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Understanding: • Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport
Passive Transport Passive transport involves the movement of material along a concentration gradient (high concentration ⇒ low concentration) Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis) There are three main types of passive transport:
Active Transport Active transport involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration) Because materials are moving against the gradient, it requires the expenditure of energy (e.g. ATP hydrolysis) There are two main types of active transport:
Types of Membrane Transport Contemporary research has focused on how selected molecules are able to enter and leave the brain and how CSF is formed. This work has led to an appreciation of the important role played by membrane transport processes in the function of the blood—brain—CSF barriers [1,2]. Monographs about the blood—brain and blood—CSF barriers are available for readers interested in a more complete review [3,4]. Physical and biological processes determine molecular movement across membranes of the blood—brain— cerebrospinal fluid barriersThe processes that determine molecular movement across membranes are diffusion, pinocytosis, carrier-mediated transport and transcellular transport [5]. The types of carrier-mediated transport are described in Chapter 5. Diffusion is the process by which molecules in solution move from an area of higher to lower concentration. With this type of transport, the net rate of solute flux is directly proportional to the difference in concentration between the two areas. In biological systems, this process is an important mechanism for the movement of molecules within a fluid compartment; however, diffusion across a lipid membrane, such as the cell membranes of the blood—brain barrier, is possible only when the solute is lipid-soluble or when the membrane contains specialized channels. Diffusion is the primary mechanism for blood—brain exchange of respiratory gases and other highly lipid-soluble compounds. Pinocytosis is a process by which extracellular fluid is engulfed by invaginating cell membranes, forming a vesicle that then separates from the membrane. This vesicle may move through the cell cytoplasm and release its contents on the other side of the cell layer by means of exocytosis. Under normal conditions, pinocytosis is thought to contribute little to the transport of solutes across the blood—brain barrier. Instead, the few vesicles that are observed within brain capillary endothelial cells are probably destined to fuse with lysosomes. Nevertheless, some proteins may traverse the brain endothelial cell through a process that has been called absorptive-mediated transcytosis [6]. Transcellular transport across a layer of cells requires the presence of carrier or channel molecules on the luminal and antiluminal sides of the cells. Facilitated and active transport are defined in Chapter 5. In transcellular facilitated diffusion, the carriers on opposite sides of the cell are usually similar and solutes are not moved against concentration gradients. Active transport across a cell layer, however, requires a special arrangement of transport proteins within the plasma membranes. The active transport system is found on only one side of the cell and usually is associated with a nonactive transport system on the other side of the cell. With this arrangement, a solute accumulates within the cell by active transport through one membrane and subsequently leaves the cell by a channel or facilitated transport process through the opposite membrane. When plasma membranes of two surfaces of a cell have different properties, that cell is said to be polar. Cellular polarity underlies active transcellular transport and secretion of fluid by epithelial cells in the choroid plexus. When fluid is secreted at one site and absorbed at another, there is bulk flow of fluid. This means that solutes of various sizes move together with the solvent as a bulk liquid. This process is important in the circulation and absorption of CSF, which is secreted by the choroid plexus, circulated through the ventricular and subarachnoid spaces and absorbed through arachnoid villi into the bloodstream. Transport processes combine to provide stability for constituents of cerebrospinal fluid and brain extracellular fluidBradbury and Stulcova [7] defined stability of the blood—CSF systems as follows. If a substance is present in CSF at concentration CCSF and in plasma at concentration Cpl, stability occurs when, as a result of a change in plasma concentration, a new steady state is reached so that At steady state, the flux of this substance from plasma to CSF, Jin, must equal its flux out, Jout, so that for any change in plasma concentration, ΔCpl stability of CSF will occur when where Jin and Jout represent transport processes that need not be identical. For instance, one process might be passive and the other active. If the carrier involved in Jin is saturated at the usual plasma concentration, then the ratio ΔJin/ΔCpl will approach zero. Such carrier-mediated transport is probably the most common mechanism controlling the flow of water-soluble substances from the capillary lumen to the brain, but carrier systems also have been found to operate for outward flux. Here, the greatest stability is achieved when the carrier system operates well below saturation so that the ratio ΔJout/ΔCCSF is a positive number. Such asymmetrical carrier mechanisms have been implicated in the maintenance of a stable K+ concentration in CSF and may exist for some amino acids and organic acids. Which process will be used to move molecules across the cell membrane to an area of higher concentration?In facilitated diffusion, substances move into or out of cells down their concentration gradient through protein channels in the cell membrane.
What process moves materials through a cell from areas of low concentration to areas of high concentration?During active transport, substances move against the concentration gradient, from an area of low concentration to an area of high concentration. This process is “active” because it requires the use of energy (usually in the form of ATP).
What processes move across the cell membrane?The processes that determine molecular movement across membranes are diffusion, pinocytosis, carrier-mediated transport and transcellular transport [5]. The types of carrier-mediated transport are described in Chapter 5.
What is it called when molecules move across the cell membrane from an area of high concentration through a carrier protein?Facilitated diffusion. Some molecules, such as carbon dioxide and oxygen, can diffuse across the plasma membrane directly, but others need help to cross its hydrophobic core. In facilitated diffusion, molecules diffuse across the plasma membrane with assistance from membrane proteins, such as channels and carriers.
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