Biology

Bioenergetics

⚡ Chapter 2 · MDCAT Biology
📅 Updated Jan 2026 ⏱️ 25 min read 🎯 High-yield

Bioenergetics — Respiration of Fats & Proteins

Complete MDCAT Biology Chapter 2 on cellular respiration of proteins and fats, correlated with glucose. Includes β-oxidation, deamination, amino acid entry points, RQ values, ATP yields, plus 145 solved MCQs with explanations and year-wise past papers.

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⚡Quick Answers to Common Searches

Direct, snippet-optimized answers to the top questions students search about Bioenergetics, fat, and protein respiration.

What is bioenergetics?

Bioenergetics is the study of energy transformations in living organisms — how cells capture, store, and use energy from nutrients like carbs, fats, and proteins.

What is cellular respiration?

Cellular respiration is the process where cells break down glucose, fats, or proteins to produce ATP. It occurs in three stages: glycolysis, Krebs cycle, and electron transport chain.

Which fuel yields most energy per gram?

Fats yield about 9 kcal/g, versus 4 kcal/g for carbohydrates and proteins. Fats are more reduced (more C-H bonds) so they release more energy.

What is β-oxidation?

β-oxidation is the mitochondrial process that breaks fatty acids into 2-carbon Acetyl-CoA units, producing 1 NADH + 1 FADH₂ per cycle.

What is RQ value?

Respiratory Quotient (RQ) = CO₂ produced ÷ O₂ consumed. Fat = 0.7, Protein = 0.8, Glucose = 1.0.

What is deamination?

Deamination is the removal of the –NH₂ group from amino acids before their carbon skeletons enter respiration. Ammonia is converted to urea in the liver.

What is the central hub of metabolism?

Acetyl-CoA is the hub — carbs, fats, and proteins all converge here before entering the Krebs cycle.

Where do amino acids enter respiration?

Alanine→Pyruvate, Leucine/Lysine→Acetyl-CoA, Glutamate→α-ketoglutarate, Valine/Isoleucine→Succinyl-CoA, Aspartate→Oxaloacetate.

What is glycerol's fate?

Glycerol (from triglycerides) is converted to DHAP and enters glycolysis as a 3-carbon intermediate.

Ketogenic vs glucogenic?

Ketogenic (Leucine, Lysine) → only Acetyl-CoA, cannot form glucose. Glucogenic → can form glucose via gluconeogenesis.

What is the carnitine shuttle?

The carnitine shuttle transports long-chain fatty acids across the inner mitochondrial membrane into the matrix for β-oxidation.

ATP from one glucose?

Traditional PTB count: 36–38 ATP. Modern textbooks: 30–32 ATP. MDCAT follows the traditional count.

Why can't fats make glucose?

Fats → Acetyl-CoA. The enzyme Pyruvate Dehydrogenase (PDH) that links pyruvate to Acetyl-CoA is irreversible, so humans can't convert Acetyl-CoA back to glucose.

What happens in starvation?

After glycogen and fat stores deplete, structural proteins are broken down. Ketone bodies from fat breakdown feed the brain.

What is acetyl-CoA?

Acetyl-CoA is a 2-carbon molecule that condenses with oxaloacetate to enter the Krebs cycle. It is produced from carbs, fats, and proteins.

❓People Also Ask — Bioenergetics

Questions students most frequently search about this chapter. Answers are direct and exam-focused.

QWhat is the RQ for fat, protein, and glucose?
RQ (Respiratory Quotient) = CO₂/O₂. Fat = 0.7, Protein = 0.8, Glucose = 1.0. RQ < 1 means more O₂ consumed than CO₂ produced.
QWhy do fats yield more energy than carbohydrates?
Fats are more reduced (more C-H bonds), so they donate more electrons to the electron transport chain, producing more ATP per gram.
QWhat is the role of carnitine?
Carnitine shuttles long-chain fatty acids across the inner mitochondrial membrane for β-oxidation.
QWhat are ketone bodies?
Ketone bodies (acetoacetate, β-hydroxybutyrate, acetone) are produced from excess Acetyl-CoA during fasting/starvation. The brain uses them when glucose is low.
QWhat is the link reaction?
The link reaction converts Pyruvate → Acetyl-CoA with release of CO₂ and reduction of NAD⁺ → NADH. Occurs in the mitochondrial matrix.
QWhere does the Krebs cycle occur?
In the mitochondrial matrix. Each Acetyl-CoA yields 3 NADH, 1 FADH₂, 1 GTP, and 2 CO₂ per turn.
QHow many ATP from palmitic acid?
Palmitic acid (16C) yields approximately 129 ATP (traditional count) — much more than glucose's 36-38 ATP.
QWhat amino acids are strictly ketogenic?
Leucine and Lysine. They can only be converted to Acetyl-CoA, never to glucose.
QWhat is the urea cycle?
A liver pathway that converts toxic ammonia (from deamination) to urea for excretion. It is linked to the Krebs cycle via fumarate.
QWhy is glucose preferred over protein?
Glucose can be respired anaerobically, produces no toxic nitrogenous waste, and is the brain's primary fuel. Proteins produce ammonia/urea as waste.
QDifference between aerobic and anaerobic?
Aerobic: requires O₂, produces 36-38 ATP, occurs in mitochondria. Anaerobic: no O₂, produces only 2 ATP via glycolysis, occurs in cytosol.
QWhat is the role of NADH?
NADH is a reduced electron carrier that donates electrons to Complex I of the ETC, driving ATP synthesis via chemiosmosis.
QWhat is oxidative decarboxylation?
A reaction that removes CO₂ and oxidizes the substrate (producing NADH). Happens in the link reaction and in Krebs cycle steps (isocitrate→α-KG, α-KG→succinyl-CoA).
QWhat is the metabolic pool?
The metabolic pool is the collection of all available intermediates, nutrients, and coenzymes in a cell that can be used for various metabolic pathways.
QWhere is the ETC located?
On the inner mitochondrial membrane. It uses NADH/FADH₂ to pump protons and produce ATP via ATP synthase (oxidative phosphorylation).
QWhat is the final electron acceptor?
Oxygen. It combines with H⁺ and electrons to form water, allowing the ETC to continue.
QWhat is substrate-level phosphorylation?
ATP production by directly transferring a phosphate group from a substrate to ADP. Occurs in glycolysis and Krebs cycle (via GTP).
QWhat is chemiosmosis?
ATP synthesis driven by a proton gradient across the inner mitochondrial membrane. Protons flow through ATP synthase, generating ATP.
QWhat is the Cori cycle?
The Cori cycle converts lactate (from anaerobic muscle) back to glucose in the liver. It links muscle and liver metabolism during intense exercise.
QWhat is gluconeogenesis?
Gluconeogenesis is the synthesis of glucose from non-carbohydrate sources (amino acids, glycerol). Fatty acids cannot contribute because Acetyl-CoA cannot convert to glucose.

📋Syllabus & Learning Outcomes

According to PMDC/PMC MDCAT syllabus, Bioenergetics covers:

📝Full Notes — Fat & Protein Respiration

Glucose Respiration (Reference)

Overall equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 36/38 ATP

  • Glycolysis (cytosol): glucose → 2 pyruvate. Net 2 ATP, 2 NADH.
  • Link reaction (matrix): pyruvate → Acetyl-CoA + CO₂ + NADH.
  • Krebs cycle (matrix): Acetyl-CoA → 2 CO₂, 3 NADH, 1 FADH₂, 1 GTP per turn.
  • ETC (inner membrane): NADH/FADH₂ donate e⁻ → proton gradient → ATP synthase.
Key entry point: Acetyl-CoA is the central hub for all fuels.

2. Cellular Respiration of Fats (β-Oxidation)

  • Occurs in mitochondrial matrix (after transport).
  • Step 1 – Activation: Fatty acid + CoA → Fatty acyl-CoA (costs 2 ATP equivalents).
  • Step 2 – Transport: Carnitine shuttle moves fatty acyl-CoA across inner mitochondrial membrane.
  • Step 3 – β-Oxidation: Each cycle removes a 2-carbon unit as Acetyl-CoA, producing 1 NADH + 1 FADH₂.
  • Glycerol (from triglyceride) is phosphorylated and enters glycolysis as DHAP.

Simplified: Fatty acid → Acetyl-CoA (into Krebs) + reduced coenzymes → ETC.

Comparison with Glucose

  • Fats yield 9 kcal/g vs glucose 4 kcal/g – more energy per gram because highly reduced.
  • RQ of fat = 0.7 (less O₂ consumed per CO₂ produced).
  • Enters respiration at Acetyl-CoA (bypasses glycolysis).

3. Cellular Respiration of Proteins

  • First step – Deamination: Amino group removed (NH₂ → ammonia → urea in liver).
  • Carbon skeletons enter at different points of the respiratory pathway:
Entry PointExamples of Amino Acids
PyruvateAlanine, Serine, Cysteine
Acetyl-CoA (ketogenic)Leucine, Lysine
α-KetoglutarateGlutamate, Glutamine
Succinyl-CoAValine, Isoleucine, Methionine
OxaloacetateAspartate, Asparagine
Important: Ketogenic amino acids (Leucine, Lysine) can only be converted to Acetyl-CoA, not glucose (precursors).

Comparison with Glucose

  • Protein RQ ≈ 0.8 (intermediate).
  • Entry after glycolysis but before/during Krebs.
  • Requires deamination – not energy-efficient as glucose.

4. Correlation of Glucose, Fats & Proteins

PropertyGlucoseFatsProteins
Entry PointGlycolysis → Acetyl-CoAAcetyl-CoA (β-oxidation)Pyruvate / Acetyl-CoA / Krebs intermediates
ATP Yield (per g)~4~9~4
RQ1.00.70.8
Initial StepGlycolysisβ-oxidationDeamination
CoenzymesNADHNADH + FADH₂NADH / FADH₂
Common hub: All three converge at Acetyl-CoA → Krebs cycle → ETC.

High-Yield Points & Common Traps

  • Fat entry: β-oxidation in mitochondria → Acetyl-CoA.
  • Protein entry: After deamination, carbon skeletons enter at various Krebs intermediates.
  • Highest energy per gram: Fats (9 kcal/g).
  • RQ values: Fat = 0.7, Protein = 0.8, Glucose = 1.0 – classic MCQs.
  • Ketogenic amino acids: Leucine, Lysine → only Acetyl-CoA.
  • Glucose vs Fat: Glucose can be respired anaerobically; fats require O₂.
  • Carnitine shuttle: Required for fatty acid entry into mitochondria.
  • Glycerol from fat: Enters glycolysis as DHAP.

💡 MDCAT tip: Know the RQ numbers cold. Expect questions on entry points of amino acids and the central role of Acetyl-CoA.

✅Practice MCQs — Test 1

🧪 Basic Concepts

30 MCQs
1How many kilocalories (kcal) of energy are released by the oxidation of 1 gram of pure fat?
  • A. 4 kcal
  • B. 7 kcal
  • C. 9 kcal
  • D. 12 kcal
Answer: C. 9 kcal
Fats yield 9 kcal/g due to high number of C-H bonds.
2What is the exact number of carbon atoms present in a single molecule of Glycerol?
  • A. 2
  • B. 3
  • C. 4
  • D. 5
Answer: B. 3
Glycerol is a 3-carbon alcohol.
3The number of carbon atoms in an Acetyl group that enters the Krebs cycle is:
  • A. 1
  • B. 2
  • C. 3
  • D. 4
Answer: B. 2
Acetyl group (CH₃CO-) contains 2 carbons.
4How many carbon atoms does α-ketoglutarate contain?
  • A. 4
  • B. 5
  • C. 6
  • D. 3
Answer: B. 5
α-ketoglutarate is a 5-carbon intermediate of the Krebs cycle.
5What is the exact value for the Respiratory Quotient (RQ) of Carbohydrates?
  • A. 0.7
  • B. 0.8
  • C. 1.0
  • D. 1.3
Answer: C. 1.0
For glucose, RQ = CO₂/O₂ = 6/6 = 1.
6For every one molecule of glucose, how many molecules of CO₂ are released during the 'Link Reaction'?
  • A. 1
  • B. 2
  • C. 4
  • D. 6
Answer: B. 2
Each pyruvate (2 per glucose) is decarboxylated to Acetyl-CoA, releasing 1 CO₂ each, total 2.
7How many NADH molecules are produced during one single turn of the Krebs cycle?
  • A. 1
  • B. 2
  • C. 3
  • D. 4
Answer: C. 3
One turn produces 3 NADH (isocitrate→α-KG, α-KG→succinyl-CoA, malate→oxaloacetate).
8How many FADH₂ molecules are produced per molecule of Acetyl-CoA entering the Krebs cycle?
  • A. 1
  • B. 2
  • C. 3
  • D. 0
Answer: A. 1
Succinate to fumarate produces 1 FADH₂ per Acetyl-CoA.
9In the breakdown of a 16-carbon fatty acid, how many cycles of β-oxidation occur?
  • A. 8
  • B. 7
  • C. 16
  • D. 15
Answer: B. 7
Number of cycles = (n/2) - 1 = 8 - 1 = 7.
10How many molecules of Acetyl-CoA are produced from one 18-carbon Stearic acid molecule?
  • A. 18
  • B. 9
  • C. 8
  • D. 6
Answer: B. 9
18/2 = 9 Acetyl-CoA.
11What is the exact number of ATP molecules consumed during the 'Energy Investment Phase' of Glycolysis?
  • A. 1
  • B. 2
  • C. 4
  • D. 38
Answer: B. 2
Hexokinase and PFK each use 1 ATP.
12The number of NADH molecules produced during the Glycolysis of one glucose molecule is:
  • A. 1
  • B. 2
  • C. 4
  • D. 10
Answer: B. 2
Glyceraldehyde-3-phosphate dehydrogenase produces 2 NADH (one per triose).
13How many carbon atoms are found in one molecule of Oxaloacetate?
  • A. 2
  • B. 3
  • C. 4
  • D. 6
Answer: C. 4
Oxaloacetate is a 4-carbon dicarboxylic acid.
14What is the approximate percentage of energy conserved in the form of ATP from the total potential energy of one glucose molecule?
  • A. 10%
  • B. 25%
  • C. 40%
  • D. 90%
Answer: C. 40%
About 40% of glucose energy is captured as ATP; rest is lost as heat.
15How many molecules of CO₂ are produced during two turns of the Krebs cycle?
  • A. 2
  • B. 4
  • C. 6
  • D. 8
Answer: B. 4
Each turn releases 2 CO₂, so 2 turns = 4 CO₂.
16How many ATP molecules (net) are produced during the anaerobic breakdown of one glucose molecule?
  • A. 0
  • B. 2
  • C. 4
  • D. 36
Answer: B. 2
Fermentation yields 2 net ATP from glycolysis.
17What is the exact number of oxygen atoms in a molecule of Glucose (C₆H₁₂O₆)?
  • A. 6
  • B. 12
  • C. 18
  • D. 2
Answer: A. 6
Glucose formula shows 6 oxygen atoms.
18How many NADH molecules are produced in total (from Glycolysis to Krebs) for one glucose molecule?
  • A. 4
  • B. 6
  • C. 8
  • D. 10
Answer: D. 10
Glycolysis:2, Link:2, Krebs:6 (2 turns ×3) = 10.
19How many turns of the Krebs cycle are required to process the products of one 12-carbon fatty acid?
  • A. 12
  • B. 6
  • C. 3
  • D. 1
Answer: B. 6
12C fatty acid yields 6 Acetyl-CoA, each requires one turn.
20The energy released by 1 gram of protein is approximately:
  • A. 4 kcal
  • B. 9 kcal
  • C. 7 kcal
  • D. 2 kcal
Answer: A. 4 kcal
Proteins yield about 4 kcal/g, similar to carbohydrates.
21How many carbon atoms are in one molecule of Succinate?
  • A. 2
  • B. 4
  • C. 5
  • D. 6
Answer: B. 4
Succinate (C₄H₄O₄) has 4 carbons.
22According to PTB/Federal standards, how many ATP are generated from one molecule of FADH₂ in the ETC?
  • A. 1
  • B. 2
  • C. 3
  • D. 4
Answer: B. 2
Traditional MDCAT/PTB uses 2 ATP per FADH₂.
23How many molecules of water (H₂O) are produced at the end of the ETC for one molecule of glucose?
  • A. 2
  • B. 4
  • C. 6
  • D. 12
Answer: C. 6
Overall, 6 H₂O are formed per glucose (from 6 O₂).
24What is the number of phosphate groups in one molecule of ATP?
  • A. 1
  • B. 2
  • C. 3
  • D. 4
Answer: C. 3
ATP has three phosphate groups.
25How many ATP/GTP molecules are produced via 'Substrate-Level Phosphorylation' in the Krebs cycle per glucose molecule?
  • A. 1
  • B. 2
  • C. 4
  • D. 34
Answer: B. 2
One GTP per Acetyl-CoA (2 turns) = 2.
26The number of carbon atoms in Pyruvate is:
  • A. 2
  • B. 3
  • C. 4
  • D. 6
Answer: B. 3
Pyruvate (CH₃COCOOH) has 3 carbons.
27In the RQ calculation for proteins, the value is typically:
  • A. 1.0
  • B. 0.8
  • C. 0.5
  • D. 1.5
Answer: B. 0.8
Protein RQ is about 0.8-0.9, typically 0.8.
28How many carbon atoms are lost as CO₂ during the conversion of one Isocitrate to α-ketoglutarate?
  • A. 1
  • B. 2
  • C. 3
  • D. 0
Answer: A. 1
Isocitrate (6C) loses one CO₂ to become α-ketoglutarate (5C).
29What is the net gain of NADH from the 'Link Reaction' only, per glucose molecule?
  • A. 1
  • B. 2
  • C. 3
  • D. 4
Answer: B. 2
Each pyruvate yields 1 NADH, so 2 per glucose.
30How many carbon atoms are in a molecule of Citrate?
  • A. 4
  • B. 5
  • C. 6
  • D. 2
Answer: C. 6
Citrate (C₆H₈O₇) has 6 carbons.

✅Practice MCQs — Test 2

🧪 Advanced Concepts

50 MCQs
1Which part of a triglyceride molecule enters the glycolytic pathway directly after being phosphorylated?
  • A. Fatty acids
  • B. Glycerol
  • C. Acetyl-CoA
  • D. Amino group
Answer: B. Glycerol
Glycerol is converted into DHAP, a 3-carbon intermediate of glycolysis.
2Fatty acids are broken down into 2-carbon fragments through a process known as:
  • A. Deamination
  • B. Glycolysis
  • C. β-oxidation
  • D. Decarboxylation
Answer: C. β-oxidation
Beta-oxidation sequentially removes 2-carbon units to form Acetyl-CoA.
3Before amino acids can be used in the respiratory pathway, they must lose their nitrogen-containing group. This is called:
  • A. Hydrogenation
  • B. Deamination
  • C. Carboxylation
  • D. Amination
Answer: B. Deamination
Deamination removes the -NH₂ group, leaving a carbon skeleton for respiration.
4Which is the common entry point for both fatty acids and certain ketogenic amino acids into the Krebs cycle?
  • A. Pyruvate
  • B. Oxaloacetate
  • C. Acetyl-CoA
  • D. Citrate
Answer: C. Acetyl-CoA
Acetyl-CoA is the central hub where carbohydrate, fat, and protein pathways converge.
5How many molecules of Acetyl-CoA are produced from the β-oxidation of a 14-carbon fatty acid?
  • A. 14
  • B. 7
  • C. 6
  • D. 2
Answer: B. 7
Each Acetyl-CoA contains 2 carbons (14/2 = 7).
6The RQ for the oxidation of pure glucose is:
  • A. 0.7
  • B. 0.8
  • C. 0.9
  • D. 1.0
Answer: D. 1.0
Glucose oxidation gives CO₂/O₂ = 1:1.
7Why do fats provide more than double the energy per gram compared to carbohydrates?
  • A. They have more oxygen atoms
  • B. They have a higher proportion of C-H bonds
  • C. They skip the Krebs cycle
  • D. They are polar molecules
Answer: B
C-H bonds are highly reduced, providing more electrons for the ETC.
8An amino acid that is converted into oxaloacetate is considered:
  • A. Ketogenic
  • B. Glucogenic
  • C. Essential
  • D. Non-respiratory
Answer: B. Glucogenic
Glucogenic amino acids produce intermediates for gluconeogenesis.
9Which intermediate of the Krebs cycle is the direct entry point for the amino acid glutamate?
  • A. Succinate
  • B. Fumarate
  • C. α-ketoglutarate
  • D. Malate
Answer: C. α-ketoglutarate
Glutamate is deaminated directly into α-ketoglutarate.
10The toxic ammonia (NH₃) produced during protein catabolism is converted into ______ in the human liver.
  • A. Uric acid
  • B. Urea
  • C. Nitric acid
  • D. Amino acids
Answer: B. Urea
Humans are ureotelic; the liver processes ammonia into urea.
11Which molecule acts as the link between the urea cycle and the Krebs cycle?
  • A. Citrate
  • B. Fumarate
  • C. Pyruvate
  • D. Acetyl-CoA
Answer: B. Fumarate
Fumarate is a byproduct of the urea cycle that enters the Krebs cycle.
12During starvation, the body utilizes proteins for energy. These amino acids enter the respiratory pathway at which stages?
  • A. Glycolysis only
  • B. Krebs cycle only
  • C. Pyruvate, Acetyl-CoA, or Krebs cycle intermediates
  • D. ETC directly
Answer: C
Depending on the R-group, amino acids enter at various points.
13What is the approximate RQ value for a typical fat molecule?
  • A. 1.0
  • B. 0.9
  • C. 0.7
  • D. 1.2
Answer: C. 0.7
Fats require more oxygen relative to CO₂ produced, giving RQ ~0.7.
14Glycerol is converted into DHAP, which then isomerizes into:
  • A. Pyruvate
  • B. Glyceraldehyde-3-phosphate (G3P)
  • C. Glucose
  • D. Phosphoenolpyruvate (PEP)
Answer: B. G3P
G3P is the direct intermediate used in glycolysis.
15Which of these cannot be used to synthesize 'new' glucose (gluconeogenesis) in humans?
  • A. Glycerol
  • B. Glucogenic amino acids
  • C. Fatty acids
  • D. Lactate
Answer: C. Fatty acids
Humans cannot convert Acetyl-CoA (from fatty acids) back into glucose.
16How many ATP are produced from one turn of the Krebs cycle (traditional count)?
  • A. 12
  • B. 24
  • C. 36
  • D. 2
Answer: A. 12
1 turn produces 3 NADH (9 ATP), 1 FADH₂ (2 ATP), and 1 GTP (1 ATP) = 12 ATP.
17In the absence of glucose, the brain can utilize ______ derived from rapid fat breakdown.
  • A. Fatty acids
  • B. Ketone bodies
  • C. Lactic acid
  • D. Starch
Answer: B. Ketone bodies
Ketone bodies cross the blood-brain barrier when glucose is scarce.
18The enzymes for β-oxidation are located in the:
  • A. Cytosol
  • B. Mitochondrial matrix
  • C. Intermembrane space
  • D. Ribosomes
Answer: B. Mitochondrial matrix
Like the Krebs cycle, fat oxidation occurs in the matrix.
19Which cofactor is reduced during the conversion of a fatty acid to Acetyl-CoA?
  • A. NAD⁺ only
  • B. FAD only
  • C. Both NAD⁺ and FAD
  • D. NADP⁺
Answer: C
Each β-oxidation cycle produces 1 NADH and 1 FADH₂.
20If a protein-rich diet is consumed, the excess nitrogen is primarily excreted as:
  • A. CO₂
  • B. Urea
  • C. Ammonia
  • D. H₂O
Answer: B. Urea
Excess nitrogen is converted to urea in the liver and excreted.
21Alanine is an amino acid that can be directly transaminated into:
  • A. Oxaloacetate
  • B. Pyruvate
  • C. Acetyl-CoA
  • D. Citrate
Answer: B. Pyruvate
Alanine transaminase converts alanine to pyruvate.
22Which of the following yields the most metabolic water per gram?
  • A. Carbohydrates
  • B. Proteins
  • C. Fats
  • D. Nucleic acids
Answer: C. Fats
High hydrogen content in fats leads to more water production in the ETC.
23The 4-carbon Krebs intermediate that accepts Acetyl-CoA is:
  • A. Citrate
  • B. Oxaloacetate
  • C. Succinate
  • D. Malate
Answer: B. Oxaloacetate
Oxaloacetate (4C) condenses with Acetyl-CoA (2C) to form citrate.
24Which process is common to the respiration of all three (Carbs, Fats, Proteins)?
  • A. Glycolysis
  • B. β-oxidation
  • C. Krebs cycle
  • D. Deamination
Answer: C. Krebs cycle
All fuels eventually converge into Acetyl-CoA or Krebs intermediates.
25An RQ of 0.8 is characteristic of which fuel source?
  • A. Pure Carbohydrate
  • B. Pure Fat
  • C. Pure Protein
  • D. Anaerobic respiration
Answer: C. Pure Protein
Protein RQ is approximately 0.8.
26Which molecule acts as a 'shuttle' to transport fatty acids into the mitochondria?
  • A. Coenzyme A
  • B. Carnitine
  • C. ATP
  • D. NAD⁺
Answer: B. Carnitine
Carnitine carries long-chain fatty acids across the inner mitochondrial membrane.
27How many ATP are required to 'activate' a fatty acid before β-oxidation?
  • A. 1 ATP
  • B. 2 ATP (equivalents)
  • C. 0 ATP
  • D. 36 ATP
Answer: B. 2 ATP
ATP is converted to AMP (equivalent to using 2 ATP).
28Aspartate is an amino acid that enters the Krebs cycle as:
  • A. Pyruvate
  • B. Oxaloacetate
  • C. Succinyl-CoA
  • D. Acetyl-CoA
Answer: B. Oxaloacetate
Aspartate transaminates to oxaloacetate.
29Which step of cellular respiration produces the majority of CO₂ when oxidizing a fat?
  • A. β-oxidation
  • B. Glycolysis
  • C. Krebs cycle
  • D. ETC
Answer: C. Krebs cycle
Krebs cycle releases CO₂ from Acetyl-CoA.
30The conversion of Glycerol to DHAP requires:
  • A. NAD⁺
  • B. FAD
  • C. NADP⁺
  • D. O₂
Answer: A. NAD⁺
Glycerol dehydrogenase uses NAD⁺.
31Which of the following is an 'amphibolic' pathway?
  • A. Glycolysis
  • B. Krebs cycle
  • C. ETC
  • D. β-oxidation
Answer: B. Krebs cycle
The Krebs cycle functions in both catabolism and anabolism.
32Excess Acetyl-CoA from glucose metabolism can be used to synthesize fatty acids. This is called:
  • A. Lipolysis
  • B. Lipogenesis
  • C. Glycogenesis
  • D. Glycolysis
Answer: B. Lipogenesis
Lipogenesis converts excess Acetyl-CoA into fatty acids.
33Which intermediate of the Krebs cycle is used for the synthesis of Heme?
  • A. Citrate
  • B. Succinyl-CoA
  • C. α-ketoglutarate
  • D. Fumarate
Answer: B. Succinyl-CoA
Succinyl-CoA is a precursor for heme synthesis.
34In humans, the irreversible step that prevents fats from becoming glucose is:
  • A. Pyruvate to Acetyl-CoA
  • B. Glucose to G6P
  • C. PEP to Pyruvate
  • D. Succinate to Fumarate
Answer: A. PDH reaction
PDH is irreversible; Acetyl-CoA cannot be converted back to pyruvate.
35A 16-carbon fatty acid (Palmitic acid) yields approximately how many net ATP?
  • A. 36
  • B. 38
  • C. 129
  • D. 2
Answer: C. 129
Palmitate yields about 129 ATP (traditional count).
36Which amino acids are strictly ketogenic (cannot form glucose)?
  • A. Alanine and Glycine
  • B. Leucine and Lysine
  • C. Glutamate and Aspartate
  • D. Serine and Valine
Answer: B. Leucine and Lysine
Leucine and lysine are the only strictly ketogenic amino acids.
37During intense exercise, the primary fuel is glucose. During low-intensity long-duration exercise, the primary fuel shifts to:
  • A. Proteins
  • B. Fats
  • C. Nucleic acids
  • D. Pure oxygen
Answer: B. Fats
Fats are used during prolonged, low-intensity activity.
38The removal of CO₂ is called decarboxylation. Where does this NOT occur during fat respiration?
  • A. Link Reaction
  • B. Krebs cycle
  • C. β-oxidation
  • D. None of the above
Answer: C. β-oxidation
Beta-oxidation releases Acetyl-CoA units without releasing CO₂.
39How many carbons are in one molecule of glycerol?
  • A. 2
  • B. 3
  • C. 4
  • D. 6
Answer: B. 3
Glycerol has 3 carbons.
40Which vitamin is a precursor for FAD?
  • A. B1 (Thiamine)
  • B. B2 (Riboflavin)
  • C. B3 (Niacin)
  • D. C (Ascorbic acid)
Answer: B. B2
Riboflavin (B2) is a component of FAD.
41Which vitamin is a precursor for NAD⁺?
  • A. B1
  • B. B2
  • C. B3 (Niacin)
  • D. B12
Answer: C. B3
Niacin (B3) is a component of NAD⁺.
42Which fuel is used last during prolonged starvation?
  • A. Liver glycogen
  • B. Muscle glycogen
  • C. Adipose fat
  • D. Structural proteins
Answer: D. Structural proteins
Structural proteins are broken down only after fat stores are depleted.
43The metabolic pool of a cell refers to:
  • A. The amount of water in the vacuole
  • B. The total ATP available
  • C. The sum of all available nutrients/intermediates in the cytosol/mitochondria
  • D. The DNA content
Answer: C
The metabolic pool is the collection of intermediates available for pathways.
44If the RQ of a person is measured as 0.7, they are primarily respiring:
  • A. Carbohydrates
  • B. Proteins
  • C. Fats
  • D. A mix of all three
Answer: C. Fats
RQ 0.7 indicates pure fat oxidation.
45Which Krebs intermediate can be transaminated to form Glutamate?
  • A. Citrate
  • B. α-ketoglutarate
  • C. Oxaloacetate
  • D. Malate
Answer: B. α-ketoglutarate
α-ketoglutarate + NH₃ → glutamate via glutamate dehydrogenase.
46How many FADH₂ are produced per molecule of glucose in the entire process?
  • A. 2
  • B. 4
  • C. 10
  • D. 0
Answer: A. 2
Krebs cycle produces 2 FADH₂ (one per Acetyl-CoA).
47Substrate-level phosphorylation in the Krebs cycle produces:
  • A. ATP/GTP
  • B. NADH
  • C. FADH₂
  • D. CO₂
Answer: A. ATP/GTP
Succinyl-CoA synthetase produces GTP (or ATP).
48Which molecule has the highest energy density?
  • A. Glycogen
  • B. Glucose
  • C. Triglyceride
  • D. Hemoglobin
Answer: C. Triglyceride
Triglycerides store about 9 kcal/g, compared to 4 kcal/g for carbohydrates.
49Fatty acid synthesis occurs in the ______, while β-oxidation occurs in the ______.
  • A. Mitochondria; Cytosol
  • B. Cytosol; Mitochondria
  • C. Nucleus; Cytosol
  • D. SER; Mitochondria
Answer: B. Cytosol; Mitochondria
Fatty acid synthesis is cytosolic; β-oxidation is mitochondrial.
50The end product of glycolysis is Pyruvate (3C). How many pyruvates are formed from one glycerol molecule entering respiration?
  • A. 1
  • B. 2
  • C. 0
  • D. 3
Answer: A. 1
Glycerol (3C) becomes G3P, which yields one pyruvate.

✅Practice MCQs — Test 3

🧪 Exam-Level Questions

50 MCQs
1If a 12-carbon fatty acid is completely oxidized, how many turns of the Krebs cycle are required to process the resulting Acetyl-CoA?
  • A. 5
  • B. 6
  • C. 12
  • D. 11
Answer: B. 6
12 carbons ÷ 2 = 6 Acetyl-CoA, each requires one turn.
2Which describes the correct fate of a carbon atom entering as Aspartate compared to a carbon atom from Glucose?
  • A. Both must undergo oxidative decarboxylation.
  • B. Aspartate enters as Oxaloacetate, bypassing the CO₂ releasing steps.
  • C. Both must be converted to Pyruvate.
  • D. Aspartate produces more FADH₂ per carbon atom.
Answer: B
Aspartate enters as oxaloacetate, which is after the decarboxylation steps.
3During starvation, the 'Glucose-Alanine Cycle' occurs. What is the net ATP cost in the liver to convert two Alanine molecules back into one Glucose?
  • A. 2 ATP
  • B. 4 ATP
  • C. 6 ATP
  • D. 12 ATP
Answer: C. 6 ATP
Gluconeogenesis from pyruvate requires 4 ATP and 2 GTP = 6 high-energy bonds.
4Which molecule, when present in high concentrations, simultaneously inhibits Phosphofructokinase (PFK) and activates Fatty Acid synthesis?
  • A. ADP
  • B. Citrate
  • C. Pyruvate
  • D. Oxaloacetate
Answer: B. Citrate
High citrate signals energy abundance, inhibiting glycolysis and promoting fat storage.
5For an 18-carbon saturated fatty acid, how many molecules of FADH₂ are produced during the β-oxidation phase ONLY?
  • A. 9
  • B. 8
  • C. 18
  • D. 17
Answer: B. 8
Number of cycles = (18/2)-1 = 8, each produces 1 FADH₂.
6In a scenario where a cell has an inhibited Alpha-ketoglutarate Dehydrogenase enzyme, which amino acid could still be partially oxidized to produce some ATP?
  • A. Glutamate
  • B. Aspartate
  • C. Proline
  • D. Arginine
Answer: B. Aspartate
Aspartate enters as oxaloacetate, after the inhibited step.
7Why is the energy yield of Fats (~9 kcal/g) so much higher than Proteins (~4 kcal/g), even though both enter the Krebs cycle?
  • A. Proteins are not fully oxidized because the urea cycle consumes ATP.
  • B. Fats have a higher Oxidation State.
  • C. Fats have a lower Oxidation State (more reduced).
  • D. Proteins cannot be used in the ETC.
Answer: C
Fats are more reduced (more C-H bonds), yielding more electrons for the ETC.
8How many oxidative decarboxylation steps occur during the complete oxidation of one molecule of Palmitic Acid (16C)?
  • A. 8
  • B. 16
  • C. 24
  • D. 0
Answer: B. 16
β-oxidation has no decarboxylation; 8 Acetyl-CoA × 2 decarboxylations each = 16.
9Which amino acid is 'strictly' ketogenic and can be used to produce Ketone bodies but NEVER Glucose?
  • A. Alanine
  • B. Leucine
  • C. Valine
  • D. Glycine
Answer: B. Leucine
Leucine and lysine are strictly ketogenic.
10A sudden increase in the NADH/NAD⁺ ratio in the matrix will most likely:
  • A. Stimulate the breakdown of Fats.
  • B. Inhibit the Krebs Cycle.
  • C. Accelerate Glycolysis.
  • D. Increase the rate of Deamination.
Answer: B. Inhibit Krebs
High NADH inhibits key Krebs cycle enzymes (e.g., isocitrate dehydrogenase).
11If a carbon atom from a fatty acid is labeled, where is it most likely to be released as CO₂?
  • A. During β-oxidation in the Matrix.
  • B. During Pyruvate → Acetyl-CoA.
  • C. During the Krebs Cycle.
  • D. During Chemiosmosis.
Answer: C. Krebs Cycle
CO₂ is released in the Krebs cycle (isocitrate→α-KG and α-KG→succinyl-CoA).
12In the comparison of Glucose (C₆H₁₂O₆) and a Fatty Acid (C₁₆H₃₂O₂), which has a higher 'Oxygen-to-Carbon' ratio?
  • A. Fatty Acid
  • B. Glucose
  • C. Both are equal
  • D. Depends on pH
Answer: B. Glucose
Glucose has O/C = 1; fatty acid has O/C ≈ 1/8, so glucose is more oxidized.
13Which enzyme is common to both Gluconeogenesis and Glycolysis?
  • A. Hexokinase
  • B. Phosphofructokinase
  • C. Phosphoglycerate Kinase
  • D. Pyruvate Kinase
Answer: C
Most middle steps of glycolysis are reversible and used in gluconeogenesis.
14What is the net ATP yield of the glycerol component of a triglyceride if it is respired anaerobically?
  • A. 1 ATP
  • B. 2 ATP
  • C. 0 ATP
  • D. 7 ATP
Answer: A. 1 ATP
Glycerol → G3P (costs 1 ATP), G3P → pyruvate (2 ATP), net = 1 ATP.
15Which amino acid catabolism would be most affected by a deficiency in the enzyme Succinate Dehydrogenase?
  • A. Alanine
  • B. Aspartate
  • C. Isoleucine (enters as Succinyl-CoA)
  • D. Glutamate
Answer: C
Succinyl-CoA is converted to succinate, then to fumarate by SDH; deficiency blocks this step.
16If 10 molecules of Acetyl-CoA enter the Krebs cycle, how many molecules of O₂ are indirectly required to process their electrons in the ETC?
  • A. 10
  • B. 20
  • C. 30
  • D. 40
Answer: B. 20
10 Acetyl-CoA → 30 NADH + 10 FADH₂; NADH needs 15 O₂, FADH₂ needs 5 O₂, total 20 O₂.
17The 'Thermogenic' effect of high-protein diets is partly due to:
  • A. Proteins having more C-H bonds.
  • B. The high ATP cost of the Urea Cycle.
  • C. Proteins bypassing the ETC.
  • D. Ammonia acting as an uncoupler.
Answer: B
Urea synthesis consumes ATP, generating heat.
18Which intermediate of the Krebs cycle can cross the mitochondrial membrane to provide carbons for fatty acid synthesis in the cytosol?
  • A. Acetyl-CoA
  • B. Citrate
  • C. Pyruvate
  • D. Oxaloacetate
Answer: B. Citrate
Citrate is exported to the cytosol and cleaved into oxaloacetate and Acetyl-CoA.
19A molecule of Glucose-6-Phosphate is produced from Glycogen. What is the net ATP yield if it is fully oxidized?
  • A. 36
  • B. 37
  • C. 38
  • D. 39
Answer: B. 37
Bypassing hexokinase saves 1 ATP, so yield becomes 37 (from 36).
20During β-oxidation, the carbon chain is oxidized at which position?
  • A. Alpha carbon
  • B. Beta carbon (Carbon-3)
  • C. Omega carbon
  • D. Gamma carbon
Answer: B. Beta carbon
Oxidation occurs at the beta carbon (third carbon from the carboxyl end).
21If a cell is performing exclusively protein respiration, and the RQ is 0.8, what does this imply about oxygen consumption?
  • A. Cell produces more CO₂ than O₂ consumed.
  • B. Cell consumes more O₂ than CO₂ produced.
  • C. O₂ consumption is zero.
  • D. Cell is in anaerobic state.
Answer: B
RQ < 1 indicates more O₂ consumed than CO₂ produced.
22Which of the following does NOT happen during the conversion of Glycerol to DHAP?
  • A. ATP hydrolysis
  • B. NAD⁺ reduction
  • C. Phosphorylation
  • D. Decarboxylation
Answer: D. Decarboxylation
Glycerol → G3P (ATP, no decarboxylation) → DHAP (NAD⁺ reduction).
23The urea cycle and Krebs cycle are linked by the 'Aspartate-Argininosuccinate' shunt. Which Krebs intermediate is released into the cytosol during this process?
  • A. Malate
  • B. Fumarate
  • C. Citrate
  • D. Succinate
Answer: B. Fumarate
Fumarate is produced in the urea cycle and enters the Krebs cycle.
24How many high-energy phosphate bonds are broken to activate a single fatty acid molecule for respiration?
  • A. 1
  • B. 2
  • C. 3
  • D. 4
Answer: B. 2
ATP → AMP + PPi (equivalent to 2 ATP).
25In the presence of Dinitrophenol (an uncoupler), what happens to the respiration of fats?
  • A. It stops completely.
  • B. It continues, but no ATP is made (only heat).
  • C. It shifts to anaerobic fermentation.
  • D. Oxygen consumption decreases.
Answer: B
Uncouplers allow ETC to continue without ATP synthesis, producing heat.
26Which molecule is the final electron acceptor in the respiration of proteins?
  • A. Urea
  • B. NAD⁺
  • C. Pyruvate
  • D. Oxygen
Answer: D. Oxygen
Oxygen is always the final electron acceptor in aerobic respiration.
27If a fatty acid has 17 carbons (odd-numbered), the final 3-carbon fragment is called Propionyl-CoA. What is its metabolic fate?
  • A. Exhaled as CO₂
  • B. Converted to Succinyl-CoA to enter Krebs
  • C. Becomes Glucose via Pyruvate
  • D. Excreted as urea
Answer: B
Propionyl-CoA is carboxylated and isomerized to succinyl-CoA.
28Which enzyme is inhibited by high levels of ATP during respiration?
  • A. Isocitrate Dehydrogenase
  • B. Citrate Synthase
  • C. PFK
  • D. All of these
Answer: D. All of these
ATP allosterically inhibits PFK, isocitrate dehydrogenase, and citrate synthase.
29Comparing 1 mole of Glucose and 1 mole of a 6-carbon Fatty Acid, which is true?
  • A. Both produce exactly 36 ATP.
  • B. The fatty acid produces more ATP because it is more reduced.
  • C. Glucose produces more ATP.
  • D. The fatty acid produces more CO₂.
Answer: B
A 6C fatty acid (e.g., hexanoic acid) yields more ATP than glucose.
30What is the role of FAD in β-oxidation?
  • A. It causes decarboxylation.
  • B. It creates a double bond between Alpha and Beta carbons.
  • C. It attaches Coenzyme A.
  • D. It produces water.
Answer: B
Acyl-CoA dehydrogenase uses FAD to form a trans-2-enoyl-CoA.
31The process by which amino acids are used to produce ATP is primarily regulated by the availability of:
  • A. Alpha-keto acids
  • B. Insulin
  • C. Cholesterol
  • D. Nitrogen gas
Answer: A. Alpha-keto acids
Alpha-keto acids (from carbon skeletons) regulate amino acid catabolism.
32Which intermediate is shared by the oxidation of both odd-chain fatty acids and the amino acids Valine/Isoleucine?
  • A. Acetyl-CoA
  • B. Succinyl-CoA
  • C. Fumarate
  • D. Citrate
Answer: B. Succinyl-CoA
Propionyl-CoA from odd-chain fats and valine/isoleucine is converted to succinyl-CoA.
33A single molecule of FADH₂ enters the ETC at which complex?
  • A. Complex I
  • B. Complex II
  • C. Complex III
  • D. Complex IV
Answer: B. Complex II
FADH₂ donates electrons to Complex II (succinate dehydrogenase).
34How many CO₂ molecules are released per molecule of Glycerol during its complete aerobic oxidation?
  • A. 1
  • B. 2
  • C. 3
  • D. 6
Answer: C. 3
Glycerol (3C) → pyruvate (3C) → 3 CO₂ (one from link, two from Krebs).
35During deamination of Glutamate, which byproduct is formed?
  • A. NADH
  • B. α-ketoglutarate
  • C. Ammonia
  • D. All of these
Answer: D
Glutamate dehydrogenase produces α-ketoglutarate, NH₃, and NADH.
36Which metabolic state would favor the production of Ketone bodies?
  • A. High insulin, low glucagon
  • B. High glucose, high ATP
  • C. Low oxaloacetate, high Acetyl-CoA
  • D. High protein with high carbohydrates
Answer: C
When oxaloacetate is low, Acetyl-CoA is diverted to ketogenesis.
37What is the P:O ratio for NADH in modern bioenergetics (Federal Board context)?
  • A. 1.5
  • B. 2.5
  • C. 3.0
  • D. 2.0
Answer: B. 2.5
Modern values give ~2.5 ATP per NADH.
38If you start with 1 molecule of Citrate and end at Succinate, how many NADH and CO₂ are produced?
  • A. 1 NADH, 1 CO₂
  • B. 2 NADH, 2 CO₂
  • C. 3 NADH, 2 CO₂
  • D. 1 NADH, 2 CO₂
Answer: B. 2 NADH, 2 CO₂
Citrate → isocitrate → α-KG (1 NADH, 1 CO₂) → succinyl-CoA (1 NADH, 1 CO₂) → succinate. Total 2 NADH, 2 CO₂.
39Why can't Acetyl-CoA be used for the net synthesis of glucose in humans?
  • A. It is too small.
  • B. There is no enzyme to convert it to Pyruvate (PDH is irreversible).
  • C. It is only found in mitochondria.
  • D. It is always converted to fat.
Answer: B
Pyruvate dehydrogenase is irreversible; Acetyl-CoA cannot be converted back to pyruvate.
40Which of the following produces the most NADH per carbon atom?
  • A. Glucose
  • B. Palmitic Acid
  • C. Alanine
  • D. Pyruvate
Answer: B. Palmitic Acid
Palmitic acid (16C) yields many NADH relative to its carbons.
41Which amino acid can enter the Krebs cycle via the 'Fumarate' entry point?
  • A. Phenylalanine
  • B. Lysine
  • C. Methionine
  • D. Proline
Answer: A. Phenylalanine
Phenylalanine is converted to fumarate.
42During β-oxidation, which step involves the addition of water (hydration)?
  • A. Step 1
  • B. Step 2
  • C. Step 3
  • D. Step 4
Answer: B. Step 2
Enoyl-CoA hydratase adds water to the double bond.
43What is the net charge of the Ammonia (NH₃) converted to Ammonium (NH₄⁺) in biological fluids?
  • A. Neutral
  • B. Negative
  • C. Positive
  • D. Variable
Answer: C. Positive
NH₃ + H⁺ → NH₄⁺, which is positively charged.
44Which enzyme catalyzes the 'Link Reaction' for Pyruvate but has no equivalent for Fatty Acids?
  • A. Pyruvate Dehydrogenase
  • B. Thiokinase
  • C. Enolase
  • D. Aconitase
Answer: A. PDH
PDH converts pyruvate to Acetyl-CoA; fatty acids use β-oxidation, not PDH.
45How many ATP are produced from the complete oxidation of one Acetyl-CoA via the ETC and Krebs cycle (traditional count)?
  • A. 10
  • B. 12
  • C. 14
  • D. 15
Answer: B. 12
MDCAT traditionally uses 12 ATP (3 NADH=9, 1 FADH₂=2, 1 GTP=1).
46A fatty acid with 10 carbons will produce how many NADH total (Beta-oxidation + Krebs)?
  • A. 5
  • B. 15
  • C. 19
  • D. 20
Answer: C. 19
4 cycles of β-ox (4 NADH) + 5 Acetyl-CoA into Krebs (5×3=15) = 19 NADH.
47Which tissue is the primary site for the Urea cycle?
  • A. Kidney
  • B. Brain
  • C. Liver
  • D. Muscles
Answer: C. Liver
Urea synthesis occurs in the liver.
48The enzyme Fatty Acyl-CoA Synthetase is located on:
  • A. Inner mitochondrial membrane
  • B. Outer mitochondrial membrane
  • C. Matrix
  • D. Nucleus
Answer: B
Fatty acid activation occurs on the outer mitochondrial membrane.
49If you compare the ATP yield of Glucose and Protein per gram, they are similar. Why is Glucose still the preferred fuel?
  • A. It is easier to store.
  • B. It can be respired anaerobically.
  • C. It doesn't produce nitrogenous waste.
  • D. All of the above
Answer: D. All of the above
Glucose can be used anaerobically and produces no toxic waste.
50Which biological molecule stores the highest amount of energy per gram?
  • A. Glucose
  • B. Protein
  • C. Fat
  • D. Nucleic acid
Answer: C. Fat
Fats yield 9 kcal/g, the highest energy density among biological molecules.

📄Past Paper MCQs

📅 Actual MDCAT / NUMS Questions

15 MCQs
1The common intermediate in the oxidation of carbohydrates, fats, and proteins is:
MDCAT 2016
  • A. Pyruvate
  • B. Acetyl-CoA
  • C. Oxaloacetate
  • D. Citrate
Answer: B. Acetyl-CoA
Acetyl-CoA is the convergence point for all major fuels.
2The value of Respiratory Quotient (RQ) for fats is approximately:
NUMS 2017
  • A. 1.0
  • B. 0.9
  • C. 0.8
  • D. 0.7
Answer: D. 0.7
Fats have an RQ of about 0.7.
3Which of the following yields the highest amount of energy per gram when oxidized?
MDCAT 2018
  • A. Glucose
  • B. Amino acids
  • C. Lipids
  • D. Glycogen
Answer: C. Lipids
Lipids yield 9 kcal/g, others ~4 kcal/g.
4Before entering the respiratory pathway, glycerol is first converted into:
MDCAT 2019
  • A. Pyruvate
  • B. Acetyl-CoA
  • C. Dihydroxyacetone phosphate (DHAP)
  • D. Oxaloacetate
Answer: C. DHAP
Glycerol → G3P → DHAP.
5The process of removing the amino group from an amino acid to use its carbon skeleton for energy is known as:
NUMS 2020
  • A. Beta-oxidation
  • B. Glycolysis
  • C. Deamination
  • D. Decarboxylation
Answer: C. Deamination
Deamination removes the -NH₂ group.
6During the oxidation of fatty acids, 2-carbon fragments are removed in the form of:
PMC National MDCAT 2021
  • A. Pyruvate
  • B. Citrate
  • C. Acetyl-CoA
  • D. Succinate
Answer: C. Acetyl-CoA
β-oxidation produces Acetyl-CoA.
7In the metabolism of proteins, which molecule enters the Krebs cycle directly after transamination of aspartate?
MDCAT 2022
  • A. α-ketoglutarate
  • B. Oxaloacetate
  • C. Fumarate
  • D. Succinyl-CoA
Answer: B. Oxaloacetate
Aspartate transaminates to oxaloacetate.
8Most of the energy in fats is stored in the form of:
NUMS 2022
  • A. C-O bonds
  • B. C-H bonds
  • C. O-H bonds
  • D. N-H bonds
Answer: B. C-H bonds
C-H bonds store high chemical energy.
9Which intermediate links the urea cycle with the Krebs cycle during protein catabolism?
MDCAT 2023
  • A. Citrate
  • B. Fumarate
  • C. Pyruvate
  • D. Acetyl-CoA
Answer: B. Fumarate
Fumarate connects the two cycles.
10How many molecules of Acetyl-CoA will be produced from a 16-carbon fatty acid (Palmitic acid)?
NUMS 2023
  • A. 16
  • B. 8
  • C. 4
  • D. 32
Answer: B. 8
16/2 = 8 Acetyl-CoA.
11Which of the following is the 'hub' of metabolism?
MDCAT 2024
  • A. Glucose-6-phosphate
  • B. Pyruvate
  • C. Acetyl-CoA
  • D. ATP
Answer: C. Acetyl-CoA
Acetyl-CoA is central to many metabolic pathways.
12Why is the RQ of proteins approximately 0.8?
MDCAT 2024
  • A. They are more oxidized than glucose.
  • B. They are more reduced than glucose but more oxidized than fats.
  • C. They produce more CO₂ than O₂ consumed.
  • D. They do not produce CO₂.
Answer: B
Proteins have an intermediate reduction state.
13A 3-carbon glycerol molecule enters the respiratory pathway at the level of:
MDCAT 2025
  • A. Acetyl-CoA
  • B. Glycolysis
  • C. Krebs cycle
  • D. ETC
Answer: B. Glycolysis
Glycerol enters as DHAP in glycolysis.
14Which amino acid is directly converted to Pyruvate?
NUMS 2024
  • A. Alanine
  • B. Glutamate
  • C. Aspartate
  • D. Leucine
Answer: A. Alanine
Alanine transaminase produces pyruvate.
15If a cell is respiring only fats, the volume of CO₂ produced will be:
MDCAT 2017
  • A. Equal to O₂ consumed
  • B. Greater than O₂ consumed
  • C. Less than O₂ consumed
  • D. Zero
Answer: C. Less than O₂
RQ < 1, so less CO₂ than O₂.

🔍Direct Search Queries — Bioenergetics

Exact search terms students use on Google for this chapter. Click any to jump to the relevant section:

bioenergetics MDCAT notes cellular respiration MDCAT fat respiration β-oxidation beta oxidation MDCAT protein respiration deamination deamination vs transamination acetyl-CoA central hub amino acid entry points Krebs ketogenic vs glucogenic amino acids carnitine shuttle fatty acid glycerol fate glycolysis DHAP RQ value fat protein glucose respiratory quotient MDCAT ATP yield from fat vs glucose why fats yield more energy ketone bodies starvation gluconeogenesis MDCAT urea cycle Krebs link fatty acids cannot form glucose pyruvate dehydrogenase irreversible what is bioenergetics biology difference between fat and protein respiration how many ATP from palmitic acid MDCAT biology chapter 2 notes high yield bioenergetics topics bioenergetics MCQs with answers bioenergetics MCQs pdf respiration of fats MCQs MDCAT past papers bioenergetics UHS MDCAT biology MCQs NUMS bioenergetics past papers bioenergetics chapter 2 class 11 β-oxidation notes class 11 deamination vs β-oxidation Krebs cycle MCQs

❔Frequently Asked Questions

What is bioenergetics?
Bioenergetics is the study of energy transformations in living organisms — how cells capture, store, and use energy from carbs, fats, and proteins.
What is the RQ of fat, protein, and glucose?
RQ = CO₂/O₂. Fat = 0.7, Protein = 0.8, Glucose = 1.0.
Why do fats yield more energy than carbohydrates?
Fats are more reduced (more C-H bonds), producing more electrons for the ETC and more ATP per gram (9 kcal/g vs 4 kcal/g).
What is β-oxidation?
β-oxidation breaks fatty acids into 2-carbon Acetyl-CoA units in mitochondria, producing 1 NADH + 1 FADH₂ per cycle.
What is deamination?
Deamination removes the –NH₂ group from amino acids before their carbon skeletons enter respiration. Ammonia is converted to urea in the liver.
What is the central hub of metabolism?
Acetyl-CoA — carbs, fats, and proteins all converge here.
Where do amino acids enter respiration?
Alanine→Pyruvate, Leucine/Lysine→Acetyl-CoA, Glutamate→α-ketoglutarate, Valine/Isoleucine→Succinyl-CoA, Aspartate→Oxaloacetate.
What is glycerol's fate?
Glycerol converts to DHAP and enters glycolysis.
What are ketogenic vs glucogenic amino acids?
Ketogenic (Leucine, Lysine) → only Acetyl-CoA. Glucogenic → can form glucose.
What is the carnitine shuttle?
The carnitine shuttle transports fatty acids into mitochondria for β-oxidation.
How many ATP from one glucose?
Traditional count: 36–38 ATP. Modern textbooks: 30–32 ATP.
Why can't fats make glucose?
Fats → Acetyl-CoA. The enzyme PDH is irreversible, so Acetyl-CoA can't become pyruvate or glucose.
What happens during starvation?
Glycogen and fats are depleted first; then structural proteins break down. Ketone bodies feed the brain.
What is acetyl-CoA?
Acetyl-CoA is a 2-carbon molecule that condenses with oxaloacetate to enter the Krebs cycle.
Where is the ETC located?
On the inner mitochondrial membrane, producing ATP via oxidative phosphorylation.
What is the final electron acceptor?
Oxygen — combines with H⁺ and electrons to form water.
What is the urea cycle?
A liver pathway converting ammonia to urea for excretion. Linked to Krebs via fumarate.
What is gluconeogenesis?
Synthesis of glucose from non-carb sources (amino acids, glycerol). Fatty acids cannot contribute.
What is the Cori cycle?
Converts lactate (from muscles) to glucose in the liver.
What is chemiosmosis?
ATP synthesis driven by proton gradient across inner mitochondrial membrane via ATP synthase.

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Biological Molecules
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Digestive System
11
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