Anabolic reactions are metabolic processes that build larger molecules from smaller ones, requiring energy input. They are considered "uphill" because they increase the complexity and order of the system, going against the natural tendency towards disorder (entropy).

Examples of anabolic reactions:

* Protein synthesis

* Photosynthesis

* Glycogenesis (formation of glycogen from glucose)

* Steroid hormone synthesis

Catabolic reactions, on the other hand, are downhill reactions that break down larger molecules into smaller ones, releasing energy. They are considered "downhill" because they decrease the complexity and order of the system.

Examples of catabolic reactions:

* Cellular respiration

* Glycolysis

* Digestion of food

* Breakdown of muscle protein during exercise

In summary, anabolic reactions are uphill because they require energy to build complex molecules, while catabolic reactions are downhill because they release energy by breaking down complex molecules.