BIO101: Introduction to Molecular and Cellular Biology (2022.A.01)

Photosynthesis is how green plants and other photosynthetic organisms use the power of sunlight to synthesize their own food from carbon dioxide and water. Photosynthesis harnesses sunlight by using green pigment chlorophyll and generates oxygen as a byproduct. The food and oxygen created by these autotrophs (organisms that make their own food) indirectly nourish and energize the whole earth.

The products of photosynthesis are sugars and oxygen. This anabolic process requires the reactants carbon dioxide and water. When there are significant amounts of these reactants, photosynthesis can nourish the plant and indirectly support life on earth. However, in environments where water and carbon dioxide are limited, there is a risk of photorespiration.

A crucial step in the Calvin cycle is the fixation step, which takes in carbon dioxide and joins it with an intermediate compound (ribulose bisphosphate), thus incorporating inorganic carbon dioxide into an organic compound. The enzyme that catalyzes this step is called RuBisCO (ribulose bisphosphate carboxylase oxygenase). RuBisCO can operate to join either carbon dioxide or oxygen to ribulose bisphosphate. However, joining oxygen instead of carbon dioxide is counterproductive because no carbon fixation (the purpose of the Calvin cycle) takes place. This counterproductive process (incorporating oxygen instead of carbon dioxide) is called photorespiration.

Two major categories of plant species have evolved ways around this problem:

1. C-4 plants separate the process of carbon dioxide intake (which occurs in superficial cells called mesophyll cells) from the process of carbon fixation in the Calvin cycle (which occurs in deeper cells called bundle sheath cells).
2. CAM plants take in carbon dioxide and store it in the form of organic acids during the night when their stomata are open. During the day, the organic acids get broken down to release the carbon dioxide for the Calvin cycle, while the stomata are closed (preventing oxygen from interfering).

These two types of plants more efficiently operate the Calvin cycle because photorespiration is minimized. As you review C-4 plants and CAM plants, notice that they accomplish the same thing in two different ways. After you have read this section, you should be able to define the purpose of the stomata, describe the two photosynthetic adaptations that minimize photorespiration, and list the types of plants that have these adaptations.

The phrase carbon cycle refers to the many chemical transformations that occur involving compounds containing carbon. The carbon cycle is cyclic because there is a continuous alternation between the carbon of organic compounds and the carbon of inorganic compounds. Inorganic carbon dioxide gets fixed (by autotrophs) into organic compounds. These organic compounds get converted into other organic compounds (including simple organic compounds like monosaccharides, nucleotides, and amino acids, as well as complex macromolecules like polysaccharides, nucleic acids, lipids, and polypeptides). 

The carbon in these organic compounds gets passed from organism to organism as they feed on each other. Some of the organic molecules get used as fuel by the organisms, and the oxidation of these organic fuels (to provide energy for the organisms) returns the carbon to inorganic form (carbon dioxide) to complete the cycle. In this cycle of transformations, carbon (matter) remains in the ecosystem (it is conserved). It is not destroyed; it is only transferred and transformed.