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2. Screwdriver
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Photosynthesis: Properties of Light and Chlorophyll
A while back, we talked about how organisms fall into different categories based on what food they eat and how they get it. Humans, for example, are heterotrophs because they get their energy from organic compounds that they didn’t make themselves—another organism did. Autotrophs, however, have special mechanisms to transform energy from the environment into a kind of energy that they can consume. Heterotrophs rely on autotrophs to make their food for them; they’re like the base of the foodchain, supporting everything else.
You’ve probably guessed what kind of autotrophs I’m talking about: plants.
Photosynthesis is the process plants use to convert light energy from the sun into chemical energy that we use in cellular respiration—glucose! Think of a plant’s leaves as solar collector, letting in light, water, and carbon dioxide: the three key ingredients for the photosynthetic process. These are allowed in through little passageways or holes called stomata. Oxygen is produced as a byproduct and is shuttled out via the same route. When the stomata are open, they can also allow the leaf to lose water vapor to the atmosphere—so to prevent plants drying out, the stomata are flanked by guard cells, which control when they open or close.
The leaves of a plant are filled with photosynthetic cells that contain chloroplast, the organelle where photosynthesis takes place. Chloroplasts are filled with stacked-up thylakoid membranes, which contain chlorophyll—pigments used in photosynthesis.

These chlorophyll pigments are actually contained within two light-harvesting protein complexes embedded in the membrane, called photosystem I and photosystem II. Their goal is to capture and pass on light energy. Just so you know, photosystem II is used first and photosystem I is used second; they’re only named I and II because that’s the order they were discovered.
To understand how we can get energy from light, we have to understand a bit about light itself. You know when you pass light through a prism and it’s separated into different colours? Each colour represents a different wavelength of light: red is the longest and violet is the shortest. Colours with shorter wavelengths are more energetic, so, for example, X-Rays and UV light have shorter wavelengths than visible light, which are in turn shorter than radio waves.

When light interacts with matter, it can be either reflected, transmitted, or absorbed. A pigment is a substance that absorbs light. Pigments are usually only good at absorbing only certain wavelengths of light. Black is good at absorbing all visible light and white isn’t—it mostly reflects colours back. There are two chlorophyll pigments (called chlorophyll a, which is the primary pigment, and chlorophyll b) and they’re are good at absorbing most wavelengths of visible light except for green—they reflect green back, which is why most plants are green. Chlorophyll a is most efficient at absorbing red light while chlorophyll b is most efficient at absorbing blue light.

There are also a couple of “accessory pigments” called carotenoids (like xanthophyll and carotene), which help pick up the wavelengths that chlorophyll doesn’t, and also helps protect them from damaging wavelengths.
Photosynthesis depends on chlorophyll capturing light energy, as we’ll see in the next article.
Cool background fact: photosynthesis most likely originated in the infolded regions of the membrane in ancient bacteria. In photosynthetic bacteria today, their membranes are folded in such a way as to act like the theylakoid membranes (remember that bacteria are prokaryotes and don’t have organelles).
Body images sourced from Wikimedia Commons
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3. Life is Short
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Photosynthesis: Calvin Cycle
You probably know that plants take in carbon dioxide and give out oxygen, but as we saw in the last article, that isn’t a neat exchange, turning O2 into CO2. Rather, oxygen is created as a byproduct of splitting water, and CO2 is consumed by being turned into sugar. This happens in the Calvin Cycle.
In the Calvin Cycle, carbon dioxide, NADPH, and ATP are put in, and a sugar called G3P comes out. There are three steps to create this sugar: carbon fixation, reduction, and regeneration. Note that none of these steps needs direct light!
The first step is carbon fixation. CO2 is taken in from the atmosphere around the plant, added to a 5-carbon sugar called RuBP (ribulose bisphosphate), and thus turned into 3-phosphoglycerate, an organic molecule. This process is catalysed by an enzyme called Rubisco—basically, it recognises CO2 and pairs it with the “CO2 acceptor”, RuBP. For every “turn” of the Calvin Cycle, three CO2 molecules are fixed into two 3-phosphoglycerate molecules.
In the second step, reduction, the cycle takes in 6 NADPH and 6 ATP (from the light reactions) to convert these molecules into glyceraldehyde 3-phosphate (G3P). The “reducing power” of NADPH is used to add electrons to the molecules, and the ATP gives them phosphate groups.
Then in the last stage, regeneration, 3 more ATP molecules are used to turn five molecules of G3P back into RuBP, the CO2 acceptor, so it can be used again at the start of the cycle. What’s leftover—a single G3P—is the output of the cycle. It’s the overall goal of photosynthesis: a sugar molecule that can then be used in cellular respiration to create energy for living cells to use.

So, a roundup of the cycle:
We put in 9 ATP, 6 NADPH, and 3 CO2.
We get out 9 ADP, 6 NADP+, and 1 G3P (plus 3 RuBP molecules).
The ADP and NADP+ are then recycled back to the light reactions, and photosynthesis begins over again.
Body images sourced from Wikimedia Commons
Further resources: 3D video or Video from Crashcourse
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4. Stick with you
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