HN&F 171 - Energy Metabolism

Metabolism

How the body obtains energy from foods and is the sum of all chemical and physical processes where the body both breaks down and builds molecules.

Anabolism

- Building body compounds

-Requires energy

- Critical for growth, repair, maintenance, and synthesis of chemical products essential for human functioning

Catabolism

- Breaking down body compounds

- Releases energy

- Breaking down proteins, lipids, carbs

- Old cells or tissues broken down for repair or replacement

Catabolic Reactions

Glycolysis, beta-oxidation, and deamination

Glycolysis

Process of breaking down carbohydrates

Beta-oxidation

Process of breaking down fats

Deamination

Process of breaking down proteins

Adenosine Triphosphate (ATP)

- Released during breakdown of glucose, fatty acids, and amino acids

- Provides energy for all cell activities

Primary sites of ATP production

Mitochondria

Condensation

- Anabolic process

- Simple units combine to form a larger, more complex molecule

- Water is released as a by-product

Hydrolysis

- Usually a catabolic process

- A large molecule is broken apart with the addition of water

Helpers in Metabolic Reactions

Enzymes and coenzymes

Enzymes

Facilitators of metabolic reactions.

Coenzymes

- Organic

- Associate with enzymes

- Without coenzyme, an enzyme cannot function

- Are not a protein

Breaking down nutrients for energy

- Digestion: Carbohydrates → glucose (and other monosaccharides), Fats (triglycerides) → glycerol and fatty acids, Proteins → amino acids

TCA Cycle

The final common metabolic pathway for carbohydrates, fats, and amino acids

Energy from Carbohydrates

§ When glucose is transported to the liver, it is: Phosphorylated and metabolized for energy (glycolysis) or stored as glycogen, released into circulation for other cells to use as fuel or stored as glycogen (muscle tissue), converted to fatty acids, if glucose exceeds energy needs, and stored as triglycerides in adipose tissue

- Fructose and galactose are converted to glucose in the liver and follow the same process

Energy from Fat

- Lipolysis: Dietary and adipose triglycerides are broken down by lipase to yield glycerol and three free fatty acids

- Glycerol is converted to pyruvate, then to acetyl CoA for entry into the TCA cycle

- Fatty acids are used for energy

Energy from Protein

- The body preferentially uses fat and carbohydrate as fuel sources

- Protein is saved for metabolic functions that cannot be performed by other compounds

- Protein is used for fuel primarily when total energy or carbohydrate intake is low

- Proteolysis: Dietary proteins are digested into amino acids or small peptides

- Amino acids are transported to the liver and made into proteins or released into the blood for uptake by other cells for building and repair functions

- Excess dietary protein os converted to acetyl-CoA and can be used for energy or stored as triglycerides

Mitochondria

- Powerhouses of the cells

- TCA cycles occurs in the inner compartment of a mitochondrion

Electron Transport Chain

- One: The electron transport chain captures energy in the high-energy bonds of ATP.

- Two: Coenzymes deliver hydrogens and electrons from the TCA cycle, glycolysis, and fatty acid oxidation

- Three: The chain is a series of proteins that carry electrons.

- Four: Oxygen accepts electrons. This combines with hydrogen atoms to form water

- ATP is synthesized as hydrogen ions rush back into the inner compartment of the mitochondria. This powers the synthesis of ATP.

Feasting - Excess Energy

- Metabolism favors fat formation regardless of excess from protein, fat, or carbohydrates

- The body will store the excess energy as fat

- Stored energy can be used during times of sleep, fasting, or exercise

Feasting - Inadequate Energy

- Carbohydrate, fat, and protein all eventually used for energy

- Begins with release of glucose and fatty acids

- Low blood glucose levels signal fat breakdown and release of amino acids from muscles

Gluconeogensis

Making new glucose from nonglucose substrates

Metabolic Response to Feeding

- Anabolic state: Bloodstream enriched with glucose, fatty acids, and amino acids

- Glucose stored as glycogen

- Glycogen stores are saturated, remaining glucose is stored as triglycerides

- Fatty acids are stored as triglycerides mostly in adipose tissues

- Amino acids are deaminated and carbon skeletons are converted to fatty acids for storage as triglycerides

Metabolic Response to Fasting

- Liver glycogen is broken down and blood glucose released

- Most cells can switch to using fatty acids as fuel to conserve glucose for brain and other cells that rely on glucose as fuel

- Gluconeogenesis

Metabolic Responses to Starvation

- The body shifts to survival mode

- Blood glucose is maintained to support brain and red blood cells

- Decline in activity, body temperature, and resting metabolic rate

- Fatty acids become the primary fuel

- Brain cells start to use ketone bodies

- Muscle protein supplies glucose