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通过一个经典童话故事学习更多关于光合作用和细胞呼吸作用: 杰克与魔豆. 里希(Dr.RIshi Desai)是儿童传染病医师,他是可汗学院的成员之一. Rishi Desai 创建




there's a classic story out there and it has to do with a character named Jack and you may have heard this story but I'm sure that there's parts of that story that you have not heard and so I'm actually gonna try to fill in those parts that you get a complete idea of what happened now Jack came across a long time ago a famous now famous Beanstalk right so this Beanstalk was growing and growing and had these huge leaves and actually Jack used these leaves to make his way up this Beanstalk and so this is how this Beanstalk became very famous because it basically allowed Jack to to use it like a ladder now the part that we don't hear about is what was going on between Jack and the Beanstalk he was exercising right so he was actually making a lot of carbon dioxide I'd he was making a lot of this gas this carbon dioxide gas as kind of a waste product as he was running scampering up the Beanstalk and the Beanstalk was helping him physically but also was actually providing him with very precious oxygen in fact if the Beanstalk didn't do that he may not have even made it and we also we don't know for sure but we think that perhaps some of the story may have taken during taking place during the day and in fact we know that sunlight is quite important for this process and we think that this process the name that we give it for the Beanstalk anyway is photosynthesis photosynthesis and so what is really happening we're actually gonna kind of write it out here between Jack and the Beanstalk and really between all plants and animals what is this process between them we know that on the one hand you have bean stalks doing photosynthesis and on the other hand you have folks like Jack doing cellular respiration right and there's this really kind of interesting symbiosis and by that I just mean that the two are kind of relying on each other to really to really work right so you kind of need both of them to work well and so let's actually take a moment to write out these processes that are happening between Jack and the Beanstalk so let's start with the the process of photosynthesis the Beanstalk so on the one hand you've got what you've got water because of course the beanstalk needs water and you've got carbon dioxide now I'm gonna do carbon dioxide in orange so it's taking in water and carbon dioxide and it's going to put out it's gonna actually take these ingredients if you want to think of it as kind of cooking it's gonna take these ingredients and it's going to put out it's gonna put out what oxygen and glucose I'll put glucose up top and oxygen down below so these are the inputs and outputs of photosynthesis right and on the other side you've got something very similar you've got inputs you've got glucose and oxygen going in you're gonna start seeing some serious similarities here you've got glucose and oxygen going in so jack is taking in those two things and he's again of course processing them and he's putting out water and carbon dioxide so this looks really really nice right it looks perfect actually because everything is nice and balanced and you can see how it makes perfect sense that not only did Jack knead the Beanstalk but actually it sounds like the Beanstalk needed jack based on how I've drawn it now remember none of this would even happen if there was no sunlight so we actually need light energy in fact that's the whole purpose of this right getting energy so you have to have some light energy I'm gonna put a big plus sign and I might even circle it because it's so important I don't want you to lose track of it and on the other side of course jack is getting something as well he's getting chemical energy chemical energy in fact he's using the chemical energy to help him climb the Beanstalk and so the chemical energy comes in the form of what we call ATP which is just a molecule of high energy and so jack is basically or Jack in the Beanstalk are basically going from light energy to chemical energy using these two equations now here's the part that people don't always appreciate and I'm actually gonna take just a moment to show you that this isn't the full story there's actually something else going on as well and that is that there's actually some cellular respiration happening on the plant side so remember not only does the does the human or the jak need energy but so does the plant right the plant needs energy as well and in fact if it takes in light energy right here it needs to find a way to actually eventually get some chemical energy itself so that it can do all the things it needs to do like you know it doesn't need to run because plants don't move in that sense but it might need to make new roots and it may need to make a flower and all these things take energy so actually photosynthesis is happening during the day right this is happening during the day but at all times at all times plants are also capable of doing cellular respiration just like humans are so humans and plants have actually more in common than you might think so this brings up an obvious question why in the world would a plant send its send its glucose and energy glucose and oxygen this way when it needs it itself you know why would it actually get rid of it well the truth is that the glucose ends up oftentimes in fruits and vegetables that we cannot eat but as far as the oxygen goes it makes an excess of oxygen so there is actually enough oxygen to go both to us or to Jack and to be used by itself so it actually has an excess of oxygen that it's making so that's actually kind of interesting good to know now if you think about it if I was to let's say sketch out a planet let's let's draw it a little planet over here and ask you the question you know this was your planet Earth and you've got thousands instead of just one Jack let's say now you have thousands of Jack's and thousands of beanstalks and in fact not even thousands let's say billions because really that's what we have right we have a planet full of humans and full of other animals and full of plants what would the atmosphere look like this is the atmosphere what would the atmosphere look like well you'd guess that the atmospheres is you know gas and so what would those gases be well the way I've kind of drawn things again it looks like I've got lots of oxygen and lots of carbon dioxide so I would say well I guess there must be I don't know maybe 5050 carbon dioxide and oxygen based on what we know so far and the truth is that's actually not true that if you actually look at air if you actually kind of breakdown the atmosphere or air I'm just gonna write air here if you actually break it down it turns out that the that the ratios are actually a little different so for example oxygen makes up about 21% of our air this is our air breakdown air and carbon dioxide makes up about less than 1% so that leaves you wondering what the heck is making up all that other parts of air well what is it made of and it turns out it's about 78% nitrogen nitrogen now you you know you've got nitrogen in your proteins we've got nitrogen in our DNA so nitrogen is part of us and it's part of you know many many living things but nitrogen gas specifically is actually n to an end to this nitrogen gas really is not too reactive it kind of just hangs out by itself does not like to react with things so looking at our little atmosphere graph if you want to now think about it knowing that we've got very little carbon dioxide and you know about 21% oxygen you could think of oxygen kind of being let's say something like that well then relative to that nitrogen would be no much more right you have much more nitrogen hanging out and so this is really what our atmosphere looks like it looks more filled with nitrogen than anything else and in terms of carbon dioxide it's just got little smidge of maybe right there that could be carbon dioxide maybe even less than that so this is really what our atmosphere looks like visually and the nitrogen again it's making up the majority and if you actually kind of wonder where all that nitrogen is coming from because I didn't mention it in any of the equations right most of that nitrogen has been around a scientist thinks since the beginning of when Earth even had an atmosphere and that nitrogen were just kind of carrying with us at all times and that's why it just kind of remains around 78% and will probably remain there for many many years to come