<P> In petunia flowers (Petunia hybrida), the ABC transporter PhABCG1 is involved in the active transport of volatile organic compounds . PhABCG1 is expressed in the petals of open flowers . In general, volatile compounds may promote the attraction of seed - dispersal organisms and pollinators, as well as aid in defense, signaling, allelopathy, and protection . To study the protein PhABCG1, transgenic petunia RNA interference lines were created with decreased PhABCG1 expression levels . In these transgenic lines, a decrease in emission of volatile compounds was observed . Thus, PhABCG1 is likely involved in the export of volatile compounds . Subsequent experiments involved incubating control and transgenic lines that expressed PhABCG1 to test for transport activity involving different substrates . Ultimately, PhABCG1 is responsible for the protein - mediated transport of volatile organic compounds, such as benezyl alcohol and methylbenzoate, across the plasma membrane . </P> <P> Additionally in plants, ABC transporters may be involved in the transport of cellular metabolites . Pleiotropic Drug Resistance ABC transporters are hypothesized to be involved in stress response and export antimicrobial metabolites . One example of this type of ABC transporter is the protein NtPDR1 . This unique ABC transporter is found in Nicotiana tabacum BY2 cells and is expressed in the presence of microbial elicitors . NtPDR1 is localized in the root epidermis and aerial trichomes of the plant . Experiments using antibodies specifically targeting NtPDR1 followed by Western blotting allowed for this determination of localization . Furthermore, it is likely that the protein NtPDR1 actively transports out antimicrobial diterpene molecules, which are toxic to the cell at high levels . </P> <P> In secondary active transport, also known as coupled transport or cotransport, energy is used to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP; instead it relies upon the electrochemical potential difference created by pumping ions in / out of the cell . Permitting one ion or molecule to move down an electrochemical gradient, but possibly against the concentration gradient where it is more concentrated to that where it is less concentrated increases entropy and can serve as a source of energy for metabolism (e.g. in ATP synthase). The energy derived from the pumping of protons across a cell membrane is frequently used as the energy source in secondary active transport . In humans, sodium (Na) is a commonly co-transported ion across the plasma membrane, whose electrochemical gradient is then used to power the active transport of a second ion or molecule against its gradient . In bacteria and small yeast cells, a commonly cotransported ion is hydrogen . Hydrogen pumps are also used to create an electrochemical gradient to carry out processes within cells such as in the electron transport chain, an important function of cellular respiration that happens in the mitochondrion of the cell . </P> <P> In August 1960, in Prague, Robert K. Crane presented for the first time his discovery of the sodium - glucose cotransport as the mechanism for intestinal glucose absorption . Crane's discovery of cotransport was the first ever proposal of flux coupling in biology . </P>

Cotransport is an active process in which the energy is supplied by