hi i'm andrew douche in this video i want to explain crassulation acid metabolism or cam photosynthesis in biology although there's a lot of really big words and big terms and acronyms usually they do a pretty good job of describing the thing that they name but this is not one of those occasions cam photosynthesis crassulation acid metabolism is one of the stupidest terms in all of biology because there's no such thing as crassulation acid i mean you could be forgiven for looking at this term crassulation acid metabolism and assuming that it has something to do with the metabolism of something called crassulation acid but that's just simply not the case there is no such thing as crastulation acid the word crassulation comes from the plant family crassulase which is where cam photosynthesis was first discovered and if you have a look at these plants you'll notice they look a little bit like cactuses don't they these are this kind of plant that you're looking at here is called a stone crop plant and it's a great example of family crassulase you notice that it's it's succulent kind of like a cactus um it looks like a desert plant and we typically find cam photosynthesis in plants that are adapted to dry environments like cactuses and in fact even though it's called crassulation acid metabolism family crassulase isn't the only family where we find cam photosynthesis we also also find it in cactaceae the cactuses we find it in bromeliace which is the family that includes pineapples and we even find it in some orchids particularly orchids that are epiphytic that is orchids that live on the branches of trees and if you think about it even though if they're living in a forest on the branch of a tree that's not the desert it's not an arid environment necessarily if if you're a plant and you're living on the branch of a tree your roots don't go down into the soil and so very often if it's not been raining there won't be a lot of water available to an orchid like that and so even though it's in a forest it still has to manage water very carefully it needs to be careful not to lose too much water and that's what cam photosynthesis does for a plant it's an adaptation that helps a plant to manage its water loss and we'll look at how that happens how that solution comes into play but in order to understand the solution we need to first understand the problem this is a summary of photosynthesis and i'm sure you know if you've learned about photosynthesis that it takes place not in one stage not in one step but in two separate stages called the light dependent stage and the light independent stage the light dependent stage takes place in the grana which is each granum is a stack of disks membrane enclosed discs called thylakoid discs and the light dependence stage of photosynthesis takes place on that membrane on the thylakoid membrane then the light independent stage of photosynthesis takes place in the stromer of the chloroplast that is the fluid surrounding those thylakoids and it's catalyzed by an enzyme called rubisco which is the most plentiful enzyme on planet earth its name stands for ribulose one five bisphosphate carboxylase oxygenase so you can understand with a name that long it's actually a really great name it really does exactly describe what what rubisco does but you can understand that with a name that long it's not surprising that most people just refer to it as rubisco it's a lot it rolls off the tongue a lot easier doesn't it anyway we'll get to that in just a moment on my diagram here this shows the inputs and outputs of both the light dependent stage and the light independent stage of photosynthesis but you'll notice some of the arrows are orange they show the inputs to each of the stages so the light dependent stage the only input apart from sunlight the only input is water and the water comes to the chloroplast to the leaves through the xylem which comes from the roots right so so the water travels up the roots right up the stem to the leaf and that's how the water gets into this process the 18 atp and the 12 nadph are the products of the light dependent stage and they go into the light independent stage so that's where they come from but the one that we're really interested in here is carbon dioxide carbon dioxide is an input to the light independent stage of photosynthesis or calvin cycle it's an input to that but it doesn't come through the roots of the plant and it doesn't come from within the plant like the atp and nadph the carbon dioxide has to be brought into the plant from the surrounding air and in order to do that in order for the carbon dioxide to get into the leaf it need that the leaf needs to provide it with some pores through which the carbon dioxide can diffuse and those pores are called stomata each stomata is surrounded by two guard cells and they're able to open and close to open and close the stomata and so that's how carbon dioxide is able to diffuse into the leaf space from the air outside is through open stomata the problem is and remember if we're looking at cam photosynthesis as a solution to a problem the problem is that when the stomata are open allowing carbon dioxide into the leaf they also allow water out of the leaf now for most plants that's not really a problem because there's plenty of water in their environment they live in soils it's damp they're able to get more water to replace the water that's lost through their stomata but desert plants or plants that are living on the branch of another tree and don't have access to much water if they're losing water through this demander that really is a problem and that's the problem that cam photosynthesis seeks to address here we're looking at a plant cell and we're looking at that plant cell at night time this is what happens in cam photosynthesis this is how cam photosynthesis works at night time the plant opens its stomata that's unusual because most plants close their stomata at night doesn't mean mean think about it for most plants stomata being open at night doesn't make sense because they can't photosynthesize at night so there's no point having the stomata open and letting water out if they can't they can't photosynthesize and don't need any carbon dioxide but in cam plants the stomata are open at night allowing carbon dioxide into the leaf but it's cooler at night and so when the stomata are open they're not going to lose so much water at night because you know it's not the heat of the day when of water tends to evaporate so quickly now notice in my diagram here that i've drawn carbon dioxide as just a single carbon atom of course you know that carbon dioxide has two oxygens as well as that carbon atom but for simplicity in this diagram i've ignored oxygen and hydrogen i mean all these molecules here have hydrogen oxygen as well they're all organic molecules but i'm just drawing the carbons here because i think it really simplifies what happens all right so here we are at night you can see that we've got carbon dioxide diffuses in through open stomata gets into the cells where it meets this molecule here called pep well his real name is phosphoenol pyruvate you can see again another big word and you can see why people thought well let's give it a name that's more peppy let's call it pep um and so phosphoenolpyruvate which is a three carbon molecule and the carbon in that one carbon molecule combines with this three carbon molecule phosphoenolpyruvate by an enzyme called phosphon or pyruvate carboxylase which converts them into a four carbon molecule called malic acid or malate now malic acid is stored in the large vacuole inside the cell as as you can see here so all of this is happening at night photosynthesis isn't really happening here at night it can't because there's no light so photosynthesis needs light in order to happen it can't happen at night all that's happening here at night is that carbon dioxide is being brought into the cell joined to pep to pep to make malic acid or malate which is stored in the large vacuole of the cell we call this carbon fixing okay we're taking that carbon in its carbon dioxide form converting it into a different form that can be stored in the cell as malic acid then when the sun comes up and it's bright and now hot the plant will close its stomata and again the advantage of that is that it's not going to be losing water to the environment now for most plants that's not an option you can't close the stomata during the day because if you do you can't get carbon dioxide in for photosynthesis but for a cam plant that's not really an issue because cam plants have stored up all this carbon dioxide in the form of malate or malic acid here in the large vacuole so even with the stomata closed this malate is broken back down into pyruvate and one carbon dioxide molecule the pyruvate is then recycled back to pep um again ready for the night time the carbon dioxide then enters the calvin cycle where this enzyme rubisco is operating which you know then is what produces glucose and water and you know the outputs of the light independent stage of photosynthesis so at the end of that we're going to end up with glucose we're going to end up with water and we're going to end up with a recycled molecule of phosphoenolpyruvate or pep so then when it becomes night again that pep is ready to be joined together with another carbon dioxide molecule to form malic acid in the large vacuole again and over and over again it goes pretty cool isn't that it's a really great adaptation because it enables a plant to keep its stomata closed during the heat of the day so that it minimizes water loss but then open them at night when it's cooler to save up all that carbon dioxide to be used in photosynthesis during the day even though the stomata are closed i hope you found that really helpful thanks for listening