Energy Systems and Metabolism Notes
Phosphocreatine system (fast, immediate energy)
Energy is limited; as you move, you consume it quickly and must recharge to continue high-intensity efforts.
Easiest recharge pathway: phosphocreatine (PCr) system.
Reaction (simplified): \text{PCr} + \text{ADP} \rightarrow \text{ATP} + \text{Cr} using creatine kinase.
Energy release is rapid and directly regenerates one ATP per PCr molecule.
Typical duration: about ~10 seconds of high-intensity effort before PCr is depleted.
No fatigue-related byproducts from this short recharge here; it’s a quick, fatigue-free recharge.
Creatine supplementation can increase muscle PCr stores, extending the time you can sustain high-intensity work; there’s a limit due to cellular space for PCr.
When PCr is depleted, we must switch to other energy systems for continued effort.
Glycolysis and anaerobic energy production (second fastest, cytosolic)
Fast energy system that does not require mitochondria or oxygen (anaerobic glycolysis).
Glucose (a six-carbon sugar) is broken down in the cytosol through glycolysis, producing two pyruvate molecules per glucose and a net yield of about 2\text{ ATP} per glucose (net).
Ten enzymatic steps in glycolysis; the first phase converts glucose to two molecules of pyruvate, with a production of 2 NADH (in the cytosol).
End products and quick energy outcomes:
Net ATP: \approx 2\;\text{ATP} per glucose (anaerobic).
NADH produced: 2\;\text{NADH} in glycolysis (cytosol).
Pyruvate produced: 2 pyruvate molecules per glucose.
When oxygen is limited or when NADH clearance is not fast enough, pyruvate is reduced to lactate to regenerate NAD+, allowing glycolysis to continue. This forms lactate and protons, contributing to acidity.
Lactate formation (anaerobic end product):
Reaction (simplified): \text{pyruvate} + \text{NADH} + H^+ \rightarrow \text{lactate} + \text{NAD}^+
Lactate itself is not simply a waste; it stores chemical energy and can be reused later.
Hydrogen accumulation and pH: hydrogens pulled off during glycolysis can acidify the intracellular environment; if acids accumulate faster than they’re cleared, enzymes lose function and fatigue sets in.
Hydrogen clearance via shuttle systems: NAD and FAD shuttle hydrogens (electrons) to the mitochondria for oxidation; when clearance is slower than production, lactate builds up and fatigue occurs.
Lactate, pH, and fatigue (important concept in performance)
Lactate buildup inside the cell (and in the bloodstream) correlates with acidification and fatigue; the pH drop inhibits enzymes and compromises muscle contraction.
Lactate in blood (plasma) can be measured as an index of fatigue risk; baseline lactate in healthy individuals is about [\text{lactate}] \approx 1\,\text{mM} in plasma due to circulating red blood cells (RBCs) which lack mitochondria and produce lactate.
Lactate threshold (LT) and OBLA (onset of blood lactate accumulation):
LT is the exercise intensity where lactate begins to accumulate in the blood faster than it can be cleared.
OBLA marks a defined level of lactate accumulation in the blood (onset of blood lactate accumulation); training can shift LT/OBLA to higher intensities, allowing sustained higher work rates.
Training adaptation: lactate can be cleared more rapidly with conditioning, allowing higher intensities before lactate accumulates.
Lactate is not simply a waste product; it represents chemical energy (pyruvate + 2 H"s; H2 = electrons, energy).
When oxygen becomes available again or exercise intensity decreases, lactate can be reconverted to pyruvate and fed into aerobic pathways.
At higher intensities where oxygen delivery lags, lactate accumulates and spills into the bloodstream (plasma), triggering chemoreceptors that contribute to fatigue perception.
From cytosol to mitochondria: aerobic metabolism (the powerhouse of the cell)
Transition to aerobic metabolism requires oxygen to be available at the end of the electron transport chain (ETC) in mitochondria.
Oxygen location matters: aerobic = oxygen is present at the end of the ETC in the mitochondria; anaerobic = oxygen not available to accept electrons at the end of the chain.
Pyruvate fate with oxygen present:
Pyruvate is transported into the mitochondria and converted to acetyl-CoA by the pyruvate dehydrogenase complex (irreversible step).
This acetyl-CoA enters the Krebs (citric acid) cycle, enabling extensive ATP generation through the ETC.
The mitochondrion: the cell’s powerhouse because it produces much more ATP per substrate than glycolysis alone, but requires oxygen and time to generate energy.
Acetyl-CoA: the universal entry substrate for the Krebs cycle, derived from all macronutrients (carbs, fats, proteins) by converting them to acetyl-CoA ( fats → acetyl-CoA via fatty acids; proteins → amino acids → acetyl-CoA).
Krebs cycle (Citric acid cycle) in mitochondria:
Acetyl-CoA enters the cycle and is oxidized in a series of enzymatic reactions; the cycle shreds acetyl-CoA completely, liberating energy.
Outputs per acetyl-CoA: 3\;\text{NADH},\;1\;\text{FADH}2,\;1\;\text{ATP (as GTP, via substrate-level phosphorylation)} and 2 molecules of CO$2$ are released.
Because glucose yields 2 acetyl-CoA (one from each pyruvate), the cycle runs twice per glucose oxidation: per glucose, 6\;\text{NADH},\;2\;\text{FADH}2,\;2\;\text{ATP} and 4 CO$2$ are produced in total from the Krebs cycle.
Electron transport chain (ETC) and oxidative phosphorylation:
NADH and FADH$_2$ donate electrons to the ETC (cytochrome complexes) to create a proton gradient across the inner mitochondrial membrane.
At the end of the chain, the final electron acceptor is oxygen, which combines with protons to form water.
Energy from electron flow drives ATP synthase to convert ADP + Pi into ATP.
NADH contributions: typically yield about 2.5\ \text{ATP} per NADH; FADH$2$ contributions: about 1.5\ \text{ATP} per FADH$2$.
There is a transport cost for moving NADH produced outside the mitochondria (cytosolic NADH) into the mitochondria; this “toll” reduces the effective ATP yield from those electrons and is why some sources use slightly different numbers (e.g., two-and-a-half vs three for NADH, and one-and-a-half vs two for FADH$2$). In your notes, the effective values are given as \text{NADH} \rightarrow 2.5\ \text{ATP} and \text{FADH}2 \rightarrow 1.5\ \text{ATP}.
Overall aerobic yield from glucose oxidation (per glucose):
Glycolysis: net 2\;\text{ATP} (substrate-level) and 2\;\text{NADH} (cytosolic).
Pyruvate to acetyl-CoA: yields NADH (per pyruvate, i.e., per glucose: 2 NADH).
Krebs cycle: per glucose, yields 6\;\text{NADH},\;2\;\text{FADH}_2,\;2\;\text{ATP} (from acetyl-CoA two times).
ETC oxidative phosphorylation: from NADH and FADH$_2$, total ATP from oxidation is typically around 30\ \text{ATP}.
Summed total for glucose under aerobic conditions: \approx 32\;\text{ATP} per glucose (2 from glycolysis, 2 from Krebs cycle substrate-level, and the rest from oxidative phosphorylation).
Practical implications of oxygen availability:
Aerobic metabolism provides large amounts of ATP with minimal byproducts that cause fatigue (water and CO$_2$).
If oxygen is limited or delivery is insufficient for the rate of NADH production (e.g., during high-intensity exercise), the ETC cannot oxidize all NADH/FADH$_2$, hydrogens accumulate, and lactate production rises to recycle NAD$^+$ and sustain glycolysis temporarily.
Aerobic metabolism allows sustained energy production at lower intensities; anaerobic metabolism supports rapid bursts but only for short durations.
Role of RBCs and baseline lactate level (illustrative example):
Red blood cells lack mitochondria; they rely on glycolysis and thus produce lactate, contributing to a baseline plasma lactate of about [\text{lactate}] \approx 1\,\text{mM}.
When exercising, lactate appears in plasma as a marker of glycolytic flux and oxygen availability.
Lactate clearance and training adaptation in real-world performance:
At lower intensities, lactate production and clearance are balanced; at rising intensities, clearance becomes rate-limiting and lactate accumulates.
As lactate accumulates, pH falls, enzymes lose efficiency, and fatigue increases; with training, LT/OBLA can shift to higher work rates, delaying fatigue.
Key connections and conceptual summary
All macronutrients can be eventually converted to acetyl-CoA, which enters the Krebs cycle; fats yield acetyl-CoA via beta-oxidation, proteins via deamination and conversion to amino acids, which feed into acetyl-CoA or Krebs intermediates.
The mitochondria are the primary site of sustained energy production through oxidative phosphorylation, but they require adequate oxygen delivery and time for electron transport and ATP synthesis.
Short, high-intensity efforts rely heavily on phosphocreatine and glycolysis; longer, moderate efforts rely on mitochondrial oxidative metabolism.
The balance between NADH production and its oxidation determines the shift between aerobic and anaerobic metabolism, the production of lactate, and ultimately fatigue risk.
From a physiological perspective, oxygen is always present in the bloodstream and inside cells, but the limitation is getting it to the specific sites where it is needed (mitochondria) quickly enough to sustain the desired energy output.
Practical takeaway for exam-style understanding
Energy system order by speed and duration: Phosphocreatine (fastest, up to ~10 seconds) → Glycolysis (fast, up to ~1–2 minutes) → Aerobic metabolism (slower, long-term).
Lactate dynamics: lactate formation serves as a quick NAD+ regeneration mechanism during anaerobic bursts; elevated blood lactate indicates a mismatch between production and clearance and is used to gauge LT/OBLA and training adaptations.
ATP yields (summary numbers from transcript):
Glycolysis (net): \approx 2\ \text{ATP} per glucose (anaerobic).
NADH produced in glycolysis: 2 (cytosol); effective ATP yield depends on shuttle: around 2.5\ \text{ATP} per NADH in the mitochondria.
Krebs cycle (per acetyl-CoA): 3\ \text{NADH},\;1\ \text{FADH}2,\;1\ \text{ATP}; per glucose (two acetyl-CoA): 6\ \text{NADH},\;2\ \text{FADH}2,\;2\ \text{ATP}.
ETC yields: NADH ≈ 2.5\ \text{ATP} each; FADH$_2$ ≈ 1.5\ \text{ATP} each; final practical total for glucose ≈ 32\ \text{ATP} under aerobic conditions.
The lactate story is not purely about waste; lactate stores energy and is reconverted to pyruvate when oxygen returns, fueling ongoing energy production during recovery and lower-intensity work.
Red blood cells’ lack of mitochondria explains baseline lactate and anaerobic metabolism in at least some cells.
Training shifts LT/OBLA to higher intensities, enabling longer bouts of higher performance before fatiguing due to acidosis and ATP regeneration limitations.