Metabolism and Metabolic Pathways

Metabolism

Living things obtain and use energy. Trees take in sunlight, and animals take in food. Both use this energy for growth and reproduction. All chemical reactions in an organism either obtain or use energy, and these reactions constitute an organism's metabolism.

Metabolic Pathways

A metabolic pathway starts with specific molecules and ends with products. The product of one reaction becomes the reactant for the next step. Each step requires a specific enzyme. Without enzymes, these pathways would either not occur or would take millions of years instead of seconds.

Metabolic pathways can be represented as follows:

A \xrightarrow{\text{enzyme 1}} B \xrightarrow{\text{enzyme 2}} C \xrightarrow{\text{enzyme 3}} D

Where:

  • A is the starting molecule.

  • D is the product.

  • B and C are intermediate molecules.

  • Each reaction is catalyzed by a specific enzyme.

Types of Metabolic Pathways

There are two main types of metabolic pathways:

  • Anabolic pathways: Combine building blocks to synthesize larger molecules.

  • Catabolic pathways: Break down large molecules into smaller building blocks.

Anabolic Pathways

  • Require an input of energy and are not spontaneous.

  • Result in products that are more complex than the reactants.

  • Decrease the entropy of life, but increase the entropy of the surroundings, resulting in an overall increase in the entropy of the universe.

For example, photosynthesis is an anabolic process:

CO2 + H2O \rightarrow C6H{12}O_6

This reaction requires sunlight and decreases the entropy of life, but increases the entropy of the environment more.

Formation of Water

The formation of water from hydrogen and oxygen is not an anabolic process.

H2 + O2 \rightarrow H_2O

  • It is spontaneous and releases energy.

  • Requires only a little activation energy to proceed.

  • The entropy of water is lower than that of separate hydrogen and oxygen molecules.

  • The energy released increases the entropy of the environment.

Catabolic Pathways

  • Release energy.

  • Result in products that are less complex than the reactants.

  • Increase the entropy of life and the environment.

For example, breaking down glucose to release energy is a catabolic process, which occurs during cellular respiration:

C6H{12}O6 \rightarrow CO2 + H_2O

Catabolic Pathways Example

Catabolic pathways involve breaking down molecules from the environment. For example, when you eat candy, the sugar (C6H{12}O6) is digested by a series of more than 20 enzymes. The metabolic pathway provides energy, and releases carbon dioxide (CO2) and water (H_2O).

C6H{12}O6 + 6O2 + \text{Enzymes} \rightarrow 6CO2 + 6H2O + \text{Enzymes} + \text{Energy}

Anabolic Pathways and Life

Anabolic pathways use the energy released from food to build the molecules needed for life. These pathways build bones, muscles, fat, and new connections in the brain.

Organisms use energy to build bigger molecules and sustain life. Their internal entropy decreases while the entropy of their environment increases even more. Anabolic pathways are a distinguishing characteristic of living things.

Anabolic pathways decrease the entropy of life, but the universe's entropy increases continuously. The processes that decrease entropy in life increase it in life's surroundings to an even greater extent, ensuring the total entropy of the universe increases.

Metabolic Pathways and Life Processes

Living requires a vast number of continuous chemical reactions. The human body has more than 20,000 different types of enzymes, with about 1 billion (1,000,000,000) chemical reactions occurring every second.