Passive transport is a mode of transport in which substances follow the concentration gradient and do not consume cellular metabolizable energy (ATP). The transport power comes from the concentration gradient potential energy or potential difference of substances inside and outside the plasma membrane.Passive transportation is divided intoSimple diffusionandFacilitating diffusion。[1]
Chinese name
Passive transportation
Foreign name
passive transport
Transport characteristics
No consumption of cell metabolic energy
Transport power
Concentration gradient potential energy or potential difference
Simple diffusion(simple diffusion) refers to the transportation of fat soluble small molecular substances through the membrane. The transported small molecular substances have a concentration difference on both sides of the membrane, which can be diffused along the concentration.The diffusion speed of small molecule substances depends on the size of molecules and their fat solubility, such as hydrophobic gas small molecule Otwo、COtwo、Ntwo, etc. can quickly pass through the cell membrane.Small molecules with high fat solubility such as ethanol can also pass through the cell membrane quickly.HtwoO、Urea, glycerol and other molecules have strong polarity and low fat solubility, so they pass through the membrane slowly.Larger hydrophilic molecules such as glucose and various charged molecules such as amino acids and nucleotides cannot be transported by simple diffusion.[1]
Facilitating diffusion
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Facilitating diffusion(facilitated diffusion), also known as assisted diffusion, is a way to transport substances along the concentration gradient with the help of transport proteins on the membrane.Hydrophilic nutrients such as glucose, amino acid, nucleotide and Na+，K+，Ca2+Isocharged ions can be transported across the membrane through facilitation diffusion.Transport proteins that facilitate diffusion areCarrier proteinAccording to this, facilitation diffusion can be divided into two types: carrier protein mediated facilitation diffusion and channel protein mediated facilitation diffusion.Proteins that mediate active transport are also called carrier proteins.
Carrier protein mediated facilitated diffusion
The transport process is realized through the reversible conformational change of the carrier protein.The carrier protein is a transmembrane protein related to material transport on the membrane, which is highly selective to the transported substances. When the specific binding site on the surface of one end of the carrier protein binds to a specific solute molecule, the spatial conformation of the carrier protein changes, and the transported solute molecules are transported from the bonded side to the other side of the membrane;The affinity of the allosteric carrier protein to the transported substance changes at the same time, so the transported solute molecule is separated from the carrier protein and released, and the carrier protein returns to its original conformation.The carrier protein is recycled through repeated conformational changes.
The rate of facilitating diffusion depends on the concentration difference of solute molecules on both sides of the membrane.With the increase of concentration difference, the transportation speed is accelerated, but when the solute molecule and carrier protein binding site are saturated, the transportation speed reaches saturation and will not increase again.Carrier proteinIts activity can be regulated, among which hormones play a major role.
Most mammalian cell membranes containGlucose transporter(glucose transporter, GLUT), which transports glucose into human cells by facilitating diffusion. For example, there are about 50000 glucose transporters on the human erythrocyte membrane, the number of which is equivalent to 5% of the total membrane proteins, and the maximum transport speed is about 180 glucose molecules per second.The glucose transporter family, including GLUT1-GLUT14, is involved in glucose transport.GLUT1 is distributed on the plasma membrane of various cells, has high affinity for glucose, and is easy to absorb glucose into human cells.GLUT2 is mainly distributed in liver cells, pancreatic islet β cells (rodents), small intestine and kidney with absorption functionepithelial cellsThe combination with glucose has the characteristics of high foaming degree and low affinity.GLUT3 is distributed in the brainNeuron cellIt has a high affinity for glucose and transport capacity, and can rapidly transport glucose from extracellular fluid even when the blood glucose level is slightly low, so as to ensure the energy supply of neurons.GLUT4 is distributed in muscle cells andfat cellsOn the membrane, insulin can adjust the quantity of GLUT4.Under normal circumstances, GLUT4 of target cells is stored in cells in the form of vesicles. When postprandial blood glucose rises, insulin stimulates islet cells to secrete insulin. Insulin stimulates target cells, so that the intracellular vesicles containing GLUT4 quickly move to the cell surface. GLUT4 is inserted into the plasma membrane to improve glucose uptake and ensure the stability of blood sugar.On the contrary, when the body is in a state of starvation, the blood sugar concentration decreases. Under the action of glucagon, liver glycogen is degraded and a large amount of glucose is produced. The intracellular glucose sugar concentration is higher than that of extracellular glucose. Glucose binds to the binding site inside the carrier protein cell and transports asparagus tang out of the cell.Diabetic patients are often accompanied by insufficient quantity or decreased function of GLUT4. When blood sugar rises, glucose cannot enter human target cells smoothly, resulting in continuous increase of blood sugarinsulin resistance One of the reasons.[1]
Channel Protein Mediated Facilitation Diffusion
The transport process is completed by the channel proteins passing through the lipid bilayer.The center of channel protein is hydrophilic pore, and different channel proteins can transport ions, water and other small molecules respectively.Channel proteins that mainly transport ions are also called ion channels, which have high affinity and selectivity for ions.The transport rate of ion channels is high, and the number of ions transported per second is up to several million,Carrier proteinThe number of molecules carried per second is less than 1000.When some ion channel protein stars are closed, they are opened due to conformational changes when receiving specific signal stimulation. The opening time does not exceed several milliseconds, and then they are closed, which is equivalent to a gate, also known as a gate ion channel.
According to the different opening signals of the gate ion channel, it can be divided into three categories.The first type is the voltage gate channel. The opening and closing of the channel are controlled by the change of membrane potential, such as Na+Channel, K+Access, etc.The second type is ligand gate channel, whose opening and closing are regulated by chemicalsacetylcholine receptorChannel.The third type is stress activated channels. Channel proteins sense mechanical stress to change conformation and open channels.For example, the auditory hair at the top of the auditory hair cell in the inner ear has this channel. The sound wave stimulation is converted into mechanical vibration in the cochlea, which can cause the auditory hair to tilt and affect the stress activated K on the auditory hair+Channel, K+It flows into the hair cells of the inner ear and depolarizes the hair cells to produce auditory signals.
The instantaneous switch of the gate ion channel is conducive to the sequential operation of various functional activities of cells.During the whole reaction process of skeletal muscle contraction caused by nerve impulse, at least four kinds of gate ion channels were activated, opened and closed in sequence.
① When the impulse reaches the nerve endings, the nerve cell membrane depolarizes and the membrane potential decreases, causing the voltage gate Ca on the nerve endings membrane2+Open channel, high concentration of Ca outside the membrane2+Quickly enter the nerve endings to stimulate the secretion of neurotransmitters——acetylcholine。
② The released acetylcholine binds to the specific site on the ligand gate channel on the muscle cell membrane, and the gate opens instantaneously+A large number of cells enter, causing changes in membrane potential of muscle cells and local membrane depolarization.
③ The local depolarization of muscle cell membrane makes the voltage gate Na on the membrane+Open channel, more Na+It enters muscle cells and depolarization extends to the whole muscle cell membrane.
④ Depolarization of muscle cell membrane causes Ca on sarcoplasmic reticulum in muscle cells2+The channel is open and flows into the cytoplasm from the sarcoplasmic reticulum. Ca in muscle cells2+The concentration rises sharply, causing myofibril contraction.[1]
