Turning light into food

Past the equation you already know, and why, indoors, light is so often 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. But "least" is not "not at all." In one study on lettuce leaves, the green LED light was calculated to be about 81 percent absorbed, against roughly 92 to 93 percent for the red and blue LEDs (Liu & van Iersel, 2021). Because green is absorbed less readily, it tends to travel deeper into the leaf and reach cells farther from the upper surface, which in bright light can raise photosynthesis (Liu & van Iersel, 2021). In McCree's measurements, made with narrow bands of light, red light gave the most photosynthesis per photon, and green generally did slightly better than blue (Liu & van Iersel, 2021, summarising 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. It's a source of the oxygen that we and many other living things depend on. The Calvin cycle runs in the stroma, the liquid around those stacks, and spends that ATP and NADPH turning CO₂ into organic molecules, which is what botanists mean by fixing carbon (Clark et al., 2018). Output: a three-carbon compound called G3P, along with simple carbohydrates that can later become glucose, sucrose and many other sugars; two G3Ps make one glucose (Clark et al., 2018).

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.

Indoors, light is often the limit

Indoors light is often what runs short: extension guidance names low light, along with overwatering, as the most common cause of stress in indoor plants and ranks adequate light as by far the most important growth factor (Pennisi, 2022). In one growth-chamber trial held at office-like warmth, humidity and CO₂, two of three foliage plants grew faster when their LED light was made brighter and left on longer each day (Sugano et al., 2024). 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 we check the light in a spot before bringing a plant home, then choose one whose light needs suit that spot (University of Minnesota Extension, n.d.). 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. Extension guidance ranks adequate light as by far the most important growth factor for indoor plants, and a shortage of any one of light, water or CO₂ can limit photosynthesis however plentiful the others are. In a dim corner we'd expect the plant to make too little food to keep growing, however good the rest of its care is.

Why are leaves green, in one sentence?

Leaves absorb green light less readily than blue and red, so more of the green is left over. But less isn't none: in one study, lettuce leaves still absorbed about 81 percent of a green LED's light, against roughly 92 to 93 percent for red and blue.

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