How do ions cross the lipid bilayer?

1 Answer
Jun 27, 2015

Answer:

Ions can cross the lipid bilayer because proteins in the membrane create pores that help ions pass through.

Explanation:

Passive Transport

In passive transport, transmembrane proteins create a water-filled pore through which ions can pass by diffusion down a concentration gradient.

This is simple diffusion and requires no energy.

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Channel proteins are more than just holes in the membrane, as they exhibit specificity and allow only certain ions to go through.

For example, the #"K"^+# channel allows only #"K"^+# ions to enter, and the #"Na"^+# channel allows only #"Na"^+# ions to pass through.

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Active Transport

In active transport, specific carrier proteins use the energy of ATP to force ions through the membrane against a concentration gradient.

They physically carry ions from one side of the membrane to the other via conformational changes.

For example, the #"Na"^+"/K"^+# pump exchanges #"3Na"^+# for #"2K"^+#.

The proteins are called ion pumps, because they use the energy from ATP hydrolysis to change their shapes and "pump" ions from one side to the other.

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Some ion pumps involve the simultaneous transport of two different ions.

One ion may be transported down the concentration gradient, providing the energy needed to transport a second ion against the gradient.

For example, the sodium-calcium channel moves three sodium ions in one direction for each calcium ion that moves in the other direction.

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