Energy Metabolism Flashcards

Energy Metabolism

• Encompasses all chemical reactions by which the body obtains and spends energy.
• Nutrients: Getting energy from food and how the body uses the energy for different processes.

Anabolic Process

• Building of substances.
• Example: Body produces glucose from a meal but doesn't need it right now, so it converts it to glycogen as storage sugar.

Catabolic Process

• Breakdown of substances.
• Example: When fasting and the body needs glucose, glycogen will break down and convert to glucose.

Oxidation and Reduction

• Oxidation: Losing electrons.
• Reduction: Gaining electrons.
• These two processes occur together; if one molecule is being oxidized in a reaction, the other is simultaneously reduced.
• Example: Glucose is oxidized to carbon dioxide as it loses hydrogen atoms (and their electrons).
• Example: Oxygen is reduced to water as it accepts the hydrogen atoms (and their electrons).
• Oxidation must be tightly controlled in biological systems to avoid dangerous rates of heat production.
• Oxidation in cellular metabolism results in ATP formation (energy currency).
• Exothermic: Gaining energy/heat; our body prefers ATP production over heat production.
• The process is never 100% efficient at producing ATP; some energy is wasted as heat.

Metabolic Coupling

• One metabolic reaction cannot proceed without the reactions it is linked to.
• For example, the reaction B -> C cannot proceed without ADP; therefore, it relies on the reaction 1 -> 2.
  1. Enzyme takes molecule 1 to produce molecule 2 (complex reaction); energy is consumed, sourced from ATP.
  2. Complex reaction A -> less complicated reaction B.
• B and C produce ATP, taking ADP and P to produce ATP, so 1 & 2 can use ATP to produce ADP.
• Both reactions happen together; neither can occur in isolation.

Energy Balance

First Law of Thermodynamics

• Energy cannot be created nor destroyed.
• In a closed system, energy is constant.
• Energy can be transformed into different types.

Steady State

• Energy intake = Energy expenditure (+ Energy storage).
• Large Energy Intake (EI) and Energy Expenditure (EE) is less -> positive balance; more energy entering the body than needed, leading to an obesogenic state as energy is stored as adipose tissue.
• EI is less than EE -> negative energy balance; extra energy comes from storage, entering an anorexigenic state.
• Carbs, fat, and protein are the primary energy sources.
• All life processes require energy.
• Extra energy is stored; insufficient energy is sourced from storage, creating a balance.

Energy Intake

• Food and beverage composition.
• 1 \text{g} \text{ carbohydrate} = 4 \text{ kcal} = 17 \text{ kj}
• A 70 kg body can store 700g glycogen (= 2800 \text{ kcal} = 11,760 \text{ kj}
• If resting metabolic rate = 2100 \text{ kcal/day}, then the body has 1.3 days of fuel to sustain resting metabolic rate.
• 1 \text{g} \text{ fat} = 9 \text{ kcal} = 38 \text{ kj}
• Adipose tissue is about 87% lipid or fat content.
• If you have 1 \text{ kg} \text{ body fat} = 870 \text{g} of lipids, (870 \text{g} \times 9 \text{ kcal/g} = 7830 \text{ kcal} = 32,886 \text{kj}) which is a high amount of energy.
• If a 70kg person has 20% fat (14kg), then they carry 12,180g lipids = 109,620 \text{ kcal}.
• If resting metabolic rate = 2100 \text{ kcal/day}, then there are 52.2 days' worth of fuel to sustain RMR.
• 1 \text{g} \text{ protein} = 4 \text{ kcal} = 17 \text{ kj}
• Used for structural/functional roles rather than energy provision, except during starvation (~15%).
• 1 \text{g} \text{ alcohol} = 7 \text{ kcal} = 30 \text{ kj}

Calorimetry

• The science of measuring heat production.
• Bomb calorimetry: Measures kcal in food.
  • Food -> C and H bonds break -> energy released as heat.
• Direct calorimetry: Heat provides a measure of food energy composition as kcalories (units of heat energy; calor = heat).
• Indirect calorimetry: Measure of O2 consumption and CO2 production as an indicator of heat production.
  • O_2 consumption is directly proportional to heat production, which comes from cellular respiration.
• Living organisms' Respiratory Quotient (RQ): What type of fuel are we predominantly burning?
  • Ratio of moles CO2 produced per moles O2 consumed at the tissue level.
    • Carbohydrate: 6O2 -> 6CO2 = 1:1 ratio.
    • Fat: 23O2 -> 16CO2 = 0.7 ratio.
    • Protein: 0.8 ratio.

Second Law of Thermodynamics

• Chemical transformations always result in a loss of free energy available to drive metabolic processes.
• ATP -> ADP produces a lot of energy.

Gibbs Free Energy: G

• Free energy available to drive metabolic processes.
• Total internal energy: E.
• Wasted energy (mainly lost as heat): T.S.
• Example: Consume glucose = small increase in total internal energy (E).
• Some of this energy is stored as glycogen (G).
• Chemical reactions converting glucose to glycogen are inefficient.
• Therefore, some E is wasted as heat (T.S).
• Chemical transformations always result in a loss of free energy.
• 1 mole glucose:
  • In bomb calorimeter: -> CO2 + H2O (completely combusts to heat, but no usable energy) = 686 \text{ kcal} (heat, not conserved energy) - maximum energy in mole.
  • In the body: Mitochondria oxidizes glucose -> CO2 + H2O + free energy stored (ATP) = 400 \text{ kcal} of free energy; rest = 286 \text{ kcal/mole} liberated as heat energy (40% of energy is lost).
  • Another way of stating the 2nd law: TS cannot = 0 (reactions are never 100% efficient); therefore, if we add 0 energy, the body’s total free energy declines.

Energy Expenditure

• Burn 1 \text{ kg} \text{ fat} = 8 \text{ hours} running or sleep for 130.5 hours.

Metabolic Rate

• Measure of energy balance.
• Resting metabolic rate: Estimate of energy required while at rest.
• Metabolism = all chemical processes involved in energy production, release, and growth.
• Healthy young 70kg human requires 2100 \text{ kcal/day} to sustain metabolism.
• The number of calories required (i.e., RMR) can rise in response to heavy exercise, cold exposure, or illness.
• Measured by indirect calorimetry: O2 consumed and CO2 produced.
• Basal Metabolic Rate: Clinical measurement of metabolism, measured under several standardized conditions.
  • AM measurement after good sleep.
  • Fasted for 12 hours.
  • 1 hour of quiet rest.
  • 25-degree air temp.
• Clinical calculation of energy expended to sustain vital functions when awake (bare minimum).
• RMR is typically higher than BMR due to the inclusion of the thermic effect of food; BMR is taken after 12 hours of fasting.

Energy Storage

• Energy Intake vs. Energy Expenditure.
• People are intermittent eaters (3 meals a day), but we are using energy all the time (continuous).
• Humans have to store energy.

How is energy stored?

• ATP: Adenosine Triphosphate.
• The last 2 phosphate molecules are held by high-energy bonds.
• To add a phosphate, it requires a lot of energy (hydrogen gradient) so ATP synthase can produce ATP.
• Stores energy from energy-releasing reactions (ADP -> ATP).
• The bond is broken, and it produces a lot of energy.
• Releases energy to deliver to other reactions (ATP -> ADP).

Summary of Metabolic Pathways

Carbohydrates

• Cellular respiration also produces heat.
  • Glycolysis (lysis = breaking down): Breaking glucose down to pyruvic acid.
  • Glycogenesis (genesis = production): Taking glucose and forming glycogen.
  • Glycogenolysis (lysis = breakdown): Breakdown of glycogen to glucose.
  • Gluconeogenesis (gluco = glucose, neo = new, genesis = creation): Creation of new glucose.
  • Krebs cycle: Mitochondria.
  • Electron transport chain: Enzymes that take electrons and add them to oxygen, eventually forming oxygen then ATP.

Lipids

• Beta Oxidation: Loss of electrons.
  • Lipolysis (breakdown): Taking triglyceride and breaking it down to fatty acids and glycerol.
  • Lipogenesis (production): Building up, while lipolysis is breaking down.

Proteins

• Transamination
• Oxidative deamination

Distribution of Metabolic Pathways

• The liver is an important supplier.
• Muscle tends to be a consumer.
• Red blood cells: Only use anaerobic glycolysis.
• Consumers of products only oxidize glucose; they don't store it or produce other things.

Study Questions

1. Describe metabolic coupling.
2. What is the first law of thermodynamics?
3. Describe oxidation and reduction and how it relates to cellular metabolism.
4. Explain how we measure energy intake.
5. What is RQ?
6. Describe the second law of thermodynamics.
7. Explain the difference between resting and basal metabolic rate.