How are photosynthesis and cellular respiration linked, and why do they form a cycle?
Explain the interrelated nature of photosynthesis and cellular respiration (NGSSS SC.912.L.18.9; Reporting Category 1, Molecular and Cellular Biology).
A benchmark-level answer on the link between photosynthesis and respiration for the Florida Biology 1 EOC: how the products of one are the reactants of the other, the cycling of matter and energy, and why both happen in plants.
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What this topic is asking
The NGSSS benchmark SC.912.L.18.9 asks you to explain the interrelated nature of photosynthesis and cellular respiration. For the Florida Biology 1 EOC this is the synthesis topic that ties the two processes together: you need to see that the products of one are the reactants of the other, that together they cycle matter and energy, and that plants do both. Items test the reactant-product relationship and the misconception that plants do not respire.
The two processes side by side
Lining up the two word equations shows the link at a glance:
- Photosynthesis: carbon dioxide + water + light energy gives glucose + oxygen
- Respiration: glucose + oxygen gives carbon dioxide + water + energy (ATP)
This is the single most-tested idea: one process builds glucose and releases oxygen, the other breaks glucose down and uses oxygen.
Cycling matter and energy
The link between the two processes is how matter and energy move through living systems.
- Matter cycles. Carbon and oxygen pass back and forth: photosynthesis pulls carbon dioxide from the air and locks the carbon into glucose and releases oxygen; respiration releases the carbon back as carbon dioxide and uses the oxygen. This is the heart of the carbon cycle.
- Energy flows (it does not cycle). Photosynthesis captures light energy and stores it as chemical energy in glucose; respiration releases that chemical energy as ATP for the cell to use, with some lost as heat. Energy enters as light and leaves as heat, so it flows one way even as matter cycles.
Why plants do both
This is why a plant kept in the dark still uses oxygen and releases carbon dioxide: it is respiring even though it cannot photosynthesize.
Try this
Q1. Explain how the products of photosynthesis are related to the reactants of respiration. [2]
- Cue. Photosynthesis produces glucose and oxygen, which are exactly the reactants (inputs) of cellular respiration; the processes are the reverse of each other.
Q2. State whether plants carry out respiration, and when. [2]
- Cue. Yes, plants respire continuously, day and night, to release the energy in glucose; they photosynthesize only in the light.
Exam-style practice questions
Practice questions written in the style of FLDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
FL Biology 1 EOC (2023 released style)1 marksHow are the reactants and products of photosynthesis and cellular respiration related? (A) They are unrelated processes. (B) The products of photosynthesis (glucose and oxygen) are the reactants of respiration, and the reverse. (C) Both processes use only carbon dioxide. (D) Both processes release light energy.Show worked answer →
A 1-point multiple-choice item on the core relationship.
The correct answer is B. Photosynthesis makes glucose and oxygen using carbon dioxide and water; respiration uses glucose and oxygen and produces carbon dioxide and water. The output of one is the input of the other, so they form a cycle. A denies the link, and C and D misstate the chemistry.
Picture the two equations as reverses of each other; the arrows point opposite ways.
FL Biology 1 EOC (2024 released style)1 marksA plant carries out both photosynthesis and cellular respiration. Which statement is correct? (A) Plants only photosynthesize and never respire. (B) Plants photosynthesize in the light and respire all the time. (C) Plants respire only in the light. (D) Plants do neither at night.Show worked answer →
A 1-point item correcting a common misconception.
The correct answer is B. Plants make glucose by photosynthesis when light is available, but like all organisms they respire continuously, day and night, to release the energy in that glucose. A and C and D all misstate when each process occurs. This is one of the most common EOC misconceptions.
Related dot points
- Identify the reactants, products, and basic functions of photosynthesis (NGSSS SC.912.L.18.7; Reporting Category 1, Molecular and Cellular Biology).
A benchmark-level answer on photosynthesis for the Florida Biology 1 EOC: the reactants and products, the chloroplast and chlorophyll, where the energy goes, and the overall equation.
- Identify the reactants, products, and basic functions of aerobic and anaerobic cellular respiration (NGSSS SC.912.L.18.8; Reporting Category 1, Molecular and Cellular Biology).
A benchmark-level answer on cellular respiration for the Florida Biology 1 EOC: the reactants and products of aerobic respiration, the role of the mitochondrion and ATP, and the two types of anaerobic respiration (fermentation).
- Explain how matter cycles through ecosystems, including the carbon, nitrogen, and water cycles, and the roles organisms play in them (NGSSS SC.912.L.17; Reporting Category 3, Organisms, Populations, and Ecosystems).
A benchmark-level answer on biogeochemical cycles for the Florida Biology 1 EOC: the carbon cycle (photosynthesis and respiration), the nitrogen cycle and nitrogen-fixing bacteria, the water cycle, and how matter cycles while energy flows.
- Use a food web to identify producers, consumers, and decomposers, and explain the transfer of energy through trophic levels and the reduction of available energy at each level (NGSSS SC.912.L.17.9; Reporting Category 3, Organisms, Populations, and Ecosystems).
A benchmark-level answer on energy flow for the Florida Biology 1 EOC: producers, consumers, and decomposers, food chains and webs, trophic levels, the energy pyramid, and the ten percent rule.
- Describe the basic molecular structures and primary functions of the four major categories of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids (NGSSS SC.912.L.18.1; Reporting Category 1, Molecular and Cellular Biology).
A benchmark-level answer on biological macromolecules for the Florida Biology 1 EOC: carbohydrates, lipids, proteins, and nucleic acids, their monomers, the elements they contain, and the function of each.
Sources & how we know this
- Next Generation Sunshine State Standards: Science (Biology 1) — Florida Department of Education (2024)
- Biology 1 End-of-Course Assessment Test Item Specifications — Florida Department of Education (2024)