Photosynthesis and Respiration Compared [H]

This Higher tier lesson brings together your knowledge of photosynthesis and respiration to make detailed comparisons between the two processes. Understanding how these reactions are connected — and how they differ — is a key skill for AQA GCSE Biology. Examiners frequently test the ability to compare and contrast these two fundamental metabolic reactions.

---

Why Compare Photosynthesis and Respiration?

Photosynthesis and respiration are two of the most important chemical reactions in biology. They are:

• Opposite in many ways (one builds glucose, the other breaks it down)
• Interdependent (the products of one are the reactants of the other)
• Both essential for life on Earth

Understanding how they relate to each other is critical for answering Higher tier questions on Bioenergetics, ecology and energy transfer.

---

Side-by-Side Comparison

Feature	Photosynthesis	Aerobic Respiration
Word equation	CO2 + water ---(light)---> glucose + O2	glucose + O2 ---> CO2 + water
Symbol equation	6CO2 + 6H2O ---> C6H12O6 + 6O2	C6H12O6 + 6O2 ---> 6CO2 + 6H2O
Type of reaction	Endothermic (takes in energy)	Exothermic (releases energy)
Energy source	Light energy (from the Sun)	Chemical energy (from glucose)
Where it occurs	Chloroplasts (in leaf cells)	Mitochondria (in all living cells)
Which organisms?	Plants, algae, some bacteria	All living organisms
When does it occur?	Only in the light	All the time (day and night)
Gas exchange	Takes in CO2, releases O2	Takes in O2, releases CO2
Effect on glucose	Produces glucose	Uses up glucose
Effect on mass	Increases plant biomass	Decreases stored energy (glucose/glycogen)

graph LR
    A[Photosynthesis] -->|Produces glucose and O2| B[Aerobic Respiration]
    B -->|Produces CO2 and water| A
    A -->|Endothermic — takes in light energy| C[Energy stored in glucose]
    B -->|Exothermic — releases energy| D[Energy for life processes]
    C --> B

Exam Tip: Notice that the equation for photosynthesis is the reverse of the equation for aerobic respiration. The reactants of one are the products of the other. This is the single most important comparison and is almost guaranteed to appear in some form in the exam.

---

The Relationship Between Photosynthesis and Respiration

Photosynthesis and respiration are not separate, isolated processes. They are deeply interconnected:

1. Photosynthesis produces glucose — this glucose is then used as the fuel for respiration.
2. Respiration produces CO2 and water — these are the raw materials for photosynthesis.
3. Photosynthesis stores energy in glucose molecules (light energy is converted to chemical energy).
4. Respiration releases energy from glucose molecules (chemical energy is transferred for life processes).

The Carbon Cycle Link

This relationship is fundamental to the carbon cycle:

• Photosynthesis removes CO2 from the atmosphere and fixes it into organic molecules (glucose)
• Respiration returns CO2 to the atmosphere by breaking down organic molecules
• This creates a continuous cycle of carbon between living organisms and the atmosphere

---

Gas Exchange in Plants: Day vs Night

Because plants both photosynthesise and respire, their overall gas exchange depends on the time of day:

During the Day (Bright Light)

Process	Rate	Net Effect
Photosynthesis	Fast (lots of light energy)	Produces lots of O2 and uses lots of CO2
Respiration	Continuous (normal rate)	Uses some O2 and produces some CO2
Overall	Photosynthesis faster than respiration	Net release of O2 and net uptake of CO2

During the Night (No Light)

Process	Rate	Net Effect
Photosynthesis	Zero (no light)	No O2 produced, no CO2 used
Respiration	Continuous (normal rate)	Uses O2 and produces CO2
Overall	Only respiration occurs	Net uptake of O2 and net release of CO2

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 by photosynthesis
• The amount of O2 produced by photosynthesis equals the amount used by respiration
• There is no net gas exchange

This occurs at dawn and dusk, when light intensity is intermediate.

graph TD
    A[Dawn - Low Light] --> B[Compensation Point]
    B --> C[Photosynthesis rate = Respiration rate]
    C --> D[No net gas exchange]
    E[Bright Daylight] --> F[Photosynthesis much faster than Respiration]
    F --> G[Net O2 release, Net CO2 uptake]
    H[Night - No Light] --> I[Only Respiration occurs]
    I --> J[Net O2 uptake, Net CO2 release]

Exam Tip: The compensation point is a favourite Higher tier question. Make sure you can explain that at the compensation point, the rate of photosynthesis equals the rate of respiration, so there is no net exchange of gases. Above this point, photosynthesis dominates; below it, respiration dominates.

---

Energy Flow Between Photosynthesis and Respiration

The energy flow between the two processes can be summarised as:

1. Light energy from the Sun is absorbed by chlorophyll during photosynthesis
2. This energy is converted to chemical energy and stored in the bonds of glucose
3. During respiration, the chemical energy in glucose is released
4. This released energy is used for life processes (movement, growth, active transport, etc.)
5. Some energy is transferred to the surroundings as heat at every stage (this is why energy transfer is never 100% efficient)

Stage	Energy Form	Process
Sunlight hits leaf	Light energy	Absorbed by chlorophyll
Photosynthesis	Light ---> Chemical	Energy stored in glucose
Respiration	Chemical ---> Kinetic/Heat/Other	Energy released for life processes
Heat loss	Thermal energy	Dissipated to surroundings

---

Measuring Net Photosynthesis

When scientists measure the rate of photosynthesis, they are usually measuring the net rate — the amount of photosynthesis minus the amount of respiration happening at the same time.

• Gross photosynthesis = the total rate of photosynthesis
• Net photosynthesis = gross photosynthesis minus respiration

Net photosynthesis = Gross photosynthesis - Respiration

This means that the amount of oxygen released by a plant (which is what we can measure) is actually less than the total amount produced, because some is used up by respiration within the plant.

---

Practical Implications

For Greenhouse Growers

Understanding the relationship between photosynthesis and respiration helps growers:

• Ensure photosynthesis rate exceeds respiration rate to maximise net glucose production
• Provide light for extended periods (or 24 hours) to maximise the time spent producing glucose
• Maintain warm temperatures to keep both photosynthesis and respiration running efficiently (but not so hot that enzymes denature)

For Ecologists

• The balance between photosynthesis and respiration across all organisms determines whether an ecosystem is a carbon sink (absorbing more CO2 than it releases) or a carbon source
• Forests are typically carbon sinks because tree photosynthesis outpaces total respiration
• Destroyed or degraded ecosystems may become carbon sources

---

Common Exam Mistakes