Answer to Question 1
Barbara Fredrickson proposed a broaden-and-build model to explain the adaptive and evolutionary value of our positive emotions. Whereas negative emotions tend to narrow our options, positive emotions tend to broaden them. These specific action tendencies of negative emotions (e.g., fight-or-flight) stand in contrast with the non-specific action tendencies of positive affect. For example, it was not very adaptive for those primitive humans to ponder and contemplate a bevy of options for dealing with an approaching saber-toothed tiger unless they wanted to be the tiger's next lunch. Instead, their negative emotions helped them act quickly, sometimes without thinking, to either fight or flee the tiger.
Answer to Question 2
Positive affect refers to an experience of positive emotions and negative affect to an experience of negative emotions. Each appears to have different evolutionary and adaptive values. Whereas positive affect is approach-oriented, moving the person toward situations or others that could yield pleasure and reward, negative affect is part of the withdrawal-oriented system designed to protect a person from threat, harm, or pain (e.g., fight-or-flight).
Model for cytoplasmic segregation
A proposed pathway for the action of the E. coli chaperonins GroEL
Heart Model
The Atkinson-Shiffrin model
Action Potentials
The sliding-filament model