Respiration in Plants

Glycolysis: the one pathway every cell runs, oxygen or not

Glycolysis, occurring in the cytoplasm and requiring no oxygen at all, is the universal first step of both aerobic and anaerobic respiration: one molecule of glucose is partially broken down into two molecules of pyruvate, netting 2 ATP and 2 NADH in the process. What happens to that pyruvate next is what actually determines whether a cell nets a small or a large amount of usable energy, and it depends entirely on whether oxygen is available.

Without oxygen: fermentation, and a low energy return

In the absence of oxygen, pyruvate is instead converted through fermentation, which regenerates the NAD+ glycolysis needs to keep running but yields no further ATP beyond glycolysis's own initial 2. Alcoholic fermentation, carried out by yeast, converts pyruvate into ethanol and carbon dioxide. Lactic acid fermentation, occurring in some bacteria and, in animals, in oxygen-starved muscle tissue, converts pyruvate directly into lactic acid instead. Either way, fermentation exists purely to keep glycolysis itself running when oxygen is unavailable, not to extract any further significant energy from the glucose molecule.

With oxygen: the Krebs cycle and the electron transport chain

Where oxygen is available, pyruvate is instead oxidised further, first converted to acetyl-CoA, then fed into the Krebs cycle (citric acid cycle), which runs in the mitochondrial matrix and completes the oxidation of the original glucose carbon skeleton all the way to carbon dioxide, in the process generating further NADH and FADH2, the cell's own electron-carrying molecules. Those electron carriers then feed the electron transport chain, embedded in the inner mitochondrial membrane, where their stored electrons are passed down a chain of carriers to finally reduce oxygen, the process that actually drives the bulk of ATP production through oxidative phosphorylation. Textbook accounting puts the total yield of complete aerobic respiration at roughly 36 to 38 ATP per glucose molecule (the exact figure depends on which shuttle mechanism moves cytoplasmic NADH's electrons into the mitochondrion), a return more than fifteen times larger than fermentation's own bare 2 ATP, the single clearest reason oxygen availability matters so much to a cell's overall energy economy.

Respiratory quotient: reading which fuel is actually being burned

The respiratory quotient (RQ) is the ratio of the volume of carbon dioxide released to the volume of oxygen consumed during respiration, and its value reveals which class of molecule a tissue is actually oxidising for energy at a given moment, since different fuels require different amounts of oxygen relative to the carbon dioxide they release. When carbohydrates are the respiratory substrate, RQ equals 1.0, since carbohydrates already contain oxygen and hydrogen in the same ratio as water, requiring no extra oxygen input beyond what is needed to oxidise the carbon itself. When fats are the substrate, RQ is lower, around 0.7, since fats are more hydrogen-rich and require proportionally more oxygen to fully oxidise them relative to the carbon dioxide they release. Proteins give an intermediate RQ of around 0.8. RQ can exceed 1 when certain organic acids are the substrate, since these are already relatively oxygen-rich to begin with and consequently need comparatively little additional oxygen.

Quick revision points

• Glycolysis (cytoplasm, no oxygen needed): glucose to 2 pyruvate, net 2 ATP and 2 NADH; runs identically whether or not oxygen is present.
• Fermentation (anaerobic, no oxygen): regenerates NAD+ so glycolysis can continue, but yields no additional ATP beyond glycolysis's own 2. Alcoholic fermentation (yeast): pyruvate to ethanol + CO2. Lactic acid fermentation (some bacteria; oxygen-starved animal muscle): pyruvate to lactic acid.
• Aerobic respiration: pyruvate to acetyl-CoA, then the Krebs cycle (mitochondrial matrix, generates NADH/FADH2 and CO2), then the electron transport chain (inner mitochondrial membrane, oxidative phosphorylation) generates the bulk of ATP. Total textbook yield: roughly 36 to 38 ATP per glucose, far more than fermentation's 2.
• Respiratory quotient (RQ) = CO2 released / O2 consumed. Carbohydrates: RQ = 1.0. Fats: RQ ≈ 0.7 (lower, since fats need more oxygen relative to CO2 released). Proteins: RQ ≈ 0.8. Certain organic acids: RQ can exceed 1.