bio3: Metabolism (v1.0)

The circulatory system delivers nutrients and oxygen to the cells and carries off waste. But to function, a cell needs energy. The process of releasing that energy from nutrients and oxygen to run cellular processes is one of the key functions of metabolism. We will mainly focus on energy release from monosaccharides in this essay. The other key function of metabolism is the conversion of nutrients to building blocks for proteins, lipids and nucleic acids and some carbohydrates like glycogen. We will focus primarily on protein synthesis in this essay and ignore nucleic acid, carbohydrate, and lipid synthesis.

Metabolic reactions may be categorized as catabolic – the breaking down of compounds (for example, of glucose to pyruvate by cellular respiration); or anabolic – the building up (synthesis) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy.

Fat metabolism involves the breakdown and storage of fats for energy and the synthesis of structural and functional lipids. We will not discuss lipid synthesis in this essay. Large fat molecules in an organism’s food must be broken down into the small fatty acids that it is comprised of. Then, for the organism to store energy for winter, large fat molecules must be created and stored. Catabolic pathways break the fats down, and anabolic pathways rebuild them. Fatty acid oxidation to release energy is called beta oxidation. Fat metabolism will not be further explored in this essay.  

Enzymes play a crucial role in metabolism. Metabolism chemical reactions occur in a series of steps facilitated by enzymes. Enzymes act as a catalyst. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy and will not occur by themselves, by coupling them to spontaneous reactions that release energy. Enzymes allow a reaction to proceed more rapidly and controls the direction of the reaction – and they also allow the regulation of the rate of a metabolic reaction, for example in response to changes in the cell’s environment or to signals from other cells.

Humans can consume a variety of carbohydrates, digestion breaks down complex carbohydrates into simple monosaccharides: glucose, fructose, mannose, and galactose. The monosaccharides are transported through the portal vein to the liver, where all non-glucose monosaccharides are transformed into glucose as well. Glucose is distributed to cells in tissues by the circulatory system, where it is broken down for energy or stored as glycogen.

Adenosine Triphosphate (ATP) is the chemical that powers many cellular processes. Every day we make and break down our body weight in ATP. Releasing energy from glucose is done in multiple steps. Pyruvate is a chemical compound and is the output of the metabolism of glucose, known as glycolysis which occurs in the cytoplasm of the cell. The chemical energy in Glucose in glycolysis is converted to the chemical energy in ATP with little energy loss. To use an analogy, ATP works like a battery storing energy and it can transfer it as needed for chemical processes. By removing a phosphate group, ATP becomes Adenosine diphosphate (ADP) transferring energy. ADP and ATP are interconvertible. ADP is also interconvertible to Adenosine monophosphate (AMP) and AMP has one less phosphate group.  ATP is continually reformed from lower-energy species ADP and AMP. Energy transfer used by all living things is a result of dephosphorylation of ATP by enzymes known as ATPases. ATP supply is especially important for muscles for energy. Creatine is a molecule created mostly in the liver and kidney to ensure a reliable ATP supply is maintained for the muscles. Creatine facilitates recycling of ATP, primarily in muscle and brain tissue. Recycling is achieved by converting ADP back to ATP via donation of phosphate group.

The Krebs citric acid cycle is important to have some understanding of since sugars, fatty acids and amino acid energy release metabolism all comes together here in a single oxygen dependent metabolic pathway. The cycle acts as a huge hub. It is a series of chemical reactions facilitated by enzymes to release stored energy through the oxidation of acetyl-CoA which fatty acids and amino acids can transform to. Even the output of glycolysis – pyruvate – can transform to acetyl-CoA and then enter the citric acid cycle. I won’t go into the details of the cycle, but the reactions occur in the mitochondria organelle of the cell and this cycle is the biggest source of cellular energy through production of ATP.

The Atwater general factor system was developed by W.O. Atwater and his colleagues at the United States Department of Agriculture (USDA). The system is based on the heats of combustion of protein, fat and carbohydrate, which are corrected for losses in digestion, absorption and urinary excretion of urea. It uses a single factor for each of the energy-yielding substrates (protein, fat, carbohydrate), regardless of the food in which it is found. The energy values are 17 kJ/g (4.0 kcal/g) for protein, 37 kJ/g (9.0 kcal/g) for fat and 17 kJ/g (4.0 kcal/g) for carbohydrates.

Now I switch to protein breakdown. Once a protein source reaches your stomach, hydrochloric acids and an enzyme called proteases break it down to smaller chains of amino acids. In the small intestines, the pancreas releases enzymes and a bicarbonate buffer that reduces the acidity of digested food. More enzymes then break down the amino acid chains in this less acidic environment to individual amino acids that are then distributed to cells through the circulation system.  

I will now switch to protein synthesis in the cells from amino acids. I will give a brief description. Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template to produce a protein (messenger RNA). RNA and deoxyribonucleic acid (DNA) are nucleic acids. RNA is assembled as a chain of nucleotides. Cellular organisms use messenger RNA (mRNA) to convey genetic information (using the nitrogenous bases of guanine, uracil, adenine and cytosine denoted by the letters G, U, A, and C) that directs synthesis of specific proteins. Messenger RNA is transcribed from a section of DNA. RNA molecules direct the synthesis of proteins on ribosomes. This process uses transfer RNA (tRNA) molecules to deliver amino acids to the ribosome, where ribosomal RNA (rRNA) then links amino acids together to form coded proteins. Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to form polypeptide chains. These then fold to form shaped proteins with structure.