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First year semester 2 > GI Metabolism > Flashcards

GI Metabolism Flashcards

1
Q

What form are nutrients absorbed from the small and large intestine as?

A

glucose, amino acids / di-/tri-peptides, triglycerides, VFAs

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2
Q

What organ plays a key role in absorption?

A

LIVER

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3
Q

How does the Liver play a key role in absorption

A
  1. Monosaccharides / amino acids absorbed from gut first pass through liver via hepatic portal vein
  2. Lipids enter systemic circulation via lymphatic system (too big for blood transport), bypassing liver
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4
Q

Briefly describe metabolism in carnivores/ omnivores:

A
  1. It varies between 2 phases: Absorptive state (when eating) and Post absorptive state (not eating)
    • Absorptive state: Energy derived from oxidation of absorbed nutrients
    • Post-absorptive state = Energy derived from oxidation of stored nutrients
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5
Q

Briefly describe metabolism in herbivores

A
  1. Almost continuous = No real distinction between absorptive & post-absorptive states
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6
Q

Metabolism in the absorptive state reason:

A
  1. ENERGY - cells use absorbed nutrients as source of energy and for PROTEIN SYNTHESIS
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7
Q

What happens to nutrients?

  1. What absorbs it
  2. What happens to nutrients
  3. What happens to surplus nutrient?
A
  1. Liver absorbs
  2. Uses glucose/ AA as necessary, but will always be a surplus as eating.
  3. surplus converted into glycogen, filling up stores
  4. finite capacity of gly stores
  5. XS glucose = converted into fat
  6. Net synthesis of protein in all cells
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8
Q

What happens to CARBOHYDRATE in the absorptive state for CARNIVORES/ OMNIVORES?

A
  1. (hydrolysed) CHO absorbed as monosaccharides, principally glucose
  2. Galactose / fructose converted to glucose in liver
  3. always hydrolysable CHO goes into glucose
  4. Fibre = non hydrolysable CHO
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9
Q

What happens to CARBOHYDRATE in the absorptive state for

A
  1. (hydrolysed) CHO absorbed as monosaccharides, principally glucose
  2. Galactose / fructose converted to glucose in liver
  3. non hydrolysable CHO (fibre) = fermented by bacteria into glucose which is used by bacteria NOT host. Metabolism of this glucose in anaerobic conditions releases VFAs.
  4. VFA can be used as host (waste product of bacteria)
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10
Q

What are plasma glucose levels maintained at?

A

5 mmol/l

millimolar

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11
Q

Why do we need to keep plasma glucose levels at 5 mmol/l?

How do we maintain?

A
  1. To supply all cells with the glucose they require
  2. Keep constant whether eating a meal or not
    MAINTAINED:
    • In absorptive state glucose removed from blood (mainly by liver via hepatic portal vein) & stored / metabolised
    • In post-absorptive state glucose stores released
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12
Q

What happens to glucose in ALL tissues

A

• Glucose used as energy source

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13
Q

What happens to glucose in the liver

A
  1. Glucose taken up and stored as glycogen
  2. This continues until glycogen represents ~5% total liver mass
  3. Glucose still remaining is converted to tri-glycerides:
    - Some tri-glycerides stored in liver
    - Most exported to blood as VLDL (very low density lipoproteins)
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14
Q

What happens to glucose in skeletal muscle?

A

• Glucose taken up and stored as glycogen

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15
Q

What happens to glucose in adipose tissue?

A
  • Glucose taken up and converted to glycerol for synthesis of tri-glycerides
  • If glucose in excess some is also converted to FFA (water insoluble)
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16
Q

What happens to LIPIDS in the absorptive state?

A
  1. Water insoluble therefore can’t be transported in blood (aq environ)
  2. Made water soluble by binding to proteins
  3. Free Fatty Acids (product of fat digestion) bind to albumin
  4. All other lipid complexes (triglycerides, phospholipids, cholesterol) bind to apoprotein = lipoprotein. = now can be transported in aq environment
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17
Q

What is one of the purposes of albumin?

A
  1. bind to FFA (water insoluble)

2. Enable them to be transported around the body

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18
Q

What are the constituents of a lipoprotein?

A
  1. Complex of lipid: triglycerides, phospholipids, cholesterol
  2. Bound to apoprotein
19
Q

Purpose of lipoprotein

What are 4 types of lipoprotien?

A

To transport FAT
• Chylomicrons – fat transported to liver after being absorbed
• Very low density lipoproteins (VLDL) - transport from liver to body
• Low density lipoproteins (LDL)
• High density lipoproteins (HDL)

20
Q

What are the similarities and differences between chylomicron and very low density lipoprotein?

A

Similarities
1. Both lipoproteins:. transport fat
Differences:
1. Chylomicron = transports fat that has been absorbed
2. VLDL = transporting fat that has been synthesised in the liver

21
Q

Fate of chylomicrons and VLDL

A
  • Converted to FFA by lipoprotein lipase (in capillary wall)
  • Taken up by adipose tissue (stored as triglycerides) or muscle (oxidised for energy)
  • Chylomicron remnants & some VLDL remnants (also referred to as IDL) taken up by liver & metabolised
  • Remaining VLDL remnants converted to LDL & transfer cholesterol to other cells (bad form of cholesterol – ideally want cholesterol to be removed in the liver)
22
Q

Fate of HDL (synthesised in liver)

A
  • Transfers apoproteins to chylomicrons & VLDL to enhance lipid uptake
  • Removes cholesterol from other cells & transfers it to liver (good form of cholesterol) as cholesterol-rich HDL
  • Cholesterol-rich HDL degraded & releases cholesterol - converted to bile salts or excreted in bile
23
Q

What happens to amino acids in the absorptive state?

A

Liver keeps level constant

  1. Most (75%) absorbed amino acids taken up by liver via hepatic portal vein
  2. Used for protein synthesis for its OWN use.
  3. SURPLUS = converted to keto acids
  4. 25% bypasses lvier and enters systemic circulation
24
Q

What proteins does the liver synthesise?

A

• Albumin, fibrinogen, enzymes, coagulation factors, globulins etc

25
Q

What are keto acids used?

A
  • Provide energy to liver cells (via CHO metabolism pathways)
  • Converted to glucose / glycogen
  • Converted to fatty acids (used in lipid synthesis)
  • used to synthesise non essential amino acids
26
Q

The 25% AA that bypass liver and enter systemic circulation, what used for?

A
  • Used for protein synthesis
  • Used for energy if glucose levels low
  • Converted to fat or glycogen for storage
27
Q

What does the breakdown of AA result in?

A

Degradation of amino acids for energy leads to NH3( TOXIC TO BRAIN) sooo:

  1. Converted to urea by liver & excreted by kidneys
  2. EXCEPT herbivores:
    a) Urea transferred to fore-stomach / large intestine via diffusion across epithelium or secreted in saliva (ruminants only)
    b) Used for microbial protein synthesis
    c) XS urea excreted by kidneys
28
Q

What is occurring in the post absorptive state?

A
  1. animal NOT eating so no nutrients coming in
  2. All cells still need energy source to carry out functions = plasma glucose will decline
  3. Dependent on mobilisation of stored substrates for energy
  4. need to keep glucose plasma level at 5mmol/l
  5. liver switches to mobilise glycogen stores (glycogenolysis) and gluconeogenesis (amino acids into glucose)
29
Q

In post absorptive state where do diff body parts get glucose from?

A
  1. Important brain, erythrocytes and kidneys use glucose
  2. anaerobic conditions skeletal muscle. Pregnancy and lactation
  3. other tissues derive energy from lipids so don’t deplete finite glucose stores
30
Q

Post absorptive state: mobilisation of glycogen

A
  1. LIVER
  2. Reduced its own anabolic activities
  3. Mobilisation of glycogen stores to release glucose into blood
  4. Liver itself will switch to sue keto acids or fats as energy source so glucose used for essential tissues
  5. glycogen stores = only for few hours = have to do gluconeogenesis
31
Q

Gluconeogenesis

A
  1. glycogen stores depleted glucose synthesised from non-carbohydrate sources (= gluconeogenesis)
  2. LIVER AND KIDNEY
  3. substrate for gluconeogenesis: PYRUVATE (VFA = fermentation) (or formed from breaking down lactate, glycerol or amino acids)
32
Q

Glucose and different animal groups: carni, herbi, omni

A
  1. Omnivores = only absorb sufficient glucose directly from diet
  2. carbi: diet = low in CHO = v dependent on gluconeogenesis from digested fat/ protein
  3. Herbivores: not enouch CHO in diet, main source from VFA after fermentation
33
Q

What is the pre cursor of glucose for animals in starvation in post absorptive state?

A

amino acids from animal’s own skeletal muscle

34
Q

Role of skeletal muscle in post absorptive state

A
  • Glycogen stores in skeletal muscle similar to liver
  • In post-absorptive state skeletal muscle also starts glycogenolysis
  • BUT can’t dephosphorylate glucose phosphate hence free glucose can’t be transported into blood stream
  • Instead glucose oxidised to pyruvate / lactate
  • Converted to glucose by liver and then can enter blood stream
  • This cycling referred to as Cori cycle (LONG WAY ROUND)
  • When skeletal muscle glycogen stores depleted amino acids from protein degradation serve as major pre-cursors for gluconeogenesis
35
Q

What used fat as a energy source in post absorptive state?

A
  1. essential organs other than brain, kidney, RBC
36
Q

How are lipids used as an energy source?

A
  1. (During absorptive state VLDL synthesised from excess glucose, then released in blood stream)
  2. During post-absorptive state VLDL synthesised from plasma FFA
    •FFA normally transported in blood bound to albumin
    • Amount of albumin limits transport of FFA
    • FFA transport capacity increased by synthesis of VLDL
  3. VLDL especially important in mobilisation of tri-glycerides from adipose tissue
    •Tri-glycerides broken down by hormone-dependant lipase
    a) Glycerol used in gluconeogenesis
    b) FFA oxidised for energy production (glucose-sparing)
    c) Nervous tissue & fetuses have very low uptake of FFA therefore heavily dependant on glucose for energy
37
Q

How are ketone bodies synthesised?

A

BY LIVER

  1. Most mobilised FFA taken up by liver
  2. Converted to Acetyl CoA and used as energy source by liver (via oxidation in citric acid cycle)
  3. Most acetyl CoA is surplus & converted to ketone bodies
38
Q

Kentone bodies in monogastric vs ruminants

A

Monogastric:
1. Ketone bodies synthesised exclusively by liver
2. Only low levels synthesised in pigs / horses (most FFAs re-esterified to tri-glycerides)
3. Brain can switch to ketone bodies as energy source enabling mobilised proteins to last longer if glucose vvv low
Ruminants:
1. Ketone bodies also synthesised from butyrate VFA in ruminal epithelial cells
a) Very important as butyrate inhibits gluconeogenesis from pyruvate
2. Unable to limit brain glucose utilization – ruminants more prone to ketosis. = smell on breath

39
Q

If the volatile fatty acid butyrate build up too much in ruminants what happens?

A
  1. inhibits gluconeogeneesis

2. R = v heavily depended on gluconeogenesis for generation of glucose supplies

40
Q

How is carbohydrate metabolism regulated

A
  1. Insulin: absorptive state
  2. Glucagon: post absorptive state
  3. Adrenaline
  4. Glucocorticoids released during starvation. Make gluconeogenesis easier to occur
41
Q

How is protein metabolism regulated?

A 
Insulin (absorptive state)
• Increased amino acid uptake
• Increased protein synthesis in
liver and muscle (anabolysis)
• Glucagon (post-absorptive state)
• INCREASED amino acid uptake
in liver only
• Proteins degraded into amino
acids that can be used in
gluconeogenesis
42
Q

How is protein metabolism regulated?

A 
• Insulin (absorptive state)
• Triglycerides synthesised from
glycerophosphates / FFA &
stored
• Inhibition of FFA release into
blood
• Glucagon (post-absorptive state &
during exercise)
• Increases lipolysis in adipose
tissue
• Mobilisation of triglycerides for
ATP production
43
Q

What effect does the ratio of insulin: glucagon have?

A
  1. determines the net effect of metabolism
  2. Absorptive state: I = high so ratio = high = anabolism
  3. Post = low I, low ratio = catabolism = breakdown to release glucose
44
Q

What is important to remember about ruminants and gluconeogenesis?

A
  1. They require gluconeogenesis to occur in both absorptive and post-absorptive state as primary source of energy is VFAs NOT glucose
    • Propionic acid / amino acids serve as pre-cursors for glucose in
    absorptive state
    • Amino acids / glycerol serve as pre-cursors for glucose in postabsorptive state
    • utilisation / production of glucose reduced
    • proteins / lipids mobilised
    • Insulin : glucagon ratio remains relatively low & constant

First year semester 2 (50 decks)

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