Metabolism is the set of life-sustaining chemical processes in organisms. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes; the conversion of food/fuel to building blocks for proteins, lipids, nucleic acids, and some carbohydrates; and the elimination of metabolic wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structure, and respond to their environments.
The study of metabolism is called physiological chemistry or biochemistry. Clinical physiology studies the effects of disease states on normal metabolism while pathophysiology investigates how metabolic abnormalities lead to disease.
In humans, metabolism occurs mostly in the liver and pancreas. Other organs with important roles in metabolism include the small intestine, kidney, muscles, brain, and lungs. Metabolic processes are also present in other tissues including white blood cells, endocrine glands (e.g., adrenal gland), placenta
Cellular respiration is a set of redox reactions that convert nutrients into ATP molecules that can be used by cells for energy (ATP synthesis). In eukaryotic cells such as animals and plants, aerobic respiration takes place in mitochondria using oxygen as the final electron acceptor; whereas anaerobic respiration takes place in cytoplasm without using oxygen but instead using other electron acceptors like sulfate (SO42−) or nitrate (NO3−). Cellular respiration includes glycolysis–the breakdown of glucose–and oxidative phosphorylation–the use of oxygen to generate ATP from ADP molecules. Glycolysis results in a net production two ATP molecules per glucose molecule while oxidative phosphorylation results in a net production thirty-six ATP molecules per glucose molecule through substrate-level phosphorylation and chemiosmosis respectively
Aerobic cellular respiration requires both pyruvate dehydrogenase complex (PDC) activity and mitochondrial respiratory chain activity whereas anaerobic cellular respiration only requires PDC activity since no mitochondrial respiratory chain is needed when no oxygen is present
PDHc is composed of multiple enzymes that transfer electrons from NADH + H+ generated during glycolysis directly to CoQ resulting in reduction of CoQH2 to CoQ (-0.32 V). This reaction couples oxidation at one site with decarboxylation at another site yielding Acetyl-CoA which enters Krebs cycle for further oxidation producing more NADH + H+ that can be used by Complex I for generating more ATP through oxidative phosphorylation . In addition , this reaction provides reducing equivalents required for biosynthetic reactions like fatty acid synthesis
Lipid catabolism refers to all biochemical pathways involved in the breakdown (catabolism) or turnover (metabolism) – usually via beta oxidation –of fats/lipids stored within an organism Lipogenesis on the other hand is defined as any process resulting in storage lipid synthesis i.e de novo triglyceride formation from simple precursors . Although there are many different lipid classes , we will focus on triglycerides here . Triglycerides are synthesized from Intermediate Acting Phospholipids -diacylglycerols -which themselves are formed by hydrolytic action carried out by Phospholipases on membrane bound Phospholipids
Fats/triglycerides can be oxidized via beta oxidation yielding fatty acids which enter mitochondria where they undergo β-oxidation yielding acetyl-CoA units that eventually enter TCA cycle Beta oxidation occurs via 4 distinct steps : Dehydration , Hydration , Oxidation & Thiolytic Cleavage These steps yield an acetyl-CoA unit which then enters tricarboxylic acid(TCA) cycle & FADH2 & NADH+H produced provide electrons required for oxidative phosphorylation leading to generation more ATP Beta oxidation not only provides means for disposal excess lipids but also necessary precursor units(acetyl CoA)for various biosynthetic pathways like ketone body formation etc