Photosynthesis: Overview, Chloroplasts, Light and Pigments

Every breath you take…

Roughly half of the oxygen produced on Earth each year is released by tiny ocean phytoplankton; most of the rest comes from land plants, like the maples outside your school. And nearly every calorie you eat began as sunlight captured in a leaf. Photosynthesis is the reverse of respiration in its overall chemistry, but its machinery is different. In this and the next two lessons you'll follow the energy from a photon of sunlight all the way into the bonds of a sugar molecule.

The overall equation

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂ ΔG = +2870 kJ/mol

In redox terms, it's the reverse of respiration: CO₂ is reduced to sugar, and water is oxidized to O₂. Because it is strongly endergonic, it needs an energy input: light.

Where does the O₂ come from: CO₂ or water? In 1941, Samuel Ruben and Martin Kamen gave algae water containing the heavy isotope ¹⁸O. The O₂ released contained ¹⁸O. When they labelled the CO₂ instead, the O₂ did not. So all the O₂ comes from water. To show this, the equation is sometimes written:

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

(12 water molecules supply the 12 O atoms of 6O₂; new water is made from the oxygen of CO₂.)

A little history: in the 1640s Jan Baptist van Helmont grew a willow tree in a pot and found the tree's mass came mostly from something other than soil (he suggested water; we now know much of it is carbon from the air). In 1771 Joseph Priestley showed that a sprig of mint could "restore" air in which a candle had burned out, and in 1779 Jan Ingenhousz showed that plants do this only in light.

The chloroplast

In a leaf, most chloroplasts are in the mesophyll cells; a single mesophyll cell may contain dozens of them. Like a mitochondrion, a chloroplast has a double membrane and compartments with different jobs:

• Outer and inner membranes enclose the organelle.
• Thylakoids are flattened membrane sacs, often stacked into grana. The thylakoid membrane holds the pigments, photosystems, electron transport chain and ATP synthase: it is the site of the light-dependent reactions.
• The thylakoid lumen is the space inside each thylakoid, where H⁺ accumulates (like the mitochondrial intermembrane space).
• The stroma is the fluid around the thylakoids. It holds the enzymes of the Calvin cycle, plus chloroplast DNA and ribosomes.

Two stages

1. Light-dependent reactions (thylakoid membranes): light energy splits water (releasing O₂) and drives electron transport, producing ATP and NADPH. These are the energy carriers for stage 2.
2. Calvin cycle (stroma): ATP and NADPH are used to fix CO₂ into organic molecules and reduce them to the sugar G3P, from which glucose, sucrose and starch are made. ADP and NADP⁺ return to the thylakoids to be recharged.

Older texts call stage 2 the "dark reactions" because it doesn't use light directly. That name is misleading: the Calvin cycle normally runs in the light, because it depends on ATP and NADPH from stage 1 and several of its enzymes are switched on by light.

Light and pigments

Sunlight is electromagnetic radiation. Visible light spans wavelengths of about 400 nm (violet) to 700 nm (red). Light travels in packets of energy called photons; the shorter the wavelength, the more energy each photon has.

A pigment absorbs some wavelengths and reflects or transmits others. The colour we see is the light that is not absorbed.

• Chlorophyll a is the main photosynthetic pigment. It absorbs strongly in the blue-violet and red and reflects green, which is why leaves look green.
• Chlorophyll b is an accessory pigment with slightly different peaks, so it widens the range of light the plant can use and passes energy on to chlorophyll a.
• Carotenoids (yellow, orange) absorb blue and blue-green light, pass some energy to chlorophyll, and protect chlorophyll from damage by excess light.

Fall colours: in autumn, maple leaves stop making chlorophyll and it breaks down, revealing the yellow and orange carotenoids that were there all along. The brilliant reds of sugar maples come from anthocyanins, pigments that the leaves make in the fall.

Absorption vs action spectrum

An absorption spectrum shows which wavelengths a pigment absorbs. An action spectrum shows how well each wavelength drives photosynthesis (measured, for example, by O₂ released).

In 1882 the German scientist Theodor Engelmann shone light split by a prism onto a filament of green alga, in water containing bacteria that gather where O₂ is highest. The bacteria clustered around the parts of the alga lit by blue-violet and red light: an early action spectrum. The action spectrum closely matches the combined absorption of the chlorophylls and carotenoids, which is strong evidence that these pigments drive photosynthesis.

Photosystems. In the thylakoid membrane, pigments are organized into photosystems. Hundreds of pigment molecules in the antenna complex absorb photons and pass the energy along, until it reaches a special pair of chlorophyll a molecules in the reaction centre. There, the energy boosts an electron to a high-energy state, and the electron is passed to a primary electron acceptor. That's the moment light energy becomes chemical energy. There are two photosystems: photosystem II (P680) and photosystem I (P700), named for the wavelength (in nm) their reaction-centre chlorophyll absorbs best. You'll see them work together in the next lesson.