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I take it back.. where does the first 4 carbon molecule come from? This is like photosynthesis.. where does the original 5 carbon molecule come from?

Glycolysis, the process of splitting a six-carbon glucose molecule into two three-carbon pyruvate molecules, is linked to the Krebs cycle. I searched things up and it says that in glycolysis, 4 atp molecules and 2 NADH are produced. But in this video, glycolysis only produces atp and pyruvate. 2 atp molecules are produced in glycolysis and 2 pyruvate. the other 2 atp molecules are produced in the krebs cycle, and the 2 NADH molecules are also produced in the krebs cycle, both of them came from the 2 pyruvate molecules from glycolysis. I think since they are linked, that's why some sources say 4 atp molecules and 2 NADH are produced in glycolysis. Go to 8:35, you can see 2 atp molecules at the top, which was produced during glycolysis, and the 2 other atp that was produced in the krebs cycle, and you can also see the 2 NADH molecules inside. That matches 4 atp molecules and 2 NADH, which was from different sources on Google! So in easier terms, glycolysis and krebs cycle is linked to each other so maybe that's why sources say that. In glycolysis, 2 ATP and 2 pyruvate are produced. In krebs cycle, the 2 pyruvates from glycolysis produces 2 more ATP molecules and 2 NADH molecules. All in all, 4 atp molecules and 2 NADH are produced. And that matches what Google and many sources say! Hope this helps and feel free to correct me!

this is from BYJU'S: "The final output or end product of Glycolysis is two molecules of pyruvate, ATP, NADH, and water. The end product of glycolysis in the aerobic condition is pyruvate and lactate in anaerobic conditions." only 2 molecules of ATP is produced in glycolysis, just like i said in my first reply. the other 2 is produced in the krebs cycle, coming from the 2 pyruvates from glycolysis. some sources may say that 4 atp is produced because they treated glycolysis and krebs cycle as one cycle, even though they are a SEPARATE CYCLE. (they are only linked) idk, i might be wrong though, feel free to correct me!

Update #3: I watched Crash Course Biology and it said that GLYCOLYSIS produces 4 atp, BUT the video was 12 years ago. And this video by beverly biology was 1 year ago and UPDATED. So it might be that things changed through further study. If you watch Crash Course Biology #7 which is about cellular respiration, Hank said that we're not even that sure about the total ATPs produced, whether its about 38+ or 30 something when you combine all the processes. but again, that video was 12 years ago. this video by beverly biology was an UPDATED overview of respiration and it's new, considering it's just 1 year ago.

This video is an overview of cellular respiration. The complete process is quite complex. In the complete process, a total of 4ATPs are created during glycolysis. But there is an early step that requires the usage of 2ATPs. So the total ATP created in glycolysis is indeed 4. But the “net” gain of ATP is 2. That’s what is being referenced in this simplistic overview.