Final answer:
Cellulose is the carbohydrate that provides sturdiness to plant stems due to its structural properties. Composed of β-D-glucose units linked by β-1→4 glycosidic linkages, cellulose forms the plant's rigid cell walls, offering strength and stability to the plant's architecture.
Step-by-step explanation:
The type of carbohydrate that helps a plant stem to be sturdy is cellulose. Cellulose is a complex polysaccharide composed of long chains of β-D-glucose units linked together by β-1→4 glycosidic linkages. Unlike starch or sucrose, which are used primarily for energy storage, cellulose provides structural support. It forms the rigid cell walls that are responsible for the plant's strength and stability. Because of its unbranched and linear structure, cellulose chains can pack tightly together, creating a fibrous and strong material that is essential in maintaining the sturdiness of plant stems.
Polysaccharides like cellulose are vital for plants not only for their structural properties but also in water transport and defense against pests and pathogens. Starches, composed of amylose and amylopectin, serve as energy storage in plants but do not contribute as much to the mechanical strength of the plant's tissues. Thus, while sucrose plays a critical role as a transport carbohydrate and an energy source for growing plants, it is the cellulose that gives plant cells and tissues their rigidity, enabling plants to grow upright and resist external forces.