Turning light into food

Past the equation you already know, and why light is almost always the thing in short supply.

Light + CO₂ + Water → Sugar + Oxygen

Why leaves are green

Chlorophyll absorbs blue and red light most strongly and green light least, which is why leaves look green: we're seeing the wavelengths the leaf used least. But "least" is not "not at all." Measured across living leaves rather than pigment in a test tube, a leaf still absorbs roughly three-quarters of the green light that reaches it, and green penetrates deeper into the leaf than red or blue does, driving photosynthesis in the lower cell layers (McCree, 1971; Terashima et al., 2009). On living leaves the efficiency peaks sit around 620 nanometers (nm) in the orange-red and 440 nm in the blue (McCree, 1971).

Two stages, briefly

The light reactions run on the thylakoids , stacked membranes inside the chloroplast. They capture light energy into two energy-carrying molecules, ATP and NADPH, and split water, releasing oxygen as a byproduct. Most of the oxygen you're breathing was put there by this exact process. The Calvin cycle runs in the stroma, the fluid those stacks sit in, and uses that energy to lock CO₂ into sugar, which is what botanists mean by fixing carbon (Taiz, Zeiger, Møller, & Murphy, 2015). Output: glucose, which is energy, building material, and storage all at once.

Labeled chloroplast: stacked thylakoids (grana), the surrounding stroma, and the outer membrane.

Thylakoids Stroma Outer membrane

Fig. 9. Inside a chloroplast: light reactions on the stacked thylakoids, the Calvin cycle in the surrounding stroma.

Light is the limit

Photosynthesis runs at the rate of whatever's in shortest supply. Indoors, that's almost always light. You can dial in water, substrate, and humidity perfectly, but without enough light the plant can't make enough sugar to cover its own running costs, so it spends down its reserves and declines by degrees. That's why light is the first thing to assess for any houseplant, and why Module II is Reading Light.

From memory Tier 1

Strengthen your understanding

Try these in your head first, then open to see.

You've dialed in perfect water, substrate, and humidity, but a plant in a dim corner keeps slowly declining. What's the limiting factor?

Light. Photosynthesis runs at the rate of whatever's scarcest, and indoors that's almost always light. The plant can't make enough sugar to cover its own running costs, so it draws down reserves and slowly declines no matter how good everything else is.

Why are leaves green, in one sentence?

Chlorophyll absorbs blue and red most strongly and green least, so green is what's left over to reach your eye. Careful, though: "least" isn't "not at all." A real leaf still absorbs about three-quarters of the green light hitting it.

Module hub