Respiration & Photosynthesis in Plants

Plants acquire energy through photosynthesis and expend it through respiration.

Photosynthesis and respiration are essential processes in plants. In the first, energy is assimilated from light sources. In the second, that energy is expended for various metabolic needs within the plant.

Photosynthesis

Photosynthesis is the process plants use to convert light energy into carbohydrates. The carbohydrates produced are used as fuel for growth, maintenance and reproduction. The chemical reaction of photosynthesis is as follows:

Light Energy + 6CO2 + 6H2O ' C6H12O6 + 6O2

Organisms use the oxygen produced during this reaction for respiration. The carbohydrate produced is called glucose. It is a type of sugar that is broken down during respiration to yield energy.

Chloroplasts

Photosynthesis occurs in chloroplasts in the cells of leaves and green stems. The chloroplasts contain fluid called stroma that is rich in protein. Within the stroma, membranes called thylakoids contain many of the photosynthetic reactions.

Photosystems

Clusters of pigments in the chloroplasts, called photosystems, absorb light. The majority of pigments are chlorophyll, which absorb high-energy red light waves and reflect low-energy green light waves. Energy is removed from the light in the form of electrons as it passes through a series of enzymes. This energy is used during the Calvin cycle, named for Melvin Calvin who researched the reactions. During this cycle, carbon from carbon dioxide absorbed through the plant leaves and electrons removed in the photosystems are used to form glucose in the stroma.

Respiration

All organisms use respiration to break down energy-containing molecules. Plants are no exception. During respiration, oxygen from the atmosphere and oxygen released as a waste product during photosynthesis is used to help break down the energy-rich glucose molecules also produced during photosynthesis. The energy released during the breakdown of carbohydrates is stored in specialized molecules for later use. This reaction is the opposite of the photosynthesis reaction and is represented by the following formula: 6O2 + C6H12O6 ' 6CO + 6H2O + Energy.

Glycolosis

Glycolosis is the first step of the respiration process. In this step, glucose is broken down into acid molecules. A small amount of energy is produced during glycolosis and is contained in energy-storing molecules. This step occurs in the cytoplasm, which is the fluid-filled space outside the organelles in a plant cell. The acid produced is transferred to a structure called the mitochondria.

Krebs Cycle

The Krebs cycle, also known as the citric acid cycle, occurs inside the mitochondria. The acid molecules produced during glycolosis is used to produce energy molecules and carbon dioxide. Some of the energy molecules contain hydrogen atoms.

Electron Transport Chain

In the electron transport chain, energy-containing molecules produced in the Krebs cycle are transported through a series of enzymes where their energy is absorbed. The hydrogen atoms on some of the molecules are removed and bond with oxygen left over from photosynthesis to form water. These enzymes are located in the cristae of the mitochondria, which is a series of folded membranes similar to the thylakoids of the chloroplast.

References

• "The Economy of Nature, 5th ed."; Robert E. Ricklefs; 2007
• Cliff Notes: Photosynthesis
• Cliff Notes: Respiration