Krebs Cycle, Calvin Cycle, light reactions and glycolysis

Created March 2020 by Bree Latner, Offline version here
Video by Paul Anderson, also on his website Bozeman Science.

    Organisms need to capture free energy from the environment so they can produce cellular energy in the form of . Organisms that can capture free energy from the environment are called . They store that energy as macromolecules, like glucose. Some autotrophs who do not have access to sunlight utilize the process of to produce using the energy of chemicals like hydrogen sulfide. Autotrophs and heterotrophs then take in those macromolecules and break them down using oxygen to produce ATP through a process called . Some heterotrophs that don’t have access to oxygen utilize the process of to break down sugars to produce ATP. In photosynthesis, free energy from the sun is stored in the form of macromolecules like glucose. This process captures free energy as can be seen by the . There are two stages in photosynthesis, which can be remembered by breaking down the word, “photo” meaning light which corresponds to the first stage, the . The “synthesis” stage of photosynthesis is the . In this stage we synthesize or build macromolecules, like glucose. Photosynthesis takes place in the of photosynthetic organisms. The light reactions occur in the thylakoid membranes and the Calvin Cycle occurs in the stroma. In the light reactions, the absorb the free energy of light to excite electrons that are passed down an electron transport chain. The electrons from electron transport are stored as NADPH at the end of the chain. The electrons for transport come from the breakdown of in PSII. As the electrons are released into the chain, H+ ions and oxygen are released. As the electrons are passed from protein to protein, protons are being pumped from the stroma into the thylakoid lumen, causing an gradient to build in the lumen. The H+ gradient that has been built up in the lumen then flows through to catalyze the formation of ATP. The Calvin Cycle takes in and uses the energy of ATP and NADPH and converts it into G3P which will go on to make sugar, like glucose. By the end of the light reactions and the Calvin Cycle the original free energy from sunlight has been stored in the bonds of . The evolution of photosynthesis as a method of capturing free energy can be seen beginning over 2 billion years ago. This can be seen as an increase in in the Earth’s atmosphere as photosynthesizers flourished. Now that the free energy from the environment has been stored as , organisms must release that energy from glucose by breaking it down through respiration. Through respiration, oxygen is used to break it down glucose into and water. This releases the energy that was stored. The reaction of respiration is , which can be seen by a negative △G, as energy is released from glucose during the breakdown. Cellular respiration takes place in the . The mitochondrion is a double membraned organelle with a highly folded inner membrane. The first stage of respiration is , and it occurs outside of the mitochondria, in the cytoplasm of the cell. On the interior of the mitochondria is an aqueous fluid called the which is the site of the Krebs Cycle, the second stage of respiration. The folding in the inner membrane provides surface area for the , the final stage of respiration. In glycolysis, we start with the C6 molecule glucose and break it down into two C3 molecules called . Pyruvate then moves into the Krebs Cycle where it is broken down into . The energy released from breaking down pyruvate is stored as NADH and FADH2 which will power the in the final stage. Electrons from NADH and FADH2 get transported down the chain to pump H+ ions across the membrane into the . The H+ gradient into the inner membrane space flows through to make ATP. The oxygen molecules that are in the matrix side of the membrane are so electronegative that they pull the electron through the chain and combine with them and H+ ions to form . By the end of photosynthesis and reparation, all the energy captured from and stored in glucose has been harvested and used to make ATP, which is the form of energy usable by cells.