If you remember, I gave a quote (Dec 7 post) from Lewis Thomas about membranes. The last portion of that quote is: "To stay alive, you have to be able to hold out against equilibrium, maintain imbalance, bank against entropy, and you can only transact this business with membranes in our kind of world".
Today, I would like to focus on the line "... you have to be able to hold against equilibrium, maintain imbalance,..."
This line usually confuses students because they think that biological systems always try to maintain equilibrium and balance. So when I tell them the contrary, they get confused. How then do we remove this confusion?
Well, what I usually do is I ask them the questions: "What are the contents of cells?, What is the composition of the fluid surrounding cells?, What is the composition of earth?" Once they give the answers, they then realize that indeed cells have different composition from their immediate surrounding and from the earth. And the only way for cells to have a different composition from their surroundings is to "hold against equilibrium and maintain imbalance." This is of course only possible because of membranes and active transport. The membranes keep the substances inside cells and active transport maintains this difference in composition. And the only way cells can do this is by having a constant supply of energy.
Energy is thus needed to go against equilibrium and maintain imbalance. These make cells alive and different from the surroundings. Hooray for imbalance!
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