Purposes of Plant Physiology as a Science
The possibility to constantly build up on available research knowledge and the potential for implementing thefinal results make plant physiology a fundamental science with practical applicability.
Plant physiology is a scientific discipline that explores the unique characteristics of various plant species, including both cultivated and wild flora Its primary aim is to understand and optimize essential processes such as growth, development, nutrition, and metabolism.
Plant physiology is a crucial scientific discipline that investigates the molecular, physiological, biochemical, and morphogenetic mechanisms underlying vital processes in plants This field of study focuses on how these processes evolve dynamically in response to varying environmental conditions.
• discovering the essence of the organism’s individual development and studying the interaction of genetic, physiological, enzymatic mechanisms during growth and development;
• elucidating regulatory and autoregulatory mechanisms under the action of external factors;
• detailing the biochemical theory concerning mineral nutrition of plants;
• elucidating the ways used by plants to improve the efficiency of solar energy utilization;
• investigating the mechanism of atmospheric nitrogenfixation and its utilization by superior plants;
• developing and detailing the theoretical bases of the use of biologically active substances;
• elucidating the laws of plant viability (mechanisms of nutrition, growth, movement, reproduction);
• improving the theoretical knowledge on maximizing crop yields;
• researching endogenous mechanisms of regulating physiological functions, including basic mechanisms of enzyme biosynthesis, transport of substances and regulatory action of biomembranes;
• decoding mechanisms that control the chronological sequence of genetic pro- gram implementation during plant ontogenesis, including intracellular interde- pendence, interaction between vegetal organs during growth, reproduction and,
• studying the regulation of secondary metabolite biosynthesis (alkaloids, rubber, phenolic compounds, etc.) which are often of great economical importance.
As an applicative science, plant physiology aims to increase plant productivity.
In wheat, an ear typically contains 3 to 5 flowers, with only 1 to 3 successfully fructifying The challenge in plant physiology is to enhance the fructification of all these flowers Understanding the factors that hinder metabolite formation and grain filling is essential for this process Key issues include the inadequate performance of the photosynthetic apparatus, often due to depleted chloroplast enzymes and diminished cellular energy reserves, such as adenosine triphosphate (ATP) To address these challenges, plant physiology conducts in-depth research.
• photosynthetic apparatus activity and efficiency of solar energy use;
• plant requirements for mineral nutrition;
• water regime and efficiency of water utilization;
• plant resistance to various unfavorable factors;
• the possibility of using growth regulators;
• physiological bases of implementing the morphogenetic program.
Understanding the absorption of nutrients by plant root systems is crucial for improving mineral nutrition and enhancing plant breeding By analyzing the rhythm and rate of nutrient uptake across various plant varieties, we can identify biological materials with optimal fertilizer absorption capabilities, which is essential for maximizing crop yields.
Understanding the functions of growth regulators, such as gibberellins and auxins, has significant practical applications in horticulture Gibberellins can be applied to tree seedlings in greenhouses to accelerate their growth in the first year, thereby minimizing greenhouse time, while auxins effectively stimulate seedling rooting Plant physiology plays a crucial role in determining the optimal exposure duration, concentrations, and age of seedlings for these phytohormones to maximize growth Additionally, it is essential to identify plants' nutrient and water requirements throughout different growth stages For instance, autumn cereals require substantial nitrogen for foliar regrowth in early spring after winter frost damage.
In winter vegetable production, obtaining healthy seedlings in greenhouses is challenging due to low light intensity caused by persistent cloud cover To prevent etiolation, characterized by long, weak stems, smaller leaves, and a pale yellow color, growers can use artificial lighting or apply diluted retardant solutions to inhibit seedling elongation.
Research Methods Used by Plant Physiology
Plant physiology is fundamentally an experimental science, with experiments serving as the primary research method, following the formulation of a hypothesis Research in this field is conducted through three essential aspects.
(1) Passing from a higher level to a more elementary one, from analyzing complex biological laws to studying simpler ones—physical and chemical This
Plant physiology serves as a crucial scientific discipline that has significantly contributed to advancements in molecular biology, including the discovery of the hereditary code and the mechanisms of protein biosynthesis Additionally, it has unveiled key principles governing the absorption and utilization of light quanta during photosynthesis Nevertheless, these findings alone do not provide a comprehensive understanding of the physiological processes that take place in plant organisms.
The V.A Engelhardt integral approach transitions from simple to complex processes, enabling the analysis of evolution at various biological levels, including DNA, RNA, proteins, enzymes, biochemical reactions, physiological processes, and cellular properties This pathway allows for regulation at each stage, supported by internal autoregulatory mechanisms that target DNA replication, RNA and protein synthesis, enzymatic activity, as well as the differentiation of cells, tissues, and organs.
(3) Physiological processes are studied in ontogenetic dynamics and in relation- ship with environmental factors.
Plant physiology research is conducted in various environments, including fields, vegetation pots, solariums, greenhouses, and laboratories, with modern studies often taking place in phytotrons—controlled environments introduced by R.A Millikan in 1949 Phytotrons feature specialized rooms equipped with natural or artificial lighting, temperature control, and adjustable humidity, allowing researchers to maintain specific vegetation parameters through automated systems This research involves conducting experiments and examining processes at multiple levels of biological organization using biochemical, biophysical, physical-chemical, and biological methods.
The study of living organisms encompasses various levels of organization, beginning at the molecular level with the examination of physical-chemical processes such as the synthesis and restructuring of proteins, nucleic acids, polysaccharides, and lipids, alongside the energetic and informational metabolism that governs these activities At the cellular level, researchers focus on the structure and properties of cells and their organelles, exploring their interrelationships Moving to the intercellular level, knowledge from diverse disciplines is integrated to understand principles like photosynthesis, respiration, and the interactions between tissues and organs Finally, at the organismal level, the coordinated functioning of organs and systems within an individual organism is analyzed, including the roles of different organs and the changes that occur due to accommodation.
Research at the population level examines the fundamental unit of evolution—populations—by investigating the interactions among individuals within a specific, often isolated, territory The dynamics and composition of these populations are closely linked to molecular, cellular, intercellular, and organismal levels of organization Additionally, studies at the biosphere level focus on the processes occurring within biogeocenoses, exploring the interactions between biotic and abiotic components of ecosystems.
Each of the mentioned levels of organization has its own specific research methods The observation of various phenomena is carried out with the naked eye
The advent of electronic microscopy and advancements in imaging technologies have significantly advanced plant physiology, ushering in the era of cellular organelle physiology This technology, combined with methods like ultracentrifugation of cellular homogenates, has enabled the exploration of submicroscopic structures of organelles Additionally, chemical micro-analysis has facilitated the understanding of their chemical composition, allowing researchers to decode the physiological functions of various cell organelles.
Scientific research in plant physiology frequently employs various techniques, including ordinary and electronic microscopy, centrifugation, chemical analysis, chromatography, radioactive labeling, gel filtration, electrophoresis, roentgen analysis, artificial system modeling, autoradiography, and in vitro culture.
Recently, alongside physiological and biochemical techniques, mathematical modeling has been increasingly applied to understand life processes and enhance plant productivity under specific growth and development conditions, employing a triad of models, algorithms, and programs.
Adaptation The evolutionary process by which the organism or species survives and reproduces in new environmental conditions.
Self-regulation, or autoregulation, is a fundamental characteristic of biological systems that enables the autonomous coordination and control of their components, ensuring the maintenance of dynamic equilibrium within the system.
Evolution The progressive development of living organisms during successive generations by means of accumulating favorable hereditary variations enforced by natural selection.
Enzyme A protein produced by the cell which controls the reactions of synthesis and degradation via its catalytic activity, playing a fundamental role in metabolic processes regulation.
Phylogenesis The history of the development of a species or other taxonomical unit during the evolutionary process.
Phytohormone A substance secreted by the plant cell in small quantities, which controls various aspects of growth, developmental transitions, organ morpho- genesis, response to various stress factors etc.
Photosynthesis is the essential process through which green plants and photosynthesizing microorganisms convert inorganic substances, such as carbon dioxide (CO2) and water (H2O), into organic compounds using light energy This process transforms solar energy into chemical energy stored in organic molecules.
1.3 Research Methods Used by Plant Physiology 11
Metabolism The totality of all the complex processes of synthesis (energy storage) and degradation (energy release) undergone by the substances in a living organism.
Morphogenesis Cyto-differentiation and development of visible structures (organs or parts) in an organism during ontogenesis.
Levels of organization Systems with a specific organization (characteristic of biological systems only) and with a character of universality.
Ontogenesis The series of transformations undergone by the organism, from egg fecundation to death, according to the scenario for the respective species.
Respiration The process of oxidative degradation of complex organic substances into inorganic ones accompanied by energy release.
Acatrinei Gh (1991) Reglarea proceselor eco fi ziologice la plante Editura Junimea, Ia ş i, p 280 Burzo I, Toma S, Cr ă ciun C ş a (1994) Fiziologia plantelor de cultur ă , vol 1 – 4 Chi ş in ă u, Ş tiin ţ a
Cr ă ciun T, Cr ă ciun L (1989) Dic ţ ionar de biologie Editura Albatros, Bucure ş ti, p 285
Der fl ing K (1985) Gormony rasteniy Mir, 304 p
Duca M (1996) Sisteme ş i mecanisme de autoreglare la plante Chi ş in ă u, USM, 199 p
Duca Gh, Z ă noag ă C, Duca M, Gladchii V (2001) Procese redox ợ n mediul ambiant Chi ş in ă u, 381 p Lebedev SI (1982) Fiziologiya rasteniy M Kolos, 544 p
Milic ă C, Doroban ţ iu N ş a (1982) Fiziologia vegetal ă Bucure ş ti, Ed Didactic ă ş i Pedagogic ă ,
Polevoy VV (1989) Fiziologiya rasteniy M Vysshaya shkola, 464 p
Polevoy VV (1982) Fitogormony L Izd Leningradskogo universiteta, 248 p
Tarhon T (1992) Fiziologia plantelor, vol I, II Chi ş in ă u, Lumina
Udovenko GV, Sheveluha VS (1995) Fiziologicheskie osnovy selektsii rasteniy, vol 2 VIRYakushina NI (1980) Fiziologiya rasteniy M Prosveshchenie, 303 p
The cell, as the smallest structural and functional unit of all living organisms, embodies the essential characteristics of life Composed of various structures, each has evolved to fulfill specific functions vital to the organism's survival and operation.
The cell wall is structured with a middle lamella, a primary cell wall, and a secondary cell wall, all formed during cell division It is primarily made of cellulose micro and macrofibrils embedded in an amorphous matrix of hemicellulose, pectic substances, and proteins, with optional components like suberin and lignin that enhance its rigidity Serving as a mechanical exoskeleton and a delimiting barrier, the cell wall features interconnected gaps that create the apoplast, a transport pathway for liquids in plants, while the symplast, formed by plasmodesmata, connects the cytoplasm of adjacent cells.
Protoplasm consists of a viscous liquid matrix known as hyaloplasma, which facilitates metabolic and energy exchange reactions, as well as substance deposition This matrix is also home to essential cellular organelles, including the nucleus, mitochondria, plastids, endoplasmic reticulum, Golgi body, ribosomes, and vacuole.
Biological membranes, including the plasmalemma, tonoplast, and organelle membranes, consist of fluid amphiphilic phospholipid bilayers embedded with proteins that serve diverse functions and define the membranes' unique properties These membranes are essential for compartmentalization, act as mechanical barriers, facilitate the transport of substances such as water through osmosis, enable ATP synthesis in chloroplasts and mitochondria, and perform receptor functions.
The Cell as a Structural, Morphological, Functional Unit
All living organisms on Earth are composed of cells, which are the smallest structural and functional units of life The term "cell" originates from the Latin word "cellula" and the Greek word "cytos," meaning "room." Unlike other life forms, viruses are classified as non-cellular entities.
Cells exhibit diverse shapes, sizes, and colors, yet all cell types meet the essential criteria of living organisms This is evidenced by successful cell cultivation on artificial media, in vitro reproduction, and the ability to regenerate entire plants.
The cell was first discovered in 1665 by English physicist Robert Hooke, who utilized an advanced microscope to examine cork samples The cell theory was developed between 1838 and 1839 by German researchers, botanist M.J Schleiden and zoologist T Schwann, who identified the cell as the fundamental structural unit of all living organisms In 1855, R Virchow reinforced this concept by stating that all cells arise from pre-existing cells, summarized by the phrase "omnis cellula e cellula." The foundational principles of cell theory emerged from these pivotal discoveries.
The cell serves as the fundamental structural and functional unit of all living organisms, functioning as an open thermodynamic system that continuously exchanges and transforms matter Eukaryotic cells contain a complex of organelles that regulate metabolism and manage energy consumption Cells can exist independently, as seen in bacteria, protozoa, and certain algae and fungi, or as part of multicellular organisms, such as plants and animals, which are composed of billions of specialized cells These cells perform various functions, including contraction, excretion, substance transport, and photosynthesis, with over 60 distinct cell types identified in plant organisms.
Vegetal and animal cells exhibit comparable structures, functions, and chemical compositions Despite being the simplest form of life, typically measuring just a few microns in diameter, cells possess a complex architecture Nonetheless, these cell types share analogous functions and structural components.
Vegetal cells possess a unique ability to harness solar energy, converting it into chemical or mechanical energy, setting them apart from animal cells This distinctive characteristic differentiates them from other eukaryotic cells.
• a system of plastids, present due to autotrophic nutrition;
• a central vacuole in the mature vegetal cell playing an essential role in osmosis regulation and maintenance of the turgor pressure;
• a cell wall (cell envelope) which confers rigidity to the tissues.
Qualitatively, the chemical composition of the vegetal and animal cells is similar (Fig.2.2, Table2.2).
Inorganic substances, such as water and inorganic ions, serve as essential structural components of various organic compounds and act as a reaction medium for cellular metabolism Organic substances in plant cells are typically categorized into distinct groups.
• structural molecules, normally not involved in cellular metabolism and forming instead the plant skeleton (cellulose, pectin, myosin, certain lipids, carbohy- drates, proteins);
• enzymes—macromolecules catalyzing cellular metabolism (ribulose-1,5- biphosphate carboxilase, phosphofructokinase, phospholipases, chlorophyllases, amylases, ribonucleases, catalase, ascorbate oxidase, superoxide dismutase);
• micromolecular active substances (pigments, vitamins and phytohormones);
• micromolecular metabolites (resulting from specific catabolic reactions, serve for the transport of electrons and protons, usually CoF);
• excretions deposited in the cell wall and vacuole (tannin, anthocyanin, lignin);
Table 2.1 Chemical composition of different plant cell organelles (% of dry matter)
Structural elements of the cell Proteins Lipids RNA
Fig 2.2 Chemical composition of the cell
2.1 The Cell as a Structural, Morphological, Functional Unit … 17
Each daughter cell is formed from the mother cell during the process of division The cellular theory ranks among the three most significant discoveries of the 19th century, alongside the laws of mass and energy conservation and transformation by A.L Lavoisier and M.N Lomonosov, as well as Charles Darwin's theory of evolution.
Structural Organization, Chemical Composition
and Function of the Cell Wall
The cell wall (cellular envelope) is a complex formation with 3 basic layers:
Table 2.2 Comparing the structure and composition of plant and animal cells (Pickering 1998)
Characteristics of an animal cell (the result of heterotrophic nutrition)
Common features of animal and vegetal cells (with regard to the processes of life maintenance)
Characteristics of a vegetal cell (the result of autotrophic nutrition)
In animal cells secretory vesicles containing cellular products like hormones or enzymes can be often found
The cell membrane surrounding the cytoplasm controls the entrance and exit of soluble substances being, thus, responsible for cell content separation from the environment
The cellulosic cell wall offers support and protection against potential lesions caused by water fl owing into cells due to the osmotic force
The cytoplasm of animal cells is more dense and contains more cellular organelles
The cytoplasm contains water, soluble substances like amino acids and carbohydrates and supports cellular organelles Various metabolic reactions occur in both the cytoplasm and cellular organelles
Chloroplasts contain a special pigment called chlorophyll (which absorbs light) and the enzymes necessary for glucose production via photosynthesis
Vacuoles are small and temporary They may be involved in digestion (e.g. phagocytosis) or excretion processes (contractile vacuoles can remove water excess from the cell)
The nucleus contains the genetic material (which makes up the genes and chromosomes and encodes genetic information).
Chromosomes become visible only during cell division
A big, permanent vacuole contains the water needed to generate the turgor pressure and is also a repository for ions and molecules
Carbohydrates are stored in the form of glycogen
Carbohydrates are stored in the form of starch (found in the cytoplasm and in chloroplasts)
During the telophase of mitosis, the middle lamella forms between two daughter cells as a result of Golgi apparatus activity Young cells from meristematic tissues typically exhibit a primary cell wall, which evolves into a secondary cell wall as the cells age, with both structures being produced by the protoplast.
Functions of the cell wall:
• represents a mechanical exoskeleton for the cell;
• confers shape to the cell and rigidity to vegetal tissues;
• prevents the rupture of the cytoplasmic membrane due to hydrostatic forces acting from the cell interior;
• represents a barrier for various infections;
• participates in the absorption and transport of water and mineral salts;
• participates in the exchange of substances;
• lectins contained in the cell wall recognize symbiotic bacteria, which cause the formation of root nodules in plants.
Properties The cell wall has a high rigidity but can also support elastic deformations The thickness of the cell wall in different plant species are ranging from 0.1 to 10àm.
The cell wall is primarily composed of cellulose, hemicellulose, pectic substances, and proteins Cellulose molecules, with the chemical formula (C6H10O5)n, form long unbranched chains that consist of 3,000 to 14,000 glucose residues.
Structure The cell wall consists of two basic components:
• the micro-and macrofibril complex;
• the amorphous complex (the matrix).
Cellulose macromolecules are interconnected by hydrogen bonds, forming microfibrils that combine into larger macrofibrils with a non-uniform structure These fibrils exist alongside well-organized, para-crystallized, and amorphous regions Within the cell wall, cellulose macro- and microfibrils are embedded in a gel matrix made up of hemicellulose, pectic substances, and proteins Hemicellulose is a soluble polymer composed of hexose and pentose units, with a lower degree of polymerization compared to cellulose, consisting of 150 to 130 monomers Pectic substances are carbohydrate-based polymeric compounds, while the proteins in the cell wall contribute to its elasticity and enzymatic functions.
The middle lamella, composed of an amorphous complex of protopectin and hemicellulose, forms a rigid mesh that cements mature cells through calcium pectate Insufficient calcium or pectic substances can lead to a mucous-like state and tissue maceration As fruits ripen, pectic substances in the middle lamella dissolve, causing softening During growth, cellulose, hemicellulose, and pectin are deposited on either side of the middle lamella, creating the primary cell wall, which consists of approximately 70% amorphous complex and 30% microfibrillar complex The relatively small cellulose molecules, linked by hydrogen bonds, result in a primary cell wall that is flexible, allowing for cell elongation during the second growth stage.
The secondary cell wall, located on the inner side of the primary cell wall, is composed of up to 80% cellulose, which forms micro- and macrofibrils with long chains containing up to 12,000 glucose residues This arrangement creates a strong network, while the gaps in the mesh can be filled with water-insoluble substances like lignin and suberin, increasing rigidity at the expense of elasticity These gaps connect the cell walls of various plant cells, forming the apoplast, which serves as a transport pathway and integrates all plant components, occupying 5% of the volume of aerial plant organs.
The cell wall features pores known as plasmodesmata, which enable the cytoplasm of adjacent cells to connect, creating a continuous network throughout the organism Each surface area of 100 µm² contains approximately 10 plasmodesmata.
Plasmodesmata, measuring 0.2 µm in diameter, facilitate the direct transport of water and substances between plant cells, creating a continuous flow known as the symplast Both the apoplast and symplast play crucial roles in transport functions and maintaining the structural integrity of the plant organism.
Structure and Ultrastructure of Cell Protoplasm
The cellular protoplasm, or cytoplasm (from gr cytos “cavity” and plasma
The cytoplasm, often referred to as the "structure" and "substance" of the cell, is a viscous, transparent, and colorless liquid that is immiscible with water It is characterized by its surface tension, constant pH and rH values, semipermeability, selectivity, excitability, viscosity, and movement Within this homogeneous medium, various organelles of differing sizes, shapes, structures, chemical compositions, and functions are suspended.
Electron microscopy, reveals cytoplasm as a heterogeneous and complex structure It consists of a cytoplasmic matrix—the hyaloplasma, in which the cel- lular organelles are suspended (Figs.2.4and 2.5).
Hyaloplasm (from gr.hyalos“transparent, glass”) represents the soluble phase, which performs a series of functions:
• a matrix for metabolic and energy exchange reactions (glycolysis);
• deposition of organic (glycogen, starch) and inorganic substances;
• cell adaptation to environmental conditions.
• a structural part, representing several types offibrillar and globular proteins, arranged in microfilaments (with the diameter of 6–10 nm) or in microtubules
Fig 2.4 The components of the vegetal cell (Yakushina N.I., 1980)
2.3 Structure and Ultrastructure of Cell Protoplasm 21
(with 25–30 nm in diameter) This is a three-dimensional structure and facili- tates vesicular and organelle transport within the cell;
The liquid within the gaps of the fibrillar network is composed of approximately 70% water and 30% organic compounds These organic compounds include carbohydrates (primarily in small amounts as monomers or oligomers), lipids (notably phospholipids), proteins, nucleotides, RNA, phytohormones, vitamins, and mineral compounds, which exist as dissociated ions such as K+, Ca2+, and Mg2+.
Fe 2+ , Mn 2+ , Cu 2+ , Zn 2+ , NH4 +
, CI − , PO4 3 −, SO4 2 −) or ions bound by organic molecules and,finally, inclusions (reserve substances).
The balance of electrolyte and non-electrolyte substances is crucial for establishing a specific physical-chemical state of the hyaloplasm This balance facilitates the autoregulation of metabolic processes within the cell and is essential for maintaining physiological and biochemical homeostasis, which is marked by optimal integrity.
The viscosity of hyaloplasm varies based on the presence of protein types, being more fluid with globular proteins and more viscous with fibrillar proteins, potentially transitioning from a sol to a gel state Metabolic processes are inversely related to viscosity and often reflect adaptation to environmental conditions such as cold and drought Protoplasm viscosity is significantly higher than that of water, showing variations throughout ontogenesis, with the lowest viscosity observed during flowering and the highest during seed maturation and anabiosis The hyaloplasm is dynamic, exhibiting circular or sliding motion and is characterized by a specific isoelectric point due to its protein content Additionally, it contains various organelles of different shapes and sizes that fulfill specific functions essential for the organism's vitality, categorized into two major groups.
(a) membranous organelles (bi-membranous and uni-membranous);
(b) non-membranous organelles, which have no phospholipid membrane on their surface.
Structure and Function of Biological Membranes
Biological membranes, derived from the Latin word for "parchment," are essential two-dimensional structures made up of lipids and proteins that compartmentalize living matter These membranes vary in origin, structure, and function, highlighting their diverse roles in cellular processes.
Cytoplasmic membrane (plasmalemma) is a thin, semipermeable film (6–
The plasmalemma, a thin membrane (10 nm thick) located beneath the cellulose envelope, separates the cell contents from the external environment and regulates the selective transport of nutrients Unlike the cell wall, the plasmalemma is inseparable from the cytoplasm, a characteristic evident during plasmolysis and deplasmolysis This flexible membrane can either adhere to or detach from the cell wall based on the cytoplasm's water content Additionally, the plasmalemma plays a crucial role in the synthesis and assembly of the cell wall The tonoplast, introduced by Hugo de Vries in 1885, is a membrane structurally similar to the plasmalemma that surrounds a vacuole.
2.3 Structure and Ultrastructure of Cell Protoplasm 23
The membranes of the cellular organelleshave a thickness of 6–7 nm and can be classified into:
Double membranes are characteristic for the nucleus, mitochondria and chlo- roplasts, while the simple membranes can be found in the endoplasmic reticulum, the Golgi apparatus, in lysosomes and vacuoles.
Biological membranes are thin, transparent films that create a system of folds, enhancing surface area, with thickness varying between organelles (5.5 to 20 nm) They exhibit selective permeability, allowing precise control over the inflow and outflow of substances For instance, the plasmalemma permits limited glucose passage, while the tonoplast remains impermeable, ensuring that photosynthesis products are effectively transported out of the cell without accumulating in the vacuole.
After cell death, biological membranes lose their property of semipermeability which proves that their structure and chemical composition are maintained by energy consumption.
Biological membranes are primarily composed of lipids (approximately 50%), proteins (about 33%), small amounts of polysaccharides, enzymes, and various ions, with the protein/lipid ratio indicating the membrane's functional activity The predominant lipids in membranes are phospholipids, followed by glycolipids and cholesterol, while sterols and unsaturated fatty acids enhance the phospholipid bilayer's porosity Phospholipids possess amphiphilic properties, featuring a hydrophilic (polar) end and a hydrophobic end, which facilitates their self-assembly into a bilayer in aqueous environments Lipids within membranes are dynamic, undergoing lateral diffusion within the monolayer and less frequently engaging in transverse diffusion ("flip-flop") between lipid layers In terms of lipid content, plasmalemma contains 35–40% lipids, mitochondrial membranes have up to 28%, and myelin membranes can contain as much as 80%.
Biomembrane proteins, including enzymes, receptors, pumps, channels, and structural proteins, primarily consist of hydrophilic (polar) amino acids that contribute to their unique properties These membrane proteins exhibit the ability to move within the fluid double lipid layer and can be categorized into three distinct groups.
• Integral—which form hydrophobic links with the lipids and pierce the double lipid layer;
• peripheric—proteins are on the surface of the biological membranes.
Biological membranes may also contain heterogeneous macromolecules (gly- coproteins, glycolipids) and several minor components (coenzymes, nucleic acids,antioxidants, carotenoids, pigments, etc.) depending on the role they perform.
Glycoproteins usually consisting of up to 15 monomers, function as receptors. Membrane components are formed in the endoplasmic reticulum and then modified in the Golgi body.
Biological membranes, as per the Singer-Nicholson model, consist of a double lipid layer that is impermeable to polar molecules and ions This fluid layer allows for rapid lateral movement of phospholipid molecules, including rotation and diffusion Integrated or partially integrated proteins, primarily globular in shape, are present on both sides of the lipid bilayer and facilitate trans-membranous transport These membranes exhibit asymmetry, featuring distinct internal and external leaflets that contain different compounds tailored to specific biochemical functions Importantly, all biological membranes arise from pre-existing membranes.
Biological membranes serve as crucial barriers that separate a cell's internal environment from the external surroundings, regulating the circulation of substances As cellular structures evolved, these membranes developed additional functions, enhancing their role in cellular processes.
• Protection Membranes serve as mechanical barriers to harmful factors of the environment, protecting the internal content of the cell, nucleus, cellular organelles, etc.;
• Transport Biological membranes contain structural and functional systems necessary for passive and active transport of substances (Fig.2.7);
• Energetic function.The internal membranes of the chloroplasts and mitochon- dria transform solar energy and the energy of redox processes into ATP macroergic bonds;
Osmosis regulation in biological membranes plays a crucial role in maintaining chemical gradients, enabling water absorption In certain aquatic plants, these membranes act as barriers to passive water diffusion, safeguarding the plants from potential damage.
Fig 2.6 Ultrastructure of the cellular membrane (Johnson and Raven 2002)
2.4 Structure and Function of Biological Membranes 25
Enzymatic activity is regulated through various biochemical reactions occurring at biological membranes, where specific enzymes catalyze these processes These membranes serve as essential matrices for assembling enzymatic systems, particularly for enzymes involved in redox reactions, hydrolysis, and biosynthesis.
Compartmentalization in living cells allows for simultaneous biosynthesis, degradation, and transport reactions, facilitated by numerous specific enzymes This complex cellular structure enables various physiological and biochemical processes to interact continuously Biological membranes play a crucial role in creating compartment systems for specific chemical reactions, such as ATP biosynthesis occurring in mitochondria and chloroplasts These compartments support autoregulatory processes in cellular metabolism, with cellular metabolites serving as key regulatory factors that influence gene expression and, consequently, enzyme concentrations and activity.
• Receptor function By means of membrane embedded receptors the information related tofluctuations of the environmental factors is perceived and, based on this, the adjustment of the metabolism is carried;
• Structural role.The cytoplasmic membrane forms a lot of folds, which together with the nuclear membrane and the endoplasmic reticulum forms a complex system of structures (Fig.2.8).
Fig 2.7 Functions of the components of biological membranes (Johnson and Raven 2002)
Exchange of Substances Between the Cell and the Medium
Ion Flow into the Cell
The living cell is capable of selective absorption and accumulation of mineral elements (see Table2.3) For instance, waterflows freely through the membranes, while macromolecular substances don’t.
Semipermeability is one of the basic mechanisms of selective accumulation of ions in the cell and is owed to a large extent to the hydrophobic layer of the
Fig 2.8 Complex membrane structures in the cell
Table 2.3 Content of several ions in sea water and vacuole sap
Chemical element Ion content (in milliequivalents) Sea water Vacuole sap
2.5 Exchange of Substances Between the Cell and the Medium 27 membrane (which prevents charged ions from passing through with varying effi- ciency depending on ion charge, diameter).
The introduction of phospholipid membranes has led to the development of transmembrane ion transfer mechanisms Membranes rich in lipids exhibit greater permeability for organic substances, whereas those primarily composed of proteins allow for increased permeability of water and mineral ions Ion transport across biological membranes can occur through either passive or active processes.
Passive transportof ions occurs according to the chemical and electrochemical gradients, without metabolic energy consumption This phenomenon is based on simple and facilitated diffusion.
Simple diffusion refers to the process where ions and molecules move from areas of higher concentration to lower concentration, driven by their kinetic energy, which rises with temperature and concentration In thermodynamics, the diffusion vector is influenced by the chemical potential of a substance, indicating that a higher concentration correlates with an increased chemical potential.
Passive transport across phospholipid membranes allows substances that are lipid-soluble or have a molecular diameter smaller than the pore diameter to move freely The transport rate is inversely related to the diameter and mass of the molecules, as described by W Ruhland's ultrafilter concept, while it is directly related to their lipid solubility, a principle established by Overton A key example of this process is the passive movement of carbon dioxide from the air into leaf tissues.
Fig 2.9 Transport of substances in the cell
Facilitated diffusion enables the transport of polar molecules and ions along their concentration gradient through the assistance of transport proteins These proteins may be integral to the cell membrane or temporarily bind to the molecules, allowing them to cross biological membranes and release their cargo inside the cell, after which the proteins return to their original state to continue the process.
Active transport is performed contrary to the concentration gradient with metabolic energy consumption (ATP, NADH, NADPH) derived from the respira- tion process.
Active transport utilizes specific transport proteins in biological membranes, functioning like enzymes to facilitate the movement of substances These transporters exhibit high specificity and affinity for their cargo, such as ions, enabling selective absorption on one side of the membrane and desorption on the other This transport mechanism is essential for the movement of carbohydrates, amino acids, and nucleotides, and can occur as simple translocation of a single substance or as coupled cotransport, where two substances are simultaneously transported in the same direction.
Fig 2.10 Sodium and potassium ion transport carried out by a single pump (the K + /Na + -ATP-ase) (Johnson and Raven 2002)
2.5 Exchange of Substances Between the Cell and the Medium 29 direction this kind of cotransport is called symport whereas if in opposite directions
The energy source facilitates a cycle that includes transporter activation, cargo-transporter complex formation, conformational changes for cargo translocation, cargo release, and the return of the transporter to its original state, with only certain events in this process requiring energy.
• ATP hydrolysis—K + /Na + -ATPase (Fig.2.10); Ca 2+ /H-ATPase;
• the electrochemical gradient of an ion used for cotransport.
Proton pumps utilize ATP energy, hydrolyzed by H+-ATPase in membranes, to create a high electrochemical potential and pH gradient This gradient drives the absorption of co-transported substances, exemplified by the active transport of mineral ions from soil through root hairs Additionally, amino acids, glucose, and ions like K+ and Na+ are effectively transported in this process.
Fig 2.11 Active transport of a single substance by the H + -pump through a series of conformational modi fi cations
Transport of substances like Na+ and K+ occurs in opposite directions, while Na+ and carbohydrates move in the same direction This process happens against the concentration gradient and requires energy, typically in symport with H+ ions (Johnson and Raven, 2002).
Ion movement is influenced by both the concentration gradient of chemical species and the electrostatic potential across the membrane, collectively known as the electrochemical gradient The symport system facilitates the transport of Na+ ions alongside amino acids or carbohydrates, while the Na+/K+ pumps contribute to this process by creating a chemical gradient, with a higher concentration of Na+ outside the cell, and establishing a charge gradient.
2 positively charged K + ions that are transported inside of the cell, 3 positively charged Na + ions are transported outside of the cell) (Fig.2.12)
Pinocytosis is a specialized form of active transport that involves membrane modifications to create evaginations, resulting in vesicles that encapsulate various substances These vesicles can release their contents either inside or outside the cell membrane (plasmalemma) or tonoplast This process facilitates the transport of anthocyanins from the cytoplasm to the vacuole via the tonoplast, as well as through Golgi vesicles to the cell wall.
Water Flow into the Cell
Water exchange between cells and the environment is achieved through the inter- vention of the following physical phenomena, characteristic of both the biological systems and the non-living matter:
• suction force of the cell.
Fig 2.13 Diffusion of ions in water solutions through a semipermeable membrane
(based on the example of urea)
2.5 Exchange of Substances Between the Cell and the Medium 31
Water currents refer to the overall movement of water driven by its free energy, with water flowing from areas of higher potential to lower potential Diffusion is the random movement of molecules from regions of higher concentration to lower concentration, influenced by factors such as temperature, particle size, and medium fluidity, and it occurs slowly over short distances Osmosis involves the movement of water or solvent molecules through a selectively permeable membrane, occurring from areas of lower concentration to higher concentration, effectively moving from higher water potential to lower water potential.
Osmosis is the essential process that facilitates water flow into cells, playing a crucial role in the regulation and distribution of water in living organisms The resistance to water entering the cell, known as osmotic pressure (P), is directly related to the concentration of cellular juice Osmosis enables water movement from tissue fluids into cells, from soil to root hairs, and from xylem to leaf mesophyll cells Aquatic plants exhibit the lowest osmotic pressure at 0.1 atm, while halophytes, which have a high mineral salt concentration, show the highest pressures Most crop plants have osmotic pressure values ranging from 0.5 to 3.0 atm, with terrestrial plants typically falling between 506–1010 kPa and aquatic plants between 101–304 kPa Fruits, vegetables, and sugar beets are characterized by significantly high osmotic pressures of 2,026–4,052 kPa.
The osmotic nature of water exchange between cells and their surroundings is evident in turgidity and plasmolysis, which are influenced by the cell's internal tonicity Various scenarios arise during this water exchange based on the chemical potential, leading to the formation of isotonic, hypertonic, and hypotonic solutions.
Isotonic solutions have equal ion concentrations inside and outside of the cell, resulting in the same osmotic pressure as the surrounding environment In these solutions, water movement is limited to diffusion and occasional short-distance travel, with no active transport occurring.
In hypertonic solutions, the concentration of solutes inside the cell is lower than that in the surrounding environment This leads to water leaving the cell through exosmosis, resulting in the detachment of the plasmalemma from the cell wall, a process known as plasmolysis, which ultimately causes wilting in plants.
Fig 2.15 Types of solution depending on the osmotic pressure a Isotonic solution b Hypertonic solution c Hypotonic solution
2.5 Exchange of Substances Between the Cell and the Medium 33
Plasmolysis can be categorized into two types: concave (early) and convex (final) In the case of convex plasmolysis, the protoplasm contracts around the vacuole, maintaining contact with the cell wall solely through fine threads known as Hecht’s filaments The formation and types of plasmolysis are influenced by the hydration level of the protoplasm and the micro-structural characteristics of the plasma membrane.
Hypotonic solutions have a higher concentration of ions inside the cell compared to the external environment, leading to endosmosis, where water molecules move into the cell, causing deplasmolysis and increased turgidity In a hypotonic medium, water enters the plant cell, causing it to expand, while the cell wall generates counter-pressure known as turgor force, which provides rigidity and tension to the cell This turgor force varies throughout the day, peaking in the morning and reaching a minimum in the afternoon, with values ranging from 5–10 atm in thallophytes to 100 atm in fungi During drought or intense transpiration, turgor force can drop to zero, resulting in wilting Plasmolysis and deplasmolysis, phenomena exclusive to living cells with semipermeability, reflect the viscosity of protoplasm and help determine osmotic pressure, which is crucial for ecological research and understanding how plants absorb and retain water Osmotic processes in plants are vital for effective water absorption, circulation, and substance exchange.
Electroosmosis refers to the movement of liquid through a membrane's pores driven by an electric field This phenomenon occurs when an electric potential forms at the cell wall of plant cells due to the dissociation of pectin and other carboxyl-containing substances into COO− and H+ ions The COO− ions impart a negative charge to the cell wall, prompting the attraction of positively charged ions to create an electrostatic balance, which establishes an electrical gradient Additionally, biological membranes possess a specific electrochemical potential that facilitates water circulation.
Imbibition forces facilitate water penetration through protein colloids and cellulose-pectin microfibrils, leading to an irreversible increase in volume and mass This process, driven by colloidal and capillary effects, is crucial during seed germination, as hydrophilic organic reserve matter can bind significant amounts of water, generating internal pressures of up to 1,000 kPa in seeds While cellular proteins exhibit a higher imbibition capacity, cellulose has a lower capacity, with each OH− radical of β-glucose residues in cellulose capable of binding three water molecules, alongside the acidic COOH group.
NH 2 amino group can retain 4 water molecules Imbibition may be limited, when the imbibed mass remains in the gel state and unlimited when the colloid transitions entirely into the sol state.
The suction force (S) in plant cells, which drives water absorption, is influenced by the balance between osmotic and turgor pressures This force varies based on internal and external factors, including plant species and environmental conditions As turgor pressure approaches its maximum, the rate of water entry decreases until equilibrium is achieved (P = T), resulting in a suction force of zero During endosmosis, cells operate as self-regulating osmotic systems, where the interplay between turgor and osmotic pressures determines both the intensity of water absorption and the volume of water intake This relationship was illustrated by German scientist K Hoffler.
The apoplast refers to the complete network of inter-fibrillar spaces within the cellular envelope, allowing for the movement of water and various substances between cells throughout the plant.
Cellulose (C6H10O5)n is a crucial polysaccharide that serves as the primary structural component of plant cell walls It typically exists in the form of fibrils, with its molecules organized into micelles, microfibrils, and macrofibrils, which collectively create a protective housing for the cell membrane.
K Hofler's diagram illustrates the relationship between cellular suction force and cell volume, progressing from plasmolysis to maximum turgor (Milica, 1982).
2.5 Exchange of Substances Between the Cell and the Medium 35 that is embedded in an amorphous matrix of hemicellulose, pectins, lipids and proteins It is the most common organic compound on earth.
Role of Water in Plants
Water is essential for all living organisms, serving as the primary mineral compound that supports vital processes and maintains cell integrity In plants, physiological functions rely heavily on water saturation within cells Due to its unique physical and chemical properties, water is crucial for plant life, playing multiple roles in homeostasis and cell composition.
• a basic solvent for mineral salts and organic compounds and, at the same time, a dispersion medium for colloidal macromolecules and a medium for biochemical reaction progression;
Maintaining stable plant temperatures is crucial for preventing tissue overheating, which can occur due to metabolic heat production or direct sunlight exposure during the summer.
Protoplasm contains electrostatically fixed elements within long polypeptide chains, which influence its physical and chemical properties This mechanism supports the formation of colloidal systems and is essential for the conformational structure of proteins, crucial for their functionality Additionally, it plays a vital role in maintaining the ultrastructure and functional activity of cell organelles Dehydration of proteins can result in coagulation and sediment deposition.
• ensures the phenomenon of osmosis and allows turgidity, contributing to sto- mata movement, to plant orientation in space and to sprout, leave and other organ positioning and orientation;
• serves as a donor of protons and electrons for CO 2 reduction in the dark phase of photosynthesis;
• is a component of the redox reactions of the Krebs cycle;
• participates in the reactions of hydrolysis, oxidation and reduction, assimilation and dissimilation;
• structural water in biological membranes ensures the assembly of the phos- pholipid bilayer, and thus, influences on the permeability of these membranes to electrons and protons;
• represents a universal carrier, ensuring the transport of dissolved substances through the xylem and phloem vessels, as well as the radial transport though the symplast and apoplast;
• ensures the integrity of plant organisms, forming a continuous flow from the root to the leaves, via which mineral salts and organic substances are transported.
Fig 3.1 a A water molecule, b hydration of polar molecules
Water Content and State in Plants
The total water content in plants varies significantly based on species and factors such as organ type, tissue, and developmental stage For instance, algae have a water content ranging from 94% to 98%, while succulent leaves typically contain about 95% water, and reserve organs exhibit different moisture levels.
The water content of plants is significantly influenced by environmental factors and the type of organ, with leaves containing approximately 85% water, dry seeds around 12-14%, and varying water retention capacities This retention is driven by osmotic forces and colloidal and capillary imbibition, with young leaves exhibiting a greater ability to retain water due to their higher protoplasmic colloid content compared to older leaves.
Water retention in plant cells occurs primarily in the cell wall, protoplasm (which can contain 90–95% water), and vacuole sap (up to 98%) The capacity of cellular envelopes to retain water is influenced by their thickness, structure, and chemical composition.
7–8 % water is bound to cellulose polymeric chains and is retained by superficial bonds The vacuolar sap contains up to 98 % water, which is retained by osmotic, electroosmotic and imbibition forces.
Water is also a structural component of biological membranes—water interact- ing with the membrane surface, water located in the space between the internal and external chondriosome (mitochondria) membranes.
Plants exhibit three states of water aggregation: liquid, gaseous, and solid Liquid water, making up 30-35% of cell membranes, is essential for cellular structure, protoplasm, and vacuoles In its gaseous form, water vapor occupies intercellular spaces and aeriferous tissues, while solid water appears as ice crystals during severe frost, which can damage cells by breaking cytoplasmic membranes Within plants, liquid water can be categorized as free (95%), acting as a solvent for minerals and organic substances, or bound (4-5%), held by hydrogen bonds or within macromolecular structures Free water circulates easily within vacuoles and conducting vessels, facilitating turgidity and serving as the medium for biochemical processes, often participating directly in reactions However, free water's susceptibility to freezing at temperatures as low as -10°C makes plants with high free water content less resilient to cold conditions.
Bound water in plants is tightly retained and consists of immobile molecules that cannot diffuse or evaporate, making it difficult for cells to release This type of water freezes at temperatures below −10°C and does not circulate within the cell or the plant, nor does it engage in biochemical processes or dissolve substances Consequently, bound water is not involved in the transfer and circulation of nutrients within the plant.
Bound water is held by:
Osmotic forces are generated by dissolved substances that attract and retain water, known as osmotic water The removal of this water from tissues through transpiration becomes increasingly challenging as the concentration of vacuolar sap rises.
Imbibition forces, generated by hydrophilic colloids, play a crucial role in water retention within cells The water held by these forces is referred to as imbibition water Numerous hydrophilic colloids, including proteins, mucilage, and cellulose, are present in cells and exhibit a strong capacity to retain water Notably, each mole of protein amino groups can bind 2.6 moles of water, while entire protein molecules, which can vary significantly in size, have the potential to bind tens of thousands of moles of water.
Water adheres to various particles, such as ions or molecules, through electrostatic forces due to the bipolar nature of water molecules This bound water, known as adsorption or hydration water, plays a crucial role in dissolving colloidal particles by creating a dense shell of water around them Additionally, organic substances found in plant cells have extensive surfaces that facilitate significant water adsorption; for instance, just 1 gram of cellulose can provide a remarkably large surface area for this process.
A surface area of 1 million cm² can bind approximately 4,000 to 5,000 water molecules per 100 average protein molecules Additionally, the thickness of the hydration layer around ions increases with a smaller ion radius and a higher electric charge.
The imbibition of proteins and mucilaginous substances in water is influenced by electrostatic phenomena Proteins exhibit stronger imbibition in electrolyte solutions due to their ability to fix ions, which carry hydration layers that enhance the overall effect Conversely, the adsorption of ions is inversely related to the size of their aqueous envelopes In contrast, mucilaginous substances are more effectively imbibed in water than in electrolyte solutions, as they fix ions weakly and must compete with dissolved ions for available water.
Fig 3.2 Hydration of NaCl molecules
Plant cells contain numerous capillary spaces, including vacuoles and areas between colloidal micelles in the membrane and protoplasm These capillary forces play a crucial role in retaining water molecules, which is referred to as capillary water, particularly within the conducting vessels at the tissue level.
There is also theconstitutional water, chemically bound by certain molecules. Release of this water by molecules implies their destruction.
The concepts of free and bound water in plants are interrelated, as these forms can shift between one another under varying environmental conditions When plants face stress due to unfavorable conditions, the amount of free water decreases while bound water increases, enhancing their stress resistance Typically, free water is more abundant than bound water, but this disparity narrows during drought The critical threshold for cell dehydration occurs at 35%, beyond which vital processes are severely diminished Water can be classified as exogenous, primarily absorbed from the soil through roots or from atmospheric vapors, or endogenous During droughts, dew significantly hydrates young leaves, contributing 50-70% of their water content, compared to only 5-7% for older leaves.
Endogenous water is synthesized during the process of respiration in mitochondria.