Final answer:
The plant cell wall, external to the plasma membrane, is primarily composed of cellulose and provides protection, support, and shape to the cell. It allows the movement of substances without impeding, and contributes to the robustness of plant cells. This feature, along with chloroplasts and central vacuoles, distinguishes plant cells from animal cells.
Step-by-step explanation:
When examining a plant cell diagram, one prominent structure that is observed exterior to the plasma membrane is the cell wall. This rigid covering is essential for the plant cell as it provides protection, structural support, and shape. Unlike prokaryotic cells whose cell walls contain peptidoglycan, plant cell walls are primarily composed of cellulose, a complex carbohydrate or polysaccharide made of glucose units. This cellulose in the cell walls is responsible for the crunch you feel when biting into raw vegetables, such as celery, due to you tearing through these rigid structures.
Not only plant cells, but also fungal and some protistan cells possess cell walls with varying compositions. However, cellulose is the predominant organic molecule in plant cell walls. The cell wall is also unique in that it does not prevent substances from entering or leaving the cell, but instead acts as a strong barrier offering strength and protection. Additional structures, such as pores in the cell wall, allow for the movement of water and nutrients in and out of the cell, aiding in maintaining the overall health of the plant.
The presence of the cell wall is a distinctive feature of plant cells, which also contain other structures unique to them such as chloroplasts for photosynthesis and a large central vacuole that aids in the expansion of the cell without producing more cytoplasm. Such features are absent in animal cells.