Question

Wild plants readily grow "all by themselves" yet the molecules of the growing plant have less entropy than the materials used to make the plant. How is it possible for there to be this decrease in entropy for a process that occurs all by itself?

Short answer

The plant decreases its internal entropy by utilizing energy from its surroundings, increasing overall system entropy.

Step-by-step solution

Understand the Concept of Entropy

Entropy is a measure of the disorder or randomness in a system. In natural processes, the total entropy tends to increase. However, locally, systems can experience a decrease in entropy as long as there is a corresponding increase in the entropy of the surroundings.

Analyze the Plant Growth Process

During the growth of wild plants, they absorb energy from their surroundings, primarily from sunlight through photosynthesis. This process converts carbon dioxide and water into glucose and oxygen, which does result in a decrease in entropy at the molecular level within the plant.

Consider the Total Entropy of the System

While the plant's internal entropy decreases, the overall entropy of the universe increases. The energy absorbed from the environment not only powers the biochemical reactions in plants but also results in energy dispersion to the surroundings, such as the release of oxygen, heat, and increased randomness in the environment.

Apply the Second Law of Thermodynamics

The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. Thus, even if a plant decreases its internal entropy, the total entropy of the system (plants plus surroundings) increases, aligning with this law.

Key concepts

photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy. During photosynthesis, these organisms take in carbon dioxide from the air and water from the soil, combining them to produce glucose (a type of sugar) and oxygen as a by-product.
This process occurs mainly in the chloroplasts of plant cells, using a pigment called chlorophyll to capture sunlight.
There are two main stages of photosynthesis:

• Light-dependent reactions: These occur in the thylakoid membranes of the chloroplasts where sunlight is absorbed, causing water molecules to split, releasing oxygen and generating energy-rich molecules like ATP and NADPH.
• Calvin cycle (light-independent reactions): These occur in the stroma of the chloroplasts where ATP and NADPH are utilized to convert carbon dioxide and water into glucose.

Photosynthesis is crucial because it provides energy for plant growth and releases oxygen into the atmosphere, sustaining life on Earth.

second law of thermodynamics

The second law of thermodynamics is a fundamental principle of physics that states that in any natural thermodynamic process, the total entropy of a system and its surroundings always increases. Entropy is often referred to as a measure of disorder. Although locally, within some parts of a system, entropy might decrease, the law assures that the overall entropy of the entire closed system will increase.
For example, consider the process of photosynthesis. As plants convert carbon dioxide and water into glucose, they increase their molecular order, thereby lowering their internal entropy.
However, this local decrease in entropy is more than compensated by the increase in entropy of the environment. Entropy increases due to:

• Energy absorbed from sunlight that disperses heat into the surroundings.
• Release of oxygen, which increases randomness in the atmosphere.

This ensures that the total entropy of the universe moves towards greater disorder, as required by the second law.

entropy and plant growth

The relationship between entropy and plant growth is quite fascinating. As plants grow, they organize simple substances such as carbon dioxide and water into complex organic molecules like glucose and cellulose. This organization represents a decrease in entropy within the plant itself. However, this does not violate the second law of thermodynamics.
Plants accomplish this organization by utilizing energy input in the form of sunlight through photosynthesis. The absorbed solar energy not only fuels this complex synthesis but also results in energy being dispersed to the environment.
Important aspects of entropy in plant growth include:

• Reduction of entropy within the plant as it grows more structured and organized.
• Overall increase in surroundings' entropy as sunlight (free energy) is absorbed and then partly converted into less organized forms such as heat and oxygen dispersal.

Thus, while plants locally decrease their entropy, they contribute to an overall increase in the entropy of the universe, remaining perfectly in accordance with the principles of thermodynamics.