synthesis

Meaning: In biochemistry, synthesis is anabolism: building larger molecules from smaller precursors, such as proteins from amino acids, glycogen from glucose, fatty acids from acetyl-CoA, and glucose from lactate (gluconeogenesis). Forming these products is usually endergonic, so cells couple it to exergonic steps, most often ATP hydrolysis, and reductive pathways draw on NADPH. In organic chemistry, synthesis instead means a multistep route to a target compound.

Equation: ΔGnet = ΔGsynthesis + ΔGATP hydrolysis, where each ΔG is a free energy change in J/mol. The coupled process is spontaneous when ΔGnet < 0.

On the MCAT: Passages ask which conditions favor synthesis versus breakdown. For example, insulin promotes storage (glycogen and fat synthesis), while glucagon promotes breakdown and gluconeogenesis. Other questions ask you to predict what happens when an enzyme is inhibited or ATP is low, or to spot the coupling step in a pathway diagram.

Common confusion: Synthesis is not simply the breakdown pathway run backward. The two pathways often share the reversible steps but bypass the irreversible ones with different enzymes, and they can occur in different compartments (fatty acid synthesis in the cytosol, β-oxidation in mitochondria). Also, dehydration synthesis releases water, whereas hydrolysis consumes it.