Cell Components and Function
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal in Principles of Physiology for the Anaesthetist, 2020
Primary active transport utilizes energy (ATP) to move substances against their electrochemical gradients at a faster rate. Active transporting membranes contain ATPase which breaks down ATP to liberate energy. An example of this is the Na+/K+ pump, a membrane protein with ATPase activity. By splitting ATP, it is alternately phosphorylated (with high affinity for Na+ and low affinity for K+) and dephosphorylated (high K+ affinity and low Na+ affinity). The exchange ratio of Na+ to K+ is 3:2; 3 Na+ are pumped out for every 2 K+ moving in. It is not responsible for the resting membrane potential (RMP). The RMP is caused by the concentration gradients of K+ and Na+ across the membrane that are maintained by the activity of the Na+/K+ pump and the membrane impermeability to Na+. The metabolic cost of the Na+/K+ pump is high and accounts for a large part of the resting oxygen consumption of cells.
The cell
Laurie K. McCorry, Martin M. Zdanowicz, Cynthia Y. Gonnella in Essentials of Human Physiology and Pathophysiology for Pharmacy and Allied Health, 2019
With active transport, energy is expended to move a substance against its concentration gradient from an area of low concentration to an area of high concentration. This process is used to accumulate a substance on one side of the plasma membrane or the other. The most common example of active transport is the sodium-potassium pump that involves the activity of Na+-K+-ATPase, an intrinsic, enzymatic membrane-bound protein. For each ATP molecule hydrolyzed by the Na+-K+-ATPase, the pump moves 3 Na+ ions out of the cell and 2 K+ ions into the cell. As will be discussed, the activity of this pump contributes to the difference in the composition of the ECF and the intracellular fluid, which is necessary for nerve cells and muscle cells to function.
Membrane Transport
Lelio G. Colombetti in Biological Transport of Radiotracers, 2020
The frog skin is clamped in an “Ussing” chamber (Figure 13) with identical electrodes on both sides of the membrane system inserted into identical solutions. Ussing found that with or even he could measure a short circuit current with JNa+ > 0. This was an important result because it had been claimed that active transport occurs through binding of the transported molecules to the intracellular matrix. There are indeed some systems where binding can contribute to concentration of molecules in cells or organelles: for instance, Ca2+ in the sarcoplasmic reticulum is bound by binding proteins such as calsequestrin; or methotrexate in leukemia cells is bound by the enzyme, dihydrofolate reductase. Even in those cases, however, bona fide active transport system has been demonstrated over and above the binding capacity of the proteins.
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.
Advance in placenta drug delivery: concern for placenta-originated disease therapy
Published in Drug Delivery, 2023
Miao Tang, Xiao Zhang, Weidong Fei, Yu Xin, Meng Zhang, Yao Yao, Yunchun Zhao, Caihong Zheng, Dongli Sun
Transporter-mediated uptake is divided into facilitated diffusion and active transport. Facilitated diffusion allows certain compounds to cross the placenta without energy. Active transport is an energy-dependent process that usually proceeds against a concentration gradient. The major superfamily of transporters found in the placenta are the SLC and ABC transporters (Al-Enazy et al., 2017; Staud et al., 2012). For instance, organic anion transporters are a family of transporters in the placenta, mediating transport in the maternal-fetal interface for metabolites, waste products, and hormones (Lofthouse et al., 2018). Similarly, transporters such as amino acid transporters, glucose transporters, and transferrin can deliver specific substrates across the placenta (Illsley, 2000; Parkkila et al., 1997). For example, iron is transported across the placenta through transferrin receptor-mediated endocytosis (Parkkila et al., 1997).
Peptide-mediated drug delivery across the blood-brain barrier for targeting brain tumors
Published in Expert Opinion on Drug Delivery, 2019
Behzad Jafari, Mohammad M. Pourseif, Jaleh Barar, Mohammad A. Rafi, Yadollah Omidi
The active transport is an energy-dependent process that requires either the ATP-utilizing sources or electrochemical gradient. The processes enable movement of substances against their concentration gradient. As reported previously, there are several active transporters that allow the non-lipophilic molecules (e.g., glucose, amino acids, small monocarboxylic acids, choline, vitamins, nucleosides, thyroid hormones, and peptides) to cross the BBB. For instance, there are several ATP-dependent transporters that are differentially expressed on the polarized BCECs. Of the active/facilitated transporters, several transporters are ascribed to the influx and/or efflux of essential nutrients, ions, and other endogenous compounds (or drugs) into the brain [21]. Generally, the main active and facilitated transporters include (i) ATP-binding cassettes (ABC) transporters, (ii) carrier-mediated transporters (CMTs), (iii) receptor-mediated transporter (RMT), (iv) adsorptive phase endocytosis (AME) and fluid phase endocytosis (FME). Some of the transport machinery entities are discussed in the following contexts.
Related Knowledge Centers
- Cell Biology
- Cell Membrane
- Fick'S Laws of Diffusion
- Adenosine Triphosphate
- Electrochemical Gradient
- Passive Transport
- Sodium–Potassium Pump
- Ion
- Glucose
- Amino Acid