For large polar molecules and ions, there are special protein called transport protein through which they cross the lipid bilayer. Some transport proteins form pores. Others are gated that close in response to a stimulus. Active transport is a transport where protein pumps a solute across a cell membrane against the solutes concentration gradient which requires energy. Passive transport does not need energy for transport to occur. Facilitated diffusion is a type of passive transport in which solute follows its concentration gradient across a membrane by moving through a transport protein. Osmosis is an example of passive transport, a membrane-crossing mechanism that requires no energy input.

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Chapter 5, Problem 13SQ

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Introduction: For large polar molecules and ions, there are special protein called transport protein through which they cross the lipid bilayer. Some transport proteins form pores. Others are gated that close in response to a stimulus. Active transport is a transport where protein pumps a solute across a cell membrane against the solutes concentration gradient which requires energy. Passive transport does not need energy for transport to occur. Facilitated diffusion is a type of passive transport in which solute follows its concentration gradient across a membrane by moving through a transport protein. Osmosis is an example of passive transport, a membrane-crossing mechanism that requires no energy input.

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