How do photosynthesis and cellular respiration cycle matter and energy through the cell?
Explain the roles of photosynthesis and cellular respiration in the cycling of matter and the flow of energy, including their reactants, products, and how the two processes connect (GSE SB1.e).
A Georgia Milestones Biology EOC answer on photosynthesis and cellular respiration: the reactants and products of each, where they occur, how energy flows and matter cycles, and why the two processes are reverse complements that link plants and animals.
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What this topic is asking
Standard SB1.e asks you to explain how photosynthesis and cellular respiration drive the cycling of matter and the flow of energy. For the Georgia Milestones Biology EOC you must know the reactants and products of each process, where each occurs, and how the two are reverse complements that connect producers and consumers. This is also the cell-level basis for the ecology standard on energy flow and matter cycling.
Photosynthesis
The summary equation is:
Photosynthesis stores light energy in the chemical bonds of glucose. The oxygen released is a by-product that nearly all other organisms then use for respiration. Plants make their own food this way, which is why they are autotrophs (self-feeders) and the base of food chains.
Cellular respiration
The summary equation is:
Respiration happens in almost all living cells, plant and animal. Plants both photosynthesize (in chloroplasts, when light is available) and respire (in mitochondria, all the time) to power their own activities. When oxygen is scarce, cells use fermentation (anaerobic) to make a little ATP, producing lactic acid (in animal muscle) or ethanol and carbon dioxide (in yeast).
The connection: reverse complements
The reason the EOC pairs the two processes is that they are near-mirror images, which links them into a cycle:
| Photosynthesis | Cellular respiration | |
|---|---|---|
| Site | Chloroplast | Mitochondrion |
| Reactants | Carbon dioxide, water, light | Glucose, oxygen |
| Products | Glucose, oxygen | Carbon dioxide, water, energy (ATP) |
| Energy | Stores light energy in glucose | Releases energy from glucose |
| Who | Producers (autotrophs) | Almost all organisms |
The products of one are the reactants of the other. Photosynthesis makes the glucose and oxygen that respiration consumes; respiration makes the carbon dioxide and water that photosynthesis consumes.
Try this
Q1. State the organelle where each process occurs: photosynthesis and cellular respiration. [2 points]
- Cue. Photosynthesis in the chloroplast; cellular respiration in the mitochondrion.
Q2. Explain why the products of photosynthesis are the reactants of cellular respiration. [2 points]
- Cue. Photosynthesis makes glucose and oxygen; respiration uses glucose and oxygen to release energy, so the two processes feed each other in a cycle of matter.
Exam-style practice questions
Practice questions written in the style of GaDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Milestones (style)1 marksWhich statement correctly pairs a process with where it occurs? (A) Photosynthesis occurs in mitochondria. (B) Cellular respiration occurs in chloroplasts. (C) Photosynthesis occurs in chloroplasts. (D) Cellular respiration occurs only in plants.Show worked answer →
A 1-point selected-response item on the sites of the two processes.
The correct answer is C. Photosynthesis occurs in chloroplasts (in plants and algae), and cellular respiration occurs in mitochondria (in plants and animals). A and B swap the organelles, and D is wrong because cellular respiration occurs in nearly all living cells, plant and animal, not only plants. Knowing which organelle hosts each process is a frequent EOC point.
Milestones (style)2 marksDrop-down. In photosynthesis the reactants are [carbon dioxide and water / glucose and oxygen] and the products are [glucose and oxygen / carbon dioxide and water].Show worked answer →
A 2-point technology-enhanced (drop-down) item.
The correct choices are "carbon dioxide and water" (reactants) and "glucose and oxygen" (products). Photosynthesis uses carbon dioxide and water plus light energy to make glucose and release oxygen: . Cellular respiration is the reverse, using glucose and oxygen to release energy plus carbon dioxide and water. Reversing the reactants and products of the two processes is the most common error.
Related dot points
- Construct an explanation of how cell structures and organelles (nucleus, cytoplasm, cell membrane, cell wall, chloroplasts, lysosome, Golgi apparatus, endoplasmic reticulum, vacuoles, ribosomes, mitochondria) interact as a system to maintain homeostasis (GSE SB1.a).
A Georgia Milestones Biology EOC answer on the eukaryotic organelles as a structure-and-function system: the nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, lysosomes, vacuoles, membrane, and cell wall, and how they work together to maintain homeostasis.
- Relate the structure of the four macromolecules (carbohydrates, lipids, proteins, nucleic acids), their monomers, and their functions in carrying out cellular processes (GSE SB1.c).
A Georgia Milestones Biology EOC answer on the four biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids, their monomers and elements, their functions, and how structure relates to function in cellular processes.
- Explain how enzymes (a type of protein) lower activation energy and carry out cellular processes, and how temperature, pH, and substrate fit affect enzyme activity (GSE SB1.c).
A Georgia Milestones Biology EOC answer on enzymes: how they lower activation energy, the lock-and-key specificity of the active site, the effect of temperature, pH, and substrate concentration, and what denaturation does to enzyme activity.
- Analyze the cycling of matter through ecosystems, including the carbon, nitrogen, and water cycles, and the roles of photosynthesis, respiration, and decomposers (GSE SB5.b).
A Georgia Milestones Biology EOC answer on the cycling of matter: the carbon cycle (photosynthesis and respiration), the nitrogen cycle (fixation by bacteria), and the water cycle, and how decomposers recycle nutrients, contrasted with the one-way flow of energy.
- Analyze the flow of energy through ecosystems using food chains, food webs, and energy pyramids, including the roles of producers, consumers, and decomposers and the ten percent rule (GSE SB5.b).
A Georgia Milestones Biology EOC answer on energy flow: producers, consumers, and decomposers, trophic levels, food chains and food webs, the ten percent rule, and why energy pyramids narrow toward the top.
Sources & how we know this
- Biology Georgia Standards of Excellence (GSE) — Georgia Department of Education (2024)
- Georgia Milestones Biology EOC Assessment Guide — Georgia Department of Education (2024)