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The second law of thermodynamics states that systems tend to proceed from ordered, low-entropy states to disordered, high-entropy states without outside input. Living cells must constantly expend energy to maintain their highly ordered structures like DNA and proteins. As cells break down molecules such as glucose for energy, they release byproducts—carbon dioxide, water, and heat—into their surroundings, increasing universal entropy while maintaining their own order.

Entropy measures the randomness or disorder within a system. High entropy indicates high disorder and low usable energy. Living organisms are highly ordered and maintain low entropy, but this requires constant energy input. As cells transform energy-storing molecules through chemical reactions, they lose some usable energy as heat, increasing the entropy of their surroundings and the universe overall.

Single DNA strands are disordered, and their entropy decreases when they reanneal into an ordered double helix structure. However, this process releases energy into the surroundings, making the surroundings more disordered and increasing its entropy. The total entropy of the system and surroundings combined increases, satisfying the second law of thermodynamics.

No energy transfer in the universe is completely efficient. During cellular metabolic reactions, some energy is lost as heat energy, which is defined as energy transferred from one system to another that is not work. This energy loss reduces the amount of usable energy available to the cell and increases disorder in the surroundings, contributing to universal entropy increase.

Passive transport of concentrated oxygen from the lungs to less oxygenated blood disperses oxygen molecules throughout the system, increasing entropy. The molecules move from a state of high concentration and order to a state of dispersal and disorder without requiring cellular energy input, exemplifying how systems naturally proceed toward higher entropy states.

Living things are highly ordered systems requiring constant energy input to maintain low entropy. As cells take in energy-storing molecules and transform them through chemical reactions, they lose usable energy and produce waste byproducts. Since every energy transfer increases universal entropy, living organisms face a continuous uphill battle against the constant increase in disorder throughout the universe.

Heat energy is the unusable energy transferred from a system to its surroundings during reactions. As cells lose heat energy during metabolic processes, the surroundings become more disordered and random. This heat loss directly increases the entropy of the surroundings, ensuring that the total entropy of the universe increases with every cellular energy transfer and transformation.