Earth's energy flows: sun, wind, water, heat and life: Photosynthesis and the productivity of life

4.1 Photosynthesis stores sunlight in sugar, but rarely more than a few percent of it

The overall reaction is easy to write: six molecules of CO₂ and six of water, with the help of light, become one molecule of glucose and six of oxygen. Each gram of sugar formed stores about 16 kJ of chemical energy, which is the energy later released when the sugar is burned in a fire or in a cell.

The reaction runs in two stages. In the first, chlorophyll absorbs light and uses it to split water, releasing oxygen and creating energy-carrying molecules. In the second, those molecules drive the fixing of CO₂ into sugar by the enzyme rubisco, probably the most abundant protein on Earth.

Plants can use only light between about 400 and 700 nanometres, called photosynthetically active radiation. That band carries about 45% of the energy in sunlight. From there the losses pile up: some light is reflected or passes through the leaf; every absorbed photon, whatever its colour, delivers only the energy of a red photon to the reaction; several photons are needed for each CO₂ fixed; rubisco sometimes grabs oxygen instead of CO₂ and wastes energy; and the plant burns part of its sugar to stay alive.

The result is that the best crops, well watered and fertilised, convert at most about 4% of incoming sunlight into stored chemical energy, and only for short periods at the peak of growth. Over a whole growing season a superb maize field might store 2 to 3%. Averaged over the year and over all the world's vegetation, including deserts, cold seasons and dry spells, the figure is a few tenths of one percent.

That single fact underlies much of Smil's scepticism about large-scale liquid biofuels. When the conversion efficiency is below 1%, the power density of the harvest is below about 1 W/m2, and any fuel made from it needs vast areas.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M2 'Photosynthesis'
• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'Photosynthesis'
• Smil, Biomass Energies: Resources, Links, Constraints (1983) · via Smil Mastery Course, M2 'Photosynthesis'

4.2 C3, C4 and CAM plants run the same reaction three ways, and respiration takes a large cut

Most plants, including wheat, rice, soybeans, potatoes and nearly all trees, use the C3 pathway, named after the three-carbon molecule that is the first product of fixation. Its weakness is rubisco's habit of reacting with oxygen, which gets worse as leaves get hotter.

C4 plants evolved a fix. They first capture CO₂ in outer cells using a different enzyme and then pump it, concentrated, into inner cells where rubisco works. The pumping costs energy, but in strong light and heat it more than pays off. Maize, sugarcane, sorghum and many millets are C4 plants, which is why they are among the most productive crops in hot climates such as India's. Sugarcane is the outstanding energy crop: well-managed cane can produce 80 tonnes or more of fresh stalks per hectare a year, and its fibrous residue, bagasse, fuels the mills that crush it, an arrangement shown on the Bagasse and black-liquor cogeneration tile.

CAM plants, such as cacti, agave and pineapple, take a third route suited to deserts. They open their pores only at night, when it is cool and less water escapes, store the CO₂ as an acid, and fix it into sugar by day with their pores closed. They save water at the cost of slow growth.

Whatever the pathway, plants must spend energy to stay alive. The total fixed by photosynthesis is gross primary production. Plants then respire roughly half of it to maintain and build their tissues. What remains, net primary production or NPP, is the new plant matter available for growth, for storage in wood and roots, and as food for animals, fungi and microbes. NPP is the true energy income of the biosphere, and it is the figure against which every human harvest should be measured.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M2 'Photosynthesis'
• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'Photosynthesis'
• Smil, The Earth's Biosphere: Evolution, Dynamics, and Change (2002) · via Smil Mastery Course, M2 'Photosynthesis'
• Smil, Harvesting the Biosphere: What We Have Taken from Nature (2013) · via Smil Mastery Course, M2 'Photosynthesis'

4.3 How much life there is, and how fast it is made

The biosphere's living mass is dominated by plants, and plant mass is dominated by wood. A census published in 2018 put the total mass of life at about 550 billion tonnes of carbon, of which about 450 billion tonnes are in plants. Smil's own survey in Harvesting the Biosphere gives a range of roughly 450 to 650 billion tonnes of carbon for land plants alone, and he stresses how uncertain all such figures are.

Annual net primary production is roughly 55 to 60 billion tonnes of carbon on land and about 50 billion tonnes in the oceans. The two are similar even though ocean phytoplankton weigh only a tiny fraction of land plants, because microscopic algae divide and are eaten within days, while a tree holds its carbon for decades or centuries. Converted to energy at roughly 40 kJ per gram of carbon, global NPP is on the order of 100 to 150 TW (calculated), about five to eight times humanity's energy use.

Productivity varies hugely by place. Tropical rain forests and well-managed, irrigated crops fix more than a kilogram of dry matter per square metre per year. Temperate forests and grasslands manage less. Tundra, desert scrub and the nutrient-poor open ocean average only about 0.1 kg, and extreme deserts almost nothing. Water, temperature and nutrients, especially nitrogen and phosphorus, decide where a site sits in that range.

Smil insists on clear units: fresh weight includes water, dry matter excludes it, and carbon is about 45 to 50% of dry matter. Mixing them up can create errors of two to five times. His wider point in Harvesting the Biosphere is that humans now take a large share of land NPP for food, feed, fibre and fuel, so any plan to grow far more biomass for energy competes directly with food and with wild ecosystems.

• Smil, Harvesting the Biosphere: What We Have Taken from Nature (2013) · via Smil Mastery Course, M2 'Productivity and the mass of life'
• Smil, The Earth's Biosphere: Evolution, Dynamics, and Change (2002) · via Smil Mastery Course, M2 'Productivity and the mass of life'
• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'Productivity and the mass of life'
• Bar-On, Phillips and Milo, 'The biomass distribution on Earth', PNAS 115 (2018) · total biomass about 550 Gt C, plants about 450 Gt C, humans about 0.06 Gt C; summary by the Weizmann Institute, https://www.weizmann.ac.il/WeizmannCompass/node/1200
• Whittaker and Likens, 'The biosphere and man', in Lieth and Whittaker (eds), Primary Productivity of the Biosphere, Springer (1975) · table of net primary production by ecosystem type, g dry matter per m2 per year (ranges and means, e.g. tropical rain forest 1,000-3,500, mean 2,200; temperate deciduous forest mean 1,200; cultivated land mean 650; open ocean mean 125; extreme desert mean 3); means also reproduced at https://www.batrachos.com/en/produktyvnist/

See it in the table

Check your understanding

Which of these crops uses the C4 pathway?

Why does ocean phytoplankton produce about as much each year as all land plants despite having a tiny fraction of their mass?

A crop stores 1% of the 200 W/m2 average sunlight it receives. What is its power density?