Hello and welcome to another complete Cambridge IGCSE biology lesson where you'll learn absolutely everything you need to know on topic 19.1 energy flow and 19.2 food chains and food webs. As always we'll be following the Cambridge syllabus exactly and we'll cover absolutely everything you need to know for your final exam. For this lesson, you need to describe the flow of energy through living organisms, construct and interpret food chains and food webs, describe the terms producer, consumer, herbivore, carnivore, decomposer, and trophic level, describe the impact of humans on food chains, and describe and interpret pyramids of numbers and pyramids of biomass. For extended, you also need to describe and interpret pyramids of energy, understand why energy transfer between organisms is inefficient, and why it's more efficient from an energy perspective to consume crop plants than animal products. The sun is the principal source of energy input to biological systems.
Photosynthesizing plants harness sunlight to produce chemical energy, which is stored in molecules like carbohydrates, proteins and fats, and then passed on from organism to organism through food chains. When an animal eats a plant, it digests, absorbs and assimilates some of the nutrients contained within it. If that animal is eaten by a larger animal, the chemical energy locked up in its tissues is passed on again.
Eventually, all the energy in organisms is transferred to the environment. Much is lost as heat, generated during respiration, or released by decomposers like fungi and bacteria. So a food chain is a diagram that shows the transfer of energy in a biological system from one organism to the next.
A producer is an organism that makes its own organic nutrients. usually using energy from sunlight through photosynthesis. Since all the energy in a biological system comes from the sun, a food chain always begins with a producer.
A consumer is an organism that gets its energy by feeding on other organisms, so every member of a food chain other than the producer is a consumer. Consumers may be classed as primary, secondary, tertiary and quaternary, according to their position in a food chain. The position of an organism in a food chain, food web or ecological pyramid, which we'll return to later, is referred to as trophic level.
The first trophic level contains producers, the second primary consumers and so on. The primary consumer is a herbivore, which is an animal that gets its energy by eating plants. The secondary, tertiary and quaternary consumers are all carnivores, which are animals that obtain their energy by eating other animals.
Organisms that obtain their energy from dead or waste organic material are called decomposers. In this food chain, the grass obtains its energy from the sun by photosynthesis. Some of the chemical energy stored in the grass is transferred to the grasshopper, then to the bird, then the snake, and finally to the owl. When the owl dies, decomposers break down its body, making nutrients available to producers. In a food chain, the arrows between organisms always point in the direction of energy transfer.
from food to consumer. Now food chains like these are generally too simplistic, as most organisms have more than one predator, and or more than one prey. For example, an owl feeds not only on snakes, but also rabbits, mice and frogs. Grass not only sustains grasshoppers, but also mice, snails and worms. In order to more accurately depict these relationships, a food web can be drawn, which is essentially a network of interconnected food chains.
The producers are placed at the bottom of the diagram and rows of consumers are placed above. First the primary consumers and then the secondary and so on. Arrows can then be added to show the transfer of energy through the system. Just like with food chains, these arrows always point from food to consumer.
Now since all the plants and animals in a food web are connected in some way, changes to the population of one organism will have knock-on effects elsewhere. This can be demonstrated by looking at the impact of human activities, like over-harvesting, and the introduction of new species to habitats. For example, over-harvesting of food species like arctic cod reduces the amount of food available to seals, affecting their population, which could in turn cause polar bear numbers to decline.
Introducing a new species like hawks to control a population of rabbits that are causing damage to agricultural crops, may cause foxes to eat more mice to make up for the lack of rabbits in their diet. Since owls also eat mice, this could have a knock-on effect on their numbers as well. Now there are usually large numbers of organisms at the beginning of a food chain, and small numbers of organisms at the other. Pyramids of numbers show this relationship, with the width of the bands representing the number of organisms at each trophic level. Here, millions of microscopic phytoplankton are consumed by larger and less numerous zooplankton, like krill, which are in turn eaten by small fish like sardines.
These sardines can only sustain a small number of dolphins, since each dolphin needs to eat a lot of sardines to survive. Now pyramids of numbers don't always take the form of a pyramid with a broad base. In this example, a single oak tree sustains huge numbers of caterpillars, so the shape of the pyramid is somewhat inverted.
To get around this issue, pyramids of biomass can be used instead. The width of the bands in a pyramid of biomass show the mass or dry weight of organisms at each trophic level. Pyramids of biomass therefore give a clearer picture of the amount of plant or animal material at each trophic level of a food chain and are nearly always pyramidal in shape. Okay so that's everything for core, so we'll move on now to the extended content.
A pyramid of energy is used to show the flow of energy from one trophic level to the next within a food chain. Starting with the producers, energy is transferred from one trophic level to the next, but only a small proportion is passed on each time. In fact, only around 10% of the original energy is available to the organism at the next level. In other words, the transfer of energy from one trophic level to another is very inefficient, so much so that biological systems rarely exceed 5 trophic levels. There simply isn't enough energy left in the system to sustain a 6th.
This is because nutrients are used up during respiration to provide energy for movement, the maintenance of a constant internal body temperature, and other life processes. In addition, much of the food cannot be digested, so a significant proportion of energy never even makes it to the next trophic level. Now since energy is lost at each trophic level, it makes sense that the shorter a food chain, the more efficient it is. For example, it's more energy efficient for humans to eat crops plants like corn or wheat than eggs, meat or dairy products from animals fed on these grains.
This is because only around 10% of plant material is converted to animal products. Well done, you've just covered absolutely everything you need to know on topic 19.1 energy flow and 19.2 food chains and food webs. If you enjoyed this video I'd really appreciate your subscription and I'll see you next time for topic 19.3 nutrient cycles.