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K_Michael

For those of you who don't know, cellulose is a complex sugar made by plant cells. It occurs in high amounts in celery and grass. Basically, if we could digest this, it would open up a new food source to humans. Unfortunately, we don't possess the enzyme to break down cellulose. 
This shouldn't stop us however. We've already genetically engineered bacteria to produce insulin for us. Why not genetically engineer humans? (obviously, with a variety of tests to ensure its safety.)

whiteflame

It's not so much that I have a problem with genetically engineering humans. Rather, I feel like if we're going to select something to engineer humans to do, digesting cellulose should be pretty low on that list. Getting enough cells in a human being to produce cellulose-degrading enzymes seems far more difficult than simply engineering certain probiotic organisms with the same capacity. Organisms like this already exist in the guts of ruminants, and we're well aware of those enzymes that impart this capability, so it seems like something that is entirely manageable, given enough time and funding. If we're going to engineer humans, let's address genetic defects and incurable diseases first, since the process to engineer any animal still isn't widely tested in humans. There are a litany of other causes to address first, and ones with a greater acceptable risk.

K_Michael

Gut bacteria would be another excellent solution. (Can't believe I didn't think of that!)

Oppolzer

It would be theoretically feasible to take a cellulase enzyme and acquire energy from grass, for example. Albeit, it would depend on how it would be digested.

If we were to digest cellulose the same way ruminant animals do, which contain symbiotic bacteria, then humans would need specific physical adaptations. Such as the required increase in body temperature, because the symbiotic bacteria generate a significant amount of heat. Our bodies would need to adapt to a higher internal temperature, which theoretically wouldn’t be possible. The human body is meant to maintain life-sustaining regulated body temperature. Other physical adaptations would be the need for a larger stomach and teeth durable enough to grind it down.

In all things, we wouldn’t get the nutritional value from digesting cellulose. Our digestive systems don’t enable the appropriate amount of mechanical digestion or time required to acquire a substantial amount of energy from cellulose. Cows, for example, possess digestive systems that allow them to process cellulose for energy.

K_Michael

huh. you know more than i do.