BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. PLANT TRANSPORT

As discussed in Section 5.9.8, material exchange between individual Cells and their surrounding environment can occur passively via diffusion and osmosis, or through energy-requiring processes such as Active Transport, endocytosis, and exocytosis. Within The Cell, substances move primarily by diffusion, although energy-dependent transport mechanisms also operate, such as cytoplasmic streaming. These transfer Methods ensure sufficient exchange rates and efficiency over short distances; therefore, unicellular organisms and even multicellular ones with a high surface-area-to-volume ratio do not require specialized transport systems. For instance, in relatively small animals like earthworms, respiratory gas exchange (oxygen and carbon dioxide) takes place via diffusion between the surrounding air and Internal Organs across the outer body surface.

In larger and more complex organisms, many cells are too far apart and too distant from the environment for effective substance exchange to be sustained solely by the processes mentioned above. These organisms require specialized systems for rapid long-distance transport. Typically, substances in such systems move via mass flow—within a fluid medium moving along a pressure gradient from a region of higher pressure to a region of lower pressure. All components within this flow travel at virtually the same speed, much like a river, whereas in diffusion, molecules spread independently of one another according to their individual diffusion gradients. Several mass flow systems in plants and animals are listed in Table 13.1. Note that animals can utilize Muscle contraction to move fluids or gases from one part of the body to another; for example, The Heart acts as a pump driving Blood through Blood Vessels. Plants lack Muscles, yet pressure gradients within them can be generated through evaporation, active transport, and osmosis, as discussed below.

Class="center">Table 13.1. Selected mass flow systems in animals and plants

Both animals and plants possess conducting, or vascular, systems. These consist of tubes filled with a gas or liquid—a fluid medium that moves via mass flow. In animals, this is the circulatory system. In the vast majority of plants, the conducting systems are formed by the xylem and phloem (sometimes considered two parts of a single vascular system). Energy is required to operate these systems. In xylem transport, for instance, energy is derived directly from the Sun. A specialized conducting system providing mass flow movement of substances must be coupled with specialized exchange systems whose function is to maintain concentration gradients between the transport system and the cells it serves.

Table 13.2 lists the main groups of substances transported through plants, along with some information on the primary pathways and mechanisms of absorption, transport, and elimination.

Table 13.2. Movement of substances in plants

The movement of substances through plant conducting Tissues is termed translocation. In vascular plants, these tissues are highly specialized and represented by the xylem and phloem. The xylem translocates primarily water, dissolved mineral salts, and certain organic nitrogen compounds and Hormones, with transport directed upwards from the roots to the aerial Organs of the plant. The phloem serves to transport mainly solutions of organic and inorganic substances, moving primarily from the leaves and storage organs to other PARTS OF THE plant.

The Study of translocation is of great economic importance. For instance, determining the optimal application methods and evaluating the potential efficacy of herbicides, fungicides, growth regulators, and fertilizers requires an understanding of how these substances enter the plant and the pathways they follow within it. Such information is equally critical for certain plant pathogens, particularly Bacteria, Fungi, and Viruses, which can also be translocated through vascular systems, as it is required to develop disease Prevention and infection control measures. For example, the 1960s saw the Introduction of a new group of fungicides known as systemic because they are absorbed by plants and distributed via translocation to all their organs. This provides long-term and more reliable protection against fungal diseases, such as powdery mildew, compared to surface treatments.

13.1. Plant Water Relations

13.1.1. Osmosis

To understand how plant water relations are maintained, one must examine processes such as osmosis and diffusion, which are covered in Section 5.9.8. It is emphasized there that osmosis can be considered a type of diffusion in which only water molecules diffuse, while all other solute molecules are blocked by a semipermeable membrane. Water molecules move across a semipermeable membrane from a region of higher water concentration (i.e., a relatively dilute solution) to a region of lower water concentration (i.e., a more concentrated solution).