Aerobic Energy Production: ATP Yield and Lactate Threshold

Cellular Respiration: Aerobic System

The ATP produced here far exceeds the other pathways. After one cycle, one glucose molecule can yield 36 ATP molecules (give or take a few depending on the fuel source). In the presence of oxygen, the Aerobic System can sustain activity for a long time until other physiological limits are reached. It involves three separate sub-pathways that begin with one glucose molecule.

1. Glycolysis – 1st Sub-Pathway

1st Stage: Glycolysis is the same as the anaerobic lactic system, except that in the presence of oxygen, Pyruvate (pyruvic acid) is converted to Acetyl CoA (rather than lactic acid). 2 ATP are produced. Acetyl CoA then enters the Krebs Cycle (Citric Acid Cycle), which is a central pathway for the metabolism of fats and proteins also.

2. Krebs Cycle – 2nd Sub-Pathway

Through a series of 8 reactions, 2 ATP are produced along with new compounds capable of storing high-energy electrons, which will be used in the third pathway: the ETC (NADH + FADH).

3. Electron Transport Chain – 3rd Sub-Pathway

During the final stage of aerobic respiration, large amounts of ATP are produced, with carbon dioxide ($\text{CO}_2$) and $\text{H}_2\text{O}$ as by-products. The “chain” is a series of electron carriers and protein complexes that accept and donate electrons in a sequential series. The final electron acceptor is oxygen. 32 ATP are made.

Lactic Acid Management in Exercise

In the context of continued physical activity, lactic acid buildup presents a serious problem. Why?

Lactate Threshold Concepts

• The point at which lactate levels in the blood increase abruptly beyond resting values is known as the Blood Lactate Threshold or Anaerobic Threshold.
• Onset Blood Accumulation (OBLA) refers to the point at which blood lactate levels begin to accumulate very rapidly.
• The Anaerobic Threshold varies from person to person and is reached at varying levels of intensities during exercise depending on the individual's fitness levels. Generally, untrained individuals have a lower anaerobic threshold when compared to elite endurance athletes.

VO2 Max and Threshold

VO2 Max = the maximum rate of oxygen consumption as measured during incremental exercise.

• Untrained athletes reach the threshold at 50–60% of VO2 Max.
• Trained athletes reach the threshold at 70–80% of VO2 Max.

Raising The Lactic Acid Threshold

Raising this threshold is a very important objective in physical training. Achieving this involves:

1. Anaerobic (power) training to extend the point at which lactate buildup occurs.
2. Aerobic-style endurance training, which improves:
   • Cardiorespiratory capacity.
   • The concentration of mitochondria.
   • $ ext{O}_2$ carrying myoglobin concentration in muscle.
   • The efficiency of $ ext{O}_2$ transfer at the cellular level.

The recovery stage is also important in limiting the effects of lactic acid, which is why most sports involve bursts of intense activity followed by periods of rest intervals.