If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:7:42

Bioenergetics: The transformation of free energy in living systems

Video transcript

will you be silent for like three seconds now chances are in those three seconds your heart beats somewhere between three and five times now your heart's a pretty amazing muscle I mean it beats all day every day for your entire life and in order to do so it requires a ton of energy but where does all this energy come from so I do a picture of a Sun here to describe that all of the energy that our body uses for work is originally derived from the Sun but humans can't just stand outside in the Sun and get energy that way energy has to be transformed into a usable form of energy for our body that will allow our muscles and things like our heart to be now where does this energy come from let's just erase some of my work here well our story starts with plants now plants take the energy from the sunlight and they convert inorganic compounds into glucose and the energy from the sunlight is added to this living system in the form of the chemical bonds of glucose now animals and humans then eat the glucose in these plants and that glucose is then converted into a usable form of energy which is known as ATP and this process of adding energy to the system and creating glucose is known as photosynthesis and this process of breaking down glucose into a usable form of energy is known as cellular respiration so let's find out how this all happens now this process starts in the chloroplasts of the plant cells and in this step known as the light reaction the energy from light disrupts the h2o or water causing it to kick off a hydrogen ion and this hydrogen ion is then bound to NADP to form NADPH and in the process of all this ETP is formed so you can see that we already have an energy form from the light but this is a relatively smaller amount of energy and it's this NADPH that's created that is a high energy electron carrier that can be used to produce a lot more energy and the next step in this process is known as the Calvin cycle and in this part of photosynthesis we start with carbon dioxide and NADPH and ATP are added and there's a series of reactions that occur and the specifics of these reactions are less important than the outcome which is glucose so in these two steps in the light reaction in the calvin cycle we take the energy that's in sunlight we store it into the bonds of NADPH and ATP and we use those to run the calvin cycle to store all of our energy in the chemical bonds of glucose now what happens when glucose is eaten by an animal and that animal then wants to use the energy I mentioned earlier that this process is called cellular respiration and cellular respiration is very similar to photosynthesis just in the backwards direction now the first step we have to break glucose down into a molecule known as pyruvate and this step gives off ATP and ATP is the usable form of energy but we're not giving off a whole lot yet we still have a lot of energy contained in the bonds of pyruvate so by pyruvate is then broken down into acetyl co a and then acetyl co a then enters the TCA cycle and TCA stands for try carboxylic acid and this cycle is also known by a couple other names like the Krebs cycle and the citric acid cycle once again there's a series of reactions that occur in this cycle but the specifics of these reactions are less important than the outcome which is the production of co2 as well as nadh and fadh2 now these molecules are similar to the NADPH in photosynthesis in that they're high energy electron carriers now they enter a series of reactions known as the electron transport chain and in this reaction the hydrogen from these high-energy electron carriers is bumped off and we have oxygen over here which is combined with that hydrogen to form water or h2o and these hydrogen's here drive an enzymatic pump that produces ATP you can see here that photosynthesis and cellular respiration are very similar reactions just in the opposite direction and although they may have different e'en intermediates the actually the products of one are the reactants of the other and vice versa and so let me demonstrate that and if photosynthesis our reactants are h2o and co2 and our products are oxygen and glucose whereas in cellular respiration we have glucose as a reactant as well as oxygen we're now producing co2 and water and you can actually see this if I write out the equations for photosynthesis and for cellular respiration Xin and photosynthesis the equation is 6 co2 + 6 h2o which produces glucose or c6 h-12 o-6 and 602 in cellular respiration is really just the opposite of that where we take glucose which is c6h12o6 plus 6 oxygen which will end up producing 6 carbon dioxide and 6 waters now there's an important reactant and product that isn't added in these chemical equations and that is energy and in photosynthesis the energy is a reactant putting this energy into the chemical bonds of glucose whereas in cellular respiration the energy is a product and we're taking that energy from the chemical bonds of glucose so let me just show you one more way to demonstrate how energy changes in these two reactions now to do this I'm going to draw our reaction diagram and on the x-axis here we have the reaction progress and on the y-axis we have free energy which is also known as G I'm going to just dim down the reaction a little bit so that we can work over the top of it so if you look at the free energy level for where the reactants of photosynthesis start to where they end with glucose you're adding energy to this so the so you have a low entered free energy level and you're going to a higher free energy level so energy is added and that's from the sunlight whereas in cellular respiration you go from the reactants of glucose with lots of free energy to low free energy with ATP you are releasing energy and it's this release and free energy that allows our body to do work like pump the heart