Metabolism
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Introduction
Metabolism is the sum of all biochemical processes of the body, including those that involve the breakdown of molecules (catabolism) and the building of new ones (anabolism).
Catabolism serve to capture chemical energy, usually as ATP, through the degradation of energy-rich molecules such as carbohydrates or proteins into a few simple molecules, including carbon dioxide, ammonia, urea, and water. These are often oxidative processes. Catabolic hormones include glucagon, epinephrine, and cortisol.
Anabolism is a divergent process, as a few simple building blocks are used to form a wide variety of complex polymeric products that may be stored as energy for future use. These reactions require energy, usually generated by the breakdown of ATP. Anabolic reactions are often reductive processes. Anabolic hormones include insulin.
Appetite can be healthy, too much, or not enough.
Following digestion and absorption in the gastrointestinal system, molecules make their way to various tissues and are processed for energy production or for storage.
The liver is a key site of processing, storage, and synthesis of molecules.
The pancreas produces and releases insulin and glucagon, two of the most important metabolic hormones
Fat
Muscle, due to its high energy demands, maintains its own energy stores.
The brain is an important user of energy
Molecules important for metabolism include:
- carbohydrates
- fats
- proteins
- vitamins
- energy molecules
Blood Glucose
main article: glucose regulation
Maintaining blood sugar levels is a fundamental cause for metabolism. It is important to maintain blood glucose levels above 4 mmol/L (70mg/dL).
- muscle metabolism
- brain metabolism
Muscle Metabolism
Muscle energy needs vary with activity. Resting muscle accounts for 30% of oxygen consumption, while in during vigorous exercise this can go up to 90%.
Most muscles store energy required for their function. As the heart is continuously active, however, it contains negligible energy stores and therefore will die rapidly if oxygen delivery is inturrupted.
Muscles derive energy from various sources. As the ATP in muscle is used up after about a second, it needs to be generated on an ongoing basis.
The Phosphocreatine Cycle
Creatine is a rapid source of energy, lasting for about 9 seconds.
Creatine is present in the space between mitochondrial membranes, where it is phosphorylated following ATP tanslocation from the matrx.
Phosphocreatine is exported to the cytosol, and creatine kinase is used to generate ATP from ADP and phosphocreatine.
Glycogenolysis
It takes over a minute to increase blood glucose and oxygen supply during exercise, and during this time anaerobic glycogenolysis of muscle glycogen occurs.
Glycogenolyis is regulated in three ways in muscle.
Neural stimulation induces calcium release from the sarcoplasmic reticulum. Calcium binds to calmodulin, activating glycogen phosphorylase kinase which in turn activates glycogen phosphorylase (a).
Epinephrine induces cAMP-PKA activity, which also activates phosphorylase kinase.
Muscle contraction and subsequent accumulation of AMP (from ADP and adenylate kinase) stimulates glycogen phosphorylase (b).
Long-term energy use
Muscle glycogen is expended within an hour, after which blood borne fatty acids and glucose are used.
Absorptive State
Carbohydrate Metabolism
- Increased glucose intake and glycolysis
- Increased glycogen synthesis due to availability of glucose 6-phosphate
Lipid Metabolism
Lipids are of secondary importance to muscle during the well-fed state, as glucose is the primary source of energy.
Amino Acid Metabolism
- Increased protein synthesis
- Increased uptake of branched amino acids, as muscle is the principal source of their metabolism
Brain Metabolism
Although the brain represents only two percent of adult body weight, it accounts for 20% of basal oxygen consumption.
The brain uses energy at a constant rate.
Normally, the brain uses glucose as a fuel source. Ketone bodies are sued during a severe fast.
Carbohydrate Metabolism
The brain uses approximately 140 g of glucose per day, completely oxidizing it to carbon dioxide and water. It has no significant stores of glycogen and is therefore completely dependent on blood glucose levels.
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