H2 Digestion

Assessment StatementsH.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.

Secretion of digestive juices digestive juices are

secreted into the alimentary canal by glands

These glands includes;salivary glands,- secrets

salivagastric glands in the

stomach wall,- secret gastric juice

pancreas, - secrets pancreatic juice

wall of the small intestine, - secrets intestinal juice

Structural features of exocrine gland cellsExocrine 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 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

Group of acini with their ducts connecting together

TEM of secretory cells of the exocrine glands of the pancreas

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

Secretion and gland

Site of action

Active ingredients, conditions required & outcome

pH enzymes &non-enzyme components

substrate or effect product

Salivasalivary glands

mouth

6.5–7.5

Amylasemucus

Starchlubricates

maltose

gastric juicegastric glands

stomach

2.0

Pepsinrennin (young mammals only)hydrochloric acid

Proteinscoagulates milk proteincreates acidicenvironment thatkills bacteria

polypeptides

pancreatic juicepancreas

small intestine

7.0

Amylaseproteases (trypsin & chymotrypsin)peptidases

Lipases

nucleases

Starchproteins

polypeptides

Triglycerides

nucleotides

MaltosePolypeptidespeptides &amino acids

fatty acids &Glycerolpentose sugars, Pi & bases

Control of gastric juice secretion by nerves & hormones.

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

The control of gastric secretion

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 molecules which are immediately absorbed into epithelial cells & pass into blood capillary

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 cellulase thus 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

Synthesis & 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 them

thus they are produced in an inactive form (precursors), pepsinogen & trypsinogen

pepsinogen is activated by HCl while trypsinogen is activated by enterokinase

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

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 gastritis i.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 lining for 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 is now thought to be primary cause of gastric ulcers & is now treated as infectious disease

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

bile is 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 neutralises the acidity of the chyme

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 exposed to 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

Revision QuestionsExplain 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]