[Music] hi and welcome back to three size lessons cold ok by the end of this video you should be able to describe what happens in photosynthesis you should then be able to describe the factors that affect the rate of photosynthesis now I should point out that this is a tricky topic so you might need to watch this video a couple of times ok I'm showing you a plant here this brings us to the first key fact about plants plants use light for their source of energy the reaction that plants use to trap this light energy is called photosynthesis so because photosynthesis takes an energy it's an example of an endothermic reaction so if you imagine that this plant grew from a tiny seed lend like this one all of the energy that the seedling needed to grow came from the light energy trapped by photosynthesis photosynthesis takes place in the leaves of a plant and leaves contain the green chemical chlorophyll chlorophyll can absorb light energy so in the first stage of photosynthesis the plant takes carbon dioxide and water into the leaf like this light energy is then absorbed by chlorophyll this light energy is then used to convert the carbon dioxide and water into the sugar glucose and here it is in this reaction oxygen is also produced now this is called the word equation for photosynthesis aren't you expected to know this you also expected to recognize the chemical formulas for the molecules in this reaction the formula for carbon dioxide is co2 and the formula for water is h2o the formula for glucose is c6h12o6 and the formula for oxygen is old too now I should point out that this equation isn't balanced but you're not required to balance it for your exam so we're not going to look at that as we saw before in order photosynthesis to take place we need carbon dioxide and light so the question is what happens if there's not enough of these we're going to look at that now imagine that I take a plant and I increase the light intensity in other words the amount of light but I keep everything else constant I then measure the rate of photosynthesis at each level of light intensity when the light intensity zero the rate of photosynthesis is zero and that makes sense as plants need light to carry out photosynthesis as we increase the light intensity the rate of photosynthesis increases that's because the plant now has more light energy to carry out the photosynthesis reaction so the reaction gets faster now there's a key point here that you need to get if we increase the light intensity and the rate of photosynthesis also increases that tells us that the light intensity was limiting in other words photosynthesis was not as fast as it could have been because there wasn't enough light scientists say that at this point light intensity is a limiting factor now if we keep increasing the light intensity that comes upon where the rate of photosynthesis no longer increases and it levels off like this now at this point light intensity is no longer the limiting factor something else is now in short supply for example the level of carbon dioxide in the air we can now do the same experiment but this time increasing the level of carbon dioxide again keeping everything else constant we get a graph like this as you can see the shape of the graph is exactly the same as the one for light intensity as we increase the carbon dioxide level the rate of photosynthesis increases this tells us that carbon dioxide is the limiting factor however at a certain point the rate of photosynthesis no longer increases telling us that carbon dioxide is no longer the limiting factor now there are two other factors that can affect the rate of photosynthesis the first is the amount of chlorophyll in the leaf I'm showing you here a leaf which has got patches of chlorophyll because these leaves can trap less light energy than normal leaves they'll have a lower rate of photosynthesis kombucha can also affect the way to photosynthesis as we increase the temperature the enzymes involved in photosynthesis work faster so the rate increases however if we keep increasing the temperature the enzymes will denature and the rate of photosynthesis Falls remember you'll find plenty of questions on photosynthesis in my vision world book and you can get up I click on the link above okay so hopefully now you should be able to describe what happens in photosynthesis you should then be able to describe the factors that affect the rate of photosynthesis [Music]