There are several ways that the photosynthesis reaction is often written. A common form shows carbon dioxide and water combining, with the help of energy from the sun, to produce sugar and oxygen.

Often the reaction will be simplified to remove the coefficients of the molecules, or the reaction will be written in words instead of chemical symbols. NADP is converted to NADPH during photosynthesis, but may or may not be included in the reaction.

The incorrect reaction in the answer choices reverses the carbon dioxide and oxygen. Remember that the reaction must contain carbon on both sides of the equation in order to be balanced; carbon dioxide cannot be on the same side as glucose. The overall purpose of photosynthesis is to convert carbon, in the form of carbon dioxide from the air, into usable sugar. In terms of a reaction equation, this means that carbon dioxide is a reactant on the left side and sugar is a product on the right side.

The Calvin cycle is the second set of biochemical reactions in photosynthesis and follows the light reactions. The light reactions function to use photons and water to generate NADPH, oxygen, and water. The NADPH can then be used to power the Calvin cycle, which uses the energy stored in ATP and NADPH during light reactions to generate organic molecules for energy. The light reactions take place in the chloroplasts, but the Calvin cycle takes place in the stroma and is not dependent on light. The final result of the Calvin cycle is to use energy to bind reactant carbon dioxide to produce glyceraldehyde 3-phosphate (G3P), a three-carbon sugar. G3P is then used to build sucrose, starch, and cellulose for energy storage and metabolism.

Rubisco is an enzyme that helps add carbon dioxide to RuBP molecules. This in turn forms an unstable inermediate compound, which immediately breaks into two 3-PGA molecules. 

The key characteristic of the C₄ pathway is that is produces oxaloacetate and four-carbon sugars from carbon dioxide, compared to the Calvin cycle of most plants, which generates glyceraldehyde 3-phosphate and three-carbon sugars.

The C₄ pathway fixes carbon dioxide into four-carbon compounds, thus the name. Pores on the plant called stomata regulate how much carbon dioxide, oxygen, and water enter and leave the plant and are usually partially closed during the hottest part of the day to conserve water. This yields a low carbon dioxide level and high oxygen level, which inhibits carbon fixation by the Calvin cycle. Plants that use the C₄ pathway have an enzyme that can efficiently fix carbon to four-carbon compounds when the carbon dioxide level is low. The four-carbon compounds are then transported to other cells, where carbon dioxide is released and can enter the Calvin cycle.

Plants that use the C₄ pathway are better adapted to hot and dry conditions, as they can fix carbon with less loss of water. Examples of C₄ plants are corn and crabgrass.

Sugar (glucose) and oxygen are the two products of photosynthesis. Methane is a gas consisting of carbon and hydrogen. ATP is the energy produced in the organelle mitochondria. Ethyl alcohol is the active substance in beer and wine that causes intoxication if too much is consumed.

An anabolic pathway is a pathway in which smaller molecules are combined to form larger ones. This type of pathway usually requires energy to complete the combinations required. Photosynthesis is an example of this, because it combines carbon dioxide molecules to form glucose. The rest of the listed processes are catabolic pathways.