KS3 Biology - Food Chains

Added: 2026-05-05

[00:00:07]In this video, we're going to learn about food chains.

[00:00:10]Specifically, we're going to find out what food chains show, how energy is transferred and lost through food chains, and also the effect of toxins in food chains as well.

[00:00:24]Starting with what food chains show, a food chain tells us the feeding relationships between certain organisms in an ecosystem, which basically means it shows us what an organism eats and what that organism is eaten by.

[00:00:38]Let's take a look at an example. Here, we can see four different organisms that live in the same ecosystem.

[00:00:44]So, in this one, we've got grass, a rabbit, a snake, and a hawk.

[00:00:48]We use arrows in food chains to show who eats who.

[00:00:51]For instance, here, grass is eaten by rabbits, so we would add an arrow pointing from the grass to the rabbit.

[00:00:58]Rabbits are eaten by snakes, so we would add an arrow pointing from the rabbit to the snake, and finally, snakes are eaten by hawks, so we would add an arrow pointing from the snake to the hawk.

[00:01:09]Another thing the arrows show, apart from simply who eats who, is the transfer of energy through an ecosystem.

[00:01:15]What this means is that when rabbits eat grass, they gain the energy from the grass and can use it themselves for things like movement.

[00:01:22]And in this way, energy is transferred throughout ecosystems by things eating each other.

[00:01:28]It's really important, however, to note that not all energy is transferred from one organism to the next in a food chain.

[00:01:33]So, not all the energy is going to be transferred from the grass to the rabbit and from the snake to the hawk, because some energy is lost at every stage.

[00:01:42]This means that less energy is available to the other organisms further along the food chain.

[00:01:47]So, at this point, let's take a look at the different ways that energy is lost.

[00:01:51]The first is that energy is used for movement.

[00:01:54]For example, as the rabbit moves around to eat different patches of grass, it's going to use up some of the energy that it gained from the grass in the first place.

[00:02:01]This means that when the snake eats the rabbit, not all of the energy that the rabbit got from the grass will be available to the snake.

[00:02:09]Energy is also released during respiration, which allows energy to be used for things like growth and reproduction.

[00:02:15]And just like before, when this energy is used up, it won't be available to the next organism in the food chain.

[00:02:21]The final way is that during movement and other processes, some energy is lost into the environment as heat as a side effect. Just like how when you run around, your body warms up. This is your body losing heat to the environment and effectively losing some energy as it does so.

[00:02:36]All of these different factors combined together to mean that organisms at the end of food chains, especially really long food chains, don't get very much of the energy that was in the food chain to begin with.

[00:02:50]Next, let's take a look at the effects of toxins in food chains.

[00:02:54]The word toxins means poisonous substances, and they're really dangerous for an organism's survival. Large amounts of toxins can even cause an organism to die.

[00:03:03]The issue with toxins is that they can enter food chains and be passed, just like energy, from one organism to the next when the organism that picked up the toxins in the first place is eaten by another organism.

[00:03:14]Toxins can first enter food chains either by being eaten or by being absorbed, such as by a plant.

[00:03:20]For instance, the leaves of this plant might have absorbed some toxins from the soil that the plant is growing in.

[00:03:26]Then, when the rabbit eats the leaves, the toxins will be passed onto the rabbits.

[00:03:30]When this lion eats those rabbits, the toxins will then also be passed onto the lion.

[00:03:36]In this case, the red arrows are showing the movement of toxins through a food chain, rather than the flow of energy.

[00:03:42]Now, you might have noticed that there's a higher level of toxins in the lion than there are in each of the rabbits, which in turn each have more toxins than each of the individual leaves do.

[00:03:51]This is because they can't be broken down or excreted by an organism in their waste fast enough, so the toxins build up in each organism more and more as we move along the food chain.

[00:04:01]The rabbit eats so many leaves that they get all of the toxins from each of the individual leaves.

[00:04:06]The lion is a top carnivore found right at the end of a food chain, which means that it isn't eaten by any other organism.

[00:04:13]Because the lion eats lots of rabbits, it gets the toxins from all of the leaves and the rabbits combined.

[00:04:19]As toxins build up when we move along food chains, top carnivores like the lion will be most affected by toxins.

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