19A/B: Digestion and Metabolism

Question 1

Define metabolism, catabolism and anabolism

Answer 1

• metabolism: sum of all chemical and physical changes occurring in the body
• catabolism: breakdown of large molecules into smaller molecules; releasing energy
• anabolism: conversion of small molecules into larger ones; consumes energy

Question 2

Name the components of the nutrient pool and their corresponding structural/functional/storage components

Answer 2

• amino acids (proteins)
• fatty acids (triglycerides)
• glucose (glycogen)

Question 3

Describe carbohydrate digestion.

Answer 3

1. carbohydrates ingested
2. in mouth: chewing, salivary amylase
3. in stomach: salivary amylase until pH < 4.5
4. in duodenum: secretin raises pH; CCK stimulates release of pancreatic enzymes; GIP stimulates insulin release from pancreas
5. indigestible carbohydrates are nutrient source for colon bacteria, generating flatus
6. epithelial cells lining jejunum secrete enzymes; monosaccharides cross epithelial cells into capillaries into hepatic portal vein
7. monosaccharides arrive at liver and either are converted to glucose and released into blood or stored as glycogen

Question 4

glycogenesis vs. glycogenolysis vs. gluconeogenesis vs. glycolysis

Answer 4

• glycogenesis: glucose to glycogen
• glycogenolysis: glycogen to glucose
• gluconeogenesis: non-carbohydrate carbon chains to glucose
• glycolysis: breakdown of glucose into 2/3-carbon chains

Question 5

Describe carbohydrate metabolism

Answer 5

• glucose broken down into pyruvate ( two molecules of 3-carbon chains)
• pyruvate converted into acetyl-CoA (2-carbon chains) and can enter mitochondria
• acetyl-CoA catabolism through citric acid cycle and electron transport chain produces ATP, water and CO2, and consumes O2

Question 6

Describe carbohydrate metabolism in muscles of different activity levels

Answer 6

• muscles at rest: ATP demand low, excess O2, surplus ATP; ATP converts glucose to glycogen
• moderate activity: mitochondria meets ATP demand; glycogen and glucose breakdown generates ATP
• peak activity: mitochondria can only provide 1/3 ATP; glycolysis provides the remaining 2/3 ATP but produces more pyruvate than mitochondria can use (which is converted to lactate during low O2); ATP hydrolysis produces H+, lowering pH and compromising muscle contractions

Question 7

Describe lipid digestion.

Answer 7

1. lipid ingestion
2. in mouth: chewing, lingual lipase
   3: in stomach: churning, lingual lipase
3. small intestine: CCK stimulates release of pancreatic enzymes and bile; emulsification by bile salts; pancreatic lipase
4. in jejunum: micelles contact epithelium; lipid diffuses across membrane, bile salts released; chylomicrons formed, travel through lymphatic vessels, gets broken down into fatty acids and monoglycerides by lipoprotein lipase and can diffuse into interstitial fluid

Question 8

After digestion and diffusion of fatty acids and monoglycerides into interstitial fluid, what are they used for?

Answer 8

• fatty acids absorbed by and broken down in resting skeletal muscle
• converted to triglycerides for storage in adipocytes

Question 9

Describe protein digestion.

Answer 9

1. ingestion of protein
2. in mouth: chewing
3. in stomach: churning, HCl, acid, pepsin (pepsinogen converted to pepsin by presence of HCl)
4. small intestine: CCK (pancreatic proteolytic enzymes), enteropeptidase (trypsinogen into trypsin, which converts proenzymes into active forms)
5. jejunum: dipeptidases on epithelial surfaces; amino acids absorbed into intestinal cell and exit into interstitial fluid

Question 10

After protein digestion and amino acids enter interstitial fluid; what are they used for?

Answer 10

• they enter liver by hepatic portal vein (from interstitial capillaries) and form plasma proteins or undergo gluconeogenesis

Question 11

role of hormones in absorptive state of metabolic activity

Answer 11

• primary hormone is insulin: glucose/amino acid uptake, anabolism
• androgens/estrogens/growth hormone: protein synthesis from amino acids
• elevation of glucose/lipid/amino acid levels in the blood
• anabolic pathways predominant

Question 12

role of hormones in postabsorptive state of metabolic activity

Answer 12

• glucagon and epinephrine in liver: stimulates synthesis and release of glucose
• glucagon/epinephrine/glucocorticoids/growth hormone stimulates catabolism of triglycerides into fatty acids
• glucocorticoids/growth hormone responsible for catabolism of proteins

Question 13

Describe short term regulation of satiety and appetite

Answer 13

• stimulation of satiety center: increase in blood glucose causes appetite decrease; CCK causes appetite decrease during absorptive state; stimulation of stretch receptors in stomach decreases appetite
• stimulation of feeding center: neuropeptide Y (NPY) causes appetite increase; ghrelin causes appetite increase (ghrelin increases when stomach empty)

Question 14

Describe long term regulation of satiety and appetite

Answer 14

• leptin binds to CNS neurons to increase satiation, decrease appetite
• leptin released by adipose tissue

Question 15

What does insulin do?

Answer 15

• helps recruit carrier proteins to transport glucose into the cell and stimulate glycolysis
• decreases blood glucose (converted to glycogen or taken up by body cells)