How does aerobic respiration depend on photosynthesis

The third step is the phosphorylation of fructosephosphate, catalyzed by the enzyme phosphofructokinase. A second ATP molecule donates a high-energy phosphate to fructosephosphate, producing fructose-1,6-bisphosphate. In this pathway, phosphofructokinase is a rate-limiting enzyme. This is a type of end product inhibition, since ATP is the end product of glucose catabolism.

Step 4. The newly added high-energy phosphates further destabilize fructose-1,6-bisphosphate. The fourth step in glycolysis employs an enzyme, aldolase, to cleave 1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone-phosphate and glyceraldehydephosphate.

Step 5. In the fifth step, an isomerase transforms the dihydroxyacetone-phosphate into its isomer, glyceraldehydephosphate. Thus, the pathway will continue with two molecules of a single isomer.

At this point in the pathway, there is a net investment of energy from two ATP molecules in the breakdown of one glucose molecule. So far, glycolysis has cost the cell two ATP molecules and produced two small, three-carbon sugar molecules.

Both of these molecules will proceed through the second half of the pathway, and sufficient energy will be extracted to pay back the two ATP molecules used as an initial investment and produce a profit for the cell of two additional ATP molecules and two even higher-energy NADH molecules. Figure 3. Step 6. The sugar is then phosphorylated by the addition of a second phosphate group, producing 1,3-bisphosphoglycerate. Note that the second phosphate group does not require another ATP molecule.

Here again is a potential limiting factor for this pathway. If oxygen is available in the system, the NADH will be oxidized readily, though indirectly, and the high-energy electrons from the hydrogen released in this process will be used to produce ATP.

Step 7. In the seventh step, catalyzed by phosphoglycerate kinase an enzyme named for the reverse reaction , 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming one molecule of ATP. This is an example of substrate-level phosphorylation. A carbonyl group on the 1,3-bisphosphoglycerate is oxidized to a carboxyl group, and 3-phosphoglycerate is formed.

Step 8. In the eighth step, the remaining phosphate group in 3-phosphoglycerate moves from the third carbon to the second carbon, producing 2-phosphoglycerate an isomer of 3-phosphoglycerate.

The enzyme catalyzing this step is a mutase a type of isomerase. Step 9. Enolase catalyzes the ninth step. This enzyme causes 2-phosphoglycerate to lose water from its structure; this is a dehydration reaction, resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bond and produces phosphoenolpyruvate PEP. Step Many enzymes in enzymatic pathways are named for the reverse reactions, since the enzyme can catalyze both forward and reverse reactions.

Two ATP molecules were used in the first half of the pathway to prepare the six-carbon ring for cleavage, so the cell has a net gain of two ATP molecules and two NADH molecules for its use.

If the cell cannot catabolize the pyruvate molecules further, it will harvest only two ATP molecules from one molecule of glucose. Mature mammalian red blood cells are not capable of aerobic respiration —the process in which organisms convert energy in the presence of oxygen—and glycolysis is their sole source of ATP.

If glycolysis is interrupted, these cells lose their ability to maintain their sodium-potassium pumps, and eventually, they die. The last step in glycolysis will not occur if pyruvate kinase, the enzyme that catalyzes the formation of pyruvate, is not available in sufficient quantities.

In this situation, the entire glycolysis pathway will proceed, but only two ATP molecules will be made in the second half. Thus, pyruvate kinase is a rate-limiting enzyme for glycolysis. Glycolysis is the first pathway used in the breakdown of glucose to extract energy. It was probably one of the earliest metabolic pathways to evolve and is used by nearly all of the organisms on earth. Glycolysis consists of two parts: The first part prepares the six-carbon ring of glucose for cleavage into two three-carbon sugars.

ATP is invested in the process during this half to energize the separation. Two ATP molecules are invested in the first half and four ATP molecules are formed by substrate phosphorylation during the second half.

If oxygen is available, aerobic respiration will go forward. In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are the sites of cellular respiration.

There, pyruvate will be transformed into an acetyl group that will be picked up and activated by a carrier compound called coenzyme A CoA. The resulting compound is called acetyl CoA. CoA is made from vitamin B5, pantothenic acid. Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next stage of the pathway in glucose catabolism.

In order for pyruvate which is the product of glycolysis to enter the Citric Acid Cycle the next pathway in cellular respiration , it must undergo several changes. The conversion is a three-step process Figure 5.

Figure 5. Upon entering the mitochondrial matrix, a multi-enzyme complex converts pyruvate into acetyl CoA.

In the process, carbon dioxide is released and one molecule of NADH is formed. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium.

The result of this step is a two-carbon hydroxyethyl group bound to the enzyme pyruvate dehydrogenase. This is the first of the six carbons from the original glucose molecule to be removed. This step proceeds twice remember: there are two pyruvate molecules produced at the end of glycolysis for every molecule of glucose metabolized; thus, two of the six carbons will have been removed at the end of both steps.

An acetyl group is transferred to conenzyme A, resulting in acetyl CoA. The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA. Note that during the second stage of glucose metabolism, whenever a carbon atom is removed, it is bound to two oxygen atoms, producing carbon dioxide, one of the major end products of cellular respiration.

In the presence of oxygen, acetyl CoA delivers its acetyl group to a four-carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule with three carboxyl groups; this pathway will harvest the remainder of the extractable energy from what began as a glucose molecule. This single pathway is called by different names, but we will primarily call it the Citric Acid Cycle. In the presence of oxygen, pyruvate is transformed into an acetyl group attached to a carrier molecule of coenzyme A.

The resulting acetyl CoA can enter several pathways, but most often, the acetyl group is delivered to the citric acid cycle for further catabolism. During the conversion of pyruvate into the acetyl group, a molecule of carbon dioxide and two high-energy electrons are removed.

The carbon dioxide accounts for two conversion of two pyruvate molecules of the six carbons of the original glucose molecule. At this point, the glucose molecule that originally entered cellular respiration has been completely oxidized. Chemical potential energy stored within the glucose molecule has been transferred to electron carriers or has been used to synthesize a few ATPs.

Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of mitochondria. This single pathway is called by different names: the citric acid cycle for the first intermediate formed—citric acid, or citrate—when acetate joins to the oxaloacetate , the TCA cycle since citric acid or citrate and isocitrate are tricarboxylic acids , and the Krebs cycle , after Hans Krebs, who first identified the steps in the pathway in the s in pigeon flight muscles.

Almost all of the enzymes of the citric acid cycle are soluble, with the single exception of the enzyme succinate dehydrogenase, which is embedded in the inner membrane of the mitochondrion. Unlike glycolysis, the citric acid cycle is a closed loop: The last part of the pathway regenerates the compound used in the first step.

This is considered an aerobic pathway because the NADH and FADH 2 produced must transfer their electrons to the next pathway in the system, which will use oxygen.

If this transfer does not occur, the oxidation steps of the citric acid cycle also do not occur. Note that the citric acid cycle produces very little ATP directly and does not directly consume oxygen.

Figure 6. In the citric acid cycle, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. Through a series of steps, citrate is oxidized, releasing two carbon dioxide molecules for each acetyl group fed into the cycle. Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants.

Prior to the start of the first step, pyruvate oxidation must occur. Then, the first step of the cycle begins: This is a condensation step, combining the two-carbon acetyl group with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate. CoA is bound to a sulfhydryl group -SH and diffuses away to eventually combine with another acetyl group.

This step is irreversible because it is highly exergonic. The rate of this reaction is controlled by negative feedback and the amount of ATP available. If ATP levels increase, the rate of this reaction decreases. If ATP is in short supply, the rate increases. In step two, citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate. Steps 3 and 4. Why do food chains rarely have more than four trophic levels? I understand that only Why should we eat from the lower part of the food chain?

What organisms are the producers, first order consumers, second order consumers, and third order What are the food web and food pyramids in the biome taiga? See all questions in Energy Flow. Impact of this question views around the world. You can reuse this answer Creative Commons License. All organisms need energy to perform life functions, and energy that is released is reused in other ways.

Photosynthesis and Aerobic Respiration : Photosynthesis consumes carbon dioxide and produces oxygen. Aerobic respiration consumes oxygen and produces carbon dioxide.

These two processes play an important role in the carbon cycle. Whether the organism is a bacterium, plant, or animal, all living things access energy by breaking down carbohydrate molecules. Living things need energy to perform life functions.

In addition, an organism can either make its own food or eat another organism; either way, the food still needs to be broken down. In nature, there is no such thing as waste. Every single atom of matter and energy is conserved, recycling over and over, infinitely. Substances change form or move from one type of molecule to another, but their constituent atoms never disappear.

CO 2 is no more a form of waste than oxygen is wasteful to photosynthesis. Both are byproducts of reactions that move on to other reactions. Photosynthesis absorbs light energy to build carbohydrates in chloroplasts, and aerobic cellular respiration releases energy by using oxygen to metabolize carbohydrates in the cytoplasm and mitochondria.

Photosynthesis consumes carbon dioxide and produces oxygen. Both processes use electron transport chains to capture the energy necessary to drive other reactions. These two powerhouse processes, photosynthesis and cellular respiration, function in biological, cyclical harmony to allow organisms to access life-sustaining energy that originates millions of miles away in the sun.

Privacy Policy. Skip to main content. Search for:. The Light-Independent Reactions of Photosynthesis. CAM and C4 Photosynthesis Some plants have evolved mechanisms to increase the CO 2 concentration in their leaves under hot and dry conditions. Key Takeaways Key Points The process of photosynthesis in desert plants has evolved mechanisms to conserve water.

Plants that use crassulacean acid metabolism CAM photosynthesis fix CO 2 at night, when their stomata are open. Key Terms crassulacean acid metabolism : A carbon fixation pathway that evolved in some plants as an adaptation to arid conditions, in which the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide CO2.

The Calvin Cycle The Calvin cycle is organized into three basic stages: fixation, reduction, and regeneration. Learning Objectives Describe the Calvin Cycle.