Final answer:
The statement is true; in cells, actin monomers bound to ATP add to the + end of actin filaments, causing growth, while ATP hydrolysis at the - end leads to shrinkage, a process known as treadmilling.
Step-by-step explanation:
The statement that both growth and shrinkage of microtubules (MTs) occur from the + end is true. Unlike equilibrium polymers, where growth or shrinkage would occur at both ends of the polymer, the polymerization of actin within cells exhibits a non-equilibrium behavior with different dynamics at the + and - ends. Actin monomers bound to ATP preferentially associate with the + end, leading to elongation, while ATP hydrolysis to ADP at the - end leads to shrinkage. This results in a phenomenon known as treadmilling, where the + end of the filament grows while the - end shrinks, leading to the illusion of motion. This differential behavior is due to the conformational changes in the monomers that are catalyzed by ATP, making the process highly dynamic and non-equilibrium in nature.