Final answer:
Substances can move across the plasma membrane through active transport, passive diffusion, facilitated diffusion, and endocytosis. Active transport uses energy, whereas passive diffusion and facilitated diffusion do not. Osmosis, a type of passive transport, along with other mechanisms, is vital for the cell's homeostasis.
Step-by-step explanation:
The four basically different ways that a substance can move across the plasma membrane are active transport, passive diffusion, facilitated diffusion, and endocytosis. Active transport requires energy (ATP) to move substances against their concentration gradient, from an area of lower concentration to higher concentration. Passive diffusion allows substances to move from an area of higher concentration to an area of lower concentration without the use of energy. Facilitated diffusion is similar to passive diffusion but involves the use of carrier proteins to move substances down their concentration gradient. Endocytosis involves the cell membrane engulfing a substance to transport it into the cell, which also requires energy.
These mechanisms are crucial for maintaining homeostasis of the cell by controlling the internal environment. For example, osmosis is a type of passive transport where water moves through a semipermeable membrane, maintaining the right balance of water inside the cell. Moreover, cells, such as those in certain salmon species, must regulate ion concentration through these mechanisms to survive in both freshwater and saltwater environments.