Amdahl's Law

Biological Parallel

Amdahl's Law states that system speedup is limited by the sequential fraction that can't be parallelized. Metabolic pathways demonstrate this constraint precisely. Glycolysis has 10 sequential enzymatic steps—each reaction requires the product of the previous step. Adding more enzyme molecules to step 5 doesn't speed the pathway if step 3 is the bottleneck. The sequential dependency limits parallelization gains. Neuromuscular contraction reveals the same mathematics. Motor neurons can fire action potentials in parallel to recruit more muscle fibers, increasing force. But the action potential propagation down each axon is strictly sequential—sodium channels must open, close, reset in order. No amount of parallelization speeds this sequential component. Maximum contraction speed is bounded by the sequential axon propagation time, not by how many fibers you recruit. Photosynthesis demonstrates bottleneck dynamics at ecosystem scale. Light reactions (photon capture) can be parallelized by adding more chlorophyll. But the Calvin cycle (carbon fixation) has rate-limiting sequential steps, particularly the RuBisCO enzyme reaction. RuBisCO is the most abundant protein on Earth precisely because it's a bottleneck—plants compensate for its slow sequential kinetics by producing massive quantities. You can't parallelize a fundamentally sequential process. Amdahl's Law emerges from causality: when B requires A's output, B can't start until A finishes. Biology encounters this constantly. Cell division has sequential checkpoints—metaphase can't begin until prophase completes. DNA replication has sequential base addition—you can't add base 1001 before base 1000. The sequential fraction dominates: doubling the parallel capacity yields diminishing returns when the bottleneck is sequential.