There are many individual plant protein sources that are not complete proteins, and some of the ones that provide complete proteins provide a low ratio of protein to other macronutrients and may not, for that reason, be suitable alone as a primary protein source. But there are plants that individually supply complete protein without this problem, and more importantly there are well-known and combinations of plant protein sources that do this -- they are often the staple foods of various cultures, since in pre-modern times very most agriculture societies didn't produce meat in sufficient quantities for it to be the main protein source, so if adequate plant sources didn't exist, neither would civilization.

(That being said, the heavy focus on soy in the market means there aren't as many convenient options meat-avoiding, soy-allergic/intolerant people as for either meat eaters or soy-tolerant folks.)

http://en.wikipedia.org/wiki/Complete_protein

Many plants have "complete" amino acid profiles but one or more amino acids are too low to be completely adequate for humans. In this case, another protein source with a supplemental amino acid profile is required in order to balance out the deficiency in the first.

You don't need to eat meat to get your protein.

So protein quality and quantity can't likely be insured by eating any old vegetables. Its not as simple as 'don't eat meat'.

http://en.wikipedia.org/wiki/Amino_acid_synthesis

The great majority of energy in the biosphere is certainly captured by way of photosynthesis, but it is silly to complain about an oversimplification and then turn around and make one.

I guess you could twist that around and say that it is produced using photosynthesis that was captured in the natural gas that is used, but I don't think the poster meant readers to unpack all that out of 'created by plants'.

This isn't accurate. The grass that cows eat doesn't contain enough amino acids to support the cow's growth.

It's actually the bacteria the cow's gut that digests the grass (mostly carbohydrate) and converts some of it into amino acids that the cow absorbs.

That's not all amino acids, since the cow's body can create some of it's own (just like humans).