Lesson 13 Which way is up?
| Site: | MoodleHUB.ca đ |
| Course: | Science 10 [5 cr] - AB Ed copy 1 |
| Book: | Lesson 13 Which way is up? |
| Printed by: | Guest user |
| Date: | Sunday, 7 September 2025, 6:46 PM |
Introduction
How do plants know which end to grow leaves on and which end to grow roots on?
A13.1 Plants need to respond to the sun and the soil.
Plants do not have eyes, so they cannot see where the sun is or where the soil is, yet it is extremely important they grow leaves in the sunlight and roots in the soil. How does a plant do this?
Plants, just like other organisms, respond to stimuli in their environment. This means they take cues from the area around them that help them know where to grow their leaves and roots. In this lesson, we will study these cues and the systems the plants use to interpret them. We will also look at how scientists discovered these systems.
Plants, just like other organisms, respond to stimuli in their environment. This means they take cues from the area around them that help them know where to grow their leaves and roots. In this lesson, we will study these cues and the systems the plants use to interpret them. We will also look at how scientists discovered these systems.
Targets
By the end of this lesson, you will be able to- explain the processes of phototropism and gravitropism
- trace the development of theories of phototropism and gravitropism
Watch This
Plant Hormones: Tropisms © YouTube FuseSchool-Global Education
This video will introduce the concepts of phototropism and gravitropism to you. Please note this video uses an older term for gravitropism and calls it geotropism instead.
The video will also introduce positive and negative tropisms as well.
Light as the Stimulus
Plants respond to light as a stimulus.
A13.2 A Trunk showing Phototropism
Have you ever noticed a plant that is bent? This often occurs because the plant was at an odd angle at one point but continued to grow toward the light. Look at image A13.2. The trunk of the tree is bent at the base because as the tree emerged from
the cliff face, it started to grow toward the sunlight. The plant did not want to grow straight out from the cliff, as only the part of the tree facing upward would get sun. By bending and growing toward the sun, all the leaves get equal access to
the sun.
This reaction to the sun is called phototropism. âPhotoâ means light and âtropismâ means the direction of growth in response to a stimulus. Together, âphototropismâ means the response of the plantâs growth to the sun. Positive phototropism means
the plant is growing toward the light. The plants stem, trunk, and leaves show positive phototropism. Negative phototropism means the plant is growing away from the light. The plantâs roots show negative phototropism. Phototropism ensures the
parts of the plant that perform
photosynthesis (the leaves) are closest to the sun.
Some key scientists were involved in discovering the mechanism for phototropism. All the scientists used the same plant at the same growing stage to carry out their experiments. This ensured their results were due to the reactions of the plant to the sun, not due to the type of plant used.
Some key scientists were involved in discovering the mechanism for phototropism. All the scientists used the same plant at the same growing stage to carry out their experiments. This ensured their results were due to the reactions of the plant to the sun, not due to the type of plant used.
A13.3 Roots growing away from the sun
A13.4 Darwin and Darwinâs experiment
Charles Darwin and his son Francis were the first to investigate phototropism. They wanted to know which part of the plant detected light and which part reacted to the light. They took some oat grass seedlings and modified the tip of the
seedlings. They found that if the tips of the seedlings were covered, the plant did not respond to the sunlight; but if the tips were not covered, the plant showed phototropism. They also found they could cover the stem of the seedling
and the seedling would still grow toward the light. This showed the Darwins that while the tip of the seedling was detecting the light, the growth was happening in the stem of the plant. They concluded the cells in the tip must be communicating
with the cells in the stem.
Peter Boysen-Jensen was determined to discover how the cells in the tip of the seedling were communicating with the cells in the stem. To do this, he cut the tips off two seedlings. With the first seedling, he glued the tip back on using
gelatin, and with the second seedling, he placed a piece of mineral mica between the tip and the rest of the seedling. He found the seedling with the gelatin still grew toward the light, while the seedling with the mineral mica did not.
Based on this observation, he suggested it must be a chemical produced in the tip that caused the stem to grow toward the light. This chemical could diffuse through the gelatin to the stem but could not diffuse through the mineral mica.
A13.5 Boysen-Jensenâs Experiment
A13.6 How auxin works
F. W. Went wanted to figure out what the chemical substance was that caused the stem to grow toward the light. His work allowed him to isolate the substance so future scientists could continue to test it. It was later determined
this chemical substance is a hormone, or a chemical messenger called auxin.
Cells with a high concentration of auxin grew longer, causing the stem to bend. The cells farthest from the sun were the ones containing the highest concentration of auxin, and so they were the cells that grew the longest. This causes
the stem to bend toward the light. Note: The way your textbook has worded this explanation makes it sounds like the plant elongates the cells where there is a low concentration of auxin. This is not the case, the cells elongate where
there is a HIGH concentration of auxin.
Try This
Houseplants and Phototropism
If you have a houseplant in your home, try moving it to a window with a strong beam of sunlight. Rotate it every week and note how the plant grows. You should observe that after a length of time, the plant can be seen to bend toward the light. Plants
that grow quicker will show this bend faster, while slower growing plants will take longer. Cacti tend to grow slowly, so it will take a couple of weeks to see a difference, while plants like a shamrock or a seedling will bend toward the light within
a couple of hours.
Read This
Please read pages 323, 326 and 327 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on what phototropism is
and how it works. Remember, if you have any questions or you do not understand something, ask your teacher!
Practice Questions
Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.- What is the difference between positive and negative phototropism? What parts of the plant exhibit each of these?
Positive phototropism is when the plant grows toward the sunlight. The leaves and stem of a plant perform positive phototropism. Negative phototropism is when the plant grows away from the sunlight. The roots of a plant perform negative phototropism.
- Complete the following chart outlining the three scientists that contributed to discovering how phototropism works.
| Scientist(s) | Experimental Question | Experiment Performed | Results |
|---|---|---|---|
| Scientist(s) | Experimental Question
|
Experiment Performed
|
Results |
|---|---|---|---|
| Charles and Francis Darwin | What part of the plant detects and reacts to light? | They modified the tips and the stems of seedlings by covering them or cutting them off. | They found the that tip detects the light and the stem bends to grow toward it. |
| Peter Boysen-Jensen | What is the signal that causes the plant to react to the light? | He cut off the tips of seedlings and glued them back on using gelatin or mica. | He found the tips with gelatin still showed phototropism while the tips with mica did not. |
| F. W. Went | What is the specific substance that causes the plant to react to light? | He isolated the exact substance that causes plants to perform phototropism. |
He found this substance causes cells to elongate and higher concentrations of it are found on the shaded side of the plant.
|
Gravity as the Stimulus
Plants respond to gravity as a stimulus.
Gravity plays an important role in how plants grow. Just like how plants respond to light, different parts of the plant respond to gravity differently. The roots of a plant grow toward the source of gravity (the centre of Earth), while the stem grows
away from that source. This response to gravity is called gravitropism. The roots show positive gravitropism while the stem shows negative gravitropism.
This tropism is not as easily observed as phototropism, but it plays just as important of a role as phototropism in making sure the roots grow where they are supposed to and the leaves grow where they are supposed to. Gravitropism helps plants that
are just germinating to know which way to send the roots to get water and minerals and which way to send the stem to break out of the soil into the light.
A13.8 Gravitropism
The current theory on gravitropism suggests specialized cells in the roots contain heavy starch particles. If a potted plant is knocked over, for example, these starch particles will settle in a new area of the cell. This signals the roots to
grow in this new direction. The growth happens in the area of the roots where the starch particles settle.
Did You Know?
A13.9 Reading from a lie detector
In the 1960s, Cleve Backster, an expert in lie detectors, decided to test a plant to see if it would have similar reactions as a human under stressful situations. He hooked a plant up to a lie detector and then injured the leaves to see how the plant would react. The plants reaction looked the same as when a human is lying. He concluded plants must have emotions just like people do. Other scientists were not able to repeat Backsterâs results, leaving Backsterâs work in question. If you are interested in watching a video on Backsterâs results, please go to the following link.
Read This
Please read pages 323 and 327 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on what gravitropism is and how
it works. Remember, if you have any questions or you do not understand something, ask your teacher!
Practice Questions
Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.- What is the difference between positive and negative gravitropism? What part of the plant exhibits each of these?
Positive gravitropism is when the plant grows toward the source of gravity. This occurs in the roots. Negative gravitropism is when the plant grows away from the source of gravity. This occurs in the stem and leaves.
- In your own words, explain how scientists currently believe gravitropism works.
Your answer should be a variation of the following: Scientists currently believe there are large starch molecules in a plantâs roots. When the plant is tipped over, these starch molecules settle on the new bottom of the roots, where they are pulled by
gravity, and promote growth in that direction.
Other Plant Responses
Plants do not only respond to light and gravity, there are other stimuli they respond to as well.
A13.10 Pea Plant sending out tendrils
Plants also respond to touch, temperature, chemicals, water, and length of darkness or light. Not all plants respond to all of these stimuli, but most respond to more than just light and gravity. For example, climbing plants such as peas or clematis
will send out tendrils looking for something to grab hold of. Once they find something strong, they will wrap the tendrils around it so the plant can climb up. This is why climbing plants are often seen climbing rock walls, fences, or trellises.
Many plants will use these other stimuli as a signal for when their needs are met and they can begin to flower or produce fruit. For example, plants that flower in the winter often need a certain amount of time in darkness every day in order to flower. Some examples of these plants are poinsettia or a Christmas cactus. Other plants need a certain amount of time in light every day in order to flower.
Many plants will use these other stimuli as a signal for when their needs are met and they can begin to flower or produce fruit. For example, plants that flower in the winter often need a certain amount of time in darkness every day in order to flower. Some examples of these plants are poinsettia or a Christmas cactus. Other plants need a certain amount of time in light every day in order to flower.
Did You Know?
The mimosa plant responds to touch in a unique way. When you brush against this plant, the leaves collapse together and the stem bends away from the touch. This is a defense mechanism that protects the plant from damage from
wind
or animals passing by. If you are interested in watching this response, please go to the following link.
Did You Know?
A13.11 A banana plant
An ornamental banana plant must leaf out 47 times (produce 47 leaves) before it will produce any fruit. This is how the plant makes sure it is strong enough to support the production of fruit.
Read This
Please read page 328 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on other stimuli that plants respond to.
Remember, if you have any questions or you do not understand something, ask your teacher!
Practice Questions
Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.- Why do you think some plants require a stimulus to signal them to begin producing flowers or fruit?
A stimulus is needed to signal the production of flowers as some plants want to make sure they are able to support the flowers. They wait until they know they have enough stored energy to produce the flower or fruit as it takes lots of energy
to create these. Flowers and fruits are needed as those create the seeds that produce the next generation of plants. Plants also want to make sure they produce the flowers or fruits at a time when those potential seeds can thrive.
- Can you think of other plants that respond to stimuli other than light and gravity?
Carnivorous plants (Venus flytrap, pitcher plant) respond to touch. Other plants, such as tulips, hyacinths, and daffodils, will not start to grow until they have gone through a period of cold weather. Still other plants require heat to start
growing. In the mountains, there are some types of pine cones that will not release their seeds until they have been through the heat of a forest fire. An example of this would be the lodgepole pine or jack pine.
Phototropism and Gravitropism
Plants use stimuli to determine which direction to grow in.
Plants may not be able to see or hear, but they can still respond to the environment around them. They respond to a variety of stimuli, and they use these stimuli to guide how they grow. The two main stimuli plants respond to are light and gravity.
Phototropism is how the plant responds to light, and gravitropism is how the plant responds to gravity. The stem of a plant will show positive phototropism and negative gravitropism. This means the stem grows toward light and away from the source
of gravity. The roots of a plant will show negative phototropism and positive gravitropism. This means the roots grow away from light and toward the source of gravity.
Problem-Solving Activity
Plants in Space
Plants in Space
Over the past few years, there have been many experiments done to see if we can grow plants on space stations. It is very important that we are able to grow plants in space for longer trips, such as to Mars. There is not enough storage space to bring all the food needed, and it is beneficial to be able to eat fresh food. If we can grow food, it would make longer space trips easier.
One of the concerns with growing plants in space is how the roots know where to grow. We can simulate sunlight so the plants can still perform phototropism, but what about gravitropism?
Write down what you think will happen to plant roots when we try to grow plants in space?
Click on the experiment tab to continue.
Watch this video titled âSpace Station Live: Cultivating Plant Growth in Spaceâ to learn about the experiments being performed around how roots grow without gravity.
Click on the analysis tab to complete the analysis questions.
Click on the analysis tab to complete the analysis questions.
©YouTube NASA Johnson
Space Station Live: Cultivating Plant Growth in Space
Space Station Live: Cultivating Plant Growth in Space
- What direction do roots grow in in space?
It has been found roots still tend to grow away from the top of the plant (or down) in space.
- How does this result compare to your hypothesis?
Your answer should be a variation of the following: My hypothesis was wrong. I believed the roots would not grow straight down but grow in a bunch of different directions without the pull of gravity to signal them. However, the results show the roots still grow down in space.
- How do scientists think roots know which way to grow in space?
Scientists are still doing experimentations to figure out how roots know which way to grow. One theory is the roots use light (or lack of light) to know which way to grow. The roots grow from where there is light into the darkness.
1.6 Assignment
Unit A Assignment Lessons 10-13
It is now time to complete the Lesson 13 portion of 1.6 Assignment. Click on the button below to go to the assignment page.
1.6 Assignment
Conclusion
Plants are fascinating examples of multicellular organisms.
AC.1 Structure of a leaf
The systems found in multicellular organisms are made up of organs that are made up of tissues that are in turn made up of cells. Without the cell, multicellular organisms could not survive, as the cells help the multicellular organism perform the basic
functions of life.
In this unit, we studied cells and their functions. We looked at both plant and animal cells as single-celled organisms and determined how they performed the basic life functions. We studied the development of the cell theory and how microscopes
played a huge part in this development. We also learned how microscopic research and knowledge about the cell membrane has impacted society.
Finally, we looked at plants as an example of a multicellular organism and we studied how cells can take on specialized functions in order to work together.
In the next unit, you will study matter and energy in chemistry.
Finally, we looked at plants as an example of a multicellular organism and we studied how cells can take on specialized functions in order to work together.
In the next unit, you will study matter and energy in chemistry.
AC.2 Animal vs. plant cell
Review Questions
Complete the following review questions to check your understanding of the concepts you have learned. Make sure you write complete answers to the review questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.
- How was the theory of spontaneous generation disproved?
The theory of spontaneous generation was disproved through experimentation. Francesco Redi did an experiment using three jars and raw meat that showed flies only appeared on the raw meat when the meat was in a jar open to the air. Then Louis
Pasteur performed his S-bend flask experiment where he stopped microorganisms from growing in broth by first boiling it and then leaving it in an S-bend flask. This showed that microorganisms only arose from other microorganisms and disproved
spontaneous generation.
- Name the three points of the cell theory.
- All living things are made up of one or more cells and the materials produced by these cells.
- All life functions take place in cells, making them the smallest unit of life.
- All cells are produced from pre-existing cells through the process of cell division.
- Why are microscopes important to the study of the cell?
Microscopes allow us to see cells in order to study them. Without microscopes, we would only be able to guess what cells look like or what is inside them. Some microscopes even allow us to observe living cells so we can observe how they perform
their functions.
- Complete the following table around the cell organelles.
| Organelle | Function |
|---|---|
| cell membrane
|
|
| nucleus | |
| lysosome | |
| vacuole | |
| mitochondria
|
|
| endoplasmic reticulum
|
|
| Gogli apparatus
|
|
| ribosomes | |
| chloroplast
|
|
| cell wall
|
| Organelle | Function |
|---|---|
| cell membrane | protective barrier around the cell
|
| nucleus | directs the cellâs processes, houses DNA |
| lysosome | digests the particles brought into the cell |
| vacuole | used for storing and moving particles around in the cell
|
| mitochondria |
|
| endoplasmic reticulum
|
|
| Golgi apparatus
|
takes the proteins, oils, and lipids made in the ER and packages them into vesicles for transport around and out of the cell |
| ribosomes | help to create proteins |
| chloroplast |
|
| cell wall
|
|
- What are the similarities and differences between plant and animal cells?
Your answer should include the following:
Similarities:
Differences:
Similarities:
- Plant and animal cells contain the same organelles, except for cell walls and chloroplasts.
- Both plant and animal cells make specialized compounds according to their functions.
- Both plant and animal cells contain DNA.
- Both plant and animal cells are made of carbon, nitrogen, hydrogen, and oxygen.
- Both plant and animal cells require water and use lipids, proteins, carbohydrates, and nucleic acids.
Differences:
- Only plant cells contain chloroplasts and cell walls.
- Only plant cells make chlorophyll.
- Only animal cells make hemoglobin.
- Plant cells store energy as starch and oils, while animal cells store energy as sugars and fats.
- Plant cells have a large central vacuole, while animal cells have small vacuoles.
- Label each of these images with the type of cell transport being pictured.
1
©ADLC
AC.3 Cell Transport 1
AC.3 Cell Transport 1
2
©ADLC
AC.4 Cell Transport 2
AC.4 Cell Transport 2
3
AC.5 Cell Transport 3
4
AC.6 Cell Transport 4
5
AC.7 Cell Transport 5
Cell transport 1âfacilitated diffusion using a protein channel.
Cell transport 2âdiffusion.
Cell transport 3âosmosis.
Cell transport 4âactive transport.
Cell transport 5âendocytosis.
Cell transport 2âdiffusion.
Cell transport 3âosmosis.
Cell transport 4âactive transport.
Cell transport 5âendocytosis.
- How does the surface area to volume ratio limit cell size?
If the cellâs volume is too big when compared to the surface area, the cell will struggle with transporting into the cell the materials it needs. The cell membrane acts as the doorway to the cell, and if it is too small compared to the volume
of the cell, there will not be enough room for all the materials to move into the cell at the rate needed. This is similar to a large crowd of people trying to all get through one doorway.
- Give one example of how microscopic research or knowledge of the cell membrane has benefited society.
Any answer from the textbook, the course content, or your own research is a great answer. Some examples include: HIV or cancer treatment, peritoneal dialysis, stem cell research, gene mapping, reverse osmosis, cell communication, and liposomes.
- How are the cells in a multicellular organism organized?
Cells â tissues â organs â systems
- Label the following leaf diagram with all the visible parts.
- Describe how the stomata and guard cells function.
Your answer should include the following:
- When sunlight hits the two guard cells surrounding a stoma, it actively transports potassium ions into itself.
- This causes water to flow into the guard cells through osmosis.
- As the cell becomes turgid, it changes shape to become more crescent-like.
- This happens because the membrane surrounding the stoma is thicker, so the pressure from the water causes the guard cell to push out away from the stoma.
- This opens the stoma.
- As the light disappears, or there is not enough water left, the potassium ions are transported out of the guard cells by active transport.
- Once again, the water follows through osmosis.
- The guard cells go back to their original shape, and the stoma is closed.
- Explain how water is transported from the roots of the plant to the leaves.
Water is transported from the roots to the leaves through root pressure and transpiration pull. The minerals in the soil are actively transported into the roots, and water follows by osmosis. As the water pressure in the roots increases, the
water molecules are pushed farther and farther up the root system toward the stem. This is root pressure. As water leaves the leaves of a plant through transpiration, the rest of the water molecules are pulled up the stem into the leaves.
This is done through the water properties of cohesion and adhesion. The water particles are attracted to each other and the walls of the xylem, so as one molecule leaves, the attraction force pulls the next molecule into place. This is called
transpiration pull.
- How are gravitropism and phototropism similar? How are they different?
Gravitropism and phototropism are both control systems in plants. They are the plantâs response to stimuli in their environment, telling them which way to grow. Phototropism is the plantâs response to sunlight. The stem and leaves grow toward
the sunlight (positive phototropism) and the roots grow away from sunlight (negative phototropism).
Gravitropism is the plantâs response to gravity. The stem and leaves grow against gravity (negative gravitropism), and the roots grow with gravity (positive gravitropism).
Gravitropism is the plantâs response to gravity. The stem and leaves grow against gravity (negative gravitropism), and the roots grow with gravity (positive gravitropism).