Respiratory substrates are organic molecules that can be broken down to generate ATP.
Common substrates include carbohydrates (e.g., glucose), lipids (e.g., fatty acids), and proteins (e.g., amino acids).
Main Focus: Efficiency of ATP Production
Cells can break down carbohydrates, lipids, and proteins to produce ATP.
The efficiency of ATP production varies between different substrates.
Proteins as Energy Sources
Proteins are composed of amino acids.
While proteins can be broken down for ATP, they are primarily needed for making antibodies, enzymes, hemoglobin, collagen, etc.
Cells prefer not to use amino acids as fuel.
Carbohydrates vs. Lipids
Examples:
Carbohydrate: Glucose (C₆H₁₂O₆)
Lipid: Fatty Acid (e.g., C₁₈H₃₄O₂)
ATP Yield:
Fatty acids have more carbon-hydrogen bonds than glucose.
More carbon-hydrogen bonds = more hydrogen atoms released upon breakdown.
Hydrogen atoms are accepted by carriers like NAD+ and FAD during respiration.
More hydrogen means more reduced NAD (NADH) and FAD (FADH₂), leading to higher ATP production during oxidative phosphorylation in fatty acids compared to glucose.
Why Cells Prefer Glucose
Despite fatty acids providing more ATP, glucose is the main energy source.
Aerobic vs. Anaerobic Conditions:
Fatty acids require oxygen for breakdown; they are only used in aerobic pathways.
Glucose can be broken down without oxygen through glycolysis (anaerobic pathway).
Glycolysis Process:
Glucose (6-carbon) is broken into 2 pyruvate molecules (3-carbon each) via glycolysis.
Pyruvate enters the mitochondrial matrix for further breakdown.
In absence of oxygen, glucose is converted to lactate in animals or ethanol and CO₂ in plants and yeast.
Ease of Breakdown:
Glucose (6-carbon) is easier to break down than fatty acids (e.g., 14-carbon), making it more efficient for quick energy release, especially during anaerobic conditions.
Misconceptions in Energy Use
During intense exercises like fast running, the body relies on glucose rather than fats due to the anaerobic demand.
Key Takeaways
Fatty Acids: More ATP from oxidative phosphorylation, but requires oxygen.
Glucose: Versatile for both aerobic and anaerobic respiration, easier to break down.
Understanding the differences in energy yield and conditions under which these substrates are used is crucial.