phototropism experiment gcse

• Plant Biology
• Human Biology
• Biology Plant Biology Phototropism Experiment

Experiment: How do plants “see” light?

Can plants really “see” the light.

Scientists call a plant’s ability to bend toward light phototropism . Even as far back as ancient Greece it’s been a big puzzle about how plants are able to do it. People experimented with how plants accomplish this amazing feat, but no one really figured out how it worked—until Charles Darwin came along, that is.

Although Darwin is most well-known for his studies on evolution, he was also a prolific scientist in general. The questions about phototropism piqued his curiosity, and he thought of an ingenious experiment to test how plants are able to see light. In this experiment, we’ll recreate what he did, and at the end we’ll dive further into the science.

3 small cups full of soil Tape, a marker, and 3 sticky notes Medium-sized box (such as a shoebox or a storage cube) 12 corn seeds Aluminum foil Small cookie sheet that fits inside the box (or another sheet of aluminum foil) 1 Straw Water

• Plant four corn seeds in each of the soil cups. Make sure they’re evenly spaced, and plant them just a half inch under the dirt.
• Water the cups, and dump out any excess water (be careful not to tip the soil and seeds out). Place the cups on the cookie sheet or aluminum foil. This will prevent moisture and dirt from soaking through the box.
• Place the cups/cookie sheet setup inside of the box. Make sure it’s open on one side so that light is coming in from an angle. Place in a windowsill, with the open side facing the sun. (You might need to stack some books underneath it to support it, if your windowsill isn’t very wide.)
• Shoot cap: Cut a small 2″ x 3″ square of aluminum foil. Wrap it around the tip of a straw to create a small, closed-ended metal cap, and slide it off. This will be placed over the tip of the growing shoot to cover any light coming in to the tip.
• Base sleeve: Cut a small 1/2″ x 3″ square of aluminum foil. Wrap it around the middle of a straw so it creates a small open-ended 1/2″ tall tube, and slide it off. This will be placed around the growing shoot so that it can grow through it.
• Check the cups each day. Once they send up a shoot about half an inch high, place either a shoot cap (on Tip seedlings) or a base sleeve (on Base seedlings) around them, depending on which cup they’re in. The control cup will get neither of the light exclusion devices. The seedlings might grow at different rates, so be sure to check each day to put the caps/sleeves on as needed. They grow fast once they germinate!
• Continue to water the seedlings as needed.
• Check the seedlings after a week. What has happened? Compare the seedlings with the caps and the sleeves to the control seedlings. Are any of them growing in certain directions?

How did the seedlings “see” the light?

If the experiment worked correctly, you should have noticed that the seedlings that were covered with caps at the tip grew straight up, while the control seedlings and the seedlings with the bases covered bent towards the light. This is phototropism in action.

Darwin correctly concluded that plants are able to “see” light using the tips of the plant shoots, rather than through the stalks. It wasn’t until a bit later that scientists figured out exactly why that was, though.

It turns out that plants are able to grow by using hormones such as auxins and gibberellins . Auxin in particular tells individual cells to reach out and grow longer, like Stretch Armstrong. It’s one of the ways that plants grow taller. Normally, plants growing with an unshaded light source will grow straight up towards the sun because auxin is evenly distributed all around the shoot.

But when the light is heavily shaded and comes in from an angle, something interesting happens. Auxin starts to concentrate on the shaded side of the plant instead, and as a result, the cells on the sunny side stay the same size but the cells on the shaded side grow longer. This causes the plant to tip and grow towards the light.

Auxin is primarily produced in the tips of the plants. This is why the plant grew straight up when you covered the tip with a cap—it couldn’t “see” the light anymore! The tips of the control seedlings and the seedlings with the bases covered could still sense the light, so they grew towards the sunlight.

Thanks to Charles Darwin and modern science, the mystery of how plants grow towards light was finally solved.

Learn more about phototropism:

To understand plant tropisms, you first have to understand plant hormones. We created an excellent page about Plant Growth Hormones  here and here .

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Lindsay graduated with a master’s degree in wildlife biology and conservation from the University of Alaska Fairbanks. She also spent her time in Alaska racing sled dogs, and studying caribou and how well they are able to digest nutrients from their foods. Now, she enjoys sampling fine craft beers in Fort Collins, Colorado, knitting, and helping to inspire people to learn more about wildlife, nature, and science in general.

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Student Sheet 8 – Phototropism: the Response of Seedlings to Light

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Seedlings growing on a windowsill will often bend towards the window as they respond to light – phototropism. But what exactly are the seedlings responding to? Which wavelengths of light stimulate the phototropic response?

The technique for this experiment helps students design an investigation to find out more about this tropic response. Students germinate seedlings in enclosed containers, with a coloured filter over the small hole allowing light in. Students predict which seedlings will demonstrate phototropism, and which will remain unaffected.

The students’ sheet contains a number of suggestions for further experiments, while the  worksheet outlines the basic technique.

Download the student sheets and teachers’ notes from the links on the right.

What's included?

• SAPS Sheet 8 - The response of seedlings to light - Student Notes
• SAPS Sheet 8 - The response of seedlings to light - Student Sheet
• SAPS Sheet 8 - The response of seedlings to light - Technical and Teaching Notes
• Plant growth
• Plant reproduction
• Plant responses

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Teaching resources.

• Investigating Thigmotropism
• Tackling tropisms: gravitropism and phototropism

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Science project, plant phototropism experiment.

As plants grow, they move up toward the light. But what is a plant’s favorite color? Do plants move toward some colors more than others?

Do plants bend toward certain colors of light?

• 2 1-foot tall cardboard boxes with lids
• Piece of cardboard
• 2 small lamps
• 2 full spectrum light bulbs
• Box cutter knife
• Masking tape
• 1 3” x 3” piece of clear, red, green, and blue cellophane
• Spray bottle
• 8 bean seeds
• 8 small pots
• First, get your plants growing. Plant two of your bean seeds in two different pots, water them, and wait for them to poke out of the ground.
• While you’re waiting, get your boxes ready.  Cut a hole 2” in diameter about 3 inches from the bottom of each box. Place the clear cellophane over the hole. This will let all of the light into the box. Over the hole in the other box, place the red cellophane. This will only let red light into the box.
• Put one plant in the first box and one in the second. Use a ruler to position each bean plant two inches away from the cellophane window.  Take a photo of the plants, looking downward from the top of the box.
• Put the boxes on different sides of the same room.
• Now it’s time to light things up! Put the lamps next to the boxes on the side with the cellophane window. Take out your ruler again and measure to make sure that the lamps are the same distance from the hole.
• Put the lids on each box.
• Every morning, turn on each lamp. Every night, turn off the lamps before you go to bed. Leave the plants to grow for a week.
• After a week has passed, remove the lid and take a photo looking downward. Then remove the plants and take a photo from the front. Do the plants look different? Is one taller than the other? Is one twisted in a different direction?
• Do the same experiment with new bean plants, but change the color of cellophane to blue. Finally, repeat the experiment with green cellophane.
• Compare the photos of each bean plant after it had been growing for a week. Did the plants turn more toward a certain color? Was there a color they didn’t like?

The control plants will do better than the plants that are only exposed to one wavelength of light. The plants will grow better in red and blue light than in green light. The plants will grow toward red and blue light but will not move toward the green light.

Plants love the light, right? Yes and no. Plants do love the light, but they like some wavelengths of light more than others.

When you look at a rainbow, you can see that the visible spectrum of light actually has different colors or wavelengths inside it.  The visible spectrum is the light that we can see. Different objects reflect different types of light. A blue bowl reflects blue light. A green plant reflects green light.

Inside a plant are chloroplasts . Inside the chloroplasts are tiny molecules called photopigments . Photopigments help the plant absorb light. A plant has different types of photopigments so it can absorb different colors of light.

When natural light shines on a plant, that plant takes in the light from the different wavelengths and uses it to make food.  This natural light is called white light, and it contains all of the types of light. If there’s only one color of light shining on a plant, then only some of the photopigments work, and the plant doesn’t grow as well. This is why your plant under the full light spectrum grew better than the plants with the cellophane filters.

Plants also move toward the light. Seeds push little leaves up from the ground into the light. A house plant in a dark room will grow toward the light. This movement in response to light is called phototropism . When a plant moves toward the light, it’s called positive tropism . When a plant moves away from light, it’s called negative tropism .

How do plants move? They do so with the help of chemicals called auxins . Think of auxins as an elastic band for cells. They help cells get longer and move. Sunlight reduces auxin, so the areas of the plant that are exposed to sunlight will have less auxin. The areas on the dark side of the plant will have more auxin. That means that they will have long, stretchy cells. This allows the plant to move toward the light.

The plants in your experiment likely showed positive tropism, except when it came to the green light. Why did the plants not move toward the green light? Plants are green, which means that they reflect green light. It bounces off the leaves. This means that they can’t use green light very well, and the green light bounces off the plant instead of encouraging movement toward the light.

Digging Deeper

What would happen if you left plants for a long time in light that was only red or blue? Would they survive? 

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Experiments to Show Phototropism (A-level Biology)

Experiments to show phototropism, investigating plant responses, phototropism experiment.

We can investigate phototropism in plants using the following method. This will allow us to see the response of plants to light.

• Use 9 plant shoots. Plant all the shoots in individual plant pots, with the same soil type in each pot. Ensure that all the shoots are roughly the same height.
• Wrap some of the shoots in foil. Now, wrap the tips of 3 shoots in foil. For another 3 shoots, wrap the base of the shoots in foil. Leave the final 3 shoots without foil.
• Place the shoots under a light source. Place all 9 shoots under a light source for 2 days. Ensure that the shoots are equally exposed to the light source. Control the temperature and moisture over the course of the experiment.
• Interpret the results after 2 days. After the shoots have been exposed to the light source for 2 days, interpret the results. The shoots with covered tips will not grow towards the light source, but the other 6 shoots will.
• Record the amount of growth. To get accurate, quantitative results, you can measure the growth of each shoot and write down the direction of growth.

Phototropism is the growth response of a plant towards or away from light.

There are several experiments that can be done to demonstrate phototropism in plants, including: The experiment with potted plants, where a plant is grown in a pot and then covered on one side with a black paper. The plant will grow towards the light source The experiment with grass seedlings, where grass seedlings are grown in a tray and exposed to light from one side. The seedlings will grow towards the light source The experiment with Avena seedlings, where Avena seedlings are grown in a test tube and exposed to light from one side. The seedlings will bend towards the light source The experiment with coleoptiles, which are the protective sheaths surrounding grass shoots. Coleoptiles are placed in a darkened room and exposed to light from one side. The coleoptiles will bend towards the light source

Phototropism works in plants through the unequal distribution of auxin, a hormone responsible for promoting growth in plants. When light is shone on one side of the plant, it stimulates the cells on that side to produce more auxin. This causes the cells on that side to grow faster, bending the plant in the direction of the light.

The knowledge of phototropism has practical implications for agriculture and horticulture, as it can be used to increase crop yields and improve the growth of ornamental plants. By manipulating the light exposure of plants, farmers and horticulturists can encourage the growth of plants in a desired direction, leading to more efficient use of space and resources.

The study of phototropism is important for future careers in Biology because it provides a fundamental understanding of plant growth and development. This knowledge is essential for careers in areas such as botany, plant sciences, agriculture, horticulture, and other related fields, where an understanding of plant growth is crucial.

The study of phototropism is approached in A-level Biology through a combination of theoretical and practical work. Students learn about the mechanisms of phototropism, the role of hormones in plant growth, and the factors that affect the direction of growth. They also conduct practical experiments to demonstrate phototropism in plants and gain hands-on experience in plant growth and development.

Some of the factors that can affect the direction of phototropism in plants include: The intensity of the light The duration of the light exposure The wavelength of the light The age of the plant The species of the plant

Phototropism can be used to grow plants in space as it provides a way to orient the plants towards a light source, even in a low-gravity environment. This can be important for growing food crops or for conducting experiments on plant growth in space.

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GCSE Biology: Plant hormones

For GCSE Biology, students need to be able to explain how plant hormones are important in the control and coordination of plant growth and development, with reference to the role of auxins in phototropism and gravitropisms. Students should be able to describe some of the effects of plant hormones, relating to auxins, gibberellins and ethane, as well as describing some of the different ways in which people use plant hormones to control plant growth.

Students often find it difficult to understand how auxins can have different effects in roots and shoots, so it is worth spending some time on this when teaching this topic. Students need to use accurate terms in this topic area,so throughout the delivery of this topic it is important to stress this, for example use light not  "the sun", for geotropism use gravity  and not just "ground" or "earth", ensure that when talking about the shoot students say this and not just plant.

Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS, SSERC or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our  Health and Safety  guidance.

Tropisms and Hormones *suitable for home teaching*

Quality Assured Category: Science Publisher: Twig

This short film (3 minutes), from Twig World, looks at plant hormones and tropisms. The key points made by the film are:

• Tropisms allow plants to seek out the best conditions for survival.
• Tropisms are controlled by hormones called auxins.
• Tropisms react to varying conditions of light, gravity and water.
• There are both positive and negative tropisms.

Students could watch this video in pairs and then be asked to identify these four key points, either from scratch or provided with a gapped handout. Alternatively this video could be used as an introduction to a practical demonstration to investigate the effect of auxin in plants – (see the protocol included in this list.)

ABPI interactive website

This Association of British Pharmaceutical industry (ABPI) website contains a range of animations which could be used within lessons to consolidate learning. This is page 9 of a resource all about hormones and their effects. This page deals specifically with plant hormones, it provides a summary about Auxins and the commercial use of plant hormones.

Towards the bottom of the page there is an interactive drag and drop activity. This would make a good short self-study/homework activity to consolidate learning for this topic.

Tackling Tropisms: Gravitropism and Phototropism

Quality Assured Category: Science Publisher: Science & Plants for Schools (SAPS)

This set of demonstrations into the effects of tropisms on seedlings is both simple and extendable, making it useful across key stages, including for GCSE Biology students.

In this investigation seeds are sown on damp cotton wool in (plastic) Petri dishes which are either flat or stuck vertically to a wall. Students enjoy the dramatic effect of sticking Petri dishes to the wall, and they will have well-grown seedlings within a week, showing the different effects of gravitropism and phototropism

Plant Hormones ( AQA GCSE Biology )

Topic questions.

Draw a ring around the correct answer to complete the sentence.

Plants grow in the right direction due to the action of

How did you do?

What is phototropism in plants?

Growth due to photosynthesis

Growth inhibited by light

Growth stimulated by temperature

Growth stimulated by light

Choose your answer

Plant roots respond to gravity by growing downwards.

What is this response called?

Positive gravitropism

Negative geotropism

Positive phototropism

Negative gravitropism

A plant is grown near a fixed light source.

The plant is then rotated by 180 · as shown in Figure 1 .

Predict the direction of growth of the plant shoots over the next 48 hours.

Did this page help you?

Name the plant hormone responsible for both phototropism and gravitropism.

What effect would positive gravitropism have on a plant?

Roots would grow towards gravity

Roots would grow away from gravity

Shoots would grow away from gravity

Shoots would grow sideways

A group of students set up an experiment to investigate plant growth.

They attached a plant pot to a machine called a clinostat so that it could be rotated slowly on its side as shown in Figure 2 . The apparatus was then placed in a darkened room.

Which of the following images would show the expected result of this experiment after two days?

Figure 3 represents a plant shoot growing, it is illuminated from the side.

Which of the following images best represents how auxins will start to accumulate in the shoot tip in response to light?

What happens to the cells in plant shoots in response to auxins?

The cell cycle is stopped.

Mitosis is increased. 

Complete the sentence.

Choose the answer from the box

__________ is beneficial for plants because they can grow __________  the __________ and absorb more __________ which __________ photosynthesis.

Draw one line from each plant part to the correct response.

Higher Tier Only

Which gas acts as a plant hormone?

Carbon Dioxide

Figure 6 shows three different hormones and their potential functions within the plant.

Draw one line from each hormone to its potential use.

A gardener wishes to remove all the dandelions (a type of weed) from their lawn without damaging the grass. 

They started to pick the dandelions out by hand but it was taking too long, so they purchased a selective weedkiller which contained a synthetic plant hormone.

Which hormone would most likely be in the weedkiller?

Gibberellin

A scientist investigated the effects of gibberellins on flowering in plants.

The plants were grown in identical environmental conditions (light, temperature, water) but half the plants were treated with gibberellins.

The results are shown in  Figure 6 .

What conclusion can the scientist make about gibberellins?

Plants respond to stimuli such as light and gravity.

State the name of a plant growth response to light.

Describe the biological mechanism that results in the shoot bending towards the light.

Auxins have multiple uses amongst gardeners, including use as a naturally derived weedkiller.

Give one other use gardeners might have for auxins

Some gardeners use synthetic plant hormones as weedkillers.

Synthetic plant hormones behave in a similar way to auxins.

Discuss the use of these hormones as weedkillers.

A group of students investigated control and coordination of growth in plants.

They planted a bean seed in a box filled with moist soil. After the seed germinated, the box was turned onto its side and placed in a dark room.

Figure 1 illustrates their experiment.

Describe the expected appearance of the seed 3 days later.

Explain the role of auxins in coordinating the response of seeds in this experiment.

Suggest a suitable control for the investigation detailed in 2(a) and explain why it is necessary.

When a seed germinates, the developing root grows downwards towards gravity and the developing shoot grows away from gravity.

These effects are controlled plant hormones called auxins.

Name this type of plant response to gravity.

Suggest why this growth response in developing roots and shoots is important.

Another important plant hormone is ethene.

Ethene is the most widely produced organic compound in the world. It is used in the manufacture of plastics and by the food industry.

Explain why ethene is widely used in the food industry.

A group of students wanted to investigate gravitropism in bean seedling roots.

Figure 1  shows part of the set-up of their experiment.

This is part of the method they planned to use:

• Measure the length of the root of 5 bean seedlings
• Pin the 5 bean seedlings to a cork mat as shown in Apparatus A
• Place apparatus A in a dark cupboard for 3 days
• After 3 days, measure the length of the root in each seedling and make a drawing of each seedling

The students' teacher told them that their method was incomplete and that they needed to set up a control experiment ( Apparatus B ) for comparison.

Describe how a suitable control could be set up so that the students could improve their investigation.

Why was it important that the seedlings in the experiment were placed into a dark room?

The students carried out their experiment using 10 bean seedlings split across two sets of apparatus; Apparatus A and Apparatus B (the control).

The results of the student’s investigation are shown in Table 1 below. The table is incomplete.

Complete the table to calculate the mean change in length for the bean seedlings in Apparatus A and Apparatus B .

Figure 2 shows the appearance of the seedlings at the end of the 3 days.

A plant hormone, made in the tip of the root, diffuses from the tip into the tissues of the root.

Explain how the hormone causes the appearance of the seedlings in Figure 2.  

You should refer to both seedlings A and B in your answer.

Plant hormones are used in horticulture to control plant growth.

Draw one line from each plant hormone to its correct use.

A group of students were given a number of young plant shoots and some powdered auxin.

Describe how the students could use these items to demonstrate how auxins cause shoots to grow towards a light stimulus.

Figure 1  shows a developing seedling.

Identify which letter represents the region which shows positive geotropism and explain why this is important for a developing seedling.

Describe three commercial uses of gibberellins for the food industry.

Explain the effect that auxins have on the cells in shoot tips.

Explain why hormonal effects on the cells in shoot tips is beneficial for the plant.

A young seedling, growing in a forest was knocked over by animal activity.

Figure 1 shows how the plant grew after being knocked over.

Identify the stimulus and the response in  Figure 1 (including its direction).

Describe the effect phototropism has on the growth of plant roots.

In the late 1800s, Charles Darwin performed an experiment where he grew maize seedlings in the dark, in a horizontal orientation. Half of the seedlings had the root tip removed ( A ), the other half of the seedlings were left intact ( B ). The results of his investigation are shown in  Figure 2 .

Explain what these results demonstrate for each experiment.

Draw an annotated diagram showing accumulation of auxins in response to light and how the shoot tip will grow.

Which method of movement most likely explains how auxins travel to other parts of the plant shoot or root?

Active transport

Mass flow in the phloem

In UK supermarkets, bananas are usually in perfect condition even though they have been imported via ships from countries with tropical climates.

Describe how plant hormones can be used to ensure bananas arrive in perfect condition.

Read the following two statements:

Evaluate the use of artificial auxins.

A company called SME agrochemicals were trailing a novel plant hormone selective herbicide in order to be able to register the product for commercial use. The agrochemical, named SME roundup ®, showed broad spectrum efficacy against dicot weeds in cereal crops. During a trial, winter wheat was grown alongside the broad leafed 'Yellow hawkweed' Hieracium pratense over four testing plots of 1 m 2 in size and different concentrations of SME roundup were applied. The only data captured was the number of weeds, two weeks post application. The results, are shown in Table 1 .

Calculate the total percentage efficacy for each application rate.

Use the following equation:

During early lab-based trials, SME Agrochemicals suggested that the recommended application rate for the herbicide SME roundup ®  should be 1000 mg/m 2 Explain why this was an incorrect assumption (Use the information from part a to support your answer).

SME Roundup ® is marketed as a selective herbicide that can be used for cereal crops such as wheat and barley.

Explain how this product acts as a selective herbicide

Suggest how the trials could be improved.

In addition to auxins, gibberellins, and ethene, there are a number of other groups of plant hormones that control many aspects of plant growth. When plant roots sense a water shortage in the soil, the plant hormone abscisic acid is released and binds to receptor proteins in guard cells.  Suggest how abscisic acid may help plants in drought conditions. 

A student wanted to investigate the mechanism of phototropism in plants. They set up their investigation as shown in Figure 4 .

In experiment A - the shoot tip was left untreated (as a control).

In experiment B - 2mm was removed from the growing end of the shoot tip.

In experiment C - The shoot tips were covered with caps of aluminium foil.

The shoot tips were initially measured and left for 48 hours next to a unidirectional light source.

Predict what differences in growth occurred in each experiment after 48 hours.

Explain why the plant shoots grow differently in experiments B and C .