How would a herbivore's digestive tract differ from a human's? What are parts (organs, tissues, etc.) or enzymes that can't be found in one but is in the other? Also, is it correct to say that a herbivore's digestive tract would be more complex than a human's because herbivore diets tend to contain a lot of complex carbohydrates to digest?
It depends on the method of digestion the herbivore utilizes and what they eat, grasses, leaves, fruit, or nuts . Ruminants are only one form of herbivore. A wonderful introduction to the various methods can be foundd here. http://vetsci.co.uk/2010/05/14/comparative-digestion/#
For comparison gorilla which are far more herbivorous than human have a much longer small intestines which is one reason they need a much wider trunk. They also of course need larger teeth with thicker enamel and much larger jaw muscles to chew the tough plant fibers. Not so much more complex digestive systems just bigger/longer.
Herbivorous animals certainly tend to have very different digestive tracts to humans, or other carni-/omnivores. In general, mammals cannot produce enzymes that will digest cellulose, so if an animal is to use cellulose as an energy source there must be another way of extracting its energy. Principally, this is fermentation.
Fermentation is the process of bacterial breakdown of a sugar-like compound to other molecules, such as gases, acids or alcohol. Herbivores provide an internal environment that is hospitable to cellulose-fermenting bacteria, and provide them with cellulose (by eating plants!). They then use the breakdown products of fermentation as an energy source - often the relevant compounds are volatile fatty acids.
Different species have solved the problem of having an internal fermentation chamber in different ways. Cattle, as many people are aware, have a complex and expanded foregut, and regurgitate their stomach contents to allow further mechanical digestion. Horses have a vastly expanded hindgut (caecum and large intestine). Rabbits have an expanded hindgut, and also eat (certain types of) their droppings to allow further mechanical digestion.
One other difference that we can observe in herbivores is in the structure of their teeth. A carnivore such as a lion will have sharp, pointed teeth and a wide opening jaw to allow catching and killing prey, and tearing of flesh. Herbivores have broad, flat teeth with rough occlusal surfaces to allow efficient crushing of prehended food. They may also have adaptations for dental longevity - the horse, for example, has hypsodont or long rooted teeth, that erupt through its lifetime to combat the wear of extended chewing of rough material.
The digestive tracts of ruminants (cud chewers) such as cattle, goats, and sheep are specialized to maximize the benefits of their endosymbiotic microorganisms. In place of the usual mammalian stomach, ruminants have a large, four chambered organ.
The first two chambers, the rumen and the reticulum, are packed with anaerobic microorganisms that break down cellulose by fermentation.
The ruminant periodically regurgitates the contents of the rumen (the cud) into the mouth for rechewing. When the more thoroughly ground-up vegetable fibers are swallowed again, they present more surface area to the microorganisms for their digestive actions.
The microorganisms in the rumen and reticulum metabolize cellulose and other nutrients to simple fatty acids, which become nutrients for their host.
The food leaving the rumen carries with it enormous numbers of cellulose-fermenting microorganisms. This mixture passes through the omasum, where it is concentrated by water absorption.
It then enters the true stomach, the abomasum, which secretes hydrochloric acid and proteases. The microorganisms are killed by the acid, digested by the proteases, and passed on to the small intestine for further digestion and absorption.
The rate of multiplication of microorganisms in the rumen is great enough to offset their loss, so a well-balanced, mutually beneficial relationship is maintained.
It would be more correct to say that Herbivores have special adaptations for digesting cellulose rather than complex as humans also have a complex digesting system.
Hope that helps you.