HS-LS2-5: Photosynthesis, Respiration and the Carbon Cycle
Also called NGSS HS-LS2-5 · Topic: Matter and Energy in Organisms and Ecosystems · Life Science
The Official Wording
Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere, hydrosphere, and geosphere.
Clarification statement: Examples of models could include simulations and mathematical models.
Assessment boundary: Assessment does not include the specific chemical steps of photosynthesis and respiration.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What HS-LS2-5 Means in Plain English
Students build a model of how carbon moves among living things, the air, the oceans and the rocks and soil. Photosynthesis pulls carbon dioxide out of the air and water into sugar, and respiration by plants, animals and decomposers puts it back.
The model can be a diagram, a simulation or a set of numbers, and it leaves out the chemical steps inside cells. It builds on HS-LS1-5 and HS-LS1-7 and leads to HS-ESS2-6, the full carbon cycle with fossil fuels and climate.
HS-LS2-5 I Can Statements
- I can model carbon moving among air, water, land and life.
- I can explain how photosynthesis and respiration move carbon in opposite directions.
- I can use carbon dioxide data to show the effect of plant growth.
HS-LS2-5 Worked Example
Students read real monitoring data from Mauna Loa in Hawaii, where air has been sampled since 1958, and look for the planet's yearly breath.
Why does the amount of carbon dioxide in the air rise and fall every year?
Set It Up: A graph of monthly carbon dioxide readings from Mauna Loa Observatory over several recent years, graph paper and colored pencils.
- Mark the highest and lowest point in each year on the graph.Highs around May, lows around September and October
- Measure the size of the swing within one year.About 6 ppm from high to low
- Match the low point to the Northern Hemisphere seasons.Lows follow summer, when forests and crops grow fastest
- Match the high point to the seasons.Highs follow winter, when decay and respiration outpace growth
- Draw a model with arrows between air, plants, soil and ocean.Photosynthesis arrow down, respiration arrow up
Carbon dioxide drops through each northern summer and climbs through each northern winter, on top of a steady rise from year to year.
Why it works: The Northern Hemisphere holds most of Earth's land plants, so its summer photosynthesis pulls in more carbon than respiration returns, and the balance flips in winter.
Common Mistakes with HS-LS2-5
Giving respiration only to animals
“Plants take in carbon dioxide and animals give it out”
Try this: Plants respire too, and decomposers return huge amounts of carbon from dead matter. Put respiration arrows on every living group in the model.
Leaving out the ocean and rocks
A model with only air and living things
Try this: Add the ocean, where algae photosynthesize and water absorbs carbon dioxide, and rocks and soil, which store carbon for very long times.
Reading the seasonal dip as the long-term trend
“Carbon dioxide is falling because the September value is lower”
Try this: Compare the same month across years. The yearly swing comes from plant growth and decay, while the steady climb comes mostly from burning fossil fuels.
Where HS-LS2-5 Fits
Practice HS-LS2-5
Quick Check
- Which process removes carbon dioxide from the air, and which adds it?
- Why is carbon dioxide lowest at Mauna Loa in early fall?
- Name two places carbon is stored outside living things.