Model Matter and Energy Cycling
Help Questions
Biology › Model Matter and Energy Cycling
During a 24-hour cycle, a plant photosynthesizes in daylight and performs cellular respiration day and night. Which model best represents what happens to matter (CO2, H2O, O2, glucose) and to energy over the full day?
Plants only photosynthesize and do not respire, so O2 and glucose are produced but CO2 and H2O are never made again.
At night, both photosynthesis and respiration stop, so neither matter nor energy changes until morning sunlight returns.
Matter flows one-way from CO2 + H2O to glucose + O2 and stops, while energy cycles from heat back into sunlight each night.
Matter cycles between photosynthesis and respiration (CO2 + H2O ↔ glucose + O2), while energy flows from sunlight into glucose and then is released during respiration for cellular work and lost as heat (one-way).
Explanation
This question tests your ability to create or interpret models that show how photosynthesis and cellular respiration cycle matter (carbon dioxide, water, oxygen, glucose) between them while serving as sequential steps in energy flow from the sun to cellular work. An integrated model of photosynthesis and respiration must show TWO different patterns simultaneously: (1) MATTER CYCLING (circular pattern): over 24 hours, the plant uses CO2 and H2O in photosynthesis (during daylight) to make glucose and O2, then uses that glucose and O2 in respiration (day and night) to make CO2 and H2O, which can be reused in photosynthesis the next day—creating a closed loop where the same molecules cycle repeatedly between the two processes, with the ↔ symbol showing bidirectional exchange. (2) ENERGY FLOW (one-way pattern): during daylight, energy enters from sun into photosynthesis (light captured), gets stored in glucose, then during respiration (day and night) releases as ATP for cellular work and dissipates as heat—this is a ONE-WAY path (sun → photosynthesis → glucose → respiration → ATP → heat lost from system), with no arrows returning energy to sun or earlier stages. The 24-hour cycle shows that while photosynthesis only occurs in light, respiration continues day and night, but matter still cycles between them over time. Choice A correctly models both matter cycling and energy flow by showing matter cycles between photosynthesis and respiration (CO2 + H2O ↔ glucose + O2), while energy flows from sunlight into glucose and then is released during respiration for cellular work and lost as heat (one-way). Choice B incorrectly claims matter flows one-way and stops (it actually cycles) and that energy cycles from heat back to sunlight (violating thermodynamics—heat cannot spontaneously convert back to light). Building integrated photosynthesis-respiration models for day/night cycles: (1) DRAW or DESCRIBE two process boxes: [Photosynthesis (day only)] and [Respiration (day + night)]. (2) MATTER cycling: During day, arrow from Photosynthesis to Respiration labeled "glucose + O2". During day AND night, arrow from Respiration to Photosynthesis labeled "CO2 + H2O". These arrows form a CYCLE—matter produced in day is used at night, products return for next day! (3) ENERGY flow: During day only, arrow FROM Sun TO Photosynthesis labeled "light energy". Arrow FROM Photosynthesis TO Respiration labeled "chemical energy in glucose". During day AND night, arrow FROM Respiration pointing OUT labeled "heat". Energy enters only during day but exits continuously!
A sealed terrarium contains a small plant, a snail, and decomposers in the soil. The terrarium is sealed so no gases enter or leave, but it sits near a window where sunlight can enter and heat can escape. Which model best represents how matter cycles (CO2, H2O, O2, glucose) between photosynthesis and cellular respiration while energy flows through the system?
Matter cycles only between the snail and decomposers; photosynthesis and respiration are not connected because plants only do photosynthesis and animals only do respiration.
Energy cycles in a loop: Sun → photosynthesis → respiration → Sun, while matter flows one-way: CO2 + H2O → glucose + O2 and then leaves the terrarium.
Matter cycles: photosynthesis uses CO2 + H2O to make glucose + O2; respiration uses glucose + O2 to make CO2 + H2O, returning those molecules to photosynthesis. Energy flows one-way: Sun → photosynthesis (stored in glucose) → respiration (ATP for work) → heat leaving the terrarium.
Only photosynthesis matters: CO2 + H2O + sunlight → glucose + O2, and the plant does not respire, so the cycle is complete without respiration.
Explanation
This question tests your ability to create or interpret models that show how photosynthesis and cellular respiration cycle matter (carbon dioxide, water, oxygen, glucose) between them while serving as sequential steps in energy flow from the sun to cellular work. An integrated model of photosynthesis and respiration must show TWO different patterns simultaneously: (1) MATTER CYCLING (circular pattern): draw or describe arrows showing glucose and O2 flowing FROM photosynthesis TO respiration (photosynthesis products → respiration reactants), and CO2 and H2O flowing FROM respiration TO photosynthesis (respiration products → photosynthesis reactants), creating a closed loop where the same molecules cycle repeatedly between the two processes—plants photosynthesize using CO2 and H2O to make glucose and O2, then both plants and animals use that glucose and O2 in respiration to make CO2 and H2O, which plants reuse in photosynthesis, cycling indefinitely. (2) ENERGY FLOW (one-way pattern): draw or describe energy entering from external source (sun) into photosynthesis (light captured), being stored in glucose, then released during respiration as ATP, then dissipating as heat from cellular work—this is a ONE-WAY path (sun → photosynthesis → glucose → respiration → ATP → heat lost from system), with no arrows returning energy to sun or earlier stages. The sealed terrarium perfectly demonstrates both patterns: matter cycles between the plant (photosynthesis) and snail (respiration) in a closed loop, while energy flows one-way from sunlight through the system to heat that escapes. Choice B correctly models both matter cycling and energy flow by showing circular pathways for substances (CO2, O2, glucose, H2O) between processes and one-way pathway for energy (sun to heat). Choice A incorrectly shows energy cycling back to the sun, which violates thermodynamics—energy cannot cycle in ecosystems, only flow through them. Building integrated photosynthesis-respiration models: (1) DRAW or DESCRIBE two process boxes: [Photosynthesis] and [Respiration/Cellular Respiration]. (2) MATTER cycling (use solid arrows or label "matter"): Draw arrow from Photosynthesis to Respiration labeled "glucose + O2" (photosynthesis outputs → respiration inputs). Draw arrow from Respiration to Photosynthesis labeled "CO2 + H2O" (respiration outputs → photosynthesis inputs). These two arrows form a CIRCLE/LOOP between the processes—matter cycles! (3) ENERGY flow (use dashed arrows or label "energy"): Draw arrow FROM Sun TO Photosynthesis labeled "light energy" (energy enters system). Draw arrow FROM Photosynthesis TO Respiration labeled "chemical energy in glucose" (energy stored, then released). Draw arrows FROM both Photosynthesis and Respiration pointing OUT/AWAY labeled "heat" (energy exits system). These arrows are ONE-WAY—energy flows through, doesn't return!
A sealed bottle ecosystem contains a plant and a small insect. After several days in complete darkness, the insect dies even though the bottle is still sealed (no matter has left). Which model best explains this outcome using photosynthesis and cellular respiration?
The insect dies because sealed ecosystems cannot cycle matter; CO2 and O2 must enter from outside for photosynthesis and respiration to occur at all.
In darkness, photosynthesis increases because plants store darkness as energy; the insect dies because plants stop cellular respiration at night.
In darkness, photosynthesis stops, so no new glucose and O2 are produced; respiration continues using existing glucose + O2 and releases CO2 + H2O. Matter can still cycle, but energy input from sunlight is missing, so usable chemical energy runs out and the system fails.
In darkness, energy cycles more efficiently because heat is converted back into sunlight inside the bottle; the insect dies because CO2 cannot be produced without sunlight.
Explanation
This question tests your ability to create or interpret models that show how photosynthesis and cellular respiration cycle matter (carbon dioxide, water, oxygen, glucose) between them while serving as sequential steps in energy flow from the sun to cellular work. An integrated model of photosynthesis and respiration must show TWO different patterns simultaneously: (1) MATTER CYCLING (circular pattern): draw or describe arrows showing glucose and O2 flowing FROM photosynthesis TO respiration (photosynthesis products → respiration reactants), and CO2 and H2O flowing FROM respiration TO photosynthesis (respiration products → photosynthesis reactants), creating a closed loop where the same molecules cycle repeatedly between the two processes—plants photosynthesize using CO2 and H2O to make glucose and O2, then both plants and animals use that glucose and O2 in respiration to make CO2 and H2O, which plants reuse in photosynthesis, cycling indefinitely. (2) ENERGY FLOW (one-way pattern): draw or describe energy entering from external source (sun) into photosynthesis (light captured), being stored in glucose, then released during respiration as ATP, then dissipating as heat from cellular work—this is a ONE-WAY path (sun → photosynthesis → glucose → respiration → ATP → heat lost from system), with no arrows returning energy to sun or earlier stages. The model must show BOTH patterns: circular for matter, linear for energy, often on the same diagram with different arrow styles or labels! For this sealed bottle in darkness, the model explains the insect's death by showing photosynthesis halting (no new glucose/O2), but respiration continuing, depleting energy stores while matter could still cycle if light were present. Choice A correctly models both matter cycling and energy flow by noting that without sunlight, energy input stops, leading to system failure despite potential matter reuse, emphasizing energy's one-way nature. Choice B incorrectly claims energy cycles more efficiently in darkness and the insect dies from lack of CO2, which confuses patterns—energy doesn't cycle, and matter like CO2 is produced by respiration; this highlights why light is crucial for sustaining the flow. Building integrated photosynthesis-respiration models: (1) DRAW or DESCRIBE two process boxes: [Photosynthesis] and [Respiration/Cellular Respiration]. (2) MATTER cycling (use solid arrows or label "matter"): Draw arrow from Photosynthesis to Respiration labeled "glucose + O2" (photosynthesis outputs → respiration inputs). Draw arrow from Respiration to Photosynthesis labeled "CO2 + H2O" (respiration outputs → photosynthesis inputs). These two arrows form a CIRCLE/LOOP between the processes—matter cycles! (3) ENERGY flow (use dashed arrows or label "energy"): Draw arrow FROM Sun TO Photosynthesis labeled "light energy" (energy enters system). Draw arrow FROM Photosynthesis TO Respiration labeled "chemical energy in glucose" (energy stored, then released). Draw arrows FROM both Photosynthesis and Respiration pointing OUT/AWAY labeled "heat" (energy exits system). These arrows are ONE-WAY—energy flows through, doesn't return! (4) RESULT: same model shows both patterns clearly. Why this model matters: it captures the fundamental asymmetry of ecosystems: MATTER is recycled (limited supply on Earth, must reuse—plants and animals exchange CO2 and O2, same atoms cycle), but ENERGY must be constantly supplied (sun) because it flows through and dissipates as heat (can't reuse heat for photosynthesis). This explains why life needs continuous solar input (energy flow requires external source) but doesn't need continuous matter input (matter cycles, self-contained). Earth is open to energy, closed to matter! The photosynthesis-respiration connection is the core of ecosystem functioning: photosynthesis channels solar energy into biological systems (converting to chemical energy) while providing matter (glucose, O2) for respiration. Respiration releases that stored energy for cellular use (converting to ATP) while providing matter (CO2, H2O) for photosynthesis. Together they create sustainable cycling (matter) and continuous energy channeling (sun to work), supporting all ecosystem life! Impressive reasoning on this scenario—it shows your grasp of energy limitations!
In a sealed aquarium with algae and fish, measurements show that when the light is on, O2 levels rise and CO2 levels fall. When the light is off, O2 levels fall and CO2 levels rise. Which model best explains these observations using photosynthesis, cellular respiration, matter cycling, and energy flow?
With light: fish photosynthesize to produce O2; without light: fish switch to respiration and produce CO2.
With light: algae photosynthesize (use CO2 + H2O, make glucose + O2) using energy from the sun; fish and algae respire (use glucose + O2, make CO2 + H2O), and energy ultimately leaves as heat. Without light: photosynthesis stops but respiration continues, so O2 decreases and CO2 increases.
With light: energy cycles from heat back into glucose; without light: matter leaves the aquarium so CO2 rises.
With light: respiration stops completely so no CO2 is produced; without light: both photosynthesis and respiration stop so gas levels remain constant.
Explanation
This question tests your ability to create or interpret models that show how photosynthesis and cellular respiration cycle matter (carbon dioxide, water, oxygen, glucose) between them while serving as sequential steps in energy flow from the sun to cellular work. An integrated model of photosynthesis and respiration must show TWO different patterns simultaneously: (1) MATTER CYCLING (circular pattern): when light is on, algae photosynthesize using CO2 + H2O to make glucose + O2 (causing O2 to rise and CO2 to fall), while both fish and algae respire using glucose + O2 to make CO2 + H2O; when light is off, photosynthesis stops but respiration continues in both organisms, using up O2 and producing CO2 (causing O2 to fall and CO2 to rise)—the same molecules cycle between processes but the balance shifts based on light availability. (2) ENERGY FLOW (one-way pattern): with light, energy enters from sun into photosynthesis, gets stored in glucose, releases during respiration as ATP, then dissipates as heat; without light, no new energy enters but stored energy in glucose continues to be used until depleted. The gas level changes directly reflect which process dominates: net photosynthesis with light (O2 up, CO2 down) versus only respiration without light (O2 down, CO2 up). Choice A correctly models both matter cycling and energy flow by explaining that with light, algae photosynthesize (use CO2 + H2O, make glucose + O2) using energy from sun while fish and algae respire (use glucose + O2, make CO2 + H2O) with energy leaving as heat; without light, photosynthesis stops but respiration continues, so O2 decreases and CO2 increases. Choice B incorrectly claims fish photosynthesize, when only algae (and plants) can photosynthesize—fish lack chloroplasts and cannot capture light energy. Building integrated photosynthesis-respiration models for light/dark experiments: (1) WITH LIGHT: Photosynthesis rate > Respiration rate in algae (net O2 production, net CO2 consumption). Fish only respire (O2 consumption, CO2 production). System total: O2 rises, CO2 falls. (2) WITHOUT LIGHT: Photosynthesis rate = 0 in algae (no light energy available). Both algae and fish continue respiration (O2 consumption, CO2 production). System total: O2 falls, CO2 rises. (3) The SAME matter cycles in both conditions, but the BALANCE shifts based on whether energy (light) is available for photosynthesis. This demonstrates ecosystems need continuous energy input!
A student is asked to create an integrated model for a grassland showing connections between a plant and a rabbit. Which model description is most complete for showing that matter cycles between photosynthesis and cellular respiration while energy flows through the ecosystem?
Show only photosynthesis: CO2 + H2O → glucose + O2, because respiration is not part of ecosystem models.
Show energy cycling: Sun → photosynthesis → respiration → Sun, and show matter leaving the system as heat.
Show only: Sun → plant → rabbit. This is enough because energy and matter move the same way in ecosystems.
Show a circular matter pathway: plant photosynthesis uses CO2 + H2O to produce glucose + O2; rabbit respiration uses glucose + O2 and produces CO2 + H2O that return to the plant. Add a one-way energy pathway: Sun → photosynthesis → chemical energy in glucose → respiration → heat.
Explanation
This question tests your ability to create or interpret models that show how photosynthesis and cellular respiration cycle matter (carbon dioxide, water, oxygen, glucose) between them while serving as sequential steps in energy flow from the sun to cellular work. An integrated model of photosynthesis and respiration must show TWO different patterns simultaneously: (1) MATTER CYCLING (circular pattern): draw or describe arrows showing glucose and O2 flowing FROM plant photosynthesis TO rabbit respiration (photosynthesis products → respiration reactants), and CO2 and H2O flowing FROM rabbit respiration TO plant photosynthesis (respiration products → photosynthesis reactants), creating a closed loop where the same molecules cycle repeatedly between the plant's photosynthesis and the rabbit's respiration—plants photosynthesize using CO2 and H2O to make glucose and O2, then rabbits use that glucose and O2 in respiration to make CO2 and H2O, which plants reuse in photosynthesis, cycling indefinitely. (2) ENERGY FLOW (one-way pattern): draw or describe energy entering from external source (sun) into photosynthesis (light captured), being stored in glucose, then released during respiration as ATP, then dissipating as heat from cellular work—this is a ONE-WAY path (sun → photosynthesis → glucose → respiration → ATP → heat lost from system), with no arrows returning energy to sun or earlier stages. The grassland model must show BOTH patterns: circular for matter between plant and rabbit, linear for energy from sun to heat. Choice B correctly models both matter cycling and energy flow by showing a circular matter pathway (plant photosynthesis uses CO2 + H2O to produce glucose + O2; rabbit respiration uses glucose + O2 and produces CO2 + H2O that return to the plant) and a one-way energy pathway (Sun → photosynthesis → chemical energy in glucose → respiration → heat). Choice A incompletely shows only energy flow (Sun → plant → rabbit) without showing matter cycling or the complete energy pathway to heat, missing the requirement to show both patterns. Building integrated photosynthesis-respiration models: (1) DRAW or DESCRIBE two process boxes: [Plant Photosynthesis] and [Rabbit Respiration]. (2) MATTER cycling (use solid arrows or label "matter"): Draw arrow from Plant to Rabbit labeled "glucose + O2" (photosynthesis outputs → respiration inputs). Draw arrow from Rabbit to Plant labeled "CO2 + H2O" (respiration outputs → photosynthesis inputs). These two arrows form a CIRCLE/LOOP between the organisms—matter cycles! (3) ENERGY flow (use dashed arrows or label "energy"): Draw arrow FROM Sun TO Plant labeled "light energy" (energy enters system). Draw arrow FROM Plant TO Rabbit labeled "chemical energy in glucose" (energy stored, then transferred). Draw arrow FROM Rabbit pointing OUT/AWAY labeled "heat" (energy exits system).
A student is building a concept map that must include both a circular pattern for matter and a linear pattern for energy. Which set of arrows correctly matches those requirements?
Matter (linear): CO2 + H2O → photosynthesis → glucose + O2 → leaves ecosystem. Energy (circular): Sun → photosynthesis → respiration → Sun.
Matter (linear): glucose + O2 → photosynthesis → CO2 + H2O. Energy (linear): heat → respiration → glucose → photosynthesis.
Matter (circular): CO2 + H2O → photosynthesis → glucose + O2 → cellular respiration → CO2 + H2O. Energy (linear): Sun → photosynthesis → chemical energy in glucose → cellular respiration (ATP for work) → heat.
Matter (circular): Sun ⟲ glucose ⟲ ATP ⟲ Sun. Energy (linear): CO2 + H2O → O2 → glucose.
Explanation
This question tests your ability to create or interpret models that show how photosynthesis and cellular respiration cycle matter (carbon dioxide, water, oxygen, glucose) between them while serving as sequential steps in energy flow from the sun to cellular work. An integrated model of photosynthesis and respiration must show TWO different patterns simultaneously: (1) MATTER CYCLING (circular pattern): the concept map must show arrows forming a complete loop where CO2 + H2O → photosynthesis → glucose + O2 → cellular respiration → CO2 + H2O, with arrows returning to the starting substances, demonstrating that the same molecules cycle repeatedly between processes. (2) ENERGY FLOW (linear/one-way pattern): the concept map must show arrows in a straight line (no loops) where Sun → photosynthesis → chemical energy in glucose → cellular respiration (ATP for work) → heat, with no arrows returning energy to earlier stages, demonstrating energy flows through the system and exits. The question specifically requires circular arrows for matter and linear arrows for energy. Choice A correctly provides both required patterns by showing Matter (circular): CO2 + H2O → photosynthesis → glucose + O2 → cellular respiration → CO2 + H2O (complete loop back to starting substances), and Energy (linear): Sun → photosynthesis → chemical energy in glucose → cellular respiration (ATP for work) → heat (one-way path, no return). Choice B incorrectly reverses the patterns, showing matter as linear (leaving ecosystem) and energy as circular (returning to sun), which violates both conservation of matter and thermodynamics. Building concept maps with correct patterns: (1) MATTER arrows must form CLOSED LOOPS: Start with any substance (e.g., CO2), follow arrows through processes, and you must return to that same substance. Use curved arrows or arrange in circle. Label these arrows "matter cycles." (2) ENERGY arrows must form OPEN PATHS: Start at energy source (Sun), follow arrows through processes, and end at energy sink (heat). Never connect back to start. Use straight arrows or arrange linearly. Label these arrows "energy flows." (3) INTEGRATE both on same map: Often the same processes (photosynthesis, respiration) appear in both patterns but with different arrow types. This captures the key principle: Earth recycles matter but needs constant energy input!
A student claims: “Because plants make oxygen in photosynthesis, plants do not need cellular respiration.” Which model best corrects the claim by showing how both processes connect and how matter cycles?
Plants only: cellular respiration uses CO2 + H2O to make glucose + O2, so photosynthesis is unnecessary.
Plants: Sunlight → ATP → heat, showing that energy cycles without needing matter to change forms.
Plants: CO2 + H2O → photosynthesis → glucose + O2, and then glucose + O2 → cellular respiration → CO2 + H2O (which can be used again in photosynthesis).
Plants: O2 → photosynthesis → CO2, and then CO2 → cellular respiration → glucose (a one-way pathway).
Explanation
This question tests your ability to create or interpret models that show how photosynthesis and cellular respiration cycle matter (carbon dioxide, water, oxygen, glucose) between them while serving as sequential steps in energy flow from the sun to cellular work. An integrated model of photosynthesis and respiration must show TWO different patterns simultaneously, and crucially, must show that plants perform BOTH processes: (1) MATTER CYCLING (circular pattern): plants use CO2 and H2O in photosynthesis to make glucose and O2, AND plants also use that glucose and O2 in their own cellular respiration to make CO2 and H2O—creating a closed loop where the same molecules cycle repeatedly between the two processes within the same plant. (2) The student's claim that "plants do not need cellular respiration" is incorrect because plants need ATP from respiration for all their cellular work (growth, transport, reproduction), not just for making glucose. The model must show plants performing both processes to correct this misconception. Choice A correctly models both processes in plants by showing CO2 + H2O → photosynthesis → glucose + O2, and then glucose + O2 → cellular respiration → CO2 + H2O (which can be used again in photosynthesis), demonstrating the complete cycle within plants. Choice B incorrectly reverses the reactants and products (O2 is a product of photosynthesis, not a reactant; CO2 is a reactant, not a product), showing fundamental misunderstanding of both processes. Building models to correct the "plants don't respire" misconception: (1) EMPHASIZE plants do BOTH: [Plant Photosynthesis] AND [Plant Respiration]. (2) Show MATTER cycling within the plant: Photosynthesis makes glucose + O2 from CO2 + H2O (in chloroplasts, during light). Respiration uses that glucose + O2 to make CO2 + H2O and ATP (in mitochondria, day and night). The CO2 + H2O can cycle back to photosynthesis. (3) EXPLAIN why plants need respiration: photosynthesis makes glucose (stored energy) but cells need ATP (usable energy) for work. Only respiration converts glucose to ATP! Plants need ATP for growth, transport, cell division—everything except photosynthesis itself. Without respiration, plants would die despite making glucose!
During the day, a plant in a meadow does photosynthesis and cellular respiration. At night, it does only cellular respiration. Which model best captures how matter (CO2, H2O, O2, glucose) cycles between photosynthesis and respiration over a 24-hour period, while energy flows one-way?
Day: CO2 + H2O → photosynthesis → glucose + O2; also glucose + O2 → respiration → CO2 + H2O. Night: glucose + O2 → respiration → CO2 + H2O. Matter can be reused the next day; energy: Sun → photosynthesis → glucose → respiration (ATP) → heat.
Day: photosynthesis makes glucose and O2 that permanently leave the ecosystem. Night: respiration stops because there is no sunlight. Energy: Sun → glucose (no heat loss).
Day and night: matter flows one-way from CO2 to glucose and never returns to CO2; energy cycles between glucose and sunlight.
Day: respiration only. Night: photosynthesis only. Matter: CO2 is produced by photosynthesis and used in respiration. Energy: Sun → heat → Sun (cycle).
Explanation
This question tests your ability to create or interpret models that show how photosynthesis and cellular respiration cycle matter (carbon dioxide, water, oxygen, glucose) between them while serving as sequential steps in energy flow from the sun to cellular work. An integrated model of photosynthesis and respiration must show TWO different patterns simultaneously: (1) MATTER CYCLING (circular pattern): draw or describe arrows showing glucose and O2 flowing FROM photosynthesis TO respiration (photosynthesis products → respiration reactants), and CO2 and H2O flowing FROM respiration TO photosynthesis (respiration products → photosynthesis reactants), creating a closed loop where the same molecules cycle repeatedly between the two processes—plants photosynthesize using CO2 and H2O to make glucose and O2, then both plants and animals use that glucose and O2 in respiration to make CO2 and H2O, which plants reuse in photosynthesis, cycling indefinitely. (2) ENERGY FLOW (one-way pattern): draw or describe energy entering from external source (sun) into photosynthesis (light captured), being stored in glucose, then released during respiration as ATP, then dissipating as heat from cellular work—this is a ONE-WAY path (sun → photosynthesis → glucose → respiration → ATP → heat lost from system), with no arrows returning energy to sun or earlier stages. The model must show BOTH patterns: circular for matter, linear for energy, often on the same diagram with different arrow styles or labels! The 24-hour plant model must show that during the day, the plant performs BOTH photosynthesis (using CO2 and H2O to make glucose and O2) AND respiration (using glucose and O2 to make CO2 and H2O for cellular energy), with photosynthesis rate exceeding respiration so net O2 is released; at night, only respiration occurs (no light for photosynthesis), using stored glucose and O2 while releasing CO2 and H2O that can be reused the next day when photosynthesis resumes—matter cycles between day and night processes! Choice A correctly models both matter cycling and energy flow by showing circular pathways for substances (CO2, O2, glucose, H2O) between processes and one-way pathway for energy (sun to heat). Choice B incorrectly claims plants do respiration during the day and photosynthesis at night—photosynthesis requires sunlight and cannot occur in darkness, while respiration occurs continuously day and night to power cellular processes! Why this model matters: it captures the fundamental asymmetry of ecosystems: MATTER is recycled (limited supply on Earth, must reuse—plants and animals exchange CO2 and O2, same atoms cycle), but ENERGY must be constantly supplied (sun) because it flows through and dissipates as heat (can't reuse heat for photosynthesis). The photosynthesis-respiration connection is the core of ecosystem functioning: photosynthesis channels solar energy into biological systems (converting to chemical energy) while providing matter (glucose, O2) for respiration. Respiration releases that stored energy for cellular use (converting to ATP) while providing matter (CO2, H2O) for photosynthesis.
A student makes a model of a forest ecosystem including trees and deer. Which model correctly shows that trees do both photosynthesis and cellular respiration, deer do cellular respiration only, and that CO2, H2O, O2, and glucose cycle between the two processes while energy flows from the Sun to heat?
Trees: respiration only (glucose + O2 → CO2 + H2O). Deer: photosynthesis only (CO2 + H2O → glucose + O2). Energy: Sun → deer → trees → heat.
Trees and deer: both do photosynthesis and respiration equally. Matter: O2 is converted into glucose during respiration. Energy: heat is recycled back into sunlight.
Trees: photosynthesis and respiration; Deer: respiration. Matter: CO2 + H2O from respiration → photosynthesis in trees → glucose + O2 → respiration in trees and deer → CO2 + H2O (cycle). Energy: Sun → photosynthesis → glucose chemical energy → respiration (ATP for work) → heat (one-way).
Trees: photosynthesis only (CO2 + H2O → glucose + O2). Deer: respiration only (glucose + O2 → CO2 + H2O). Energy: Sun → trees → deer → heat.
Explanation
This question tests your ability to create or interpret models that show how photosynthesis and cellular respiration cycle matter (carbon dioxide, water, oxygen, glucose) between them while serving as sequential steps in energy flow from the sun to cellular work. An integrated model of photosynthesis and respiration must show TWO different patterns simultaneously: (1) MATTER CYCLING (circular pattern): draw or describe arrows showing glucose and O2 flowing FROM photosynthesis TO respiration (photosynthesis products → respiration reactants), and CO2 and H2O flowing FROM respiration TO photosynthesis (respiration products → photosynthesis reactants), creating a closed loop where the same molecules cycle repeatedly between the two processes—plants photosynthesize using CO2 and H2O to make glucose and O2, then both plants and animals use that glucose and O2 in respiration to make CO2 and H2O, which plants reuse in photosynthesis, cycling indefinitely. (2) ENERGY FLOW (one-way pattern): draw or describe energy entering from external source (sun) into photosynthesis (light captured), being stored in glucose, then released during respiration as ATP, then dissipating as heat from cellular work—this is a ONE-WAY path (sun → photosynthesis → glucose → respiration → ATP → heat lost from system), with no arrows returning energy to sun or earlier stages. The model must show BOTH patterns: circular for matter, linear for energy, often on the same diagram with different arrow styles or labels! The forest ecosystem model must accurately represent that trees perform BOTH photosynthesis (making glucose and O2 from CO2 and H2O using sunlight) AND respiration (using glucose and O2 to make ATP, releasing CO2 and H2O), while deer only perform respiration since they cannot photosynthesize—this creates matter cycling where tree respiration and deer respiration both contribute CO2 and H2O that trees use in photosynthesis, while tree photosynthesis provides O2 and glucose (through leaves deer eat) for both tree and deer respiration. Choice C correctly models both matter cycling and energy flow by showing circular pathways for substances (CO2, O2, glucose, H2O) between processes and one-way pathway for energy (sun to heat). Choice B incorrectly reverses the organisms' capabilities, claiming deer do photosynthesis and trees only do respiration—deer lack chloroplasts and cannot capture sunlight to make glucose! Building integrated photosynthesis-respiration models: (1) DRAW or DESCRIBE two process boxes: [Photosynthesis] and [Respiration/Cellular Respiration]. (2) MATTER cycling (use solid arrows or label "matter"): Draw arrow from Photosynthesis to Respiration labeled "glucose + O2" (photosynthesis outputs → respiration inputs). Draw arrow from Respiration to Photosynthesis labeled "CO2 + H2O" (respiration outputs → photosynthesis inputs). These two arrows form a CIRCLE/LOOP between the processes—matter cycles! (3) ENERGY flow (use dashed arrows or label "energy"): Draw arrow FROM Sun TO Photosynthesis labeled "light energy" (energy enters system). Draw arrow FROM Photosynthesis TO Respiration labeled "chemical energy in glucose" (energy stored, then released). Draw arrows FROM both Photosynthesis and Respiration pointing OUT/AWAY labeled "heat" (energy exits system). These arrows are ONE-WAY—energy flows through, doesn't return! (4) RESULT: same model shows both patterns clearly.
A student is asked to compare two models of an ecosystem.
Model A: shows CO2 + H2O → photosynthesis → glucose + O2 → cellular respiration → CO2 + H2O (a loop), but no energy arrows.
Model B: shows the same matter loop and also shows Sun → photosynthesis → glucose chemical energy → cellular respiration (ATP) → heat.
Which choice best evaluates which model better represents both matter cycling and energy flow?
Model B is better because it includes matter cycling between photosynthesis and respiration and also shows one-way energy flow from the Sun to heat.
Model A is better because energy cycles in a loop just like CO2 and O2, so energy arrows would be circular.
Model B is worse because it incorrectly shows glucose and O2 moving from photosynthesis to respiration; those should move from respiration to photosynthesis.
Model A is better because energy should not be included when modeling ecosystems; only matter cycles.
Explanation
This question tests your ability to create or interpret models that show how photosynthesis and cellular respiration cycle matter (carbon dioxide, water, oxygen, glucose) between them while serving as sequential steps in energy flow from the sun to cellular work. An integrated model of photosynthesis and respiration must show TWO different patterns simultaneously: (1) MATTER CYCLING (circular pattern): draw or describe arrows showing glucose and O2 flowing FROM photosynthesis TO respiration (photosynthesis products → respiration reactants), and CO2 and H2O flowing FROM respiration TO photosynthesis (respiration products → photosynthesis reactants), creating a closed loop where the same molecules cycle repeatedly between the two processes—plants photosynthesize using CO2 and H2O to make glucose and O2, then both plants and animals use that glucose and O2 in respiration to make CO2 and H2O, which plants reuse in photosynthesis, cycling indefinitely. (2) ENERGY FLOW (one-way pattern): draw or describe energy entering from external source (sun) into photosynthesis (light captured), being stored in glucose, then released during respiration as ATP, then dissipating as heat from cellular work—this is a ONE-WAY path (sun → photosynthesis → glucose → respiration → ATP → heat lost from system), with no arrows returning energy to sun or earlier stages. The model must show BOTH patterns: circular for matter, linear for energy, often on the same diagram with different arrow styles or labels! Model A shows the complete matter cycle (CO2 + H2O ↔ glucose + O2) but lacks energy flow arrows, making it incomplete for ecosystem representation; Model B shows the same matter cycle PLUS the complete energy flow pathway (Sun → photosynthesis → glucose → respiration → heat), making it the superior model that captures both essential patterns of ecosystem function. Choice B correctly identifies Model B as better because it includes both matter cycling and energy flow, while Model A only shows half the story. Choice D incorrectly claims glucose and O2 should move from respiration to photosynthesis—this reverses the actual flow, as photosynthesis produces glucose and O2 that respiration consumes! Building integrated photosynthesis-respiration models: (1) DRAW or DESCRIBE two process boxes: [Photosynthesis] and [Respiration/Cellular Respiration]. (2) MATTER cycling (use solid arrows or label "matter"): Draw arrow from Photosynthesis to Respiration labeled "glucose + O2" (photosynthesis outputs → respiration inputs). Draw arrow from Respiration to Photosynthesis labeled "CO2 + H2O" (respiration outputs → photosynthesis inputs). These two arrows form a CIRCLE/LOOP between the processes—matter cycles! (3) ENERGY flow (use dashed arrows or label "energy"): Draw arrow FROM Sun TO Photosynthesis labeled "light energy" (energy enters system). Draw arrow FROM Photosynthesis TO Respiration labeled "chemical energy in glucose" (energy stored, then released). Draw arrows FROM both Photosynthesis and Respiration pointing OUT/AWAY labeled "heat" (energy exits system). These arrows are ONE-WAY—energy flows through, doesn't return! (4) RESULT: same model shows both patterns clearly.