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How do proteins help cells move materials across membranes?
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Protein
The molecular structure of a peptide (a small protein) consists of a sequence of amino acids.
Because the cell membrane is composed of a lipid bilayer, it acts as a barrier that repels large, water-soluble molecules and electrically charged ions. To maintain life, cells utilize specific intrinsic proteins embedded within this membrane to facilitate the transport of these vital substances.12
These proteins function through three primary systems:
- Channels: These proteins form open pathways that allow specific ions to diffuse directly across the membrane. Many are "gated," opening or closing in response to stimuli like voltage changes or chemical binding.13
- Facilitators: These transporters assist larger molecules, such as glucose and amino acids, in moving down their concentration gradient. They bind to specific substrates on one side of the membrane and undergo conformational changes to release them on the other side.14
- Pumps: When substances must move against their concentration gradient, these proteins use energy—often derived from ATP hydrolysis—to force solutes through the membrane, a process known as active transport.152