Mechanisms of macromolecule and particle transport across plant cell membranes

The transport of macromolecules and large particles is carried out via "membrane packaging" transport, i.e., through the formation of membrane-enclosed vesicles. Depending on the direction in which substances are transported (into or out of the cell), membrane-packaging transport is subdivided into endocytosis and exocytosis.

Endocytosis (from the Greek for "inside" and "receptacle/cell") is the process of a cell engulfing macromolecules and larger particles (viruses, bacteria, cell fragments). Endocytosis occurs via phagocytosis and pinocytosis.

Phagocytosis (from the Greek for "eater") is the process of active capture and engulfment by a cell of microscopic living objects (microbes, cell fragments) and other solid microparticles. During phagocytosis, the cell uses receptors to recognize specific molecular groups of the phagocytosed particle. Then, at the site where the particle touches the cell membrane, outgrowths of the plasmalemma called pseudopodia form, which envelop the microparticle on all sides. As a result of the pseudopodia fusing, such a particle becomes enclosed inside a membrane-bound vesicle called a phagosome. As it sinks into the cytoplasm, the phagosome can fuse with a primary lysosome, whereby the organic microparticle engulfed by the cell is broken down by hydrolytic enzymes into amino acids, sugars, and nucleotides and utilized by the cell.

Pinocytosis (from the Greek for "to drink") is the capture by the cell surface and engulfment by the cell of liquid (in the form of true and colloidal solutions) (Fig. 46; Yagodin B.A., 1980). This process is divided into 4 stages: 1) adsorption of molecules or ions of the substance to be absorbed onto the membrane; 2) invagination of the membrane inward and formation of a pinocytic vesicle, and its detachment from the membrane, which requires ATP energy; 3) migration of the pinocytic vesicle into the protoplast or an organelle; 4) dissolution of the vesicle membrane by enzymes (the pinocytic vesicle fuses with a lysosome) or simply its rupture. Thus, pinocytosis is the invagination of the membrane and its subsequent detachment, which results in the uptake of substances together with droplets of liquid. In pinocytosis, substances do not cross the membrane.

Fig. 46. Mechanisms of pinocytosis: A – the membrane invaginates into the cell, forming a narrow channel. Vesicles containing the captured substance pinch off from the end of the channel. B-Zh – a portion of the membrane on which macromolecules have adsorbed (V) invaginates (G). At the site of invagination, the membranes fuse (D) and the resulting pinocytic vesicle detaches from the cell membrane (E). Deep inside the cell, the vesicle membranes are degraded by enzymes (Zh)

Since different proteins are pinocytosed only in specific areas of the plasmalemma, it can be assumed that the membrane contains certain receptors onto which the absorbed substances are adsorbed. Consequently, the specificity of pinocytosis is determined at the first stage: substances for which there are no suitable receptors are not pinocytosed.

Pinocytosis is the primary method for the transport of macromolecules—proteins, lipids, and glycoproteins—into the cell. It is possible that chloride ions also enter the cell via pinocytosis. They penetrate through the plasmalemma in pinocytic vesicles. Then, these vesicles can either be directed straight to the tonoplast and open into the vacuole, or they can enter the endoplasmic reticulum. Consequently, chloride ions can reach the vacuole without essentially ever having been in the mesoplasm, i.e., by passing through it within pinocytic vesicles. If the mesoplasm is already saturated with ions, they enter the vacuole. The transport of ions into the vacuole is associated with overcoming yet another barrier—the tonoplast. The tonoplast and the plasmalemma possess different permeability. In particular, data on the electrical resistance of both membranes show that the electrical conductivity of the tonoplast can be 10 times higher. Unlike the plasmalemma, the tonoplast is positively charged, with a potential of +26 mV. The transport of ions across the tonoplast occurs with the help of pumps and carriers and in most cases requires the expenditure of energy.

Phagocytosis and pinocytosis can also occur in the opposite direction. Reverse endocytosis is called exocytosis. Exocytosis (from the Greek "exo" — outside) is the process of removing substances from the cell. In eukaryotic cells, various types of molecules are constantly secreted via the process of exocytosis. Some of them may remain on the cell membrane and become part of it, while others exit into the extracellular space. For example, secretory proteins are packaged into transport vesicles in the Golgi apparatus and then transported directly to the membrane. The formation of exocytic vesicles can occur rhythmically, at a constant rate, engulfing extracellular fluid and the components contained within it. In some cases, the initiating factor for vesicle formation is contact with a specific substance, or it becomes possible due to the presence of specific receptors in the membrane that capture ligands complementary to them. A number of contractile proteins play an important role in the invagination of the membrane and the formation of vesicles.

Macromolecular proteins that cannot penetrate the cell via transporters may enter through the plasmalemma via endocytosis. Since the functioning of contractile proteins requires ATP energy, the process of exocytosis can be classified as a mechanism of active transmembrane transport of substances.

Thus, when considering the existing variety of plant nutrition theories, it is necessary to distinguish between three forms of nutrient passage through the plasmalemma: 1) passive diffusion; 2) facilitated diffusion, provided by selective molecular transporters; 3) active transport of substances. In passive diffusion, nutrients pass through the membrane as a result of random molecular motion, and the magnitude of the flux depends linearly on the concentration and the permeability coefficient of the membrane for the given substance. Diffusion through the membrane can occur via hydrophobic regions for non-polar compounds and via hydrophilic pores for polar compounds. Facilitated diffusion implies the presence of specific transporters that bind to the transported substances and thus "facilitate" their passage through the plasmalemma. Such "facilitated" diffusion occurs along the concentration gradient without energy expenditure and is not accompanied by the accumulation of substances in the cell. The mechanism of facilitated diffusion is based on the reversible binding of the transported substance with a specific transporter, and the resulting "substance-transporter" complex diffuses within the membrane from the outer surface to the inner one, where the complex dissociates, releasing the substance into the cell. The free transporter then diffuses back to the outer surface of the membrane, where it binds with a new substance, and the cycle repeats. The transport of ions against the electrochemical gradient, i.e., active transport, is carried out by transport ATPases using ATP. ATPases received their name due to their inherent ability to break down ATP. The released energy is used for the transport of the substance, and the transport ATPase is re-phosphorylated. K+-ATPase, H+-ATPase, Ca2+-ATPase, and anion-sensitive ATPase are known. The active transport of H+ ions can also be supported by the energy of NADPH. The transport of H+ ions across the plasmalemma using ATP or NADPH is called a proton pump (H+-pump).

Active ion uptake across the plasmalemma is also possible via vector metabolism. The latter concept characterizes the transformation of substances carried out by enzyme ensembles, in which various enzymes are oriented in space relative to each other in a strictly defined manner. Sequential transformations of substances cause their transfer from one enzyme to another, i.e., simultaneously with the transformation of a substance, its directed movement in space occurs.

It should be noted that nutrients that have entered the cell from the soil solution, regardless of the method of their transfer across the cell membrane, are practically not involved in metabolism at the plasmalemma level, and after entering the internal space of the cell, they may follow the next path: after undergoing a cycle of sequential transformations, they may be incorporated into cellular structures; be concentrated in vacuoles or transported via xylem vessels to the above-ground part of the plant, or they may be excreted back.