7. transport in plants
Characteristics and Classification of Dicotyledonous Plants
Dicotyledonous plants, commonly referred to as dicots, are characterized by several distinct anatomical and morphological features. Their seeds contain two cotyledons, which are also known as seed leaves. The vascular architecture in their leaves typically presents as a complex network of veins branching out from a central midrib. These leaves generally possess broad blades, providing a large surface area for photosynthesis, and are attached to the stem via petioles or stalks. Below the surface, dicots typically develop a tap root system featuring a primary root with various lateral branches. Dicots are classified into two broad categories based on their structural composition: woody dicots and herbaceous dicots, the latter being non-woody in nature.
The Necessity for Specialized Transport Systems in Plants
Plants require specialized transport systems to meet their diverse and demanding metabolic needs. These systems are essential for the movement of vital substances including glucose, water, hormones, and various mineral ions throughout the plant body. Because plants possess a relatively small surface area to volume ratio, simple diffusion is insufficient to sustain their life processes across large distances. Consequently, they utilize a vascular system composed of a network of specialized vessels. This network consists of xylem tissue, which is responsible for transporting water and mineral ions from the roots to the upper parts of the plant, and phloem tissue, which facilitates the translocation of organic substances from sources, such as the leaves, to sinks, such as the roots.
Structural Organization of Vascular Bundles
The xylem and phloem tissues are organized together into structures known as vascular bundles. The specific arrangement of these bundles varies significantly depending on the plant organ. In the leaves, vascular bundles form the midrib and veins, spreading from the center in parallel lines. Within these bundles, the xylem is positioned on the upper side, closer to the upper epidermis, while the phloem is located on the lower side, closer to the lower epidermis.
In the stem, vascular bundles are arranged around the periphery. The xylem tissue is situated on the inner side, closest to the center or pith of the stem, to provide optimal structural support. Conversely, the phloem tissue is found on the outer edges, closest to the cortex and epidermis. Other tissues present in the stem include the sclerenchyma and cambium.
In the root, the vascular bundle is centrally located to help the plant withstand the pulling strains caused by upward water transport and growth. This central core is primarily composed of xylem tissue, with phloem tissue located around the edges of this center core. The root structure also includes the endodermis, pericycle, cortex, epidermis, and root hairs.
Detailed Anatomy and Physiology of Xylem Vessels
Xylem tissue serves three primary functions: the transport of dissolved minerals and water up the plant, the provision of structural support, and the storage of food. This complex tissue is composed of four distinct cell types working in unison. Tracheids are long, narrow, tapered cells that contain pits. Vessel elements are large cells with thickened walls and no end plates when they reach maturity. Xylem parenchyma facilitates food storage and assists in the conduction of water. Sclerenchyma cells, which include both fibres and sclereids, provide essential structural support.
The structure of xylem vessels is perfectly adapted to its function. The cell walls are lignified, meaning they are impregnated with lignin, which makes them impermeable to water and provides the strength necessary to withstand hydrostatic pressure without collapsing. Mature xylem vessels have no end plates, rendering them hollow tubes of dead cells that allow for the unimpeded mass flow of water and solutes, facilitated by cohesive forces between water molecules and adhesive forces between water and the vessel walls. The lack of protoplasm further ensures that the transpiration stream is not obstructed. Pits in the non-lignified regions of the walls allow for the lateral movement of water, which is crucial for maintaining flow if an air bubble occurs. Additionally, the small lumen of the vessels helps prevent the water column from breaking and assists in capillary action.
Anatomy and Physiology of Phloem Vessels
The primary role of phloem tissue is the translocation of organic compounds, most notably sucrose, from source organs to sink organs. Phloem is composed of sieve tube elements, which act as the main conducting cells, and companion cells. Sieve tube elements are lined up end-to-end to form a continuous tube. They possess sieve plates with sieve pores to allow the movement of organic compounds. Although they are living cells, mature sieve tube elements lack a nucleus, vacuole, and ribosomes to maximize space for translocation. They have a thin layer of cytoplasm to reduce friction against moving assimilates, and their cellulose cell walls are strengthened to withstand hydrostatic pressures.
Each sieve tube element is associated with a companion cell that controls the metabolism of the sieve tube. Companion cells are characterized by the presence of a nucleus and other standard organelles, a large number of mitochondria to generate for active transport, and specialized transport proteins in their plasma membranes. They are linked to sieve tube elements via plasmodesmata, allowing organic compounds to flow between the two. Companion cells often feature infoldings in their cell surface membrane to increase the surface area available for the active loading and unloading of solutes.
Fundamental Differences Between Xylem and Phloem
Xylem and phloem exhibit several key differences in their biological and functional characteristics. Xylem tissue consists of dead, hollow cells at maturity, whereas phloem tissue contains living cells, specifically the companion cells. Xylem transports water and mineral ions via the process of transpiration, moving substances in a one-way, upward direction from the roots to the leaves. In contrast, phloem transports organic compounds or assimilates via active translocation, allowing for a two-way flow between sources and sinks. Structurally, xylem vessels lack end walls, whereas phloem sieve tubes possess sieve plates with pores. The cell walls of xylem are composed of both lignin and cellulose, while phloem cell walls are made of cellulose only.
Mechanisms of Water Transport in Plants
The uptake of water by plant roots is a passive process occurring through osmosis. Root hairs, which are long and thin extensions of the epidermis, significantly increase the surface area for absorption. Water and dissolved solutes move across the root cortex toward the xylem via two distinct pathways: the apoplastic pathway and the symplastic pathway.
The apoplastic pathway involves the movement of water through the cell walls and intercellular spaces. This movement occurs via diffusion and does not involve the cell surface membrane, offering little resistance to flow. Most water travels this way when transpiration rates are high. However, when water reaches the endodermis, it encounters the Casparian strip, a waterproof band made of suberin that blocks the apoplastic route. This forces water into the symplastic pathway, allowing the plant to control which mineral ions enter the xylem and helping to generate root pressure.
The symplastic pathway involves the movement of water through the living parts of the cell, including the cytoplasm, vacuoles, and plasmodesmata. Water moves into the cell across the cell surface membrane by osmosis and travels between cells through the plasmodesmata. This pathway is inherently slower than the apoplastic pathway due to the resistance of the membranes and cytoplasm.
The Cohesion-Tension Theory and Transpiration
Transpiration is the evaporation of water from the spongy mesophyll cells into the air spaces, followed by the diffusion of water vapor through the stomata into the atmosphere. This process provides evaporative cooling for the plant, assists in mineral ion uptake, and maintains cell turgor pressure. The movement of water is driven by the polar nature of water molecules, which form hydrogen bonds leading to cohesion (attraction between water molecules) and adhesion (attraction between water and the cellulose in cell walls).
According to the cohesion-tension theory, the evaporation of water in the leaves creates a transpiration pull that puts the xylem under tension. Because of the cohesive forces between water molecules, a continuous column of water is pulled from the roots, through the xylem, and into the mesophyll cells to replace lost water. Soil typically has a higher water potential than the cytoplasm of root hair cells, causing water to enter via osmosis. Mineral ions enter either with the water or through facilitated diffusion and active transport. Root pressure further assists in pushing water into the xylem. The adhesive forces prevent the water column from pulling away from the xylem walls, ensuring the high tensile strength of the water column.
Translocation of Assimilates and the Mass Flow Hypothesis
Translocation is the active transport of assimilates through the phloem from source to sink, requiring metabolic energy in the form of . These assimilates are dissolved in water to form phloem sap, which contains sucrose, amino acids, hormones, and minerals. Sucrose is the preferred transport carbohydrate because it is a disaccharide with higher energy storage