Metabolism and Energy Production

Metabolism

• Metabolism is the sum total of all chemical reactions in the human body.
• It's the balance between:
  • Anabolism: Building things up, repairing, or storing.
  • Catabolism: Breaking things down, usually for energy.

Breakdown of Food for Energy or Storage

Carbohydrates

• Carbohydrates are eaten as starches or sugars.
• They are broken down into glucose, the major single-unit currency.
• $Glucose \rightarrow Pyruvate \rightarrow Acetyl-CoA$
• Acetyl-CoA is a central molecule for energy production.

TCA Cycle (Tricarboxylic Acid Cycle or Krebs Cycle)

• Acetyl-CoA moves around the cycle, forming slightly different compounds.
• High-energy carriers are bounced off during this process.

Electron Transport Chain

• High-energy carriers move down the electron transport chain.
• $ATP$ (adenosine triphosphate) is produced, which is the energy currency of the body.
• $H_2O$ (water) is also given off.
• Oxygen is required for the TCA cycle and electron transport chain to function.
• Carbon dioxide ($CO_2$) is given off.

Proteins

• Proteins are broken down into amino acids.
• Amino acids enter the system at different levels, depending on the type.
• When amino acids are processed, an amino group is given off.
• The amino group becomes a waste product in urine.

Fats

• Fats consist of a glycerol backbone and three fatty acid chains.
• When fats are used for energy, they are broken down into two-carbon units from the fatty acids.
• Fatty acids enter the system at the level of acetyl-CoA.
• Glycerol enters the system earlier.

Building Up Reactions (Anabolism)

• Amino acids are used to build muscle and other lean tissue.
• They are also used for repair and as protein carriers in the blood.
• Glucose is stored as glycogen.
• Excess fats are stored as adipose tissue.

Hormonal Control of Metabolism

Catabolic Hormones (Breaking Down)

• Glucagon: Breaks down glycogen stores.
• Epinephrine: Fight or flight hormone.
• Glucocorticoids: Stress hormones.

Anabolic Hormones (Building Up)

• Insulin: Leads to storage.
• Sex hormones/steroids.
• Thyroxin (T4) and Growth Hormone (GH): Favors the building up of protein stores (protein synthesis) but also the breakdown of fats and carbohydrate stores.

Reversibility of Reactions

• $Glucose \rightleftharpoons Pyruvate$ (Reversible)
• Amino acids can be used to make glycogen.
• Glycerol can be used to make glycogen or glucose.
• $Fatty Acids \nrightarrow Pyruvate$ (Irreversible - One-Way Reaction)
• Fatty acids cannot be converted to pyruvate, glucose, or glycogen.

ATP Demand and Energy Use

• The TCA cycle continues if ATP is needed (e.g., during exercise).
• If there is no energy demand (sedentary lifestyle), acetyl-CoA builds up.
• Excess acetyl-CoA favors the production of adipose tissue.
• Excess carbohydrate, fat, or protein can be converted to adipose tissue.