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Photosynthesis

In this unit, students explore the phenomena of forest ecosystems, studying the science phenomena of interactions between living things and the environment. In this lesson, students investigate how plants carry out photosynthesis to convert the sun’s energy into a form of energy they can use to grow and develop. This page highlights key parts of this lesson.

Science Background for Teachers:

The science background section gives teachers more in-depth information on the phenomena students explore.. Below is an excerpt of the science background information from this lesson on phenomena of photosynthesis. 

How Plants Make Food

Different parts of the plant work together so that photosynthesis can happen. Photosynthesis happens in the chloroplasts of plant cells. Remember that plant cells are the only kind of cells with chloroplasts, which make chlorophyll, a pigment that absorbs sunlight.

Plant leaves also have pores called stomata that allow plants to take in carbon dioxide from the environment and release oxygen. The stomata open and close so that plants can constantly exchange these gasses, similar to how people always breathe in oxygen and breathe out carbon dioxide. The back-and-forth exchange of carbon dioxide and oxygen between plants, animals, and the environment is called the oxygen cycle.

The roots collect water and nutrients from the soil, and the stem transports the water and nutrients between the roots and the rest of the plant. The stem also absorbs some water, although not as much as the roots.

In scientist Suzanne Simard’s experiment, she put plastic bags over individual trees. She then injected carbon-14 into one bag covering a birch tree to see if it would transfer some of the carbon to other trees nearby and waited for an hour. She then used a Geiger counter to detect whether the carbon-14 had been transferred to the other trees.

Simard’s results showed that at that time of year, in the summer, the birch trees sent extra carbon to the fir trees. Suzanne believes this benefited the fir trees because the fir trees tended to be more shaded than the birch trees, so they received less sunlight to carry out photosynthesis. Suzanne and her team did follow-up experiments that showed at later times of the year, the fir trees sent more carbon to the birch trees. She believes this is because birch trees begin to lose their leaves in the fall, making it harder to photosynthesize, while firs keep their leaves.

Simard and other researchers have found that trees in a forest are connected through a complex underground network made up of fungi mycelium that carry carbon molecules between the trees. A forest is any area covered with trees. “They're actually sending messages back and forth that balance the resource distribution among the community,” Simard said in a 2017 interview with TED Radio Hour.

Sunlight in Forests

Simard’s research has also led her to conclude that there are “hub trees,” which she calls mother trees because they nurture their young. The mother trees have grown tall enough to reach the forest’s canopy, which is the upper layer of the forest where the treetops meet and form a thick cover. Canopy trees access most of the forest’s energy because of the amount of sunlight they receive.

The mother trees help their young, which grow in the understory. The understory of a forest exists below the canopy, which means that less sunlight reaches this layer because it has to filter through the canopy. As a result, plants in this layer must be able to make food with a limited amount of sunlight. Suzanne’s research showed that mother trees send extra carbon to the seedlings growing in the understory, which helps the seedlings survive because plants use some carbon atoms to build and repair their different structures. It also benefits the mother trees. When their offspring survive, they are more likely to reproduce and pass their genes along to future offspring.

There is even less sunlight on the forest floor than in the understory. The forest floor is blanketed with decaying leaves, twigs, fallen trees, animal scat, moss, and other organic particles.

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Science Lesson: Exploring Photosynthesis

In this lesson, students explore the phenomena of how plants cycle matter and energy through photosynthesis. They focus on plants in a forest as they create an experiment to test how different light conditions affect the rate of photosynthesis phenomena in aquatic plants. 

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Science Big Ideas

  • Like all living things, plants need energy to grow and develop. However, unlike most other living things, including fungi and animals, plants make their own food using sunlight, carbon dioxide, and water. 
  • Plants cycle both matter and energy in photosynthesis.
  • Plants need sunlight to carry out photosynthesis because it is a chemical reaction. The sunlight is the input of energy that turns carbon dioxide and water into glucose and oxygen.
  • Different parts of the plant work together to collect what the plant needs to carry out photosynthesis.
  • There are layers in any forest, and different amounts of energy reach each layer.
  • A plant’s ability to carry out photosynthesis depends on how much sunlight it can access, which is partly dependent on which layer of the forest it is in.

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Science Essential Questions

  • How do plants get the energy they need to survive?
  • What kinds of matter do plants need to take in to carry out photosynthesis?
  • Where do plants get their source of energy to carry out photosynthesis?
  • Why are a plant’s leaves important for photosynthesis?
  • How does a plant get the water it needs to carry out photosynthesis?
  • Why do plants take in minerals from the soil if they make their own food?
  • Why do trees that grow in the canopy access most of the forest’s energy?
  • Why is there less sunlight available in the understory?
  • Why does the forest floor have even less sunlight than the understory?

Common Science Misconceptions

close-circle-red Misconception: Food must come from outside of an organism.

checkmark-circle-green Fact: Plants make their own food through photosynthesis, unlike animals, which must eat other organisms for food.

close-circle-red Misconception: Healthy ecosystems do not change.

checkmark-circle-green Fact: Healthy ecosystems are dynamic and constantly changing.

Science Vocabulary

Canopy : the upper layer of a forest where the treetops of most of the trees meet to form a layer of habitats for insects, birds, and tree-climbing mammals

Forest : an area of land covered by trees

Forest Floor : the part of the forest that is blanketed with decaying leaves, twigs, fallen moss, and other organic particles

Oxygen Cycle : the back-and-forth exchange of carbon dioxide and oxygen between plants, animals, and the environment

Photosynthesis : the process of turning sunlight, carbon dioxide, and water into glucose and oxygen

Understory : the middle layer of the forest that contains a mixture of small and immature trees that provide shelter for animals

Lexile(R) Certified Non-Fiction Science Reading (Excerpt)

Studying How Trees Communicate

Scientist Suzanne Simard has been studying forests in Canada for 30 years. A forest is an area of land covered with trees. In 1997, Suzanne had a question: Do trees in a forest communicate with one another and share resources?

Her hypothesis was that trees in a forest are connected together in some kind of network. Suzanne designed an experiment to test her hypothesis. She focused on an area of the forest that had two kinds of trees: birch trees and fir trees.

Testing Trees and Carbon

She put plastic bags over individual trees. She then injected carbon-14 into one bag covering a birch tree to see if it would transfer some of the carbon to other trees nearby.

Carbon-14 is an isotope of carbon. This means it is one form of carbon. All isotopes of an element have the same number of protons, but they have a different number of neutrons. Carbon-12 is the most common form of carbon, making up 99 percent of all carbon in Earth’s atmosphere. Carbon-13 makes up 1 percent, and carbon-14 is found in trace amounts. Suzanne knew that carbon-14 molecules would bond with oxygen molecules in the environment to form carbon dioxide (CO2). She also knew that plants need carbon dioxide to survive.

Making Food

Plants need carbon dioxide because it is an important part of photosynthesis. Photosynthesis is the process of turning sunlight, carbon dioxide, and water into glucose and oxygen. All plants carry out photosynthesis. This is how they make food. The glucose is a kind of sugar that holds chemical potential energy. Plants need this energy for growth and development.

Plants are constantly taking in carbon dioxide and releasing oxygen. This is similar to how people are always breathing, but people breathe in oxygen and release carbon dioxide. The back- and-forth exchange of carbon dioxide and oxygen between plants, animals, and the environment is called the oxygen cycle.

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Hands-on Science Activity

For the hands-on activity of this lesson, students explore investigative phenomena on different light conditions and how they affect the rate of photosynthesis in aquatic plants. Students do this by measuring how much carbon dioxide aquatic plants remove from water. They then use the data to determine the relationship between the intensity of light and the ability of plants to carry out the phenomena of photosynthesis.

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Standards citation: NGSS Lead States. 2013. Next Generation Science Standards: For States, By States. Washington, DC: The National Academies Press. Neither WestEd nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.