The big picture
Photosynthesis builds it, respiration burns it, hormones decide when.
Twelve processes, one loop · energy flows through; carbon goes in circles
This mnemonic, unpacked word by word →
The whole story at cruising altitude. Plants reduce CO₂ to glucose with sunlight; animals store it, release it and burn it back to CO₂, and the O₂ one half releases is what the other half breathes. Insulin points the loop toward storage, glucagon reverses it, and when glycogen runs dry, the refill routes kick in: new glucose from muscle protein, acetyl-CoA and ketones from fat. The quiz here draws on every pathway at once.
Amino acid degradation at body scale, driven by cortisol and falling insulin. Muscle barely listens to glucagon. Transaminases strip the nitrogen (bound for the urea cycle) and the carbon skeletons travel to the liver, much of it as alanine. The expensive phase: every gram of new glucose costs muscle.
Lactate and glycerol feed in alongside the amino acids. Gluconeogenesis exists for red blood cells, which can never switch fuels, and the brain, which switches only slowly and partially. Acetyl-CoA can never make this trip, which is why fat cannot rescue blood glucose.
Hormone-sensitive lipase, phosphorylated ON by the same PKA signal that empties glycogen. The glycerol heads to gluconeogenesis; the fatty acids fuel nearly everything else.
Muscle, liver and heart switch to fat, sparing glucose for the brain. The acetyl-CoA flood also inhibits PDH and activates pyruvate carboxylase, pushing pyruvate toward new glucose instead of burning it.
Days into a fast, the liver turns its acetyl-CoA surplus into a fuel the brain can import. As the brain adapts, glucose demand falls and muscle protein is spared, the adaptation that makes long starvation survivable.
Light reactions plus the Calvin cycle: light strips electrons from water (releasing the O₂ you breathe), the Calvin cycle parks them on carbon. Every glucose an animal ever burns was built here first.
The fed state banks the meal: insulin activates glycogen synthase by dephosphorylation, and the surplus beyond glycogen becomes fat. Liver stores for the blood; muscle stores for itself.
Between meals the liver spends the pantry, about a day’s supply. Muscle glycogen never reaches the blood (no glucose-6-phosphatase), so the liver carries blood sugar alone.
Ten cytosolic steps, no oxygen required, every cell on Earth. The first fork: with no O₂, fermentation takes the pyruvate and the story ends here; with O₂, it heads into the mitochondrion.
The one-way door: irreversible, which is why acetyl-CoA can never become glucose. The first CO₂ you exhale is released here.
The carbon is fully oxidized and leaves for the lungs; the energy rides out on the carriers. Also the merge point: β-oxidation and the ketogenic amino acids enter here as acetyl-CoA.
Electrons fall to O₂, the very O₂ photosynthesis released, making the water photosynthesis will split again. The loop closes: carbon cycles between air and sugar; energy flows through and leaves as heat.