The most abundant organic compound in the terrestrial environment is cellulose. Cellulose is a complex carbohydrate found in plant cell walls and provides structural support to plants. It is made up of long chains of glucose molecules linked together. Unlike other organic compounds like lactose, fructose, and sucrose, which are found in certain foods and have specific functions, cellulose is widely distributed in plant matter and is a major component of plant biomass. Lactose is a sugar found in milk, fructose is a sugar found in fruits, and sucrose is a commonly used table sugar. Therefore, while these compounds may be abundant in certain sources, they are not as prevalent in the overall terrestrial environment as cellulose.