As I mentioned in the first post, it is the job of the mashing enzymes to break down both amylose and amylopectin into fermentable sugars and non-fermentable dextrins. It is the balance between the fermentable and non-fermentable components of wort that is of most concern to brewers because, obviously, this determines the level of residual sugars that will be left in the finished product.
I would like to start first with the action of beta-amylase. Beta-amylase is an exo-enzyme, meaning it attacks the starch molecules from the ends of the chains of glucose residues. It exclusively hydrolizes every second glucose molecule in the chain starting from the non-reducing end leaving the dissacharide maltose. With amylose, this means that almost the entire starch molecule can be broken down into maltose with only beta-amylase action. With amylopectin it is a bit more tricky. Beta-amylase will still hydrolize the branches of the amylopectin molecule in the exact same way as with amylose, except, beta-amylase cannot hydrolize the glucose residues closest to the branch points. In a theoretical mash with only beta-amylase, what we would end up with is a wort containing maltose as virtually the only fermentable sugar, with very large dextrins of what is left over of the amylopectin molecules.
Luckily, there is also alpha-amylase. Alpha-amylase is an endo-enzyme, meaning it cleaves the starch molecules (both in amylose and amylopectin) from the inside of the glucose chains instead of from the outside. This opens up new non-reducing ends from which beta-amylase can continue hydrolizing maltose sugars. Acting in concert with beta-amylase, there is a much more complex array of fermentable sugars and dextrins that are created. It is important to remember that even though alpha-amylase opens up new non-reducing ends for beta-amylase, beta-amylase still cannot hydrolize the glucose links close to the branch points on the amylopectin molecules. Therefore, even if there is full action of both beta-amylase and alpha-amylase, you will still have larger glucose linked molecules in the wort. Some of these would still be fermentable, such as the trisaccharide maltotriose, but most would not. In addition, in both amylose and amylopectin molecules, when alpha-amylase cleaves the starch it can leave odd numbers of glucose residues. Remember that beta-amylase can reduce those glucose residues in pairs of two (generally leaving a maltotriose molecule when it gets to the end of the chain) but occasionally it releases a singular glucose molecule. So even in a theoretical scenario, there is a wide range of sugar molecules produced in a wort from unfermentable dextrins to smaller fermentable sugars.