Passive, primary active, and secondary active transport are the three ways of transferring species across a membrane.
Passive transport requires no external energy input beyond heat (which is naturally present in any living system). By allowing substances to flow down their concentration gradient, directly through the membrane or a transport protein, the cell can easily take up substances required in low concentrations. Aquaporins are one example of a channel protein being used for facilitated diffusion. This is a highly selective transmembrane protein, allowing the flow of only $$\text{H}_2\text{O}$$ molecules through, preventing the loss of a proton gradient. This is attained through two half-helices found within the pore, forming a selectivity filter. The NPA motif, formed of asparagine-proline-alanine residues, forms hydrogen bonds with the water molecules. This causes the ‘wire’ of molecules to break as they flow through the pore, preventing any hydronium ions from passing through a dissipating the proton gradient. By not necessitating any conformational change in the protein, transport is able to occur at the maximum rate of diffusion, enabling an increased rate of water transport across the membrane. This is used in renal water reabsorption, allowing for greater uptake and thus reduced water loss.
Primary active transport involves the coupling of the energy source directly to the transport protein. An example of this would be the Na+/K+ ATPase, however other ATPases also exhibit primary active transport properties. The Na+/K+ ATPase allows the cell to maintain its resting potential, pumping Na+ out, and K+ in to the cell. This process has to be active, due to the high differences in concentrations (12mM to 150mM Na+, for example). By using ATP hydrolysis, the conformational change from the phosphorylation of a tyrosine residue is able to occur, enabling the transport of 3 Na+ out and 2 K+ in. The conformational change prevents the risk of loosing a membrane potential through a ion channel protein, which is of utmost importance in nerve cells where electrochemical gradients must be carefully regulated in the initiation and transmission of an action potential.
Secondary active transport involves the indirect coupling of the source of energy to the transport process. Lac permease, an enzyme transporting lactose across the membrane against the concentration gradient, couples H+ transport (down the gradient) with lactose transport (against the gradient). This is secondary transport, as the H+ gradient is produced by a different protein, separating the energy source from the transport process. Lac permease requires the binding of a proton (H+) first, inducing a conformational change and increasing the transport protein’s affinity for lactose. This allows the binding of lactose, even in very low concentrations. Once lactose has bound to the H+-protein complex, a second conformational change occurs, closing access to the extracellular region, and then allowing access to the cytoplasm. By having this varying conformation, there is never direct access from the inside of the cell to the outside - this is highly beneficial, as due to the width of the channel, ions and solutes would easily be able to pass through. The affinity to lactose decreases, lactose dissociates, and then the H+ ion. This allows the protein to adopt its original conformation, ready to bind another H+ ion and then transport a lactose across the membrane.
Passive: Aquaporin, GLUT1 Primary: Na+/K+ ATPase (incl. other ATPases as well) Secondary: Lac permease