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Enzymes

The chemical catalysts in human cells (and, for that matter: in all cells) are proteins. There are about 1000 known enzymes in the human body. Groups of enzymes are lipases (enzymes that split triglycerides), proteases (enzymes that split proteins), dehydrogenases (enzymes that remove hydrogen atoms from molecules), anhydrases (enzymes that remove water molecules), oxidases (enzymes that add oxygen atoms to molecules), or kinases. A kinase adds phosphate groups to molecules.

For chemical reactions to occur, molecules of reactants have to collide. But they don't just have to collide. They have to collide with certain parts of their structures, and they may have to collide in a certain angle. Enzymes bind reactants (called substrates) and align them in a certain manner so that collisions with other reactants do not occur randomly, but in a controlled manner. The part of the enzyme that accomplishes this is called the active site of the enzyme. Enzymes can cause reactions to occur 100 million to 10 billion times faster or more often than they would occur without the enzyme. Typically, an enzyme can convert 1 to 10,000 substrate molecules per second; however, in some cases, the rate can be as high as 600,000 molecule conversions per second.

Apart from catalyzing synthesis reactions, enzymes can also break down substrates, or rearrange the atoms of a substrate. Sometimes, an enzyme can catalyze reversible reaction in bodth directions. For example, the enzyme carbonic anhydrase can catalyze the formation of carbonic acid (H2CO3) from carbon dioxide and water, or the breakdown of carbonic acid into carbon dioxide and water.

Galactosemia is a inherited disorder in which the enzyme galactose-1-phosphate uridylyl transferase (which converts galactose into glucose) is absent. The milk sugar galactose therefore remains unprocessed, and builds up in the blood; this can imp[act health in many ways and result in mental retardation. Treatment is as easy as avoiding milk and milk products.

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Nucleic acids

Nucleic acids are the material, our genes are made of. But strictly speaking, genes do not code for our traits (such as whether we quickly get angry or not, or whether we are good lovers; strictly speaking, the nucleic acids DNA and RNA only code for protein synthesis, and for how proteins are arranged in the human body. On the next level, it is the interplay of (genetically coded) proteins with other factors that determine both our physical and mental properties. It is a dangerous, and wrong, position to assume that everything we are is determined by our genes. By and large, our genes code only for the protein frame of our physical and mental existence. Effects as versatile as the air we breath (including pollutants) and our experiences of love and hate, or as versatile as the food we eat, or the geographic location where we have been born, will modify the way the assemblage of genetically coded-for proteins exists and behaves. Because the science of genetics is so much en vogue, with biologists coming up with genes that allegedly code for faithfulness, or with those that allegedly determine whether a boy enjoys basketball, the actual facts are obscured. A set of genes may actually just determine that a young man is physically not particularly attractive, or that he is neurologically labile, with a proneness for anxiety. People with a corresponding assemblage of proteins may be more likely to be faithful than better looking young men with no neurological problems. And a tall healthy boy will probably enjoy basketball more than a short fat one. But we ought not forget that genes code for protein structures, and not for what we think, enjoy, or believe in. That would be taking too many steps at once.

Like carbohydrates and proteins, nucleic acids are arranged as polymers, repeated units that are joined together. The monomers of nucleic acids are called nucleotides. While human proteins are synthesized from 20 amino acids, the nucleic acids that code for proteins are assembled from just 4 nucleotides (monomers): adenine, thymine, cytosine, and guanine.

Actually, these four names belong to just one component of nucleotides, the nitrogenous base. But it's only the nitrogenous base that differentiates nucleotides one from the other. Apart from the nitrogenous base, each nucleotide still incorporates a five-carbon (pentose) sugar (deoxyribose for DNA, ribose for RNA), and a phosphate group.

Chemically, adenine and guanine are double-ring purines, while thymine (or uracil in RNA) and cytosine are pyrimidines. In DNA, the nucleotides are arranged in a double helix (two intertwined strands), with adenine in one strand always pairing with thymine in the other strand, and guanine always pairing with cytosine. The double helix structure of DNA was discovered in 1953 by the British scientist Crick and the American scientist Watson.

Because the double helix structure of DNA allows us (and nature) to determine the complementary strand from a single DNA strand, the genetic material is preserved in cell divisions.

In all higher forms of life, the DNA of a cell nucleus is translated into the single-stranded nucleic acid RNA, which is then used in ribosomes to assemble proteins from amino acids.

Adenosine triphosphate

Adenosine triphosphate (ATP) is an organic compound in a category by itself, even though it is structurally very similar to the adenine nucleotide of RNA (not DNA), featuring three phosphate group in a row where the nucleotide only has one.

But it's not so much the structure that puts ATP into a category by itself, but rather it's function as energy currency of cells, and actually, whole organisms. ATP allows the coupling of exergonic with endergonic reactions. Most of the energy released in cellular respiration ends up as ATP molecules which are energized ADP (adenosine diphosphate) molecules.

However, all ATP is consumed within a second or within seconds for cellular processes that require energy. Both the energy-requiring dehydration synthesis of ATP from ADP and the energy-releasing hydrolysis of ATP into ADP are catalyzed by enzymes, the energy-releasing process by ATPase and the energy-requiring process by ATP synthase.

The energy for the synthesis of ATP is obtained from both anaerobic and aerobic cellular respiration. Anaerobic respiration breaks glucose down into pyvuric acid (a process that generates 2 ATP per glucose molecule). Aerobic respiration (in the presence of oxygen) completely dissolves glucose into carbon dioxide and water and generates 36 to 38 ATPs per glucose molecule.

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