Turning light into matter

The balance of photosynthesis

Nearly all the energy circulating through living things comes from a single reaction: photosynthesis. Plants use it to turn light into organic matter, releasing along the way the oxygen we breathe.

The overall balance

6 CO₂  +  6 H₂O  + light energy  ->  C₆H₁₂O₆  +  6 O₂
(carbon    (water)                   (glucose)   (dioxygen)
 dioxide)

Two gases and water go in, sugar and oxygen come out. A living organism builds organic matter from mineral matter: this is what being autotrophic means.

AUTOTROPH                          HETEROTROPH
--------------------------------   --------------------------------
builds its organic matter          must take it from other
from CO₂ and water                 living organisms
plants, algae, cyanobacteria       animals, fungi, most bacteria

It is the most structuring distinction in all of biology: it separates the producers from everyone else.

Where it happens

In the chloroplasts, organelles found in leaf cells. Their internal layout is built to capture light:

      ____________________________
     /                            \
    |   ▬▬▬▬▬   ▬▬▬▬▬   ▬▬▬▬▬     |   <- thylakoids (stacked into grana)
    |   ▬▬▬▬▬   ▬▬▬▬▬   ▬▬▬▬▬     |      membranes rich in CHLOROPHYLL
    |                              |
    |        stroma (fluid)        |   <- where the sugar is made
     \____________________________/

Chlorophyll is the pigment that absorbs light. It absorbs strongly in the blue and the red, and reflects green — that, and nothing else, is why leaves look green to us. The colour of plants is literally the part of the spectrum they do not use.

Two linked phases

LIGHT-DEPENDENT PHASE               CARBON-FIXING PHASE
(in the thylakoids)                 (in the stroma)
--------------------------------    --------------------------------
needs LIGHT                         does not need light
splits water (photolysis)           fixes CO₂
releases O₂                         builds glucose
produces ATP and NADPH      ---->   consumes ATP and NADPH

The first phase converts light energy into transportable chemical energy (ATP and NADPH). The second uses that energy to assemble sugar. Light therefore never touches CO₂ directly: it is converted first.

The oxygen comes from water, not CO₂

This is the most widespread misconception. Reading the equation, one seems to see CO₂ giving up its oxygen. That is wrong: the released O₂ comes from the photolysis of water, the splitting of H₂O molecules by light energy.

2 H₂O  --[light]-->  4 H⁺  +  4 e⁻  +  O₂

This was proved in 1941 by Ruben and Kamen, who supplied plants with water labelled with a heavy oxygen isotope. The labelled oxygen turned up in the released gas — and not in the sugar.

Summary