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Imaging of Intracellular Targets
Published in George C. Kagadis, Nancy L. Ford, Dimitrios N. Karnabatidis, George K. Loudos, Handbook of Small Animal Imaging, 2018
In order to cross the hydrophobic interior of the cell membrane, hydrophilic and large molecules need the help of membrane transport proteins. These integral membrane proteins provide a continuous protein-lined pathway through the bilayer. Two major classes can be distinguished: channel proteins, which form a narrow pore through which ions can pass (passive transport), and carrier proteins, which translocate specific molecules across (passive or active transport). However, these transport mechanisms are less relevant to imaging of intracellular targets. Indeed, ion channels are important in muscle and neuronal excitation, and a well-known example of a carrier protein that carries out passive transport is the glucose transporter, exploited by the most used extracellular molecular imaging marker [18F]-FDG. Active transporters include ATP-driven ion pumps, coupled transporters (or secondary active transporters), and ABC transporters. This final type of transporter is specialized in pumping small molecules, including drugs and other toxins, out of the cells and is not only involved in multidrug resistance in cancers but also contributes to the blood-brain barrier.
Physical properties of the body fluids and the cell membrane
Published in Ronald L. Fournier, Basic Transport Phenomena in Biomedical Engineering, 2017
The transport of essential water-soluble molecules across the cell membrane is achieved through the use of special transmembrane proteins that have a high specificity for a certain type or class of molecules. These membrane transport proteins come in two basic types: carrier proteins and channel proteins. The carrier proteins bind to the solute and then undergo a change in shape or conformation (ding to dong; see Figure 3.7), which allows the solute to traverse the cell membrane. The carrier protein, therefore, changes between two shapes, alternately presenting the solute-binding site to either side of the membrane. Channel proteins actually form water-filled pores that penetrate across the cell membrane. Solutes that cross the cell membrane by either carrier or channel proteins are said to be passively transported.
Structure and diversity of bacterial communities in the water column of three reservoirs in Yun-Gui Plateau, China
Published in Inland Waters, 2023
The gene functions in these reservoirs suggested that the bacterial community is involved in various biogeochemical processes, including carbohydrate, energy, and lipid metabolisms. The dominant abundance of the ‘environmental information processing’ category may indicate the presence of different types of compounds in the water, such as organic nutrients, heavy metals, or other pollutants. Some of these compounds would be uptaken by cells, which requires membrane transport (belongs to environmental information processing). Membrane transport proteins play an essential role in efficient metabolism and the translocation of solutes, such as nutrients, ions, drugs, and endogenous bioactive substances (Piepenbreier et al. 2017). Moreover, metabolism dominance proved the numerous activities bacteria involved in various pathways concerning metabolism. These results highlighted the gene diversity of bacteria and the multi-facet roles played by bacteria in such plateau reservoirs. To some extent, the increasing nutrient contents could increase the bacterial diversity, which implies the eutrophic status of these reservoirs.