Passive and active transport are two categories of transport, with passive not requiring external energy input, and active requiring a source of energy.
Passive transport uses the existing heat energy present in the environment, taking advantage of the random movement of molecules to allow substances to transport down concentration gradients. This allows cells to absorb substances from their surroundings without expending energy. Some molecules, such as water or other uncharged and small molecules, are able to diffuse directly across the membrane. However, this may not be the fastest or most efficient mode of transport. Where a greater rate is required, a protein may be employed. One example of a passive transport protein is the aquaporin. This allows the selective and rapid transport of water molecules, while preventing the loss of proton gradients. Using two NPA motifs (asparagine-proline-alanine), hydrogen bonds can be formed with the two lone electron pairs present on the oxygen in each water molecule, flipping the molecule. This breaks the ‘wire’ of water molecules, preventing hydronium ions and protons from passing through the channel. Aquaporins are used in renal water reabsorption, helping to maintain blood pressure and salt concentrations, and are induced into the membrane by vasopressin. Carrier proteins are a form of facilitated diffusion, including the GLUT1 transport protein. This protein binds a glucose molecule, which induces a conformational change in the protein structure. Following Jardetzky’s model of alternating access, only one side of the protein can be open at a time, preventing the loss of proton gradients during transport. After the conformational change, the protein releases the glucose molecule to the other side of the membrane, transporting the glucose down the concentration gradient. This form of passive transport is better for larger molecules, due to the inability to use a selectivity filter with such large molecules.
Active transport requires the input of energy from the cell, and can be split into two categories. Primary active transport is linked directly to the energy source, such as ATP, while secondary active transport is indirectly linked to the source of energy (such as a proton gradient). An example of primary active transport would be a sodium-potassium pump. This transport protein involves the binding of three sodium ions to the transport protein, before the protein is then phosphorylated by an ATP molecule. The phosphorylation induces a conformational change in the protein, transporting the sodium ions against the concentration gradient and releasing them on the other side of the membrane. Two potassium ions then bind to the transporter, and on dephosphorylation of the protein they are transported in the opposite direction, against the concentration gradient. The pump is now in its original conformation, ready to accept three new sodium ions. A secondary active transport process includes the lac permease. A proton binds to the transport protein, on $$-\text{COO}^-$$ groups, causing a protonation. This increases the protein’s affinity for lactose, presenting a high affinity binding site to the low concentration side of the membrane. On binding of a lactose molecule, the thermal energy present allows a conformational change to occur, switching the open side. The lactose binding site decreases in affinity, and the lactose is released into the cell, where there is a high concentration. The proton is also released into the cell, allowing the original conformation to be adopted. This has a net effect of transporting protons down their concentration gradient, and moving lactose against its concentration gradient.