Final answer:
When sketching a bean nodule with Rhizobium under a microscope, label areas with darker, 'Y'-shaped bacteroids as Rhizobium-infected cells and lighter areas without these structures as uninfected cells, illustrating the symbiosis and structure changes that occur.
Step-by-step explanation:
When observing Rhizobium within legume root nodules under a microscope, you would sketch the nodule with an emphasis on cells with and without Rhizobium. Infected cells will contain bacteroids, which are modified bacterial cells adapted for nitrogen fixation. These bacteroids are typically darker, irregularly shaped or 'Y'-shaped, and encased in vesicles called symbiosomes.
In contrast, uninfected cells will not show these darkly-stained structures. You would label these regions respectively, and potentially note the presence of leghemoglobin within the cells, the protein responsible for maintaining low oxygen levels to facilitate nitrogen fixation. Additionally, the vascular connections supplying the nodules with carbohydrates can be indicated, emphasizing the mutualistic relationship between the plant and bacteria.
The typical arrangement of bacterial cells, which can be rod-shaped in nature with possible flagellae, changes once the bacteria enter the plant tissues. This is due to the unique environment that influences bacterial shapes and arrangement, leading to the aforementioned 'Y'-shaped bacteroids within the plant cells. During infection, the bacteria induce the formation of an infection thread, a structure that allows them to proliferate and enter the root cortex where they form nodules.