1. In a reciprocal manner; so that each affects the other, and is equally affected by it; interchangeably; mutually. These two particles to reciprocally affect each other with the same force. (Bentley)
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... the size, can cross the whole membrane (think channels for example) but they cannot invert the side that is facing out with the one facing in and reciprocally. Basically because the extremity popinting out are hydrophilic and cannot cross the hydrophobic membrane.
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