Plants Make Glucose from CO₂

Glucose is among the most common sugars in the biosphere because plants make most of the sugars in the biosphere and glucose is the most abundant sugar in plant matter (Buchanan et al., 2015). Plants make glucose by reducing atmospheric CO₂. In other words, plants close the cycle of glucose oxidation performed by heterotrophs (like humans and octopi and yeast and E. coli). If we examine the reduction of CO₂ to glucose using eQuilibrator

6 CO₂ + 6 H₂O ⇌ Glucose + 6 O₂

It’s clear that the reaction is extremely unfavorable, with a positive Δ_rG’ of roughly 2900 kJ / mol. In fact, it has to be unfavorable because it is the reverse of glucose oxidation (see here) and we know that the complete oxidation of glucose is highly thermodynamically favorable. In plants, this reaction is coupled to light energy that is absorbed by chlorophyll (Nelson et al., 2008; Buchanan et al., 2015). Chlorophyll can absorb photons best when they have a wavelength of about 700 nm, which corresponds to a per-photon energy of 1

\[E = \frac{h c}{λ} \approx 180 \frac{kJ}{mol\text{ photons}}\]

As a result, we expect that plants need at least 16 moles of photons to make 1 mole of glucose from CO₂. In fact, estimates indicate that the carbon fixation pathways of contemporary plants are much less efficient than our lower-bound, using about 10 photons energy per CO₂ or 60 photons per glucose. As we saw above, biological processes often leave a bunch of energy “on the table.” It is often suggested that this excess energy or “overpotential” is used to drive reactions in the forward direction more quickly (Milo, 2009).

1

Try to verify this calculation yourself, remembering that “h” is Planck’s constant, c is the speed of light in a vacuum and λ is the photonic wavelength.