We've all heard the word, but how many of us really know what is going on inside our plants? What is chlorophyll and how does it convert light into energy?
Chlorophyll is a collection of green pigments that makes plant (and algae) life possible. Pigments are materials that change the appearance of reflected light by absorbing specific wavelengths. As light strikes a leaf, first the blue wavelength and then the red wavelength are absorbed, and the green is reflected. That’s why, to us, most plants look green.
Discovery of chlorophyll
The word chlorophyll (you can spell it with one “l” if you prefer), comes from two Greek words that mean green (chloros) and leaf (phyllon). Chlorophyll is the reason photosynthesis works. This was first figured out in 1817 by two men, Pierre Joseph Pelletier and Joseph Bienaimé Caventou, ages 29 and 22, respectively. (Certainly not what I was doing in my 20’s…)
Plant cell review
Before we get into the amazing way that chlorophyll helps plants collect energy from the sun, let’s have a quick review of plant cells: Plant cells are very similar to animal cells in that they both have a nucleus that contains DNA, a storage vacuole, a mitochondria to keep everything functioning, a cell membrane that filters what goes in and out, and a jelly-like cytoplasm that holds all the parts. Plant cells are different from animal cells because they have rigid, rectangular cell walls and organelles called chloroplasts. Chloroplasts contain a lot of chlorophyll. This is where the really amazing stuff happens.
Chloroplasts have their own DNA and they are something like living solar panels within plant cells. According to Wikipedia, “Chloroplasts cannot be made by the plant cell and must be inherited by each daughter cell during cell division.” How crazy is that? I wonder if that makes them the oldest living thing?
Photosynthesis and chlorophyll molecules
You can learn a lot more about photosynthesis and chlorophyll molecules elsewhere, but the basic idea is that light energy is absorbed by chlorophyll molecules that are held along and in structures called photostems, in a process called resonance energy transfer. (Stay with me! We’re almost through the tough part!) The absorbed energy is handed over to an electron in something called charge separation. The energy is then oxidized (or rusted) off the electron and handed over to other molecules in an electron transport chain. The original electron is then grounded with the aid of a water molecule. It’s really mind-boggling, isn’t it? So what does this have to do with gardening?
Chlorophyll in the garden
If you understand how plants get their energy, you can help them stay healthy. For example, if your plants are dusty, the process of photosynthesis doesn’t work as well as it might because the chlorophyll can’t absorb light energy. (And spider mites will be more of a problem.) Or, if you are growing endive, celery, or fennel, you may want to block light from reaching the chlorophyll, for white, tender leaves, stalks, or bulbs. If chlorosis (or yellowing) is seen, you will know that something is causing a lack of chlorophyll. Chlorosis can indicate bacterial or fungal disease, physical injury, improper soil pH, iron or manganese deficiency (the former is very common in the Bay Area, the latter is not). Excessive levels of other nutrients, such as potassium, magnesium, and phosphorus can also cause chlorosis.
The more we learn about the amazing processes that are going on in the garden, the better we can care for the plants in our gardens and landscapes. (And you'll be even better at Scrabble!)
You can grow a surprising amount of food in your own yard. Ask me how!
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