Importance of Metabolism
Characteristics of metabolic chemical reactions
One of the basic reactions of metabolism is the transformation of chemical substances : two or more substances can be joined together to give rise to more complex molecules and a molecule can be broken down into its basic components.
Unlike spontaneous chemical reactions that occur in the physical world, the chemical reactions of cellular metabolism are induced and highly controlled in all their steps by complex cellular machinery.
When two molecules join together they can generate different compounds, depending on the way in which they associate.
For cells, not just any form of molecule is the same ; some work while others do not perform their function well or do not work at all. This is why the steps of the chemical reaction must be very well controlled so that the molecule is produced with the specific form required, and not any other.
Some chemical reactions require energy, and they never occur spontaneously . For example, sunlight does not spontaneously cause carbon dioxide to join with water to form carbohydrates; this union requires a lot of energy .
A reactor of some sort would be needed to supply enough energy for this connection to occur; powering such a reactor would consume a lot of electricity and would be very expensive.
However, in plants, this reaction occurs all the time . Their leaves are like these reactors, and they have cellular machinery capable of converting the light energy of the sun into electrical energy. Photosynthesis is an example of an induced reaction: the cell causes chemical reactions to occur that would never occur outside .
Chemical reactions in metabolism are usually chain reactions : one reaction produces the precursors needed by the next. This chain of metabolic reactions is called a metabolic pathway. A metabolic pathway will stall and stop working if any of its steps fail .
There are two main types of metabolism: anabolism and catabolism .
Anabolism
Anabolism refers to the metabolic processes of building new molecules from other molecules, which are known as precursors . The “building” process is called synthesis .
Anabolism uses nutrients from food to create new molecules. These molecules will then be used to create or repair new tissues, or to replenish “inputs” for cellular activities.
There are metabolic synthesis pathways for all known types of organic molecules . The precursors of almost all of them are organic molecules synthesized in reactions that are earlier in the metabolic pathway.
Ultimately, the precursors to the entire variety of molecules that can be synthesized are the simple carbohydrates synthesized by photosynthesis .
Photosynthesis, which is essentially a glucose synthesis route , unlike other anabolic pathways, does not use organic but inorganic precursors and its energy source is not provided by the cell, but by the sun.
Catabolism
Under this name are grouped the routes of decomposition of molecules .
All molecules store chemical energy in the bonds between the atoms that compose them ; when the molecule breaks down, that energy is released (just as when something burns, the chemical energy of the fuel is released as heat during the combustion reaction).
The cell cannot contain a fire inside itself: such an uncontrolled reaction would destroy it. The release of energy is tightly controlled in all catabolic processes .
Catabolic pathways break down all molecules into smaller units, until the simplest organic molecule of all remains: glucose.
Glucose is literally burned by the cell, releasing its energy. That energy from the sun that was used to force atoms together during photosynthesis is the same energy that is obtained when the glucose molecule is broken apart. This process is called cellular respiration, and it breaks down glucose into its elemental components: carbon dioxide, water, and energy. Cellular respiration requires oxygen, just as oxygen is needed to start a fire.
Energy is used to maintain the life of all organisms, the rest of the products of respiration are discarded.
Without oxygen available, breathing cannot continue and without energy, the organism dies. Cyanide, a very powerful poison, blocks cellular respiration and causes death within minutes.
Respiration can be thought of as the reverse metabolic pathway of photosynthesis. The balance of both is essential for the maintenance of life.
While photosynthesis uses water, carbon dioxide and energy to produce glucose and oxygen (the latter is discarded), respiration uses oxygen and glucose to produce energy, carbon dioxide and water.
Another important catabolic pathway is lipolysis , which is responsible for breaking down fats into glucose to supply cellular respiration and obtain energy.
Body fat is a reservoir of glucose, where it is stored for when it is needed . Lipolysis breaks down lipid molecules to release glucose units when needed.
Protein catabolism is responsible for breaking down proteins into amino acids, and these into their constituent compounds.
These “amino acid fragments” can then be used in anabolic pathways to synthesize new amino acids and proteins, or they can be broken down to glucose and burned for energy.
Given the important role of proteins, they are never used as glucose reserves, and except in situations of severe malnutrition, proteins are never used in cellular respiration.
The decomposition of amino acids gives rise to nitrogenous waste , which is eliminated by the excretory systems in the form of urea, uric acid or ammonia.
All metabolic pathways work in a coordinated manner to maintain a balance of chemicals in our body in the right amounts and to provide precursors and energy to the anabolic pathways. This energy and material balance achieved by metabolism is what makes life possible.
Alterations in any metabolic pathway affect this balance and cause serious health problems.