Energy sustains life

All life on Earth depends on energy to sustain the seven life processes.

The form of energy that the Sun produces is called radiant energy. Although the Sun provides us with both light and warmth, plants only use the light energy from the Sun to photosynthesise.

Most organisms cannot directly use the energy from the sun to perform the seven life processes. For example, a reptile can lie in the Sun to warm up from the heat energy, but this does not provide the necessary energy for that animal to move, reproduce or excrete waste.

Except for a few sea slugs, plants are the only organisms on Earth that can absorb the Sun's radiant energy and convert it into food for themselves and for other living organisms.

Radiant energy to chemical potential energy

What is potential energy? Do you remember that we spoke about energy for movement (kinetic energy) and energy that is stored (potential energy) in Energy and Change in Gr. 6 and 7? What are some things that have kinetic energy and some that have potential energy? Remember to take down some notes in the margins of your workbook as you discuss things in class.

All living organisms can use energy in the form of chemical potential energy for the life processes. This is the energy that is stored in the food that organisms eat. Plants are able to capture the radiant energy from the Sun and transfer it to chemical potential (stored) energy for other organisms to use. They do this through the process of photosynthesis. All organisms release the stored potential energy from the food that they eat to support their life processes. This process is called respiration.

Photosynthesis takes place in small structures called chloroplasts, which are inside the cells of the leaves and stems of green plants. Inside the chloroplasts are green pigments called chlorophyll.This is what gives plants their green colour. Photosynthesis is the process in which chlorophyll molecules absorb the radiant energy from the sun and transfers it into chemical potential energy. The only function of chlorophyll is to trap the sunlight energy; chlorophyll is not produced or used up during photosynthesis.

Photosynthesis has other requirements besides light energy from the Sun. What are these? Look at the following diagram which summarises the process of photosynthesis.

Plants use radiant energy from the Sun in a series of chemical reactions to change carbon dioxide from the air and water from the soil into glucose. The process releases oxygen.

Requirements and products of photosynthesis

INSTRUCTIONS:

1. Summarise what you have learnt about photosynthesis in the diagram below.
2. Fill in the requirements of photosynthesis in the block on the left and fill in what type of energy is needed and the name of the pigment that absorbs the energy.
3. Fill in the products of photosynthesis in the block on the right.

The learner's diagram should look as follows:

The process of photosynthesis can be presented in the form of an equation:

What happens to the glucose that plants produce during photosynthesis?

Glucose storage and use

The glucose that a plant produces when it photosynthesises is the food for the plant. The plant can use this glucose directly, and release the energy during its own respiration or it can store the glucose or convert it into other chemical compounds.

Glucose is soluble in water. As we learnt in Matter and Materials in Gr. 6, this means that glucose can dissolve in water. This is useful to the plant as it means it can transport the glucose in water to where it is needed elsewhere in the plant. However, in order to store large amounts of glucose, plants need to convert it into compounds which are insoluble in water. Therefore the plant converts glucose into starch, which is insoluble in water. Why do you think the plant might need to store some glucose?

In addition to starch, plants also convert glucose into cellulose. Cellulose is used to support and strengthen plants. Animals do not have cellulose for support. Instead animals have something else to provide support and protect the body. Do you remember what this is?

Glucose is also converted into other chemical compounds that enable processes in the plant such as reproduction and growth.

We have now learnt about how plants produce glucose and store it as starch, but how do we know for sure? As young scientists we also need to question whether this explanation of photosynthesis is accurate. Is there an investigation we can do to test for the presence of these compounds? Let's find out!

We have learnt that plants produce glucose during photosynthesis and store this in the form of starch. Therefore, to see if a plant photosynthesises, we can test to see if the plant produced starch.

Study the following properties of starch and glucose with your class. Think of possible tests that can be done to determine whether a plant has produced either starch or glucose. Record some of your discussion points.

• Glucose tastes sweet but starch does not taste sweet at all.

• Glucose will dissolve in water while starch will not dissolve in water.

• Iodine changes from brownish-orange to dark blue-black when it comes into contact with starch. Have a look at the following photos which illustrate this.

Now that we know that plants produce glucose and change this into starch, we can find out if all leaves produce the same amount of starch through photosynthesis.

Which leaves photosynthesise?

You will need variegated and normal leaves for this investigation. Variegated leaves have white patterns (areas lacking chlorophyll) on them. There are many examples of South African plants that have variegated leaves, such as some geraniums, African violets, ivy, etc. You can also take a walk around your school property and surrounds to see if you can find any variegated leaves. Get learners to look at the leaves and discuss the investigation first. They can also do this in groups. This investigation can be done over 2 lessons. You should place one set of pot plants in the cupboard the day before you want to do part 1 of the investigation. After you have done part 1, you can do part 2 in the following lesson. In part 2, learners will need to write up an experimental report. If you do not have time to do both parts of the investigation, a suggestion is to get your learners to read through Part 1, and then to conduct Part 2 where they have to write up their own report.

There are two parts to this investigation. First, we want to find out which leaves are able to photosynthesise. We will place some pot plants in the light for a day, and some other pot plants in a dark cupboard for a day, and then perform the investigation on the leaves of plants from both groups.

In the second part of the investigation, we will use what we have learnt to investigate which parts of variegated leaves photosynthesise.

Part 1: Leaves in light and dark

AIM:

What do you wish to establish by conducting this investigation?

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Learner-dependent answer

Possible answers include: 'To determine whether leaves photosynthesise in the dark or the light', or 'To investigate whether Light is necessary for Photosynthesis'.

HYPOTHESIS:

What do you think or predict will happen when you conduct this investigation?

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Learner-dependent answer

The leaves in the light will test positive for starch as they photosynthesised, whereas the leaves in the dark will not photosynthesise and will test negative for starch.

MATERIALS AND APPARATUS:

• gloves
• a range of pot plants that can be easily moved around
• 100 ml beaker or glass jar in a saucepan with water
• bunsen burner, spirit lamp or a stove
• tweezers
• ethyl alcohol (or methylated spirits)
• glass petri dishes, white saucer or white tile
• stopwatch or timer
• glass pipette or dropper
• iodine solution

METHOD:

Before starting this investigation, all plants must be placed in a dark cupboard for up to 48 hours prior to starting the investigation in order to ensure that the plants do not have starch in the leaves before the process begins.

1. Work in groups of three or four.
2. Place half of the plants in the dark for at least 24-48 hours and the others in a well-lit area of the class that is exposed to lots of natural sunlight.
3. After 24 hours, pour 50 ml of the ethyl alcohol into the beaker and place it in the saucepan with water. Heat the saucepan over the bunsen burner or the stove. The water in the saucepan will distribute the heat evenly to warm the ethyl alcohol evenly.

Instead of a saucepan with water and a beaker with alcohol, teachers may use a beaker with water and a test tube with meths or alcohol. As long as the container holding the alcohol is safely contained within the water to prevent it from being directly heated or coming into contact with the flame. Teachers may also feel safer if they demonstrate this experiment.

1. Remove one healthy looking leaf from the pot plants that were in the well-lit area exposed to direct sunlight.
2. Using the tweezers, dip a leaf into the boiling water for 1-2 minutes. This helps to remove the waxy cuticle that covers the leaf and breaks down the cell walls.
3. After this, place the leaf into the beaker with the ethyl alcohol.

Warning: The alcohol needs to be heated as well but it cannot be heated directly because it is extremely flammable. It therefore needs to be heated in a water bath.

1. Leave the leaf in the alcohol until all the chlorophyll has been removed from the leaf and the alcohol turns green.
2. Place the leaf into warm water to soften it.
3. Remove the leaf from the warm water and place it on a white tile or a petri dish on top of a white surface.
4. Use the pipette or dropper to carefully drop 2 or 3 drops of iodine solution on the leaf in the petri dish and record your observations.

The amount of iodine solution required to observe a result depends on the leaf tested. Some leaves may need to be flooded with iodine solution. Place the leave in a petri dish and cover the leaf with iodine.

1. Repeat this process for two more leaves that were in the well-lit area.
2. Remove the plants that were in the dark for at least 24 hours. Use the test above to test whether there is starch present in the leaves from the plants that were kept in the dark.
3. Record your observations.

A really good demonstration of the various experiments in this and subsequent sections is found at . It starts off with carbon dioxide through limewater test and then demonstrates the starch test.

RESULTS AND OBSERVATIONS:

Keep a record of your observations. Draw a table to record and compare your results.

Learners must draw their own tables to record their observations. This being the first investigation that they will perform in high school, this activity will allow teachers to gauge their level of proficiency and abilities in this regard.

The hot water removes the waxy cuticle and the alcohol dissolves the chlorophyll, releasing the green colour of the leaf. After the leaves are taken from the ethyl alcohol they should be white. The chlorophyll needs to be removed from the leaf so it does not mask the colour change we expect with the iodine solution. When iodine is dropped on the leaf it turns blue-black in the presence of starch.This is an indicator to show that the leaf was photosynthesizing and producing glucose that was turned into starch.

CONCLUSION:

What did you learn from doing this investigation?

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Plants that were given sunlight were able to photosynthesise and therefore tested positive for starch (iodine solution turned blue-black). Those that did not receive radiant energy did not photosynthesise so they tested negative for starch presence (iodine solution remained yellow-brown). Therefore, light is necessary or required for photosynthesis to occur.

QUESTIONS:

Why were some plants placed in a well-lit area with direct sunlight and others in the dark?

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This was done to allow some plants to photosynthesise and the others not. Thus those that were able to photosynthesise could produce starch and those that didn't only had small amounts of starch or no starch.

Explain what the results of the iodine test indicates.

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If the iodine changes from brownish-orange to dark blue-black that indicates that the leaf or other part of the plant contains starch and must have photosynthesised. If the iodine solution does not change colour then the leaf or other part of the plant does not contain starch and did not photosynthesise.

Part 2 : Which parts of variegated leaves photosynthesise?

Learners need to apply what they have learnt in Part 1 to plan their own experiment in part 2 and write up an experimental report.

Have a look at the following photos of different plants. What do you notice about the leaves?

Geranium leaves are an excellent option for this investigation. The ivy has a very thick cuticle and the iodine can't penetrate the leaf easily. Variegated mint-leaf is also very good for this investigation, and is a common plant in Kwa-Zulu Natal.

We call these leaves variegated as they have green and white sections. We want to find out which parts of these leaves photosynthesise in this part of the investigation.

INSTRUCTIONS:

1. You need to design this investigation yourself.
2. First decide what question you are trying to answer and the aim of your investigation.
3. Make a hypothesis for your investigation.
4. You then need to think back to part 1 and design the method for your investigation.
5. After conducting the investigation, you need to write up an experimental report of your findings.
6. In your report, you must have the following headings:
   1. Aim
   2. Hypothesis
   3. Materials and apparatus
   4. Method
   5. Results
   6. Discussion
   7. Conclusion
7. In your results section you need to record your observations in a scientific way. You can do this using a table, diagrams or a combination of both. Think carefully about what information you need to record in order to come to conclusions at the end of your experiment.
8. In your discussion, you need to explain your results and what they mean. You also need to evaluate your investigation and explain if there were any unusual results and suggest ways that you could have improved your investigation for future researchers who might want to repeat what you have done.
9. Present your report on separate paper.

Leaves are not the only parts of plants that store starch. Starch is also stored in the stems, roots and fruit. Have you ever wondered why fruit becomes sweeter as it ripens? Think of an unripe green banana and a ripe yellow banana. Which one is sweeter? Let's find out why.

Why do bananas become sweeter as they ripen?

It is not crucial to do this investigation if you do not have time. This is an optional extension of the starch test.

In this investigation we will taste the bananas to determine if they have more glucose or more starch. We will also conduct a starch test on the ripe and unripe bananas to see which contain more starch.

AIM:

What do you wish to establish by conducting this investigation?

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Learner-dependent answer

An example of a possible answer is: To investigate the presence of starch in ripe and unripe bananas; To investigate why bananas becomes sweeter as they ripen; etc.

HYPOTHESIS:

What do you think or predict will happen when you conduct this investigation?

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If the unripe banana does not taste so sweet compared to the very sweet tasting ripe banana then perhaps it contains more starch, and less glucose. The starch test might show that the unripe banana contains more starch.

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MATERIALS AND APPARATUS:

• ripe and unripe bananas cut into discs
• petri dish or saucer
• iodine solution
• dropper

METHOD:

Work in groups of three or four. Take a piece of the ripe banana and a piece of the unripe banana and compare the tastes and textures of each. Record your observations in a table. Which banana do you think contains the most starch and the least glucose (a sugar) based on the taste test?

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Learner-dependent answer

Learners should be able to describe the taste and texture of each banana: the ripe one being soft and sweet, and the unripe one being firm and not too sweet. Learners might say that the unripe banana contains more starch and less glucose as it is not as sweet as the ripe banana. But, this is not a very accurate test. The learners might also experience the tastes differently, making the results unreliable.

1. Use the iodine starch test identify which banana, the ripe or the unripe one, contains the most starch. Record your observations in the table.

Learners should be able to quite easily see this based on the speed with which the iodine changes colour in the unripe banana.

1. Compare this test to the results from your taste and texture test to identify which banana contained the most starch.

OBSERVATIONS:

Draw a table to record your observations from the taste and iodine test for starch.

Learner-dependent answer

Allow learners the freedom to tabulate their results in any way they choose as long as it is easy to interpret and understand. Use this as a teaching opportunity to help those who cannot do this and when everyone is done compare the different methods used in the class.

QUESTIONS:

Compare your observations of ripe and unripe bananas with those of the other learners in the class. Did you all make the same observations?

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Learner-dependent answer.

You should use this comparison to help learners understand that reliability of an experiment rests on the fact that although different people perform the same test they should all reach very similar results.

What do you conclude from these results? Which method of testing is better to use and why do you say so?

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The conclusion is that unripe bananas contain more starch than ripe bananas. This question was specifically included to introduce learners to the concept of validity and teachers are encouraged to allow learners to debate this issue in the class.

Explain what you think happens to the starch as the bananas ripen.

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As the bananas ripen, the starch is converted into glucose.

Now that we have looked at how green plants produce their own food, let's find out how all living things release the energy stored in food in order to perform the life processes.

Energy from food

Our bodies need energy to move and do work. Where do we get our energy from? The energy is obtained from the food that we and all other organisms eat.

If you think back to the work you did on fuel and energy in previous grades in Energy and Change, you will remember that fuels, such as wood, coal, and oil, contain chemical potential energy. When this fuel is burned in the presence of oxygen, the chemical potential energy is transferred into light and heat energy. In the same way, the glucose from the food that you eat is combined with oxygen in a series of chemical reactions to release the energy. The glucose is broken down and the energy is released. This energy is then used to drive all the other processes in your body. This process is called respiration. We can define respiration in all living organisms as the process by which energy is released from glucose in a series of chemical reactions.

Respiration takes place in all organisms, even plants. However, plants do not need to eat any food as they make their own food during photosynthesis.

Products of respiration

Do you remember how we represented photosynthesis as an equation to show what goes in and what comes out? We can represent respiration as an equation in the same way as we did for photosynthesis.

We know what is required for respiration to take place in all organisms. List the two ingredients for respiration.

However, respiration does not only produce energy. It also produces water and carbon dioxide as by-products. We can write an equation for respiration as follows:

glucose + oxygen → carbon dioxide + water + energy

During photosynthesis in plants, oxygen is produced as a by-product. We call it a by-product as it is not the main product that is wanted from the process. In photosynthesis, the main product that is required from the process is glucose. What are the by-products in respiration?

The carbon dioxide that is produced in the body of an organism during respiration needs to be removed. In humans, we do this by breathing out carbon dioxide-rich air. We will learn more about the whole respiratory system next year in Gr. 9, and how breathing, our blood circulation system and respiration all work together as one system within our bodies.

We can test for the products of respiration using our own breath. So how do we test that our breath contains carbon dioxide? It is a colourless gas, so we cannot see it directly.

There is a very well known test for detecting carbon dioxide using clear limewater. To test if a gas contains carbon dioxide, simply bubble the gas through limewater. If the clear limewater turns milky, then the gas contains carbon dioxide. Next term in Matter and Materials, we will look at this again and find out about the chemical reaction taking place in the test. For now, let's use this test to show that our breath contains carbon dioxide.

Does our breath contain carbon dioxide?

Watch a video of this activity.

You will need to prepare limewater prior to this activity. Here are instructions on how to do this:

1. Place a few tablespoons of calcium hydroxide, Ca(OH)₂, in a clear 500 ml reagent bottle and fill with water. Shake or stir to make a cloudy suspension.
2. Leave the suspension to settle for a few days. The clear liquid above the solid Ca(OH)₂ is a saturated solution of Ca(OH)₂, also known as clear limewater.
3. Carefully decant as much of this as you need, without stirring up the solid Ca(OH)₂ sludge at the bottom.
4. To make more, simply add more water, shake it up and let it settle again. When the sludge dissolves completely, add more solid Ca(OH)₂.

MATERIALS:

• small beakers (or test tubes)
• rubber tubes or drinking straws

For health reasons, there should be one straw or rubber tube per learner.

• limewater
• 20 ml syringe (or larger if available)

Pharmacies are generally quite helpful and will assist schools in purchasing low cost syringes. Technology teachers at your school might also have syringes for their syringe mechanics lessons. An alternative to a large syringe is to use a bicycle pump or balloon blower. Safety warning: remove the needles from the syringes, if there are any, before you hand out the syringes in class.

INSTRUCTIONS

1. Work in groups of three.
2. Mark one beaker AIR and the other BREATH.
3. Pour clear limewater into each beaker until they are half full.
4. Blow bubbles through the rubber tube into the beaker marked BREATH, as shown in the diagram. Do this for at least 1 minute. Notice what happens to the clear limewater.

1. Attach a rubber tube to the front of a syringe. Draw air into the syringe from the atmosphere.
2. Place this rubber tube into the beaker marked AIR and push out the air inside the syringe slowly and carefully into the limewater as shown in the diagram. Notice what happens to the clear limewater.

QUESTIONS

Describe what you observed when you blew air from your lungs through the limewater. What does this mean?

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The clear limewater turned cloudy white. This means our breath contains carbon dioxide from respiration.

Describe what you observed when you used the syringe to bubble air from the atmosphere through clear limewater.

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The clear limewater did not change and remained clear. Some might notice a very slight change, which indicates that there is a small amount of carbon dioxide present in atmospheric air.

A very small percentage of atmospheric air is carbon dioxide gas (0.03). Why do you think you observed the result you did when you pushed air from the atmosphere through the limewater?

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As air contains such a small percentage of carbon dioxide, there is not enough to cause a noticeable difference in the limewater. Discuss with your learners why they think that the limewater turned cloudy with your breath but not with the air, even though air does contain carbon dioxide. Point out that air from your lungs contains a much higher percentage of carbon dioxide from respiration and so this is able to turn the limewater cloudy in a shorter time than atmospheric air will.

Think about respiration.

1. What are the requirements for respiration?

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2. What are the products of respiration?

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1. Glucose and oxygen.
2. Energy, carbon dioxide and water.

Requirements and products of respiration

INSTRUCTIONS:

1. Summarise what you have learnt about respiration in the summary diagram below.
2. Fill in the requirements of respiration in the block on the right.
3. Fill in the products of respiration in the block on the left.

This is what learners' diagrams should look like:

Have you noticed the VISIT boxes in the margins which contain links? You simply need to type this whole link into the address bar in your internet browser, either on your PC, tablet or mobile phone, and press enter, like this:

It will direct you to our website where you can watch the video or visit the webpage online. Be curious and discover more online at our website!