Digestion And Metabolism

Question 1

Discuss the digestion of carbohydrates

Answer 1

Carbohydrates, long complex chains cannot be absorbed
Broken down by enzymes into single units (monosaccharides) to be absorbed
Salivary amylase starts off process
Pancreatic amylase coverts all digestible polysaccharides not acted on by salivary amylase to disaccharides
Enzymes eg sucrase, maltase and lactase split the disaccharides into their monosaccharides

Question 2

Discuss the digestion of proteins

Answer 2

Proteins, long chains of polypeptides, broken down to amino acids
Broken down by protease enzymes e.g. pepsin
Problem with digesting ingested proteins is the gut is also made of proteins
So proteases are stored in an inactive form and only activated in a safe environment
Pancreatic enzymes are activated by micro villi to do this

Question 3

Discuss the digestion of fats

Answer 3

To digest fat need bile and enzymes
Bile salts break fat up into smaller droplets (it emulsifies it), they are released into the duodenum
This increases SA
Lipases are enzymes that then break the fat into smaller units which can be digested

Question 4

Discuss the lock and key hypothesis of enzyme action

Answer 4

Enzymes are protein molecules which act as catalysts to speed up reactions. They are not used up
Enzymes have a specific shape, with a specific active site. This is where the enzyme-substrate complexes are formed.
The molecules that enzymes act upon are called substrates.
An enzyme is specific for its substrate like a key for its lock
This is called the lock and key model
The enzyme remains unchanged to can be reused

Question 5

Discuss the three phases of gastric secretion

Answer 5

Cephalic phase: secretion occurs before food reaches the stomach due to reflex stimulation of vagus nerve, initiated by sight, smell, taste or thought of appealing food.

Gastric phase: stimulated by presence of food, the enteroendocrine cells in gastric glands secrete hormone gastric into blood. Stimulates more gastric juice, this way secretion of juice continues after eating is finished

Intestinal phase: when the partially digested contents of stomach reach small intestine, 2 hormones- secretin and CCK are produced by enteroendocrine cells in intestinal mucosa. They slow down secretion of gastric juice

Question 6

Discuss the constituents of bile and its role in digestion

Answer 6

Consists of:
water, mineral salts, mucus, bile salts, pigments (mainly bilirubin) and cholesterol
Functions:
Emulsification of fats in small intestine (bile salts)
Making cholesterol and fatty acids soluble (bile salts)
Excretion of bilirubin (waste product from breakdown of red blood cells), this prevents jaundice

Question 7

Discuss lipid metabolism, identifying the roles of fatty acids, triglycerides, phospholipids, and cholesterol within the body

Answer 7

Fat ingested is emulsified into smaller particles by bile salts and then lipase is secreted by the pancreas & small intestine, hydrolysing the triglycerides into free acids and monoglycerides

Triglycerides- energy store for later use
Phospholipids- prevent accumulation of fat in the liver
Cholesterol- helps make cell membranes and hormones

Question 8

Discuss carbohydrate metabolism to include monosaccharides, disaccharides and polysaccharides

Answer 8

Monosaccharides are the simplest form of carbohydrate, composed of a single module
Disaccharides are composed of 2 monosaccharides linked together
Polysaccharides are composed of 3 or more monosaccharides linked together

Question 9

Define the terms glycolysis, glycogenesis and gluconeogensis

Answer 9

Glycolysis: the break down of glucose by enzymes, realising energy and pyruvic acid
Glycogenesis: the formation of glycogen (from glucose), stored in the liver and muscle cells
Gluconeogenesis: the formation of new glucose. When glycogen stores are low and more glucose is needed, the body can make it from non-carbohydrate sources e.g. amino acids and glycerol

Question 10

Identify the role of insulin and glucagon in carbohydrate metabolism

Answer 10

Insulin- lowers blood glucose by increasing glucose transport in muscle and adipose tissue, stimulates the synthesis of glycogen, fat and protein
Glucagon- promotes hepatic (liver) conversion of glycogen to glucose and inhibits glucose break down and glycogen formation

Question 11

Discuss protein metabolism

Answer 11

Proteins are built from amino acids, 9 are essential as the body cannot synthesise them
In the small intestine digestive enzymes break down dietary proteins into amino acids for absorption into blood capillaries of the villi
Amino acids transported from small intestine to liver then general circulation
From amino acids available, different cells choose from the particular ones required for own needs