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

2. Structural Components of the Plant Cell
2.6 Structure and Functions of Membranes in the Plant Cell

The elements of the protoplast include membranes, which compartmentalize The Cell. Membranes feature a symmetrical, linear, trilaminar Structure. The core of this architecture is a lipid bilayer sandwiched between two protein monolayers. The total thickness of membranes is 6-10 nm. Evidence suggests that membranes contain pores, or channels. The walls of these channels are formed exclusively by protein substances.

In 1972, S. Singer and G. Nicolson proposed the fluid-mosaic model of the membrane. Today, the fluid-mosaic model of Introduction/36.html">Biological Membranes enjoys the widest recognition.

Membranes are lipoprotein complexes consisting of approximately 60% Proteins and 40% Lipids, predominantly Phospholipids (along with some galactolipids, sterols, and Fatty acids). The lipids are oriented with their hydrophobic tails facing each other. In phospholipids, two hydroxyl groups of the glycerol molecule are esterified with fatty acids, while the third is esterified with phosphoric acid. Various compounds, typically amino alcohols such as ethanolamine or Choline, can be attached to the phosphoric acid. The phospholipid molecule is polar, containing a hydrophilic HEAD (phosphoric acid, amino alcohol) and two hydrophobic hydrocarbon tails. The Unsaturated fatty acids of polar lipids ensure a somewhat loose (fluid) state of the bilayer at physiological temperatures, a property also supported by sterols. Lipids are not rigidly anchored; instead, they continuously exchange places. Lipid movement occurs in two ways: within their own monolayer and through The transfer of two lipid molecules between monolayers (flip-flop). During material diffusion, lipid molecules swap places a million times per second, whereas the translocation of lipid molecules from one monolayer to the other occurs much less frequently. The lipid layer is stabilized by hydrophobic interactions (Van der Waals forces and molecular cohesion), while hydrophilic interactions develop between lipids and proteins. Specialized Protein Complexes reside within the fluid layers of lipid membranes. These Lipoproteins are embedded in the lipid phase and held in place by Hydrophobic bonds (integral proteins).

Hydrophilic (peripheral) proteins are anchored to the inner and outer surfaces of membranes via Electrostatic Interactions with the hydrophilic heads of polar lipids. Hydrophobic interactions—lipid-lipid, lipid-protein, and protein-protein—play a crucial role in membrane formation. Membranes Contain Proteins that perform diverse Functions, acting as Enzymes, pumps, transporters, and Ion Channels, as well as structural and regulatory proteins.

Depending on the specific composition of lipids and proteins, the Structural Features of a membrane may vary.

Membranes perform the following functions:

1. Barrier function. Membranes partition the cell into distinct compartments. By surrounding the Cytoplasm and Organelles, they make it possible to confine essential enzymes and metabolites within small volumes, and to establish diverse—and sometimes opposing—biochemical reactions on opposite sides of the membrane.

2. Transport function. Membranes mediate the Transmembrane Transport of ions, substrates, and metabolites. Passive diffusion, Facilitated Diffusion, and active Ion transport across membranes are all distinguished. Thanks to membranes, the Organism functions as an open, dynamic system.

3. Osmotic function. The osmotic influx of Water into a plant does not cause cell bursting because the hydrostatic turgor pressure is counterbalanced by the pressure of elastically stretched cell walls. By regulating the concentration of osmotically active substances, plant Cells absorb water from their surroundings even when the environmental water content is low.

4. Structural function. Membranes organize the spatial arrangement of multienzyme complexes, achieving high catalytic efficiency through the close proximity of active sites.

5. Energy accumulation and transformation. On chloroplast membranes, light energy is transformed into the chemical bond energy of NAD⁺ and ATP, and ultimately into the chemical bond energy of sugars, organic acids, and Amino Acids. On mitochondrial membranes, ATP is generated and stored in high-energy bonds during Respiration.

6. Receptor-regulatory function. Membranes incorporate protein-based chemo-, photo-, and mechanoreceptors that are sensitive to chemical and physical stimuli. These receptors receive signals from the external and internal environments, enabling the cell to mount a response to changing conditions. Receptor systems also mediate functional interactions between individual Components of the cell protoplast.

7.  Various substances are concentrated and adsorbed on membranes.

8. Membranes possess the capacity for self-assembly. If membranes are treated with alcohol or chloroform (extracting the lipids) and the lipids are subsequently reintroduced, the Membrane Structure is restored.