10.4 Thermodynamics of Living Systems

In heat, the energy is in random motion, while in work it is in an ordered motion.

The ordering of all the thermal motion is very unlikely.
The second law of thermodynamics states that it is impossible because the probability of completely converting heat to work is small.

It is clear from this equation that heat can only be converted into work if the heat is rejected at absolute zero temperature.

The idea that animals need energy because they consume it is incorrect.
The first law of thermodynamics says that energy is conserved.
The body can change from one form to another.
The first law could lead us to believe that animals should not have a source of external energy.
The body takes in energy from the food and converts it to heat.
The energy input to the body is equal to the heat energy leaving the body if the weight and temperature of the body remain constant.
The body could survive without food if the heat outflow could be stopped.
This supposition is not correct.
The second law of thermodynamics shows the need for energy by examining the functioning of the body.

The functions of the body are dependent on the structure and location.
The second law of thermodynamics states that a highly ordered system, left to itself, tends to become disorganized and ceases to function once it is.
The system needs constant work to prevent it from falling apart.
The flow of blood is slowed due to the fact that the veins and arteries are subject to friction.
The blood would stop flowing if a force was not applied.

The natural tendency is to equalize with the environment.
The contents of the cell need to be kept out of the water.
New tissue must be manufactured if the animal is growing, and cells that die must be replaced.
Smaller, more random subcomponents must be put together for replacement and growth.
This activity requires work in the face of disorder.
The situation is similar to a pillar made of small, slippery blocks that tend to fall out of the structure.
Blocks are pushed back so that the pillar is still standing.

The chemical energy in food can be used to maintain the ordered structures in the body.

Except for the energy utilized in external work done by the muscles, all the energy provided by food is converted into heat by other dissipative processes in the body.
All the heat generated by the body must leave through the various cooling mechanisms if the temperature is to be desired.
The body doesn't have the ability to get work from heat energy, so the heat must be dissipated.

The limit is set by the second law.
The temperature differences between the inside and outside of the body are small.
The efficiency of heat conversion to work would be 1 at 303 K.

The body doesn't have a way to convert the other forms of energy into work.
A person can bask in the sun indefinitely and still die of starvation.
Plants are able to use energy from the sun.
Plants use solar radiation to provide the energy for the ordering processes necessary for life.

The organic materials produced in the life cycle of plants provide food energy for herbivorous animals, which in turn are food for the animals that eat them.

Since living systems create order out of relative disorder, it may appear that they violate the second law of thermodynamics, but this is not the case.

To determine if the second law is valid, we must look at the whole process of life, which includes not only the living unit but also the energy that it consumes and the by-products that it rejects.
The food that is consumed by an animal has a lot of order.
The atoms in the food are ordered in a certain way.
The ordered structures are broken down when the chemical energy in the food is released.
The waste products are more disorganized than the food.
The body converts the chemical energy into heat.

The increase indisorder in the surroundings produced by the living system is always greater than the decrease indisorder in the living system.