Biology Quiz: Interpret Matter And Energy Cycles

What this quiz covers

This quiz focuses on Interpret Matter And Energy Cycles, giving you a quick way to practice the rules, question types, and explanations that matter most for Biology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Plants convert atmospheric N2 directly into nitrate during photosynthesis.
2. Bacteria convert nitrate in soil into nitrogen gas, returning nitrogen to the atmosphere. (correct answer)
3. Animals release nitrate gas into the atmosphere during respiration.
4. Nitrate moves from the atmosphere into the soil without any biological process.

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). For ENERGY FLOW, arrows are ONE-WAY—starting from sun, pointing through trophic levels (sun → producers → consumers), with additional arrows showing energy LEAVING as heat from each box (dissipating, never returning to sun or previous levels). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! This nitrogen cycle diagram highlights the denitrification arrow from soil NO3- to atmospheric N2, labeled as a bacterial process, closing the cycle by returning nitrogen gas to the air. Choice B correctly interprets the diagram by explaining that bacteria convert soil nitrate back to N2, completing the return pathway in the cycle. Choice A fails because plants do not convert N2 directly during photosynthesis; the arrow shows bacterial denitrification, not plant action—always check arrow labels for the process and actor! Reading ecosystem diagrams—the arrow-following method: (1) IDENTIFY what's being shown: Matter (C, N, water, nutrients) or Energy? (check title, labels). (2) LOCATE starting point: For matter: atmosphere, soil, or any reservoir. For energy: always SUN (external input). (3) FOLLOW arrows: Trace path by following arrow directions. Note what each arrow represents (process labels: photosynthesis, feeding, respiration, decomposition). (4) CHECK for CIRCULAR vs LINEAR: Matter: do arrows loop back to start? (yes = cycling). Energy: do arrows return to sun? (no = one-way flow). (5) IDENTIFY key processes at arrows: Photosynthesis (CO2 to plant), Feeding (organism to organism), Respiration (organism to CO2), Decomposition (dead to soil/atmosphere). This systematic arrow-following reveals complete pathways! Impressive understanding— you're nailing these interpretations!

Question 2

1. Decomposers
2. Atmosphere CO2 (correct answer)
3. Carnivores
4. Plants

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). For ENERGY FLOW, arrows are ONE-WAY—starting from sun, pointing through trophic levels (sun → producers → consumers), with additional arrows showing energy LEAVING as heat from each box (dissipating, never returning to sun or previous levels). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! In this carbon cycle diagram, the atmosphere CO2 box serves as the central reservoir with arrows entering from respiration and decomposition, and leaving via photosynthesis to plants. Choice B correctly interprets the diagram by identifying the atmosphere CO2 as the main atmospheric reservoir where carbon is stored as gas, connecting to all other components. Choice A is misleading because decomposers are organisms that process dead matter, not the atmospheric reservoir—focus on box labels to distinguish storage from actors! Reading ecosystem diagrams—the arrow-following method: (1) IDENTIFY what's being shown: Matter (C, N, water, nutrients) or Energy? (check title, labels). (2) LOCATE starting point: For matter: atmosphere, soil, or any reservoir. For energy: always SUN (external input). (3) FOLLOW arrows: Trace path by following arrow directions. Note what each arrow represents (process labels: photosynthesis, feeding, respiration, decomposition). (4) CHECK for CIRCULAR vs LINEAR: Matter: do arrows loop back to start? (yes = cycling). Energy: do arrows return to sun? (no = one-way flow). (5) IDENTIFY key processes at arrows: Photosynthesis (CO2 to plant), Feeding (organism to organism), Respiration (organism to CO2), Decomposition (dead to soil/atmosphere). This systematic arrow-following reveals complete pathways! Fantastic effort— you're getting better with every question!

Question 3

1. Both energy and carbon cycle in closed loops because arrows return to the starting box.
2. Carbon cycles among reservoirs, while energy flows one way and leaves as heat. (correct answer)
3. Energy cycles back to the Sun, while carbon flows one way to decomposers and stops.
4. Neither energy nor carbon moves between boxes; arrows only show where matter is stored.

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). For ENERGY FLOW, arrows are ONE-WAY—starting from sun, pointing through trophic levels (sun → producers → consumers), with additional arrows showing energy LEAVING as heat from each box (dissipating, never returning to sun or previous levels). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! This combined diagram contrasts matter (carbon) arrows forming a closed loop through atmosphere, producers, consumers, and decomposers, while energy arrows go one-way from sunlight to producers to consumers with heat loss at each step, without returning. Choice B correctly interprets the diagram by recognizing the circular pattern for carbon cycling among reservoirs and the linear flow for energy dissipating as heat. Choice A fails because energy arrows do not form closed loops—they point outward as heat without returning, so only carbon cycles while energy flows one-way. Reading ecosystem diagrams—the arrow-following method: (1) IDENTIFY what's being shown: Matter (C, N, water, nutrients) or Energy? (check title, labels). (2) LOCATE starting point: For matter: atmosphere, soil, or any reservoir. For energy: always SUN (external input). (3) FOLLOW arrows: Trace path by following arrow directions. Note what each arrow represents (process labels: photosynthesis, feeding, respiration, decomposition). (4) CHECK for CIRCULAR vs LINEAR: Matter: do arrows loop back to start? (yes = cycling). Energy: do arrows return to sun? (no = one-way flow). (5) IDENTIFY key processes at arrows: Photosynthesis (CO2 to plant), Feeding (organism to organism), Respiration (organism to CO2), Decomposition (dead to soil/atmosphere). This systematic arrow-following reveals complete pathways! You're doing great—keep tracing those arrows to build your skills!

Question 4

1. Carbon is converted from CO2 into glucose in the animal's chloroplasts.
2. Carbon moves from plant organic molecules into animal organic molecules when the animal eats the plant. (correct answer)
3. Carbon moves from the atmosphere into animals directly through breathing.
4. Carbon leaves the ecosystem as heat energy during feeding.

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). For ENERGY FLOW, arrows are ONE-WAY—starting from sun, pointing through trophic levels (sun → producers → consumers), with additional arrows showing energy LEAVING as heat from each box (dissipating, never returning to sun or previous levels). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! In this carbon cycle diagram, the feeding arrow transfers carbon from plant biomass to animal biomass, continuing the cycle with respiration returning it to the atmosphere. Choice B correctly interprets the diagram by describing the feeding process as moving carbon from plant organic molecules to animal ones through consumption, without creating or losing carbon atoms. Choice A fails because animals lack chloroplasts for photosynthesis; the arrow is labeled feeding, not conversion of CO2 to glucose—pay attention to process labels on arrows! Reading ecosystem diagrams—the arrow-following method: (1) IDENTIFY what's being shown: Matter (C, N, water, nutrients) or Energy? (check title, labels). (2) LOCATE starting point: For matter: atmosphere, soil, or any reservoir. For energy: always SUN (external input). (3) FOLLOW arrows: Trace path by following arrow directions. Note what each arrow represents (process labels: photosynthesis, feeding, respiration, decomposition). (4) CHECK for CIRCULAR vs LINEAR: Matter: do arrows loop back to start? (yes = cycling). Energy: do arrows return to sun? (no = one-way flow). (5) IDENTIFY key processes at arrows: Photosynthesis (CO2 to plant), Feeding (organism to organism), Respiration (organism to CO2), Decomposition (dead to soil/atmosphere). This systematic arrow-following reveals complete pathways! Wonderful job—you're mastering matter transfers!

Question 5

1. Atmosphere (N2) → plant uptake → animals → denitrification → atmosphere (N2)
2. Atmosphere (N2) → nitrogen fixation → soil (NH4+) → nitrification → soil (NO3-) → plant uptake → animals → decomposition → soil (NH4+) → nitrification → soil (NO3-) → denitrification → atmosphere (N2) (correct answer)
3. Atmosphere (N2) → decomposition → soil (NH4+) → animals → plant uptake → atmosphere (N2)
4. Atmosphere (N2) → nitrification → soil (NO3-) → animals → photosynthesis → atmosphere (N2)

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). For ENERGY FLOW, arrows are ONE-WAY—starting from sun, pointing through trophic levels (sun → producers → consumers), with additional arrows showing energy LEAVING as heat from each box (dissipating, never returning to sun or previous levels). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! In this nitrogen cycle diagram, arrows trace nitrogen from atmospheric N2 through bacterial fixation to soil NH4+, nitrification to NO3-, plant uptake, consumption by animals, decomposition back to soil NH4+, and denitrification returning to N2, forming a complete cycle. Choice B correctly interprets the diagram by following the full arrow pathway from atmosphere to animal and back, including all key processes like fixation, nitrification, uptake, decomposition, and denitrification. Choice A is incomplete because it skips essential steps like fixation and nitrification, misreading the arrows by jumping directly to plant uptake without bacterial transformations in soil. Reading ecosystem diagrams—the arrow-following method: (1) IDENTIFY what's being shown: Matter (C, N, water, nutrients) or Energy? (check title, labels). (2) LOCATE starting point: For matter: atmosphere, soil, or any reservoir. For energy: always SUN (external input). (3) FOLLOW arrows: Trace path by following arrow directions. Note what each arrow represents (process labels: photosynthesis, feeding, respiration, decomposition). (4) CHECK for CIRCULAR vs LINEAR: Matter: do arrows loop back to start? (yes = cycling). Energy: do arrows return to sun? (no = one-way flow). (5) IDENTIFY key processes at arrows: Photosynthesis (CO2 to plant), Feeding (organism to organism), Respiration (organism to CO2), Decomposition (dead to soil/atmosphere). This systematic arrow-following reveals complete pathways! Great job tracing cycles—you've got this!

Question 6

1. Respiration (Plants → Atmosphere)
2. Decomposition (Dead organisms → Atmosphere)
3. Feeding (Plants → Animals)
4. Photosynthesis (Atmosphere → Plants) (correct answer)

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). For ENERGY FLOW, arrows are ONE-WAY—starting from sun, pointing through trophic levels (sun → producers → consumers), with additional arrows showing energy LEAVING as heat from each box (dissipating, never returning to sun or previous levels). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! In this carbon cycle diagram, the arrows show processes like photosynthesis removing CO2 from the atmosphere to plants, feeding transferring it to animals, and respiration or decomposition returning it to the atmosphere, forming a cycle. Choice D correctly interprets the diagram by identifying the photosynthesis arrow as the process that removes CO2 from the atmospheric reservoir and incorporates it into plant biomass, which is living matter. Choice A is incorrect because respiration arrows point from plants to the atmosphere, adding CO2 rather than removing it—remember to follow arrow directions to see what is moving where! Reading ecosystem diagrams—the arrow-following method: (1) IDENTIFY what's being shown: Matter (C, N, water, nutrients) or Energy? (check title, labels). (2) LOCATE starting point: For matter: atmosphere, soil, or any reservoir. For energy: always SUN (external input). (3) FOLLOW arrows: Trace path by following arrow directions. Note what each arrow represents (process labels: photosynthesis, feeding, respiration, decomposition). (4) CHECK for CIRCULAR vs LINEAR: Matter: do arrows loop back to start? (yes = cycling). Energy: do arrows return to sun? (no = one-way flow). (5) IDENTIFY key processes at arrows: Photosynthesis (CO2 to plant), Feeding (organism to organism), Respiration (organism to CO2), Decomposition (dead to soil/atmosphere). This systematic arrow-following reveals complete pathways! Keep practicing, and you'll master interpreting these cycles with ease!

Question 7

1. Plants (proteins/DNA)
2. Soil NO$_3^-$
3. Atmosphere (N$_2$) (correct answer)
4. Decomposers

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). For ENERGY FLOW, arrows are ONE-WAY—starting from sun, pointing through trophic levels (sun → producers → consumers), with additional arrows showing energy LEAVING as heat from each box (dissipating, never returning to sun or previous levels). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! In the nitrogen cycle diagram, the Atmosphere (N2) box is the primary atmospheric reservoir, with arrows showing it as the starting pool for fixation into soil before cycling through other components. Choice C correctly interprets the diagram by identifying Atmosphere (N2) as the largest atmospheric storage pool of nitrogen, from which it cycles into soil via fixation. Choice A distracts by selecting Plants, which store nitrogen in biomass but aren't atmospheric reservoirs, confusing biotic storage with the main atmospheric pool. Terrific improvement—utilize this arrow-following method: (1) IDENTIFY what's being shown: Matter (C, N, water, nutrients) or Energy? (check title, labels). (2) LOCATE starting point: For matter: atmosphere, soil, or any reservoir. For energy: always SUN (external input). (3) FOLLOW arrows: Trace path by following arrow directions. Note what each arrow represents (process labels: photosynthesis, feeding, respiration, decomposition). (4) CHECK for CIRCULAR vs LINEAR: Matter: do arrows loop back to start? (yes = cycling). Energy: do arrows return to sun? (no = one-way flow). (5) IDENTIFY key processes at arrows: Photosynthesis (CO2 to plant), Feeding (organism to organism), Respiration (organism to CO2), Decomposition (dead to soil/atmosphere). This systematic arrow-following reveals complete pathways! Carbon cycle reading example: Find ATMOSPHERE (CO2) box. Arrow labeled "photosynthesis" points to PLANTS box (CO2 removed from air, incorporated into plant). Arrow labeled "feeding/consumption" from plants to ANIMALS (carbon transferred in food). Arrows labeled "respiration" from both plants AND animals back to ATMOSPHERE (CO2 released, carbon returned). Arrow from dead organisms to DECOMPOSERS. Arrow labeled "decomposition" from decomposers to ATMOSPHERE (more CO2 released). Result: circular path: atmosphere → plants → animals → decomposers → atmosphere → (repeat). Carbon cycles! Same carbon atoms go round and round. Energy flow reading example: Find SUN (energy source). Arrow to PRODUCERS (energy captured ~1%). Arrow from producers to PRIMARY CONSUMERS (energy transferred ~10%). Arrow to SECONDARY CONSUMERS (~10% of previous). From EACH box (producers, consumers), arrows labeled "heat" point OUT of system (energy dissipated). NO arrows returning to sun. Result: linear path with branches out: sun → producers → (consumers) → heat. Energy flows through, doesn't return. Needs continuous sun input!

Question 8

1. Arrow 1 (correct answer)
2. Arrow 2
3. Arrow 3
4. Arrow 4

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). For ENERGY FLOW, arrows are ONE-WAY—starting from sun, pointing through trophic levels (sun → producers → consumers), with additional arrows showing energy LEAVING as heat from each box (dissipating, never returning to sun or previous levels). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! In this nitrogen cycle diagram, Arrow 1 from Atmosphere (N2) to Soil NH4+ via nitrogen fixation is the initial step that converts unusable atmospheric nitrogen into a soil form accessible for further cycling to plants. Choice A correctly interprets the diagram by identifying Arrow 1 as the process moving nitrogen into the soil, making it available to living things through subsequent steps like nitrification and uptake. Choice D distracts by focusing on denitrification (Arrow 4), which returns nitrogen to the atmosphere rather than making it available from there to soil. Impressive effort—employ this arrow-following method: (1) IDENTIFY what's being shown: Matter (C, N, water, nutrients) or Energy? (check title, labels). (2) LOCATE starting point: For matter: atmosphere, soil, or any reservoir. For energy: always SUN (external input). (3) FOLLOW arrows: Trace path by following arrow directions. Note what each arrow represents (process labels: photosynthesis, feeding, respiration, decomposition). (4) CHECK for CIRCULAR vs LINEAR: Matter: do arrows loop back to start? (yes = cycling). Energy: do arrows return to sun? (no = one-way flow). (5) IDENTIFY key processes at arrows: Photosynthesis (CO2 to plant), Feeding (organism to organism), Respiration (organism to CO2), Decomposition (dead to soil/atmosphere). This systematic arrow-following reveals complete pathways! Carbon cycle reading example: Find ATMOSPHERE (CO2) box. Arrow labeled "photosynthesis" points to PLANTS box (CO2 removed from air, incorporated into plant). Arrow labeled "feeding/consumption" from plants to ANIMALS (carbon transferred in food). Arrows labeled "respiration" from both plants AND animals back to ATMOSPHERE (CO2 released, carbon returned). Arrow from dead organisms to DECOMPOSERS. Arrow labeled "decomposition" from decomposers to ATMOSPHERE (more CO2 released). Result: circular path: atmosphere → plants → animals → decomposers → atmosphere → (repeat). Carbon cycles! Same carbon atoms go round and round. Energy flow reading example: Find SUN (energy source). Arrow to PRODUCERS (energy captured ~1%). Arrow from producers to PRIMARY CONSUMERS (energy transferred ~10%). Arrow to SECONDARY CONSUMERS (~10% of previous). From EACH box (producers, consumers), arrows labeled "heat" point OUT of system (energy dissipated). NO arrows returning to sun. Result: linear path with branches out: sun → producers → (consumers) → heat. Energy flows through, doesn't return. Needs continuous sun input!

Question 9

1. Decomposers convert CO$_2$ into glucose by photosynthesis, so they start the cycle.
2. Decomposers return carbon to the atmosphere as CO$_2$, helping close the cycle. (correct answer)
3. Decomposers store carbon permanently in soil and stop carbon from moving to the atmosphere.
4. Decomposers move carbon directly from animals to plants by feeding.

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). For ENERGY FLOW, arrows are ONE-WAY—starting from sun, pointing through trophic levels (sun → producers → consumers), with additional arrows showing energy LEAVING as heat from each box (dissipating, never returning to sun or previous levels). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! Examining the carbon cycle diagram, note the arrows from Decomposers (soil) via decomposition/respiration back to Atmosphere (CO2), which returns carbon rather than removing it, countering the student's claim by showing decomposers facilitate the cycle's closure. Choice B correctly interprets the diagram by highlighting how decomposers release CO2, closing the loop and recycling carbon instead of removing it. Choice C distracts by confusing decomposition with permanent storage, but the arrow shows carbon moving to the atmosphere, not staying in soil forever. You're making wonderful strides—practice this arrow-following method: (1) IDENTIFY what's being shown: Matter (C, N, water, nutrients) or Energy? (check title, labels). (2) LOCATE starting point: For matter: atmosphere, soil, or any reservoir. For energy: always SUN (external input). (3) FOLLOW arrows: Trace path by following arrow directions. Note what each arrow represents (process labels: photosynthesis, feeding, respiration, decomposition). (4) CHECK for CIRCULAR vs LINEAR: Matter: do arrows loop back to start? (yes = cycling). Energy: do arrows return to sun? (no = one-way flow). (5) IDENTIFY key processes at arrows: Photosynthesis (CO2 to plant), Feeding (organism to organism), Respiration (organism to CO2), Decomposition (dead to soil/atmosphere). This systematic arrow-following reveals complete pathways! Carbon cycle reading example: Find ATMOSPHERE (CO2) box. Arrow labeled "photosynthesis" points to PLANTS box (CO2 removed from air, incorporated into plant). Arrow labeled "feeding/consumption" from plants to ANIMALS (carbon transferred in food). Arrows labeled "respiration" from both plants AND animals back to ATMOSPHERE (CO2 released, carbon returned). Arrow from dead organisms to DECOMPOSERS. Arrow labeled "decomposition" from decomposers to ATMOSPHERE (more CO2 released). Result: circular path: atmosphere → plants → animals → decomposers → atmosphere → (repeat). Carbon cycles! Same carbon atoms go round and round. Energy flow reading example: Find SUN (energy source). Arrow to PRODUCERS (energy captured ~1%). Arrow from producers to PRIMARY CONSUMERS (energy transferred ~10%). Arrow to SECONDARY CONSUMERS (~10% of previous). From EACH box (producers, consumers), arrows labeled "heat" point OUT of system (energy dissipated). NO arrows returning to sun. Result: linear path with branches out: sun → producers → (consumers) → heat. Energy flows through, doesn't return. Needs continuous sun input!

Question 10

1. Both carbon and energy cycle in closed loops because arrows connect all components.
2. Carbon cycles among reservoirs, but energy flows one-way and exits as heat. (correct answer)
3. Energy cycles among organisms, but carbon flows one-way and is lost as heat.
4. Neither carbon nor energy moves between components; arrows only show where matter is stored.

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). For ENERGY FLOW, arrows are ONE-WAY—starting from sun, pointing through trophic levels (sun → producers → consumers), with additional arrows showing energy LEAVING as heat from each box (dissipating, never returning to sun or previous levels). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! In this combined model, observe the carbon arrows forming a closed loop from atmosphere through producers, consumers, decomposers, and back, contrasting with energy arrows going one-way from sun to producers to consumers with heat exiting, showing no return. Choice B correctly interprets the diagram by distinguishing the circular cycling of carbon among reservoirs and the linear, one-way flow of energy exiting as heat. Choice C fails because it reverses the patterns, claiming energy cycles (but arrows show no loop) and carbon flows one-way (but carbon arrows loop back). Excellent progress—try this arrow-following method: (1) IDENTIFY what's being shown: Matter (C, N, water, nutrients) or Energy? (check title, labels). (2) LOCATE starting point: For matter: atmosphere, soil, or any reservoir. For energy: always SUN (external input). (3) FOLLOW arrows: Trace path by following arrow directions. Note what each arrow represents (process labels: photosynthesis, feeding, respiration, decomposition). (4) CHECK for CIRCULAR vs LINEAR: Matter: do arrows loop back to start? (yes = cycling). Energy: do arrows return to sun? (no = one-way flow). (5) IDENTIFY key processes at arrows: Photosynthesis (CO2 to plant), Feeding (organism to organism), Respiration (organism to CO2), Decomposition (dead to soil/atmosphere). This systematic arrow-following reveals complete pathways! Carbon cycle reading example: Find ATMOSPHERE (CO2) box. Arrow labeled "photosynthesis" points to PLANTS box (CO2 removed from air, incorporated into plant). Arrow labeled "feeding/consumption" from plants to ANIMALS (carbon transferred in food). Arrows labeled "respiration" from both plants AND animals back to ATMOSPHERE (CO2 released, carbon returned). Arrow from dead organisms to DECOMPOSERS. Arrow labeled "decomposition" from decomposers to ATMOSPHERE (more CO2 released). Result: circular path: atmosphere → plants → animals → decomposers → atmosphere → (repeat). Carbon cycles! Same carbon atoms go round and round. Energy flow reading example: Find SUN (energy source). Arrow to PRODUCERS (energy captured ~1%). Arrow from producers to PRIMARY CONSUMERS (energy transferred ~10%). Arrow to SECONDARY CONSUMERS (~10% of previous). From EACH box (producers, consumers), arrows labeled "heat" point OUT of system (energy dissipated). NO arrows returning to sun. Result: linear path with branches out: sun → producers → (consumers) → heat. Energy flows through, doesn't return. Needs continuous sun input!

Question 11

1. Atmosphere (CO$_2$) → Animals → Plants → Decomposers
2. Atmosphere (CO$_2$) → Plants → (feeding) herbivore → (feeding) carnivore (correct answer)
3. Plants → Atmosphere (CO$_2$) → Carnivore → Decomposers
4. Decomposers → Plants → Atmosphere (CO$_2$) → Carnivore

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). For ENERGY FLOW, arrows are ONE-WAY—starting from sun, pointing through trophic levels (sun → producers → consumers), with additional arrows showing energy LEAVING as heat from each box (dissipating, never returning to sun or previous levels). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! Tracing the carbon cycle diagram, follow from Atmosphere (CO2) via photosynthesis to Plants, then feeding to Animals (herbivores), and implicitly another feeding to carnivores, as the diagram supports transfers between animals. Choice B correctly interprets the diagram by sequencing the path from air to plants to herbivores to carnivores via feeding arrows, accurately tracing carbon incorporation into a carnivore. Choice A distracts by starting correctly but then reversing directions, like animals to plants, which doesn't follow the arrow flows. Superb job—implement this arrow-following method: (1) IDENTIFY what's being shown: Matter (C, N, water, nutrients) or Energy? (check title, labels). (2) LOCATE starting point: For matter: atmosphere, soil, or any reservoir. For energy: always SUN (external input). (3) FOLLOW arrows: Trace path by following arrow directions. Note what each arrow represents (process labels: photosynthesis, feeding, respiration, decomposition). (4) CHECK for CIRCULAR vs LINEAR: Matter: do arrows loop back to start? (yes = cycling). Energy: do arrows return to sun? (no = one-way flow). (5) IDENTIFY key processes at arrows: Photosynthesis (CO2 to plant), Feeding (organism to organism), Respiration (organism to CO2), Decomposition (dead to soil/atmosphere). This systematic arrow-following reveals complete pathways! Carbon cycle reading example: Find ATMOSPHERE (CO2) box. Arrow labeled "photosynthesis" points to PLANTS box (CO2 removed from air, incorporated into plant). Arrow labeled "feeding/consumption" from plants to ANIMALS (carbon transferred in food). Arrows labeled "respiration" from both plants AND animals back to ATMOSPHERE (CO2 released, carbon returned). Arrow from dead organisms to DECOMPOSERS. Arrow labeled "decomposition" from decomposers to ATMOSPHERE (more CO2 released). Result: circular path: atmosphere → plants → animals → decomposers → atmosphere → (repeat). Carbon cycles! Same carbon atoms go round and round. Energy flow reading example: Find SUN (energy source). Arrow to PRODUCERS (energy captured ~1%). Arrow from producers to PRIMARY CONSUMERS (energy transferred ~10%). Arrow to SECONDARY CONSUMERS (~10% of previous). From EACH box (producers, consumers), arrows labeled "heat" point OUT of system (energy dissipated). NO arrows returning to sun. Result: linear path with branches out: sun → producers → (consumers) → heat. Energy flows through, doesn't return. Needs continuous sun input!

Question 12

1. Both carbon and energy cycle in loops because arrows connect all groups.
2. Carbon cycles among reservoirs, but energy flows one-way and is lost as heat. (correct answer)
3. Energy cycles among organisms, but carbon flows one-way from the Sun to decomposers.
4. Neither carbon nor energy moves between organisms; only the atmosphere changes.

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! In this combined model, carbon arrows loop circularly (Atmosphere → Plants → Animals → Decomposers → Atmosphere), while energy arrows go one-way (Sun → Plants → Animals → Decomposers) with heat leaving each, showing flow. Choice B correctly interprets the diagram by properly reading arrow directions, identifying carbon's cycle among reservoirs and energy's one-way flow with heat loss. Choice A fails by claiming both cycle in loops, ignoring energy's linear path and heat arrows indicating no return. Practice the strategy: (1) IDENTIFY matter vs energy, (2) LOCATE starts (atmosphere for matter, Sun for energy), (3) FOLLOW arrows, (4) CHECK circular vs linear, (5) Note processes—this comparison skill is key, and you're building it brilliantly!

Question 13

1. Energy increases at higher trophic levels because consumers concentrate energy.
2. About 10% of energy is transferred to the next trophic level, with most lost as heat. (correct answer)
3. All energy captured by producers is recycled back to the Sun after consumption.
4. Energy is conserved within the pyramid, so each level must have equal kJ.

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). For ENERGY FLOW, arrows are ONE-WAY—starting from sun, pointing through trophic levels (sun → producers → consumers), with additional arrows showing energy LEAVING as heat from each box (dissipating, never returning to sun or previous levels). In this energy pyramid, energy decreases from 10,000 kJ (producers) to 1,000 kJ (primary) to 100 kJ (secondary), illustrating ~10% transfer with most lost as heat. Choice B correctly interprets the diagram by properly reading the decreasing values, recognizing the 10% transfer rule and heat loss pattern. Choice A fails by claiming energy increases at higher levels, which contradicts the pyramid's narrowing shape and values. Use the strategy: (1) IDENTIFY energy, (2) LOCATE base (producers), (3) FOLLOW levels upward, (4) CHECK linear decrease (~10%), (5) Note heat loss—this quantifies flow, and you're acing it!

Question 14

1. Both models show cycling because arrows connect multiple components.
2. Matter (carbon) is shown cycling in a loop, while energy is shown flowing one-way and leaving as heat. (correct answer)
3. Energy cycles through decomposers back to the Sun, while matter leaves ecosystems as heat.
4. Matter and energy both flow one-way because decomposers stop all movement.

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Reading the arrow pattern tells you whether it's cycling (closed loops, circular) or flowing (open path, linear)! In these models, Model 1 shows carbon looping circularly (Atmosphere → Plants → Animals → Decomposers → Atmosphere), while Model 2 shows energy one-way (Sun → Plants → Animals → Decomposers) with heat leaving, no return. Choice B correctly interprets the diagrams by properly reading arrow directions, distinguishing matter's cycle from energy's flow with heat loss. Choice A fails by claiming both show cycling, overlooking energy's linear arrows and heat dissipation. Apply the strategy: (1) IDENTIFY matter vs energy per model, (2) LOCATE starts, (3) FOLLOW arrows, (4) CHECK circular vs linear, (5) Note heat for energy—this comparison is crucial, and you're excelling at it!

Question 15

1. Soil: NO$_3^-$ → Atmosphere (N$_2$) (denitrification) (correct answer)
2. Atmosphere (N$_2$) → Soil: NH$_4^+$ (nitrogen fixation)
3. Soil: NO$_3^-$ → Plants (plant uptake)
4. Decomposers → Soil: NH$_4^+$ (ammonification)

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). In this nitrogen cycle diagram, trace arrows: from Atmosphere (N₂) to Soil NH₄⁺ (fixation), to NO₃⁻ (nitrification), to Plants (uptake), to Animals (consumption), to Decomposers (death), back to Soil NH₄⁺ (ammonification), and from Soil NO₃⁻ back to Atmosphere (denitrification), forming a cycle. Choice A correctly interprets the diagram by properly reading arrow directions, identifying the denitrification arrow from Soil NO₃⁻ to Atmosphere (N₂) as the return process. Choice B fails by describing fixation, which moves nitrogen from atmosphere to soil, not returning it, misreading the direction. Use the arrow-following method: (1) IDENTIFY matter (nitrogen), (2) LOCATE start (atmosphere), (3) FOLLOW arrows through soil forms and organisms, (4) CHECK for circular (yes, via denitrification), (5) Note processes like denitrification—this will help you spot return paths easily, keep up the excellent work!

Question 16

1. Carbon in plants can be transferred to animals by feeding or released to the atmosphere as CO$_2$ by respiration. (correct answer)
2. Carbon in plants can only move to animals; respiration moves oxygen, not carbon.
3. Carbon moves from the atmosphere into plants during respiration and then to animals by photosynthesis.
4. Carbon leaves plants only when decomposers convert it directly into glucose in the atmosphere.

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). For MATTER CYCLES, arrows form CIRCULAR pathways—you can trace from any component and eventually return to where you started (example: atmosphere → plant → animal → decomposer → atmosphere → complete circle). In this carbon model, arrows from Plants to Animals (feeding) transfer carbon in biomass, and to Atmosphere (respiration) release it as CO₂, showing two exit paths. Choice A correctly interprets the diagram by properly reading arrow directions, identifying both transfers and recognizing carbon's movement options from plants. Choice B fails by claiming carbon only moves to animals and respiration involves oxygen, ignoring the labeled respiration arrow for carbon. Practice tracing: (1) IDENTIFY matter (carbon), (2) LOCATE plants box, (3) FOLLOW outgoing arrows, (4) CHECK paths (to animals or atmosphere), (5) Note processes—this reveals multiple fates, you're progressing wonderfully!

Question 17

1. Energy cycles back to the Sun after passing through consumers.
2. Energy flows one-way through trophic levels and is lost as heat at each step. (correct answer)
3. Energy increases at higher trophic levels because consumers make energy by eating.
4. Energy is recycled by decomposers back into producers with no losses.

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). The energy pyramid shows ONE-WAY arrows from Sun through trophic levels with heat arrows leaving each level—this pattern indicates energy flows through the ecosystem without cycling back, losing some as heat at every transfer! Choice B correctly interprets the diagram by recognizing the linear flow pattern (sun → producers → consumers) and the heat loss arrows at each level, showing energy doesn't cycle but flows through and exits. Choice A incorrectly suggests energy cycles back to the Sun (no return arrows exist); Choice C wrongly claims energy increases at higher levels (pyramids show decreasing energy); Choice D incorrectly states decomposers recycle energy with no losses (heat arrows show continuous losses). Reading energy flow diagrams—the key indicators: (1) START at the Sun (external energy source). (2) FOLLOW arrows through trophic levels (always one direction). (3) LOOK for heat arrows leaving each box (energy dissipation). (4) CHECK for return paths to Sun (none exist—energy doesn't cycle). (5) NOTE the pyramid shape shows decreasing energy at each level. This one-way flow with continuous heat loss explains why ecosystems need constant solar input!

Question 18

1. Atmosphere (CO2) → Animals → Plants → Atmosphere (CO2)
2. Atmosphere (CO2) → Plants (photosynthesis) → Animals (feeding) → Atmosphere (CO2) (respiration) (correct answer)
3. Plants → Atmosphere (CO2) (photosynthesis) → Animals → Decomposers
4. Atmosphere (CO2) → Decomposers (photosynthesis) → Plants → Animals

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). To trace the complete carbon pathway, start at Atmosphere (CO2), follow the photosynthesis arrow to Plants (carbon fixed into organic molecules), then the feeding arrow to Animals (carbon transferred in food), and finally the respiration arrow back to Atmosphere (CO2 released)—this forms a complete cycle! Choice B correctly interprets the diagram by properly following arrow directions and identifying each labeled process in the correct sequence: atmosphere → plants (via photosynthesis) → animals (via feeding) → atmosphere (via respiration). Choice A incorrectly shows animals before plants, which reverses the photosynthesis step; Choice C wrongly shows photosynthesis releasing CO2 from plants to atmosphere (it actually removes CO2); Choice D incorrectly shows decomposers doing photosynthesis (only plants and some bacteria photosynthesize). Reading ecosystem diagrams—the arrow-following method: (1) IDENTIFY what's being shown: Matter (C, N, water, nutrients) or Energy? (check title, labels). (2) LOCATE starting point: For matter: atmosphere, soil, or any reservoir. (3) FOLLOW arrows: Trace path by following arrow directions, noting process labels. (4) CHECK for CIRCULAR vs LINEAR: Matter cycles should loop back to start. This systematic approach reveals that carbon moves in a continuous cycle through living and non-living components!

Question 19

1. About 1% (one hundredth)
2. About 10% (one tenth) (correct answer)
3. About 50% (one half)
4. About 100% (all of it)

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). The energy values show a pattern: 1,000 → 100 → 10, decreasing by a factor of 10 at each transfer—this represents the ~10% rule where only about one-tenth of energy transfers to the next trophic level! Choice B correctly interprets the diagram by calculating that 100/1,000 = 0.1 (10%) transfers from producers to primary consumers, and 10/100 = 0.1 (10%) transfers to secondary consumers. Choice A (1%) would show 1,000 → 10 → 0.1; Choice C (50%) would show 1,000 → 500 → 250; Choice D (100%) would show no energy loss between levels. Reading energy transfer efficiency: (1) COMPARE values between adjacent trophic levels. (2) CALCULATE the fraction: (energy at next level) ÷ (energy at current level). (3) The ~10% rule means 90% is lost as heat, movement, and life processes. (4) This explains why food chains rarely exceed 4-5 levels—not enough energy remains! The 10% rule demonstrates why it takes about 10 pounds of grain to produce 1 pound of beef, and why top predators are rare!

Question 20

1. Both carbon and energy cycle in closed loops because arrows return to the starting boxes.
2. Carbon flows one-way from the Sun to consumers, but energy cycles between atmosphere and plants.
3. Carbon cycles among reservoirs and organisms, while energy flows one-way and exits as heat. (correct answer)
4. Energy cycles through decomposers back into the Sun, but carbon is lost as heat.

Explanation: This question tests your ability to interpret diagrams and models showing how matter cycles through ecosystems (in circular pathways through atmosphere, organisms, soil) and how energy flows through food webs (in one-way paths from sun to heat). Ecosystem cycle diagrams use arrows and boxes to show movement: BOXES represent reservoirs or components (atmosphere, plants, animals, soil, decomposers—where matter is stored or organisms are located), and ARROWS show transfers or transformations (photosynthesis arrow from atmosphere CO2 to plants, feeding arrow from plants to animals, respiration arrows from organisms back to atmosphere, decomposition from dead material to soil/atmosphere). The combined model shows two distinct patterns: carbon arrows form CLOSED LOOPS (atmosphere → producers → consumers → decomposers → atmosphere), while energy arrows are ONE-WAY (sun → producers → consumers → heat), never returning to the sun! Choice C correctly interprets both patterns by recognizing that carbon cycles in closed loops among components while energy flows linearly and exits as heat at each transfer. Choice A incorrectly claims both cycle (energy doesn't return to sun); Choice B reverses the patterns (carbon doesn't come from sun, energy doesn't cycle); Choice D confuses the two (energy doesn't cycle through decomposers, carbon isn't lost as heat). Reading combined matter-energy diagrams—distinguish the patterns: (1) TRACE carbon arrows: Do they form loops? Yes = cycling matter. (2) TRACE energy arrows: Do they return to sun? No = one-way flow. (3) IDENTIFY heat arrows: These show energy leaving, not cycling. (4) NOTE the difference: Matter atoms are conserved and reused; Energy is transformed and dissipated. This fundamental difference explains why ecosystems need continuous energy input (sunlight) but can recycle the same matter indefinitely!