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Life Science: Session 7

Photosynthesis

What is photosynthesis?

Photosynthesis refers to the process by which light energy is harnessed and transformed into chemical energy in the form of the food sugar. More specifically, carbon dioxide and water are combined and their atoms are rearranged to form a sugar molecule. The bonds within the sugar molecule store the energy obtained from light. Oxygen from water is released as a by-product in this process.

The following is the chemical reaction for photosynthesis:

light energy + CO2 + H2O -> CH2O + O2

light energy + carbon dioxide + water -> sugar + oxygen

ellison's experiment
Dr. Ellison's photosynthesis
experiment

Dr. Aaron Ellison demonstrates photosynthesis in the video. A leaf from a pitcher plant is placed in a closed chamber that measures the amount of carbon dioxide in the air. As photosynthesis occurs, carbon dioxide is removed from the air and its level falls. As this happens, sugar is formed and oxygen is released.

Where does photosynthesis occur?

Photosynthesis is a process that is carried out within the cells of plants as well as certain protists, bacteria, and archaea. In the eukaryotes (e.g., the plants and protists), photosynthesis is carried out in organelles called chloroplasts. Chloroplasts contain chlorophyll, a lipid that gives plants their green color. Chlorophyll is what actually captures light energy. The photons in light (i.e., the light particles) “jostle” the electrons in the chlorophyll molecule, which starts the process of energy transformation.

Interestingly, the chloroplast is actually derived from a group of bacteria known as cyanobacteria. This is one of the examples of symbiosis mentioned by our host, Dr. Doug Zook. In this case, the cyanobacteria were taken up by eukaryotic host cells billions of years ago and the ability to photosynthesize was eventually passed onto upon the host cells. In addition to being important as ancient symbionts, cyanobacteria in the ocean are responsible for most of the photosynthesis occurring on Earth today.

How does photosynthesis reflect the properties of energy?

In photosynthesis, light energy is changed into chemical energy. The first law of thermodynamics is “obeyed” in that the amount of light that is the energy input equals the amount of chemical energy that is the output. However, the second law of thermodynamics seems to be defied. Energy emerges from photosynthesis in a more organized form as chemical energy, which is a higher-quality energy than light. This is an example of “life battling against disorder” and is possible only by the addition of energy.

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