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When molecules are too large or too charged to cross the plasma membrane by diffusion or osmosis, there are other methods to accomplish their transport into or out of the cell. For example, some proteins in the plasma membrane are known as protein channels and they form a way for large or charged molecules to pass through the membrane. These transport proteins are sandwiched between the phospholipid bilayer and are involved in facilitated diffusion. No energy input is required for this particular process. Sometimes, substances that are large require energy to help facilitate their movement across the membrane. This is known as active transport and ATP supplies the energy currency. This type of transport involves cell organelles known as vesicles. In vesicular transport, vesicles allow large molecules to exit the cell because the vesicle fuses with the plasma membrane and its contents spill out. This is called exocytosis. In endocytosis, a vesicle forms as an indentation on the plasma membrane and the cell engulfs a large substance from outside and brings the molecules in. Pinocytosis or cell drinking is another mode of endocytosis by which particles are brought into the cell due to an invagination of the membrane and these particles will be suspended inside a vesicle. Phagocytosis means to devour and it is when a cell engulfs a solid particle to form a vesicle called a phagosome. This is also a type of endocytosis and is a way of obtaining nutrients in some cells and a way of removing pathogens and cellular debris in others.
Larger molecules wold require a transport protein in order to cross the cell membrane.
There are three modes of transport across the cell membrane: diffusion, facilitated diffusion, and active transport.
Diffusion is the movement of substances down a concentration gradient (from higher to lower concentration). There is no transport protein involved. Smaller ions and molecules are transported across the membrane in this way.
Facilitated diffusion also transports substances down a concentration gradient. However, it requires a transmembrane integral protein, which helps transport larger molecules. The proteins are specific to allow certain things in and keep harmful substances out (hence, we call the membrane "selective").
Becuase diffusion and facilitated diffusion do not require energy, they are referred to as modes of passive transport.
The last form of transportation is active transport. It carries substances against the concentration gradient (from low to high concentrations). It, too, requires a transport protein.
Thus, because the larger protein would require a transport protein, it would travel via either facilitated diffusion or active transport.
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