What happens to a human body once a sugary snack is consumed?
The human/animal digestive tract breaks down food chemically (with low pH/acid), enzymatically (like proteases and glycolytic enaymes which break down protein and sugars respectively), as well as symbiotically (bacteria participates in the breakdown of some compounds in the gut).  The results are released into the blood stream for the most of the body to assimilate. 

With foods with complex carbohydrates (scenario 1), the results can be only a modest change in the glucose level in the serum.  This depends upon the specific food which you can understand better by researching the 'glycemic index'.  Highly glycemic foods result in rather short term release of simple carbohydrates glucose/sucrose into the blood.  Lower glycimic food contain complex carbohydrates (which are elaborate chains of sugars) which need to be broken up into simple carbohydrates before they are metabolized the the cells.

Foods which contain a lot of simple carbohydrates change the sugar levels in the blood nearly immediately and it can go quite high.  simple carbohydrates are monomers or dimers of sugars.  BTW complex carbohydrates have few pictures on the web, but you can imagine them as chains and networks of many many simple carbohydrates linked together.  

Not all simple carbohydrates are used by human beings for energy.  For instance the glycosamine in 'joint juice' is the sort that makes cartilidge, which just to show you how different some complex carbohydrates can be. BTW i don't recommend joint juice, just trying to give a familiar example.  Its unlikely that the glucosamine you drink will be directly used for your joints!

So glucose is the energy currency in the blood.  when the glucose level goes up insulin is secreted by the pancreas which tells the cells to take up the glucose for glycogen (internal cell energy storage in a complex carbohydrate) or to be metabolized directly.  Diabetes results when insulin is not produced (type I) or when the cells stop responding to insulin (type II).  

Human tolerance to glucose in the blood is estimated to be up to about 100 mg/dl long term. higher than this on the average is not healthy and can go up to several hundreds and cause lots of degeneration in the kidney, liver, and eyes, and more.  Exercise and fasting increase insulin sensitivity and athletes have blood glucose that is quite low even if they have a sugary beverage.  

hope this helps - many keywords embedded within...

One last note.  Fructose, which is about 45-60% of high fructose corn syrup (the rest is sucrose) is almost entirely metabolized in the liver.  This is why fructose can be harmful to people who drink too much sweetened beverages.  With the liver taking in the vast majority of fructose, it tends to make fat out of it, sort of overdosing it to make it fatter than the rest of you relatively quickly.  Fatty liver is a marginally disfunctional liver and can cause health problems down the way.  Abdominal body fat (not on your love handles, but amongst your chest cavity and internal organs) is particularly harmful as it interferes with all sorts of organ function.  Probably something everyone should be aware of.