This is a picture of me and taken a few years ago at Muir Woods in California. I'm fairly tall. I'm five, eight and so it's fairly obvious in this picture, how small I am compared to this tree. How did this tree get so big? And for that matter, where does the mass of any tree or plant come from? That question was a subject of a viral video a few years ago. Interviewers pose this exact question to Harvard graduates right after commencement. The reason why the video went viral is that none of the graduates could answer the question correctly. The misconception is that the mass of the tree comes from dirt and that dirt is food, so to speak, for plants. It is true that there are nutrients that come from the soil, but that is not where the mass of a tree comes from. The mass of the tree actually comes from air, carbon dioxide to be more specific. Plants have the unique ability to perform photosynthesis. So let's break the word photosynthesis down. Photo means light and synthesis means creation. So photosynthesis is creating with light, plants use energy from the Sun, the photo part, and carbon dioxide from the atmosphere to make food for itself, that's the synthesis piece. A plant creates its own food and food for us, but more on that in a minute, using light and gas. That's pretty neat. The super power of plants is important for almost all life on Earth. This is also why when considering climate change, deforestation can be thought of as twice as bad for the environment. When you remove trees you're taking away an organism that takes carbon dioxide a greenhouse gas out of the atmosphere. When those trees are burned, that stored carbon is also released back into the atmosphere. So it's so bad because you're removing an organism that's actively taking carbon dioxide out of the atmosphere and then when you process it, you're releasing all the stored carbon dioxide back into the atmosphere at the same time. Since photosynthesis is so essential for life, for a long time it was thought that all life on earth was dependent on the sun. However, when deep sea exploration started, life was found on the bottom of the ocean far beyond the reach of the sun. This is a nice example of something we talked about in course one, that science knowledge changes in light of new evidence. Whether the energy source id the Sun or the Earth's core, the transfer of energy and consequently the presence of life is dependent on a group of organisms called autotrophs. So let's break it down again. So autotroph, auto means self, like an autobiography is a book about one's self. And troph means to feed. So an autotroph is an organism that self feeds or makes it's own food. Plants are autotrophs, they use carbon dioxide and sunlight to generate their own food. Plants don't need to go to the supermarket, just a little bit of sunlight and carbon dioxide gas, and they're all set. Imagine how nice it would be if we were autotrophs. Rather than going grocery shopping, all you would need to do is go and spend time outside in the sun everyday. But how do we actually go from sunlight to actually fueling our bodies? Plants and other autotrophs are also called producers. Producers, so produce, you're making something. So a movie producer is producing a movie for example. So when we think about autotrophs and plants, they're producers because they are producing energy. So our energy here. Okay, so let's represent the energy that producers make as a box. So here's our box and label this as our producers. And then just as a reminder to ourselves that these are things like plants, so there's our nice flower. So plants or producers get eaten by something else, like a bug for example. So let's draw in this bug here. So this is also called our primary consumer. Draw a little picture of a bug here. Okay, so note right now that the bug's box is smaller than the producer's box. So remember these boxes here represent how much energy is available. So we're losing energy as we move up. So we get to our secondary consumers, so this is the creature that is eating the bug. Secondary consumers, so something like a bird, the box is smaller. Now when we get to the top, so I'm just going to abbreviate this as a tertiary consumer, say something like a cat that eats the bird. Now there's even less energy available. So the higher up we go, the more energy is being lost. So when we get up to the top right here, this is where the least amount of energy is available. So you can also draw an analogy here to a budget. So how much energy is available is like how much money you have available at the start of the month. So maybe here we have $100 and so by the time the producer meets its own energy needs, so maybe the first bill that you pay every month is your mortgage or your rent. Now by the time we get to the next level, okay, we have $75 left, so that goes to utility bills, and then maybe we spend some money on groceries because you've got to eat. And then by the time we get to the top, well, we have about $25 left for our fun money. So you have to meet your own needs first, and then the rest can be shared as you go up. So how does this relate back to our daily lives? One of the solutions to feeding the hungry is to eat a plant based diet. Because when you have a smaller food chain, so let's say we're only talking about this bottom level or maybe the secondary level, there's more energy available, there's less expenses going back to our budget analogy. There's more to go around. And so it's easier to feed more people by eating lower on the food chain than if everybody's eating meat at the very top. Now let's look at another application of this idea in our daily lives. Energy isn't the only thing that gets moved around. Certain chemicals can also be passed around or bioaccumulate. Bioaccumulate, okay, so let's break this down. So we have bio, meaning it's something living. So again biology is the study of life, and then accumulate. So accumulate, we're getting more of something, so this has to do with life getting more of something. And in this case we're going to be talking about mercury. So maybe you've heard recommendations about not eating certain seafood like sharks or swordfish, particularly among pregnant women or children. And so what is it about sharks and swordfish above all seafood that's available? Why do we care so much about sharks and swordfish? And it's because they're at the top of the food chain. So if you remember back to our little diagram with the boxes, there were producers at the bottom, primary, secondary, and tertiary consumers. Our sharks and swordfish are up here, they are at the top. And also remember that these guys are eating these guys are eating these guys are eating these guys. So as we just talked about before, as we go up the pyramid, the amount of energy we have it's going down. However, things like mercury will go back to green increase as we go up. So how does that work? How do we accumulate toxins like mercury? Well, first of all, to bioaccumulate, the compound has to meet two different criteria. The first one is it needs to be fat soluble. This is important because it can't be excreted. So a lot of things that we eat, so if you have too much vitamin C for example, you're going to pass it in your urine. It doesn't stick around. Things that are fat soluble can't pass out of our bodies in a normal way. So it sticks around, stays in our body for a long time. The second criteria that needs to be met rather, is that it has to be stable. Stable means it won't degrade or break down over time. So it sticks around long enough that it moves up the food chain, so that's going back to this green arrow here, starting to move up the food chain. So in the case of mercury, it's starts with pollution and pollution that's running off to our oceans. So here's our water. So we have mercury coming in, and then if you want to be very particular it's actually methyl mercury. That's the toxin bioaccumulates, so it's running into the ocean. So where does this mercury come from? One example is that it comes from coal fired power plants. So we have mercury that's coming in to the water. It's at a low concentration, it doesn't really matter. Maybe some of it gets it's on our plankton. So these little pink dots will represent plankton that's picked up a little bit of mercury. Again, not really at a high enough level that it really matters. So now let's say we have some small fish that come in and they look at this and they say, ooh, some tasty plankton. So now we have small fish with mercury. And again they don't have that much. But now let's say we have a bigger fish that comes along, here's your bigger fish. He sees these guys and he's going to eat them. So now we have bigger fish that's got all the mercury that the smaller fish did. Okay, so let's say an even bigger fish comes along and we've got these medium sized fish here and let's say they all have between two and three pieces of mercury. Well, now that the big fish eats these guys, then we're left with the big fish with lots of mercury. So by the time you get to this level, there's now potentially dangerous levels of mercury that are present. And so because there's potentially dangerous levels of mercury here, this is why it's not recommended to eat these fish at the top of the food chain. So remember this guy is up here, so the least energy available but the most mercury. So what do we do about this problem? If you love to eat seafood and you want to continue eating seafood it's better to eat lower on the food chain. It's better to eat these guys for example that don't have as much mercury. It also is a good idea to support water friendly initiatives that decrease or at least seek to decrease the amount of mercury pollution that's going into the water in the first place. To summarize what we just learned, we learned about how energy and other things like mercury are transferred through the environment. This illustrates how interconnected life is, something that we'll return to in course three. What we eat has consequences both on our health and on the environment, but how do we turn what we eat into energy that fuels our bodies?