Photosynthesis vs Respiration

Team Biology at
Created by: Team Biology at, Last Updated: May 8, 2024

Photosynthesis vs Respiration

Photosynthesis and plant respiration are two fundamental processes in plant biology, each playing a crucial role in the life cycle of plants. While both are vital for the survival of plants, they serve very different functions. Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, carbon dioxide, and water to produce glucose and oxygen. This process is essential for the production of energy in the form of glucose, which serves as a primary energy source for the plant. On the other hand, plant respiration is the process where plants consume oxygen and use it to turn glucose into ATP, releasing carbon dioxide as a byproduct. This occurs in the mitochondria of the cells and is crucial for energy conversion that fuels all plant activities.

What is Photosynthesis?

Photosynthesis is the incredible biological process by which plants, algae, and some bacteria convert light energy into chemical energy. This process not only sustains the organism performing it but also supports most life forms on Earth by producing oxygen and organic compounds.

Chlorophyll and Sunlight

At the heart of photosynthesis lies chlorophyll, the pigment that gives plants their green color. Chlorophyll absorbs sunlight, which serves as the energy source required to drive this process.

The Stages of Photosynthesis

Photosynthesis occurs in two main stages: the light-dependent reactions and the Calvin cycle (also known as the light-independent reactions).

1. Light-Dependent Reactions: In this stage, sunlight strikes the chlorophyll in the chloroplasts, exciting electrons to a higher energy state. These high-energy electrons travel through the electron transport chain, a series of reactions that ultimately generates ATP (adenosine triphosphate) and NADPH. These molecules are rich in energy and critical for the next stage of photosynthesis.

2. The Calvin Cycle: The Calvin Cycle does not require light and takes place in the stroma of the chloroplasts. Here, the energy from ATP and NADPH powers the conversion of carbon dioxide from the atmosphere into glucose, a simple sugar. Enzymes catalyze the reactions that convert inorganic carbon into organic compounds, essentially capturing carbon in a usable form.

Oxygen and Glucose

The primary outputs of photosynthesis are oxygen and glucose. Oxygen is produced as a byproduct of the light-dependent reactions when water molecules are split to release electrons. Plants release this oxygen into the atmosphere, providing the essential element that supports most terrestrial life. Glucose, on the other hand, serves as an energy source for plants and, indirectly, for animals that consume them.

What is Plant Respiration?

Plant respiration is a crucial biological process where plants convert glucose and oxygen into energy. This process occurs in every living cell of a plant, providing the energy necessary for growth, reproduction, and other vital functions.

The Mechanics of Respiration

Plant respiration takes place in the mitochondria, often referred to as the “powerhouse” of the cell. Here, glucose molecules break down through a series of enzymatic reactions.

The Stages of Plant Respiration

Plant respiration can be divided into three main stages: glycolysis, the Krebs cycle, and the electron transport chain.

1. Glycolysis: In glycolysis, glucose molecules split into two three-carbon compounds called pyruvate. This process occurs in the cytoplasm and produces small amounts of ATP and NADH, which are used in further stages of respiration.

2. The Krebs Cycle: The Krebs cycle takes place in the mitochondria. Here, pyruvate undergoes a series of reactions that produce carbon dioxide, ATP, NADH, and FADH2. These products are critical for the final stage of respiration.

3. Electron Transport Chain: In this final stage, NADH and FADH2 generated from the previous stages donate electrons to the electron transport chain in the inner mitochondrial membrane. The flow of electrons through this chain releases energy, which is used to form a large amount of ATP. Oxygen plays a key role here, acting as the final electron acceptor and combining with hydrogen to form water.

Carbon Dioxide, Water, and ATP

The main outputs of plant respiration are carbon dioxide, water, and ATP. Carbon dioxide is released as a waste product during the Krebs cycle and is expelled through the stomata during gas exchange. Water is formed in the electron transport chain, and ATP is produced throughout respiration, providing essential energy for cellular activities.

Differences Between Photosynthesis and Plant Respiration

Differences Between Photosynthesis and Plant Respiration
AspectPhotosynthesisPlant Respiration
Primary FunctionConverts light energy into chemical energy (glucose).Converts chemical energy (glucose) into usable energy (ATP).
Occurs InChloroplastsMitochondria
Input MaterialsCarbon dioxide (CO2), water (H2O), and sunlightGlucose (C6H12O6) and oxygen (O2)
Output ProductsOxygen (O2) and glucose (C6H12O6)Carbon dioxide (CO2), water (H2O), and ATP
Energy RequirementRequires energy (light energy from the sun).Releases energy (used to power cellular activities).
Time of OccurrencePrimarily during daylight when sunlight is available.Occurs continuously, peaking during night time.
Role in Energy CycleEnergy-storing process.Energy-releasing process.
Impact on AtmosphereReduces atmospheric CO2 and increases atmospheric O2.Increases atmospheric CO2 and consumes atmospheric O2.
Dependence on SunlightDirectly dependent on sunlight.Not dependent on sunlight.
Overall ContributionCritical for the synthesis of organic compounds; supports biomass accumulation.Essential for breaking down sugar to provide energy for cellular functions.
Thermodynamic ProcessEndothermic (absorbs energy).Exothermic (releases energy).
Chemical ProcessReduction (gains electrons).Oxidation (loses electrons).
EfficiencyLess efficient in immediate energy yield; stores energy for later use.More efficient in immediate energy release for cellular needs.
Environmental InfluenceInfluenced by external factors such as light intensity, CO2 concentration, and temperature.Less influenced by external light conditions but affected by energy needs and oxygen availability.
Global Ecological RoleDrives the carbon cycle, supports oxygenic atmosphere crucial for aerobic life.Part of the cellular respiration cycle, impacting global carbon and oxygen cycles.

Key Similarities Between Photosynthesis and Plant Respiration

Despite their distinct roles in plant biology, photosynthesis and plant respiration share several important similarities that underline their interconnectedness within the life processes of plants:

  • Involvement in Energy Transformation: Both processes are crucial for the energy management of plants. Photosynthesis captures energy from sunlight to produce glucose, while respiration breaks down glucose to release energy stored in its chemical bonds.
  • Involvement of Carbon Compounds: Each process utilizes and produces carbon-containing compounds. Photosynthesis takes in carbon dioxide to produce glucose, a carbohydrate, whereas respiration converts this glucose back into carbon dioxide.
  • Cyclic Nature: Both processes are part of a larger biological cycle. The products of photosynthesis (oxygen and glucose) are the reactants in respiration, and conversely, the products of respiration (carbon dioxide and water) are reactants in photosynthesis.
  • Occurrence in Cells: Both photosynthesis and respiration occur at the cellular level, involving complex organelles specialized for each process—chloroplasts for photosynthesis and mitochondria for respiration.
  • Importance to Life: Both processes are fundamental to sustaining life, not just for plants but also for other organisms. Photosynthesis provides the oxygen and organic compounds necessary for the survival of most life forms on Earth, while respiration is essential for energy production in plants and many other organisms.
  • Regulation by Enzymes: Both processes are enzyme-driven, relying on specific proteins to catalyze the various chemical reactions involved in transforming molecules and releasing energy.
  • Influence by Environmental Factors: Both photosynthesis and plant respiration are influenced by environmental conditions, such as temperature and water availability, which can affect their rates and efficiency.


What is the Main Difference Between Photosynthesis and Respiration?

Photosynthesis converts sunlight into glucose and oxygen, while respiration turns glucose into energy (ATP), releasing carbon dioxide.

What is the Difference Between ETC in Photosynthesis and Cellular Respiration?

The Electron Transport Chain (ETC) in photosynthesis generates energy by capturing light, while in respiration, it releases energy by breaking down glucose.

Which is Faster, Respiration or Photosynthesis?

Respiration is generally faster than photosynthesis as it occurs constantly at high rates to meet energy demands.

What Happens if Respiration Exceeds Photosynthesis?

If respiration exceeds photosynthesis, plants may deplete their glucose reserves, leading to reduced growth and vitality.

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