HS-LS2-3: Matter and Energy With and Without Oxygen
In plain English: HS-LS2-3 is the NGSS high school performance expectation that asks students to construct and revise explanations, based on evidence, of how matter cycles and energy flows when oxygen is present (aerobic) or absent (anaerobic). Students compare decomposition, gases and heat in compost, wetlands, lakes and yeast cultures. It is usually taught in Biology.
Construct and revise an explanation based on evidence for the cycling of matter and flow of energy in aerobic and anaerobic conditions.
Clarification Statement: Emphasis is on conceptual understanding of the role of aerobic and anaerobic respiration in different environments.
Assessment Boundary: Assessment does not include the specific chemical processes of either aerobic or anaerobic respiration.
Next Generation Science Standards · Disciplinary Core Idea: Ecosystems: Interactions, Energy, and Dynamics · Official standard
Why does a compost pile get hot while a bog can hold dead plants for thousands of years? In this lesson students build explanations, backed by data, of how matter cycles and energy flows in aerobic conditions (oxygen present) and anaerobic conditions (no oxygen). They compare how fast dead matter breaks down, which gases carry its carbon back to the air, and how much of its stored energy is released, in settings from a yeast bottle to a lake. Then they revise their explanations when new evidence arrives, as scientists do.
The NGSS Clarification Statement says: "Emphasis is on conceptual understanding of the role of aerobic and anaerobic respiration in different environments." The Assessment Boundary says: "Assessment does not include the specific chemical processes of either aerobic or anaerobic respiration." So students work with what goes in, what comes out, how fast and how much energy, and never with the reaction steps inside cells, which are beyond this standard. Energy values are standard values, rounded, and every data set is invented for teaching and labeled as invented.
Learning Objectives
By the end of this lesson, students will be able to:
Use data on mass loss, gases and temperature to explain how dead matter is broken down in aerobic and anaerobic conditions
Explain, with evidence, where the carbon in dead matter goes in each condition and why organic matter builds up where oxygen is missing
Use energy data to explain why organisms release far less of the energy in food without oxygen, and where the rest of the energy remains
Explain how matter cycles while energy flows through an ecosystem and leaves as heat
Revise an explanation when new data do not fit it, and name the evidence that changed it
Prior Knowledge Required
Students should already be comfortable with:
Models of how matter cycles and energy flows among living and nonliving parts of an ecosystem MS-LS2-3
Food molecules are rearranged in chemical reactions that release energy MS-LS1-7
Cellular respiration uses oxygen and food molecules and releases energy HS-LS1-7
Show two photos: steam rising from a compost pile on a cold morning, and a dark, waterlogged bog with layers of old plant matter.
Warm-Up Prompt
"Both places are full of dead plants and decomposers. Why is one steaming hot and breaking down in weeks, while the other has kept dead plants for thousands of years? Write your best explanation, and name one measurement that could test it."
Collect explanations and sort them on the board (air, water, temperature, kinds of decomposers). Many students do not think of oxygen at first. Keep the list visible: students return to it and revise it at the end of the lesson.
Direct Instruction15-20 minutes
Two conditions, one set of atoms. When oxygen is present, decomposers and other organisms break food molecules down completely: the carbon leaves as carbon dioxide, the hydrogen as water, and most of the stored energy is released. The organisms use some of that energy to live and grow, and all of it ends up as heat. When oxygen is missing, many microbes, yeast and even muscle cells can still break down sugar, but only partly. Their products, such as ethanol, lactic acid and methane, still hold most of the energy, so each gram of food gives the organism much less energy. Anaerobic breakdown is slow, so dead matter builds up. In both cases the atoms are conserved and keep cycling, but the energy flows one way and leaves as heat. Diagram 1 summarizes the model.
Energy released when 100 g of each substance burns (standard values, rounded)
Substance
Energy released (kJ per 100 g)
Glucose (a sugar)
1,560
Ethanol
2,970
Methane
5,550
Mass loss with and without oxygen (matter)
Mesh bags with 10.0 g of dry leaves are buried for 12 months (invented). In well-drained forest soil 3.5 g remain; in soil that stays waterlogged 8.0 g remain.
Result: Mass lost: 6.5 ÷ 10.0 = 65% in drained soil against 2.0 ÷ 10.0 = 20% in waterlogged soil. Water fills the soil pores and blocks oxygen, so decomposers work slowly; the carbon that is not released as gas stays in the soil and builds up.
Where the energy goes (energy)
Using the table: yeast working without oxygen turn 100 g of glucose into about 51 g of ethanol and 49 g of carbon dioxide. Microbes in a swamp turn 100 g of glucose into about 27 g of methane and 73 g of carbon dioxide.
Result: Energy left in the ethanol: 0.51 × 2,970 ≈ 1,515 kJ, or about 97% of the 1,560 kJ in the glucose. Left in the methane: 0.27 × 5,550 ≈ 1,500 kJ, about 96%. With oxygen, the products are CO₂ and water, which hold almost none, so nearly all 1,560 kJ is released.
Oxygen in a lake (a different environment)
Dissolved oxygen in a lake in late summer (invented), from the surface to 20 m: 0 m: 8.4, 4 m: 8.2, 8 m: 5.5, 12 m: 2.0, 16 m: 0.6, 20 m: 0.2 mg/L.
Result: Oxygen falls from 8.4 to 0.2 mg/L. Algae that die and sink are decomposed in the deep water, which is not mixed with air in summer, so aerobic decomposers use up the oxygen. Below 12 m the water is nearly anaerobic: breakdown slows, methane forms and organic mud builds up.
Revising an explanation: a compost pile
An unturned compost pile's temperature on days 0, 4, 8, 12, 16, 20: 20, 45, 62, 58, 41, 33 °C (invented). Explanation 1: "It cooled because the food ran out." New data: the pile had lost only 30% of its mass by day 20, an oxygen probe read 15% on day 4 and 2% on day 16 (air is 21%), and after the pile was turned on day 20 it reached 55 °C by day 24.
Result: Plenty of matter was left, oxygen had fallen, and mixing in air brought the heat back. Revised explanation: the decomposers ran short of oxygen, not food. With oxygen they release much more energy as heat; without it, breakdown and heating slow down.
Carbon leaving a rice field in two forms
Carbon released from rice fields over one season, per m² (invented). Flooded all season: 12 g as methane and 80 g as CO₂. Drained for two weeks mid-season: 5 g as methane and 95 g as CO₂.
Result: Methane share: 12 ÷ 92 ≈ 13% when flooded, 5 ÷ 100 = 5% when drained. Carbon cycles back to the air either way, but flooding (no oxygen in the mud) sends more of it out as methane.
After each example, ask two questions: "Where did the atoms go?" and "Where did the energy go?" Correct the idea that matter is "used up" or turned into energy: the atoms are all still there, in gases, water, products or new cells.
Guided Practice15 minutes
Pairs analyze an invented experiment. Two flasks each hold 100 mL of broth with 1.0 g of sugar and the same small amount of yeast. Flask A is shaken all day with a loose foam plug, so air keeps reaching the yeast. Flask B is sealed with a valve that lets gas out but no air in.
Guided practice data after 24 hours (invented)
Flask
Yeast cells at start (millions per mL)
Yeast cells at 24 h (millions per mL)
Sugar left (g)
Ethanol made (g)
A, with air
1
61
0.1
trace
B, sealed
1
13
0.2
0.4
Find how much sugar each flask used and how many new yeast cells (millions per mL) each flask grew.
Compute new cells per gram of sugar used for each flask. How many times as much growth did each gram of sugar give in Flask A?
Use the energy table from Direct Instruction to explain where most of the energy of the sugar used in Flask B ended up.
Write an explanation of the difference between the flasks that covers both matter (where the carbon went) and energy.
Independent Practice10-15 minutes
Each student analyzes invented data from a coastal marsh with thick peat soil. Part of the marsh was drained with ditches 40 years ago to make farmland.
Independent practice data (invented)
Measurement
Undrained marsh
Drained field
Water table below the surface
5 cm
80 cm
CO₂ released (g per m² per year)
250
2,000
Methane released (g per m² per year)
25
1
Change in ground height over 40 years
no change
50 cm lower
Students compute how many times more CO₂ the drained field releases (8 times) and how fast its surface sank (1.25 cm per year), then write an explanation of why draining turned stored peat into CO₂ and why the undrained marsh releases more methane. Ask them to name one more measurement that would test their explanation.
Closure5 minutes
Exit ticket: "A fallen log on a dry forest floor rots away in about 20 years. The same kind of log sunk in the cold, oxygen-poor bottom of a deep lake can last for centuries. Use matter and energy to explain the difference." (On land, oxygen lets decomposers break the wood down, releasing its carbon as CO₂ and its energy as heat. On the lake bottom, little oxygen and the cold slow decomposers, so the wood keeps its carbon and its stored energy.) Return to the warm-up list and have students revise their compost-and-bog explanation.
Differentiation Strategies
For Struggling Students
Give a two-column organizer, "Oxygen present" and "No oxygen", with rows for speed of breakdown, gases, energy released and what builds up
Provide the frame: "In ___ conditions, the carbon leaves as ___, and the energy ___. The evidence is ___."
Work the percent calculations with the class before students do them alone
For Advanced Students
Ask students to use the energy table to compare how much of the energy in 100 g of glucose ends up as heat in a compost pile and in a sealed biogas tank
Have students research why draining peatlands is discussed in climate policy and present the evidence to the class
Ask students to design an investigation that separates the effect of oxygen from the effect of temperature on decomposition
Assessment Guidance
What to Look For
Check that explanations follow both the matter and the energy: the carbon goes somewhere (CO₂, methane, new cells, stored peat), and the energy is either released as heat or remains in the products. Listen for the ideas that matter is "used up" or "turned into energy", and that nothing lives or decomposes without oxygen. When students revise, look for the specific data that forced the change. Stay out of reaction steps: explanations should not need them.
02
Classroom Activities
3 Activities
1
Yeast Balloon Lab: What the Gas Tells Us
30 min (plus 40 min of waiting, with readings every 10 min)Groups of 3-4
Groups set up four bottles with balloons over their mouths and measure how much gas yeast release under different conditions. Once the yeast use up the little oxygen in each bottle, they keep working without it. The results show that the gas comes from the sugar (matter) and that the yeast need sugar and warmth to release energy.
Procedure
Bottle A: 1 packet (7 g) of yeast, 2 spoons of sugar and 100 mL of warm tap water. Bottle B: yeast and warm water, no sugar. Bottle C: sugar and warm water, no yeast. Bottle D: like A but with cold water from the refrigerator.
Stretch a balloon over each mouth, swirl gently and set the bottles side by side. Every 10 minutes for 40 minutes, measure each balloon around its widest part with a string and a ruler.
Safety: wear goggles. Use warm tap water no hotter than about 40 °C, checked with a thermometer; never use boiling water or a kettle. Use plastic bottles, not glass, and never screw a cap on a bottle with yeast in it, because gas pressure builds up. Use latex-free balloons if anyone has a latex allergy. Do not taste or drink anything. Pour the mixtures down the sink with running water, wipe up sugar spills so no one slips, and wash hands.
Sample Data (invented)
Balloon circumference (cm) at 10, 20, 30 and 40 minutes: A: 14, 22, 29, 34. B: flat at every reading. C: flat at every reading. D: flat, flat, 9, 12.
Discussion Questions
Only the bottles with both yeast and sugar produced gas. Where did the carbon atoms in the gas come from?
Bottle D inflated later and less than A (12 cm against 34 cm at 40 minutes). What does temperature do to the rate at which yeast break down sugar?
The air in each bottle held only a little oxygen, which the yeast used up early, yet the balloon on A kept growing. What does that tell you about yeast in anaerobic conditions, and where is most of the sugar's energy still stored?
Modification for Distance Learning
Students run Bottle A and Bottle B at home with an adult, using a plastic bottle and a balloon, and send photos with a ruler in view; or they graph the sample data above.
2
Decomposition Cups: Air Against Water
20 min setup, then 5 min weekly checks for 4 weeksGroups of 3
Groups bury mesh bags of dry leaves in two kinds of cups: moist soil with air holes, stirred twice a week (aerobic), and soil covered with water and sealed with plastic wrap (anaerobic). After 4 weeks they measure the mass lost and explain the difference.
Procedure
Weigh 5.0 g of air-dried leaves into each of two mesh bags. Bury one in moist potting soil in a cup with holes poked in the sides; stir the soil gently twice a week. Bury the other in soil in a cup filled with water to 2 cm above the soil, then cover it with plastic wrap.
After 4 weeks, remove the bags, rinse them gently, let them air-dry for 3 days on paper towels and weigh them again.
Safety: wear gloves and goggles when handling soil and wash hands afterward. Use bagged potting soil and plant material only: no food scraps, meat or dairy. Keep the sealed cups covered in a ventilated spot and do not sniff them. If a sealed cup smells like rotten eggs, keep it closed and let the teacher dispose of it sealed in the trash. Students with asthma or mold allergies can record data while others handle the soil.
Sample Data (invented)
After 4 weeks: aerobic cup bag 3.6 g, anaerobic cup bag 4.6 g. The anaerobic cup had small gas bubbles under the plastic wrap by week 3.
Discussion Questions
The aerobic bag lost 1.4 g (28%) and the anaerobic bag 0.4 g (8%), so the aerobic leaves lost 3.5 times as much. Explain the difference in terms of oxygen.
Where did the lost mass go in each cup? Which gases would you expect in each?
What would happen to leaves that stayed in the waterlogged cup for many years? Connect your answer to peat bogs.
Modification for Distance Learning
Students set up the aerobic version in a yard or a pot at home and analyze the sample data for the anaerobic cup.
3
Evidence Cards: Why Do Bogs Store Carbon?
25 minGroups of 3-4
Groups build an explanation of why peat bogs store so much carbon, using three rounds of invented evidence cards. After each round they keep, change or replace their explanation and record which evidence made them do it.
Evidence Cards
Round 1: The top meter of bog soil holds about 50 kg of carbon per m²; nearby forest soil holds about 10 kg. Plants add about 300 g of new plant matter per m² each year in the bog and about 900 g in the forest.
Round 2: Bog soil is waterlogged to 10 cm below the surface, and its water holds almost no oxygen (0.1 mg/L at 30 cm); forest soil pores hold air. Litter bags left for 1 year keep 85% of their mass in the bog and 40% in the forest.
Round 3: Part of the bog was drained with ditches in 1980. There the water table is now 60 cm down, the surface has sunk 40 cm, and the soil releases 4 times as much CO₂ as the intact bog, but almost no methane.
Discussion Questions
After Round 1: the bog stores 5 times as much carbon although plants add only a third as much matter each year. What must be different about decomposition?
After Round 2: how do the litter bags and the oxygen reading support an explanation based on anaerobic conditions?
After Round 3: the drained bog sank 40 cm and releases 4 times as much CO₂. Revise your explanation to say what happens to stored carbon when oxygen reaches it. (Cold and acidity also slow decomposition in many bogs.)
Challenge Variation
Groups predict what would happen to the carbon and to methane release if the ditches were blocked and the drained part flooded again, and describe the data that would test the prediction.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: A Model of Decomposition With and Without Oxygen
A conceptual model of what happens to dead organic matter. With oxygen, decomposers break it down fast and fully and release most of its energy, which ends up as heat. Without oxygen, breakdown is partial and slow, most of the energy stays in products such as methane, and matter builds up. In both cases the atoms keep cycling. The reaction steps inside cells are beyond this standard.
Diagram 2: Dissolved Oxygen by Depth in a Lake in Late Summer
The invented lake data from the third worked example, drawn to scale with depth increasing downward. Oxygen stays high in the mixed surface water and falls to 0.2 mg/L at 20 m, where decomposers breaking down sinking algae have used it up. The dashed line marks 2 mg/L.
04
Homework Assignment
~30 min
HS-LS2-3 Homework: Explaining Matter and Energy With and Without Oxygen
Directions: All data sets are invented for practice. Energy values are standard values, rounded. In every explanation, say where the matter goes and where the energy goes, and cite specific numbers. Where a problem asks you to revise, name the evidence that changed your explanation.
Part 1: Constructing Explanations (Problems 1-3)
Two compost bins start with 80 kg of garden waste. Bin A is turned every week; Bin B is packed wet and never turned. Temperatures in weeks 1-4: Bin A 55, 60, 58, 52 °C; Bin B 30, 32, 29, 28 °C. After 8 weeks Bin A holds 44 kg and Bin B 68 kg. (a) Find the percent of mass lost in each bin. (b) Where did the lost mass go? (c) Explain the temperature difference in terms of energy. (d) Bin B smells bad. What does that suggest about the conditions inside it?
Organic carbon, as a percent of dry soil mass (invented): upland forest soil 5% at the surface and 1% at 50 cm deep; bog soil 48% at the surface, 50% at 50 cm and 49% at 150 cm. (a) How many times more organic carbon does bog soil hold at 50 cm? (b) Construct an explanation, based on these data, of why organic matter builds up in the bog and not in the forest soil.
A dairy farm feeds manure into a sealed tank with no oxygen. The tank makes 400 m³ of biogas per day, and 60% of it is methane. Burning methane releases about 36 MJ per m³. (a) How much methane does the tank make per day? (b) How much energy could the methane release per day? (c) Trace the energy from sunlight to the flame, and the carbon from the air back to the air. (d) What would happen to the carbon and the energy if the manure were instead spread in an open, well-aired pile?
Part 2: Revising Explanations (Problems 4-6)
Many fish in a lake died in late August. Explanation 1: "The hot water killed them." Data: the surface water reached 26 °C, and the same fish live in nearby lakes that reach 29 °C. An algae bloom made algae 5 times as abundant as in June. At dawn after a cloudy week, oxygen was 2.5 mg/L at the surface and 0.3 mg/L near the bottom; these fish need at least 4 mg/L. (a) Do the data support Explanation 1? (b) Write a revised explanation that uses aerobic and anaerobic conditions. (c) Name one more measurement that would test your revised explanation.
Two 150 mL balls of bread dough, made with yeast and sugar, rise for 1 hour: one at 30 °C grows to 270 mL, the other at 5 °C to 165 mL. Explanation 1: "The dough rises because air gets trapped when it is kneaded." (a) Find the percent increase in volume for each. (b) Use the data to revise Explanation 1. (c) Inside the dough there is very little oxygen. Where do the carbon atoms in the gas come from, and where is most of the sugar's energy after rising?
A student claims: "Without oxygen, decomposition stops completely." In an 8-week test, leaves in an aerated jar lost 40% of their mass. Leaves in a sealed, waterlogged jar lost 10%, and the gas collected from that jar contained methane. (a) Find the average percent lost per week in each jar. (b) How many times faster was decomposition with air? (c) Revise the student's claim so it fits all the data, and explain where the carbon from the sealed jar went.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Using the Data
Calculations (percents, rates, energy) are correct and the explanation cites specific numbers
Correct method with one calculation error, or numbers not cited
Matter and Energy
Explains where the carbon goes and where the energy goes, in both aerobic and anaerobic conditions
Explains matter or energy, but not both
Revision
Revised explanation fits all the data and names the evidence that forced the change
Explanation is changed but the evidence is not named
05
Quiz: 20 Questions
Interactive, with answers
Instructions
All data sets are invented, and energy values are standard values, rounded. 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
A fallen leaf is broken down by decomposers in well-aired soil. Where do most of the carbon atoms from the leaf end up?
Answer: A
With oxygen present, decomposers break the leaf down fully and release its carbon as CO₂. Choice B is a common misconception: atoms are conserved, and energy is not made from matter in this process. Choice C is impossible, since carbon atoms cannot become oxygen atoms. Choice D describes waterlogged soil better, and even there some carbon leaves as gas.
Question 2 of 20 · Multiple Choice
Bags with 20 g of dry leaves were left for a year (invented data). In a dry forest floor 6 g remained; in a marsh flooded all year 17 g remained. Which explanation do the data best support?
Answer: C
The forest bags lost 14 of 20 g (70%) and the marsh bags 3 of 20 g (15%). Slow loss in flooded soil fits a lack of oxygen. Choice A contradicts the data. Choice B is not supported: the marsh bags still lost mass. Choice D rejects data without evidence.
Question 3 of 20 · Multiple Choice
A student draws a model of a forest in which arrows show energy moving from plants to animals, from both to decomposers, and then from decomposers back to the plants. What is wrong with the model?
Answer: C
Carbon atoms can travel back to plants as CO₂, but energy cannot: each organism releases energy that ends up as heat, and plants need new energy from sunlight. Choice A treats energy like matter. Choice B is wrong because decomposers get their energy from dead matter. Choice D does not fix the problem: energy still would not cycle.
Question 4 of 20 · Multiple Choice
A soil bacterium that can live with or without oxygen is grown in two flasks, each with 2 g of sugar (invented data). With air it makes 80 units of new cells; without air it makes 16 units from the same sugar. Which explanation fits these data?
Answer: D
The same 2 g of sugar gave 80 ÷ 16 = 5 times as much growth with oxygen, which fits the model that aerobic breakdown releases far more of the sugar's energy. Choice A contradicts the setup: the sugar was the same. Choice B is wrong because oxygen is not food; it allows the food to be broken down fully. Choice C has no support in the data.
Question 5 of 20 · Multiple Choice
Burning 100 g of lactose (milk sugar) releases about 1,650 kJ. Bacteria in yogurt break lactose down without oxygen, and the lactic acid they make from 100 g of it could still release about 1,600 kJ if burned (standard values, rounded). About what share of the lactose's energy did the bacteria release?
Answer: A
The bacteria released about 1,650 - 1,600 = 50 kJ, and 50 ÷ 1,650 ≈ 0.03, or 3%. Choice B is the share that stays in the lactic acid. Choice C mistakes the 50 kJ released for 50%. Choice D would be true only if the products held no energy, as with full aerobic breakdown to CO₂ and water.
Question 6 of 20 · Multiple Choice
Dissolved oxygen in a pond in summer (invented data): 0 m: 8.5 mg/L, 6 m: 7.0 mg/L, 12 m: 1.5 mg/L, 18 m: 0.1 mg/L. Where is decomposition most likely to be anaerobic, and why?
Answer: C
At 18 m the oxygen is nearly zero (0.1 mg/L), so decomposers there must work without it. Choice A ignores the high surface oxygen. Choice B picks a depth that still has 7.0 mg/L. Choice D is contradicted by the 18 m reading.
Question 7 of 20 · Multiple Choice
Flooded wetland soils release a lot of methane, while the same soil drained releases mostly carbon dioxide. Which explanation fits?
Answer: B
The form in which carbon leaves depends on whether oxygen is present. Choice A is wrong because draining lets stored carbon leave as CO₂; it does not add carbon. Choice C is wrong: the methane comes from microbes in the soil. Choice D breaks conservation of matter.
Question 8 of 20 · Multiple Choice
The center of a well-aired compost pile reaches 60 °C on a cool day. What is the best explanation of the heat?
Answer: D
Aerobic breakdown releases most of the stored energy, and the decomposers lose much of it as heat, which the pile traps. Choice A cannot heat the inside of a pile more than the surface. Choice B is wrong: nothing burns, and a well-aired pile makes little methane. Choice C breaks conservation of matter.
Question 9 of 20 · Multiple Choice
A student explains that the bottom water of a lake lost its oxygen each summer because the fish there used it up. Which new data would most clearly require revising that explanation?
Answer: B
If a lake with no fish loses its bottom oxygen just as fast, the fish cannot be the main cause, and the sinking algae point to decomposers using the oxygen. Choices A and C are consistent with the fish explanation and do not test it. Choice D describes the surface, not the bottom water the explanation is about.
Question 10 of 20 · Multiple Choice
A sealed plastic bottle with fruit juice and yeast, with a balloon on top, has a mass of 350 g at the start and still 350 g, to the nearest gram, a day later, although the balloon has partly filled with gas (invented data). What does this show?
Answer: B
The sealed system kept the same mass, so the atoms of the gas came from the sugar: matter was rearranged, not created or destroyed. Choice A is wrong: the gas has mass, but it came from the bottle's contents. Choice C breaks conservation of matter. Choice D has no support in the data.
Question 11 of 20 · Multiple Choice
A peat layer in a bog is 3 m deep and builds up about 0.5 mm per year (invented data). About how long did it take to form?
Answer: C
3 m is 3,000 mm, and 3,000 ÷ 0.5 = 6,000 years. Choice A misplaces the decimal (it divides 300 by 0.5). Choice B multiplies 3,000 by 0.5 instead of dividing. Choice D converts meters to millimeters wrongly (30,000 mm).
Question 12 of 20 · Multiple Choice
Cows digest grass with the help of microbes in a stomach chamber that has no oxygen, and they burp out methane. What does the methane show about energy?
Answer: D
Anaerobic breakdown leaves energy in its products, and methane holds a lot of it, so the energy in the burped methane is lost to the cow. Choice A breaks conservation of energy. Choice B is wrong: methane burns and releases energy. Choice C is wrong because the methane leaves the body.
Question 13 of 20 · Multiple Choice
Soil cores are kept in jars at the same temperature (invented data). Cores with air release 12 mg of CO₂ per hour; waterlogged cores release 3 mg per hour. How many times faster is CO₂ released with air?
Answer: A
12 ÷ 3 = 4. Choice B subtracts (12 - 3) instead of dividing. Choice C divides the wrong way (3 ÷ 12). Choice D adds the two rates.
Question 14 of 20 · Multiple Choice
An ocean area off a river mouth has very low oxygen near the bottom each summer. Which data would best support the explanation that decomposition of algae causes it?
Answer: C
The explanation predicts that low oxygen follows blooms and matches where dead algae collect, which is exactly what choice C describes. Choices A and D are true of many areas that stay well oxygenated. Choice B describes winter, when the low oxygen is not present.
Question 15 of 20 · Short Answer
Cut grass left in a wet open pile lost 50% of its dry mass in 2 months and heated to 60 °C. The same grass packed into sealed, airless bags (silage) lost 8% and stayed near 25 °C (invented data). Construct an explanation of the difference that covers matter and energy, and say why farmers store cattle feed as silage.
In the open pile, oxygen let decomposers break the grass down quickly: much of its carbon left as CO₂ (50% of the mass was lost), and the energy they released heated the pile. In the sealed bags, without oxygen, microbes broke down only a little (8% lost), released little energy (no heating), and most of the matter and energy stayed in the grass. Farmers use silage because it keeps the feed's matter and energy for the cattle instead of losing it to decomposers.
Question 16 of 20 · Short Answer
A student explains: "Yeast need oxygen to release gas." A bottle filled to the brim with sugar water and yeast, with no air space, inflated its balloon to 30 cm around in 60 minutes; a bottle with an air space reached 32 cm (invented data). Revise the explanation using the data.
The bottle with no air space made almost as much gas (30 cm against 32 cm), so oxygen is not needed for the yeast to release gas. Revised explanation: yeast can break down sugar with or without oxygen. Without oxygen they break it down only partly, releasing CO₂ gas and leaving most of the sugar's energy in the ethanol they make. The small extra gas in the bottle with air may come from the yeast using the oxygen in the air space first.
Question 17 of 20 · Short Answer
A landfill collects 2,000 m³ of gas per day, and half of it is methane. Burning methane releases about 36 MJ per m³, and 1 kWh = 3.6 MJ. (a) How much energy could the methane release per day, in MJ and in kWh? (b) Explain where this energy came from and why it is still available.
(a) 2,000 × 0.5 = 1,000 m³ of methane; 1,000 × 36 = 36,000 MJ, and 36,000 ÷ 3.6 = 10,000 kWh per day. (b) The energy was captured from sunlight by plants and stored in food, paper and yard waste. Buried in the landfill with little oxygen, the waste is broken down only partly, and the microbes release methane that still holds much of that energy, so it can be burned for electricity.
Question 18 of 20 · Short Answer
Near the bottom of a lake, dissolved oxygen was 0.2 mg/L in August and 8.0 mg/L in November, after cool weather and wind mixed the whole lake (invented data). Explain both readings, and say how the mixing changes what happens to dead matter on the lake bottom.
In August the deep water was cut off from the air, and decomposers breaking down sinking algae used up its oxygen (0.2 mg/L), so conditions were nearly anaerobic and breakdown was slow. In November, mixing carried oxygen-rich surface water to the bottom (8.0 mg/L). With oxygen back, aerobic decomposers can break down the dead matter faster, releasing its carbon as CO₂ rather than methane and releasing more of its energy as heat.
Question 19 of 20 · Short Answer
Wetland soil is kept in jars for a month (invented data). Jars with air released 50 mg of carbon, all as CO₂. Jars without air released 20 mg of carbon: 14 mg as CO₂ and 6 mg as methane. (a) How many times more carbon did the jars with air release? (b) What share of the carbon left the jars without air as methane? (c) Explain both results.
(a) 50 ÷ 20 = 2.5 times. (b) 6 ÷ 20 = 30%. (c) With air, aerobic decomposers break organic matter down quickly and fully, so more carbon leaves, all as CO₂. Without air, breakdown is slower and done by microbes that release part of the carbon as methane. The carbon that stays behind builds up in the soil, which is how wetlands store carbon.
Question 20 of 20 · Short Answer
A student says: "The mud at the bottom of a deep lake has no oxygen, so nothing lives there and dead matter just stays forever." Data: the mud gives off bubbles of methane, a gram of it contains about 100 million bacteria, and the organic matter in the mud decreases by about 0.5% per year (invented data). Revise the student's explanation.
The data show that the mud is alive: methane bubbles and 100 million bacteria per gram mean microbes are breaking down dead matter without oxygen. The 0.5% loss per year shows that breakdown continues, but slowly. Revised explanation: anaerobic microbes live in the mud and decompose dead matter slowly, releasing some of its carbon as methane and CO₂; because breakdown is so slow, organic matter builds up faster than it disappears.
0 of 20 answered · 0 correct
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Frequently Asked Questions
10 Questions
What does HS-LS2-3 mean?
HS-LS2-3 asks students to construct and revise explanations, backed by evidence, of how matter cycles and energy flows in aerobic (oxygen present) and anaerobic (no oxygen) conditions. The code means High School, Life Science, core idea 2 (Ecosystems: Interactions, Energy, and Dynamics), performance expectation 3.
What is the assessment boundary for HS-LS2-3?
The official Assessment Boundary says that assessment does not include the specific chemical processes of either aerobic or anaerobic respiration. Students explain inputs, outputs, rates and energy at the level of organisms and environments; the stages and reaction steps inside cells are beyond this standard.
What is the difference between aerobic and anaerobic conditions?
Aerobic conditions have oxygen available, as in well-aired soil, a turned compost pile or the surface of a lake. Anaerobic conditions have little or no oxygen, as in waterlogged soil, deep lake mud, a sealed jar, a cow's stomach or buried trash. The same dead matter breaks down very differently in the two.
Can organisms get energy without oxygen?
Yes. Yeast, many bacteria and even human muscle cells for short periods can break down sugar without oxygen. They release only a small share of the sugar's energy, and the rest stays in products such as ethanol, lactic acid or methane. That is why anaerobic organisms tend to grow slowly and why their products can still be used as fuel.
Why does matter cycle but energy flows?
Atoms are never used up: the carbon in a dead leaf ends up in carbon dioxide, methane, new cells or stored peat, and plants can take the carbon up again. Energy changes form at every step, and some leaves as heat each time. Heat spreads into the surroundings and cannot be captured by producers again, so ecosystems need a constant input of energy from sunlight.
Why do wetlands and peat bogs store so much carbon?
Their soil is waterlogged, and water blocks oxygen, so decomposers break dead plants down very slowly. Plant matter piles up faster than it disappears and becomes peat. In many bogs cold and acidic water slow decomposition further. When a wetland is drained, oxygen reaches the peat and the stored carbon is released as carbon dioxide.
Is HS-LS2-3 the same as HS-LS1-7?
No. HS-LS1-7 is about cellular respiration inside organisms: food and oxygen molecules are broken down and new bonds form, releasing energy. HS-LS2-3 moves to the ecosystem scale and compares aerobic and anaerobic conditions in different environments: how fast matter breaks down, where its carbon goes and how much energy is released. Teaching HS-LS1-7 first helps.
What labs are safe for HS-LS2-3?
A yeast and sugar balloon lab in plastic bottles, with warm (not hot) water and no caps, and decomposition cups with plant material in potting soil are both classroom-safe with goggles, gloves and hand washing. Avoid food scraps, meat or dairy in sealed containers, never open or sniff sealed anaerobic containers, and do not collect or burn any gas.
Where does methane in nature come from?
Much of it comes from microbes that break down organic matter without oxygen: in wetlands, rice fields, lake mud, landfills and the stomachs of cattle and other grazing animals. Methane holds a lot of energy, which is why landfill gas and biogas can be burned as fuel. In the air, methane is slowly changed to carbon dioxide.
How does HS-LS2-3 connect to other standards?
It builds on MS-LS2-3, where students model how matter cycles and energy flows in ecosystems, and on HS-LS1-7 on cellular respiration. HS-LS2-4 adds math on matter and energy moving between trophic levels, and HS-LS2-5 models the carbon cycle among the biosphere, atmosphere, hydrosphere and geosphere. HS-ESS2-6 takes the carbon cycle to the scale of Earth's systems.
07
Related Standards
5 standards
These standards connect to HS-LS2-3: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
MS-LS2-3Prerequisite
Model the cycling of matter and flow of energy among parts of an ecosystem
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