Lecture 26: Chemical Digestion Flashcards
What is chemical digestion?
when we release the nutrients necessary for our own growth and energy needs
Motility carries out mechanical digestion which is essential for chemical digestion. How?
Mechanical digestion breaks up the food into small pieces which is needed for chemical digestion
How does motility aid in chemical digestion?
mechanical digestion mixes the food with digestive enzymes
How does motility aid absorption?
by moving the food through the GI tract at a rate that allows for maximum absorption
How does secretion aid mechanical digestion?
the mucous coats the food and stops abrasion of epithelium
Why is secretion essential for chemical digesiton?
secretions contain digestive enzymes and electrolytes which ensure optimal pH for the enzymes
How does secretion aid in absorption?
By suspending food molecules in solution so they can be exposed to absorptive surfaces
What are the three main nutrients that undergo chemical digestion?
- carbohydrates (sugars)
- proteins
- lipids (fats)
What is the main form of carbohydrates that we ingest?
storage polysaccharides
Give an example of a storage polysaccharide in grains and meat
starch in grains and glycogen in meat
What are storage polysaccharides?
very large complex chains of monosaccharides such as glucose
A large number of ________ molecules are bound together in long chains which is bound together in further chains to form _______ _______
glucose
storage polysaccharides
Starch and glycogen are storage carbohydrates that are joined by what type of bonds?
α 1-4 glycosidic bonds
As well as ingesting storage carbohydrates, we can also ingest _______
disaccharides
Give three examples of disaccharides and what monosaccharides they are made of
- sucrose made from glucose and fructose
- lactose made from glucose and galactose
- maltose made from two glucose molecules
We also ingest a small number of monosaccharides. What monosaccharide do we ingest?
glucose
Proteins are required to be ingested for us to have ______ ______
amino acids
According to the lecture slide, how many amino acids are there and how many can we synthesise?
21 amino acids
12 can be synthesised
What is an essential amino acid?
amino acids that we need to access from our diet
Where are our two sources of protein?
- diet
- endogenous proteins
What are endogenous proteins?
proteins secreted into our intestine such as enzymes and immunoglobins which we digest and reabsorb
Ingested proteins are long chains of ______ ______ linked by ______ bonds
amino acids
peptide
The lipids that we ingest are mainly
triglycerides
What does a triglyceride consist of?
a glycerol back bone with 3 fatty acids attached
Fatty acids can be variable in chain length. How many carbons make up short chain fatty acids, medium chain fatty acids, long chain fatty acids respectively?
- short: less than 6 carbons
- medium: 6 - 12 carbons
- long: 12 - 24 carbons
Why do we need chemical digestion?
because we ingest nutrients in the form of large complex molecules (carbs, proteins, lipids) but we can absorb nutrients as small molecules
What is the purpose of chemical digestion?
to reduce the size of nutrients to allow them to be absorbed
Where does chemical digestion occur?
At the surface of food particles
During chemical digestion, we break up molecules into its constituent parts. What are the constituent parts of sugars, proteins and fats?
- sugars: monosaccharides
- proteins: amino acids
- fats: monoglycerides and free fatty acids
Chemical digestion utilises ________ enzymes
chemical
What are digestive enzymes?
Extracellular, organic catalysts
Are digestive enzymes specific?
yes
they need different enzymes for different substrates
Give the different substrates for amylase, protease and lipase
Amylase: carbohydrates
Proteases: proteins
Lipases: fats
Enzymes have an optimal pH. Give the optimal pH for salivary, gastric and small intestinal enzymes
Salivary: alkaline
Gastric: acidic
Small intestinal: alkaline
What are the two stages of chemical digestion?
luminal digestion
contact digesiton
What is luminal digestion?
the initial digestion involving enzymes secreted into the lumen
Give examples of enzymes involved in luminal digestion in the mouth, stomach and small intestine
salivary amylase
pepsin
pancreatic enzymes
What is contact digestion?
involved enzymes produces by enterocytes and attached to brush border of the enterocytes
Where does contact digestion only occur?
in the small intestine
What are enterocytes?
epithelium of the small intestine
Describe the luminal digestion of carbohydrates
salivary amylase in the mouth converts polysaccharides to oligosaccharides and pancreatic amylase converts oligosaccharides to disaccharides in the small intestine
Describe contact digestion of carbohydrates
Disaccharides are converted to monosaccharides by different enzymes attached to the brush border
What are three enzymes involved in the contact digestion of carbohydrates and what do they break down?
- Sucrase breaking down sucrose to glucose and fructose
- lactase breaking down lactose to glucose and galactose
- maltase breaking down maltose into two glucose molecules
Describe the luminal digestion of proteins
Pepsin in the stomach and trypsin, chymotrypsin, carboxypeptidase in the small intestine convert proteins to polypeptides
Describe the contact digestion of proteins in the small intestine
peptidases attached to the brush border convert polypeptides into individual amino acids
What is different about the chemical digestion of fats compared to the digestion of proteins and carbohydrates?
there is no contact digestion
What is the main digestive enzyme involved in digestion of lipids?
pancreatic lipase
What are the four phases of the chemical digestion of fat?
- emulsification
- stabilisation
- digestion
- formation of micelles
Describe the emulsification stage of fat digestion
- motility breaks up fat droplets into small droplets which forms an emulsion
Where does the emulsification phase of fat digestion occur and what are the processes occurring here that allow that to happen?
stomach - retropulsion
small intestine - segmentation
Where does the stabilisation phase of fat digestion occur?
in the small intestine
What is involved in emulsification?
breaking up large fat droplets into smaller droplets
What is the role of bile salts in the stabilisation phase of fat digestion?
Bile salts secreted by the liver and concentrated in the gallbladder are released into the small intestine with the arrival of food. They have both a hydrophobic side and a hydrophilic side. They stabilise the emulsion in the small intestine by preventing the droplets from coalescing due to like charges repelling
Describe the hydrolysis stage of lipid digestion
colipase binds to the bile salt around the small droplets and anchors pancreatic lipase at the surface of the droplet to convert the triglycerides to monoglycerides and free fatty acids
What is the purpose of micelle formation around free fatty acids and monoglycerides?
FFA and MAGs are insoluble in water so they need to be kept in solution through the formation of the micelles.
What are micelles?
small droplets made from bile salts surrounding the FFA and MAGs
