Plant protien and animal protien both get absorbed by the body.  
As to which is better for building muscle, it is generally believed animal protien has a slight edge.

It is a matter of amino acid stereochemistry, (what a great topic!) I believe.

BRIEFLY:

There are twenty naturally occuring AA's, and nearly all (Glycine isn't chiral) exist naturally in the "L" form, that is, they rotate polarized light to the left.  "D" AA's appear in bacteria and in cell walls, and rotate polarized light to the left.  (You can investigate isomers if you are interested)
 
Under conditions of high temperatures and alkali conditions, the AA's can undergo racemization from the L form to the D form.  An analysis of the stereochemical designation of compounds to determine the degree of racemization is one way to evaluate food processing and bacterial contamination.

Your friend may have been referring to the fact that only L AA's can be incorporated into human tissues due to the limitations of our enzymes.  D's can, though, be used for energy.  

Stereochemistry is also important in a discussion of carbohydrate metabolism.  Again, human enzymes recognize only L's, and so although we can taste the "sweetness" of D-sugars, they are metabolized in the same way.  

I could be wrong, but this is my memory.

 [b]Dietary proteins are, with very few exceptions, not absorbed. Rather, they must be digested into amino acids or di- and tripeptides first.[/b] Absorption of intact proteins occurs only in a few circumstances. In the first place, very few proteins get through the gauntlet of soluble and membrane-bound proteases intact. Second, "normal" enterocytes do not have transporters to carry proteins across the plasma membrane and they certainly cannot permeate tight junctions.


Whey protein, made from milk, is the highest quality protein for muscle building. It has a fast absorption rate and for that reason, it is a good idea to also use some other form of protein that is absorbed slower.  Soy, is easy to digest, even for infants and will keep you in a positive nitrogen balance over a longer period.

Soy will also:
Lowers blood cholesterol and triglyceride levels
Delays osteoporosis/ bone fractures
Helps reduce menopausal symptoms
Reduces risk of breast cancer
Reduces risk of prostate cancer


When protein is consumed, the body breaks it   down into amino acids, the building blocks of all proteins. Some of the amino acids are designated nonessential. This does not mean that they are unnecessary, but rather that they do not have to come from the diet because they can be synthesized by the body from other amino acids. Other amino acids are considered essential, meaning that the body cannot synthesize them, and therefore must obtain them from the diet.

Whenever the body makes a protein - when it builds muscle, for instance - it needs a variety of amino acids for the protein-making process. These amino acids may come from dietary protein or from the body's own pool of amino acids. If a shortage of amino acids becomes chronic, which can occur if the diet is deficient in essential amino acids, the building of protein in the body stops, and the body suffers.

Because of the importance of consuming proteins that provide all of the necessary amino acids, dietary proteins are considered to belong to two different groups, depending on the amino acids they provide. Complete proteins, which constitute the first group, contain ample amounts of all of the essential amino acids. These proteins are found in meat, fish, poultry, cheese, eggs, and milk.

Incomplete proteins, which constitute the second group, contain only some of the essential amino acids. These proteins are found in a variety of foods, including grains, legumes, and leafy green vegetables.

Although it is important to consume the full range of amino acids, both essential and nonessential, it is not necessary to get them from meat, fish, poultry, and other complete-protein foods. Mutual supplementation enables you to combine partial-protein foods to make complementary protein-proteins that supply adequate amounts of all the essential amino acids. For instance, although beans and brown rice are both quite rich in protein, each lacks one or more of the necessary amino acids. However, when you combine beans and brown rice with each other, or when you combine either one with any of a number of protein-rich foods, you form a complete protein that is a high-quality substitute for meat.

To make a complete protein, combine beans with any of the following: Brown rice Nuts Seeds Wheat. Or combine brown rice with any one of the following: Beans Nuts Seeds Wheat.

Be aware that a combination of any grains, any nuts and seeds, any legumes (such as beans, peanuts, and peas), and a variety of mixed vegetables will make a complete protein. All soybean products, such as tofu and soymilk, are complete proteins. They contain the essential amino acids plus several other nutrients.

Yogurt is an animal-derived complete-protein source recommended for frequent use in the diet. Made from milk that is curdled by bacteria, yogurt contains Lactobacillus acidophilus and other "friendly" bacteria needed for the digestion of foods and the prevention of many disorders, including candidiasis. Yogurt also contains vitamins A and D, and many of the B-complex vitamins. Do not buy the sweetened, flavored yogurts that are sold in supermarkets. These products contain added sugar and, often, preservatives. Instead, either purchase fresh unsweetened yogurt from a health food store or make the yogurt yourself, and sweeten it with fruit juices and other wholesome ingredients.


 Mmmmmm good-apple snails-->http://www.applesnail.net/content/various/eating_snails.htm

These are terms of stereochemistry, the study of the three dimensional structure of molecules.

Chiral Molecules are not superimposable on their mirror image, having the property of chirality, or “handedness”.  The token example is a comparison of the right and left hands.  They are similar; and when facing, palm-to-palm, are mirror images of one another.  If one places the right hand’s palm on top of the back of the left hand, though, we see that the fingers don’t align.  The hands are mirror images, but cannot be superimposed.  Your hands (and feet) are chiral.
 
I’m not sure what your background in O. Chemistry is, but the Carbon based science depends upon Carbon’s ability to make four bonds (due to hybridization, and you can investigate hybridization yourself if you want).  To explain for this thread, if Carbon (C) is bonded to four different molecules, or four different groups of molecules, then it is said to be a chiral, or a chiral center (stereocenter).  C bound to four different groups means no internal plane of symmetry, and so chirality.

The L and D forms of a protein refer to the property of optical activity.  If a substance is capable of rotating the plane of polarized light, then it is said to be optically active.  There are two directions of rotation, and one method of nomenclature uses the letters L and D to refer to the direction of rotation.  L and D forms are non superimposable mirror images of one another, they are enantiomers. This is an outdated system that has been largely replaced by the R and S method of notation.  The composition of the stereoisomers are the same, the physical properties (freezing, boiling, etc.) are the same, BUT, they are not recognized by our enzymes the same way.  Enzymes require a specific “shape” and composition specific sites to do their work, and our enzymes recognize only the L forms of AA’s.  Maybe this is what your colleague was referring to, maybe not.

Molecules can, with time, and heat, and under other conditions, change, at random, from one “hand” to the other.  This process is known as racemization.  If the trend is a shift from the predominant L to the D form, eventually the concentrations of the two will equalize, and reach a 1:1 ratio.  This may be one method, like C dating, to date organic materials.

Glycine is an amino acid, and, like TxDoc indicated, a building block of a protein.  It is unusual because it contains an “extra” Hydrogen (H) group.  All AA’s are bound, at the central (alpha) C, to one H, to one amine group, and to one carboxylic acid group.  The difference in the 20 AA’s come from different “side chains” attached to the C.  Glycine’s side chain is a Hydrogen, so because there are two hydrogens, the AA doesn’t have bonds to four different groups, and so isn’t chiral.  The L and D notation doesn’t apply.    

TxDoc, do you think that there is sufficent evidence in the lit. to support a general warning not to overconsume Soy?

DoubleFeed,
Have you asked your co-worker for a clarification?  I'm interested.

Oh.  My guess is that he heard this about sugars.  Don't know.

I've left some protein with some fine womenz in the past. They were happy when I left them[:D]