Cellulose is an important structural polysaccharide. It occurs as a major component of the cell wall in which of the following organisms?

Introduction / Context:
Cellulose is a long chain polysaccharide composed of glucose units linked by beta 1,4 glycosidic bonds. It is best known as the main structural component of plant cell walls, but it also appears in the cell walls of some other groups of organisms. This question focuses on identifying which of the listed organisms has cell walls with cellulose as a major component, contrasting it with bacteria and fungi that use different cell wall materials.

Given Data / Assumptions:

- The question asks about a major component of the cell wall. - The options include Pythium, Pseudomonas, Xanthomonas, and Saccharomyces. - We assume standard, textbook level descriptions of cell wall composition in these groups. - The focus is on which organism has cellulose based walls.

Concept / Approach:
Different groups of organisms build their cell walls from different structural polymers. Most true fungi have cell walls made mainly of chitin and glucans. Bacteria use peptidoglycan, also known as murein, a polymer made of sugars and amino acids. Plants and some protist like organisms use cellulose as the main structural framework. Pythium belongs to the oomycetes, which are fungus like organisms but differ from true fungi in several ways, including having cell walls rich in cellulose rather than chitin. Therefore, among the options given, Pythium is associated with cellulose containing walls.

Step-by-Step Solution:
1. Recognise that Pythium is an oomycete, often called a water mould, which is not classified as a true fungus. 2. Oomycetes are known to have cell walls composed mainly of cellulose and beta glucans, similar to plants in that regard. 3. Pseudomonas is a genus of gram negative bacteria. Bacterial cell walls are composed primarily of peptidoglycan, not cellulose. 4. Xanthomonas is also a genus of gram negative bacteria with cell walls built from peptidoglycan and an outer membrane, again with no major cellulose component. 5. Saccharomyces is a genus of yeast, a true fungus. Fungal cell walls are mainly made of chitin and glucans rather than cellulose. 6. Comparing these structures, only Pythium, as an oomycete, has cell walls in which cellulose is a major structural component.

Verification / Alternative check:
Botany and microbiology texts usually highlight oomycetes as being different from true fungi because of two features: they have diploid dominant life cycles and cell walls rich in cellulose rather than chitin. Bacteria are always described as having peptidoglycan walls, and yeasts like Saccharomyces are frequently used as classic examples of chitin containing cell walls. These contrasts confirm that Pythium is the correct choice for cellulose rich walls among the listed organisms.

Why Other Options Are Wrong:
- Pseudomonas, a bacterium with a peptidoglycan cell wall: Bacterial cell walls are based on peptidoglycan and lack cellulose as a major component. - Xanthomonas, a bacterium with a peptidoglycan cell wall: Like other bacteria, Xanthomonas uses peptidoglycan and does not build its wall from cellulose. - Saccharomyces, a yeast with mainly chitin and glucan in the cell wall: As a true fungus, Saccharomyces uses chitin, not cellulose, for cell wall rigidity.

Common Pitfalls:
Many learners automatically associate cellulose only with higher plants and forget that some plant like or fungus like organisms also use it. Another common error is to assume all microbes have similar cell walls, which is not correct. Distinguishing between bacteria, true fungi, and oomycetes based on their wall composition is a frequent exam theme. Remember that peptidoglycan is bacterial, chitin is fungal, and cellulose is associated with plants and oomycetes like Pythium.

Final Answer:
Cellulose is a major component of the cell wall of Pythium, an oomycete.