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View All Categories. This occurs in several ways using one of three energy systems: Phosphagen immediate source Anaerobic somewhat slow, uses carbohydrates Aerobic slow, uses either carbohydrate or fat Phosphagen This system uses creatine phosphate CP and has a very rapid rate of ATP production.
Anaerobic Glycolysis Anaerobic glycolysis does not require oxygen and uses the energy contained in glucose for the formation of ATP. Aerobic Glycolysis This pathway requires oxygen to produce ATP, because carbohydrates and fats are only burned in the presence of oxygen. Buy Now. Stay Informed Sign up to receive relevant, science-based health and fitness information and other resources. Enter your email. Why do cancer cells choose this inefficient metabolic pathway when they could obtain 16 times as much ATP per molecule of glucose by opting for normal respiration?
The answer is twofold. First, although cancer cells produce far less ATP per molecule of glucose, they produce it much faster. Cancer cells produce ATP almost a hundred times faster than normal cells. It is essentially a cost-benefit calculation, where the benefits of speedy ATP production outweigh the costs associated with inefficient glucose breakdown.
Second, it is not just about ATP production. Cancer cells undergoing aerobic glycolysis also produce many intermediate biosynthetic precursors. These molecules are used as building blocks for the production of proteins, lipids and DNA required by the rapidly dividing cells.
Cancer cells consume more glucose than normal cells. This is exploited when imaging cancer. In this image, besides the normal accumulation of the FDG molecule in the heart, bladder, kidneys, and brain, liver metastases of a colorectal tumor are visible in the abdominal region.
Image credit: Jens Maus, Wikimedia Commons. How do cancer cells satisfy their voracious appetite for glucose? Glucose typically enters cells through protein channels known as glucose transporters, which act as gateways through the surface of the cell membrane, selectively allowing glucose molecules to enter the cell.
Cancer cells actively produce more glucose transporters on their cell surface membranes, so more glucose is brought inside the cell. Once inside the cell, the glucose is broken down by aerobic glycolysis into lactic acid, in order to speedily produce ATP and metabolic precursors through various metabolic pathways.
These pathways are tightly controlled, requiring specific enzymes for processing the molecules from each step to the next. Cancer cells are addicted to these metabolic precursors; the enzymes that control these pathways are often over-expressed or mutated in cancer cells.
This addiction is exploited in chemotherapy strategies. For example, the drugs 5-fluorouracil, methotrexate and pemetrexed inhibit the biosynthesis of DNA precursor molecules. The high glucose consumption of cancer cells is also exploited when imaging cancer. What is the utility of this alternative metabolic pathway to healthy cells?
The ability to grow and divide rapidly is useful in the context of wound healing and immune responses. When an immune response is required, immune cells massively increase their glucose uptake, switch from metabolizing glucose through normal respiration to aerobic glycolysis, and switch on the multitude of enzymes that control the biosynthesis of proteins, lipids and DNA. Therefore, there is a strong evolutionary basis for rapid cell division and faster growth despite the inefficient use of glucose in the process.
Cancer cells hijack this metabolic switch in order to fuel their own uncontrolled growth. We therefore know why cancer cells opt to switch from normal respiration to aerobic glycolysis. The next question is how they do this. This is an active area of research, and we are still deciphering the exact mechanisms of this metabolic switch.
In a previous article , I explained how tumors often develop regions of low oxygen. This activates the hypoxia stress response, mediated by the hypoxia inducible factor HIF. However, over recent years, it has become clear that HIF activity is not merely a response to low oxygen levels. HIF can be activated in response to a variety of triggers, such as radiation induced DNA damage, signaling from other proteins, growth factors and the presence of pyruvate.
Once activated, HIF can go on to activate genes that support aerobic glycolysis and repress genes involved in normal respiration. Autotrophs like plants produce glucose during photosynthesis. Heterotrophs like humans ingest other living things to obtain glucose. While the process can seem complex, this page takes you through the key elements of each part of cellular respiration.
Cellular respiration is a collection of three unique metabolic pathways: glycolysis, the citric acid cycle, and the electron transport chain. Glycolysis is an anaerobic process, while the other two pathways are aerobic.
In order to move from glycolysis to the citric acid cycle, pyruvate molecules the output of glycolysis must be oxidized in a process called pyruvate oxidation. Glycolysis is the first pathway in cellular respiration. This pathway is anaerobic and takes place in the cytoplasm of the cell. This pathway breaks down 1 glucose molecule and produces 2 pyruvate molecules. There are two halves of glycolysis, with five steps in each half.
This half splits glucose, and uses up 2 ATP. If the concentration of pyruvate kinase is high enough, the second half of glycolysis can proceed. Some cells e. However, most cells undergo pyruvate oxidation and continue to the other pathways of cellular respiration.
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