How does photosynthesis work? Light into sugar, step by step

Photosynthesis is how plants, algae and some bacteria use the energy of light to turn carbon dioxide and water into sugar, releasing oxygen. The work is done in two linked stages: the light reactions capture sunlight and split water, storing the energy in two carrier molecules, ATP and NADPH; the Calvin cycle then spends those carriers to pull carbon dioxide out of the air and build it into sugar. Almost all food comes from it, and so does almost all the oxygen in the air.

In short

• Light is used to split water; the oxygen plants release comes from that water, not from carbon dioxide.
• The energy caught from light is passed to ATP and NADPH, which the Calvin cycle spends to turn carbon dioxide into sugar.
• Even in theory, a plant can store under 5% of the sunlight that falls on it as biomass, and real crops store far less.

What goes in and what comes out?

The overall reaction is usually written as 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂: six molecules of carbon dioxide and six of water, with light, give one molecule of glucose and six of oxygen. That version balances, but it hides where the atoms go. The more honest version, which biochemists use, puts water on both sides:

6CO₂ + 12H₂O + light → C₆H₁₂O₆ + 6O₂ + 6H₂O

Following each atom

1. The oxygen gas. The 12 water molecules on the left are split. Their 12 oxygen atoms pair up into the 6 O₂ molecules released. None of the oxygen a plant releases comes from carbon dioxide. Samuel Ruben, Martin Kamen and colleagues showed this in 1941 by giving green algae water labeled with oxygen-18, a heavy form of oxygen, and finding the label in the gas released.
2. The carbon. All 6 carbon atoms in glucose come from the 6 CO₂ molecules, one at a time.
3. The oxygen from CO₂. CO₂ brings 12 oxygen atoms. Six end up in glucose and six in the 6 new water molecules on the right.
4. The hydrogen. The 24 hydrogen atoms from the split water end up 12 in glucose and 12 in the new water.

Everything balances: 6 carbon, 24 oxygen and 24 hydrogen atoms on each side. What the reaction adds is energy. Glucose holds far more chemical energy than the CO₂ and water it is made from, and all of that difference came from light. Photosynthesis is a chemical reaction that runs uphill, and it is the main way energy from the Sun enters living things.

Where does photosynthesis happen?

In plants and algae it happens in chloroplasts, organelles packed into leaf cells. A single leaf cell can hold dozens of them. Inside each chloroplast are stacks of flattened sacs called thylakoids, where the light reactions happen, surrounded by a fluid called the stroma, where the Calvin cycle runs. Like mitochondria, chloroplasts descend from bacteria that were swallowed by another cell, in their case cyanobacteria, and they still carry their own DNA.

The thylakoid membranes hold chlorophyll, the pigment that catches light. Chlorophyll absorbs red and blue light strongly and reflects much of the green, which is why leaves look green. The pigments are organized into two light-catching complexes, photosystem II and photosystem I, named in the order they were discovered rather than the order they work.

Stage one: how do the light reactions work?

The light reactions turn light into chemical energy, in a chain of handoffs.

1. Light excites an electron. A photon absorbed by photosystem II lifts an electron in chlorophyll to a high-energy state, and the electron is passed away to a carrier.
2. Water is split to replace it. Chlorophyll that has lost an electron is a powerful electron-grabber. An enzyme cluster pulls electrons from water to refill it: 2H₂O → O₂ + 4H⁺ + 4 electrons. The oxygen leaves the leaf as a by-product.
3. The electron's energy pumps protons. The electron travels down a transport chain in the thylakoid membrane, and its energy is used to pump protons, hydrogen ions, into the thylakoid. Protons build up inside.
4. A second boost. At photosystem I, a second photon re-energizes the electron, which is finally handed to NADP⁺ to make NADPH, an electron carrier.
5. ATP is made. Protons flow back out through ATP synthase, the same kind of rotating enzyme mitochondria use, and it builds ATP.

So the light reactions output ATP and NADPH. OpenStax's biology textbook notes that these carriers last only millionths of a second to moments before being used; the plant cannot store energy in them, so it spends them straight away in the second stage.

Stage two: how does the Calvin cycle build sugar?

The Calvin cycle, worked out at the University of California, Berkeley, in the late 1940s and 1950s by Melvin Calvin, Andrew Benson and James Bassham, runs in the stroma and does not need light directly. It has three steps, repeated in a loop.

1. Fixation. The enzyme RuBisCO attaches one CO₂ to a five-carbon sugar called RuBP. The unstable six-carbon result splits at once into two three-carbon molecules.
2. Reduction. ATP and NADPH from the light reactions are spent to turn those molecules into G3P, a three-carbon sugar.
3. Regeneration. Most of the G3P is rebuilt into RuBP, using more ATP, so the cycle can take in the next CO₂.

Because each turn fixes only one carbon atom, the cycle must turn three times to export one three-carbon G3P, and six times for the six carbons of one glucose. The cost per glucose is 18 ATP and 12 NADPH. The electron count fits the light reactions exactly: each NADPH holds 2 electrons, so 12 NADPH need 24 electrons, which takes splitting 12 water molecules, which releases 6 O₂. The ATP is less tidy: the main electron route makes slightly too little, and a side loop around photosystem I, called cyclic electron flow, makes up the difference. The glucose is then turned into sucrose to move around the plant, starch to store, or cellulose to build cell walls.

RuBisCO is probably the most abundant protein on Earth, and it is slow and imperfect. It sometimes grabs O₂ instead of CO₂, starting a wasteful side process called photorespiration that undoes part of the work. In hot, dry conditions, when leaves close their pores to save water and CO₂ inside runs low, the waste gets worse. Two groups of plants evolved workarounds. C4 plants, such as maize and sugarcane, first capture CO₂ in a four-carbon molecule in outer cells and pump it to RuBisCO in inner cells at high concentration. CAM plants, such as cacti and pineapple, open their pores only at night, store CO₂ as an acid, and run the Calvin cycle by day with the pores shut.

How efficient is photosynthesis?

Not very, by the standard of a solar panel. Only a little under half of sunlight's energy is in the wavelengths chlorophyll can use, some is reflected, and each step loses some as heat. A widely cited analysis by Xin-Guang Zhu, Stephen Long and Donald Ort, published in 2008, worked through every step and put the maximum conversion of solar energy into biomass at 4.6% for ordinary C3 plants at 30 °C and the carbon dioxide levels of the time, and 6% for C4 plants. Real fields do worse: plants spend energy on respiration, and leaves cannot use the full strength of midday sun. Estimates of crop efficiency put typical crops at about 1% to 2% of sunlight turned into biomass, and the share stored in the harvested part, such as corn kernels or potato starch, at about 0.25% to 0.5%.

Across the whole planet, the total is still enormous. Global photosynthesis captures energy at an average rate of about 130 terawatts, around eight times the power human civilization uses, and turns around 100 to 115 billion tonnes of carbon a year into living matter, according to figures summarized in Wikipedia's article.

Why does photosynthesis matter beyond plants?

Photosynthesis is where almost every food chain starts. Plants and algae are the producers in an ecosystem: a cow eating grass, and a person eating the cow, are both living on sugar first built by photosynthesis. The oxygen in the air is its by-product too: until oxygen released by cyanobacteria began to build up about 2.4 billion years ago, Earth's atmosphere had almost no free oxygen. Most life on Earth's surface, in other words, runs on sunlight caught this way.

It is also half of the carbon cycle. Photosynthesis pulls CO₂ out of the air each growing season, and respiration and decay put it back; the yearly rise and fall in measured CO₂, as northern forests leaf out and drop their leaves, is that breathing. Burning fossil fuels returns carbon that photosynthesis locked away millions of years ago, faster than plants and oceans take it up, which is the core of climate change. Planting trees helps, but slowly, and a forest only holds its carbon while it stands.

A common misconception is that a plant's mass comes from the soil. Almost all of a tree's dry weight is carbon, oxygen and hydrogen taken from air and water; the soil supplies water and small amounts of minerals such as nitrogen and phosphorus. A log, in a real sense, is built mostly out of thin air.

Check yourself

• Without looking back, explain where the oxygen released by a plant comes from, and how that was shown.
• What do the light reactions hand to the Calvin cycle, and why can't the plant store them?
• Why does a tree's mass come mostly from the air rather than the soil?

Sources

• 8.1 Overview of Photosynthesis — OpenStax, Biology 2e, 2018
• 8.3 Using Light Energy to Make Organic Molecules — OpenStax, Biology 2e, 2018
• What is the maximum efficiency with which photosynthesis can convert solar energy into biomass? — Zhu, Long and Ort, Current Opinion in Biotechnology, 2008
• Photosynthesis — Wikipedia
• Photosynthetic efficiency — Wikipedia