OCR Specification focus:
‘Relate the raw materials and products of photosynthesis and respiration, showing how the two processes are interdependent.’
Photosynthesis and respiration are complementary biological processes essential for energy flow in living organisms. Each relies on the other’s products, ensuring a continuous exchange of gases, energy, and organic compounds within ecosystems.
The Relationship Between Photosynthesis and Respiration
Overview of Energy Conversion
Energy transformation in living systems centres on two major processes: photosynthesis and respiration.
Photosynthesis converts light energy into chemical energy stored in organic molecules such as glucose.
Respiration releases the stored energy by oxidising glucose to produce ATP (adenosine triphosphate) — the universal energy currency of cells.
These processes are interconnected in a biological energy cycle, maintaining the balance of oxygen and carbon dioxide in the biosphere.
Photosynthesis: The Source of Organic Energy
Raw Materials and Products
Photosynthesis occurs in the chloroplasts of plant cells. It requires:
Carbon dioxide (CO₂) from the atmosphere
Water (H₂O) from the soil
Light energy absorbed by chlorophyll pigments
The products are:
Glucose (C₆H₁₂O₆) — an energy-rich carbohydrate
Oxygen (O₂) — released as a by-product into the atmosphere
A concise diagram showing a plant using light, carbon dioxide, and water to produce oxygen and carbohydrates. This directly illustrates the overall photosynthesis reaction emphasised in A-Level courses. Extra mechanistic details are intentionally absent to keep focus on inputs and outputs. Source.
EQUATION
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Overall Photosynthesis Equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
CO₂ = Carbon dioxide (gas absorbed from air)
H₂O = Water (from soil via roots)
C₆H₁₂O₆ = Glucose (used in respiration or storage)
O₂ = Oxygen (released into atmosphere)
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This process fixes carbon from inorganic to organic form, creating the primary energy source for almost all life forms.
Respiration: The Release of Energy
Cellular Respiration Overview
Respiration takes place in mitochondria and involves the oxidation of glucose to produce ATP, which fuels metabolic reactions. It requires oxygen and releases carbon dioxide and water.
A simplified equation diagram for aerobic respiration converting glucose and oxygen into carbon dioxide, water, and ATP. This mirrors the photosynthesis equation in reverse at the level of overall reactants and products. The image focuses only on whole-cell inputs/outputs, without pathway detail, matching OCR depth. Source.
EQUATION
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Overall Respiration Equation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
C₆H₁₂O₆ = Glucose (oxidised for energy)
O₂ = Oxygen (electron acceptor)
CO₂ = Carbon dioxide (waste product)
H₂O = Water (by-product)
ATP = Adenosine triphosphate (energy currency of the cell)
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The ATP produced in respiration is used for active transport, biosynthesis, and muscle contraction, among other essential biological activities.
Interdependence of Photosynthesis and Respiration
The Biochemical Connection
The two processes are biochemically interlinked:
The products of photosynthesis (glucose and oxygen) are the reactants for respiration.
Schematic of a leaf indicating CO₂ uptake, O₂ release, and carbohydrate formation during photosynthesis, alongside respiration using O₂ and returning CO₂. It clearly depicts the reciprocal gas exchange and energy use linking the two processes. The figure includes day/night timing, an extra detail beyond the syllabus but helpful context. Source.
The products of respiration (carbon dioxide and water) are the raw materials for photosynthesis.
This creates a cyclical relationship ensuring a stable environment and continuous energy transfer between autotrophs (producers) and heterotrophs (consumers).
Gas Exchange and Balance
Oxygen produced during photosynthesis supports aerobic respiration in plants and animals.
Carbon dioxide released during respiration is reused in photosynthesis to maintain atmospheric balance.
This gas exchange underpins global carbon and oxygen cycles, vital for ecosystem stability.
Energy Flow Through Ecosystems
The Role of ATP
Although photosynthesis produces glucose, ATP generated in respiration provides the immediate, usable form of energy.
ATP (Adenosine Triphosphate): A nucleotide molecule that stores and transfers energy within cells, releasing energy when its terminal phosphate bond is broken.
ATP drives metabolic pathways such as DNA replication, protein synthesis, and active transport across membranes.
Trophic Interconnections
Producers (plants and algae) harness solar energy through photosynthesis.
Consumers (animals) obtain energy by feeding on producers or other consumers, depending on respiration to release the energy stored in food.
Decomposers return carbon to the atmosphere through respiration, completing the carbon cycle.
This transfer ensures that energy captured by chlorophyll ultimately supports all forms of life.
Cellular Complementarity
Chloroplasts and Mitochondria
Both organelles cooperate within plant cells:
Chloroplasts synthesise glucose and oxygen during the day using light energy.
Mitochondria respire continuously, converting glucose into ATP needed for cellular processes, even at night when photosynthesis ceases.
Starch and sucrose may serve as storage and transport forms of glucose, ensuring a constant supply for respiration.
Recycling of Molecules
This interdependence ensures no waste of vital substances:
CO₂ from respiration is fixed again in photosynthesis.
O₂ from photosynthesis is reused in respiration.
H₂O acts both as a reactant and a product, maintaining hydration and osmotic balance.
Ecological and Evolutionary Importance
Stability of the Biosphere
The dynamic equilibrium between photosynthesis and respiration regulates:
Atmospheric composition — stabilising CO₂ and O₂ levels
Global temperature — through control of greenhouse gases
Energy availability — supporting food webs and productivity
Without this interdependence, aerobic life would not persist, as energy flow and gas exchange would collapse.
Evolutionary Adaptation
The rise of photosynthetic organisms billions of years ago increased atmospheric oxygen, enabling the evolution of aerobic respiration, which yields much more ATP than anaerobic alternatives.
The coevolution of these processes allowed multicellular life to thrive, illustrating their fundamental biological partnership.
Summary of Interdependence
Photosynthesis provides: glucose and oxygen.
Respiration provides: carbon dioxide, water, and ATP.
The cycle continues as each process sustains the other, creating a self-regulating system vital to life on Earth.
Both are essential components of energy transfer and matter recycling, illustrating the profound interrelationship between photosynthesis and respiration in all living organisms.
FAQ
Photosynthesis removes carbon dioxide from the atmosphere and releases oxygen, while respiration does the reverse.
This reciprocal exchange stabilises atmospheric concentrations of both gases, preventing excessive accumulation of either.
In ecosystems, plants act as net oxygen producers during the day, but respiration in all organisms ensures continuous carbon dioxide release. This balance is vital for supporting aerobic life and regulating global climate.
Plant cells contain both chloroplasts and mitochondria, allowing both processes to occur independently yet concurrently.
During daylight, chloroplasts capture light energy for photosynthesis, generating glucose and oxygen.
Simultaneously, mitochondria respire, using some of that glucose and oxygen to produce ATP for immediate cellular needs.
Even when photosynthesis stops at night, respiration continues, ensuring the plant has a constant energy supply.
At night, photosynthesis ceases due to the absence of light, but respiration continues.
Plants still use oxygen and release carbon dioxide.
The rate of respiration remains relatively constant, though overall gas exchange reverses from daytime patterns.
This means that during the night, plants are net producers of carbon dioxide rather than oxygen, although this output is small compared with their daytime oxygen production.
Temperature influences the enzyme-controlled reactions in both processes.
At higher temperatures, respiration generally increases faster than photosynthesis, as enzymes in respiration remain active at slightly higher ranges.
At very high or very low temperatures, photosynthetic enzymes such as RuBisCO become less efficient, reducing glucose supply for respiration.
Thus, environmental temperature changes can temporarily disturb the balance between the two processes, impacting energy availability and carbon cycling.
Although both processes involve ATP, the ATP molecules produced in photosynthesis are used immediately within the chloroplasts for the Calvin cycle and other reactions.
Respiration generates its own ATP in the mitochondria to supply energy for the whole cell.
Instead of transferring ATP, photosynthesis provides glucose, which acts as the energy carrier between the two processes — the chemical link that sustains their interdependence.
Practice Questions
Question 1 (2 marks)
Explain how the processes of photosynthesis and respiration are interdependent in plants.
Mark scheme:
1 mark for stating that the products of photosynthesis (glucose and oxygen) are used as reactants in respiration.
1 mark for stating that the products of respiration (carbon dioxide and water) are used as reactants in photosynthesis.
(Allow alternative phrasing such as “the two processes form a cycle of gas and energy exchange.”)
Question 2 (5 marks)
Describe and explain how the raw materials and products of photosynthesis and respiration demonstrate the interrelationship between these two processes in living organisms.
Mark scheme:
1 mark for correctly identifying the raw materials of photosynthesis (carbon dioxide and water) and respiration (glucose and oxygen).
1 mark for correctly identifying the products of photosynthesis (glucose and oxygen) and respiration (carbon dioxide, water, and ATP).
1 mark for explaining that each process provides the reactants required for the other, forming a biochemical cycle.
1 mark for stating that this interrelationship maintains a balance of oxygen and carbon dioxide in the atmosphere.
1 mark for noting that the energy captured by photosynthesis (in glucose) is released by respiration as ATP for metabolic processes.
(Allow clear, logical alternative explanations using equivalent terminology such as “autotroph–heterotroph interdependence.”)
