Why do we need cellular respiration

Aerobic respiration requires oxygen, however, there are many organisms that live in places where oxygen is not readily available or where other chemicals overwhelm the environment. Extremophiles are bacteria that can live in places such as deep ocean hydrothermal vents or underwater caves. Rather than using oxygen to undergo cellular respiration, these organisms use inorganic acceptors such as nitrate or sulfur, which are more easily obtainable in these harsh environments.

This process is called anaerobic respiration. When oxygen is not present and cellular respiration cannot take place, a special anaerobic respiration called fermentation occurs.

Fermentation starts with glycolysis to capture some of the energy stored in glucose into ATP. However, since oxidative phosphorylation does not occur, fermentation produces fewer ATP molecules than aerobic respiration. In humans, fermentation occurs in red blood cells that lack mitochondria, as well in muscles during strenuous activity generating lactic acid as a byproduct, therefore it is named lactic acid fermentation.

Some bacteria carry out lactic acid fermentation and are used to make products such as yogurt. In yeast, a process known as alcoholic fermentation generates ethanol and carbon dioxide as byproducts, and has been used by humans to ferment beverages or leaven dough. Cellular respiration together with photosynthesis is a feature of the transfer of energy and matter, and highlights the interaction of organisms with their environment and other organisms in the community.

Cellular respiration takes place inside individual cells, however, at the scale of ecosystems, the exchange of oxygen and carbon dioxide through photosynthesis and cellular respiration affects atmospheric oxygen and carbon dioxide levels.

Interestingly, the processes of cellular respiration and photosynthesis are directly opposite of one another, where the products of one reaction are the reactants of the other. Photosynthesis produces the glucose that is used in cellular respiration to make ATP. This glucose is then converted back into CO 2 during respiration, which is a reactant used in photosynthesis. More specifically, photosynthesis constructs one glucose molecule from six CO 2 and six H 2 O molecules by capturing energy from sunlight and releases six O 2 molecules as a byproduct.

Cellular respiration uses six O 2 molecules to convert one glucose molecule into six CO 2 and six H 2 O molecules while harnessing energy as ATP and heat. Scientists can measure the rate of cellular respiration using a respirometer by assessing the rate of exchange of oxygen. Understanding the Ideal Gas Law is of fundamental importance for knowing how the respirometer functions. The Ideal Gas Law states that the number of gas molecules in a container can be determined from the pressure, volume, and temperature.

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. Respiration happens in cells. All organisms need energy to live. This energy is used:. As animals respire, heat is also released. Other animals then eat the plant-eating animals, passing the energy from one living thing to the next.

All forms of life need energy, from humans and animals to plants, fungi and algae. Energy released during respiration is used in several ways.

For example, it may be used to create larger molecules from smaller ones, such as when plants make amino acids from sugars, nitrates and other nutrients, which are then used to make proteins. Animals and humans use energy to contract their muscles to allow them to move. Energy also helps to maintain a steady body temperature. Anaerobic respiration is another type of cellular respiration. Aerobic respiration needs oxygen, but anaerobic respiration does not.

The importance of anaerobic respiration in humans relates to muscles during exercise. When the body doesn't get enough oxygen during exercise, it relies on anaerobic respiration for an energy supply.