What is the relationship between photosynthesis and respiration?

Both cellular respiration and photosynthesis are parts of a mutually beneficial relationship. Photosynthesis cannot occur without cellular respiration and cellular respiration certainly cannot occur without photosynthesis.

Photosynthesis and cellular respiration are in many respects the “reverse” of one another. Photosynthesis is an anabolic process, whereas cellular respiration is a catabolic process. Let us explore more differences between cellular respiration and photosynthesis

The rate of dark CO2 efflux from mature wheat (Triticum aestivum cv Gabo) leaves at the end of the night is less than that found after a period of photosynthesis. After photosynthesis, the dark CO2 efflux shows complex dependence on time and temperature. For about 30 minutes after darkening, CO2 efflux includes a large component which can be abolished by transferring illuminated leaves to 3% O2 and 330 microbar CO2 before darkening. After 30 minutes of darkness, a relatively steady rate of CO2 efflux was obtained. The temperature dependence of steady-state dark CO2 efflux at the end of the night differs from that after a period of photosynthesis. The higher rate of dark CO2 efflux following photosynthesis is correlated with accumulated net CO2 assimilation and with an increase in several carbohydrate fractions in the leaf. It is also correlated with an increase in the CO2 compensation point in 21% O2, and an increase in the light compensation point. The interactions between CO2 efflux from carbohydrate oxidation and photorespiration are discussed. It is concluded that the rate of CO2 efflux by respiration is comparable in darkened and illuminated wheat leaves.

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The relationship between photosynthesis and cellular respiration is such that the products of one system are the reactants of the other. Photosynthesis involves the use of energy from sunlight, water and carbon dioxide to produce glucose and oxygen. Cellular respiration uses glucose and oxygen to produce carbon dioxide and water. To emphasize this point even more, the equation for photosynthesis is the opposite of cellular respiration.

Humans, animals and plants depend on the cycle of cellular respiration and photosynthesis for survival. The oxygen produced by plants during photosynthesis is what humans and animals inhale for the blood to transport to the cells for respiration. The carbon dioxide produced during respiration is released from the body and absorbed by plants to help provide the energy they need for growth and development. This is the never ending cycle that sustains life on earth.

The process of photosynthesis is used by plants and other photosynthetic organisms to produce energy, whereas the process of cellular respiration breaks down the energy for use. Despite the differences between these two processes, there are some similarities. For example, both processes synthesize and use ATP, the energy currency.

Below are some of the similarities and differences between these two systems.

Similarities between Photosynthesis and Cellular Respiration

Comparative Features	Photosynthesis	Cellular Respiration
Reduction-Oxidation Reaction
Electron Transport Chain (ETC)
Synthesize ATP
Utilize ATP
Phosphorylation
Electron Carriers

Differences between Photosynthesis and Cellular Respiration

Comparative Features	Photosynthesis	Cellular Respiration
Occurs in…	Plants, Algae and Photosynthetic Bacteria	All living organisms
Function/Purpose	Capture, convert and store energy – light energy from the sun is converted into chemical energy and stored in the bonds of glucose	Release energy – chemical energy stored in glucose is released to produce ATP for the cell
Reactants/Inputs	Carbon dioxide, Water and Light energy	Glucose and Oxygen
Metabolic Process	Anabolic - CO2 and energy from ATP and NADPH are used to build glucose molecules	Catabolic - glucose is broken down to produce CO2 and energy in the form of ATP, NADH and FADH2
Location	Chloroplast of the plant cell	Glycolysis occurs in the Cytoplasm while the Mitochondria is the site of the Kreb’s Cycle and Electron Transport Chain (ETC)
Source of Energy	Sunlight	Glucose (sunlight is not required)
Electron Carriers	NADPH	NADH and FADH2
Stages	Light dependent Reactions – Light energy and water are used to produce ATP and NADPH Calvin Cycle – Carbon dioxide and the energy from ATP and NADPH are used to produce glucose	Glycolysis – Glucose is combined with ATP to produce pyruvate molecules, NADH and more ATP Pyruvate Oxidation - Pyruvate molecules are broken down into Acetyl-CoA (2-carbon compound) and CO2 is produced Kreb’s Cycle – Acetyl-CoA is combined with 4-carbon molecules to form the 6-carbon citrate molecules used to generate NADH, CO2, FADH2 and ATP Electron Transport Chain – Energy stored in NADH and FADH2 during the Krebs cycle is used to generate ATP, Water, NAD+ and FAD
Products/Outputs	Glucose and Oxygen	Carbon dioxide and Water
Equation	6CO2 + 6H2O –––> C6H12O6 + 6O2	C6H12O6 + 6O2 –––> 6CO2 + 6H2O

The cycle of photosynthesis and respiration maintains the balance of oxygen and carbon dioxide. Photosynthesis produces the oxygen to replenish oxygen that is used up by living organisms during respiration. Carbon dioxide produced during respiration is one of the reactants plants need to perform photosynthesis.

Photosynthesis and cellular respiration are both part of a mutually beneficial relationship. Cellular respiration cannot occur without photosynthesis, and photosynthesis certainly cannot occur without the help of its partner.