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10.3: Carbohydrate Metabolism - Biology

10.3: Carbohydrate Metabolism - Biology


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Learning Objectives

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

  • Explain the processes of glycolysis
  • Describe the pathway of a pyruvate molecule through the Krebs cycle
  • Explain the transport of electrons through the electron transport chain
  • Describe the process of ATP production through oxidative phosphorylation
  • Summarize the process of gluconeogenesis

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms. The family of carbohydrates includes both simple and complex sugars. Glucose and fructose are examples of simple sugars, and starch, glycogen, and cellulose are all examples of complex sugars. The complex sugars are also called polysaccharides and are made of multiple monosaccharide molecules. Polysaccharides serve as energy storage (e.g., starch and glycogen) and as structural components (e.g., chitin in insects and cellulose in plants).

During digestion, carbohydrates are broken down into simple, soluble sugars that can be transported across the intestinal wall into the circulatory system to be transported throughout the body. Carbohydrate digestion begins in the mouth with the action of salivary amylase on starches and ends with monosaccharides being absorbed across the epithelium of the small intestine. Once the absorbed monosaccharides are transported to the tissues, the process of cellular respiration begins (Figure 1). This section will focus first on glycolysis, a process where the monosaccharide glucose is oxidized, releasing the energy stored in its bonds to produce ATP.

Glycolysis

Glucose is the body’s most readily available source of energy. After digestive processes break polysaccharides down into monosaccharides, including glucose, the monosaccharides are transported across the wall of the small intestine and into the circulatory system, which transports them to the liver. In the liver, hepatocytes either pass the glucose on through the circulatory system or store excess glucose as glycogen. Cells in the body take up the circulating glucose in response to insulin and, through a series of reactions called glycolysis, transfer some of the energy in glucose to ADP to form ATP (Figure 2). The last step in glycolysis produces the product pyruvate.

Glycolysis begins with the phosphorylation of glucose by hexokinase to form glucose-6-phosphate. This step uses one ATP, which is the donor of the phosphate group. Under the action of phosphofructokinase, glucose-6-phosphate is converted into fructose-6-phosphate. At this point, a second ATP donates its phosphate group, forming fructose-1,6-bisphosphate. This six-carbon sugar is split to form two phosphorylated three-carbon molecules, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, which are both converted into glyceraldehyde-3-phosphate. The glyceraldehyde-3-phosphate is further phosphorylated with groups donated by dihydrogen phosphate present in the cell to form the three-carbon molecule 1,3-bisphosphoglycerate. The energy of this reaction comes from the oxidation of (removal of electrons from) glyceraldehyde-3-phosphate. In a series of reactions leading to pyruvate, the two phosphate groups are then transferred to two ADPs to form two ATPs. Thus, glycolysis uses two ATPs but generates four ATPs, yielding a net gain of two ATPs and two molecules of pyruvate. In the presence of oxygen, pyruvate continues on to the Krebs cycle (also called the citric acid cycle or tricarboxylic acid cycle (TCA), where additional energy is extracted and passed on.

Watch this video to learn about glycolysis:

A YouTube element has been excluded from this version of the text. You can view it online here: pb.libretexts.org/aapii/?p=166

Glycolysis can be divided into two phases: energy consuming (also called chemical priming) and energy yielding. The first phase is the energy-consuming phase, so it requires two ATP molecules to start the reaction for each molecule of glucose. However, the end of the reaction produces four ATPs, resulting in a net gain of two ATP energy molecules.

Glycolysis can be expressed as the following equation:

This equation states that glucose, in combination with ATP (the energy source), NAD+ (a coenzyme that serves as an electron acceptor), and inorganic phosphate, breaks down into two pyruvate molecules, generating four ATP molecules—for a net yield of two ATP—and two energy-containing NADH coenzymes. The NADH that is produced in this process will be used later to produce ATP in the mitochondria. Importantly, by the end of this process, one glucose molecule generates two pyruvate molecules, two high-energy ATP molecules, and two electron-carrying NADH molecules.

The following discussions of glycolysis include the enzymes responsible for the reactions. When glucose enters a cell, the enzyme hexokinase (or glucokinase, in the liver) rapidly adds a phosphate to convert it into glucose-6-phosphate. A kinase is a type of enzyme that adds a phosphate molecule to a substrate (in this case, glucose, but it can be true of other molecules also). This conversion step requires one ATP and essentially traps the glucose in the cell, preventing it from passing back through the plasma membrane, thus allowing glycolysis to proceed. It also functions to maintain a concentration gradient with higher glucose levels in the blood than in the tissues. By establishing this concentration gradient, the glucose in the blood will be able to flow from an area of high concentration (the blood) into an area of low concentration (the tissues) to be either used or stored. Hexokinase is found in nearly every tissue in the body. Glucokinase, on the other hand, is expressed in tissues that are active when blood glucose levels are high, such as the liver. Hexokinase has a higher affinity for glucose than glucokinase and therefore is able to convert glucose at a faster rate than glucokinase. This is important when levels of glucose are very low in the body, as it allows glucose to travel preferentially to those tissues that require it more.

In the next step of the first phase of glycolysis, the enzyme glucose-6-phosphate isomerase converts glucose-6-phosphate into fructose-6-phosphate. Like glucose, fructose is also a six carbon-containing sugar. The enzyme phosphofructokinase-1 then adds one more phosphate to convert fructose-6-phosphate into fructose-1-6-bisphosphate, another six-carbon sugar, using another ATP molecule. Aldolase then breaks down this fructose-1-6-bisphosphate into two three-carbon molecules, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. The triosephosphate isomerase enzyme then converts dihydroxyacetone phosphate into a second glyceraldehyde-3-phosphate molecule. Therefore, by the end of this chemical- priming or energy-consuming phase, one glucose molecule is broken down into two glyceraldehyde-3-phosphate molecules.

The second phase of glycolysis, the energy-yielding phase, creates the energy that is the product of glycolysis. Glyceraldehyde-3-phosphate dehydrogenase converts each three-carbon glyceraldehyde-3-phosphate produced during the

energy-consuming phase into 1,3-bisphosphoglycerate. This reaction releases an electron that is then picked up by NAD+ to create an NADH molecule. NADH is a high-energy molecule, like ATP, but unlike ATP, it is not used as energy currency by the cell. Because there are two glyceraldehyde-3-phosphate molecules, two NADH molecules are synthesized during this step. Each 1,3-bisphosphoglycerate is subsequently dephosphorylated (i.e., a phosphate is removed) by phosphoglycerate kinase into 3-phosphoglycerate. Each phosphate released in this reaction can convert one molecule of ADP into one high- energy ATP molecule, resulting in a gain of two ATP molecules.

The enzyme phosphoglycerate mutase then converts the 3-phosphoglycerate molecules into 2-phosphoglycerate. The enolase enzyme then acts upon the 2-phosphoglycerate molecules to convert them into phosphoenolpyruvate molecules. The last step of glycolysis involves the dephosphorylation of the two phosphoenolpyruvate molecules by pyruvate kinase to create two pyruvate molecules and two ATP molecules.

In summary, one glucose molecule breaks down into two pyruvate molecules, and creates two net ATP molecules and two NADH molecules by glycolysis. Therefore, glycolysis generates energy for the cell and creates pyruvate molecules that can be processed further through the aerobic Krebs cycle (also called the citric acid cycle or tricarboxylic acid cycle); converted into lactic acid or alcohol (in yeast) by fermentation; or used later for the synthesis of glucose through gluconeogenesis.

Anaerobic Respiration

When oxygen is limited or absent, pyruvate enters an anaerobic pathway. In these reactions, pyruvate can be converted into lactic acid. In addition to generating an additional ATP, this pathway serves to keep the pyruvate concentration low so glycolysis continues, and it oxidizes NADH into the NAD+ needed by glycolysis. In this reaction, lactic acid replaces oxygen as the final electron acceptor. Anaerobic respiration occurs in most cells of the body when oxygen is limited or mitochondria are absent or nonfunctional. For example, because erythrocytes (red blood cells) lack mitochondria, they must produce their ATP from anaerobic respiration. This is an effective pathway of ATP production for short periods of time, ranging from seconds to a few minutes. The lactic acid produced diffuses into the plasma and is carried to the liver, where it is converted back into pyruvate or glucose via the Cori cycle. Similarly, when a person exercises, muscles use ATP faster than oxygen can be delivered to them. They depend on glycolysis and lactic acid production for rapid ATP production.

Aerobic Respiration

In the presence of oxygen, pyruvate can enter the Krebs cycle where additional energy is extracted as electrons are transferred from the pyruvate to the receptors NAD+, GDP, and FAD, with carbon dioxide being a “waste product” (Figure 3). The NADH and FADH2 pass electrons on to the electron transport chain, which uses the transferred energy to produce ATP. As the terminal step in the electron transport chain, oxygen is the terminal electron acceptor and creates water inside the mitochondria.

Figure 3. Click to view a larger image. The process of anaerobic respiration converts glucose into two lactate molecules in the absence of oxygen or within erythrocytes that lack mitochondria. During aerobic respiration, glucose is oxidized into two pyruvate molecules.

Krebs Cycle/Citric Acid Cycle/Tricarboxylic Acid Cycle

The pyruvate molecules generated during glycolysis are transported across the mitochondrial membrane into the inner mitochondrial matrix, where they are metabolized by enzymes in a pathway called the Krebs cycle (Figure 4). The Krebs cycle is also commonly called the citric acid cycle or the tricarboxylic acid (TCA) cycle. During the Krebs cycle, high-energy molecules, including ATP, NADH, and FADH2, are created. NADH and FADH2 then pass electrons through the electron transport chain in the mitochondria to generate more ATP molecules.

Watch this animation to observe the Krebs cycle.

A YouTube element has been excluded from this version of the text. You can view it online here: pb.libretexts.org/aapii/?p=166

The three-carbon pyruvate molecule generated during glycolysis moves from the cytoplasm into the mitochondrial matrix, where it is converted by the enzyme pyruvate dehydrogenase into a two-carbon acetyl coenzyme A (acetyl CoA) molecule. This reaction is an oxidative decarboxylation reaction. It converts the three-carbon pyruvate into a two-carbon acetyl CoA molecule, releasing carbon dioxide and transferring two electrons that combine with NAD+ to form NADH. Acetyl CoA enters the Krebs cycle by combining with a four-carbon molecule, oxaloacetate, to form the six-carbon molecule citrate, or citric acid, at the same time releasing the coenzyme A molecule.

The six-carbon citrate molecule is systematically converted to a five-carbon molecule and then a four-carbon molecule, ending with oxaloacetate, the beginning of the cycle. Along the way, each citrate molecule will produce one ATP, one FADH2, and three NADH. The FADH2 and NADH will enter the oxidative phosphorylation system located in the inner mitochondrial membrane. In addition, the Krebs cycle supplies the starting materials to process and break down proteins and fats.

To start the Krebs cycle, citrate synthase combines acetyl CoA and oxaloacetate to form a six-carbon citrate molecule; CoA is subsequently released and can combine with another pyruvate molecule to begin the cycle again. The aconitase enzyme converts citrate into isocitrate. In two successive steps of oxidative decarboxylation, two molecules of CO2 and two NADH molecules are produced when isocitrate dehydrogenase converts isocitrate into the five-carbon α-ketoglutarate, which is then catalyzed and converted into the four-carbon succinyl CoA by α-ketoglutarate dehydrogenase. The enzyme succinyl CoA dehydrogenase then converts succinyl CoA into succinate and forms the high-energy molecule GTP, which transfers its energy to ADP to produce ATP. Succinate dehydrogenase then converts succinate into fumarate, forming a molecule of FADH2. Fumarase then converts fumarate into malate, which malate dehydrogenase then converts back into oxaloacetate while reducing NAD+ to NADH. Oxaloacetate is then ready to combine with the next acetyl CoA to start the Krebs cycle again (see Figure 4). For each turn of the cycle, three NADH, one ATP (through GTP), and one FADH2 are created. Each carbon of pyruvate is converted into CO2, which is released as a byproduct of oxidative (aerobic) respiration.

Oxidative Phosphorylation and the Electron Transport Chain

The electron transport chain (ETC) uses the NADH and FADH2 produced by the Krebs cycle to generate ATP. Electrons from NADH and FADH2 are transferred through protein complexes embedded in the inner mitochondrial membrane by a series of enzymatic reactions. The electron transport chain consists of a series of four enzyme complexes (Complex I – Complex IV) and two coenzymes (ubiquinone and Cytochrome c), which act as electron carriers and proton pumps used to transfer H+ ions into the space between the inner and outer mitochondrial membranes (Figure 5). The ETC couples the transfer of electrons between a donor (like NADH) and an electron acceptor (like O2) with the transfer of protons (H+ ions) across the inner mitochondrial membrane, enabling the process of oxidative phosphorylation. In the presence of oxygen, energy is passed, stepwise, through the electron carriers to collect gradually the energy needed to attach a phosphate to ADP and produce ATP. The role of molecular oxygen, O2, is as the terminal electron acceptor for the ETC. This means that once the electrons have passed through the entire ETC, they must be passed to another, separate molecule. These electrons, O2, and H+ ions from the matrix combine to form new water molecules. This is the basis for your need to breathe in oxygen. Without oxygen, electron flow through the ETC ceases.

Watch this video to learn about the electron transport chain.

A YouTube element has been excluded from this version of the text. You can view it online here: pb.libretexts.org/aapii/?p=166

The electrons released from NADH and FADH2 are passed along the chain by each of the carriers, which are reduced when they receive the electron and oxidized when passing it on to the next carrier. Each of these reactions releases a small amount

of energy, which is used to pump H+ ions across the inner membrane. The accumulation of these protons in the space between the membranes creates a proton gradient with respect to the mitochondrial matrix.

Also embedded in the inner mitochondrial membrane is an amazing protein pore complex called ATP synthase. Effectively, it is a turbine that is powered by the flow of H+ ions across the inner membrane down a gradient and into the mitochondrial matrix. As the H+ ions traverse the complex, the shaft of the complex rotates. This rotation enables other portions of ATP synthase to encourage ADP and Pi to create ATP. In accounting for the total number of ATP produced per glucose molecule through aerobic respiration, it is important to remember the following points:

  • A net of two ATP are produced through glycolysis (four produced and two consumed during the energy-consuming stage). However, these two ATP are used for transporting the NADH produced during glycolysis from the cytoplasm into the mitochondria. Therefore, the net production of ATP during glycolysis is zero.
  • In all phases after glycolysis, the number of ATP, NADH, and FADH2 produced must be multiplied by two to reflect how each glucose molecule produces two pyruvate molecules.
  • In the ETC, about three ATP are produced for every oxidized NADH. However, only about two ATP are produced for every oxidized FADH2. The electrons from FADH2 produce less ATP, because they start at a lower point in the ETC (Complex II) compared to the electrons from NADH (Complex I) (Figure 5).

Therefore, for every glucose molecule that enters aerobic respiration, a net total of 36 ATPs are produced (see Figure 6).

Gluconeogenesis

Gluconeogenesis is the synthesis of new glucose molecules from pyruvate, lactate, glycerol, or the amino acids alanine or glutamine. This process takes place primarily in the liver during periods of low glucose, that is, under conditions of fasting, starvation, and low carbohydrate diets. So, the question can be raised as to why the body would create something it has just spent a fair amount of effort to break down? Certain key organs, including the brain, can use only glucose as an energy source; therefore, it is essential that the body maintain a minimum blood glucose concentration. When the blood glucose concentration falls below that certain point, new glucose is synthesized by the liver to raise the blood concentration to normal.

Gluconeogenesis is not simply the reverse of glycolysis. There are some important differences (Figure 7). Pyruvate is a common starting material for gluconeogenesis. First, the pyruvate is converted into oxaloacetate. Oxaloacetate then serves as a substrate for the enzyme phosphoenolpyruvate carboxykinase (PEPCK), which transforms oxaloacetate into phosphoenolpyruvate (PEP). From this step, gluconeogenesis is nearly the reverse of glycolysis. PEP is converted back into 2-phosphoglycerate, which is converted into 3-phosphoglycerate. Then, 3-phosphoglycerate is converted into 1,3 bisphosphoglycerate and then into glyceraldehyde-3-phosphate. Two molecules of glyceraldehyde-3-phosphate then combine to form fructose-1-6-bisphosphate, which is converted into fructose 6-phosphate and then into glucose-6-phosphate. Finally, a series of reactions generates glucose itself. In gluconeogenesis (as compared to glycolysis), the enzyme hexokinase is replaced by glucose-6-phosphatase, and the enzyme phosphofructokinase-1 is replaced by fructose-1,6-bisphosphatase. This helps the cell to regulate glycolysis and gluconeogenesis independently of each other.

As will be discussed as part of lipolysis, fats can be broken down into glycerol, which can be phosphorylated to form dihydroxyacetone phosphate or DHAP. DHAP can either enter the glycolytic pathway or be used by the liver as a substrate for gluconeogenesis.

Figure 7. Gluconeogenesis is the synthesis of glucose from pyruvate, lactate, glycerol, alanine, or glutamate.

Aging and the Body’s Metabolic Rate

The human body’s metabolic rate decreases nearly 2 percent per decade after age 30. Changes in body composition, including reduced lean muscle mass, are mostly responsible for this decrease. The most dramatic loss of muscle mass, and consequential decline in metabolic rate, occurs between 50 and 70 years of age. Loss of muscle mass is the equivalent of reduced strength, which tends to inhibit seniors from engaging in sufficient physical activity. This results in a positive-feedback system where the reduced physical activity leads to even more muscle loss, further reducing metabolism.

There are several things that can be done to help prevent general declines in metabolism and to fight back against the cyclic nature of these declines. These include eating breakfast, eating small meals frequently, consuming plenty of lean protein, drinking water to remain hydrated, exercising (including strength training), and getting enough sleep. These measures can help keep energy levels from dropping and curb the urge for increased calorie consumption from excessive snacking. While these strategies are not guaranteed to maintain metabolism, they do help prevent muscle loss and may increase energy levels. Some experts also suggest avoiding sugar, which can lead to excess fat storage. Spicy foods and green tea might also be beneficial. Because stress activates cortisol release, and cortisol slows metabolism, avoiding stress, or at least practicing relaxation techniques, can also help.

Chapter Review

Metabolic enzymes catalyze catabolic reactions that break down carbohydrates contained in food. The energy released is used to power the cells and systems that make up your body. Excess or unutilized energy is stored as fat or glycogen for later use. Carbohydrate metabolism begins in the mouth, where the enzyme salivary amylase begins to break down complex sugars into monosaccharides. These can then be transported across the intestinal membrane into the bloodstream and then to body tissues. In the cells, glucose, a six-carbon sugar, is processed through a sequence of reactions into smaller sugars, and the energy stored inside the molecule is released. The first step of carbohydrate catabolism is glycolysis, which produces pyruvate, NADH, and ATP. Under anaerobic conditions, the pyruvate can be converted into lactate to keep glycolysis working. Under aerobic conditions, pyruvate enters the Krebs cycle, also called the citric acid cycle or tricarboxylic acid cycle. In addition to ATP, the Krebs cycle produces high-energy FADH2 and NADH molecules, which provide electrons to the oxidative phosphorylation process that generates more high-energy ATP molecules. For each molecule of glucose that is processed in glycolysis, a net of 36 ATPs can be created by aerobic respiration.

Under anaerobic conditions, ATP production is limited to those generated by glycolysis. While a total of four ATPs are produced by glycolysis, two are needed to begin glycolysis, so there is a net yield of two ATP molecules.

In conditions of low glucose, such as fasting, starvation, or low carbohydrate diets, glucose can be synthesized from lactate, pyruvate, glycerol, alanine, or glutamate. This process, called gluconeogenesis, is almost the reverse of glycolysis and serves to create glucose molecules for glucose-dependent organs, such as the brain, when glucose levels fall below normal.

Self Check

Answer the question(s) below to see how well you understand the topics covered in the previous section.

Critical Thinking Questions

  1. Explain how glucose is metabolized to yield ATP.
  2. Discuss the mechanism cells employ to create a concentration gradient to ensure continual uptake of glucose from the bloodstream.

[reveal-answer q=”294369″]Show Answers[/reveal-answer]
[hidden-answer a=”294369″]

  1. Glucose is oxidized during glycolysis, creating pyruvate, which is processed through the Krebs cycle to produce NADH, FADH2, ATP, and CO2. The FADH2 and NADH yield ATP.
  2. Upon entry into the cell, hexokinase or glucokinase phosphorylates glucose, converting it into glucose-6-phosphate. In this form, glucose-6-phosphate is trapped in the cell. Because all of the glucose has been phosphorylated, new glucose molecules can be transported into the cell according to its concentration gradient.

[/hidden-answer]

Glossary

polysaccharides: complex carbohydrates made up of many monosaccharides

monosaccharide: smallest, monomeric sugar molecule

salivary amylase: digestive enzyme that is found in the saliva and begins the digestion of carbohydrates in the mouth

cellular respiration: production of ATP from glucose oxidation via glycolysis, the Krebs cycle, and oxidative phosphorylation

glycolysis: series of metabolic reactions that breaks down glucose into pyruvate and produces ATP

pyruvate: three-carbon end product of glycolysis and starting material that is converted into acetyl CoA that enters the

Krebs cycle: also called the citric acid cycle or the tricarboxylic acid cycle, converts pyruvate into CO2 and high-energy FADH2, NADH, and ATP molecules

citric acid cycle or tricarboxylic acid cycle (TCA): also called the Krebs cycle or the tricarboxylic acid cycle; converts pyruvate into CO2 and high-energy FADH2, NADH, and ATP molecules

energy-consuming phase, first phase of glycolysis, in which two molecules of ATP are necessary to start the reaction

glucose-6-phosphate: phosphorylated glucose produced in the first step of glycolysis

Hexokinase: cellular enzyme, found in most tissues, that converts glucose into glucose-6-phosphate upon uptake into the cell

Glucokinase: cellularenzyme, found in the liver, which converts glucose into glucose-6-phosphate upon uptake into the cell

energy-yielding phase: second phase of glycolysis, during which energy is produced

terminal electron acceptor: ATP production pathway in which electrons are passed through a series of oxidation-reduction reactions that forms water and produces a proton gradient

Krebs cycle: also called the citric acid cycle or the tricarboxylic acid cycle, converts pyruvate into CO2 and high-energy FADH2, NADH, and ATP molecules

electron transport chain (ETC): ATP production pathway in which electrons are passed through a series of oxidation-reduction reactions that forms water and produces a proton gradient

oxidative phosphorylation: process that converts high-energy NADH and FADH2 into ATP

ATP synthase protein: pore complex that creates ATP

Gluconeogenesis: process of glucose synthesis from pyruvate or other molecules


A quick look at biochemistry: carbohydrate metabolism

In mammals, there are different metabolic pathways in cells that break down fuel molecules to transfer their energy into high energy compounds such as adenosine-5'-triphosphate (ATP), guanosine-5'-triphosphate (GTP), reduced nicotinamide adenine dinucleotide (NADH2), reduced flavin adenine dinucleotide (FADH2) and reduced nicotinamide adenine dinucleotide phosphate (NADPH2). This process is called cellular respiration. In carbohydrate metabolism, the breakdown starts from digestion of food in the gastrointestinal tract and is followed by absorption of carbohydrate components by the enterocytes in the form of monosaccharides. Monosaccharides are transferred to cells for aerobic and anaerobic respiration via glycolysis, citric acid cycle and pentose phosphate pathway to be used in the starvation state. In the normal state, the skeletal muscle and liver cells store monosaccharides in the form of glycogen. In the obesity state, the extra glucose is converted to triglycerides via lipogenesis and is stored in the lipid droplets of adipocytes. In the lipotoxicity state, the lipid droplets of other tissues such as the liver, skeletal muscle and pancreatic beta cells also accumulate triacylglycerol. This event is the axis of the pathogenesis of metabolic dysregulation in insulin resistance, metabolic syndrome and type 2 diabetes. In this paper a summary of the metabolism of carbohydrates is presented in a way that researchers can follow the biochemical processes easily.

Keywords: Carbohydrate Gluconeogenesis Glycogenesis Glycogenolysis Glycolysis Oxidative pathway Pentose phosphate pathway Pyruvate decarboxylation.

Copyright © 2013 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.


10.3: Carbohydrate Metabolism - Biology

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Carbohydrate digestion

Dietary carbohydrates of greatest importance are composed of hexoses such as sucrose (saccharose or table sugar), lactose (milk sugar), galactose (derived from fermented products) and maltose (derived from hydrolysis of starch) and also pentoses such as xylose and arabinose (from fruits) [2]. Food digestion starts in the mouth through secretion of salivary alpha-amylase (or ptyalin) that hydrolyses alpha-1,4 (α-1,4) linkage of starch (or amylum) and converts it to maltose. The next enzyme is


Results

Aerobic Exercise Intensity Determines Relative Usage of Fat and Carbohydrate as Fuel Substrates During Running

Working muscles consume a mixture of metabolic substrates, and the relative contributions of fat and carbohydrate to this mixture dynamically depend on exercise intensity and the size of available glycogen reservoirs: Carbohydrates account for a greater proportion at higher intensities, while fat accounts for a greater proportion as available glycogen is depleted [24]. These trends reflect the significantly greater efficiency of carbohydrate relative to fat as a fuel for aerobic exercise, as discussed in greater detail in the Methods section: Carbohydrate oxidation typically generates approximately per mole of respired oxygen, whereas fatty acid oxidation typically generates only approximately per mole of oxygen. As a consequence, total carbohydrate consumption over the course of a marathon, and therefore the crucial question of whether the body can store enough carbohydrate fuel to complete the race, depends not only on the distance to be run but also on the intensity at which the race is run. Moreover, the rate at which ATP can be generated through physiologic processes depends on both the fuel substrate and the reaction end product (carbon dioxide in aerobic processes, regardless of the substrate lactate or creatine in anaerobic glycolysis or hydrolysis of phosphocreatine, respectively). More precisely, there is a hierarchy of metabolic processes, defined by the rate at which ATP can be produced to power muscle contractions: The anaerobic processes, hydrolysis of phosphocreatine and conversion of glycogen to lactate, produce at most and , respectively by contrast, the aerobic processes, which involve the complete oxidation of muscle glycogen, liver glycogen, or adipose-tissue-derived fatty acids, produce at most , or , respectively [25]. The maximal rate of ATP extraction tends to decrease as the size of the fuel reservoir increases.

Such considerations of substrate and efficiency underscore the importance of adequate carbohydrate reserves for endurance runners. Low glycogen and plasma glucose levels during exercise lead to an elevated ratio of glucagon to insulin, promoting lipolysis and the release of fatty acids from adipose tissue. In active muscle, fatty acids undergo -oxidation to acetyl CoA and eventually carbon dioxide. The resulting elevated levels of acetyl CoA partially suppress carbohydrate metabolism, reducing the flux of pyruvate into the citric acid cycle by inhibiting the conversion of pyruvate to acetyl CoA [25]. This biochemical feedback network forestalls complete glycogen depletion, but simultaneously decreases the energy efficiency of oxygen utilization.

The work of Romijn and colleagues [24] has made it possible to estimate the composition of the metabolic mixture consumed during exercise as a function of exercise intensity, as discussed in the Methods section: Figure 1 shows fractional usage of carbohydrate (plasma glucose plus muscle glycogen, ) and fat (plasma free fatty acids plus muscle triglycerides, ) as functions of relative exercise intensity, . These functions and the stoichiometry of muscle oxygen metabolism, reflected in the parameters and , permit the expression of in terms of power output as in Equation 1, derived in the Methods section.


Carbohydrate Metabolism Basic Overview:

1. Glycolysis:

During exercise, hormonal levels shift and this disruption of homeostasis alters the metabolism of glucose and other energy-bearing molecules. The breakdown of glucose to provide energy begins with glycolysis. To begin with, glucose enters the cytosol of the cell, or the fluid inside the cell not including cellular organelles.

Next, glucose is converted into two, three-carbon molecules of pyruvate through a series of ten different reactions.

  • A specific enzyme catalyzes each reaction along the way and a total of two ATP are generated per glucose molecule.
  • Since ADP is converted to ATP during the breakdown of the substrate glucose, the process is known as substrate-level phosphorylation.
  • During the sixth reaction, glyceraldehyde 3-phosphate is oxidized to 1,3 bisphosphoglycerate while reducing nicotinamide adenosine dinucleotide (NAD) to NADH, the reduced form of the compound.
  • NADH is then shuttled to the mitochondria of the cell where it is used in the electron transport chain to generate ATP via oxidative phosphorylation.
  • The most important enzyme in glycolysis is called phosphofructokinase (PFK)and catalyzes the third reaction in the sequence. Since this reaction is so favorable under physiologic conditions, it is known as the “committed step” in glycolysis. In other words, glucose will be completely degraded to pyruvate after this reaction has taken place.
  • With this in mind, PFK seems as if it would be an excellent site of control for glucose metabolism. In fact, this is exactly the case.

When ATP or energy is plentiful in the cell, PFK is inhibited and the breakdown of glucose for energy slows down.Therefore, PFK can regulate the degradation of glucose to match the energy needs of the cell. This type of regulation is a recurring theme in biochemistry .

2.Krebs Cycle:

Kreb’s Cycle is the central metabolic cycle of the Carbohydrate metabolism and all metabolic pathways. There are many compounds that are formed and recycled during the Krebs Cycle (Citirc Acid Cycle). These include oxidized forms of Nictotinamide adenine dinucleotide (NAD+) and Flavin adenine dinucleotide (FAD) and their reduced counterparts: NADH and FADH2. NAD+ and FAD are electron acceptors and become reduced while the substrates in the Krebs Cycle become oxidized and surrender their electrons.

The Krebs Cycle begins when the pyruvate formed in the cytoplasm of the cell during glycolysis is transferred to the mitochondria, where most of the energy inherent in glucose is extracted. In the mitochondria, pyruvate is converted to acetyl CoA by the enzyme pyruvate carboxlase.

In general, Acetyl-CoA condenses with a four carbon compound called oxaloacetate to form a six carbon acid. This six-carbon compound is degraded to a five and four carbon compound, releasing two molecules of carbon dioxide. At the same time, two molecules of NADH are formed.

Finally, the C-4 carbon skeleton undergoes three additional reactions in which guanosine triphosphate (GTP), FADH2 and NADH are formed, thereby regenerating oxaloacetate. FADH2 and NADH are passed on to the electron transport chain (see below) that is embedded in the inner mitochondria membrane.

3. Oxidative Phosphorylation / Electron Transport Chain:

GTP is a high-energy compound that is used to regenerate ATP from ADP. Therefore, the main purpose of the Krebs Cycle is to provide high-energy electrons in the form of FADH2 and NADH to be passed onward to the electron transport chain.

The high-energy electrons contained in NADH and FADH2 are passed on to a series of enzyme complexes in the mitochondrial membrane.

Three complexes work in sequence to harvest the energy in NADH and FADH2 and convert it to ATP: NADH-Q reductase, cytochrome reductase and cytochrome oxidase. The final electron acceptor in the electron transport chain is oxygen. Each successive complex is at lower energy than the former so that each can accept electrons and effectively oxidize the higher energy species.

In effect, each complex harvests the energy in these electrons to pump protons across the inner mitochondria membrane, thereby creating a proton gradient. In turn, this electro-potential energy is converted to chemical energy by allowing proton flux back down its chemical gradient and through specific proton channels that synthesize ATP from ADP.

Approximately two molecules of ATP are produced during the Kreb’s cycle reactions, while approximately 26 to 30 ATP are generated by the electron transport chain. In summary, the oxidation of glucose through the reduction of NAD+ and FADH is coupled to the phosphorylation of ADP to produce ATP. Hence, the process is known as oxidative phosphorylation.


3.2 Carbohydrates

Carbohydrates provide energy for the cell and structural support to plants, fungi, and arthropods such as insects, spiders, and crustaceans. Consisting of carbon, hydrogen, and oxygen in the ratio CH2O or carbon hydrated with water, carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides depending on the number of monomers in the macromolecule. Monosaccharides are linked by glycosidic bonds that form as a result of dehydration synthesis. Glucose, galactose, and fructose are common isomeric monosaccharides, whereas sucrose or table sugar is a disaccharide. Examples of polysaccharides include cellulose and starch in plants and glycogen in animals. Although storing glucose in the form of polymers like starch or glycogen makes it less accessible for metabolism, this prevents it from leaking out of cells or creating a high osmotic pressure that could cause excessive water uptake by the cell. Insects have a hard outer skeleton made of chitin, a unique nitrogen-containing polysaccharide.

Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 4 of the AP ® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 4 Biological systems interact, and these systems and their interactions possess complex properties.
Enduring Understanding 4.A Interactions within biological systems lead to complex properties.
Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule.
Science Practice 7.1 The student can connect phenomena and models across spatial and temporal scales.
Learning Objective 4.1 The student is able to refine representations and models to explain how the subcomponents of a biological polymer and their sequence determine the properties of that polymer.
Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule.
Science Practice 1.3 The student can refine representations and models of natural or man-made phenomena and systems in the domain.
Learning Objective 4.2 The student is able to refine representations and models to explain how the subcomponents of a biological polymer and their sequence determine the properties of that polymer.
Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule.
Science Practice 6.1 The student can justify claims with evidence.
Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Learning Objective 4.3 The student is able to use models to predict and justify that changes in the subcomponents of a biological polymer affect the functionality of the molecules.

The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 4.15] [APLO 2.5]

Molecular Structures

Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have enough energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet grains, fruits, and vegetables are all natural sources of carbohydrates. Carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many staple foods. Carbohydrates also have other important functions in humans, animals, and plants.

Carbohydrates can be represented by the stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”). Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.

Monosaccharides

Monosaccharides (mono- = “one” sacchar- = “sweet”) are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix -ose. If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is known as an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R'), it is known as a ketose. Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons). See Figure 3.5 for an illustration of the monosaccharides.

The chemical formula for glucose is C6H12O6. In humans, glucose is an important source of energy. During cellular respiration, energy is released from glucose, and that energy is used to help make adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water, and glucose in turn is used for energy requirements for the plant. Excess glucose is often stored as starch that is catabolized (the breakdown of larger molecules by cells) by humans and other animals that feed on plants.

Galactose (part of lactose, or milk sugar) and fructose (found in sucrose, in fruit) are other common monosaccharides. Although glucose, galactose, and fructose all have the same chemical formula (C6H12O6), they differ structurally and chemically (and are known as isomers) because of the different arrangement of functional groups around the asymmetric carbon all of these monosaccharides have more than one asymmetric carbon (Figure 3.6).


Anaerobic and Aerobic Metabolism

Anaerobic metabolism occurs in the cytosol of the muscle cells. As seen in Figure 10.1., a small amount of ATP is produced in the cytosol without the presence of oxygen. Anaerobic metabolism uses glucose as its only source of fuel and produces pyruvate and lactic acid. Pyruvate can then be used as fuel for aerobic metabolism. Aerobic metabolism takes place in mitochondria of the cell and is able to use carbohydrates, protein, or fat as fuel sources. Aerobic metabolism is a much slower process than anaerobic metabolism, but it can produce much more ATP and is the process by which the majority of the ATP in the body is generated.

Figure 10.1. Anaerobic vs aerobic metabolism. Note that carbohydrate is the only fuel utilized in anaerobic metabolism, but all three macronutrients can be used for fuel during aerobic metabolism.


Gene pools and speciation HL quiz 10.3

This is a quiz of multiple choice style questions about the gene pools and speciation topic 10.3 for HL students.They are self-marking questions, so you can click on "check" to see whether you have the answer correct.Each question has a helpful note written by an examiner. Great for revision.Teachers can control access to this quiz for their groups in the "student access" section.Students - If this is an assignment.

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Methods

Plant materials and treatment

L. radiata materials used in this study were originally obtained from the Lycoris germplasm repository of Nanjing Botanical Garden Mem. Sun Yat-Sen (NBG) by YCZ and QZL under legal permission on May 20, 2016, and a voucher specimen (samples NO. NAS00571237) of this material has already been deposited at the herbarium of NBG. These bulbs were transplanted into the experimental base at the Shanghai Academy of Agricultural Sciences (Qingpu District, Shanghai, China) for more than 2 years. When L. radiata plants were dormant and without leaves at the end of July, bulbs with a diameter of 2.7 ± 0.2 cm were collected. After removal of the roots and dried scales and surface sterilization, the bulbs were chipped into four sections on average and placed on enamel trays, as described previously [6]. The sections were covered with a gauze that was sprayed with distilled water twice daily to keep it moist. After these pre-treatments, the sections were placed in plant growth chambers for bulblet formation, under a 14-h light/10-h dark photoperiod (6000 lx), at 25/20 °C (day/night), and with a relative humidity of 80%. The experiment was started on July 31, 2018, and lasted 2 months. Two hundred bulb sections were prepared in the experiment, including three replicates, and each replicate consisted of 66 or 67 sections.

Sampling

We previously observed that bulblets appear and develop at junctions of the innermost layer of scales and the basal plate, where axillary buds are formed that gradually develop. Bulb sections were collected on days 0, 1, 3, 7 DAT, and samples of tissues surrounding the zones of axillary bud emergence were collected. After the axillary buds grew out, the newly formed bulblets of sections were collected weekly during the two-month experiment. Our previous study revealed that the scales of L. radiata bulbs can be separated into three layers based on a morphological index, and each layer may have different roles in regulating bulb development [24]. Thus, the three layers of each section were also separately sampled. All the materials were frozen in liquid nitrogen for 30 min and stored at − 70 °C.

Starch and soluble sugar content measurements

The contents of starch and total soluble sugars were measured by traditional anthrone colorimetry [42]. Samples were ground in liquid nitrogen, and approximately 0.5 g of powder was incubated with 4 ml of 80% ethanol at 80 °C for 30 min. Then, the extraction solution was centrifuged at 8000×g for 20 min. After decolorization with activated carbon, soluble sugars in the supernatant were measured. The precipitates were successively suspended in 9.2 M and 4.6 M HClO4 to extract the starch after removing the ethanol-soluble sugar residues. Total soluble sugar and starch concentrations were then determined using the anthrone reaction.

Starch synthesis enzyme activity measurements

Enzyme extraction and determination of the activities of starch synthesis enzymes (AGPase, SS, and GBSS) were carried out as previously reported [61, 62]. Thoroughly mixed frozen bulb tissues (0.5 g) were prepared for each sample. All procedures were conducted at a temperature of 0 °C to 4 °C. Samples were ground as described above and.

extracted with buffer solution [5 ml g − 1 sample fresh weight (FW)] containing 100 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)-NaOH frozen extraction buffer (pH 7.5), 8 mM MgCl2, 2 mM ethylenediaminetetraacetic acid (EDTA), 50 mM 2-mercaptoethanol, 12.5% glycerol, and 1% (0.01 g/ml) insoluble.

polyvinylpyrrolidone-40). The homogenate was then centrifuged at 30000×g for 30 min, and the supernatant was used for the determination of AGPase and SSS, while the sediment was used for GBSS. The activities of AGPase, SSS, and GBSS were measured following the method described previously [61, 62]. The enzymes were compared based on soluble protein content, which was determined by a modified Bradford method [63]. All treatment experiments consisted of three independent replicates.

Endogenous plant hormone measurements

The levels of IAA, ZR, GA3, and ABA were determined at Qingdao Sci-tech Innovation Quality Testing Co., Ltd. (Qingdao, China). Sample extraction and purification were carried out as described previously [64], with a modification. Approximately 0.2 g of sample was first ground in liquid nitrogen. After adding 1 ml of cold 50% acetonitrile (v/v) at 4 °C, the samples were further ground in a vibration mill at 50 Hz for 2 min, and then ultrasound-extracted for 3 min. After incubation at 4 °C for 4 h, the samples were centrifuged at 12,000×g at 4 °C for 10 min. The supernatant was purified using an Oasis HLB purification column (Waters) and was collected in a plastic microtube. The samples were dried under N2, dissolved in 200 μl of 30% acetonitrile (v/v), and filtered using 0.22-μm membrane filters.

The purified product was subjected to high-performance liquid chromatography-tandem mass spectrometry (TSQ Quantum Ultra, Thermo) analysis, using a C18 (Agilent Technologies) column (2.1 mm × 100 mm, 1.8 μm) at a flow rate of 0.3 ml min − 1 , with the gradients of solvent A (0.1% methanoic acid) and B (acetonitrile) set according to the following profile: 1 min, 95% A + 5% B 15 min, 20% A + 80% B 16 min, 100%B 19 min, 95% A + 5% B. The column temperature was set at 40 °C and the injection volume was 5 μl. MS conditions were as follows: the spray voltage was 3500 V (ESI –) and 4000 V (ESI +) respectively, and the atomizing temperature was 330 °C.

The external standard method was used to determine the hormone contents. Calibration curves for IAA, ZR, GA3 and ABA standards were obtained using seven or eight concentrations (0, 1, 5, 10, 50, 100, 500, and 1000 ng/ml) (Additional file 5: Fig. S1). TIC chromatograms of standards are shown in Additional file 6: Fig. S2.

RNA extraction

Total RNA extraction and cDNA synthesis were carried out as described previously [65]. Total RNA was extracted according using an RNAprep Pure Plant Kit (Tiangen Biotech, Beijing, China) per the manufacturer’s instructions. After measuring the RNA quantity and quality using a NanoDrop 2000 Spectrophotometer (Thermo Fisher Scientific, USA), we selected those samples with A260/280 = 1.8–2.2 for library preparation. Three biological replicates were used for RNA-Seq.

CDNA library construction and sequencing

The cDNA library was constructed using an mRNA-Seq Sample Preparation Kit (MGIEasy™ mRNA Library Prep Kit, MGI, Shenzhen, China), according to the manufacturer’s instructions as described previously [66]. Poly(A) mRNA was enriched by oligo magnetic adsorption. The enriched mRNA was fragmented and reverse-transcribed into double-stranded cDNAs with an N6 random primer. Sequencing adaptors were linked to the purified cDNA, and 15 double-strand libraries were generated by PCR amplification. The libraries were sequenced on a BGISEQ-500 platform at the Beijing Genomics Institute (www.genomics.org.cn, Shenzhen, China).

De-novo transcriptome assembly

Low-quality sequences, including sequences with ambiguous bases, low-quality reads, and reads with adaptors, were removed from the paired-end raw reads. Only clean reads were used in subsequent analyses. The high-quality reads were assembled using Trinity with default parameters to construct unique consensus sequences [67, 68].

Analysis of differential gene expression

Unigene expression levels were calculated based on FPKM values. Then, DEGs among the sample groups were identified using the NOISeq package [69]. DEGs were identified based on a false discovery rate < 0.05 and |log2 foldchange| ≥1.

Functional annotation of unigenes

Unigenes were annotated by BLASTx against seven public databases, including Nr, Nt, SwissProt, KOG, Pfam, GO, and KEGG. GO annotation was performed using the Blast2GO software, as described previously [70].

QRT-PCR assays

Approximately 500 ng of total RNA was used to prepare first-strand cDNA using the PrimeScript RT Reagent Kit (TaKaRa, Dalian, China) per the manufacturer’s instruction. The cDNA was used for qRT-PCR, which was carried out on an ABI 7500 Fast sequencer using SYBR Premix Ex Taq™ (Takara, Kyoto, Japan), as described previously [15]. Actin7 (Uni_17610) was used as a reference gene. Three biological replicates were included per treatment. Primers used are listed in Additional file 7: Table S5.

Statistical analyses

All statistical analyses were conducted using SPSS 16.0. Means of values were compared by standard analysis of variance followed by least significant difference tests, P < 0.05 was considered significant.


Watch the video: Carbohydrate metabolism (June 2022).