3. What is the protein or gluten content of various flours?

Cake flour is typically 7-9% protein; pastry, or cookie, ~9-10%; all
purpose, 10-12%, bread, 12.5-13.5%, clear and high gluten, 14-15%;
gluten "flour" (actually refined gluten), 45%.  The protein consists
of ~80% gluten, and the gluten of cake flour is weakest, and bread
and high gluten flour the strongest, and the intermediate ones
increasingly stronger. Gluten is more of less made up of equal parts
of gliadin and glutenin.

Gluten strength definition and measurement are not entirely well
understood, even by cereal chemists.  Generally, you want the most
protein and strongest gluten for bagels and breads that also use
other poor or non-gluten flours such as rye or oat; moderately strong
for all wheat bread; weaker for pastries and cookies; still weaker
for cakes such as pound cakes; yet still weaker for "high-ratio,"
rich cakes;  and weakest for angel food cakes.  One of the quality
tests for soft wheat flour is the "cookie spread test, which is one
measure of this.  There are also farinographs, elastographs, and
whatnot to further attempt to measure this elusive property.
Fortunately, with most flours, increasing protein content goes along
with increasing gluten strength.

-Jeff

See also:

Flour -- A Treatise: http://www.theartisan.net/Flours_One.htm
Flour Test -- Flours of America: http://www.theartisan.net/flour_test.htm