why do we need oxygen for cellular respiration

Cellular respiration has three main steps: glycolysis, the citric acid cycle, and oxidative phosphorylation, where oxygen is used. Step 1: Glycolysis Glycolysis is the first step in cellular respiration, and it occurs in the main compartment of the cell: the cytoplasm. Cells let glucose in from the blood--the glucose comes from the food we eat. Next, cells convert glucose through several different compounds to make two ATP molecules and a molecule called pyruvate. A compound called NADH (nicotinamide adenine dinucleotide + hydrogen) is also created. This molecule stores electrons harvested from the glucose, which will be used later to create a larger amount of ATP. Step 2: Citric Acid Cycle Next, the cell takes the pyruvate made in glycolysis and converts it to a molecule called acetyl Co-A. This happens in the powerhouse of the cell, the mitochondria. Acetyl Co-A is also converted to several different compounds but, ultimately, acetyl Co-A is regenerated, hence the 'cycle' part of the citric acid cycle. The citric acid cycle also creates another molecule of ATP, additional NADH, and the molecule FADH (flavin adenine dinucleotide + hydrogen), which also transports electrons. Step 3: Oxidative Phosphorylation The entire point of cellular respiration up until now has been to get a few ATP, but now it focuses on the electrons housed in the NADH. The NADH is taken to the mitochondrial membrane, or barrier of the mitochondria. There are actually two membranes--an inner and an outer membrane--and a small space in between called the intermembrane space.

Here, electrons are transferred between proteins in the membrane in the electron transport chain. The proteins act like factory workers, passing down the electrons in a chain. As the electrons pass through, four proteins use the energy stored in the electrons to move hydrogen ions into the intermembrane space. At the end of the chain is the ultimate electron acceptor: oxygen. Oxygen loves electrons more than any of the other proteins in the chain, so the proteins keep passing them down so oxygen can have them all. When oxygen finally gets the electrons, it also picks up two hydrogen ions. When the electrons, hydrogen ions and oxygen combine, they make water! After the oxygen is used up, the electrons have no place to go at the last protein, and the chain stops. As a result, the other steps stop, too, like backed up traffic at a light. The cell is no longer able to make energy and dies. You might still be wondering where all the ATP gets made. So far, we only made a couple in glycolysis and the citric acid cycle, but the real pay day has yet to come. Let's look at how this happens. In the intermembrane space, the hydrogen ions build up like water behind a dam. When we have water behind a dam, the water can flow through a designated space, and we harvest the energy released as the water moves to make electricity. The cell has a similar method in place for the hydrogen ions.

An important protein called ATP synthase acts like the dam. It has space for the hydrogen ions to flow into the mitochondria. As the hydrogen ions flow, ATP synthase harvests the energy stored and uses it to make ATP. Then, that energy can be used for all processes in the cells. Everything we do needs energy, which is ultimately made using oxygen and glucose. Cellular respiration is the process cells use to make energy. Cells in our body combine glucose and oxygen to make ATP and carbon dioxide. Cellular respiration starts with
glycolysis, where glucose enters the cell, is converted to pyruvate, and makes a few ATP and NADH. Next, the pyruvate moves into the citric acid cycle, as acetyl Co-A and creates more ATP and NADH. Finally, during the third step, oxidative phosphorylation, the NADH moves to the inner mitochondrial membrane to transfer electrons to proteins in the electron transport chain. As the electrons are transferred between the proteins, the proteins pump hydrogen ions into the intermembrane space. After each of the four proteins, the electrons end with oxygen. Oxygen combines with the electrons and two hydrogen ions to make water. Lastly, the hydrogen ions flow through ATP synthase to make ATP. Aerobic cellular respiration is the process by which cells use oxygen to help them convert glucose into energy. This type of respiration occurs in three steps: glycosis; the Krebs cycle; and electron transport phosphorylation.

Oxygen is not needed for glycosis but is required for the rest of the chemical reactions to take place. Cellular respiration is the process by which cells release energy from glucose and change it into a usable form called ATP. ATP is a molecule that provides a small amount of energy to the cell, which provides it fuel to do specific tasks. There are two types of respiration: anaerobic and aerobic. Anaerobic respiration does not use oxygen. Anaerobic respiration produces yeast or lactate. When exercising, the body more quickly than it is taken in; anaerobic respiration provides lactate to keep the muscles moving. Lactate buildup and lack of oxygen are the reasons for muscle fatigue and labored breathing during hard exercise. Aerobic respiration occurs in three stages. The first stage is called glycolysis and does not require oxygen. In this stage, ATP molecules are used to help break down glucose into a substance called pyruvate, a molecule that transports electrons called NADH, two more ATP molecules, and carbon dioxide. Carbon dioxide is a waste product and is removed from the body. The second stage is called the Krebs cycle. This cycle consists of a series of complex chemical reactions that generate additional NADH. The final stage is called electron transport phosphorylation. During this stage, NADH and another transporter molecule called FADH2 carry electrons to the cells. Energy from the electrons is converted to ATP. Once the electrons have been used, they are donated to atoms of hydrogen and oxygen to make water.

Glycolysis is the first stage of all respiration. During this stage, every molecule of glucose is broken down into a carbon-based molecule called pyruvate, two ATP molecules, and two molecules of NADH. Once this reaction has occurred, the pyruvate goes through a further chemical reaction called fermentation. During this process, electrons are added to the pyruvate to generate NAD+ and lactate. In aerobic respiration, the pyruvate is further broken down and combined with oxygen to create carbon dioxide and water, which are eliminated from the body. Pyruvate is a carbon-based molecule; each molecule of pyruvate contains three carbon molecules. Only two of these molecules are used to create carbon dioxide in the final step of glycolysis. Thus, after glycolysis there is loose carbon floating around. This carbon binds to various enzymes to create chemicals used in other capacities in the cell. The Krebs cycle reactions also generate eight more molecules of NADH and two molecules of another electron transporter called FADH2. NADH and FADH2 carry electrons to specialized cell membranes, where they are harvested to create ATP. Once the electrons are used, they become depleted and must be removed from the body. Oxygen is essential for this task. Used electrons bind with oxygen; these molecules eventually bind with hydrogen to form water.

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