Figure 1. Active transport. (a) Maltose uptake by the high-affinity ABC transporter MalFGK ₂. (b) K ⁺ uptake by the TrkG/TrkH symporter. ABC is a commonly used abbreviation for the ATP-binding cassette, the ATP-binding region of the protein. The conformational changes in the protein that are caused by ATP hydrolysis bring about the uptake of maltose.

Figure 2. Fermentation of glucose to two molecules of lactate via the Embden-Meyerhof-Parnas pathway. 1, PEP: glucose phosphotransferase; 2, glucose phosphate isomerase; 3, phosphofructokinase; 4, fructose bisphosphate aldolase; 5, triose phosphate isomerase; 6, glyceraldehyde 3-phosphate dehydrogenase; 7, 3-phosphoglycerate kinase; 8, phosphoglycerate mutase; 9, enolase; 10, pyruvate kinase, 11, lactate dehydrogenase. Note that the dihydroxyacetonephosphate formed in reaction 4 is isomerized in reaction 5 to glyceraldehyde-3-phosphate so that two molecules of the latter are converted by reactions 7 to 9 to two molecules of PEP.

Figure 3. The aerobic respiratory chain. (a) Scheme of the oxidation of glucose to CO ₂ and reducing equivalents involving the EMP pathway, the pyruvate dehydrogenase reaction and the TCA cycle. (b) Translocation of protons during transfer of reducing equivalents from NADH and FADH ₂ to oxygen. The 20 H represent the 10 NADH generated in the course of glucose oxidation to CO ₂, as do the 4 H of the 2 FADH ₂ generated. The oval represents the cytoplasmic membrane.

Figure 4. Chemotaxis induced by aspartate (asp) that interacts with the MCP Tar. Intracellularly, the signal is transferred from Tar to CheY, which is kept in the unphosphorylated state. The flagellar motor rotates counterclockwise, so the cells swim smoothly. There is a continuous phosphorylation of CheY. At a certain level of CheY-P, the motors switch to clockwise and the cells tumble. CheR methylates Tar. In its totally methylated form, Tar does not interfere with the CheY/CheY-P ratio, so the cells swim and tumble. Upon demethylation by CheB-P, the tumbling frequency decreases again. Mot, flagellar motor; the methyl groups are provided by S-adenosylmethionine (SAM), they are released as methanol.

Figure 5. The ArcAB two-component system. ArcB is the sensor kinase, which is inactive in the presence of oxygen. In the absence of oxygen, it is phosphorylated by ATP and the phosphate residue is transferred to the response regulator ArcA that then activates the expression of the genes for pyruvate formate lyase (pfl), an enzyme involved in fermentation (see Figure S30). Furthermore, the expression of genes for the respiratory chain is repressed. Phosphorylation of the sensor kinase occurs at one particular histidine residue. The phosphate residue is transferred to an aspartate constituent of the response regulator.

Figure 6. Quorum sensing (QS) and toxin production in P. aeruginosa. (a) Low cell density, in which the AHL (acylhomoserine lactone) concentration inside and outside the cells is low, and the QS-regulated genes are not expressed. (b) High cell density under which AHLs are abundant and interact with receptors that serve as activators of gene transcription. The products are toxins, proteases and polysaccharides for biofilm formation.