What type of molecules sit within a membrane and contain an aqueous channel that spans the membrane’s hydrophobic region?
mole
osmotic fluid
microorganisms
channel
you could prove that movements are occurring even in the absence of a gradient. In a similar manner, there are analogous carrier systems that move hydrophobic molecules through water. Channel molecules sit within a membrane and contain an aqueous channel that spans the membrane’s hydrophobic region. Hydrophilic molecules of particular sizes and shapes can pass through this aqueous channel and their movement involves a significantly lower activation energy than would be associated with moving through the lipid part of the membrane in the absence of the channel. Channels are generally highly selective in terms of which particles will pass through them. For example, there are channels in which 10,000 potassium ions will pass through for every one sodium ion. Often the properties of these channels can be regulated; they can exist in two or more distinct structural states. For example, in one state the channel can be open and allow particles to pass through or it can be closed, that is the channel can be turned on and off. Channels cannot, however, determine in which direction an ion will move - that is determined by the ion gradient across the membrane. The transition between open and closed states can be regulated through a number of processes, including the reversible binding of small molecules to the protein and various other molecular changes (which we will consider when we talk about proteins). Another method of channel control depends on the fact that channel proteins are i) embedded within a membrane and ii) contain charged groups. As we will see cells can (and generally do) generate ion gradients, that, is a separation of charged species across their membranes. For example if the concentration of K+ ions is higher on one side of the membrane, there will be an ion gradient where the natural tendency is for the ions to move to the region of lower K+ concentration.222 The ion gradient in turn can produce an electrical field across the plasma membrane. As these fields change, they can produce (induce) changes in channel structure, which can switch the channel from open to closed and vice versa. Organisms typically have many genes that encode specific channel proteins which are involved in a range of processes from muscle contraction to thinking. As in the case of carriers, channels do not determine the direction of molecular motion. The net flux of molecular movement is determined by the gradients of molecules across the membrane, with the thermodynamic driver being entropic factors. That said, the actual movement of the molecules through the channel is driven by thermal motion. Questions to answer & to ponder: • What does it mean to move up a concentration gradient? • Are there molecules that can move up their concentration gradients spontaneously? • Where does the energy involved in moving molecules come from? Is there a force driving the movement of molecules "down" their concentration gradient? • If there is no net flux of A, even if there is a concentration gradient between two points, what can we conclude? • Draw a picture of valinomycin’s position and movements within a typical membrane. What drives the movement of valinomycin in the membrane and what factors lead to a net flux in K+ movement? • What happens to the movement of molecules through channels and transporters if we reverse the concentration gradients across the membrane? 222.