Comparing Photosynthesis and Respiration

This final lesson in the Bioenergetics topic brings everything together by comparing photosynthesis and respiration side by side. Understanding the similarities, differences and interconnections between these two fundamental processes is essential for the AQA GCSE Combined Science Trilogy specification (8464) exam.

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Why Compare Them?

Photosynthesis and respiration are the two most important energy-transfer reactions in biology. They are interconnected:

• Photosynthesis produces glucose and oxygen — these are the reactants for respiration.
• Respiration produces carbon dioxide and water — these are the reactants for photosynthesis.
• Together, they form a cycle that drives energy flow through ecosystems.

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Side-by-Side Comparison

Feature	Photosynthesis	Aerobic Respiration
Type of reaction	Endothermic	Exothermic
Energy	Takes in light energy	Releases energy
Reactants	CO₂ + H₂O	Glucose + O₂
Products	Glucose + O₂	CO₂ + H₂O
Where it occurs	Chloroplasts (in plant cells)	Mitochondria (in all living cells)
When it occurs	Only in the light (daytime)	All the time (day and night)
Organisms	Plants, algae, some bacteria	All living organisms
Word equation	CO₂ + H₂O → glucose + O₂	Glucose + O₂ → CO₂ + H₂O
Symbol equation	$6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$6CO2​+6H2​O→C6​H12​O6​+6O2​	$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$C6​H12​O6​+6O2​→6CO2​+6H2​O

Exam Tip: The equations for photosynthesis and aerobic respiration are the reverse of each other. This is the single most important observation for this topic — if you know one equation, you know the other.

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The Equations Side by Side

Photosynthesis

$$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light energy}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$6CO2​+6H2​Olight energy​C6​H12​O6​+6O2​

Aerobic Respiration

$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$$C6​H12​O6​+6O2​→6CO2​+6H2​O

Anaerobic Respiration (Animals)

$$\text{glucose} \rightarrow \text{lactic acid}$$glucose→lactic acid

Anaerobic Respiration (Yeast/Plants)

$$\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}$$glucose→ethanol+carbon dioxide

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Energy Transfer

graph LR
    A["Light energy from the Sun"] -->|"Absorbed by chlorophyll"| B["Photosynthesis"]
    B -->|"Stored as chemical energy in glucose"| C["Glucose"]
    C -->|"Broken down"| D["Respiration"]
    D -->|"Energy released"| E["Life processes"]
    D -->|"CO₂ + H₂O released"| F["Back to atmosphere / environment"]
    F -->|"Used by plants"| B

This diagram shows the fundamental energy cycle:

1. Light energy is converted to chemical energy (glucose) by photosynthesis.
2. Chemical energy is released by respiration to power life processes.
3. The waste products of respiration (CO₂ and H₂O) are recycled as reactants for photosynthesis.

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Gas Exchange in Plants: Day vs Night

One of the most commonly examined aspects of this topic is how gas exchange in plants differs between day and night.

Time	Photosynthesis	Respiration	Net CO₂	Net O₂
Bright daylight	Fast (high rate)	Continuous	Net uptake (more used than produced)	Net release (more produced than used)
Dim light	Slow	Continuous	Depends on light level	Depends on light level
Compensation point	Rate = rate of respiration	Continuous	Net exchange is zero	Net exchange is zero
Night (dark)	None	Continuous	Net release	Net uptake

The Compensation Point

The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration. At this point:

• The amount of CO₂ produced by respiration equals the amount used in photosynthesis.
• The amount of O₂ produced by photosynthesis equals the amount used in respiration.
• There is no net gas exchange with the environment.

graph TD
    A["Light intensity increases"] --> B{"Below compensation point?"}
    B -->|"Yes"| C["Respiration > Photosynthesis"]
    C --> D["Net release of CO₂"]
    B -->|"No"| E{"At compensation point?"}
    E -->|"Yes"| F["Respiration = Photosynthesis"]
    F --> G["No net gas exchange"]
    E -->|"No — above"| H["Photosynthesis > Respiration"]
    H --> I["Net uptake of CO₂, net release of O₂"]

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The Carbon Cycle Connection

Photosynthesis and respiration are two key processes in the carbon cycle:

• Photosynthesis removes CO₂ from the atmosphere and locks carbon into organic molecules (glucose, starch, cellulose, proteins, lipids).
• Respiration (by all organisms) returns CO₂ to the atmosphere.
• These processes help regulate atmospheric CO₂ concentration and, by extension, global temperatures through the greenhouse effect.

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Key Similarities

Despite being "opposite" reactions, photosynthesis and respiration share some features:

Similarity	Detail
Both involve gas exchange	CO₂ and O₂ are involved in both
Both are enzyme-controlled	Both are affected by temperature
Both involve energy transfer	One stores energy, the other releases it
Both occur in plant cells	Plants carry out both processes
Both involve glucose	It is a product of one and a reactant of the other

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Worked Example

Question: A student places a plant in a sealed container with a CO₂ sensor. During the day, CO₂ levels fall. At night, CO₂ levels rise. Explain these observations.

Answer:

• During the day, the plant carries out both photosynthesis and respiration.
• The rate of photosynthesis exceeds the rate of respiration, so the plant has a net uptake of CO₂ — CO₂ levels in the container fall.
• At night, the plant cannot photosynthesise (no light), but continues to respire.
• Respiration releases CO₂, so CO₂ levels in the container rise.

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Common Mistakes

Mistake	Correction
Saying plants do not respire during the day	Plants respire all the time — day and night
Saying photosynthesis and respiration are "the same but opposite"	They share features but occur in different organelles and serve different purposes
Mixing up the organelles	Photosynthesis = chloroplasts; respiration = mitochondria
Forgetting about anaerobic respiration when comparing	Include anaerobic respiration in your comparison if the question asks for it
Saying the products of photosynthesis "go directly" to respiration	Glucose may be stored as starch first, then converted back to glucose for respiration later

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All Key Equations — Summary Table

Reaction	Equation
Photosynthesis	$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$6CO2​+6H2​Olight​C6​H12​O6​+6O2​
Aerobic respiration	$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$C6​H12​O6​+6O2​→6CO2​+6H2​O
Anaerobic (animals)	glucose → lactic acid
Anaerobic (yeast/plants)	glucose → ethanol + carbon dioxide

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Final Revision Checklist

Use this checklist to ensure you have covered everything in the Bioenergetics topic for AQA 8464: