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Lipid Metabolism Essentials
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1
Question
True or False: Triacylglycerols (TAGs) are the primary form of dietary lipids, making up approximately 98% of dietary lipids.
Answer
True. Dietary lipids are approximately 98% triacylglycerols (TAGs).
2
Question
Describe the initial physical change that dietary lipids undergo in the stomach during digestion.
Answer
Physical churning breaks large fat masses into small droplets forming a semi-liquid material called chyme; this increases surface area for enzymatic action.
3
Question
What fraction of ingested TAGs is hydrolyzed in the stomach by gastric lipase?
Answer
About 10% of ingested TAGs are hydrolyzed in the stomach by gastric lipase.
4
Question
Multiple choice: Which hormone triggers bile release and stimulates emulsification of fats when chyme enters the small intestine? A) Insulin B) Cholecystokinin (CCK) C) Glucagon D) Epinephrine
Answer
B) Cholecystokinin (CCK). CCK triggers bile release from the gallbladder and aids emulsification.
5
Question
What role do bile salts play in fat digestion in the small intestine?
Answer
Bile salts act as emulsifying agents that break large fat globules into smaller particles and solubilize them in the aqueous environment, facilitating enzyme access.
6
Question
Identify the major pancreatic enzyme responsible for hydrolyzing TAGs into monoacylglycerols and free fatty acids in the small intestine.
Answer
Pancreatic lipase is the major enzyme that hydrolyzes TAGs to monoacylglycerols and free fatty acids in the small intestine.
7
Question
Explain micelle formation and its importance for intestinal absorption of lipids.
Answer
Monoacylglycerols and free fatty acids combine with bile salts to form micelles—tiny spherical droplets with hydrophobic interiors and hydrophilic exteriors—allowing lipid components to be soluble and absorbed across enterocyte membranes.
8
Question
True or False: Micelles are large lipoprotein particles that enter the bloodstream directly after formation in the intestinal lumen.
Answer
False. Micelles are small soluble aggregates that deliver lipids to enterocyte membranes; the lipids are reassembled into chylomicrons before entering the lymphatics, not the bloodstream directly.
9
Question
After absorption in enterocytes, into what form are lipids reassembled before export, and what is their core composition percentage?
Answer
Absorbed monoacylglycerols and fatty acids are reassembled into TAGs and packaged into chylomicrons, which have a core of approximately 95% TAGs.
10
Question
Describe why chylomicrons enter the lymphatic system before reaching the bloodstream.
Answer
Chylomicrons are too large to pass through capillary walls, so they enter lymphatic vessels (lacteals) and travel via the thoracic duct to reach the bloodstream.
11
Question
Multiple choice: Which enzyme located on the vascular endothelium hydrolyzes TAGs in chylomicrons to release glycerol and free fatty acids? A) Hormone-sensitive lipase (HSL) B) Pancreatic lipase C) Lipoprotein lipase (LPL) D) Lipase A
Answer
C) Lipoprotein lipase (LPL). LPL on capillary endothelium hydrolyzes TAGs within chylomicrons.
12
Question
Identify the primary storage cell for excess fat in the body and one structural feature that allows it to store large quantities of TAGs.
Answer
Adipocytes are the primary fat-storage cells; they have large lipid droplets occupying most of the cell volume, allowing large TAG storage.
13
Question
True or False: Adipose tissue is located primarily beneath the skin (subcutaneous) and around vital organs, providing both energy storage and mechanical protection.
Answer
True. Adipose tissue is subcutaneous and visceral and provides energy storage, insulation, and mechanical protection.
14
Question
Explain the hormonal trigger and intracellular messenger that initiates triacylglycerol mobilization from adipocytes.
Answer
Hormones such as epinephrine and glucagon bind adipocyte receptors, stimulate adenylate cyclase to raise cyclic AMP (cAMP), which activates hormone-sensitive lipase (HSL) to hydrolyze stored TAGs into glycerol and free fatty acids.
15
Question
What are the three parts of fatty acid breakdown for energy as described in the material?
Answer
1) Activation: fatty acid is activated to acyl-CoA on the outer mitochondrial membrane. 2) Transport: acyl-CoA is transported into the mitochondrial matrix by the carnitine shuttle. 3) Oxidation: acyl-CoA undergoes repeated β-oxidation to produce acetyl-CoA, FADH2, and NADH.
16
Question
How is a fatty acid activated before β-oxidation and what energy cost is required?
Answer
A fatty acid is activated by conjugation with coenzyme A to form acyl-CoA on the outer mitochondrial membrane; this activation consumes the energy equivalent of two ATP molecules (ATP → AMP + PPi).
17
Question
Identify the transport mechanism that moves acyl groups into the mitochondrial matrix for oxidation.
Answer
The carnitine shuttle transports acyl groups into the mitochondrial matrix: acyl-CoA is converted to acyl-carnitine, transported across the inner membrane, and reconverted to acyl-CoA inside the matrix.
18
Question
Enumerate the four recurring chemical steps of the β-oxidation cycle inside the mitochondrial matrix.
Answer
1) Dehydrogenation (formation of a trans double bond; FAD → FADH2). 2) Hydration (addition of water across the double bond). 3) Dehydrogenation (oxidation of hydroxyl to keto; NAD+ → NADH). 4) Thiolysis (cleavage by CoA releasing acetyl-CoA and a shortened acyl-CoA).
19
Question
True or False: The first dehydrogenation step in β-oxidation reduces NAD+ to NADH.
Answer
False. The first dehydrogenation uses FAD as the oxidizing agent and reduces it to FADH2; the second dehydrogenation reduces NAD+ to NADH.
20
Question
Explain what is produced each cycle during β-oxidation from cleavage of a fatty acyl-CoA chain.
Answer
Each β-oxidation cycle yields one molecule of acetyl-CoA and an acyl-CoA shortened by two carbons; plus reduced cofactors (one FADH2 and one NADH per cycle, except for final cleavage depending on chain length).
21
Question
Calculate (from the notes) the net ATP yield from complete β-oxidation of an 18-carbon saturated fatty acid (stearic acid).
Answer
Net ATP yield for stearic acid (C18) is 120 ATP after subtracting the 2 ATP equivalents used for initial activation.
22
Question
Break down the components of the gross ATP production for C18 fatty acid listed in the text (how many Acetyl-CoA, FADH2, NADH and approximate ATP equivalents?).
Answer
C18 yields 9 acetyl-CoA (9 × 10 ATP = 90 ATP), 8 FADH2 (8 × 1.5 ATP = 12 ATP), and 8 NADH (8 × 2.5 ATP = 20 ATP), giving a gross total of 122 ATP before activation cost.
23
Question
True or False: On a per-gram basis, the oxidation of fat yields more than twice the energy of carbohydrate oxidation.
Answer
True. On an equal-mass basis, fats produce more than twice the energy — roughly 2.5 times the ATP compared to carbohydrates, which supports their higher kcal/g value.
24
Question
From the provided energy reserves table: Which energy reserve contains the largest percentage of total stored energy in a 70-kg person, and what is the approximate amount?
Answer
Triacylglycerol reserves contain the largest percentage, about 84.3% of stored energy, roughly 135,000 kcal for a 70-kg person.
25
Question
List two metabolic fates of acetyl-CoA mentioned in the notes.
Answer
1) Energy production via oxidation in the citric acid cycle (TCA) to generate ATP. 2) Conversion to ketone bodies during carbohydrate/lipid imbalance. (Additional fates: fatty acid biosynthesis and cholesterol biosynthesis.)
26
Question
True or False: In humans, acetyl-CoA can be converted back into pyruvate for net gluconeogenesis.
Answer
False. Humans cannot convert acetyl-CoA back to pyruvate for net synthesis of glucose; acetyl-CoA cannot serve as a net gluconeogenic precursor.
27
Question
Define ketogenesis and name the primary organ where it occurs.
Answer
Ketogenesis is the metabolic process converting excess acetyl-CoA into ketone bodies; it occurs primarily in the liver mitochondria.
28
Question
Enumerate the three primary ketone bodies produced during ketogenesis.
Answer
1) Acetoacetate (a C4 ketoacid), 2) β-hydroxybutyrate (a C4 hydroxyacid), and 3) Acetone (a C3 ketone produced by decarboxylation of acetoacetate).
29
Question
Outline the four-step metabolic pathway of ketogenesis from acetyl-CoA described in the material.
Answer
1) First condensation: two acetyl-CoA condense to form acetoacetyl-CoA. 2) Second condensation: acetoacetyl-CoA reacts with a third acetyl-CoA to form HMG-CoA. 3) Chain cleavage: HMG-CoA is cleaved to produce acetoacetate and one acetyl-CoA. 4) Hydrogenation: Acetoacetate can be reduced by NADH to form β-hydroxybutyrate.
30
Question
True or False: Ketone bodies are water soluble and can be used as energy substrates by heart, muscle, and brain during prolonged starvation.
Answer
True. Ketone bodies are water-soluble and can be transported in blood to fuel heart, muscle, and the brain during prolonged fasting.