Botany for Gardeners: Photosynthesis

Cibotium schiedei - Mexican Tree Fern @DC LAH 042cIf you ever took high school biology, you’ve heard about photosynthesis. You know that it’s the way a plant takes in sunlight and carbon dioxide and produces oxygen. You probably remember it has something to do with chlorophyll—the pigment that causes plants to be green. But how often have you considered photosynthesis since you passed your final exam that year?

Most people, even gardeners, take photosynthesis for granted. It’s just something plants do. But knowing a bit more about the process can help us have healthier gardens—more, tastier veggies, sweeter fruit, prettier flowers. How so? Let’s take a look.

I don’t plan to go into great detail about how the various atoms and molecules move around inside the plant. I had to learn every complicated step in college; that was plenty for a lifetime! But understanding the process in general terms helps us to know what plants need and why they need it.

Photosynthesis needs more than CO2 and sunlight. Water is another necessary ingredient. It’s also a by-product—the plant actually takes the water molecules apart, then later creates new ones.

Chlorophyll is a green pigment found in small bodies (think water balloons) inside the cell called chloroplasts. Chlorophyll has to be created inside the plant cells, so they need the raw materials for that as well—carbon, oxygen, hydrogen, nitrogen, and magnesium. There are six different kinds of chlorophyll (four are only found in bacteria and algae) but they all need these same elements.

There are a few other elements used in the chemical reactions of photosynthesis, and they’re needed in varying amounts: phosphorus (in the form of phosphate), manganese, copper, calcium, sulfur, boron, iron, molybdenum, and zinc.

Now that we have the participants, it’s time to start the show. The sun is shining. Light energy (photons) strikes a leaf full of chloroplasts and the red and blue wavelengths are absorbed (green is reflected, which is why we see it).

Inside those chloroplasts, a lot of complicated chemistry takes place involving chlorophyll, beta carotene (the stuff that makes carrots orange), carbon dioxide, water, nitrogen, magnesium, the extra elements mentioned above, and a bunch of free electrons.

If you love chemistry, this process is explained in great detail in Wikipedia. Here’s the simple version:

During the daylight hours, photons provide enough energy to move electrons from water molecules (where they were perfectly content) to carbon dioxide molecules. Throw in some extra atoms—nitrogen, phosphorus, etc.—and you end up with molecules of ATP and NADP. These molecules are how the cells store energy. You also get oxygen. This called the Light Reaction because it depends on the energy of light.

Now the cell uses that stored energy, along with water and oxygen, to create glucose (sugar) molecules. Sugars contain carbon, hydrogen, and water. The plant may store these sugars as is, convert them to more complex sugars, or string them together to make starches. (The process of taking carbon from the air and sticking it into a sugar molecule is called carbon fixation—a hot topic these days in light of the controversy over climate change)

This process is called the Calvin Cycle. (It’s also called the Dark Reaction, but scientists recently discovered that some essential enzymes are only activated by light, so this may be a misnomer.) There is oxygen and water left over, and they get released to the atmosphere.

Here’s a diagram that depicts most of what I just explained:
Diagram from

Why is it important to understand all this? I’ll cover that next week!

One thought on “Botany for Gardeners: Photosynthesis

  1. Pingback: Phloem is for “Phood” | Mountain Plover

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