Photosynthesis

Almost all the energy that flows through living things begins as sunlight captured by photosynthesis. Plants, algae and cyanobacteria use light energy to turn carbon dioxide and water into sugar, releasing oxygen as a by-product. That sugar feeds the photosynthesizers themselves and, directly or indirectly, nearly every other organism, including you. The oxygen in every breath you take, and the fossil fuels that power much of the economy, are products of photosynthesis. This lesson explains how chloroplasts capture light, how they use it to make ATP and NADPH, how the Calvin cycle builds sugar, and how some plants adapt to heat and drought.

The overall reaction

Photosynthesis is, in broad terms, respiration run backwards, driven by light:

Where does the oxygen come from: the carbon dioxide or the water? Experiments in the 1940s using water labeled with the heavy isotope oxygen-18 settled it: the labeled oxygen appeared in the oxygen gas released, so the O2 comes from splitting water. The carbon dioxide's oxygen ends up in the sugar and in new water molecules. Photosynthesis is a redox process: water is oxidized (it loses electrons) and carbon dioxide is reduced (it gains electrons, with hydrogen) to sugar. Because reducing CO2 requires a lot of energy, the reaction is strongly endergonic, the reverse of respiration's ΔG of about −2,870 kJ per mole of glucose. Light supplies that energy.

The chloroplast

In plants, photosynthesis takes place in chloroplasts, concentrated in the mesophyll cells inside leaves; a single leaf cell may contain 30 to 40 of them. Carbon dioxide enters the leaf through pores called stomata, and water arrives from the roots through the xylem. Each chloroplast has a double envelope membrane surrounding a fluid called the stroma. Suspended in the stroma is a third membrane system, the thylakoids: flattened sacs often stacked into grana. The thylakoid membranes hold the light-capturing pigments and the electron transport chain; the stroma holds the enzymes of the Calvin cycle.

Photosynthesis has two linked stages. The light-dependent reactions (light reactions) in the thylakoid membranes capture light energy, split water, release oxygen and produce ATP and NADPH. The Calvin cycle in the stroma uses that ATP and NADPH to fix carbon dioxide into sugar. The two stages are coupled: the Calvin cycle returns ADP, phosphate and NADP+ for the light reactions to recharge.

Light and pigments

Light travels as particles of energy called photons, and the energy of each photon depends on its wavelength: shorter wavelengths carry more energy. Visible light runs from violet (about 400 nm) to red (about 700 nm). The energy of a photon is \(E = hc/\lambda\), where \(h\) is Planck's constant and \(c\) the speed of light.

# Energy of photons of different colours: E = h c / wavelength
h, c, NA = 6.626e-34, 2.998e8, 6.022e23
for band, nm in [("blue", 450), ("green", 550), ("red", 680), ("far red", 700)]:
    E = h * c / (nm * 1e-9)                     # joules per photon
    print(f"{band:<8} {nm} nm: {E:.2e} J per photon, {E * NA / 1000:5.0f} kJ per mole of photons")

blue     450 nm: 4.41e-19 J per photon,   266 kJ per mole of photons
green    550 nm: 3.61e-19 J per photon,   218 kJ per mole of photons
red      680 nm: 2.92e-19 J per photon,   176 kJ per mole of photons
far red  700 nm: 2.84e-19 J per photon,   171 kJ per mole of photons

Pigments are molecules that absorb particular wavelengths. The main photosynthetic pigment is chlorophyll a, with a ring structure containing a magnesium atom at its center. Chlorophyll b and carotenoids (yellow and orange pigments) are accessory pigments that absorb other wavelengths and pass the energy on, broadening the range of light a plant can use; carotenoids also protect against damage from excess light. Chlorophylls absorb blue and red light strongly and reflect green, which is why leaves look green. In autumn, as chlorophyll breaks down, the yellow and orange carotenoids that were there all along become visible, and some trees make new red pigments called anthocyanins.

An absorption spectrum shows which wavelengths a pigment absorbs; an action spectrum shows which wavelengths drive photosynthesis. In 1883 Theodor Engelmann produced an early action spectrum with a beautiful experiment: he shone a spectrum of light across a strand of alga and added bacteria that gather where oxygen is plentiful. The bacteria clustered in the blue and red regions, exactly where chlorophyll absorbs.

The light reactions

Chlorophyll and accessory pigments are organized with proteins into large complexes called photosystems in the thylakoid membrane. Each photosystem has an antenna of hundreds of pigment molecules that collect light energy and funnel it to a reaction center containing a special pair of chlorophyll a molecules. When the special pair receives the energy, one of its electrons is boosted to a high energy level and passed to an electron acceptor. That transfer converts light energy into chemical energy.

Plants have two photosystems, named in the order they were discovered: photosystem II (whose reaction center absorbs best at 680 nm, P680) and photosystem I (P700). They work in series in what is called linear electron flow:

This closely resembles the mitochondrion (lesson 9). Both use an electron transport chain to pump protons across a membrane and an ATP synthase to make ATP as protons flow back. The differences are the source and destination of the electrons: in respiration, electrons flow from food to oxygen, releasing energy; in photosynthesis, light energy pushes electrons from water uphill to NADP+. The proton gradient in chloroplasts is huge: the thylakoid space reaches about pH 5 while the stroma is near pH 8, a thousandfold difference in hydrogen ion concentration.

The Calvin cycle

The ATP and NADPH from the light reactions power the Calvin cycle, worked out by Melvin Calvin, Andrew Benson and James Bassham in the 1950s by feeding algae radioactive carbon-14 dioxide and tracking where the label went (Calvin received the 1961 Nobel Prize in Chemistry). The cycle has three phases:

For every three turns, fixing three CO2, the cycle produces one G3P as net output and uses 9 ATP and 6 NADPH. Two G3P are needed to make one glucose, so building one glucose takes 6 turns, 18 ATP and 12 NADPH. G3P is used to make glucose, sucrose (the sugar plants transport), starch (for storage), cellulose (for walls), and the carbon skeletons of amino acids and fats.

# Calvin cycle bookkeeping
def calvin(co2_fixed):
    turns = co2_fixed                 # one CO2 per turn
    return {"G3P made (net)": co2_fixed // 3,
            "ATP used": 3 * turns,
            "NADPH used": 2 * turns}

for n in [3, 6, 12]:
    print(f"{n:>2} CO2 fixed:", calvin(n))

# Light reactions: about 8 photons per O2 released (4 electrons, each excited twice)
glucose = 1
o2 = 6 * glucose
print("Photons needed per glucose (at least):", 8 * o2)

 3 CO2 fixed: {'G3P made (net)': 1, 'ATP used': 9, 'NADPH used': 6}
 6 CO2 fixed: {'G3P made (net)': 2, 'ATP used': 18, 'NADPH used': 12}
12 CO2 fixed: {'G3P made (net)': 4, 'ATP used': 36, 'NADPH used': 24}
Photons needed per glucose (at least): 48

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) is probably the most abundant protein on Earth, making up as much as half of the soluble protein in leaves. It needs to be abundant because it is remarkably slow, fixing only a few CO2 molecules per second, and because it makes mistakes: it sometimes grabs O2 instead of CO2. That side reaction, called photorespiration, wastes energy and releases previously fixed carbon. It becomes worse in hot, dry conditions, when plants close their stomata to save water and oxygen builds up inside the leaf relative to carbon dioxide.

C4 and CAM plants: adapting to heat and drought

Some plants have evolved ways to reduce photorespiration. C4 plants, such as corn (maize), sugar cane and many tropical grasses, first fix CO2 in outer mesophyll cells using a different enzyme, PEP carboxylase, which does not react with oxygen. The four-carbon product is pumped into inner bundle-sheath cells, where CO2 is released at high concentration around rubisco. The pump costs extra ATP but pays off in hot, bright conditions. CAM plants, such as cacti, pineapples and jade plants, separate the steps in time instead of space: they open their stomata only at night, fixing CO2 into organic acids, and close them during the day, releasing the CO2 for the Calvin cycle while saving water.

Corn, the largest crop in the United States by area and value, is a C4 plant suited to the hot summers of the Midwest, while wheat, soybeans and most forest trees are C3 plants. Researchers worldwide are trying to engineer C4 features or faster rubisco into C3 crops such as rice to raise yields.

Photosynthesis and the planet

Photosynthesis transformed Earth. Cyanobacteria began releasing oxygen more than 2.5 billion years ago, and by about 2.4 billion years ago oxygen started to accumulate in the atmosphere (the Great Oxidation Event), eventually making aerobic respiration, and complex life, possible. Today, photosynthesis by land plants and ocean phytoplankton together fixes on the order of 100 billion metric tons of carbon each year, roughly half on land and half in the oceans. The balance between photosynthesis, which removes CO2 from the air, and respiration and combustion, which return it, governs atmospheric carbon dioxide and therefore climate. Burning fossil fuels, the stored products of ancient photosynthesis, releases carbon far faster than photosynthesis currently removes it.