In this lesson, we will explore how organisms manage the complex metabolic processes that sustain life. For these processes to function effectively, cells need to acquire materials from their environment and dispose of waste products. This requires the transport of substances to and from the cells.
One way that molecules move is through diffusion, but diffusion alone is not sufficient for all organisms. It can take a long time for materials in solution to diffuse even a short distance. In unicellular and simple multicellular organisms, diffusion works well because every part of the organism is in close contact with the environment. However, in complex multicellular organisms, the cells are often far from the external environment, necessitating a more comprehensive system to transport materials efficiently.
Transport in Plants
Water is essential for plant life, as it is necessary for photosynthesis, maintaining turgor pressure, and supporting various cellular activities. It also helps regulate the internal temperature of the plant. Land plants absorb water and minerals from the soil through their roots, which then need to be transported to the aerial parts of the plant. Additionally, food produced in the leaves through photosynthesis must be transported to other parts of the plant for use and storage.
To facilitate this transport, most land plants (except mosses and liverworts) have developed complex vascular tissues known as xylem and phloem. The xylem is responsible for transporting water and dissolved substances from the roots to the rest of the plant, consisting of vessel elements and tracheids. The phloem, on the other hand, conducts dissolved organic matter (food) between different parts of the plant, made up of sieve tube cells and companion cells.
Water moves from areas of higher water potential to areas of lower water potential. There is an inverse relationship between the concentration of solute and water potential: when solute concentration is high (in a hypertonic solution), water potential is low, and vice versa.
Water and Ion Uptake
Roots have three essential functions: they anchor the plant, absorb water and salts from the soil, and provide conducting tissues to distribute these substances to the stem. The conducting tissues, known as xylem and phloem, are grouped in the center of the root, forming a rod-shaped core that runs the length of the root. Surrounding the core is a layer of thin-walled cells called the pericycle, followed by the endodermis, which is a single layer of cells. Outside of this is a broad cortex made up of large, thin-walled cells, which is protected by a single layer of epidermal cells.
Roots also have tiny extensions called root hairs, which increase the surface area for absorption. These root hairs grow into the spaces between soil particles, allowing them to make direct contact with water. Because the cytoplasm of root hairs has a higher concentration of salts than the surrounding soil water, water moves into the root hairs by osmosis. Salts enter the root hairs through diffusion or active transport. Once inside, water and salts move through intercellular spaces or through cells via channels called plasmodesmata, eventually reaching the xylem. From the xylem, water and salts are transported to all parts of the plant.
Transpiration
Transpiration is the process by which plants lose water through evaporation. This can occur through stomata on the leaves, the cuticle on the leaf epidermis, and special openings called lenticels on some stems. Most transpiration happens through the stomata, a process known as stomatal transpiration. The mesophyll cells in the leaves provide a large surface area for evaporation. Water is drawn from the xylem into these mesophyll cells, where it creates a water film on the cell walls. The water then evaporates into the air spaces within the leaf, and water vapor diffuses from these spaces through the stomata and into the outside air.
Opening and Closing of Stomata
Most plants keep their stomata open during the day and close them at night to regulate transpiration, and this process is controlled by guard cells. Each stoma is flanked by two guard cells that are attached at their ends. The inner sides of these cells, which face the stoma, are thicker than the outer sides. When guard cells absorb water and become turgid, they take on a shape similar to two beans, causing the stoma to open. Conversely, when guard cells lose water and become flaccid, their inner sides touch, and the stoma closes.
The concentration of solutes, such as glucose, in guard cells plays a crucial role in this process. Recent studies show that light triggers the movement of potassium ions from nearby epidermal cells into the guard cells. Water follows these ions, which increases the turgidity of the guard cells and opens the stoma. Throughout the day, guard cells produce glucose, becoming hypertonic, which helps retain water. At the end of the day, potassium ions move back to the epidermal cells, and the glucose concentration in guard cells decreases. As a result, water leaves the guard cells, leading to a loss of turgor and the closure of the stoma.
