lipid digestion. [2]