The only step of the citric acid cycle (also known as the Krebs cycle, or the TCA cycle) that directly produces ATP or GTP is the conversion of succinyl-CoA to succinate.

In this reaction, succinyl-CoA is converted to succinate with the assistance of the enzyme, succinyl-CoA synthetase. During this reaction, ADP + Pi (or GDP + Pi) is also converted to ATP (or GTP) using the energy from the breaking of the bond between CoA and succinate. Thus, the overall reaction appears as:

While side products of some of the other reactions listed produce intermediaries that may be used to produce ATP in the future, these reactions do not directly produce ATP.

The purpose of the citric acid cycle is to produce energy (both directly via GTP, and indirectly via NADH and . As such, energy can be though of to be on the products side of the sum of the reactions of the Krebs cycle. From Le Chatelier's principle, we know that if we want to inhibit a forward reaction, we can increase the concentration of the products. This will inhibit the forward reaction, and push the equilibrium to the left. Thus, in a high energy state, the ratio of ATP:ADP, like that of NADH: is high since both ATP and NADH are products of metabolism.

Because an amino acid has been altered in sickle cell anemia, we can say that the amino acid sequence for hemoglobin has been changed. The amino acid sequence is defined as the primary structure for a protein, so that is the level that has been altered. It should be noted that the subsequent levels of protein structure would be altered as well, but the manipulation of the amino acid sequence is a changing of the primary structure first.

Trypsin will cleave the primary sequence after the lysine residue (on its carboxyl side). Thus, Lys will be the new carboxyl terminal and Glu will be the new amino terminal. Remember that a protein's primary sequence is written from N to C.

ATP synthase can indeed produce more than 100 ATP molecules per second, and in the process, it only requires a few -- three or four -- protons, per ATP. These protons pass down a gradient through the membrane. Hence, the protein is membrane-bound. The protons cause the rotor of 10-14 subunits to spin. The protein's head itself has six subunits, three of which have ADP binding and phosphate binding sites.

A peptide bond connects two amino acids. This is the result of a condensation reaction (water is lost) and a new nitrogen-carbon bond forms between two amino acids. Note that amino acid synthesis occurs in the direction. Peptide bonds are covalent bonds that are responsible for the primary structure of amino acids.

The backbone of DNA is made up of alternating phosphate groups and sugar groups, linked together via phosphodiester bonds. The nitrogenous bases jut off of the backbone and form bonds with nitrogenous bases on other strands of DNA to become double stranded. A nucleotide consists of a sugar, nitrogenous base, and one or more phosphate groups.

Cholesterol is made up of multiple rings, including three six-carbon rings and one five-carbon ring. This characteristic structure is also seen in steroid hormones and metabolites, as many biologically relevant molecules are derived from cholesterol. Fatty acids are long hydrocarbons (typically between ten and thirty carbons long) with carboxylic acid functional groups on one end. Glycolipids are lipids that have carbohydrate moieties attached, which play a role in cellular and molecular communication. Sphingolipids are a class of lipids that contain a sphingoid base backbone and include sphingosine, sphingomyelin, ceramides, gangliosides and others.

The electron transport system is the only metabolic process listed that directly requires molecular oxygen. Oxygen is the final electron acceptor (it is one of the most electronegative atoms in our bodies) in the electron transport chain. This is the same as saying that oxygen has the highest reduction potential, and is capable of receiving electons. If oxygen is not present to accept the electron from the final enzyme complex in the inner mitochondrial membrane, then electron transport will be inhibited and thus no ATP will be produced via chemiosmosis.

Note that the Krebs cycle, citric acid cycle, and tricarboxylic acid cycle (TCA cycle) all refer to the same process, and do not directly require oxygen (oxygen is neither a reactant nor a product in any of the steps). However, oxygen is indirectly required, as there is no point to this cycle without subsequent oxidative phosphorylation. Thus in the absence of oxygen, of the choices shown, only glycolysis will proceed uninhibited.

Primary structure: linear sequence of amino acids

Secondary structure: hydrogen bonds, alpha-helices and beta-pleated sheets

Tertiary structure: disulfide bonds, single polypeptide chain

Myoglobin is a monomer, and is made of a single polypeptide chain. Thus, its highest level of protein structure is tertiary. While collagen does contain different polypeptide chains, it is an example of a protein with quaternary structure, not an explanation of what this means.