Question

List the three variations of carbon-fixation reactions (C3, C4, and CAM) and write their basic characteristics (locations or times of C4 and C3 pathway in each) with one or two examples for each.

Answer 1

C3 plants: The C3 pathway gets its name because the first molecule produced in the cycle is a 3-carbon molecule that is called 3-phosphoglyceric acid. Around 85% of the plants on Earth use C3 pathway to fix carbon via the Calvin Cycle. During the one-step process, the enzyme RuBisCO is responsible to cause an oxidation reaction in which some of the energy used in photosynthesis is lost in a process called as photorespiration. These plants can survive in all environments. The C3 plant species are spinach, peanuts, cotton, wheat and rice.

C4 plants: The C4 process is also called as the Hatch-Slack pathway and is named for the 4-carbon intermediate molecules are produced. These molecules are malic acid or aspartic acid. C4 has one step in the pathway before the Calvin Cycle which is responsible to reduce the amount of carbon that is lost in the overall process. In C4, the RuBisCO enzymes are less likely to catalyze oxidation reactions and waste carbon molecules because the oxygen content inside bundle sheath cells is very low. The typical Environments for C4 plants are tropical, elevated daytime temperatures and drought. C4 plants include corn, sugar cane, millet, sorghum and pineapple.

CAM plants: Plants that use crassulacean acid metabolism are called as CAM plants. They are succulents, and they are efficient at storing water due to the dry and arid climates they live in. CAM plants such as cacti have leaves that are thick and full of moisture and they have a waxy coating to reduce evaporation. In these plants, stoma are closed during the day to prevent water loss, and they are opened at night to take in carbon dioxide. The CO₂ is converted to malate which is stored until the daylight returns and photosynthesis begins through the Calvin Cycle. CAM plants include cacti, sedum, jade, orchids and agave.