Final answer:
Humans are unable to digest cellulose, the major component of plant cell walls, because they lack the necessary enzymes. However, humans can still derive nutrients from fruits and vegetables through the digestion of other plant components and the fermentation of indigestible parts by gut bacteria.
Step-by-step explanation:
How Humans Digest Plant Materials
The structural components that make plants difficult to digest are predominantly related to the presence of cellulose in plant cell walls. Cellulose is a carbohydrate composed of glucose units bonded together in a way that human digestive enzymes cannot break down. Consequently, it traverses the human digestive tract and is classified as dietary fiber. Herbivorous animals, such as cows and horses, and pseudo-ruminants like camels, have symbiotic bacteria in their digestive systems that produce the enzyme cellulase, which breaks down cellulose. In humans, while the cellulose itself is not digested, the human digestive system still derives benefit from it in the form of fiber, and some digestion of cellulose occurs thanks to bacteria in our large intestine.
In stark contrast to plants with lignified secondary cell walls, herbaceous plants with just primary cell walls are softer and easier to digest after cellular membranes are compromised, such as during cooking. For instance, cooking spinach makes it easier to digest by breaking down the cells' structural integrity.
Different diets demand distinct digestive mechanisms. Humans obtain nutrients from fruits and vegetables through a variety of ways, including the breakdown of other plant components like starches that are more easily digested than cellulose, the fermentation of some indigestible parts by gut bacteria, and the absorption of minerals and vitamins that are not bound within the cellulose matrix.