Science · 6th Grade · Energy Flow in Ecosystems · Weeks 19-27

Photosynthesis: Capturing Sunlight

Students investigate the chemical processes that allow plants to make food using sunlight.

• Common Core State Standards
• MS-LS1-6

About This Topic

Photosynthesis and Cellular Respiration are the two fundamental processes that power life on Earth. Students learn how plants act as energy converters, taking in sunlight, water, and carbon dioxide to create glucose (food) and oxygen. Conversely, they explore how both plants and animals use cellular respiration to break down that food and release the energy needed for growth and repair. This aligns with MS-LS1-6 and MS-LS1-7.

These processes are often taught as a cycle: the products of one are the reactants of the other. This helps students understand the flow of energy and the cycling of matter in ecosystems. It also highlights the critical role of the sun as the ultimate energy source for almost all life on our planet.

This topic comes alive when students can physically model the chemical equations using manipulatives or participate in simulations that track the movement of carbon atoms through a plant and an animal.

Key Questions

1. Explain how plants turn sunlight and air into solid wood and leaves.
2. Analyze the role of chlorophyll in the process of photosynthesis.
3. Construct a diagram illustrating the inputs and outputs of photosynthesis.

Learning Objectives

• Explain the chemical equation for photosynthesis, identifying reactants and products.
• Analyze the role of chlorophyll in absorbing light energy for photosynthesis.
• Construct a diagram illustrating the inputs (sunlight, water, carbon dioxide) and outputs (glucose, oxygen) of photosynthesis.
• Compare the energy conversion in photosynthesis to the energy release in cellular respiration.

Before You Start

Introduction to Cells

Why: Students need to know that plants have specialized structures like chloroplasts where photosynthesis occurs.

Basic Chemical Reactions

Why: Understanding that chemical reactions involve rearranging atoms and forming new substances is foundational to grasping photosynthesis.

Key Vocabulary

photosynthesis	The process plants use to convert light energy, water, and carbon dioxide into chemical energy in the form of glucose (sugar) and oxygen.
chlorophyll	The green pigment found in plant cells that absorbs light energy, primarily red and blue wavelengths, needed for photosynthesis.
glucose	A simple sugar that is the primary product of photosynthesis, serving as food for the plant and a source of energy.
stomata	Small pores, usually on the underside of leaves, that allow for gas exchange (carbon dioxide in, oxygen out) and transpiration.

Watch Out for These Misconceptions

Common Misconception

Students often think that plants get their 'food' from the soil.

What to Teach Instead

Clarify that soil provides minerals and water, but the actual 'food' (glucose) is made from air (CO2) and sunlight. Using a 'mass' activity where students see that a tree's weight comes mostly from the air helps correct this.

Common Misconception

Many students believe that only animals perform cellular respiration and plants only do photosynthesis.

What to Teach Instead

Emphasize that plants also need energy to grow, especially at night when there is no sun. Peer discussion about how a seed grows underground (without light) can help surface the need for stored energy and respiration in plants.

Active Learning Ideas

Peer Teaching: The Carbon Cycle Comic

Students create a comic strip where a carbon atom is the main character. They must show the atom moving from the air into a plant (photosynthesis) and then into an animal (respiration), explaining the change at each step.

45 min·Pairs

Inquiry Circle: Elodea Bubbles

Groups place aquatic plants (Elodea) in water under different light conditions. They count the oxygen bubbles produced to see how light intensity affects the rate of photosynthesis, then share their data to find patterns.

50 min·Small Groups

Think-Pair-Share: The Oxygen Mystery

Students discuss with a partner: 'If plants make oxygen, why do they also need to perform cellular respiration?' They then share their conclusions about how plants use the food they make to stay alive.

15 min·Pairs

Real-World Connections

• Forestry professionals, like forest rangers, understand photosynthesis to predict tree growth rates and manage forest health, as the wood of trees is essentially stored plant energy.
• Farmers and agricultural scientists utilize knowledge of photosynthesis to optimize crop yields, adjusting factors like light exposure, water availability, and CO2 levels to maximize sugar production in plants.

Assessment Ideas

Quick Check

Present students with a partially completed photosynthesis diagram showing only sunlight and water as inputs. Ask them to identify the missing reactant and label the two products of the process.

Discussion Prompt

Pose the question: 'If a plant is kept in complete darkness but has plenty of water and carbon dioxide, what will happen to its growth and why?' Guide students to connect the absence of light to the inability to perform photosynthesis and create glucose.

Exit Ticket

On an index card, have students write the chemical formula for glucose and explain in one sentence how plants obtain the energy to create it. They should also list one gas that enters the plant and one gas that leaves the plant during this process.

Frequently Asked Questions

What is the formula for photosynthesis?

The basic formula is: Carbon Dioxide + Water + Sunlight yields Glucose + Oxygen. In 6th grade, focus on the 'ingredients' and 'products' rather than the complex chemical balancing.

Why are plants green?

Plants contain a pigment called chlorophyll, which is excellent at absorbing red and blue light for energy but reflects green light. That reflected green light is what we see.

How can active learning help students understand photosynthesis?

Active learning, like the 'Elodea Bubbles' experiment, provides visual proof of a chemical process that is otherwise invisible. By manipulating light levels, students see the direct cause-and-effect relationship between energy input and oxygen output. Creating 'Carbon Cycle Comics' also forces students to synthesize their knowledge and explain the transition of matter in their own words.

Do humans perform photosynthesis?

No, humans are consumers. We cannot make our own food from sunlight. We must eat plants (or animals that ate plants) to get the glucose we need for cellular respiration.

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