black paper experiment diagram

Investigating Phototropism & Geotropism ( OCR A Level Biology )

Revision note.

Biology & Environmental Systems and Societies

Investigating Phototropism & Geotropism

Phototropism in plant shoots.

• Plant shoots are positively phototropic , meaning they grow towards light
• This ensures they maximise the amount of light they can absorb for photosynthesis
• Many of the experiments were conducted using coleoptiles (a sheath that surrounds the young growing shoot of grass plants)
• Darwin discovered that removing the tip of a coleoptile stopped the phototropic response to a unidirectional light source (light coming from one side) from occurring
• To ensure this was not simply due to the wounding caused to the plant, he covered the tip of a coleoptile with an opaque cover or 'cap' instead, to block out the light. This also stopped the phototropic response from occurring, showing that the tip of the coleoptile was responsible for detecting light
• Boysen-Jensen found that if he replaced the cut tip back on top of the coleoptile and inserted a gelatin block as a barrier in between, the phototropic response was restored
• This showed that the stimulus for growth was a chemical (hormone) , which was able to travel through the gelatin block
• Bosen-Jensen then inserted a mica barrier (mica is impermeable to chemicals) halfway through the coleoptile just below the tip, first on the lit side and then on the shaded side
• When the mica barrier was inserted into the lit side , the phototropic response occurred
• When the mica barrier was inserted into the shaded side , the phototropic response did not occur
• This confirmed that the stimulus for growth was a chemical (hormone) and showed that it was produced at the tip , before travelling down the coleoptile on the side opposite to the stimulus (i.e. the shaded side)
• It also showed that the stimulus acted by causing growth on the shaded side (rather than inhibiting growth on the lit side)
• Paál cut off the tip of a coleoptile and then replaced it off-centre in the dark
• The side of the coleoptile that the tip was placed on grew more than the other side, causing the coleoptile to curve (similar to a phototropic response)
• This showed that, in the light, the phototropic response was caused by a hormone diffusing through the plant tissue and stimulating the growth of the tissue
• Went placed the cut tip of a coleoptile on a gelatin block, allowing the hormones from the tip to diffuse into the block
• The block was then placed on the coleoptile, off-centre and in the dark
• As in Paál's experiment, the side of the coleoptile that the block was placed on grew more than the other side, causing the coleoptile to curve
• The greater the concentration of hormone present in the block, the more the coleoptile curved

Four historical phototropism experiments were conducted to investigate the process by which phototropism occurs

Controlling growth by elongation

• Indole-3-acetic acid (IAA), which is an auxin , is a specific growth factor found in plants
• IAA is synthesised in the growing tips of roots and shoots (i.e. in the meristems , where cells are dividing )
• IAA coordinates phototropisms in plants by controlling growth by elongation
• IAA molecules are synthesised in the meristem and pass down the stem to stimulate elongation growth
• The IAA molecules activate proteins in the cell wall known as expansins , which loosen the bonds between cellulose microfibrils , making cell walls more flexible
• The cell can then elongate

The phototropic mechanism

• Phototropism affects shoots and the top of stems
• The concentration of IAA determines the rate of cell elongation within the region of elongation
• If the concentration of IAA is not uniform on either side of a root or shoot then uneven growth can occur
• It is described as positive because growth occurs towards the stimulus
• Experiments have shown that IAA moves from the illuminated side of a shoot to the shaded side
• The higher concentration of IAA on the shaded side of the shoot causes a faster rate of cell elongation
• This causes the shoot to bend towards the light

Higher concentrations of IAA on the shaded side increases the rate of cell elongation so that the shaded side grows faster than the illuminated side

Geotropism in plant shoots and roots

• Gravity affects both plant shoots and roots , but in different ways
• Gravity modifies the distribution of IAA so that it accumulates on the lower side of the shoot
• As seen in the phototropic response, IAA increases the rate of growth in shoots , causing the shoot to grow upwards
• In roots, higher concentrations of IAA results in a lower rate of cell elongation
• The IAA that accumulates at the lower side of the root inhibits cell elongation
• As a result, the lower side grows at a slower rate than the upper side of the root
• This causes the root to bend downwards

Practical: investigating the effect of IAA on root growth

• Experiments can be carried out to investigate the effect of IAA on root growth in seedlings
• Seedlings (of the same age and plant species)
• Cutting tile
• Light source
• Lightproof container
• Blocks of agar (all the same volume)
• Use the scalpel to cut a 1cm section from the root tip of each seedling
• Mark the root tips at 2mm marks
• The water helps to keep the plant tissue alive
• Remove the ends of the root tips using the scalpel
• Transfer root cuttings with the end removed to an agar block
• A uniform light source is present
• Transfer intact root tips to an agar block
• A light-proof container is placed over the seedlings to prevent light from entering
• Apply a directional light source to one side of the root tips
• Leave all the roots in their treatment conditions for 3 hours
• Use the 2mm marker lines to determine if growth has occurred
• Note if the growth has been even on both sides

Results and analysis

• IAA is synthesised in the root tips so removing them means that no IAA is produced
• There is no inhibition of cell elongation
• There is an equal concentration of IAA on both sides of the root tip
• The inhibition of cell elongation is equal on both sides of the root tip
• The roots do not grow as long as those in group A due to the presence of IAA
• There is a greater concentration of IAA on the shaded side
• This results in greater inhibition of cell elongation on the shaded side
• So the illuminated side grows at a faster rate
• The roots bend away from the light – negative phototropism

Limitations

• Certain genotypes may be more prone to bending or have slightly different sensitivities to IAA
• If the root is mishandled the marks can be altered, which will affect the results
• Only general comments can be made about whether there has been even growth on both sides of the roots

The different treatments produce different levels of growth in the root tips. The IAA molecules inhibit cell elongation in roots

You may sometimes see IAA simply referred to as auxin. IAA is a particular type of auxin, which is a more general term for a particular group of plant hormones.

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Author: Alistair

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

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Experiment on Photosynthesis-Sunlight is necessary for Photosynthesis

Photosynthesis is a physiological process of green plants. One of the important factor required for photosynthesis is sunlight. In this resource I have explained in detail how an experiment can be conducted to prove the importance of the factor sunlight for this process. Along with the experiment I gave some important questions as well as answers related to the experiment which will be useful for those preparing for final practical or theory exams of various Boards.

Introduction

Aim of the experiment:-, requirements:-.

Procedure for the experiment

How to conduct starch test on the experimental leaf.

• First the experimental leaf is dipped in boiling water in the beaker for few minutes so that the cells of the leaf will be killed and metabolic process of the leaf cells will be stopped.
• Now the boiled leaf in water is removed from the beaker with the help of a forceps and is placed in a test tube containing mentholated spirit. Then the leaf is boiled in mentholated spirit or ethanol over a water bath by keeping it in boiling water for about 10 minutes. The leaf looses its chlorophyll by boiling it in mentholated spirit and becomes pale-white.
• Then the pale-white leaf is removed from the test tube and then it is washed in warm water to soften it.
• Then this leaf is spread over a white porcelain tile and iodine drops are poured over at different places of the leaf.

Result of Iodine test

Questions and answers related to the experiment.

• What is the aim of the above experimental set up? Ans. The aim of the above experimental setup is to prove that sunlight is essential for photosynthesis.
• What is the first step done before starting of this experiment? What is the importance of this step? Ans. Before starting of this experiment De-starching of the leaves of the plant to be done. This step will be done to ensure that there is no starch in the leaves of the experimental plant.
• What do you understand by the De-starching? How the leaves of plant will be De-starched? Ans. De-starching is the elimination of starch from the leaves of an experimental plant by keeping it in dark room for about two days.
• Write the steps you follow to conduct Iodine test on the experimental leaf? Ans. Write the four steps mentioned above under the heading "How to conduct starch test on the experimental leaf" and also mention the result of iodine test.
• Why the experimental leaf was dipped in hot boiling water? The experimental leaf was dipped in hot boiling water to kill the cells of the leaf so that the metabolic process of the cells of the leaf will be stopped.
• Why the leaf is boiled in mentholated spirit? Ans. To remove chlorophyll pigment from the cells of the leaf.
• Why there is a difference in result of the iodine test in the covered and uncovered black paper regions of the experimental leaf? Iodine test in the black paper uncovered region show a positive result for the presence of starch i.e iodine in this region turns into blue-black color. The reason for this is that uncovered region gets sunlight and so this part of the leaf is able to synthesize starch. The black paper covered region give a negative result for starch i.e. iodine in this region turns into brown color. The reason for this is black paper covered region gets no sunlight and so this part of the leaf is not able to synthesize starch.

What is the control for this experiment? Covered part or uncovered part & why? Please explain.

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One half of a convex lens is covered with black paper. Will this lens produce a complete image of the object? Explain your observations.

When one-half of a convex lens is covered with a black paper, this lens produces a complete image of the object. to prove it we perform an experiment as given below: take a convex lens and cover half part of it by using a black paper. place it vertically in a stand. on one side of it place a burning candle. on opposite side of the lens fix a white screen. adjust the position of candle or screen till clear image of burning candle is formed on the screen. we observe that the image is a complete image of the object (burning candle). from the experimental observations, we find that image formation does not depent upon the size of a lens. a smaller lens can also form complete image of an object placed in front of it. however, brightness of the image decreases when some part of lens is blocked. it is because now lesser number of rays pass through the lens..

One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

Question 9 One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

FREE K-12 standards-aligned STEM

curriculum for educators everywhere!

Find more at TeachEngineering.org .

• TeachEngineering
• Do Different Colors Absorb Heat Better?

Hands-on Activity Do Different Colors Absorb Heat Better?

Grade Level: 1 (K-2)

(freeze ice cubes [~5 minutes] a day before doing the activity)

Less than $10 per class.

Group Size: 3

Activity Dependency: None

Subject Areas: Earth and Space

NGSS Performance Expectations:

TE Newsletter

Engineering connection, learning objectives, materials list, worksheets and attachments, introduction/motivation, vocabulary/definitions, investigating questions, activity scaling, additional multimedia support, user comments & tips.

The study of light and its behavior is an important component in the design of many items, everything from optical instruments to roofing materials to solar cells. The performance and characteristics of light guide engineers to come up with different forms of light detection for lenses in cameras, microscopes, CD players and medical devices. Different sources of light carry different quantities of energy. For example, powerful lasers can cut through stone or even metal.

• Certain colors absorb light better than others.
• The sun produces heat and light.
• Why ice cubes melt.
• (optional) The purpose of solar panels.

Educational Standards Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards. All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN) , a project of D2L (www.achievementstandards.org). In the ASN, standards are hierarchically structured: first by source; e.g. , by state; within source by type; e.g. , science or mathematics; within type by subtype, then by grade, etc .

Ngss: next generation science standards - science.

Common Core State Standards - Math

View aligned curriculum

Do you agree with this alignment? Thanks for your feedback!

International Technology and Engineering Educators Association - Technology

State standards, massachusetts - math, massachusetts - science.

• colored paper 4 sheets per group (white, yellow, red, black)
• scissors (one per student if you want the them to cut out the boxes [cube templates] from the colored paper)
• clear tape, to make the cube boxes from colored paper
• 4 ice cubes per group
• sunny day or a heat lamp
• Activity Worksheet , one per group

Imagine that it is 100 degrees outside. How do you stay cool? What kinds of clothing do you wear? Any thought to color? (Listen to student ideas.)

What might be the influence of color and its relationship to heat? Can you think of any instances in which the color of something makes a difference in how hot it gets in the sun? (Listen to student ideas. Possibilities: Wearing white vs. black clothing on super hot days. Flat rooftops sealed in black tar vs. white polymer material. Walking barefoot across a black asphalt roadway vs. lighter concrete roadway. Choosing a white car instead of a black car if you live where it is sunny and hot all the time.)

The sun emits energy in the form of electromagnetic waves. We see part of the electromagnetic wave as light and we feel part of it as warmth. Darker colors absorb more sunlight than lighter colors, which is why darker colors get warmer more quickly in the sunlight than lighter colors. The lighter colors reflect more of the sun's radiant energy, so they remain cooler to touch in the sunlight.

Let's do our own testing to find out.

Before the Activity

• Make enough ice cubes so that each group can have four. Try to make them the same size for experiment consistency.
• To save time, pre-cut and assemble (using tape) the colored paper into five-sided boxes each big enough to fit an ice cube. Otherwise, have students cut, fold and tape together their own boxes. See the Additional Multimedia Support section for Internet resources on how to make a cube from a piece of paper.
• Gather the rest of the materials.
• Make copies of the Activity Worksheet , one per group.

With the Students

• With the class, talk through the Introduction/Motivation section.
• Once the class is thinking about the influence of color and its relationship to heat, divide the class into small groups.
• Give each group four sheets of colored paper (white, yellow, red, black) and have them cut and fold their sheets into boxes.
• Hand out newspaper and have each group spread the newspaper in an exposed, sunny place outside, or under a heat lamp.
• On the newspaper, place the boxes side by side with the opening facing away from the sun/light so students can see inside.
• Give each group four ice cubes and instruct them to place one ice cube in the center of each colored box.

• Direct groups to record their data in the worksheet chart.
• Have students create a bar graph representing the time it took the ice to melt for each color of paper.
• Discuss with the class their observations, touching on the different colors and their ability to reflect light and heat. Also, talk about how these color characteristics help to melt the ice.
• Ask students the Investigating Questions. Discuss some real-world examples in which engineers use their understanding of how different colors reflect light and heat to design products and find solutions. (Example: Asphalt roads and tar roofs are dark surfaces that absorb heat from the sun. Measurements show that white roofs reflect some of the sun's heat back into space and cool temperatures, much as wearing a white shirt on a sunny day can be cooler than wearing a dark shirt. So, designing white roofing materials or paint for roofs has the effect of cooling temperatures within buildings.)

absorb: To take in; to transform (radiant energy) into a different form usually with a resulting rise in temperature.

energy: The capacity for doing work; raising weight, for example.

heat: A form of energy that causes substances to rise in temperature or to go through associated phase changes (as melting, evaporation, or expansion).

radiant energy: Energy (as heat waves, light waves, radio waves, x-rays) transmitted in the form of electromagnetic waves.

reflect: To bounce waves of light, sound, or heat off a surface.

solar cell: A photo-electric cell that converts sunlight directly into electrical energy and can be used as a power source.

solar energy: Energy derived from sunlight.

solar panel: A group of solar cells forming a flat surface (as on a spacecraft).

Rubric: Use the  Rubric for Performance Assessment .

Making Sense: Have students reflect about the science phenomena they explored and/or the science and engineering skills they used by completing the Making Sense Assessment .

• Why do ice cubes melt?
• How does the sun affect ice?
• What kind of clothes do people wear outside in the winter? In summer?
• On which color did the first ice cube completely melt? Why?
• If an ice cube was placed on a blue piece of paper, how much time do you think it would take to completely melt?
• Which color absorbs heat the quickest in the sun?
• Which color would be the best to help keep ice cubes from melting too quickly in the sun?

For a more challenging activity, add a discussion of heat as a form of energy, including the solar panels and solar energy.

Refer to WikiHow's instructions on How to Make a Paper Cube at http://www.wikihow.com/Make-a-Paper-Cube (includes a printable cube template) and How to Make a 3D Cube at http://www.wikihow.com/Make-a-3D-Cube .

Cool Roof Resources for Federal Agencies. Federal Energy Management Guide, U.S. Department of Energy. http://www1.eere.energy.gov/femp/features/cool_roof_resources.html

Do Different Colors Absorb Heat Better? Grades PreK-2. Education Resources Information Center. Office for Technology and Industry Collaboration, Tufts University and Department of Education. (alternate online location for activity) http://www.eric.ed.gov/ERICWebPortal/search/detailmini.jsp?_nfpb=true&_&ERICExtSearch_SearchValue_0=ED480661&ERICExtSearch_SearchType_0=no&accno=ED480661

Richards, Roy. An Early Start to Technology from Science. London, UK: Simon & Schuster, 1990, page 64.

White Roofs May Successfully Cool Cities: Computer Model Simulates Impact of White Roofs on Urban Areas. Posted January 28, 2010. Press release 10-016, National Science Foundation News. http://www.nsf.gov/news/news_summ.jsp?cntn_id=116283

Supporting Program

Last modified: July 31, 2021

• 3 index cards
• small piece of modeling clay or sticky tack
• hole puncher
• science journal
• For each index card, use a ruler to draw lines connecting opposite corners of the card.
• At the intersection of the two lines, use a hole puncher to punch a hole in the center of the index cards.
• For each card, use a small piece of modeling clay and place the card into the clay to create a "stand" for the card. Place the cards so that they stand vertically and at an equal distance from each other. See Diagram.
• Place the flashlight at one end of the row of index cards and turn off the light in the room.
• Arrange the index cards so that light can be seen through all the holes.
• Observe and record your observations.
• How can light be seen through all the index cards?
• What does the experiment prove about the path light travels?
• What would happen if the holes were smaller?

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Question asked by Filo student

Q12. The diagram below shows a leaf that was covered by piece of black paper for a period of 3 days. After 3 days the paper was removed. On testing, it was found that the area under the black paper tested negative for starch and the rest tested positive for starch. What was the experiment trying to test? ( a ) if plants make their own food

• if light is required for plants to make
• if plants can respire in the absence of light
• if plants can survive even in the absence of light

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David Leonhardt, who has written about affirmative action for The Times, explains the extent and nature of that change as the new academic year gets underway.

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David Leonhardt , a senior writer who runs The Morning , The Times’s flagship daily newsletter.

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