Biology STAAR Practice Test 2026 – Comprehensive All-in-One Guide to Mastering the State of Texas Readiness Exam!

The process in which substances move from an area of low concentration to an area of high concentration is known as active transport. This movement is against the concentration gradient, meaning it requires energy input from the cell, usually in the form of ATP. This is crucial for maintaining concentrations of different substances necessary for cellular function, such as ions and nutrients.

Active transport allows cells to obtain essential molecules that are found in lower concentrations outside of the cell compared to the inside. This process is vital for various cellular activities, including nutrient uptake and maintaining homeostasis.

In contrast, passive transport and facilitated diffusion involve the movement of substances from high to low concentration and do not require energy, while osmosis specifically refers to the movement of water across a semipermeable membrane. Therefore, the correct answer aligns with the definition of active transport as it specifically involves the energy-dependent movement of substances against their natural concentration gradient.