2Assessment 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.
3Secretion of digestive juices digestive juices are secreted into the alimentary canal by glandsThese glands includes;salivary glands,- secrets salivagastric glands in the stomach wall,- secret gastric juicepancreas, - secrets pancreatic juicewall of the small intestine, - secrets intestinal juice
4Structural features of exocrine gland cells Exocrine glands secrets into a space, lumen or ductThe 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 cyrtoplasm is packed with:rough endoplasmic reticulum (rER) which synthesis of proteins to be packaged for exportmany mitochondria – the source of the ATP necessary for the protein synthesisseveral Golgi apparatus – processing & packaging proteins from rERnucleoli in the nucleus – synthesis ribosomes for rER
5Group of acini with their ducts connecting together
6TEM of secretory cells of the exocrine glands of the pancreas
7By use of a table, compare the composition of saliva, gastric juice & pancreatic juice.
8Secretion and glandSite of actionActive ingredients, conditions required & outcomepHenzymes &non-enzyme componentssubstrate or effectproductSalivasalivary glandsmouth6.5–7.5AmylasemucusStarchlubricatesmaltosegastric juicegastric glandsstomach2.0Pepsinrennin (young mammals only)hydrochloric acidProteinscoagulates milk proteincreates acidicenvironment thatkills bacteriapolypeptidespancreatic juicepancreassmall intestine7.0proteases (trypsin & chymotrypsin)peptidasesLipasesnucleasesproteinsTriglyceridesnucleotidesMaltosePolypeptidespeptides &amino acidsfatty acids &Glycerolpentose sugars, Pi & bases
9Control of gastric juice secretion by nerves & hormones.
10Control of gastric juice secretion by nerves & hormones chemoreceptors & stretch receptors in gastric mucosa detect presence of food in the stomachimpulses from these receptors are sent to brain, which sends impulses to gastric glandscausing the release more gastric juiceimpulses are also sent to endocrine glands in gastric mucosa to release gastrin into the blood streamgastrin stimulates gastric glands to increase secretions of gastric juicegastric juices contains hydrochloric acid & pepsinogenlow pH of stomach & hormones inhibits gastrin production through negative feedback mechanismsight & smell of food initiate release of gastric juice before food is taken into the mouthi.e. before food reaches stomach, gastric juice is already secreted by reflex action
12Role of membrane-bound enzymes some digestive enzymes such as maltase are immobilised in the plasma membrane of epithelial cells on the surface of intestinal villienzyme immobilisation is when the enzyme molecule is attached to a fixed surfacebeing 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 transportmaltose binds into the active site of maltase on the cell membrane enzyme.maltose is hydrolysed into glucose molecules which are immediately absorbed into epithelial cells & pass into blood capillary
13Reasons for cellulose not being digested in the alimentary canal cellulose exists in plant cell wallsit is insoluble in water, causing a problem with it’s digestionhumans do not have enzyme cellulase thus they are not able to digest cellulosecellulose makes up the roughage or dietary fibre that is an essential component of our dieteventually, cellulose is egested along with other undigested materials as faeces
14Synthesis & activation of Pepsin & Trypsin pepsin and trypsin are protease enzymesif they were produced in an active form they would digest the exocrine cells that make themthus they are produced in an inactive form (precursors), pepsinogen & trypsinogenpepsinogen is activated by HCl while trypsinogen is activated by enterokinase
15Helicobacter 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 liningin 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 bicarbonatebacteria cells can't be destroyed by the body’s immune system because the mucus layer prevents antibodies from reaching the bacteria
16Roles of gastric acid & Helicobacter pylori in the development of stomach ulcers & stomach cancers. H. pylori survives in the stomach mucosa by producing urease which neutralizes gastric acidcolonization by H. pylori opens up & weakens the protective mucus lining for digestive attack by gastric acid (HCl), causing ulcerslinking H. pylori to stomach ulcers was a paradigm shift in medicinecause was previously thought to be stress, lifestyle, or dietH. pylori is now thought to be primary cause of gastric ulcers & is now treated as infectious diseaseabout 80 % of gastric ulcers are caused by Helicobacter pylori (H. pylori)H. pylori is a bacterium which causes gastritis i.e. inflammation of stomach liningstomach ulcers are open sores in the stomach wall liningprolonged presence of ulcers may lead to the formation of tumourscancer of the stomach is a malignant tumour in the stomach wall
17Problem 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) mediumlipid molecules tend to coalesce forming droplets of fatlipase is water soluble but has an active site to which its hydrophobic substrate bindsbile is a strongly alkaline yellow green, mucous fluid containing the bile salts, bile pigments & cholesterolbile’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 lipidbile also neutralises the acidity of the chyme
18Emulsification of fats bile salts are molecules with both hydrophilic & hydrophobic propertiestiny spheres of lipid are formed, with the hydrophobic part of bile salts embedded in their surfaces, and the hydrophilic parts exposed to interact with water & prevent lipid molecules from coalescing with each otherthe droplets in this condition, known as micelles, remain suspended in an aqueous medium.this process is called emulsification
19Revision QuestionsExplain the structural features of exocrine gland cells Compare gastric juice and pancreatic juice Outline the control of the secretion of gastric juice by nerves and hormones Outline the reason for one named substance found in food not being digested and absorbed by humans Discuss the roles of gastric acid and Helicobacter pylori in the development of stomach ulcers and cancers Explain why trypsin is initially synthesized as an inactive precursor and how it is activated State two components of bile Explain the role of bile in lipid digestion