Although some small molecules without electrical charge can pass through the semipermeable membrane, some hydrophilic large molecules also diffuse to the other side of membrane over a sufficiently long time, but the slow rate is clearly unable to meet active metabolism. There are some transport proteins on plasma membrane and endomembrane system to accelerate the shuttling process. Nevertheless, this still belongs to passive transport that consumes no energy. It follows the same rules as simple diffusion and osmosis: the greater the concentration difference, the faster the rate of passage.
Channel protein
Ion channel
The biggest difference between channel proteins and carrier proteins is that substrates only bind to channel briefly and lightly, so conformation doesn’t change significantly. Channel proteins are significantly more efficient than carrier proteins, allowing 10⁷~10⁸ ions to pass per second.
Its core region is a hydrophilic corridor that spans membrane. Brief interaction between ions and hydrophilic corridor is believed to be the key to selection. At the entrance, amino acid residues attract ions electrostatically to discard their water molecule shells. When the hydration layer disappears, ions interact with narrow corridor precisely. The force that pushes them forward is not only chemical force (concentration difference) but also electric force, because unevenly distributed ions have both concentration gradients and electric fields in the environment. This combination is called electrochemical potential. Ion channel certainly can’t accommodate overly large particles, and too small particles can’t squeeze in due to the failure of dehydration. Charged amino acid residues will also block ions with same charge. Each type of protein is designed to recognize only one substance or one group of very similar substances, so there are various types of ion channels on membrane.
Ion channels would be meaningless for regulating life activities if they are open permanently. At the entrance, there is a selectively opening gate to control the flow. These gates are open or close in response to a stimulus, voltage or ligands. Voltage-gated gates are most common in nerve cells. The voltage difference opens gate and a stream of sodium ions enter the cell to make biological electrical current. Signal transmission is also related to ligand-gated gates. At the synapse, the neurotransmitter binds to sodium ion channels in the next nerve cell and opens them to transmit the electrical signal. Mechanically-gated ion channels are rare. Changes in pressure or membrane tension trigger gate opening or closing. They play roles in touch, hearing, and balance.
Aquaporin
Seventy percent of components in living organisms are water. Every cell consumes a lot of water in life activities. Therefore, they need a more efficient channel protein to transport water molecules specifically. Aquaporins are widely present in animal and plant cells. They don’t have gates to restrict flow, so they remain open permanently. They are more efficient than ion channels, and 3x10⁹ water molecules pass through them per second. Single water molecules bind to channel via hydrogen bonds to reorient themselves and quickly pass through. Other ions bound by water, even the H₃O⁺, can’t pass through. Aquaporins play an important role in kidneys that reabsorb 180 liters water to concentrate urine to 1 liter daily. Otherwise, you would need to replenish the same amount of water every day.
Carrier protein
Because channel proteins may be too narrow for some molecules, they rely on carrier proteins to cross cell membrane. The main difference between carrier proteins and ion channels is that they bind more tightly to their cargo and trigger shape changes obviously. Their transport efficiency is significantly lower than ion channels, and only a few hundred or thousand molecules penetrate them per second.
Almost all cells have glucose transporters that consist of multiple transmembrane domains and switch between at least two different conformations. For example, when blood sugar levels rise after a meal, the binding sites are exposed extracellular environment to capture glucose. Then, they change shape to release glucose into cytoplasm. Glucose transporters are a bidirectional transport protein. When blood sugar levels are low, the breakdown of glycogen creates more glucose inside liver cells, so binding site appears in cytoplasm. This time, they transport glucose from cytoplasm to the extracellular environment.
Other carrier proteins operate similarly to glucose transporters, but some are not bidirectional. Driving force for substrate movement is still the concentration gradient. Although diffusion from high concentration to low concentration doesn’t consume cellular energy, the process is highly selective. For instance, glucose transporters only bind to D-glucose, not its isomer L-glucose, let alone other substances like amino acids or sucrose.