Final answer:
The two broad types of transport across the plasma membrane are passive transport, which requires no energy and moves substances down their concentration gradient, and active transport, which requires energy to move substances against their concentration gradient.
Step-by-step explanation:
Cells need to transport substances across the plasma membrane, and there are two broad types of transport mechanisms they use: passive transport and active transport.
Passive transport is a way for substances to cross the plasma membrane without any energy input. It exploits the natural progression of substances from an area of higher concentration to one of lower concentration.
Simple diffusion and facilitated diffusion are types of passive transport. Simple diffusion allows small nonpolar molecules to directly traverse the membrane, while facilitated diffusion requires transport proteins like channels and carriers to help larger or polar substances move across.
Active transport, on the other hand, requires energy, typically from ATP, to move substances against their concentration gradient, from an area of lower concentration to a higher concentration.
This type of transport often involves transport proteins such as pumps. There are also specialized forms of active transport for large molecules or particles, known as vesicle transport, which include endocytosis and exocytosis.