Nutrient absorption in the cell cytoplasm -
Each phospholipid molecule has a blue circular head and two red tails, and the tails face each other within the membrane. A series of protein complexes are positioned along the inner mitochondrial membrane, represented by colored shapes. The proteins that make up the electron transport chain start on the left and continue to the right.
At the far left, NADH dehydrogenase is represented by a light green rectangular structure that spans the membrane. Next, succinate dehydrogenase is represented by a dark green bi-lobed shape embedded in the half of the inner membrane and facing the matrix. Next, acyl-CoA dehydrogenase, electron transfer flavoprotein ETFP , and ETFP-ubiquinone oxidoreductase form a complex, and are represented by three yellow and orange ovals on the matrix-facing side of the inner membrane.
Next, ubiquinone is represented by a lime green circle labeled with a Q located in the side of the inner membrane facing the intermembrane space.
Next, cytochrome c reductase is represented by a light blue oval-shaped structure that spans the membrane. Next, cytochrome c oxidase is represented by a pink oval-shaped structure that spans the inner membrane.
Next, the ATP synthase complex is represented by an upside-down lollipop-shaped structure that traverses the inner membrane and contains a channel through the membrane; the round, purple head enters the mitochondrial matrix, and the lilac-colored stem spans the membrane.
These electrons are transferred to ubiquinone. Succinate dehydrogenase converts succinate to fumarate and transfers additional electrons to ubiquinone via flavin adenine dinucleotide FAD. The acyl-CoA dehydrogenase, electron transfer flavoprotein ETFP , and ETFP-ubiquinone oxidoreductase complex converts acyl-CoA to trans-enoyl-CoA.
During this reaction, additional electrons are transferred to ubiquinone by the FAD domain in this protein complex. Next, the electrons are transferred by ubiquinone to cytochrome c reductase, which pumps protons into the intermembrane space.
The electrons are then carried to cytochrome c. Next, cytochrome c transfers the electrons to cytochrome c oxidase, which reduces oxygen O 2 with the electrons to form water H 2 O.
During this reaction, additional protons are transferred to the intermembrane space. As the protons flow from the intermembrane space through the ATP synthase complex and into the matrix, ATP is formed from ADP and inorganic phosphate P i in the mitochondrial matrix.
Oxidative phosphorylation depends on the electron transport from NADH or FADH 2 to O 2 , forming H 2 O. The electrons are "transported" through a number of protein complexes located in the inner mitochondrial membrane, which contains attached chemical groups flavins, iron-sulfur groups, heme, and cooper ions capable of accepting or donating one or more electrons Figure 2.
These protein complexes, known as the electron transfer system ETS , allow distribution of the free energy between the reduced coenzymes and the O 2 and more efficient energy conservation.
The electrons are transferred from NADH to O 2 through three protein complexes: NADH dehydrogenase, cytochrome reductase, and cytochrome oxidase. Electron transport between the complexes occurs through other mobile electron carriers, ubiquinone and cytochrome c.
FAD is linked to the enzyme succinate dehydrogenase of the TCA cycle and another enzyme, acyl-CoA dehydrogenase of the fatty acid oxidation pathway. During the reactions catalyzed by these enzymes, FAD is reduced to FADH 2 , whose electrons are then transferred to O 2 through cytochrome reductase and cytochrome oxidase, as described for NADH dehydrogenase electrons Figure 2.
These observations led Peter Mitchell, in , to propose his revolutionary chemiosmotic hypothesis. The reaction catalyzed by succinyl-CoA synthetase in which GTP synthesis occurs is an example of substrate-level phosphorylation.
Acetyl-CoA enters the tricarboxylic acid cycle at the top of the diagram and reacts with oxaloacetate and water H 2 O to form a molecule of citrate and CoA-SH in a reaction catalyzed by citrate synthase. Next, the enzyme aconitase catalyzes the isomerization of citrate to isocitrate.
Succinyl-CoA reacts with GDP and inorganic phosphate P i to form succinate and GTP. This reaction releases CoA-SH and is catalyzed by succinyl-CoA synthetase. In the next step, succinate reacts with FAD to form fumarate and FADH 2 in a reaction catalyzed by succinate dehydrogenase.
Fumarate combines with H 2 O in a reaction catalyzed by fumerase to form malate. Then, oxaloacetate can react with a new molecule of acetyl-CoA and begin the tricarboxylic acid cycle again. The diagram shows the molecular structures for citrate, isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate.
The enzymes that act at each of the eight steps in the cycle are shown in yellow rectangles. In aerobic respiration or aerobiosis, all products of nutrients' degradation converge to a central pathway in the metabolism, the TCA cycle.
In this pathway, the acetyl group of acetyl-CoA resulting from the catabolism of glucose, fatty acids, and some amino acids is completely oxidized to CO 2 with concomitant reduction of electron transporting coenzymes NADH and FADH 2.
Consisting of eight reactions, the cycle starts with condensing acetyl-CoA and oxaloacetate to generate citrate Figure 3. In addition, a GTP or an ATP molecule is directly formed as an example of substrate-level phosphorylation.
In this case, the hydrolysis of the thioester bond of succinyl-CoA with concomitant enzyme phosphorylation is coupled to the transfer of an enzyme-bound phosphate group to GDP or ADP. Also noteworthy is that TCA cycle intermediates may also be used as the precursors of different biosynthetic processes.
The TCA cycle is also known as the Krebs cycle, named after its discoverer, Sir Hans Kreb. Krebs based his conception of this cycle on four main observations made in the s.
The first was the discovery in of the sequence of reactions from succinate to fumarate to malate to oxaloacetate by Albert Szent-Gyorgyi, who showed that these dicarboxylic acids present in animal tissues stimulate O 2 consumption. The second was the finding of the sequence from citrate to α-ketoglutarate to succinate, in , by Carl Martius and Franz Knoop.
Next was the observation by Krebs himself, working on muscle slice cultures, that the addition of tricarboxylic acids even in very low concentrations promoted the oxidation of a much higher amount of pyruvate, suggesting a catalytic effect of these compounds.
And the fourth was Krebs's observation that malonate, an inhibitor of succinate dehydrogenase, completely stopped the oxidation of pyruvate by the addition of tricarboxylic acids and that the addition of oxaloacetate in the medium in this condition generated citrate, which accumulated, thus elegantly showing the cyclic nature of the pathway.
When 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate, substrate-level phosphorylation occurs and ATP is produced from ADP. Then, 3-phosphoglycerate undergoes two reactions to yield phosphoenolpyruvate. Next, phosphoenolpyruvate is converted to pyruvate, which is the final product of glycolysis.
During this reaction, substrate-level phosphorylation occurs and a phosphate is transferred to ADP to form ATP. Interestingly, during the initial phase, energy is consumed because two ATP molecules are used up to activate glucose and fructosephosphate.
Part of the energy derived from the breakdown of the phosphoanhydride bond of ATP is conserved in the formation of phosphate-ester bonds in glucosephosphate and fructose-1,6-biphosphate Figure 4.
In the second part of glycolysis, the majority of the free energy obtained from the oxidation of the aldehyde group of glyceraldehyde 3-phosphate G3P is conserved in the acyl-phosphate group of 1,3- bisphosphoglycerate 1,3-BPG , which contains high free energy.
Then, part of the potential energy of 1,3BPG, released during its conversion to 3-phosphoglycerate, is coupled to the phosphorylation of ADP to ATP. The second reaction where ATP synthesis occurs is the conversion of phosphoenolpyruvate PEP to pyruvate.
PEP is a high-energy compound due to its phosphate-ester bond, and therefore the conversion reaction of PEP to pyruvate is coupled with ADP phosphorylation. This mechanism of ATP synthesis is called substrate-level phosphorylation. For complete oxidation, pyruvate molecules generated in glycolysis are transported to the mitochondrial matrix to be converted into acetyl-CoA in a reaction catalyzed by the multienzyme complex pyruvate dehydrogenase Figure 5.
When Krebs proposed the TCA cycle in , he thought that citrate was synthesized from oxaloacetate and pyruvate or a derivative of it. Only after Lipmann's discovery of coenzyme A in and the subsequent work of R.
Stern, S. Ochoa, and F. Lynen did it become clear that the molecule acetyl-CoA donated its acetyl group to oxaloacetate. Until this time, the TCA cycle was seen as a pathway to carbohydrate oxidation only. Most high school textbooks reflect this period of biochemistry knowledge and do not emphasize how the lipid and amino acid degradation pathways converge on the TCA cycle.
The cell is depicted as a large blue oval. A smaller dark blue oval contained inside the cell represents the mitochondrion. The mitochondrion has an outer mitochondrial membrane and within this membrane is a folded inner mitochondrial membrane that surrounds the mitochondrial matrix. The entry point for glucose is glycolysis, which occurs in the cytoplasm.
Glycolysis converts glucose to pyruvate and synthesizes ATP. Pyruvate is transported from the cytoplasm into the mitochondrial matrix. Pyruvate is converted to acetyl-CoA, which enters the tricarboxylic acid TCA cycle.
In the TCA cycle, acetyl-CoA reacts with oxaloacetate and is converted to citrate, which is then converted to isocitrate. Isocitrate is then converted to alpha-ketoglutarate with the release of CO 2. Then, alpha-ketoglutarate is converted to succinyl-CoA with the release of CO 2.
Succinyl-CoA is converted to succinate, which is converted to fumarate, and then to malate. Malate is converted to oxaloacetate. Then, the oxaloacetate can react with another acetyl-CoA molecule and begin the TCA cycle again.
In the TCA cycle, electrons are transferred to NADH and FADH 2 and transported to the electron transport chain ETC. The ETC is represented by a yellow rectangle along the inner mitochondrial membrane.
Figure 5. White, speckled red , and brown chicken eggs. CC-SA-BY 3. Unless you are eating it raw, the first step in egg digestion or any other protein food involves chewing. The teeth begin the mechanical breakdown of the large egg pieces into smaller pieces that can be swallowed. The salivary glands provide some saliva to aid swallowing and the passage of the partially mashed egg through the esophagus.
The mashed egg pieces enter the stomach through the esophageal sphincter. The stomach releases gastric juices containing hydrochloric acid and the enzyme, pepsin , which initiate the breakdown of the protein. The acidity of the stomach facilitates the unfolding of the proteins that still retain part of their three-dimensional structure after cooking and helps break down the protein aggregates formed during cooking.
Pepsin, which is secreted by the cells that line the stomach, dismantles the protein chains into smaller and smaller fragments. Egg proteins are large globular molecules and their chemical breakdown requires time and mixing. The powerful mechanical stomach contractions churn the partially digested protein into a more uniform mixture called chyme.
Protein digestion in the stomach takes a longer time than carbohydrate digestion, but a shorter time than fat digestion.
Eating a high-protein meal increases the amount of time required to sufficiently break down the meal in the stomach. Food remains in the stomach longer, making you feel full longer. The stomach empties the chyme containing the broken down egg pieces into the small intestine, where the majority of protein digestion occurs.
The pancreas secretes digestive juice that contains more enzymes that further break down the protein fragments. The two major pancreatic enzymes that digest proteins are chymotrypsin and trypsin. The cells that line the small intestine release additional enzymes that finally break apart the smaller protein fragments into the individual amino acids.
The muscle contractions of the small intestine mix and propel the digested proteins to the absorption sites. The goal of the digestive process is to break the protein into dipeptides and amino acids for absorption.
In the lower parts of the small intestine, the amino acids are transported from the intestinal lumen through the intestinal cells to the blood. This movement of individual amino acids requires special transport proteins and the cellular energy molecule, adenosine triphosphate ATP.
Once the amino acids are in the blood, they are transported to the liver. As with other macronutrients, the liver is the checkpoint for amino acid distribution and any further breakdown of amino acids, which is very minimal.
Recall that amino acids contain nitrogen, so further catabolism of amino acids releases nitrogen-containing ammonia. Because ammonia is toxic, the liver transforms it into urea, which is then transported to the kidney and excreted in the urine.
Other examples include tissues of the eyes, such as the lens, which is almost totally devoid of mitochondria; and the outer segment of the retina, which contains the photosensitive pigment. You may have already guessed that these cells and tissues then must produce ATP by metabolizing glucose only.
In these situations, glucose is degraded to pyruvate, which is then promptly converted to lactate Figure 2. This process is called lactic acid fermentation.
Although not highly metabolically active, red blood cells are abundant, resulting in the continual uptake of glucose molecules from the bloodstream. Additionally, there are cells that, despite having mitochondria, rely almost exclusively on lactic acid fermentation for ATP production.
This is the case for renal medulla cells, whose oxygenated blood supply is not adequate to accomplish oxidative phosphorylation. Finally, what if the availability of fatty acids to cells changes? The blood-brain barrier provides a good example. In most physiological situations, the blood-brain barrier prevents the access of lipids to the cells of the central nervous system CNS.
Therefore, CNS cells also rely solely on glucose as fuel molecules Figure 2. In prolonged fasting, however, ketone bodies released in the blood by liver cells as part of the continual metabolization of fatty acids are used as fuels for ATP production by CNS cells.
In both situations and unlike red blood cells, however, CNS cells are extremely metabolically active and do have mitochondria. Thus, they are able to fully oxidize glucose, generating greater amounts of ATP. Indeed, the daily consumption of nerve cells is about g of glucose equivalent, which corresponds to an input of about kilocalories 1, kilojoules.
However, most remaining cell types in the human body have mitochondria, adequate oxygen supply, and access to all three fuel molecules. Which fuel, then, is preferentially used by each of these cells?
Virtually all cells are able to take up and utilize glucose. What regulates the rate of glucose uptake is primarily the concentration of glucose in the blood. Glucose enters cells via specific transporters GLUTs located in the cell membrane.
There are several types of GLUTs, varying in their location tissue specificity and in their affinity for glucose. Adipose and skeletal muscle tissues have GLUT4, a type of GLUT which is present in the plasma membrane only when blood glucose concentration is high e. The presence of this type of transporter in the membrane increases the rate of glucose uptake by twenty- to thirtyfold in both tissues, increasing the amount of glucose available for oxidation.
Therefore, after meals glucose is the primary source of energy for adipose tissue and skeletal muscle. The breakdown of glucose, in addition to contributing to ATP synthesis, generates compounds that can be used for biosynthetic purposes.
So the choice of glucose as the primary oxidized substrate is very important for cells that can grow and divide fast. Examples of these cell types include white blood cells, stem cells , and some epithelial cells.
A similar phenomenon occurs in cancer cells, where increased glucose utilization is required as a source of energy and to support the increased rate of cell proliferation. Interestingly, across a tumor mass, interior cells may experience fluctuations in oxygen tension that in turn limit nutrient oxidation and become an important aspect for tumor survival.
In addition, the increased glucose utilization generates high amounts of lactate, which creates an acidic environment and facilitates tumor invasion. Another factor that dramatically affects the metabolism is the nutritional status of the individual — for instance, during fasting or fed states.
After a carbohydrate-rich meal, blood glucose concentration rises sharply and a massive amount of glucose is taken up by hepatocytes by means of GLUT2. This type of transporter has very low affinity for glucose and is effective only when glucose concentration is high.
Thus, during the fed state the liver responds directly to blood glucose levels by increasing its rate of glucose uptake. In addition to being the main source of energy, glucose is utilized in other pathways, such as glycogen and lipid synthesis by hepatocytes.
The whole picture becomes far more complex when we consider how hormones influence our energy metabolism. Fluctuations in blood levels of glucose trigger secretion of the hormones insulin and glucagon. How do such hormones influence the use of fuel molecules by the various tissues? Demands by one cell type can be met by the consumption of its own reserves and by the uptake of fuel molecules released in the bloodstream by other cells.
Energy use is tightly regulated so that the energy demands of all cells are met simultaneously. Elevated levels of glucose stimulate pancreatic β-cells to release insulin into the bloodstream.
Virtually all cells respond to insulin; thus, during the fed state cell metabolism is coordinated by insulin signaling. Figure 3: Blood glucose concentration after carbohydrate-rich and carbohydrate-poor meals. An extraordinary example is how insulin signaling rapidly stimulates glucose uptake in skeletal muscle and adipose tissue and is accomplished by the activity of GLUT4.
In the absence of insulin, these transporters are located inside vesicles and thus do not contribute to glucose uptake in skeletal muscle and adipose tissue. Insulin, however, induces the movement of these transporters to the plasma membrane, increasing glucose uptake and consumption.
As different tissues continue to use glucose, the blood glucose concentration tends to reach the pre-meal concentration Figure 3. This, in turn, decreases the stimulus for insulin synthesis and increases the stimulus for the release of glucagon, another hormone secreted by the α-pancreatic cells.
Therefore, during fasting, cell metabolism is coordinated by glucagon signaling and the lack of insulin signaling. As a consequence, GLUT4 stays inside vesicles, and glucose uptake by both skeletal muscle cells and adipocytes is reduced.
Now, with the low availability of glucose and the signals from glucagon, those cells increase their use of fatty acids as fuel molecules. Therefore, the use of fatty acids during fasting clearly contributes to the maintenance of adequate blood glucose concentration to meet the demands of cells that exclusively or primarily rely on glucose as a fuel.
But, mentioned above, glucose is used at an apparently high rate by the brain and constantly by red blood cells. And, under physiological conditions, blood glucose is maintained at a constant level, even during fasting.
How, then, is that delicate balance achieved? The liver is a very active organ that performs different vital functions. In Greek mythology, Prometheus steals fire from Zeus and gives it to mortals. As a punishment, Zeus has part of Prometheus's liver fed to an eagle every day.
Since the liver grows back, it is eaten repeatedly. This story illustrates the high proliferative rate of liver cells and the vital role of this organ for human life.
One of its most important functions is the maintenance of blood glucose. The liver releases glucose by degrading its glycogen stores. This reserve is not large, and during overnight fasting glycogen reserves fall severely. However, only the liver supplies the blood with glucose since it has an enzyme that make it possible for glucose molecules to be transported across cell membranes.
Since glycogen stores are limited and are reduced within hours of fasting, and blood glucose concentration is kept within narrow limits under most physiological conditions, another mechanism must exist to supply blood glucose. Indeed, glucose can be synthesized from amino acid molecules.
This process is called de novo synthesis of glucose, or gluconeogenesis. Amino acids, while being degraded, generate several intermediates that are used by the liver to synthesize glucose Figure 2. Alanine and glutamine are the two amino acids whose main function is to contribute to glucose synthesis by the liver.
The kidneys also possess the enzymes necessary for gluconeogenesis and, during prolonged fasting, contribute to some extent to the supply of blood glucose. Furthermore, since de novo glucose synthesis comes from amino acid degradation and the depletion of protein stores can be life-threatening, this process must be regulated.
Insulin, glucagon, and another hormone, glucocorticoid, play important roles in controlling the rate of protein degradation and, therefore, the rate of glucose production by the liver.
Alterations in factors that control food intake and regulate energy metabolism are related to well-known pathological conditions such as obesity, type 2 diabetes and the metabolic syndrome , and some types of cancer.
In addition, many effects and regulatory actions of well-known hormones such as insulin are still poorly understood. The consideration of adipose tissue as a dynamic and active tissue, for instance, raises several important issues regarding body weight and the control of food intake.
These factors point to the importance of further studies to expand our understanding of energy metabolism, thereby improving our quality of life and achieving a comprehensive view of how the human body functions.
Extracellular phototropic digestion is a process in which saprobionts cytoplaam by secreting enzymes through the cell membrane onto Citrus aurantium and cardiovascular health food. The enzymes catalyze the digestion Thd the food absorpgion lifestyle modifications for wakefulness, absorpiton, osmotrophy or phagocytosis. Since digestion occurs outside the cell, it is said to be extracellular. It takes place either in the lumen of the digestive systemin a gastric cavity or other digestive organ, or completely outside the body. During extracellular digestion, food is broken down outside the cell either mechanically or with acid by special molecules called enzymes.
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