Miss Casillo
Welcome to 6th grade
Matter and Energy
One of the most general distinctions among organisms is between plants, which use sunlight to make their own food (photosynthesis) and animals, which consume energy-rich foods. Photosynthesis and cellular respiration are complementary processes. Plants carry on photosynthesis and cellular respiration where food is broken down into energy. The requirements of one process are the products of the other.
Photosynthesis Cellular Respiration
Food accumulated Food broken down
Energy from sun stored in glucose Energy of glucose released
Carbon dioxide taken in Carbon dioxide given off
Oxygen given off Oxygen taken in
Produces glucose Produces carbon dioxide and water
Goes on only in light Goes on day and night
Occurs only in the presence of chlorophyll Occurs in all living cells
Plants and photosynthesis
Before we look at food chains we will go over the way green plants capture energy from the Sun to make food. This is the start of all the food chains we will look at.
Plants and photosynthesis
Animals eat food to get their energy. But green plants don't. Instead they make their own food, glucose, in a process called photosynthesis. We say that plants can photosynthesise.
These are the things that plants need for photosynthesis:
-
carbon dioxide
-
water
-
light
These are the things that plants make by photosynthesis:
-
glucose
-
oxygen
We can show photosynthesis in a word equation, where light energy is shown in brackets because it is not a substance:
carbon dioxide + water (+ light energy) → glucose + oxygen
Plants get carbon dioxide from the air through their leaves, and water from the ground through their roots. Light energy comes from the sun.
The oxygen produced is released into the air from the leaves. The glucose produced can be turned into other substances, such as starch, which is used as a store of energy. This energy can be released by respiration.
Getting carbon dioxide, light and water
Remember that the equation for photosynthesis is:
carbon dioxide + water (+ light energy) → glucose + oxygen
Let's see how plants get the carbon dioxide and water they need for this process.
Carbon dioxide
Plants get carbon dioxide from the air through their leaves. The carbon dioxide diffuses through small holes in the underside of the leaf. The lower part of the leaf has loose-fitting cells, to allow carbon dioxide to reach the other cells in the leaf. This also allow the oxygen produced in photosynthesis to leave the leaf easily.
Light
A leaf usually has a large surface area, so that it can absorb a lot of light. Its top surface is protected from water loss, disease and weather damage by a waxy layer.
Water
Plants get the water they need for photosynthesis through their roots. If a plant does not absorb enough water, it will wilt or go floppy. Without water it may also not photosynthesise quickly enough, and it may die.
Respiration in plants
All living things get the energy they need to live from a chemical reaction called respiration. This process needs glucose as a starting point. First we'll see how respiration works, and then look at how plants use respiration in combination with photosynthesis.
Respiration
Living cells respire. Aerobic respiration is the chemical reaction used to release energy from glucose. It is called aerobic because oxygen from the air is also needed.
Here is the word equation for aerobic respiration. Energy is put in brackets because it not a substance:
glucose + oxygen → carbon dioxide + water (+ energy)
Notice that the word equation for respiration is the reverse of the word equation for photosynthesis.
Plants
Plant cells respire, just as animal cells do. If they stop respiring, they will die. Remember that respiration is not the same as breathing, so take care - plants do not breathe.
As we can see from the word equations respiration and photosynthesis are opposites. Respiration uses oxygen and produces carbon dioxide. Photosynthesis uses carbon dioxide and produces oxygen.
So what happens to a plant overall? This depends on whether it is in the dark or the light, and how bright the light is.
Plants respire all the time, whether it is dark or light. They photosynthesise only when they are in the light.
Food chains
A food chain shows the different organisms that live in a habitat, and what eats what.
Producers and consumers
A food chain always starts with a producer, which is an organism that makes food. This is usually a green plant, because plants can make their own food by photosynthesis.
A food chain ends with a consumer, which is an animal that eats a plant or another animal. Here is an example of a simple food chain:
grass → cow → human
Take care - the arrow points to the organism that is doing the eating. If you get the arrows the wrong way round, instead of showing that humans eat cows, you are showing that cows eat humans, and that grass eats cows.
| Grass | → | Grasshopper | → | Frog | → | Hawk |
|---|---|---|---|---|---|---|
Predators and prey
A predator is an animal that eats other animals, and the prey is the animal that gets eaten by the predator. In the food chain above:
-
the frog is a predator and the grasshopper is its prey.
-
the hawk is a predator and the frog is its prey.
Pyramids of numbers
The population of each organism in a food chain can be shown in a sort of bar chart called a pyramid of numbers. The more organisms there are, the wider the bar. The producer in the food chain always goes at the bottom of the pyramid of numbers.
Think about this food chain:
clover → snail → thrush → hawk
Clover is a plant and it is the producer in this food chain. Its bar goes at the bottom of the pyramid:
Energy is lost to the surroundings as we go from one level to the next, so there are fewer organisms at each level in this food chain. A lot of clover is needed to support the snail population. A thrush eats lots of snails, and a hawk eats lots of thrushes, so the population of hawks is very small.
Food webs
When all the food chains in a habitat are joined up together they form a food web. Here is an example of a food web:
Although it looks complex, it is just several food chains joined together. Here are some of the food chains in this food web:
grass → insect → vole → hawk
grass → insect → frog → fox
grass → insect → vole → fox
Notice that the frogs, voles and insects have more than one predator, but the rabbits and slugs have just one predator.
This leads to some interesting effects if the population of a particular organism in the food web decreases. Some animals can just eat more of another organism if food is in short supply, while others may starve and die. This in turn can affect the populations of other organisms in the food web.
Changes to food webs
Let's look at the food web again and ask some questions about its predator-prey relationships, with some possible answers.
What would happen if the grass died?
The grass is the producer, so if it died the consumers that feed on it - rabbits, insects and slugs - would have no food. They would starve and die unless they could move to another habitat. All the other animals in the food web would die too, because their food supplies would have died out. The populations of the consumers would fall as the population of the producer fell.
What would happen if the population of slugs decreased?
Slugs, rabbits and insects all eat grass. If there were fewer slugs there would be more grass for the rabbits and insects. With more food the populations of rabbits and insects would increase. However, the thrushes would have to eat more insects to maintain their population, so it is also possible that the population of insects could decrease. This is turn may reduce the populations of voles and frogs.
What would happen if the population of insects decreased?
There would be more food for the rabbits and slugs, so their populations would increase. However, there would be less food for the frogs and voles, so their populations would decrease. This means less food for the foxes and hawks. However, there are likely to be more rabbits and thrushes for them to eat, so their populations are likely to stay the same.
