Answer the following question given this paragraph: 

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...


Q: What type of molecules sit within a membrane and contain an aqueous channel that spans the membrane’s hydrophobic region?


A:
channel