HS-LS2-5: Modeling How Photosynthesis and Respiration Cycle Carbon
In plain English: HS-LS2-5 is the NGSS high school performance expectation that asks students to develop a model of how photosynthesis and cellular respiration move carbon among the biosphere, atmosphere, hydrosphere and geosphere. Photosynthesis takes carbon dioxide from air and water into living things, and respiration returns it. It is usually taught in Biology or Environmental Science.
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.
Next Generation Science Standards · Disciplinary Core Idea: Ecosystems: Interactions, Energy, and Dynamics · Official standard
Students develop and use models of the carbon cycle that show where photosynthesis and cellular respiration fit. Photosynthesis moves carbon from carbon dioxide in the atmosphere or dissolved in water into the sugars of living things (the biosphere). Cellular respiration by plants, animals and decomposers returns it as carbon dioxide. Other transfers link these to the geosphere: remains and shells are buried in sediments, fossil fuels formed long ago from buried organisms, and burning fuels or volcanoes release carbon back to the air. Students build three kinds of models: a box-and-arrow diagram, a dice simulation and a mathematical box model with numbers.
The NGSS Clarification Statement says: "Examples of models could include simulations and mathematical models." The Assessment Boundary says: "Assessment does not include the specific chemical steps of photosynthesis and respiration." So this lesson treats photosynthesis and respiration as whole processes with inputs and outputs of carbon. The light-dependent reactions, the Calvin cycle, glycolysis and the Krebs cycle are beyond this standard and do not appear in any activity, homework or quiz item. All data sets are invented, and the global numbers are rounded values for a class model.
Learning Objectives
By the end of this lesson, students will be able to:
Develop a box-and-arrow model that shows carbon moving among the biosphere, atmosphere, hydrosphere and geosphere
Identify photosynthesis as the process that moves carbon from the atmosphere and hydrosphere into the biosphere, and cellular respiration as a process that returns it
Use indicator or sensor data from a plant and animal investigation as evidence for the roles of photosynthesis and respiration
Use a simple mathematical model (a carbon budget in Gt C per year) to predict how a change in one process changes the carbon in the atmosphere
Critique a carbon cycle model and revise it to include missing processes or spheres
Prior Knowledge Required
Students should already be comfortable with:
Photosynthesis and cellular respiration as processes with inputs and outputs HS-LS1-5
The role of photosynthesis in the cycling of matter MS-LS1-6
Conservation of atoms in chemical reactions
Adding and subtracting rates and reading a line graph
Hold up a piece of chalk (limestone), a wooden pencil and a clear glass of sparkling water. Ask students to write an answer on their own before discussing:
Warm-Up Prompt
"All three of these objects contain carbon. Which of Earth's spheres is each one part of, and how might the same carbon atom travel from the glass to the pencil, or from the chalk into the air?"
List the four spheres on the board (biosphere, atmosphere, hydrosphere, geosphere) and record the paths students suggest as arrows between them. Leave the sketch up: students revise it at the end.
Direct Instruction15-20 minutes
Two processes, four spheres. In photosynthesis, producers on land take in carbon dioxide from the air, and algae and phytoplankton take in carbon dioxide dissolved in water, and they build sugars with the carbon (overall: carbon dioxide + water, with light energy, gives sugar + oxygen). In cellular respiration, every organism, including plants, breaks down sugars and releases the carbon as carbon dioxide (overall: sugar + oxygen gives carbon dioxide + water). Carbon dioxide also dissolves into, and escapes from, ocean and lake water. Over long times, shells and remains buried in sediments move carbon into the geosphere, and volcanoes and the burning of fossil fuels move it back to the atmosphere. Use Diagram 1 as the class model and Diagram 2 as evidence from a sealed chamber.
Mathematical model: carbon atoms in and out
A plant takes in 264 g of carbon dioxide and uses it to make 180 g of sugar. Later, respiration breaks down that sugar. Use C = 12 and O = 16, so CO₂ has a mass of 44 per unit of carbon (12).
Result: Carbon in 264 g of CO₂ = 264 × 12 ÷ 44 = 72 g. The 180 g of sugar holds the same 72 g of carbon. Respiration returns 264 g of CO₂, so the same 72 g of carbon goes back to the air.
Box model of the atmosphere
Class model (Diagram 1), in Gt C per year: into the atmosphere: respiration and decay on land 117, release from water 77, fossil fuels 10, volcanoes 0.1. Out of the atmosphere: photosynthesis on land 120, dissolving 80.
Result: In = 117 + 77 + 10 + 0.1 = 204.1. Out = 120 + 80 = 200. Net change = +4.1 Gt C per year, so carbon builds up in the atmosphere.
Sealed chamber data (simulation of a small system)
A plant sits in a sealed chamber with a CO₂ sensor (invented data, Diagram 2). With the lamp on, CO₂ falls from 800 to 740 ppm in 60 minutes. With the lamp off, it rises from 740 to 760 ppm in the next 60 minutes.
Result: Light: -60 ppm per hour (photosynthesis minus respiration). Dark: +20 ppm per hour (respiration only). If respiration is the same in the light, photosynthesis alone removes 60 + 20 = 80 ppm per hour.
Tracing one carbon atom through four spheres
A carbon atom in the air dissolves into the ocean, where a phytoplankton cell takes it in by photosynthesis. The cell dies, sinks with other remains and is buried in seafloor mud.
Result: Atmosphere → hydrosphere (dissolving) → biosphere (photosynthesis) → geosphere (the buried remains become part of sedimentary rock, or over millions of years part of oil) → atmosphere (a volcano or the burning of fuel releases it).
Average time in the atmosphere
In the class model the atmosphere holds about 880 Gt C, and photosynthesis on land and dissolving together remove 200 Gt C per year.
Result: 880 ÷ 200 = 4.4 years: on average a carbon atom stays in the air only a few years before photosynthesis or the ocean takes it in, even though the total amount is growing.
After each example ask: "Which sphere does the carbon leave, which sphere does it enter, and which process moves it?" Stress that plants respire all the time, day and night, and that photosynthesis and respiration move the same carbon atoms in opposite directions.
Guided Practice15 minutes
Pairs build a model of one hectare of an invented temperate forest from the carbon flows below. They draw each flow as an arrow between spheres, label it with its process and compute the net change.
Guided practice data (invented): one hectare of forest, tonnes of carbon per year
Flow
Tonnes of carbon per year
Photosynthesis by trees and understory plants
14
Respiration by the plants
7
Respiration by animals
0.5
Respiration by decomposers in the soil
5.5
Dissolved carbon carried away by the stream
0.2
Draw the four spheres as boxes. Which box does each row move carbon out of, and which into? (Photosynthesis: atmosphere to biosphere. All three respiration rows: biosphere to atmosphere. Stream: biosphere and soil to hydrosphere.)
Compute the total leaving the forest: 7 + 0.5 + 5.5 + 0.2 = 13.2 tonnes.
Compute the net change in carbon stored in the forest: 14 - 13.2 = +0.8 tonnes per year.
Which respiration row is the largest? What does that tell you about who respires in a forest? (The plants themselves, at 7 tonnes.)
Write one sentence that uses your numbers to support or refute the claim "Forests store all the carbon they take in."
Independent Practice15 minutes
Each student develops a model for a new invented system, a lake. Phytoplankton and water plants take in 300 g of carbon per m² per year by photosynthesis. Respiration by all the lake organisms returns 285 g, and the rest, 15 g, is buried in the lake-bottom sediments.
Students draw a box-and-arrow model with all four spheres, label each arrow with its process and value, and compute the share of the photosynthesis carbon that enters the geosphere (15 ÷ 300 = 5%). They add one arrow that connects the lake water to the atmosphere and explain which way it would run on a sunny afternoon when photosynthesis is fast.
Closure5-10 minutes
Return to the warm-up sketch. Students add or correct arrows, then complete an exit ticket: "Name one arrow that moves carbon into the atmosphere and one that moves it out. For each, name the process and the sphere at the other end." Collect tickets and check that plants appear on both a photosynthesis arrow and a respiration arrow.
Differentiation Strategies
For Struggling Students
Provide a partly drawn model with the four sphere boxes and blank arrows, plus a word bank of processes to place
Use a two-column organizer, "carbon in" and "carbon out", for each sphere before computing any net change
Pair the lab color results with a simple key: yellow means more CO₂, blue means less
For Advanced Students
Ask students to add a time scale to each arrow of their model (years, centuries or millions of years) and explain why the geosphere arrows are so slow
Have students extend the mathematical box model to 20 years in a spreadsheet and compare two scenarios of their own design
Ask students to explain why carbon dioxide in the air rises and falls each year in the Northern Hemisphere, using photosynthesis and respiration
Assessment Guidance
What to Look For
Check that every model includes both processes, all four spheres and arrows that point the right way. Look for plants on a respiration arrow as well as a photosynthesis arrow, and for decomposers. Listen for students who think only animals respire, that plants get their carbon from the soil, or that burning fossil fuels is a form of respiration. Check that mathematical models add inputs and subtract outputs with units.
02
Classroom Activities
3 Activities
1
Bromothymol Blue Lab: Plants and Snails in Light and Dark
25 min, plus a check the next dayGroups of 3-4
Bromothymol blue (BTB) is an indicator that turns from blue to green to yellow as more carbon dioxide dissolves in the water. Groups set up five sealed tubes and use the color changes as evidence for which organisms add carbon dioxide to the water and which remove it.
Safety
Wear safety goggles for the whole lab. Use only dilute (0.04%) BTB from a science supplier. It can stain skin and clothes, so wear gloves and wipe up spills.
When bubbling breath through BTB, use your own straw, only breathe out gently and never suck liquid up. Stop as soon as the color changes.
Keep the lamp cord and plug away from water, and dry your hands before touching the lamp. Use an LED lamp so the tubes do not heat up.
Handle snails gently, keep each one in the indicator no more than 24 hours, then rinse it and return it to aquarium water. Never release snails or Elodea outdoors: Elodea is invasive in many regions. Seal used plants in a bag and put them in the trash, not down a drain.
Wash hands with soap after the lab.
Procedure
Make a green starting solution: add dechlorinated water to BTB until blue, then one student gently breathes out through a straw until it just turns green.
Fill five tubes with the green solution. Tube 1: Elodea, light. Tube 2: Elodea, wrapped in foil (dark). Tube 3: one snail, light. Tube 4: Elodea and one snail, light. Tube 5: solution only, light (control).
Stopper the tubes, place them 20 cm from the LED lamp and record the colors after 1 hour and the next day.
Tube 2 turned yellow even though it holds only a plant. What process caused that, and what does it show about plants?
Why is Tube 4 bluer than Tube 3 but not as blue as Tube 1?
Which spheres does this lab model, and which arrow of Diagram 1 does Tube 1 represent?
Modification for Distance Learning
Students analyze the sample data and a short teacher video of the tubes, or use a free online photosynthesis and respiration simulation, then draw the arrows the evidence supports.
2
Carbon Atom Journey: A Dice Simulation
20 minWhole class, moving between 6 stations
Each student plays one carbon atom. Six stations stand for places carbon is stored: Atmosphere, Land plants, Land animals, Ocean water, Marine plankton and Rocks and fossil fuels. At each station the student rolls a die, follows the station sign and records the station and the process on a travel log. After 10 rolls the class pools its logs.
Station Signs
Atmosphere: 1-2 Land plants (photosynthesis); 3-4 Ocean water (dissolving); 5-6 stay.
Land plants: 1-2 Atmosphere (plant respiration); 3 Land animals (eaten); 4 Atmosphere (plant dies, decomposers respire); 5-6 stay (stored in wood).
Marine plankton: 1-3 Ocean water (respiration); 4 Rocks and fossil fuels (shells and remains buried); 5-6 stay.
Rocks and fossil fuels: 1 Atmosphere (volcano or burning of fossil fuels); 2-6 stay.
Sample Class Results (invented)
28 students each rolled 10 times, 280 rolls in all. The class counted 58 moves by photosynthesis, 61 moves by respiration (by plants, animals or decomposers), 27 moves between air and water, 6 moves from plants to animals by eating, 4 moves into rocks, 1 move out of rocks and 123 rolls that stayed put.
Discussion Questions
Photosynthesis and respiration together made up 119 of the 157 moves. What does that say about their role in the carbon cycle?
In the simulation a roll of 1 releases carbon from rocks. In reality that can take millions of years. How would you change the rules to make the model more realistic?
Classify each station into one of the four spheres. Which station could belong to two spheres?
Variation: Adding Fossil Fuel Burning
Run a second round in which the Rocks and fossil fuels sign changes to "1-3 Atmosphere (burning)". Compare how many atoms end in the Atmosphere station with the first round.
3
Box Model Scenarios: What If One Arrow Changes?
20 minPairs
Pairs use the class model in Diagram 1 as a mathematical model. For each scenario they change one arrow, recompute the net change in atmospheric carbon per year and write a claim about the role of photosynthesis or respiration.
Scenarios
Scenario 1 (baseline): all arrows as in Diagram 1.
Scenario 2 (drought year): photosynthesis on land falls by 4 Gt C per year; assume everything else stays the same.
Scenario 3 (fuel cut): burning of fossil fuels is cut in half; everything else as in the baseline.
Expected Results
Scenario 1: +4.1 Gt C per year. Scenario 2: +8.1 Gt C per year. Scenario 3: -0.9 Gt C per year.
Discussion Questions
Only Scenario 3 makes the atmosphere lose carbon. Which arrows are then removing more carbon than all the arrows adding it?
In a real drought, respiration by soil decomposers may also slow down. How would that change the Scenario 2 result, and why is the "everything else stays the same" assumption a limit of the model?
Which of the three scenarios involves a change in photosynthesis, and which involves a change in a geosphere arrow?
Modification for Distance Learning
Share a spreadsheet with one row per arrow. Pairs change one cell for each scenario and post a screenshot with their claim.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: A Class Model of the Carbon Cycle
A box-and-arrow model of carbon moving among the four spheres. The values are rounded, in Gt C (billion tonnes of carbon) per year, and chosen for a class model; published estimates differ and change from year to year. Photosynthesis arrows (blue) move carbon into the biosphere, and respiration arrows (navy) move it out. With these values, more carbon enters the atmosphere each year than leaves it.
Diagram 2: Carbon Dioxide in a Sealed Plant Chamber
Invented CO₂ sensor data for a plant in a sealed chamber, drawn to scale. With the lamp on, CO₂ falls because photosynthesis removes more than respiration releases. In the shaded dark period, CO₂ rises because only respiration continues.
04
Homework Assignment
~30 min
HS-LS2-5 Homework: Building and Using Carbon Cycle Models
Directions: All data are invented or rounded for practice. Show each calculation with units. For every model, label the sphere at both ends of each arrow and the process that moves the carbon.
Part 1: Building the Model (Problems 1-3)
For each process, name the sphere the carbon leaves, the sphere it enters and whether the process is photosynthesis, respiration or neither: (a) an apple tree making sugar in its leaves; (b) a squirrel breathing out; (c) carbon dioxide dissolving into a lake; (d) a mushroom breaking down a fallen log; (e) a car engine burning gasoline made from oil; (f) a dead fern buried in swamp mud that, over millions of years, becomes part of a coal seam; (g) a volcano erupting; (h) seaweed growing in seawater. Then draw all eight as arrows on one four-sphere model.
A corn plant gains 450 g of dry mass in a season, and about 45% of that dry mass is carbon. (a) How many grams of carbon did the plant take in? (b) Carbon dioxide is 12/44 carbon by mass. How many grams of CO₂ did the plant remove from the air? (c) Name the process and the two spheres involved.
A potted plant sits in a sealed chamber with a CO₂ sensor (invented data). With a lamp on, CO₂ falls from 620 ppm to 530 ppm in 30 minutes. With the lamp off, it rises from 530 ppm to 560 ppm in the next 30 minutes. (a) Find each rate in ppm per hour. (b) Estimate the rate of photosynthesis alone and state the assumption you made. (c) Explain the result as a model of what happens in a forest over a day and a night.
Part 2: Using the Model (Problems 4-6)
In an invented ocean region, phytoplankton take in 120 g of carbon per m² per year by photosynthesis. Respiration by marine organisms returns 108 g to the water, 9 g sinks as dead material into deep ocean water and 3 g is buried in seafloor sediments. (a) Show that the carbon is conserved. (b) What percentage of the photosynthesis carbon reaches the geosphere? (c) Draw the model with the sphere at each end of each arrow.
Use the class model in Diagram 1 (atmosphere about 880 Gt C). A worldwide tree-planting effort raises photosynthesis on land by 1.5 Gt C per year, and everything else stays the same. (a) What is the new net change in atmospheric carbon per year? (b) If that rate lasts 10 years, how much carbon will the atmosphere hold? (c) Does the effort stop the build-up of carbon in the air? Use your numbers.
A student's model has only two arrows: "plants take in CO₂ (photosynthesis)" and "animals give out CO₂ (respiration)". (a) List at least three processes the model leaves out, including one that involves the hydrosphere and one that involves the geosphere. (b) The model has no decomposers. Predict what would happen to the carbon in fallen leaves and dead animals if there were no decomposers, and name the arrow the model would then lose. (c) Draw a revised model with all four spheres.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Model Components
All four spheres, both processes and correctly directed arrows
One sphere or process missing, or one arrow reversed
Calculations
Correct carbon amounts, rates and net changes with units
Correct method with one calculation or unit error
Using the Model
Claims are supported by the model and its numbers, and limits of the model are named
Claim stated but not linked to the model or its numbers
05
Quiz: 20 Questions
Interactive, with answers
Instructions
All data sets are invented, and global values are rounded for a class model. Gt C means billions of tonnes of carbon. Your score updates as you answer, and Reset quiz clears everything so you or your students can try again.
Multiple choice: pick an option to check it. Short answer: write your answer, then reveal the model answer.
0 of 20 answered · 0 correct
Question 1 of 20 · Multiple Choice
Which process moves carbon from the atmosphere into the biosphere?
Answer: B
Photosynthesis takes carbon dioxide from the air and builds its carbon into sugars in living things. Choices A and D move carbon the other way, from the biosphere to the atmosphere. Choice C moves carbon from the geosphere to the atmosphere.
Question 2 of 20 · Multiple Choice
Which organisms carry out cellular respiration?
Answer: C
All living cells respire to release energy from sugars, so plants respire day and night as well as animals and decomposers. Choice A is a frequent student error. Choice D confuses respiration with the absence of photosynthesis in the dark.
Question 3 of 20 · Multiple Choice
A snail is sealed in a tube of green bromothymol blue in the light. The next day the solution is yellow. What is the best explanation?
Answer: A
Yellow means more dissolved CO₂, and the snail releases CO₂ by respiration. Choice C is wrong because snails do not photosynthesize. Choice D reverses the color change: removing CO₂ would turn the solution blue.
Question 4 of 20 · Multiple Choice
A tube with Elodea in the light turns from green to blue. What does this evidence support?
Answer: D
Blue means less dissolved CO₂, so photosynthesis took in more than respiration gave out. Choice A goes beyond the evidence: the plant still respires, but photosynthesis is faster. Choice C is not what the indicator measures.
Question 5 of 20 · Multiple Choice
In an invented forest, photosynthesis takes in 12 tonnes of carbon per hectare per year and respiration by plants, animals and decomposers releases 10.5 tonnes. What is the net change in carbon stored in the forest?
Answer: C
Net = in - out = 12 - 10.5 = +1.5 tonnes per year, so the forest stores a little carbon. Choice A adds the two flows. Choice B subtracts in the wrong order. Choice D is only the respiration flow.
Question 6 of 20 · Multiple Choice
How can carbon move from the hydrosphere into the geosphere?
Answer: B
Organisms build shells and bodies from carbon taken from seawater, and buried remains become part of sediments and rocks such as limestone. Choice A moves carbon to the atmosphere, choice C moves it out of the geosphere, and choice D moves it from the biosphere to the hydrosphere.
Question 7 of 20 · Multiple Choice
Burning coal in a power plant moves carbon between which spheres?
Answer: A
Coal is part of the geosphere, and burning it releases carbon dioxide into the air. Choice B is a common confusion: burning is combustion, not cellular respiration, and coal is no longer living. Choice C reverses the direction.
Question 8 of 20 · Multiple Choice
The carbon in coal and oil first entered living things by which process?
Answer: C
Fossil fuels formed from the buried remains of organisms whose carbon came from photosynthesis millions of years ago. Choice A releases carbon from living things instead of bringing it in. Choice D moves carbon into water, not into living things.
Question 9 of 20 · Multiple Choice
Carbon dioxide is 12/44 carbon by mass. How many grams of carbon are in 88 g of carbon dioxide?
Answer: D
88 × 12 ÷ 44 = 24 g of carbon. Choice A is the mass of the oxygen (88 - 24). Choice B divides 88 by 12. Choice C uses 12/32, the ratio of carbon to oxygen, instead of carbon to the whole molecule.
Question 10 of 20 · Multiple Choice
A student's carbon cycle model has these arrows: air to land plants (photosynthesis); land plants, animals and decomposers to air (respiration); air to ocean water (dissolving); and ocean water to air (release). Which change would make it a model of all four spheres?
Answer: B
The model already has the atmosphere (air), the biosphere (plants, animals and decomposers) and the hydrosphere (ocean water), but no rocks, sediments or fossil fuels, so the geosphere is missing along with the arrows into and out of it. Choice A is wrong because ocean water is already the hydrosphere (a phytoplankton arrow would help, but it adds no new sphere). Choice C is wrong because plants, animals and decomposers are the biosphere. Choice D confuses energy with matter: sunlight carries no carbon.
Question 11 of 20 · Multiple Choice
In a sealed chamber, CO₂ falls by 40 ppm per hour with a lamp on and rises by 15 ppm per hour with the lamp off. If respiration is the same in the light, how fast does photosynthesis alone remove CO₂?
Answer: C
With the lamp on, the 40 ppm fall is photosynthesis minus respiration. Adding back the 15 ppm per hour of respiration gives 40 + 15 = 55 ppm per hour. Choice A subtracts instead of adding. Choice B ignores the respiration that is happening at the same time.
Question 12 of 20 · Multiple Choice
Bacteria in the deep ocean break down dead material that has sunk from the surface, and the carbon dioxide they release stays dissolved in the deep water. Which process is this, and between which spheres does it move carbon?
Answer: A
The bacteria are decomposers (biosphere). They respire the carbon in the dead material and release it as carbon dioxide into the water (hydrosphere). Choice B reverses the direction and names the wrong process: there is no light in the deep ocean for photosynthesis. Choice C is wrong because the carbon dioxide stays dissolved in deep water and does not reach the air. Choice D describes material that is buried instead of broken down.
Question 13 of 20 · Multiple Choice
In the class model of Diagram 1, suppose photosynthesis on land falls by 3 Gt C per year and nothing else changes. What is the new net change in atmospheric carbon?
Answer: D
Less photosynthesis means 3 Gt C per year less leaving the atmosphere, so the net change grows from +4.1 to 4.1 + 3 = +7.1 Gt C per year. Choice A subtracts the change instead of adding it. Choice B ignores the change.
Question 14 of 20 · Multiple Choice
In the Northern Hemisphere, carbon dioxide in the air falls during spring and summer and rises during fall and winter. Which explanation is best supported by the carbon cycle model?
Answer: B
In the growing season, photosynthesis removes more carbon dioxide than respiration returns; when leaves fall and growth stops, respiration and decay continue, so carbon dioxide rises. Choice C is false: animals respire all year. Choice D has no seasonal basis.
Question 15 of 20 · Short Answer
Develop a model: sketch or describe a carbon cycle with all four spheres and at least five labeled arrows. At least one arrow must be photosynthesis and at least two must be cellular respiration by different kinds of organisms.
A full answer names the atmosphere, biosphere, hydrosphere and geosphere and includes arrows such as: photosynthesis (atmosphere to land plants), photosynthesis (dissolved CO₂ to phytoplankton), respiration by plants and animals (biosphere to atmosphere), respiration by decomposers (dead matter to atmosphere), dissolving and release (atmosphere and hydrosphere), burial of remains (biosphere or hydrosphere to geosphere) and burning of fossil fuels or volcanoes (geosphere to atmosphere). Every arrow points from the sphere that loses carbon to the sphere that gains it.
Question 16 of 20 · Short Answer
A tree adds 30 kg of dry wood in a year. Wood is about 50% carbon by mass, and carbon dioxide is 12/44 carbon. How much carbon did the tree store, and how much carbon dioxide did it remove from the air?
Carbon stored = 30 × 0.50 = 15 kg. CO₂ removed = 15 × 44 ÷ 12 = 55 kg. Photosynthesis moved this carbon from the atmosphere into the biosphere.
Question 17 of 20 · Short Answer
A pond freezes over for the winter and snow covers the ice, so almost no light reaches the water. Use the carbon cycle model to predict what happens to the dissolved carbon dioxide in the pond water, and explain why.
Dissolved CO₂ rises. With almost no light, photosynthesis by algae and water plants nearly stops, but fish, plants and decomposers keep respiring and releasing CO₂. The ice also blocks the exchange of gases between the water and the air, so the CO₂ stays in the hydrosphere.
Question 18 of 20 · Short Answer
Explain how a carbon atom that is now in a limestone cliff could later become part of a leaf.
The atom must leave the geosphere first: weathering of the rock or a volcano can release it as carbon dioxide or dissolved carbon. As carbon dioxide in the atmosphere (or dissolved in water), it can be taken in by a plant by photosynthesis and built into sugar in a leaf, which is part of the biosphere. This path can take a very long time.
Question 19 of 20 · Short Answer
Evaluate the claim: "Planting trees removes carbon from the atmosphere permanently."
The claim is only partly supported. A growing tree does move carbon into the biosphere by photosynthesis. But when the tree dies and decomposes or is burned, respiration by decomposers or burning returns the carbon to the atmosphere. The carbon stays out of the air only as long as the wood lasts, unless it is buried and moves into the geosphere.
Question 20 of 20 · Short Answer
A simple model gives these yearly values: fossil fuels add 9.5 Gt C to the atmosphere, land plants and soils take in 3.2 Gt C more than they release, and the ocean takes in 2.8 Gt C more than it releases. What is the net change in atmospheric carbon per year?
Net = 9.5 - 3.2 - 2.8 = +3.5 Gt C per year. The atmosphere gains carbon because the geosphere source is larger than the combined net uptake by the biosphere and the hydrosphere.
0 of 20 answered · 0 correct
06
Frequently Asked Questions
10 Questions
What does HS-LS2-5 mean?
HS-LS2-5 asks students to develop a model that shows how photosynthesis and cellular respiration move carbon among the biosphere, atmosphere, hydrosphere and geosphere. The model can be a diagram, a simulation or a mathematical model. The code means High School, Life Science, core idea 2 (Ecosystems: Interactions, Energy, and Dynamics), performance expectation 5.
Is HS-LS2-5 taught in Biology or Earth Science?
It is usually taught in the ecology unit of Biology, and it also fits Environmental Science. Earth Science courses cover the closely related HS-ESS2-6, which asks for a quantitative carbon cycle model, so the two are often taught together.
What is the assessment boundary for HS-LS2-5?
Assessment does not include the specific chemical steps of photosynthesis and respiration. Students need the inputs and outputs of each process and the direction it moves carbon. The light-dependent reactions, the Calvin cycle, glycolysis and the Krebs cycle are beyond this standard.
Do plants carry out cellular respiration?
Yes, all the time. Plant cells respire to release energy from the sugars they made, in the light and in the dark. In the light, photosynthesis is usually faster, so a plant takes in more carbon dioxide than it gives out. In the dark, only respiration happens, which the bromothymol blue lab shows with a foil-wrapped tube.
What kinds of models count for HS-LS2-5?
The Clarification Statement names simulations and mathematical models. A box-and-arrow diagram, a dice or computer simulation of a carbon atom's path, and a budget that adds the flows into and out of a reservoir all work, as long as the model shows both processes and all four spheres.
How do the geosphere and hydrosphere fit into the carbon cycle?
Carbon dioxide dissolves into ocean and lake water, where algae and phytoplankton use it for photosynthesis. Shells and remains that are buried in sediments carry carbon into the geosphere, where it can stay for millions of years as rock or fossil fuel until volcanoes, weathering or burning release it.
How is HS-LS2-5 different from HS-ESS2-6?
HS-LS2-5 centers on the biological processes, photosynthesis and respiration, and how they move carbon between spheres. HS-ESS2-6 asks for a quantitative model of carbon cycling among the same spheres from an Earth systems view, including human activity. A mathematical box model like the one in this lesson is a bridge between them.
Is the bromothymol blue lab safe for students?
Yes, with simple precautions. Use dilute (0.04%) indicator, goggles and gloves, and have students breathe out gently through their own straws without ever sucking. Use LED lamps, keep cords away from water, return snails to aquarium water and throw used Elodea in the trash rather than releasing it, because it is invasive in many places.
Is burning fossil fuels a kind of respiration?
No. Both release carbon dioxide, but respiration happens inside living cells, while burning (combustion) is a chemical reaction outside any organism. In a carbon cycle model, burning is an arrow from the geosphere to the atmosphere, and respiration is an arrow from the biosphere.
What mistakes do students make in carbon cycle models?
Many students leave out plant respiration and decomposers, draw sunlight as a source of carbon, or say plants get their carbon from the soil. Others forget the hydrosphere and geosphere entirely. Asking "which sphere loses carbon and which gains it?" for every arrow catches most of these errors.
07
Related Standards
5 standards
These standards connect to HS-LS2-5: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
MS-LS1-6Prerequisite
Explain with evidence the role of photosynthesis in the cycling of matter and flow of energy
NGSS is a registered trademark of WestEd. 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.
