Travel Deep Inside a Leaf

Grade level: 5-12+
Length: 3 minutes
Next Generation Science Standards: MS-LS1.A, MS-LS1.C, HS-LS1.A, HS-LS1.C (DCIs); Systems, Scale, Energy and Matter (CCCs)

Take a journey inside a leaf of a redwood tree! Enter the stoma and view the inside of a plant cell, translucent enough to capture light from the sun.  Fly by familiar structures like the nucleus and mitochondria, and settle into the chloroplast to watch photosynthesis at work. Reflect on the change in scale as you travel down to the molecular level!

Turn on the subtitles of the annotated version of this video by clicking the CC icon, or read through this transcript, to better understand what is being illustrated and what is intentionally left out.

Pair this video clip with the Modelling Photosynthesis and Cellular Respiration (Grades 5-10) lesson plan. Prepare all the materials first!

Before Watching
Explain that the classroom will represent a leaf, and that each table within the classroom will represent a cell within the leaf. To visualize this "classroom as a leaf," watch the brief video.

While Watching
Clarify that in order to see parts of the system well, the animators removed other elements of this visual molecular model, including water, oxygen, carbon dioxide,  and photons. So, the class will visualize the movement of those key players in an active simulation.

After Watching
Proceed to explain the rules of the active model, in which students will use ping pong balls and egg cartons to simulate the production of sugar molecules to store energy (photosynthesis).

Need to jazz up your cellular structure and function unit? Start by allowing students to sit in the inquiry seat. For this exercise, show the video, and consider stopping the clip at 1:45 so that you don't dive all the way to the single molecule level.

Before Watching
Explain that they are aliens who have come to visit Earth to study it, and their technology allows them to change size to view things at smaller scales. After watching this video, each table group will be responsible for describing what they witnessed, creating new names for things using made-up terms in the alien language.

While Watching
Stop at one or more of some key freeze frames (suggestions: 0:37, 1:00, 1:06, 1:35). Have students make a quick sketch in their notebook.

After Watching
Have table groups pick the view most interesting to them for the alien assignment: make a funny description of at least one structure they see, being sure to describe the structure and make a prediction as to its function--even if wild! Groups should also name the structure. You can model this at the stoma view (0:37) by saying something like "As we approached the green wall, we entered what looked to be a volcano with a wide rim stuffed with pea-shaped thingamabobs. We might guess this volcano is used to spit out arrows at intruders, so we'll call it the Pea-Spitter for now." 

As groups share out, students can jot down these fake names and functions into their notebook. The idea here is to highlight the important biological tenet of structure and function and harness the creativity of your students to create a shared language as you enter what some find a tedious unit. Then, proceed to your standard curriculum!

Before Watching
Share how scientists learn different things by observing a phenomenon or system at different scales. The clip starts viewing the branch of a redwood tree, familiar to our naked eye as being less than a meter long. Several times throughout the animation the view changes scale, as indicated by the flashing magnifying glass.

Set a focus question such as: To what degree do you think the view changed, by orders of ten, compared to the previous "scene"?

While Watching

• Allow students to individually ponder the focus question before discussing with nearby peers, and expect to replay portions of the video multiple times.
• Hear some arguments from students, and encourage them to point to evidence they viewed in the clip and connect this to any prior knowledge.
• Reveal answers, but focus on the sense of awe -- the "correct" answer is not important:
  • Redwood tree: meters
  • Leaf blade: centimeters, or one hundredth of a meter
  • Stoma (~0:37): millimeters, or one thousandth of a meter
  • Palisade cells (~0:50): 30-60 micrometers (μm) wide, so on the order of a millionth of a meter
  • After entering chloroplast (~1:05): the view jumps to the nanometer (nm) level, so all the molecular views are a billion times smaller than a meter

After Watching
Spend several class periods completing the How Big is Big? (Grades 6-12) lesson, which allows students to practice mathematics and computational thinking to create scale models on their own.

This animation is a model, and has its strengths and limitations. In order to model something well, visual artists have to make decisions about what to represent and how best to do so. What’s present in this model, and what’s intentionally missing or altered?

• To animate a scientifically accurate leaf, artists studied the texture of a redwood leaf specimen on a glass slide at high resolution. They even counted the stomata, and used that exact count for this film!

• Because light is required for photosynthesis, the plant cells and their associated organelles must be translucent to allow light to pervade the space. That's why you'll see translucent organelles in this visualization. Animators, however, "lit up" the color to make the interior of the cell appear bright.

• To highlight this photosystem well, animators simplified the process by emphasizing the role played by one plant pigment: chlorophyll, which functions to collect light from the sun to fuel photosynthesis. Because chlorophyll absorbs light in the blue and red portions of the visible light spectrum, reflecting the green wavelengths, most of the plant’s organelles appear green, since the animation is designed for humans vision. Many other plants pigments—red, yellow, orange, even blue!—exist in nature and have particular roles in photosynthesis or plant structure. They are not evident here.

• During the last scene, we see the light-dependent reactions of photosynthesis at the thylakoid membrane assuming a continuous source of light energy from the leaf’s external environment. To accurately visualize movement at the molecular level, animators had to slow down time. The molecules shown are moving at least a million times slower than they would in real life! What does this tell you about the nature of energy and matter?

• Look closely to see bright pulses traveling through the lipid layer of the thlyakoid's surface: this represents electrons passing energy along in the classic bucket brigade!

• In order to see parts of the system well, the animators removed other elements of this visual molecular model, including water, oxygen, various proteins, ADP, photons, and electrons. Instead of seeing all reactants and products of the light-dependent reactions of photosynthesis, the animation focuses on the cellular machinery that allows for the creation of ATP.

• You won’t spot glucose or carbon dioxide, either. These molecules play a role in the Calvin Cycle, which takes place in the fluid-filled space outside of the thylakoids.

While this video doesn't necessarily cover the following standards in depth, it is a compelling resource you can use to supplement your curriculum that does.

Disciplinary Core Ideas (Grades 6-12)
LS1.A: Structure and Function
LS1.C: Organization for Matter and Energy Flow in Organisms

Crosscutting Concepts 
Systems and System Models
Scale, Proportion, and Quantity
Energy and Matter