If you are ever lucky enough to have a major scientific discovery, you’d probably name it after yourself too.
If you are ever lucky enough to have a major scientific discovery, you’d probably name it after yourself too. Melanie recaps each scientist's background (1:05) (4:26) before discussing the main events of the Calvin Cycle (1:40) and Krebs Cycle (5:02). The Calvin cycle forms sugars in the stroma of chloroplasts while the Krebs cycle oxidizes sugars in the matrix of mitochondria.
The Question of the Day asks (7:19) What group of biological molecules does the coenzyme NADH belong to?
Thank you for listening to The APsolute RecAP: Biology Edition!
(AP is a registered trademark of the College Board and is not affiliated with The APsolute RecAP. Copyright 2020 - The APsolute RecAP, LLC. All rights reserved.)
Website:
EMAIL:
Follow Us:
Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap the Calvin Cycle and the Krebs Cycle.
Zoom out:
Unit 3 - Cellular Energetics
Topics 3.5 Photosynthesis and 3.6 Cellular Respiration
Big idea - Energetics
Deep breath in, deep breath out. Or if you are a plant - inspiration through a stomata, and hopefully not too much transpiration out. So here we are with Calvin vs. Krebs and their infamous cyclic pathways. If you are ever lucky enough to have a major scientific discovery, you’d probably name it after yourself too.
Let’s Zoom in:
Melvin Calvin was an American biochemist who tinkered with algae and radioactive carbon-14 at The University of California, Berkeley. It was there (along with other scientists of course) that he unearthed the intermediate compounds of photosynthesis. This process is now termed the dark reaction, or Calvin cycle, or Calvin hyphen Bensen cycle depending on the textbook you’re reading. Calvin received the Nobel Prize for Chemistry in 1961 for discovering the chemical pathways of photosynthesis. Time magazine even referred to him as Mr. Photosynthesis!
The Calvin Cycle occurs in the stroma of the chloroplast (that's the fluid portion of the organelle) and is the second part of photosynthesis. This means that the light reaction has already occurred - chlorophyll has captured light energy, water was split, electrons were excited, oxygen released, and the coenzyme NADPH produced. Now the organelle will use the other reactant (CO2) and produce the intended product (sugars). Through a series of steps and unique structural molecules, Carbon dioxide is rearranged or fixed to form carbohydrates using chemical energy from ATP and NADPH. The Calvin cycle can be divided into three phases: fixation, reduction, and regeneration. First, Carbon is fixed (or joins up with) a 5 carbon molecule from the previous cycle called RuBP, or ribulose bisphosphate, to form a 6 carbon sugar, which splits in half. The reaction is catalyzed by the enzyme rubisco (one of my favorites, and always reminds me of nabisco cookies). During the reduction phase, the 3 carbon sugar is rearranged using energy from ATP and NADPH. It's called reduction because electrons are donated by NADPH (you know reduction, coupled with an oxidation - redox reaction for short).
Spoiler alert - we haven’t actually made glucose, but a 3 carbon molecule called PGAL or G3P. Which brings us to our final phase, regeneration. This three carbon molecule can be used by the cell to make glucose (with another turn of the Calvin Cycle) or other biological molecules. We are organic, after all, and need carbon chains. The calvin cycle is able to continue through the regeneration of a 5 carbon sugar ready to once again join up with carbon dioxide.
Next up - Krebs, Sir Hans Krebs to be exact. Originally from Germany, Krebs left for England during Nazi occupation. The biochemist continued his work at the University of Cambridge, where experimented with pigeon livers and breast muscles, discovering (along with other scientists) the reactions involved in the breakdown of sugars. This process is known as the citric acid cycle, TCA cycle, or Krebs cycle (no apostrophe, it's his last name - he doesn’t own the cycle even if he figured it out). Krebs received the Nobel Prize for Physiology in 1953 for discovering the chemical pathways of cellular respiration.
Glucose is gradually oxidized through several steps during the process of glycolysis in the cytosol of cells and during pyruvate oxidation and the Krebs cycle in the inner mitochondrial matrix (the fluid portion of the organelle). 2 Carbon acetyl Co-A produced from pyruvate oxidation joins with 4 carbon oxaloacetate from the previous cycle, forming 6 carbon citrate. (2 + 4 = 6). This is why the Krebs cycle is also called the citric acid cycle, named after its first formed compound. Through a series of steps, intermediate molecules, and enzymes - carbons are removed from citrate and released as carbon dioxide. The bond energy is captured by ATP and the coenzymes NADH and FADH2. These electron carriers will have a large role in the electron transport chain, where most ATP is produced by oxidative phosphorylation. (Check out episode 57 for more details). 4 carbon oxaloacetate is regenerated, ready to join with the next acetyl Co-A. Since glucose has 6 carbons, the Krebs cycle will need to turn twice for complete oxidation of the sugar.
Time for unit connections. Watch for energy connections to Unit 8 Ecology and biological molecule associations with Unit 1 Chemistry of Life. Alright - what about the exam? Diagrams of the Calvin cycle and Krebs cycle can be incredibly overwhelming, but remember that you are not required to know the specific steps, molecular structures, intermediate molecules or enzymes involved for the AP exam. Be prepared to review experimental data and draw conclusions - specifically related to graphing, gas production/consumption and the correlation between photosynthesis and cellular respiration. Remember plants do both processes, as they have chloroplasts and mitochondria inside of their cells.
To recap……
The Calvin cycle forms sugars in the stroma of chloroplasts while the Krebs cycle oxidizes sugars in the matrix of mitochondria. Named after discovering scientists, they each rearrange carbon, occur in the fluid portion of their respective organelles, involve ATP and coenzymes, and are regenerative by design.
Coming up next on the Apsolute RecAP Biology Edition: Cyclins and CDKS - Oh My!
Today’s question of the day is about biological molecules
Question of the day: What group of biological molecules does the coenzyme NADH belong to?