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The cell and tissues
Published in Peate Ian, Dutton Helen, Acute Nursing Care, 2020
Diffusion is the movement of a substance from an area of higher concentration to an area of lower concentration. Figure 3.4 illustrates this principle. If dye is added to a beaker of liquid, the molecules will gradually spread throughout the liquid (Figure 3.4a). This happens because the molecules of dye move around in the solution and are constantly moving and bumping into each other. The consequence of this is that they gradually spread until they are evenly distributed throughout the solution (Figure 3.4b).
Membrane Transport
Published in Lelio G. Colombetti, Biological Transport of Radiotracers, 2020
Active transport has similar characteristics to facilitated diffusion. There is, however, one important difference: the molecules are transported against their gradient of electrochemical potential. There are a variety of ways molecules can be transported against their gradient. Active transport is, however, defined for the case in which direct coupling between transport and a metabolic reaction can be demonstrated.
Lung transporters and absorption mechanisms in the lungs
Published in Anthony J. Hickey, Heidi M. Mansour, Inhalation Aerosols, 2019
Mohammed Ali Selo, Hassan H.A. Al-Alak, Carsten Ehrhardt
Transporters are membrane-bound proteins that facilitate the translocation of their substrates across biological membranes. They can be classified into passive and active transporters. Passive transporters, also known as facilitated transporters, allow diffusion of solutes (e.g., glucose, amino acids, and urea) across biological membranes down their concentration gradient without the need for metabolic energy. Active transporters, on the other hand, require energy to transport their substrates across cell membranes (often against a concentration gradient) and are further classified as primary or secondary active transporters according to the source of energy used. Primary active transporters use energy derived directly from the hydrolysis of adenosine triphosphate (ATP), whereas secondary active transporters use the energy derived from the movement of one molecule down its concentration gradient to power the uphill movement of another molecule against its concentration gradient (23,24).
Merits and advances of microfluidics in the pharmaceutical field: design technologies and future prospects
Published in Drug Delivery, 2022
Amr Maged, Reda Abdelbaset, Azza A. Mahmoud, Nermeen A. Elkasabgy
Fabrication of nanoparticles in a continuous flow (single-phase) system involves the combination of fluids by diffusion in laminar flow streams that uses single or multiple solvents (Rhee et al., 2011; Clark et al., 2017). Diffusion is described as the process of molecules spreading from a higher concentration region to a lower concentration region through Brownian motion, resulting in gradual material mixing. Diffusion is explained mathematically using Fick’s law (Fick, 1855): j is the diffusive flux (mol /m2s1), D is the diffusion coefficient (m2 /s1), /m3), and x is the position of the species (m).
Anti-ageing peptides and proteins for topical applications: a review
Published in Pharmaceutical Development and Technology, 2022
Mengyang Liu, Shuo Chen, Zhiwen Zhang, Hongyu Li, Guiju Sun, Naibo Yin, Jingyuan Wen
The transcellular pathway refers to the transportation of solutes through a cell, including transcellular passive diffusion, transcellular active transport, and transcytosis (Kasting et al. 2019). Diffusion is the movement of chemicals from a region of higher concentration to a region of lower concentration. Active transport, also known as carrier-mediated transport, involves using energy to help specific molecules move across the barrier and against the concentration gradient (Fung et al. 2018). Since the cell membrane is lipophilic, it might resist the passive diffusion of hydrophilic or charged compounds. Transcytosis is another type of transcellular route, where macromolecules are carried across the cell membranes (Liu et al. 2019). These macromolecules are captured in vesicles on the side of the cell, drawn across the cell, and then ejected on the other side (Liu et al. 2019). However, most experimental studies suggest that the primary pathway across SC is the intercellular pathway, as described below.
Intraperitoneal chemotherapy for peritoneal metastases: an expert opinion
Published in Expert Opinion on Drug Delivery, 2020
Wim Ceelen, Helena Braet, Gabrielle van Ramshorst, Wouter Willaert, Katrien Remaut
The rate of drug diffusion is proportional to a concentration gradient, according to Fick’s first law of diffusion. The rate of diffusion depends on temperature, the physicochemical drug properties, and on the stromal architecture [40]. The temperature dependence is explained by the Einstein-Stokes equation, which states that diffusion is proportional to temperature and inversely proportional to the viscosity of the medium. The extent of diffusive transport depends on properties of the drug (molecular weight, size, charge, configuration) and the extracellular matrix (cellular composition, density, visco-elasticity, geometrical arrangement, electrostatic properties) [41]. Since the shape of a molecule will affect its diffusive behavior more than its molecular weight, the Stokes-Einstein radius of a molecule is often used as a measure of its size.