Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

2. Structural Components of the Plant Cell
2.5 Cytoplasm, Its Structure and Main Properties: Viscosity, Elasticity, Isoelectric Point, Irritability, Movement, and Selective Permeability

Cytoplasm is a semi-fluid, transparent, and viscous homogeneous mass located beneath The Cell wall (comprising a system of boundary membranes—the Plasmalemma, tonoplast, and mesoplasm). Chemical composition of the cytoplasm: Water – 80–90%, Proteins – 10–12%, Lipids – 2–3%, sugars – 1–2%, Mineral Substances – 1–1.5%.

The plasmalemma is the cytoplasmic membrane separating the cytoplasm from the external environment, whereas the tonoplast surrounds the vacuole. The plasmalemma is thicker (7.5 nm) than the tonoplast (6 nm) due to its more complex Structure AND Functions, which include barrier, enzymatic, acceptor, and regulatory roles. The tonoplast contains specific transport proteins and is less permeable than the plasmalemma.

The mesoplasm consists of an unstructured portion of the cytoplasmic matrix (hyaloplasm) and structural elements embedded within it. Chemically, the hyaloplasm is a colloidal protein solution. According to N. I. Yakushnina, it consists of 80–85% water. The dry matter of the cytoplasm is approximately 75% protein. Additionally, the cytoplasm contains 15–20% fats and fat-like substances, a minor amount of CARBOHYDRATES (1–2%), and 1% mineral salts. The cytoplasmic matrix exhibits The properties of a colloidal system and readily alters its physicochemical state under the Influence of External and internal factors: shifting from a liquid (sol state) to a nearly solid, gel-like state (gel state). It is believed that the mixotropic Properties of the cytoplasmic matrix are determined by the presence of thread-like structures known as microfilaments. Each microfilament consists of two strands of the globular protein Actin, spirally twisted around one another.

Microfilaments can form a complex molecular network, thereby imparting solid-like properties to the matrix, or they can dissociate, conferring liquid-like properties. This occurs when proteins are in a highly dispersed state—meaning they carry charges of the same sign and are surrounded by Hydration and diffusion shells that prevent them from clumping together. Microtubules, together with microfilaments, form The structure of the Cytoskeleton.

The magnitude and sign of the charge depend on the pH of the cellular content. Reversible transitions from sol to gel are determined by pH values. It is well established that for every protein There is a specific pH value (the isoelectric point) at which its particle becomes neutral. The isoelectric point of most plant tissue proteins lies within mildly acidic pH values, whereas the intracellular pH is close to neutrality (6.3–7.0). Consequently, the Proteins of the cytoplasmic matrix usually carry a negative charge. If the pH of The Cell sap deviates in either direction from the corresponding isoelectric point, protein molecules acquire a net positive or negative charge, repelling one another and thereby helping to maintain the highly dispersed state of the cytoplasmic colloids.

The degree of dispersion of the cytoplasmic matrix colloids changes under METABOLISM/18.html">The Influence of factors that disrupt the hydration shells of protein molecules (acids, alkalis, high temperatures). As a result of dehydration, protein molecules lose their native Spatial Structure and coagulate. In this process, the cytoplasm loses its vital properties.

Viscosity is the ability of the cytoplasm to resist the displacement of certain components (ions, molecules, Organelles) relative to others. The viscosity of the cytoplasm is determined by its physicochemical state (sol and gel), which is based on the interaction between microfilaments. Therefore, the cytoplasm possesses so-called structural viscosity, the degree of which is determined by its hydration level and the structural specifics of its microfilaments.

At various stages of plant development, the cytoplasmic viscosity of their Cells changes. During Cell Division, viscosity is high; during elongation, it decreases due to extensive hydration of the cytoplasmic matrix, and during Cell Differentiation, it increases again as water is diverted from the cytoplasmic colloids into the vacuole. The degree of cytoplasmic viscosity in cells of different Organs also depends on their age. Changes in cytoplasmic viscosity during organ ontogeny reflect changes occurring during cell ontogeny. Throughout plant ontogeny, cytoplasmic viscosity increases until the budding phase, then decreases during flowering, and increases again after flowering ends. High cytoplasmic viscosity is characteristic of organ cells in a dormant state (seeds, tubers, bulbs). A decrease in cytoplasmic viscosity corresponds to more intense metabolism, whereas elevated (though not excessively high) viscosity correlates with greater organismal resistance to unfavorable environmental conditions.

The degree of cytoplasmic viscosity also depends on the plant genotype, which determines the structural Specificity of microfilament proteins.

Cytoplasmic viscosity is further conditioned by the peculiarities of the plant ecotype. In species from steppes and deserts, it is high; in mesophytes, it is somewhat lower; and in aquatic plants, it only slightly exceeds the viscosity of water.

Cytoplasmic viscosity is influenced not only by internal but also by external factors. At low temperatures, the thermal motion of microfilaments slows down. This promotes the stabilization of their molecular network and increases viscosity. With rising temperatures, the microfilament network breaks down, and the cytoplasm transitions into a liquid state. Cytoplasmic viscosity also depends on the presence of specific cations in the medium: monovalent cations (K⁺, Na⁺, Li⁺) decrease it, whereas divalent and trivalent cations (Ca²⁺, Mg²⁺, Al³⁺) increase it.

Elasticity is the ability of the cytoplasm to restore its shape following a deforming action. It is determined by the elasticity of microfilaments as well as the ability of The cell membrane to alter its surface area through the rapid breakdown (during plasmolysis) and subsequent formation (during deplasmolysis) of specific regions. Elasticity has an adaptive significance. Plants with higher cytoplasmic elasticity tolerate drought conditions better. In xerophytes, it is approximately three times higher than in mesophytes, enabling the former to endure prolonged dehydration without significant cytoplasmic damage.

Cytoplasmic elasticity changes throughout the plant ontogeny. Specifically, during flowering, it decreases slightly in most plants. The higher the cytoplasmic elasticity, the more difficult it is to plasmolyze.

Streaming (movement) is a property characteristic of practically all actively functioning living cells. In some plants, the cytoplasm moves at high velocity (cells of aquatic plant leaves, epidermal hairs of cucurbits and gloxinia, hairs of Tradescantia staminal filaments), whereas in others, movement is barely noticeable.

Depending on Cell Structure, several types of cytoplasmic streaming are distinguished.

Oscillatory movements are the most widespread. In this case, some cytoplasmic particles remain at rest, others move toward the periphery, and some toward the center of the cell. These movements are non-permanent and random, and are sustained by energy released through metabolism.

Rotational movements (cyclosis or circular streaming) are characteristic of cells where the cytoplasm is restricted to the periphery because of a large central vacuole. Organelles, including Chloroplasts clearly visible under a Light Microscope, are carried along with the cytoplasmic stream.

Circulatory movements are inherent to the cytoplasm of cells containing several large vacuoles (e.g., in the hairs of Tradescantia staminal filaments). This movement consists of cytoplasmic displacement in various directions along the cytoplasmic strands that separate the vacuoles. Fountain streaming occupies an intermediate position between circulatory and rotational movements. It can be observed in ROOT hairs and in the pollen tubes of many plants. The basis of cytoplasmic movement is the wave-like contraction of microfilaments, which occurs as a result of the chemical interaction of their proteins—Actin and Myosin. Elongated myosin molecules are either freely distributed in the cytoplasm, bound to membranes, or linked to actin filaments. Microfilament contraction takes place during The formation of the Actomyosin complex. Due to myosin, which exhibits ATPase activity, the chemical energy of ATP is converted into the mechanical energy of sliding of these two protein filaments relative to each other.

Cytoplasmic streaming activates the transformation of metabolites, thereby accelerating the metabolism of substances and energy within the cell.

Cytoplasmic streaming is an active process accompanied by the expenditure of ATP energy. Therefore, it proceeds at a specific Temperature optimum and corresponding ambient pH (4.5–5.0). The velocity of cytoplasmic streaming increases under the influence of factors that promote ATP production during Photosynthesis or Respiration, namely illumination and oxygen.

Selective permeability is the ability of the cytoplasm to allow various substances to pass through at unequal rates. Selective permeability is caused by the presence of boundary membrane layers and is characteristic exclusively of living cytoplasm.

Cytoplasmic permeability increases with rising temperature and illumination, under water deficit conditions, and also during cell Aging as a result of the disruption of the native Membrane Structure.

Irritability is the ability of the cell to respond appropriately to environmental and internal stimuli. In the absence of a stimulus, the cytoplasmic membrane maintains a so-called Resting Potential. It arises due to the asymmetric distribution of certain ions on either side of the membrane resulting from its differential permeability. Ca²⁺ and Na⁺ ions accumulate on the outer side, whereas K⁺ and Cl^- ions gather on the inner side, causing the latter to become negatively charged relative to the outer side, and sometimes even turning briefly positive. A potential of the opposite sign that arises in the membrane as a result of stimulation is called an Action Potential (AP). The duration of an AP is several seconds. Following a brief increase in membrane permeability to Ca²⁺ and Cl^- ions, its permeability to K⁺ ions increases. They move down their concentration gradient from the cell into the environment and accumulate on the outer side of the membrane, such that its positive charge relative to the inner side gradually increases. The restoration of the resting potential then begins.

Irritability is of great adaptive significance. It is driven by the expenditure of ATP energy.

Consequently, factors that inhibit ATP synthesis (low temperatures, oxygen deficiency) reduce the ability of the cytoplasm to perceive stimuli.