An Introduction to Metabolism pt. 1

Estimated Reading Time: 5 Minutes

Hey All!

This blog post is a part of STEMTalksNC’s ever-expanding General Biology Series.  In the next series of posts, we will discuss metabolism, which will help you understand how matter and energy flow during life’s processes and how that flow is regulated.

An Organism’s Metabolism Transforms Matter and Energy

Metabolism includes all the chemical reactions of an organism.

The Chemistry of Life is Organized Into Metabolic Pathways

A cell’s metabolism is all of the thousands of chemical reaction in the cell, arranged as intersecting metabolic pathways. A Metabolic Pathway begins with specific molecules, which is then changed in a series of reactions to a product. Each step of a pathway is catalyzed by a specific enzyme. Processes that regulate enzymes also balance metabolic supply and demand, to make sure there is not too much or too less of important cellular molecules.

There are two main metabolic pathways: catabolic pathways and anabolic pathways. Catabolic Pathways release energy by breaking down complex molecules to simpler compounds. This is similar to breaking a building (complex molecules) into individual bricks (simpler compounds). This released energy becomes available to do the work of the cell. An example of a catabolic pathway is cellular respiration. (picture of cellular respiration)

Image result for cellular respiration
An example of a Catabolic Pathway is cellular respiration. Sugar glucose is broken down in the presence of oxygen to carbon dioxide and water, releasing energy.

Anabolic pathways, on the other hand, use energy to build complicated molecules from simpler ones. The energy released from catabolic pathways can be stored and then used to drive the uphill reactions of anabolic pathways. An example of an Anabolic pathway is photosynthesis. (picture of photosynthesis)

Image result for photosynthesis equation
An example of an Anabolic Pathway is Photosynthesis. In photosynthesis, carbon dioxide and water in the presence of light energy from the sun (the energy needed) are combined to form the complex molecule of glucose.


Because energy is fundamental to all metabolic processes, a basic knowledge of energy is necessary to understand how the living cell works. Energy is the capacity to cause change. It exists in many forms, and the work of life depends on the ability of cells to transform energy from one form into another.

The Law of Energy Transformation

The energy transformations of life are subject to two laws of thermodynamics. Thermodynamics is the study of energy transformations that occur in a collection of matter. The system is the matter under study; the surroundings are the rest of the universe exclusive to the system. There are two ways of describing the “system”: isolated and open system. An isolated system is unable to exchange either energy or matter with its surroundings. An open system is when energy and matter can be transferred between the system and surroundings. An example of an open system is organisms. They absorb energy in the form of organic molecules and release heat and metabolic waste products such as urea to the surrounding environment. Two laws of thermodynamics govern energy transformations in organisms and all other collections of matter.

The First Law of Thermodynamics

The first law of thermodynamics states that energy is constant and can be transferred and transformed, but it cannot be created or destroyed. An example of this law is plants converting light energy (sunlight) to chemical energy; thus acting as an energy transformer.

The Second Law of Thermodynamics

During every energy transfer or transformation, some energy becomes unusable energy, unavailable to do work. More usable forms of energy are partly converted to heat, the energy associated with the random motion of atoms or molecules. In this context, in the process of carrying out chemical reactions, living cells partly convert usable forms of energy to heat, which is the energy associated with the random motion of atoms or molecules. This heat can only put to work only when there is a temperature difference that makes the organism warm up in a cold environment.

With each loss of energy during an energy transfer, scientists describe the universe as becoming more “disordered.” This disorder is described as entropy (you might have learned this in your General/AP Chemistry class). The more randomly arranged or more disordered a collection of matter is, the greater its entropy. The Second Law of thermodynamics is linked to entropy; it states that every energy transfer or transformation increases the entropy of the universe.

The Link of Entropy to Spontaneity

For a process to occur on its own (spontaneous), it must increase the entropy of the universe. For a process to be spontaneous (water going downhill), it must occur without an input of energy or work. It does not mean that it should occur quickly; some are instantaneous while some take forever. On the other hand, a process that cannot occur on its own is nonspontaneous; it needs energy added to make it work.

We can link catabolic and anabolic pathways to spontaneity; catabolic pathways are spontaneous and anabolic pathways are nonspontaneous.

That’s all for this post. In the next post of this series on metabolism, we will discuss free energy! Feel free to share this post to people that might benefit.

-Tahmid Islam

Co-Founder and Editor of StemTalksNC


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