hey everybody dr sammy here your friendly neighborhood entomologist and today we're going to talk about photosynthesis there's very little life on this planet that could exist without photosynthesis it is the prerequisite for pretty much everything you see around you it's how you get from the intangible light of the sun to physical bodies like those of humans or hungry hungry caterpillars but what does photosynthesis actually mean i hear people say all the time the photosynthesis is the process by which plants make sugar from light and it almost seems like magic light is not a substance it is not made up of the molecular building blocks that compose all matter and thus it doesn't have mass you can fill a room with light and never run out of space so how could you possibly make something physical out of it well you can't so instead light is a form of energy energy being the capacity to do work so this is where it's helpful to know word origin the word photosynthesis is made up of two greek words it literally means light and to put together that's right in addition to being an entomologist i dabble a little bit in etymology just to make sure i'm maximally confusing to people anyway you're literally using light to drive reactions that combine ingredients into new products a form of work so you're not turning light itself into material sugar you're taking matter that already exists in the form of six molecules of carbon dioxide six molecules of liquid water and using the energy of the sun to power a reaction that combines them into a new substance with molecular oxygen as a byproduct think of it this way when you bake a cake you don't say that you made cake from heat it would be more accurate to say that you took flour eggs sugar and butter and used heat to combine them into something new so sticking with our analogy the ingredients for photosynthesis are just carbon dioxide from the atmosphere and water from the ground with light to do the heavy lifting carbon dioxide and water are put together to produce a carbohydrate which literally means watered carbon as you can see with this typical carbohydrate molecule glucose your carbon is attached to the same atoms that compose water to hydrogens and oxygen this carbohydrate has more chemical energy or bond energy than the molecules of water or carbon dioxide that served as ingredients thus the energy at the end of the process is much greater than the energy that the ingredients had at the outset this means that photosynthesis is a sort of useful reaction that stores energy like a big biochemical solar cell we call these endergonic reactions and you might be able to see already why they would be so valuable in biological systems you take molecules that don't have much energy and use them to produce something that can do work and the kitchen where all this work happens is the chloroplast literally the green maker the chloroplast is a little organ or organelle present in some plant cells and it's what makes plant cells and ultimately plants green and this is because the chloroplasts bear the green pigment chlorophyll now the same way that the word photosynthesis can be broken down neatly into its two base words the process itself can be separated into two neat little segments i like to think of them as a charging step where energy from light is converted into chemical energy and a synthesis step where that energy is used to do the work of actually synthesizing the end product typically a carbohydrate so the first segment requires the direct input of light and as such is referred to as the light dependent reactions these happen in a section of the chloroplast called the thylakoid they form these neat little pouches the inside of which is called the lumen in the outside called the stroma when a photon of light ends its eight minute journey from the surface of the sun to the surface of a leaf its energy is absorbed by the chlorophyll embedded in the thylakoid membrane this energy powers a pump which literally charges the inside of the thylakoid like a battery by moving the ions inside as the charge builds up the energy can be used to do work but the next segment of this process happens outside of the thylakoid in the stroma so to get that energy where it's needed the thylakoid transfers the energy to a molecule called adp adenosine diphosphate by adding another phosphate bond and making it atp adenosine triphosphate now you might have heard of this one it's often called the energy currency of the cell pretty much wherever energy is needed for a cell to do work atp is involved you might have also heard that the energy is stored inside of the phosphate bonds and that breaking them releases the energy but try to remember that these bonds depicted by the lines in these diagrams are just a convention it represents an adherence of these atoms together via attraction and it shows you where they adhere but the energy isn't actually in the bond the attraction between the atoms builds up potential energy like a rubber band that's being pulled really tightly when you let it go it could hit a paper cup and do the work of displacing that cup the rubber band then falls to the ground in a low energy state because its energy has been released so when you put the energy into a system great enough to overcome the attraction between the atoms and force them apart thereby breaking the bonds atoms or in this case a whole phosphate group can go spiraling off taking what was potential energy with it if it hits something that it can bond with that energy is released as the bond is formed and can be used to do work such as the magic of atp it's described as energetic because it's easy to break the bond between it and the last phosphate group meaning you don't have to put much energy in but you get a ton of energy out so back to the thylakoid those photons of light were able to net us a highly energetic atp molecule but the next segment of photosynthesis is going to need some electrons too that light energy is used to do the work of loading up a mobile electron carrier with electrons and a proton this carrier is called nadp plus and when it's got a full load to take to the next set of reactions it's called nadph and that's pretty much the light dependent reactions in a nutshell the only other thing you should probably remember is that well this is where all the oxygen in your lungs comes from so you know no big deal no big deal when chlorophyll gets excited by that photon of light it turns into a real bully the work it's doing creates such a powerful electrochemical imbalance and the chlorophyll balances the equation by just stealing an electron from water this causes water to fall apart releasing its oxygen which the plant just lets go of so now that we've taken care of the section that's dependent on light let's discuss the section that isn't the light independent reactions or the calvin cycle occur in the stroma of the chloroplast and this is where the earth-shattering chemical reaction occurs that allows for all life on this planet the fixation of gaseous carbon or inorganic carbon into carbon chains organic carbon and this is so important because fixation doesn't just happen on its own co2 in the atmosphere doesn't form organic chains or sugars when it bumps into more co2 in the atmosphere it requires the help of enzymes and energetic molecules made by living organisms it's why when we first landed a rover on mars we immediately started looking for the evidence of organic molecules it would be evidence that something is or was living there in the light independent reactions a plant enzyme fixes carbon dioxide from the air into a chain of carbon so atp and nadph which were produced in the light dependent reactions provide the energy and the electrons to create two energetic reactive molecules that can be combined to make glucose or other useful molecules and the beauty of it is that the byproducts of the light independent reactions adp and nadp plus are shuttled off from the stroma back to the thylakoid for more light dependent reactions where they can be recharged and recycled for use again later and voila photosynthesis the takeaway here is that photosynthesis allows you to go from the intangible energy of the sun to the stored chemical energy that life on this planet is based on the sun's energy is converted to the chemical energy of a carbohydrate molecule in the chloroplast that molecule can later be broken down and most of that energy reclaimed either by the plant or by creatures that eat that plant all life on earth is carbon-based and every single molecule of that carbon once existed in the atmosphere in gaseous form as carbon dioxide until some enterprising plant or microorganism synthesized it into something you can use and while they were at it many of them filled the atmosphere with the oxygen that we all need to breathe so the next time you see a plant shake its leaf and say thank you