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Where do we get our energy from? Most of you would say food, carbohydrate, or sugars, which is right. But those answers may be enough to satisfy a preschooler, not those taking biology, or having even the mildest interest in its branches.
A simple question sometimes require an extensive answer, and this being the study of life, the answer is never simple. |
"Without energy life would be extinguished instantaneously, and the cellular fabric would collapse."
Albert Szent-Gyorgyi, Nobel Lecture, 1937
Cellular respiration is what cells do to break up sugars into a form that the cell can use as energy. This happens in all forms of life. Cellular respiration takes in food and uses it to create ATP, a chemical which the cell uses for energy. Usually, this process uses oxygen, and is called aerobic respiration.
It has three stages known as glycolysis, the Krebs cycle, and the electron transport chain. This produces ATP which supplies the energy that cells need to do work. When they don't get enough oxygen, the cells use anaerobic respiration, which doesn’t require oxygen. However, this process produces lactic acid, and is not as efficient as when oxygen is used. Aerobic respiration, the process that does use oxygen, produces much more energy and doesn’t produce lactic acid. It also produces carbon dioxide as a waste product, which then enters the circulatory system. The carbon dioxide is taken to the lungs, where it is exchanged for oxygen.
The simplified formula for aerobic cellular respiration is:
It has three stages known as glycolysis, the Krebs cycle, and the electron transport chain. This produces ATP which supplies the energy that cells need to do work. When they don't get enough oxygen, the cells use anaerobic respiration, which doesn’t require oxygen. However, this process produces lactic acid, and is not as efficient as when oxygen is used. Aerobic respiration, the process that does use oxygen, produces much more energy and doesn’t produce lactic acid. It also produces carbon dioxide as a waste product, which then enters the circulatory system. The carbon dioxide is taken to the lungs, where it is exchanged for oxygen.
The simplified formula for aerobic cellular respiration is:
Cellular Energy Sources
The goal of cellular respiration and metabolism in animals and plants is, ultimately, the conversion of one type of energy source to another. Presumably, the original energy source comes in a form that cannot be immediately used to support cellular activities. For humans, our external energy sources are the foods we eat. Once we ingest and digest the food, our cells metabolic processes convert the energy contained within the food into a form of energy that can function in our cells. These constant conversions are what allow us to perform our day-to-day activities.
Since energy is the ultimate goal of metabolism, it will be helpful to understand what these various external and internal energy sources really are. As we have mentioned, food is the external energy source for humans. Different foods are composed primarily of one of the following three macromolecules: carbohydrates (breads and pastas), lipids (fats and oils), or proteins (meats and beans). During digestion of food, when the food is first broken down internally, these large molecules are broken into subunits. Depending on their type, subunits can be metabolized in different ways and then used as internal energy sources.
Basics of Metabolism
Metabolism is a process of energy acquisition and conversion. It is necessary because organisms are constantly undergoing cellular changes--they are not in a state of equilibrium. Metabolism is an attempt to regulate cellular conditions by making internal changes to maintain a steady cellular state. As a general rule, nature's tendency is towards conditions of disorder. This means that disorderly conditions are energetically favorable--they release energy. Highly ordered and organized conditions are not energetically favorable and require energy to occur. As a result, the thousands of reactions that constantly occur inside us to maintain cellular organization need energy. The body produces this needed energy by breaking down ATP, and then using this energy to promote energetically unfavorable, but biologically necessary reactions.
In order to begin any of these processes, cells need an external energy source. The breakdown of the external source can provide the energy that can couple to drive other reactions. Cells acquire this external energy in one of two ways. Phototrophs get their energy from the sun through photosynthesis. Plants are phototrophs. Plants use light energy to convert carbon dioxide and water into carbohydrates and oxygen. Chemotrophs, such as humans, derive energy from the breakdown of organic compounds such as carbohydrates, lipids, and proteins. Our focus in discussing cell respiration and metabolism will be on this second, chemical type of energy acquisition. The relationship between phototrophs and chemotrophs is complimetary: chemotrophs require oxygen and expire carbon dioxide while phototrophs require carbon dioxide and expire oxygen. Additionally, many of the carbohydrates ingested by chemotrophs derive from the metabolic carbohydrate products of phototrophs.
Among chemotrophs, there are two major categories of metabolic pathways. The distinction between the two is that one involves degradation reactions while the other involves synthesis reactions. Catabolic pathways involve the breakdown of ingested food molecules. Anabolic pathways involve the synthesis of essential biomolecules. Along each of these pathways, a number of enzymes work in combination to help drive the reactions. The catabolic pathways are involved in breaking down carbohydrates and proteins into their polysaccharide, or sugar, and amino acid subunits. These reactions release energy needed by the cell (this is why food, the source of carbohydrates and proteins, is essential for survival). Anabolic pathways take the simple products of catabolic degradation--ATP, for example--and use energy from their degradation to synthesize complex biomolecules.
The goal of cellular respiration and metabolism in animals and plants is, ultimately, the conversion of one type of energy source to another. Presumably, the original energy source comes in a form that cannot be immediately used to support cellular activities. For humans, our external energy sources are the foods we eat. Once we ingest and digest the food, our cells metabolic processes convert the energy contained within the food into a form of energy that can function in our cells. These constant conversions are what allow us to perform our day-to-day activities.
Since energy is the ultimate goal of metabolism, it will be helpful to understand what these various external and internal energy sources really are. As we have mentioned, food is the external energy source for humans. Different foods are composed primarily of one of the following three macromolecules: carbohydrates (breads and pastas), lipids (fats and oils), or proteins (meats and beans). During digestion of food, when the food is first broken down internally, these large molecules are broken into subunits. Depending on their type, subunits can be metabolized in different ways and then used as internal energy sources.
Basics of Metabolism
Metabolism is a process of energy acquisition and conversion. It is necessary because organisms are constantly undergoing cellular changes--they are not in a state of equilibrium. Metabolism is an attempt to regulate cellular conditions by making internal changes to maintain a steady cellular state. As a general rule, nature's tendency is towards conditions of disorder. This means that disorderly conditions are energetically favorable--they release energy. Highly ordered and organized conditions are not energetically favorable and require energy to occur. As a result, the thousands of reactions that constantly occur inside us to maintain cellular organization need energy. The body produces this needed energy by breaking down ATP, and then using this energy to promote energetically unfavorable, but biologically necessary reactions.
In order to begin any of these processes, cells need an external energy source. The breakdown of the external source can provide the energy that can couple to drive other reactions. Cells acquire this external energy in one of two ways. Phototrophs get their energy from the sun through photosynthesis. Plants are phototrophs. Plants use light energy to convert carbon dioxide and water into carbohydrates and oxygen. Chemotrophs, such as humans, derive energy from the breakdown of organic compounds such as carbohydrates, lipids, and proteins. Our focus in discussing cell respiration and metabolism will be on this second, chemical type of energy acquisition. The relationship between phototrophs and chemotrophs is complimetary: chemotrophs require oxygen and expire carbon dioxide while phototrophs require carbon dioxide and expire oxygen. Additionally, many of the carbohydrates ingested by chemotrophs derive from the metabolic carbohydrate products of phototrophs.
Among chemotrophs, there are two major categories of metabolic pathways. The distinction between the two is that one involves degradation reactions while the other involves synthesis reactions. Catabolic pathways involve the breakdown of ingested food molecules. Anabolic pathways involve the synthesis of essential biomolecules. Along each of these pathways, a number of enzymes work in combination to help drive the reactions. The catabolic pathways are involved in breaking down carbohydrates and proteins into their polysaccharide, or sugar, and amino acid subunits. These reactions release energy needed by the cell (this is why food, the source of carbohydrates and proteins, is essential for survival). Anabolic pathways take the simple products of catabolic degradation--ATP, for example--and use energy from their degradation to synthesize complex biomolecules.
Respiration
Metabolism is an over-arching term to describe all of the different pathways involved in energy acquisition and conversion. In this section, we will introduce one of these specific pathways, the respiratory pathway. Respiration refers to the acquisition of energy from food. It is a process common to all eukaryotic cells. In studying respiration, we will focus on the metabolic pathways that convert glucose into ATP. Among the components of the respiratory pathway are the processes of glycolysis, the citric acid cycle, and the electron transport chain.
If we think about respiration in day-to-day terms, we immediately think of breathing and oxygen. From an early age, we learn how crucial oxygen is to our survival and that breathing is how we obtain oxygen from the air. We will now begin to see exactly how oxygen plays a role inside our bodies. Using oxygen, organisms can break down ingested foods more easily through oxidation, an energetically favorable reaction. However, though oxygen is a vital component of respiration, respiration can occur without oxygen. Respiration that involves oxygen is called aerobic respiration while that lacking oxygen is called anaerobic. Of the two, aerobic respiration is far more efficient, producing more energy per gram of glucose. We will explore the specifics of how these processes differ in the coming sections.
The energy gained from glucose breakdown can be used to synthesize ATP, which in turn can help with the synthesis of other, more complex molecules that would otherwise be too unfavorable to occur. The mitochondria is the main cellular structure involved in respiration, and we will cover this unique cellular element in another article.
Metabolism is an over-arching term to describe all of the different pathways involved in energy acquisition and conversion. In this section, we will introduce one of these specific pathways, the respiratory pathway. Respiration refers to the acquisition of energy from food. It is a process common to all eukaryotic cells. In studying respiration, we will focus on the metabolic pathways that convert glucose into ATP. Among the components of the respiratory pathway are the processes of glycolysis, the citric acid cycle, and the electron transport chain.
If we think about respiration in day-to-day terms, we immediately think of breathing and oxygen. From an early age, we learn how crucial oxygen is to our survival and that breathing is how we obtain oxygen from the air. We will now begin to see exactly how oxygen plays a role inside our bodies. Using oxygen, organisms can break down ingested foods more easily through oxidation, an energetically favorable reaction. However, though oxygen is a vital component of respiration, respiration can occur without oxygen. Respiration that involves oxygen is called aerobic respiration while that lacking oxygen is called anaerobic. Of the two, aerobic respiration is far more efficient, producing more energy per gram of glucose. We will explore the specifics of how these processes differ in the coming sections.
The energy gained from glucose breakdown can be used to synthesize ATP, which in turn can help with the synthesis of other, more complex molecules that would otherwise be too unfavorable to occur. The mitochondria is the main cellular structure involved in respiration, and we will cover this unique cellular element in another article.
Ponder this
The human body derives energy from metabolising sugars. Why? Other organic chemicals also stores vast amount of energy, alcohol for instance. Should our body evolve to get as much energy from anywhere?
Continuing from the previous question, why don't animals leverage on photosynthesis to directly produce the sugars it needs? Where and when did we branch off from our herbaceous cousins (and this very advantageous ability) on the tree of life?
Discuss
Discuss the parallels between energy production and process in animal cells and our civilisation. As our cells require energy to function (pump blood, send signals, digest food), how is this similar to the systems and infrastructure of human civilisation?
Further readings
Glycolysis, the first step to converting sugars into usable chemical energy.
Citric acid cycle, also known as the Krebs cycle.
Electron transport chain, the last of the process to produce ATP.
Adenosine triphosphate, if your cells are like businesses and consumers, the ATP is the money that drives the exchange.
