Compare and contrast between these two plant polysaccharides : starch and cellulose.

To compare and contrast between starch and cellulose, we need to understand the nature and characteristics of both plant polysaccharides.

Starch:
1. Structure: Starch is composed of glucose units joined together in a linear or branched arrangement. It consists of two main components: amylose (linear chains) and amylopectin (branched chains).
2. Function: Starch serves as a storage form of glucose in plants. It acts as an energy reserve, providing energy during plant growth and development.
3. Digestibility: Starch is easily broken down by enzymes in the digestive tract of humans and many other animals, making it a significant source of dietary carbohydrates.
4. Solubility: Starch is relatively insoluble in water, forming semisolid or gel-like structures when hydrated.
5. Appearance: Starch typically appears as a white or off-white, powdery substance.

Cellulose:
1. Structure: Like starch, cellulose is also composed of glucose units. However, the glucose units in cellulose are linked together in a linear arrangement, forming long, straight chains.
2. Function: Cellulose provides structural support to plant cell walls, making them rigid and sturdy. It is an essential component of plant cell walls and helps plants maintain their shape and resist mechanical stress.
3. Digestibility: Unlike starch, cellulose is indigestible by most animals, including humans, due to the lack of enzymes required to break its unique linkage. However, some animals, such as cows and termites, possess specialized gut microorganisms that can break down cellulose and extract energy from it.
4. Solubility: Cellulose is insoluble in water, and it forms fibers that are strong and resistant to breakdown.
5. Appearance: Cellulose is usually found in plant cell walls and appears fibrous and tough.

In summary, starch and cellulose are both plant polysaccharides made of glucose units. However, starch serves as an energy storage molecule, is easily digestible, and is soluble in water, while cellulose provides structural support, is indigestible for many animals, and is insoluble in water.