High School Level Photosynthesis



NADPH? Calvin Cycle? Krebs Cycle? acetyl-CoA? pyruvate? Thylakoids? I can’t wrap my head around all these new names and their functions–I’m about to take one of the most important tests of my life and they *love* including photosynthesis. Anyone really good at the whole thing and can explain what’s important for me to remember?

In: Biology

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Also, don’t stress too much. As a university educator, no test that you take in high school (or university for that matter) is even close to the most important anything in your life. Most people who say otherwise are either a tad self-important, or need to drastically reevaluate their perspectives on life.

Thylakoids are the parts of chloroplasts where first part of photosynthesis takes place, basically lots of folded inner membrane to increase the area.

When photon from visible light hits chlorophyll, its causes an electron to be “emited”, this electron then basically bounces all over the place through the protein complexes in the membrane and ends up dragging protons (that have opposite charge) across the membrane. These protons come from water and are basically a hydrogen atom minus electron, what is left from the water is oxygen. By concentrating protons on one side of the membrane an electric potential is formed. This is then used to move a turbine-like molecule (it literally works as a water dam!) that synthesizes ATP from ADP (ATP is the cell’s “batteries” and this is how they get charged so other enzymes can later use the stored energy to function). The bouncing electron ends up on a molecule called NADP+ which becomes NADPH (it becomes “reduced”, i.e. has the extra electron, this is another type of “cell batteries”). All these are called light-dependent reactions. They did not produce any sugar yet but prepared “batteries” (ATP and NADPH) to power the next steps.

What follows are light-independent reactions (sometimes called dark but this is a bit confusing as they also happen under light). This is where the Calvin cycle comes. Its main point is to take a carbon from atmospheric CO2 (again, releasing the oxygen) and put it inside a sugar molecule (thereby fixating atmospheric carbon into biomass). At start you have a sugar with 5 carbons and the 1 (from CO2) is added by RubisCO (super important enzyme, the name is worth remembering), a 6 carbon sugar is created. A few tweaks happen to it, then it gets split to two 3 carbon sugars. Some of these are the final product, some are used to make the initial 5 carbon sugar (five to make three, so that the maths adds up) and the cycle repeats. To power the enzymes that do all of this, the ATP and NADPH prepared in light reactions are used (converted back to their uncharged forms, ADP and NADP+).

Krebs cycle is something that occurs in mitochondria to prepare ready-to-use energy from sugars, so it’s not a part of photosynthesis and it occurs in all cells, not just plants. It’s basically where these output 3 carbon sugars end up.