Photosynthesis

Today we will be talking about photosynthesis. Now, before I start, if you haven’t already, I highly recommend that you listen to the cellular respiration episode from 2 weeks ago, because you’ll notice something cool about this when you do, trust me. Anyways, let’s get started. So, photosynthesis takes in carbon dioxide and water, and using light, creates sugar and oxygen. This is literally the reverse of cellular respiration, which means that plants, which are what do photosynthesis, can produce what they need for cellular respiration all on their own! They’re self-sustaining factories!

Now, on the topic of plants, plants are what we call autotrophs, which are organisms that make their own food, and specifically photoautotrophs, which are autotrophs that make their own food using light. Plants are super important for humans: they supply us with the oxygen we need to do cellular respiration that keeps us alive, they feed us, they shelter us (think wood), and they do a bunch of other things.

Ok, back to photosynthesis. The carbon dioxide used for photosynthesis comes in via the stoma, which is this small opening that lets in gases. Water comes in from the soil, is absorbed by the roots, and then travels up the stem of the plant into the leaves, which is where photosynthesis takes place. The light comes from the sun.

Let’s get into where photosynthesis takes place now. I know I said the leaves before, but I think being a little more specific wouldn’t hurt. Photosynthesis takes place inside plant cells. Cells are the basic unit of life which we have millions and millions of. More specifically, photosynthesis occurs in chloroplasts, which are small structures inside of plant cells. We don’t have chloroplasts because we don’t perform photosynthesis.

Photosynthesis is divided into 2 main phases: the light-dependent reaction and the Calvin Cycle.

Let’s talk about the light-dependent reactions first. These need light, as the name suggests. However, not all light is created equal. Light-dependent reactions work with blue and red wavelengths of light, but not green. The light-dependent reaction takes place in thylakoids, which are these pancake-like structures in chloroplasts. Even more specifically, it takes place in the thylakoid membrane, which is this layer of proteins and phospholipids.

Now let’s talk about how light-dependent reactions happen. They start with water coming in and being split by a water-splitting enzyme into hydrogen ions, electrons, and oxygen in a process called photolysis. It takes 2 molecules of water being split to make 1 oxygen molecule. The electrons released in this process move into Photosystem 2, which is a cluster of pigments in that thylakoid membrane I was talking about earlier. Now, Photosystem 2 has this place called the reaction center in it. This reaction center absorbs all the energy from the sunlight coming in. When the electrons reach this reaction center, they become excited, and they bounce up. When they bounce up, they get captured by the ETC or Electron Transport Chain. The ETC is a series of proteins that moves these electrons from Photosystem 2 to Photosystem 1. Now, as these electrons move along the ETC, their charge draws those hydrogen ions that were released earlier through the thylakoid membrane into the hollow inside of the thylakoid. Then the electrons reach Photosystem 1, where they are excited by its reaction center, and move along yet another ETC. Finally, the electrons will combine with hydrogen ions and NADP+ to make NADPH which will be very useful for the next stage of photosynthesis, the Calvin Cycle. Those hydrogen ions that moved into the inside of the thylakoid will now move into an enzyme called ATP Synthase, which uses the energy of these flowing ions to make ATP, a type of energy.

Ok, now let’s get into the Calvin Cycle. Unlike the light-dependent reaction, the Calvin Cycle doesn’t need light. The Calvin Cycle has 3 main phases: carbon fixation, reduction of G3P, and regeneration of RuBP. The Calvin Cycle takes place in the stroma, which is this thick fluid in the chloroplasts which surrounds those thylakoids where the light-dependent reaction takes place. The Calvin Cycle starts by taking carbon dioxide and RuBP and combining them with the help of an enzyme called RuBisCo. This new molecule is unstable, so it splits into some PGA molecules. Then, the ATP and NADPH made in the light-dependent reaction are broken down, forming ADP and NADP+ again, and electrons and a phosphorus are donated, forming G3P. Some of this G3P is transformed back into RuBP, concluding the Calvin Cycle.

So, the light-dependent reaction forms NADPH and ATP, which are needed to power the Calvin Cycle, and the Calvin Cycle forms NADP+ and ADP, which are needed to power the light-dependent reaction. Photosynthesis is kind of one big cycle.

Now, let’s talk about taking in carbon dioxide in different types of plants. Photosynthesis cannot happen at night, because there's no sunlight to power light-dependent reactions and because enzymes get deactivated in the dark. In other words, the Calvin Cycle won’t work either. A type of plant called C3 plants open their stomata (plural of stoma) during the day and close them during the night, and thus take in carbon dioxide during the day. C4 plants open their stomata when they choose, and CAM plants close them during the day and open them during the night. CAM plants want to conserve water, so they close their stomata during the scorching daytime.