Electrolytes: Their function and movement
Bernie Garrett in Fluids and Electrolytes, 2017
Facilitated diffusion is another form of passive transport, but here the movement of some molecules (such as fatty acids or polar ions, which move less easily) is facilitated by carrier proteins. Larger molecules, especially those that are not soluble in lipids (such as amino acids), do not pass so easily through cell membranes. Likewise, polarized molecules or charged ions (e.g., glucose and sodium and chloride ions) are water soluble, but cannot easily diffuse across cell membranes due to the hydrophobic nature of the cell membrane itself, which repels them. Carrier proteins (also called permeases or transporters) bind to specific molecules and undergo a series of configuration changes that have the effect of carrying the solute to the other side of the cell membrane. The carrier then discharges the solute and reorients in the membrane back to its original state. A specific carrier will transport only a small range of related molecules (see Figure 3.4).56
The microcirculation and solute exchange
Neil Herring, David J. Paterson in Levick's Introduction to Cardiovascular Physiology, 2018
In stark contrast to most capillaries, cerebral capillaries rely on specific carrier proteins in the endothelial cell membrane to transport essential, lipid-insoluble solutes between the blood and brain parenchyma. There are specific carrier proteins for D-glucose, the natural, dextrose form of glucose (carrier glucose transporter 1, GLUT-1), lactate, pyruvate, amino acids and adenosine. This form of transport is transcellular, as opposed to paracellular. The transport is not active, but is brought about by the diffusion of the carrier-bound solute down its concentration gradient (facilitated diffusion). In addition, the cerebral endothelium can regulate the K+ concentration of cerebral interstitial fluid by active transport, via Na+/K+-ATPase in the ablumi- nal membrane (Section 15.4).
Finding a Target
Nathan Keighley in Miraculous Medicines and the Chemistry of Drug Design, 2020
Transport across plasma membranes is a crucial part of a cells existence. The cell membrane presents a barrier to most polar molecules, which is important for maintaining concentrations of solutes in the cytoplasm. Likewise, the membrane-bound organelles within the cell can have a specific concentration of molecules contained within; different from that of the cytoplasm or extracellular medium. However, critical substances required by the cell must have a means of entering the cell as well as the removal of waste products. This is where the key role of transmembrane transport protein comes into fruition; as they are responsible for transporting these water-soluble molecules across the plasma membrane. A given transport protein will be responsible for assisting the movement of closely related groups of organic molecule, or a specific ion, across the membrane. There are two classes of membrane transport protein: carrier proteins and channel proteins. Carrier proteins have moving parts, activated by the chemical energy source ATP, that mechanically move small molecule across the membrane. This is known as active transport. Channel proteins form a narrow hydrophilic pore that enables the passive movement of inorganic ions, known as facilitated diffusion. By these mechanisms, the cell can create large differences in composition between the internal environment and extracellular medium. This is essential for specialised cells to perform their role in the body.
Evaluating the safety profile of focused ultrasound and microbubble-mediated treatments to increase blood-brain barrier permeability
Published in Expert Opinion on Drug Delivery, 2019
Dallan McMahon, Charissa Poon, Kullervo Hynynen
Drug delivery to the brain is limited by the BBB. Gaseous or small hydrophobic molecules (<400 Da) can diffuse across the BBB; however, more than 98% of the small-molecule drugs are excluded from entry into the brain parenchyma [1]. A major constituent of the BBB that limits entry of substances into the brain is the presence of tight junction proteins between endothelial cells, contributing to the physical barrier between the systemic circulation and brain [2]. Astrocytic endfeet processes, which enwrap almost all brain capillaries [3], as well as pericytes [4] and the anionic extracellular matrix also act to limit drug penetration and diffusion into the CNS [5,6]. Carrier proteins allow the transport of specific molecules into and out of the brain parenchyma, such as glucose and neutral amino acids [7]. The BBB limits exposure of the brain parenchyma to pathogens and aids in the maintenance of homeostasis, ensuring optimal conditions for neural function (Figure 1).
Influences of different sugar ligands on targeted delivery of liposomes
Published in Journal of Drug Targeting, 2020
Changmei Zhang, Zhong Chen, Wenhua Li, Xiaoying Liu, Shukun Tang, Lei Jiang, Minghui Li, Haisheng Peng, Mingming Lian
The movement of glucose in the blood or tissue is mediated by facilitative Glucose Transporter (GLUT) on cell membrane. GLUT is a kind of carrier protein embedded on the cell membrane. It transports glucose along the concentration gradient by using a way of facilitated diffusion without consuming energy. Therefore, GLUT can be specifically identified and bound by glucose or glycoconjugate [3]. The specific substrates of GLUT include glucose, galactose, mannose and their derivatives. These substrates can be modified relevant ligand complexes, which will occur cluster and invagination, then be swallowed by cells into lysosomes to release the drug at last [4]. Therefore, conjugation of some different ligands on the surface of liposome can achieve the purpose of active targeted drug delivery and improve the therapeutic effect of drugs.
Thyroid Feedback Quantile-based Index correlates strongly to renal function in euthyroid individuals
Published in Annals of Medicine, 2021
Sijue Yang, Shuiqing Lai, Zixiao Wang, Aihua Liu, Wei Wang, Haixia Guan
The secretion of thyroid hormone is regulated by hypothalamic-pituitary-thyroid (HPT) axis. Thyrotropin-releasing hormone (TRH) from the hypothalamus promotes the synthesis and release of TSH from the anterior pituitary, which plays an important role in all stages related to the production and secretion of thyroid hormones from the thyroid gland. The levels of TRH and TSH are in turn modulated by the negative feedback of thyroid hormones. Thyroid hormones are mainly secreted in the form of T4, which is catalyzed by deiodinases to form the bioactive T3. Both will bind to carrier proteins in the circulation and enter cells via membrane transporters. T3 then further binds to the nuclear thyroid hormone receptors. Therefore, thyroid function is regulated by the HPT axis and other factors associated with thyroid hormone conversion and bioactivity [26].
Related Knowledge Centers
- Facilitated Diffusion
- Integral Membrane Protein
- Macromolecule
- Membrane Protein
- Molecule
- Protein
- Transmembrane Protein
- Active Transport
- Ion
- Biological Membrane