Why does cellular respiration involve so many molecules and steps based on how life obeys and exploits the Laws of Thermodynamics?

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akindtexan eNotes educator| Certified Educator

The laws of thermodynamics are as follows.

The first law is that the increase in energy in a system is the sum of energy added to the system. This means that a system will not change energy unless some is introduced from outside the system. This applies to the body when you have to eat to gain weight, for example. You expend energy naturally through your metabolism, moving, breathing, and even digesting. You intake energy by eating. 

Even cells obey the first law. Cells use various pumps to gain the nutrients they need from the bloodstream. They use pumps to push out things they no longer need or can use. Cells cannot take in an infinite amount of energy, and without constant feeding, will die.

The second law states that if two bodies in equilibrium with themselves, but not with each other, are introduced, then they will attain a new equilibrium together. Basically, the energy will try to spread itself evenly. If you go outside on a cold night, you will get cold. Unless your body burns energy to keep you warm, you will freeze. The second law also states that the entropy of the universe is always increasing.

The third law governs what happens to entropy as temperature approaches absolute zero. I don't think we need to worry about a life form with zero entropy and zero heat.

The zeroth law of thermodynamics states that if two systems are in equilibrium with a third, they are in equilibrium with each other. This is important in relating things like cells to the bloodstream.

Now with that out of the way we can discuss cellular respiration. I will use the Krebs cycle, although these principles apply to fermentation and anaerobic respiration as well.

To start, the Krebs occurs in the mitochondria. This organelle has a starting energy and concentration of nutrients, and the cell surrounding it has a different concentration. Therefore, the mitochondria outer wall is semi permeable, allowing small molecules to pass in and out freely, such as water and sugar, and uses active transport to move larger molecules, like proteins. This is a perfect example of the first and second laws of thermodynamics, as the organelle uses active transport to move the proteins, but is in equilibrium with the rest of the cell with water and sugar.

When reactions occur during the Krebs cycle, such as the oxidation of pyruvate, energy is released into the cell in the form of heat. This heat can be used to start other reactions, or move things around the cell. Either way, the energy released in the reaction is added to the total heat of the cell, and when a reaction absorbs this energy, the total heat in the cell will decrease.

You will notice the use of catalysts in the metabolism of a cell. This lowers the activation energy of a reaction, and allows the cell to work at much lower temperatures than the reactions would normally need to take place.