Question

By looking at a plant externally can you tell whether a plant is ${C_3}$ or ${C_4}$. Why and how?

Answer 1

The light-dependent reactions are different in ${C_3}$ and ${C_4}$plants because of the different cellular structure of leaves. In ${C_4}$ plants, the calvin cycle occurs in bundle sheath cells, whereas the light-dependent reactions take place in mesophyll cells.

Complete answer: No, it is not possible to distinguish ${C_3}$ and ${C_4}$ plants by their external morphological features of leaves. But at the cellular level, ${C_4}$ plants show a different type of anatomy known as kranz anatomy. In kranz anatomy, the mesophyll cells are clustered around the bundle-sheath cells in a ring-like manner. The bundle-sheath cells have a greater number of chloroplast than the bundle sheath cells. Because of the kranz anatomy, both the plants differ in their primary acceptor of carbon dioxide and the presence of the RuBisCO enzyme.
-In the ${C_3}$ cycle, carbon dioxide reduces to form a first three carbon stable compound, 3-phosphoglyceric acid (3-PGA) The initial carbon dioxide acceptor is a five-carbon molecule which is RUBP or ribulose-1,5-bisphosphate and RuBP carboxylase catalyze this reaction. The plants in which carbon dioxide is fixed in this way are known as ${C_3}$ plants. One molecule of glucose is synthesized by the fixation of six molecules of carbon dioxide in six turns.
-In the ${C_4}$ cycle, phosphoenolpyruvate (PEP) which is a three-carbon molecule acts as a primary acceptor of carbon dioxide and is present in the mesophyll cells. The carbon dioxide fixation is catalyzed by PEP carboxylase or PEPcase. As a result of fixation, oxaloacetic acid (OAA) is formed which then converts into other four-carbon compounds such as malic acid or aspartic acid in the mesophyll cells. In bundle sheath cells, ${C_4}$ acids are broken down and carbon dioxide is released. Then, the PEP is transported to the mesophyll cells therefore completing the cycle.

Note: Kranz anatomy is commonly observed in maize and few dicots. The Kranz anatomy develops in three stages which are as follows- procambium initiation, specification of bundle sheath and mesophyll cells, and chloroplast development and integration of the ${C_4}$ cycle.