Energy for food process: According to estimates, a retail food product requires between 50 and 100 MJ (megajoules) of energy to produce and package each kilograms. Energy is needed in the food processing sector for power, heating, and cooling.
How Cells Obtain Energy from Food
As we just saw, for cells to create and sustain the biological order that keeps them alive, they need a steady flow of energy. Food molecules’ chemical bonds, which act as the fuel for cells, are the source of this energy.
Particularly significant fuel molecules include sugars, which are gradually converted to carbon dioxide (CO2) and water (Figure 2-69). In this part, we outline the key processes involved in the catabolism, or breakdown, of sugars and demonstrate how ATP, NADH, and other activated carrier molecules are created in animal cells as a result. Since it accounts for the majority of energy generation in most animal cells, we concentrate on glucose breakdown. Additionally, fungi, many bacteria, and plants all use a very similar mechanism. Various more compounds, such Proteins and fatty acids can also be used as energy sources if the proper enzymatic pathways are followed.
Food Molecules Are Broken Down in Three Stages to Produce ATP:
Before our cells can utilize the proteins, lipids, and polysaccharides that make up the majority of the food we consume—either as a source of energy or as building blocks for other molecules—they must be divided into smaller molecules. Food consumed from the outside must be broken down, but not the macromolecules found inside our own cells. Therefore, digestion is the first stage of the enzymatic breakdown of food molecules and takes place either outside of cells, in the gut, or inside of cells, in the lysosome, a specialized organelle. (A membrane around the lysosome prevents the digesting enzymes from mixing with the cytoplasm, In any case, during digestion, enzymes break down the huge polymeric molecules in food into their monomer subunits—proteins into amino acids, polysaccharides into sugars, and fats into fatty acids and glycerol. Small organic molecules from meals reach the cell’s cytosol after being digested to start the process of progressive oxidation. Figure shows two further phases of cellular catabolism in which oxidation takes place: stage 2 begins in the cytosol and concludes in the mitochondrion, the main organelle responsible for turning food into energy; stage 3 is exclusively restricted to the mitochondrion.
Each molecule of glucose is split into two smaller molecules of pyruvate during stage 2, a process known as glycolysis. When sugars other than glucose are transformed to one of the sugar intermediates in this glycolytic pathway, they are then similarly reduced to pyruvate. ATP and NADH are two different classes of active carrier molecules that are created during pyruvate synthesis. After that, the pyruvate moves from the cytosol into the mitochondria. Each pyruvate molecule there undergoes a transformation into CO2 and a two-carbon acetyl group, which is then joined to coenzyme A (CoA) to create acetyl CoA, another active carrier molecule . The sequential oxidation and breakdown of fatty acids formed from lipids, which are transported in the circulation and brought into cells as fatty acids, also produces significant quantities of acetyl CoA.
All of stage 3 of the oxidative disintegration of food molecules occurs in mitochondria. Since coenzyme A and acetyl CoA are connected by a high-energy bond, the acetyl group in acetyl CoA is readily transferred to other molecules. The acetyl group enters the citric acid cycle after being transferred to the four-carbon oxaloacetate molecule. In these processes, the acetyl group is oxidized to CO2, as we will explore momentarily, and significant quantities of the electron carrier NADH are produced.In the mitochondrial inner membrane, the high-energy electrons from NADH are then sent through an electron-transport chain, where the energy produced during their transmission is used to power a process that generates ATP and uses molecule oxygen (O2). The majority of the energy generated by oxidation is used in these last processes to make the majority of the cell’s ATP.
The phosphorylation of ADP to produce ATP that is fueled by electron transport in the mitochondrion is known as oxidative phosphorylation because the energy to drive ATP production in mitochondria ultimately originates from the oxidative breakdown of food molecules. Chapter 14 primarily focuses on the intriguing processes that take place within the mitochondrial inner membrane during oxidative phosphorylation. The energy released during the breakdown of carbohydrates and fats is redistributed as packets of chemical energy in a form that is useful for usage in other parts of the cell through the generation of ATP. A normal cell has around 109 ATP molecules in solution at any given time, and in many cells, all of this ATP is changed over (that is, used up and replenished) every one to two minutes.
The energetically unfavourable process Pi + ADP ATP is driven by approximately half of the energy that might, in theory, come from the oxidation of glucose or fatty acids to H2O and CO2. (In comparison, a normal combustion engine, like one in a car, can only convert up to 20% of the fuel’s potential energy into usable work.) Our bodies get heated as a result of the cell discharging the remaining energy as heat.
Glycolysis Is a Central ATP-producing Pathway:
The decomposition of glucose through the series of processes known as glycolysis—from the Greek glukus, “sweet,” and lusis, “rupture”—is the most significant step in stage 2 of the breakdown of food molecules. ATP is created during glycolysis without the help of molecule oxygen (O2 gas). Most cells, including many anaerobic bacteria (those that can survive without using molecular oxygen), include it in their cytoplasm. Prior to the introduction of oxygen into the atmosphere by photosynthetic organisms, glycolysis likely originated early in the history of life. A glucose molecule with six carbon atoms is split into two pyruvate molecules, each with three, during the glycolytic process.
Two molecules of ATP are hydrolyzed for every molecule of glucose to give energy for the early processes, whereas four molecules of ATP are created in the latter phases. As a result, towards the end of glycolysis, there is a net gain of two ATP molecules for each glucose molecule that was broken down.
Figure presents the glycolytic route in broad strokes. A series of 10 distinct processes, each of which produces a different sugar intermediate and is catalyzed by a different enzyme, make up glycolysis. These enzymes, like the majority of enzymes, all have names that end in ace, such as dehydrogenase and isomerase, which describe the sort of process they catalyze.
Although there is no molecular oxygen involved in glycolysis, oxidation does take place because some of the carbons released from the glucose molecule are subjected to electron removal by NAD+ (forming NADH). Since the process is stepwise, the energy of oxidation can be released in modest amounts, allowing for the storage of some of it in activated carrier molecules rather than its whole release as heat . As a result, part of the energy generated during oxidation is used to directly synthesize ATP molecules from ADP and Pi, while some of it is stored as electrons in the highly energetic electron transporter NADH.
In the process of glycolysis, two molecules of NADH are created for every molecule of glucose. These NADH molecules transfer their electrons to the electron-transport chain outlined in aerobic organisms, and the NAD+ produced from the NADH is then utilized once more for glycolysis.
Fermentations Allow ATP to Be Produced in the Absence of Oxygen:
Glycolysis is often just the first step in the third and final stage of the breakdown of food molecules in most animal and plant cells. The pyruvate produced in these cells at the last stage of stage 2 is swiftly carried into the mitochondria where it is transformed into CO2 plus acetyl CoA and fully oxidized to CO2 and H2O.
In contrast, glycolysis serves as the main source of ATP in the cells of many anaerobic species, which do not use molecular oxygen and can grow and divide without it. This is also true for some animal tissues, such skeletal muscle, which may continue to work even when there is a shortage of molecular oxygen. The NADH electrons and pyruvate remain in the cytosol under these anaerobic circumstances. Pyruvate is transformed into compounds that are expelled from the cell, such as lactate in muscle or ethanol and CO2 in yeasts used in brewing and breadmaking. The NADH surrenders its electrons during this phase, turning back into NAD+. To keep the glycolysis processes going, NAD+ must be replenished.
These types of anaerobic energy-producing pathways are known as fermentations. Early biochemistry was greatly influenced by studies of the commercially significant fermentations carried out by yeasts. The discovery that these processes could be investigated in cell extracts rather than living beings was a remarkable development in 1896 as a result of work done in the nineteenth century. Eventually, this ground-breaking finding allowed for the dissection and examination of each individual fermentation reaction. The discovery of the whole glycolytic pathway in the 1930s was a significant biochemical achievement, and it was soon followed by the identification of ATP’s crucial function in cellular functions. As a result, the majority of the fundamental ideas covered in this chapter have been known for more than 50 years.
Glycolysis Illustrates How Enzymes Couple Oxidation to Energy Storage:
We have previously used a “paddle wheel” analogy to illustrate how cells employ enzymes to connect an energetically unfavorable process to an energetically favorable one in order to obtain usable energy from the oxidation of organic molecules. We now return to a stage in glycolysis that we have previously covered to further show how coupled reactions take place. Enzymes serve as the paddle wheel in our example. The three-carbon sugar intermediate glyceraldehyde 3-phosphate (an aldehyde) is transformed into 3-phosphoglycerate (a carboxylic acid) through two key events in glycolysis (steps 6 and 7). This involves the two-step oxidation of an aldehyde group to a carboxylic acid group. While still releasing enough heat to the environment to make the overall reaction energetically favorable (G° for the overall reaction is -3.0 kcal/mole), the overall reaction releases enough free energy to convert a molecule of ADP to ATP and to transfer two electrons from the aldehyde to NAD+ to form NADH.
Figure 2-73 depicts the process through which this amazing achievement is performed. Two enzymes, to which the sugar intermediates are firmly linked, direct the chemical processes. Through a reactive -SH group on the enzyme, the first enzyme (glyceraldehyde 3-phosphate dehydrogenase) establishes a transient covalent link with the aldehyde and then catalysis its oxidation while still attached. An inorganic phosphate ion subsequently displaces the high-energy enzyme-substrate bond formed by the oxidation to generate a high-energy sugar-phosphate intermediate, which is then released from the enzyme. This intermediate then joins with the phosphoglycerate kinase, the second enzyme. In order to produce ATP and complete the reaction, this enzyme catalysis the energetically advantageous transfer of the recently formed high-energy phosphate to ADP.
Sugars and Fats Are Both Degraded to Acetyl CoA in Mitochondria:
We now discuss catabolism’s third step, which calls for a lot of molecule oxygen (O2 gas). Since abundant life-forms have been known to exist on Earth for 3.5 billion years and the Earth’s atmosphere is thought to have developed between one and two billion years ago, the use of O2 in the reactions we will discuss next is thought to be of relatively recent origin. Depicts a process for making ATP that does not require oxygen, suggesting that cousins of this beautiful pair of linked events may have first appeared relatively early in the evolution of life on Earth.
In aerobic metabolism, a massive trio of enzymes known as the pyruvate dehydrogenase complex quickly decarboxylates the pyruvate generated by glycolysis. A molecule of CO2 (waste product), a molecule of NADH, and acetyl CoA are the byproducts of pyruvate decarboxylation. The structure and method of action of the three-enzyme complex, which is found in the mitochondria of eukaryotic cells, are described.
Summary
By using regulated stepwise oxidation to break down glucose and other dietary components, chemical energy in the form of ATP and NADH is produced. The citric acid cycle, which takes place in the mitochondrial matrix, glycolysis, which takes place in the cytosol, and oxidative phosphorylation, which occurs on the inner mitochondrial membrane, are the three primary sets of processes that function in sequence, with the byproducts of one serving as the raw material for the next. In addition to serving as sources of metabolic energy, the intermediate products of glycolysis and the citric acid cycle are also employed to create a large number of the tiny molecules that serve as the building blocks for biosynthesis. Animal and plant cells store sugar molecules as glycogen and starch, respectively; both plants and animals also extensively employ lipids as a food storage. Together with the proteins that make up the majority of the dry mass of the cells we consume, these storage resources in turn serve as a significant source of food for us.
