MCAT Biochemistry · Lesson 12
Bioenergetics and Regulation of Metabolism
3 min read7 sectionsUpdated
7 sections
12.1 Thermodynamics and Bioenergetics
Biological systems exchange energy and matter with the environment. Bioenergetics quantifies energy changes in biological reactions using concepts of enthalpy, entropy, and free energy.
- Biological Systems
- Open systems: exchange energy & matter (e.g., food, respiration)
- Cellular/subcellular: closed systems, no matter exchange
- Energy
- Internal energy : sum of interactions
- Enthalpy, Entropy, Free Energy
- ∆H: heat change at constant pressure/volume
- ∆S: disorder (J/K)
- ∆G: Gibbs free energy =
- Physiological Conditions
- Standard free energy: (1 M, 1 atm, 25ºC)
- Standard state at pH 7:
- More products → more negative ∆G
12.2 Role of ATP
ATP is the primary energy currency, storing energy in high-energy phosphate bonds. Energy is used in substrate-level and oxidative phosphorylation.
- Energy Content
- Fats: 9 kcal/g, carbs/proteins/ketones: 4 kcal/g
- ATP: 30 kJ/mol
- cAMP = -50.4 kJ/mol, creatine phosphate = -43.3 kJ/mol
- ATP Cycling
- ADP + Pi → ATP via ATP synthase
- Daily turnover: 90% body weight used; only 50 g ATP available at a time
- Hydrolysis and Coupling
- ATP transfers phosphate to activate/inactivate targets
- Example: ATP → glucose 6-phosphate
- Hess's law for ∆G calculations
12.3 Biological Oxidation and Reduction
Redox reactions are divided into half-reactions. High-energy electron carriers facilitate energy transfer in metabolism.
- Half-Reactions
- Separate oxidation & reduction components
- Electron Carriers
- High-energy: NADH, NADPH, FADH2, ubiquinone, cytochromes, glutathione
- Flavoproteins: coenzymes in mitochondria/chloroplasts
- Roles: FA oxidation, pyruvate decarboxylation, glutathione reduction
12.4 Metabolic States
Metabolism shifts according to feeding status: absorptive (anabolic), postabsorptive (fasting), and prolonged fasting (starvation).
- Postprandial (Absorptive) State
- 3–5 hours post-meal
- ↑ insulin → glycogen synthesis (liver/muscle)
- Excess glucose → fatty acids → triacylglycerols
- Protein synthesis ↑ in muscle
- Postabsorptive (Fasting) State
- ↑ glucagon, cortisol, E, NE, GH
- Glycogenolysis: rapid glucose release
- Hepatic gluconeogenesis: slower, max ~12 h
- Lipolysis & AA release from muscle
- Prolonged Fasting (Starvation)
- Rapid glycogen depletion
- Lipolysis → acetyl CoA → ketone bodies
- Brain uses ketones; RBCs rely on glucose
12.5 Hormonal Regulation of Metabolism
Hormones (peptide, steroid, thyroid) regulate nutrient uptake, storage, and mobilization. Insulin and glucagon are key postprandial/postabsorptive regulators.
- Insulin
- β-cells; stimulates glucose uptake/storage
- Affected tissues: adipose, skeletal muscle
- ↑ glycogen synthase, glucokinase, AA uptake
- ↑ triacylglycerol synthesis; lipoprotein lipase activation
- ↓ triacylglycerol breakdown, ketone body formation
- Glucagon
- α-cells; stimulates liver glycogenolysis, gluconeogenesis, ketogenesis
- ↓ lipogenesis; ↑ lipolysis
- Secretion ↑ low plasma glucose, inhibited by high glucose
- Glucocorticoids
- Cortisol: stress response, ↑ hepatic glucose output, inhibits glucose uptake, enhances other hormones
- Long-term exposure → hyperglycemia, fat storage
- Catecholamines
- E, NE: ↑ glycogen phosphorylase, glycogenolysis
- ↑ metabolic rate via sympathetic system
- Thyroid Hormones
- T4: slower, long-term ↑ basal metabolic rate
- T3: faster, short-term ↑ BMR
- Effects: lipid & carb metabolism, cholesterol clearance, glucose absorption
- Required for catecholamine activity
12.6 Tissue Specific Metabolism
Different tissues utilize fuels differently depending on feeding state, activity, and oxygen availability.
- Liver
- Maintains blood glucose, produces ketones
- Insulin ↑ glycogen/fatty acid synthesis → triacylglycerols → VLDL
- Releases glucose between meals
- Adipose Tissue
- Insulin ↑ glucose uptake, LPL activity → triacylglycerol storage
- Fasting → ↓ insulin, ↑ E → HSL activation → FA release
- Skeletal Muscle
- Resting: glucose uptake via insulin, glycogen/AA replenishment, excess oxidized
- Fasting: uses fatty acids/ketones
- Active: short-term creatine phosphate, anaerobic glycolysis, glycogen for high-intensity activity
- Cardiac Muscle
- Prefers fatty acids; ketones in fasting
- Failing heart: ↑ glucose oxidation, ↓ beta-oxidation
- Brain
- 15% CO, 20% O₂, 25% glucose
- Glucose essential; fatty acids cannot cross BBB
- Relies on glycogenolysis or gluconeogenesis during fasting
12.7 Integrative Metabolism
Whole-body metabolism integrates tissue-specific fuel use, hormone signaling, and energy expenditure. Respirometry and calorimetry assess metabolic rate and substrate utilization.
- Respirometry
- Measures RQ = CO₂ produced / O₂ consumed
- Carbs: 1, Lipids: 0.7, Mixed diet: ~0.8
- Calorimeters
- Measure BMR via heat exchange
- Estimate BMR: age, weight, height, gender
- Regulation of Body Mass
- Mass: water, carbs, proteins, lipids
- Lipids = main factor in gradual weight change
- Excess intake → ↑ mass until equilibrium
- Activity ↑ → hunger ↑
- Hormonal Control of Hunger
- Ghrelin: ↑ appetite, triggers orexin
- Orexin: ↑ appetite, alertness, sleep-wake
- Leptin: ↓ appetite, suppresses orexin
- BMI
- Normal: 18.5–25, Overweight: 25–30, Obese: >30
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