Investigations are some of the hardest questions that you will get in an exam. Not to mention when they're paired with something like photosynthesis, which is also a really challenging topic to answer. In this video, I'm going to make it super easy and super simple to do. Well, I'm going to give you all the tools that you need to identify the different variables, what's a control, what's validity, reliability, and I'm going to break down exactly what you need to know so that when you explain yourself in an exam, you are going to get full marks every time. Hi everybody and welcome back to Miss Angller's biology class. I am Miss Angler. In today's video, we are going to be focusing in on photosynthesis investigations. This is the second video in the series, and we specifically are focusing in on light, whether you need it for photosynthesis, and then on top of that, light intensity. And so, how light intensity affects photosynthesis and the rate at which they do it. Now, with all of these investigations, I can show you one or two variations of the different experiments that your teacher might ask you, but there are so many variations of the same experiments. So, if you grasp these basic experiments, you will be golden in exams. Now, if you are at this point in the video and you haven't watched the destarching video before this, like why do we destarch and what is a starch test, I suggest you rather go and watch that video first. It'll give you context as to what I'm doing here. But this is the first example of a investigation and often there's like a question that's associated with the investigation. So this investigative question is, is light needed for photosynthesis? And there are two main ways that they can test this. And I'm going to show you the first way now. So first things first, we have a pot plant over here that we're going to use. And remember step one, we're always going to darch the plant, right? Because we want an initial value and we want validity. We want our variables to be all the same when we begin. The next thing we do is we are going to attach some foil. So, we're going to attach some foil. You could use black paper. You could use anything that's opaque, so it's not going to allow light through. And so, you attach it onto the plant's like a little pattern there. So, you can see like it's actually like working in the center of the leaf. And then you are going to allow it to photosynthesize. So, you're going to sit the plant in sunlight, right? and you're going to leave it there for 40 minutes, 2 hours, whatever the investigation suggests. Now, this next step to test whether or not anything has happened, step four is you are going to do a starch test. And when you do a starch test, as we can see in the result here, what you can perceive is this yellowy area. there is no starch whereas this green area has these like blue black areas on it which means that um photosynthesis did take place. Now, some errors that we make, and I did mention this in our uh destarching video, but I'm going to do it again in case you miss it, is that the color change. So, it going from uh brown to blue black is not it is not the dependent variable. Okay? It is not the dependent variable. Please, I can't tell you how many pupils. Write it down. The dependent variable actually is whether or not photosynthesis took place. So, it's actually just photo synthesis itself, like whether or not it actually took place. And so, you can write that out in a short couple of words or like a term or a phrase, but it's not to test um for starch itself. The starch is the indicator. And so we can see here that light is needed for photosynthesis because wherever we covered up the leaf, no photosynthesis took place. That's why it remained that yellowy color over here. But these like blue black areas does prove that uh light is needed because we then put a destarched pot plant in the sunlight and now we can see the results via a starch test. Now, I wanted to include this particular slide in this lesson because I wanted to see whether or not you were able to match the test with the outcome. So, this is one common way that examiners might set this up for you. You've got all of these leaves here and this is the test number one represents the test and they've told you they put a black paper strip on here. Now, what you need to do is you need to look at all the possible outcomes. So, we have got what? 2 3 four and five. These are the possible outcomes. Which one matches with which? Now remember what we're looking for is blue, black means that photosynthesis has taken place, right? That equals photosynthesis because it's positive for starch. Now we look back at our leaf over here and we see that well wherever the piece of paper is, there's no photosynthesis taking place. So it should be like a yellow color, right? Or a kind of brownish color. So if it's yellow, then there is no photosynthesis. So if we run through the options and we look at the yellow options here, we should easily be able to identify it almost instantly as number one because you see here it's got the yellow and so uh yellow indicates that um no photosynthesis take place took place there. Now, I know you might look over here and go, "Oh, but what about the green ones?" Well, if you successfully destarch the plant, you shouldn't get this outcome. And so, if they ask you, "Well, why are these ones green?" That's because you didn't dearchch the plant. And so, if they ask you for an error, that may be your error. The other issue here is in those same pictures is where the yellow is. It's actually lower down here, which doesn't make sense because as we look back at the uh the example leaf over here at number one, those were exposed to sunlight. So that doesn't make any sense. And what they've also done here with two and three is to confuse you because they're perfectly opposites of one another. Once again, we're looking for a blue black, which means photosynthesis took place. And you can see here that blue black is on the um outer areas of the leaf that was exposed to light. And that makes sense. So our correct answer is going to be number two. Number three is actually just a distractor to see whether or not you know your color changes because as you can see they've inverted them. Now the next way we do a light test is we are going to use a different investigation question. This time we are going to look at how does light intensity affect photosynthesis. Now once again we follow the steps. Step one, we're always going to de starch any plant that we are going to use. Right? So we put it in a cupboard for 48 hours. Right now step two in this instance we are going to place a pond weed into water. And I'm going to tell you why this is an easy one and why we do it in water. So we're going to place plant in water. The third thing that we are going to do is we are going to set up a lamp as you can see in the diagram at different distances. And that's why there is a ruler here so that we know how far away because the distance is going to equal the intensity of the light because the closer it gets towards our plant technically the faster the photosynthesis should occur. And as we move away, it should get technically slower. There are some other bits of information I'm leaving out here, like how does temperature affect this process, but let's assume that we're ignoring everything else. We're only looking at light intensity. And so, you would move this lamp towards or away from the plant. Now, what are you actually measuring here? So, in this instance, you don't actually have to do a starch test in this one. What you are looking for is the bubbles that are produced. So you're going to count the bubbles. And the bubbles are an indication of photosynthesis, but they are an indication of the rate. In other words, the speed of photosynthesis. because the plant is getting light regardless of where the lamp is, right? Whether it's close or far away. But for this experiment, we are focusing our energy on the rate or the speed at which it's happening. And so answering our invest investigative question, how does light intensity affect photosynthesis? Well, it affects it by changing the rate or the speed. Now to help you out as well, I also want to include what you would give for your independent variable and what would you give for a control. So let's start off with the variables. So for the independent variable, remember this is the variable that we are testing, right? And in this instance it is all about the light intensity because remember that is what we are going to see what is going to affect our plant. Now when it comes to the dependent variable this is the thing that we are measuring. Now it's really important to not get this one right. We are measuring the bubbles, but that is an indicator. So on the side here, I'm going to say the bubbles or the rate of the bubbles are an indicator. So they equal an indicator, but they indicate the dependent variable, which is actually the rate of photosynthesis. I'm just going to write here photo. A lot of pupils make this error where they use the speed or the rate of the bubbles as the dependent variable. And I understand why you would do that because aren't you measuring? So, don't you count the bubbles? Well, you do count the bubbles, right? You count how many are made in a certain amount of time. But that's an indication of photosynthesis. It's an indication of the speed at which photosynthesis is occurring. Um, and so that's a really tricky one for a lot of people. It's the same if you were thinking of something along the lines of the growth of a plant. Measuring its height is a measurement of its height. That's an indicator of its growth. And if we are trying to see what is growing the fastest or what produces the fastest growth, we would look at this indicator showing that. So, a lot of people get this wrong and so that's why I'm going over what the independent and dependent variables are. And then if we were to do a control for this experiment, remember a controlled experiment is one in which we can compare because that's the purpose of our control is to compare. Now, generally speaking, not always, but the most common way to do a control is to remove something. And in this instance, if we want a control experiment, we have to uh control or remove the light because that's what we're testing, right? In the independent variable. So, we remove the independent variable. And that means we would have a plant with no light. Either this plant is in a cupboard um or it's in like a dark room or something like that. But basically, you need to then have this plant in a space where there's no light. Now that might be difficult because counting bubbles would require you to be able to see the bubbles and therefore the light. But you kind of get the idea of the fact that you know the control is removing something so that you have something to compare to. Now let's move on to a variation of um an experiment we've just done now with the bubbles. And we're still looking at light but this time we're looking at does photosynthesis require light? And this experiment has already been done for us. So what we have here is a before and after. So on the left hand side, this is the before setup and this is the after setup. I want to start off with the before. Now remember in all photosynthesis experiments, what do you do? Step one, we darch, right? So, we take a pot plant, we put it in a cupboard for 48 hours. We um pluck some leaves off and we place them into some of these test tubes. And we do that, remember, because we want to make sure that all of the products of photosynthesis were produced during the investigation. The second thing is we are going to use an indicator liquid. Now in this instance we are going to use hydrogen carbonate which starts off orange and it will change color depending on how much CO2 there is. So the indicator is actually the CO2 content has it gone up down or remained the same and that's going to help us determine whether or not um photosynthesis has taken place. Now, if we look at the setup and we start off with test tube A. That is our control. There's no leaf inside of there, but there is the carbonate. We're going to use that later on, and I'll tell you why. The next setup is test tube B. We now have one of those leaves that we destarched earlier. It's positioned on top of some cotton wool, so that doesn't fall down into the uh indicator all the time and potentially stop the photosynthesis from happening. Then we have test tube C. And you'll notice on the outside of test tube C, we've wrapped it with tin foil. And the reason why we've wrapped it for tin foil or uh aluminum uh foil is we want to control the amount of light. And if you wrap something up in foil, there is going to be no light entering that test tube. Now, the last test tube, test tube D, is covered in gauze. And gauze, if you don't know what they are, it's like very like thin, almost seethrough material. And so it will allow some light to move into the test tube. So now we set up this experiment and we put this in a bright sunny place and we allow photosynthesis to occur for let's say a day or 12 hours and we come back. Right now we have the results on the other side. First things first, you will notice in test tube A there is no color change and that's because this first test was the control, right? It's the thing we want to compare to. We need to know if was there anything else inside that test tube that could have potentially affected the outcome of this experiment. And no, there isn't because it's remained orange. We then move into the next test tube B. And we're told that a purple color means that the carbon dioxide levels have dropped. Now, let's put two and two together. Photosynthesis needs carbon dioxide. Therefore, if it needs carbon dioxide, carbon dioxide is going to go into the leaf. It's going to leave the test tube. The air in the test tube is going to go into the leaf and that is going to cause the indicator fluid to change color because the carbon dioxide levels are dropping. Right? So, we've got a very successful or positive test to show that photosynthesis does require light because of the carbon dioxide levels dropping. Now, we know this also because if we look at the next test tube, test tube C, this was the test tube that was covered in tin foil. Remember, it's going to allow basically no light. No light whatsoever is going in. And you'll see that the color change indicates that the carbon dioxide level has risen. And now this is an important note for us to think here. If the carbon dioxide level has risen that means there is no photosynthesis happening here. Right? I'm just going to put here no photosynthesis. And actually what's happened is respiration of the leaf is actually higher than the rest the than the rate in which it's making oxygen which remember I mean using oxygen which remember they're doing both at the same time. Leaves are living things. They respire and they photosynthesize. But in this test tube, it can't photosynthesize. So, it can't drop the carbon dioxide levels. So, instead, all it's doing is going through respiration. It's making carbon dioxide and it's adding that carbon dioxide. So, it's leaving the leaf and it's going into the indicator, therefore changing the color of the indicator to yellow. Now, the most interesting one for us is probably D. D is showing no change. It's exactly the same as the control. But what does that actually mean? Well, this is where you really need to use your interpretation skills. Does that mean that there was a failure in this experiment because it remained or orange? Well, the answer is no. Because it was wrapped in that gauze, you know, like in that slightly transparent but not completely transparent substance. What that can indicate is that the amount of carbon dioxide being used by the leaf, so going into the leaf, is equal to the amount of carbon dioxide being made by the leaf because of cellular respiration. And they've equaled each other out or they've cancelled each other out. And so that's why there's no color change because the amount of carbon dioxide going into the leaf and the rate at which it's using it for photosynthesis is equal to the amount of oxygen that's being produced um when going through photosynthesis, right? And and then using that oxygen for for cellular respiration. So it's kind of canceled itself out. Now, if you're worried about getting something like this, don't worry. You don't have to know all of these indicators off by heart. They often give you a table of results and they tell you what the color change mean. You have to be able to interpret um the information given. So you have to know which is which and what does it mean. Now the last thing that I want to cover because again we always make this mistake and it's a very very very common mistake is what are the variables? What are the independent and dependent variables? We know what the control experiment was but what were the variables? So let's just recap that very quickly. Remember that the independent is the thing that we are testing and again in this one we are testing light. Do we require light for photosynthesis? But the dependent variable once again is not the carbon dioxide level because the carbon dioxide level is an indicator. Just like the bubbles from the previous experiment were an indicator. This is an indicator. Carbon dioxide. So please please please do not no write the color change. The color change is not your dependent variable. The dependent variable once again is photosynthesis or you want to be very specific the rate of photosynthesis or the efficiency of photosynthesis but it is not the color change that is just an indicator. Now as you can see photosynthesis is pretty tricky as an investigation question. There is a lot of moving parts and variables you need to keep your eyes on. I suggest that if you haven't already watched the destarching video, you go and click the link above now so that you understand how we always use the same test over and over again to determine whether this was a successful experiment. And I also suggest that you marry this knowledge with my cellular respiration videos which also can be found in the grade 11 playlist.