Typically, only 1% to 2% of the sun’s energy that lands on a leaf ends up in carbohydrates. Does this mean that photosynthesis is incredibly wasteful? Or is this process, the product of billions of years of evolution, surprisingly efficient? This is not an idle question. Photosynthesis is relevant to solving several pressing global issues: the effects of rising CO₂ concentrations on Earth’s climate, the search for a renewable, carbon-neutral fuel to power our transportation sector, and the agricultural demands of our skyrocketing human population.

Photosynthetic efficiency is typically calculated relative to the total energy output of the sun (Fig. 8.18). However, only visible light has the appropriate energy levels to raise the energy state of electrons in chlorophyll. Most of the sun’s output (∼60%) is not absorbed by chlorophyll and thus cannot be used in photosynthesis. In addition, leaves are not perfect at absorbing visible light—about 8% is either reflected or passes through the leaf. Finally, even under optimal conditions, not all of the light energy absorbed by chlorophyll can be transferred to the reaction center and instead is given off as heat (also ∼8%). As we have seen, when light levels are high, excess light is actively converted into heat by xanthophyll pigments.

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The photosynthetic electron transport chain therefore captures at most about 24% of the sun’s usable energy arriving at the surface of a leaf (100% – 60% – 8% – 8% = 24%). While this number may appear low, it is on a par with the efficiency of high-performance photovoltaic cells in solar panels, which convert sunlight into electricity. This comparison is even more impressive when you consider that photosynthetic organisms must build and maintain all their biochemical machinery. However, energy is lost at a later step as well. The incorporation of CO₂ into carbohydrates results in considerable loss in free energy, equivalent to ∼20% of the total incoming solar radiation. Some of this loss in free energy is due to photorespiration.

In total, therefore, the maximum energy conversion efficiency of photosynthesis is calculated to be around 4% (24% – 20%). Efficiencies achieved by real plants growing in nature, however, are typically much lower, on the order of 1% to 2%. In Chapter 29, we explore the many factors that can constrain the photosynthetic output of land plants, and see how some plants have evolved ways to minimize losses in productivity due to drought and photorespiration.