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4.8: Anabolic and Catabolic Pathways - Biology

4.8: Anabolic and Catabolic Pathways - Biology



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Anabolic pathways require an input of energy to synthesize complex molecules from simpler ones. Synthesizing sugar from CO2 is one example. These biosynthetic processes are critical to the life of the cell, take place constantly, and demand energy provided by ATP and other high-energy molecules like NADH (nicotinamide adenine dinucleotide) and NADPH (Figure 1).

ATP is an important molecule for cells to have in sufficient supply at all times. The breakdown of sugars illustrates how a single molecule of glucose can store enough energy to make a great deal of ATP, 36 to 38 molecules. This is a catabolic pathway. Catabolic pathways involve the degradation (or breakdown) of complex molecules into simpler ones. Molecular energy stored in the bonds of complex molecules is released in catabolic pathways and harvested in such a way that it can be used to produce ATP. Other energy-storing molecules, such as fats, are also broken down through similar catabolic reactions to release energy and make ATP (Figure 1).

It is important to know that the chemical reactions of metabolic pathways don’t take place spontaneously. Each reaction step is facilitated, or catalyzed, by a protein called an enzyme. Enzymes are important for catalyzing all types of biological reactions—those that require energy as well as those that release energy.

Remember: Anabolic pathways assemble large molecules form smaller ones. Catabolic pathways break large molecules into small pieces.

Anabolism

Anabolism collectively refers to all the processes of chemical reactions that build larger molecules out of smaller molecules or atoms these processes are also known as anabolic processes or anabolic pathways. The opposite of anabolism is catabolism, the set of processes that breaks down larger molecules into smaller ones. Anabolism and catabolism are the two types of metabolic pathways. Metabolic pathways are series of chemical reactions that take place in the cell. Anabolic pathways use energy, while catabolic pathways release energy.


Metabolism of Carbohydrates

The metabolism of sugar (a simple carbohydrate) is a classic example of the many cellular processes that use and produce energy. Living things consume sugar as a major energy source, because sugar molecules have a great deal of energy stored within their bonds. The breakdown of glucose, a simple sugar, is described by the equation:

Carbohydrates that are consumed have their origins in photosynthesizing organisms like plants (Figure 2). During photosynthesis, plants use the energy of sunlight to convert carbon dioxide gas (CO2) into sugar molecules, like glucose (C6H12O6). Because this process involves synthesizing a larger, energy-storing molecule, it requires an input of energy to proceed. The synthesis of glucose is described by this equation (notice that it is the reverse of the previous equation):

During the chemical reactions of photosynthesis, energy is provided in the form of a very high-energy molecule called ATP, or adenosine triphosphate, which is the primary energy currency of all cells. Just as the dollar is used as currency to buy goods, cells use molecules of ATP as energy currency to perform immediate work. The sugar (glucose) is stored as starch or glycogen. Energy-storing polymers like these are broken down into glucose to supply molecules of ATP. Plant cells use solar energy, energy from the sun, to synthesize the ATP they need to power the reactions of photosynthesis.

Figure 2. Plants, like this oak tree and acorn, use energy from sunlight to make sugar and other organic molecules. Both plants and animals (like this squirrel) use cellular respiration to derive energy from the organic molecules originally produced by plants. (credit “acorn”: modification of work by Noel Reynolds credit “squirrel”: modification of work by Dawn Huczek)


Anabolic and Catabolic Processes

Anabolic processes use simple molecules within the organism to create more complex and specialized compounds. This synthesis, the creation of a product from a series of components, is why anabolism is also called "biosynthesis." The process uses energy to create its end products, which the organism can use to sustain itself, grow, heal, reproduce or adjust to changes in its environment. Growing in height and muscle mass are two basic anabolic processes. At the cellular level, anabolic processes can use small molecules called monomers to build polymers, resulting in often highly complex molecules. For example, amino acids (monomers) can be synthesized into proteins (polymers), much like a builder can use bricks to create a large variety of buildings.

Catabolic processes break down complex compounds and molecules to release energy. This creates the metabolic cycle, where anabolism then creates other molecules that catabolism breaks down, many of which remain in the organism to be used again.

The principal catabolic process is digestion, where nutrient substances are ingested and broken down into simpler components for the body to use. In cells, catabolic processes break down polysaccharides such as starch, glycogen, and cellulose into monosaccharides (glucose, ribose and fructose, for example) for energy. Proteins are broken down into amino acids, for use in anabolic synthesis of new compounds or for recycling. And nucleic acids, found in RNA and DNA, are catabolized into nucleotides as part of the body's energy needs or for the purpose of healing.

Hormones

Many of the metabolic processes in an organism are regulated by chemical compounds called hormones. In general, hormones can be classified as anabolic or catabolic based on their effect within the organism.

Anabolic hormones include:

  • Estrogen: Present in males as well as in females, estrogen is produced mainly in the ovaries. It regulates some female sexual characteristics (growth of breasts and hips), regulates the menstrual cycle, and plays a role in strengthening bone mass.
  • Testosterone: Present in females as well as males, testosterone is produced mainly in the testes. It regulates some male sexual characteristics (facial hair, voice), strengthens bones, and helps build and maintain muscle mass.
  • Insulin: Produced in the pancreas by beta cells, it regulates the blood level and use of glucose. The body cannot use glucose, a main source of energy, without insulin. When the pancreas cannot create insulin, or when the body struggles to process the insulin it makes, this leads to diabetes.
  • Growth hormone: Produced in the pituitary, growth hormone stimulates and regulates growth during the early stages of life. After maturity, it helps regulate bone repair.

Catabolic hormones include:

  • Adrenaline: Also called "epinephrine," adrenaline is produced by the adrenal glands. It is the key component of the "fight or flight" response that accelerates heart rate, opens up bronchioles in the lungs for better oxygen absorption and floods the body with glucose for fast energy.
  • Cortisol: Also produced in the adrenal glands, cortisol is known as the "stress hormone." It is released during times of anxiety, nervousness or when the organism feels prolonged discomfort. It increases blood pressure, blood sugar levels and suppresses the body's immune processes.
  • Glucagon: Produced by the alpha cells in the pancreas, glucagon stimulates the breakdown of glycogen into glucose. Glycogen is stored in the liver and when the body needs more energy (exercise, fighting, high level of stress), glucagon stimulates the liver to catabolize glycogen, which enters the blood as glucose.
  • Cytokines: This hormone is a small protein that regulates communication and interactions between cells. Cytokines are constantly being produced and broken down in the body, where their amino acids are either reused or recycled for other processes. Two examples of cytokines are interleukin and lymphokines, most often released during the body's immune response to invasion (bacteria, virus, fungus, tumor) or injury.

What Is the Difference Between Catabolic and Anabolic Pathways?

Catabolic pathways break down molecules to release energy, while anabolic pathways use energy to create new molecules. Both types of pathways are important parts of an organism's metabolism.

Cellular respiration is an important catabolic pathway necessary for the creation of ATP molecules. ATP is a molecule high in energy vital to the work done by cells. Cellular respiration involves the breakdown of glucose, a six-carbon sugar, into two three-carbon molecules. In the process, ATP molecules are produced that provide energy for further reactions in other systems.

The building of proteins utilizes some of the ATP produced through cellular respiration. Proteins are built through the linking of amino acids into chains called polypeptides. Each amino acid links to the chain using one ATP molecule. Bone-building cells use ATP in the production of calcium phosphate crystals. These crystals incorporate into the matrix of bones to strengthen them.

A fine balance exists between catabolism and anabolism in the body. The body must take in enough precursor molecules to be broken down by catabolism, so catabolic pathways can produce the energy required for the building and reparation needs of cells. Studies suggest these metabolic pathways are inextricably linked to human circadian rhythms, explains the Cell Press journal.


What is an example of a catabolic pathway?

Catabolic pathways involve the degradation of complex molecules into simpler ones, releasing the chemical energy stored in the bonds of those molecules. Some catabolic pathways can capture that energy to produce ATP, the molecule used to power all cellular processes.

Subsequently, question is, what is a biochemical pathway and give an example? A biochemical pathway (also called a metabolic pathway) is a series of enzyme-mediated reactions where the product of one reaction is used as the substrate in the next. Each enzymes is coded by a different gene. For instance, lets assume enzyme A is coded for by gene A. Similarly enzyme B is coded for by the gene B.

Keeping this in view, what is an example of anabolic pathway?

Anabolic pathways build complex molecules from simpler ones and typically need an input of energy. Building glucose from carbon dioxide is one example. Other examples include the synthesis of proteins from amino acids, or of DNA strands from nucleic acid building blocks (nucleotides).

What is an example of a metabolic pathway?

A good example of a metabolic pathway would be the cellular respiration equation where glucose is oxidized by oxygen to produce ATP, adenosine triphosphate. The ATP molecule is used by virtually all animal cells as the primary energy source for the cells life functions.


Catabolic and Anabolic Reactions

How do we differenciate between catabolic and anabolic reactions?

According to my researches

Catabolic Reactions

Catabolic reactions (also called “catabolism”) break down larger, more complex molecules into smaller molecules and release energy in the process. The smaller end products of a catabolic reaction may be released as waste or they may be fed into other reactions. The energy that is released by catabolic reactions can be captured and used in many ways. Some of the energy is released as heat and increases the temperature of the cell. Sometimes the energy is stored in the chemical bonds of another molecule. And sometimes it can be used to do work, such as movement of cellular machinery to power the active transport of materials across cell membranes. Catabolic reactions are central to biological processes such as cellular respiration and the digestion of food molecules.

Anabolic Reactions

Anabolic reactions (also referred to as "anabolism") use energy to build more complex molecules from relatively simple raw materials. “Anabolic” and “catabolic” sound similar but are opposites. To remember the difference, it may help to think about how “anabolic steroids” promote the buildup of muscle mass. All of the complex molecules of life — carbohydrates, lipids, proteins, nucleic acids — are generated by anabolic reactions. Anabolic reactions are central to processes like photosynthesis, protein synthesis, and DNA replication.


The processes of Catabolism and Anabolism

All anabolic processes are constructive, using basic molecules within an organism, which then create compounds that are more specialized and complex. Anabolism is also known as ‘biosynthesis’, whereby an end product is created from a number of components. The process requires ATP as a form of energy, converting kinetic energy into potential energy. It is considered an endergonic process, meaning it is a nonspontaneous reaction, that requires energy 2 . The process uses up energy to create the end product, such as tissues and organs. These complex molecules are required by the organism, as a means of growth, development and cell differentiation 3 . Anabolic processes do not use oxygen.

Catabolic processes on the other hand are destructive, where more complex compounds are broken down and energy is released in the form of ATP or heat – instead of consuming energy as in anabolism. Potential energy is converted into kinetic energy from stores in the body. This results in the formation of the metabolic cycle, whereby catabolism breaks down the molecules that are created through anabolism. An organism then often uses many of these molecules, which are used again in a variety of processes. Catabolic processes do utilize oxygen.

At a cellular level, anabolism uses monomers to form polymers, resulting in the formation of more complex molecules. A common example is the synthesis of amino acids (the monomer) into larger and more complex proteins (the polymer). One of the most common catabolic processes is digestion, where ingested nutrients are converted into more simple molecules, that an organism can then use for other processes.

Catabolic processes act to break down many different polysaccharides, such as glycogen, starches and cellulose. These are converted into monosaccharides, which include glucose, fructose and ribose, used by organisms as a form of energy. Proteins that are created by anabolism, are converted to amino acids through catabolism, for further anabolic processes. Any nucleic acids in DNA or RNA, become catabolized into smaller nucleotides, that are a component of the natural process of healing as well as used for energetic needs.

Organisms are classified on the basis of type of Catabolism they use 4 :

  • Organotroph An organism that acquires its energy from organic sources
  • Lithotroph → An organism that acquires its energy from inorganic substrates
  • Phototroph → An organism that acquires its energy from sunlight

27 Energy and Metabolism

By the end of this section, you will be able to do the following:

  • Explain metabolic pathways and describe the two major types
  • Discuss how chemical reactions play a role in energy transfer

Scientists use the term bioenergetics to discuss the concept of energy flow ((Figure)) through living systems, such as cells. Cellular processes such as building and breaking down complex molecules occur through stepwise chemical reactions. Some of these chemical reactions are spontaneous and release energy whereas, others require energy to proceed. Just as living things must continually consume food to replenish what they have used, cells must continually produce more energy to replenish that which the many energy-requiring chemical reactions that constantly take place use. All of the chemical reactions that transpire inside cells, including those that use and release energy, are the cell’s metabolism .

Carbohydrate Metabolism

Sugar (chemical reactions) metabolism (a simple carbohydrate) is a classic example of the many cellular processes that use and produce energy. Living things consume sugar as a major energy source, because sugar molecules have considerable energy stored within their bonds. The following equation describes the breakdown of glucose, a simple sugar:

Consumed carbohydrates have their origins in photosynthesizing organisms like plants ((Figure)). During photosynthesis, plants use the energy of sunlight to convert carbon dioxide gas (CO2) into sugar molecules, like glucose (C6H12O6). Because this process involves synthesizing a larger, energy-storing molecule, it requires an energy input to proceed. The following equation (notice that it is the reverse of the previous equation) describes the synthesis of glucose:

During photosynthesis chemical reactions, energy is in the form of a very high-energy molecule scientists call ATP, or adenosine triphosphate. This is the primary energy currency of all cells. Just as the dollar is the currency we use to buy goods, cells use ATP molecules as energy currency to perform immediate work. The sugar (glucose) is stored as starch or glycogen. Energy-storing polymers like these break down into glucose to supply ATP molecules.

Solar energy is required to synthesize a glucose molecule during the photosynthesis reactions. In photosynthesis, light energy from the sun initially transforms into chemical energy that temporally stores itself in the energy carrier molecules ATP and NADPH (nicotinamide adenine dinucleotide phosphate). Photosynthesis later uses the stored energy in ATP and NADPH to build one glucose molecule from six molecules of CO2. This process is analogous to eating breakfast in the morning to acquire energy for your body that you can use later in the day. Under ideal conditions, energy from 18 molecules of ATP is required to synthesize one glucose molecule during photosynthesis reactions. Glucose molecules can also combine with and convert into other sugar types. When an organism consumes sugars, glucose molecules eventually make their way into each organism’s living cell. Inside the cell, each sugar molecule breaks down through a complex series of chemical reactions. The goal of these reactions is to harvest the energy stored inside the sugar molecules. The harvested energy makes high-energy ATP molecules, which perform work, powering many chemical reactions in the cell. The amount of energy needed to make one glucose molecule from six carbon dioxide molecules is 18 ATP molecules and 12 NADPH molecules (each one of which is energetically equivalent to three ATP molecules), or a total of 54 molecule equivalents required for synthesizing one glucose molecule. This process is a fundamental and efficient way for cells to generate the molecular energy that they require.

Metabolic Pathways

The processes of making and breaking down sugar molecules illustrate two types of metabolic pathways. A metabolic pathway is a series of interconnected biochemical reactions that convert a substrate molecule or molecules, step-by-step, through a series of metabolic intermediates, eventually yielding a final product or products. In the case of sugar metabolism, the first metabolic pathway synthesized sugar from smaller molecules, and the other pathway broke sugar down into smaller molecules. Scientists call these two opposite processes—the first requiring energy and the second producing energy—anabolic (building) and catabolic (breaking down) pathways, respectively. Consequently, building (anabolism) and degradation (catabolism) comprise metabolism.

Evolution of Metabolic Pathways There is more to the complexity of metabolism than understanding the metabolic pathways alone. Metabolic complexity varies from organism to organism. Photosynthesis is the primary pathway in which photosynthetic organisms like plants (planktonic algae perform the majority of global synthesis) harvest the sun’s energy and convert it into carbohydrates. The by-product of photosynthesis is oxygen, which some cells require to carry out cellular respiration. During cellular respiration, oxygen aids in the catabolic breakdown of carbon compounds, like carbohydrates. Among the products are CO2 and ATP. In addition, some eukaryotes perform catabolic processes without oxygen (fermentation) that is, they perform or use anaerobic metabolism.

Organisms probably evolved anaerobic metabolism to survive (living organisms came into existence about 3.8 billion years ago, when the atmosphere lacked oxygen). Despite the differences between organisms and the complexity of metabolism, researchers have found that all branches of life share some of the same metabolic pathways, suggesting that all organisms evolved from the same ancient common ancestor ((Figure)). Evidence indicates that over time, the pathways diverged, adding specialized enzymes to allow organisms to better adapt to their environment, thus increasing their chance to survive. However, the underlying principle remains that all organisms must harvest energy from their environment and convert it to ATP to carry out cellular functions.

Anabolic and Catabolic Pathways

Anabolic pathways require an input of energy to synthesize complex molecules from simpler ones. Synthesizing sugar from CO2 is one example. Other examples are synthesizing large proteins from amino acid building blocks, and synthesizing new DNA strands from nucleic acid building blocks. These biosynthetic processes are critical to the cell’s life, take place constantly, and demand energy that ATP and other high-energy molecules like NADH (nicotinamide adenine dinucleotide) and NADPH provide ((Figure)).

ATP is an important molecule for cells to have in sufficient supply at all times. The breakdown of sugars illustrates how a single glucose molecule can store enough energy to make a great deal of ATP, 36 to 38 molecules. This is a catabolic pathway. Catabolic pathways involve degrading (or breaking down) complex molecules into simpler ones. Molecular energy stored in complex molecule bonds release in catabolic pathways and harvest in such a way that it can produce ATP. Other energy-storing molecules, such as fats, also break down through similar catabolic reactions to release energy and make ATP ((Figure)).

It is important to know that metabolic pathway chemical reactions do not take place spontaneously. A protein called an enzyme facilitates or catalyzes each reaction step. Enzymes are important for catalyzing all types of biological reactions—those that require energy as well as those that release energy.

Section Summary

Cells perform the functions of life through various chemical reactions. A cell’s metabolism refers to the chemical reactions that take place within it. There are metabolic reactions that involve breaking down complex chemicals into simpler ones, such as breaking down large macromolecules. Scientists refer to this process as catabolism, and we associate such reactions an energy release. On the other end of the spectrum, anabolism refers to metabolic processes that build complex molecules out of simpler ones, such as macromolecule synthesis. Anabolic processes require energy. Glucose synthesis and glucose breakdown are examples of anabolic and catabolic pathways, respectively.

Multiple Choice

Energy is stored long-term in the bonds of _____ and used short-term to perform work from a(n) _____ molecule.

  1. ATP : glucose
  2. an anabolic molecule : catabolic molecule
  3. glucose : ATP
  4. a catabolic molecule : anabolic molecule

DNA replication involves unwinding two strands of parent DNA, copying each strand to synthesize complementary strands, and releasing the parent and daughter DNA. Which of the following accurately describes this process?

  1. This is an anabolic process.
  2. This is a catabolic process.
  3. This is both anabolic and catabolic.
  4. This is a metabolic process but is neither anabolic nor catabolic.

Critical Thinking Questions

Does physical exercise involve anabolic and/or catabolic processes? Give evidence for your answer.

Physical exercise involves both anabolic and catabolic processes. Body cells break down sugars to provide ATP to do the work necessary for exercise, such as muscle contractions. This is catabolism. Muscle cells also must repair muscle tissue damaged by exercise by building new muscle. This is anabolism.

Name two different cellular functions that require energy that parallel human energy-requiring functions.

Energy is required for cellular motion, through beating of cilia or flagella, as well as human motion, produced by muscle contraction. Cells also need energy to perform digestion, as humans require energy to digest food.

Glossary


Electron Transport Chain

Specific enzymes of the electron transport chain are unaffected by feedback inhibition, but the rate of electron transport through the pathway is affected by the levels of ADP and ATP. Greater ATP consumption by a cell is indicated by a buildup of ADP. As ATP usage decreases, the concentration of ADP decreases, and now, ATP begins to build up in the cell. This change is the relative concentration of ADP to ATP triggers the cell to slow down the electron transport chain.

For a summary of feedback controls in cellular respiration, see the table below.


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