The movement of molecules specifically, water and solutes is vital to the understanding of plant processes. This tutorial will be more or less a quick review of the various principles of water motion in reference to plants.
Bryophytes nonvascular plants are a plant group characterized by lacking vascular tissues. They include the mosses, the liverworts, and the hornworts. These groups of plants require external water, usually in the form of dew or rain. Some of them grow exclusively in dark, damp environments in order to provide moisture. Find out more about them here Ferns and their relatives are vascular plants, meaning they have xylem and phloem tissues.
Because of the presence of vascular tissue, the leaves of ferns and their relatives are better organized than the mosses and liverworts. Skip to content Main Navigation Search. Dictionary Articles Tutorials Biology Forum. Become strengthened by a chemical called lignin. The cells are no longer alive. Lignin gives strength and support to the vessel.
Phloem Phloem moves sugar that the plant has produced by photosynthesis to where it is needed for processes such as: growing parts of the plant for immediate use storage organs such as bulbs and tubers developing seeds respiration Transport in the phloem is therefore both up and down the stem. The cells that make up the phloem are adapted to their function: Sieve tubes - specialised for transport and have no nuclei.
Each sieve tube has a perforated end so its cytoplasm connects one cell to the next. Companion cells - transport of substances in the phloem requires energy. One or more companion cells attached to each sieve tube provide this energy. A sieve tube is completely dependent on its companion cell s. Comparison of transport in the xylem and phloem Xylem Phloem Type of transport Physical process Requires energy Substances transported Water and minerals Products of photosynthesis, including sugars and amino acids dissolved in water Direction of transport Upwards Upwards and downwards Plant organisation The xylem and phloem are distributed differently in roots and stems.
Physical process. From the companion cells, the sugar diffuses into the phloem sieve-tube elements through the plasmodesmata that link the companion cell to the sieve tube elements. Phloem sieve-tube elements have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap.
Phloem is comprised of cells called sieve-tube elements. Phloem sap travels through perforations called sieve tube plates. Neighboring companion cells carry out metabolic functions for the sieve-tube elements and provide them with energy. Lateral sieve areas connect the sieve-tube elements to the companion cells.
Image credit: OpenStax Biology. This increase in water potential drives the bulk flow of phloem from source to sink. Unloading at the sink end of the phloem tube can occur either by diffusion , if the concentration of sucrose is lower at the sink than in the phloem, or by active transport , if the concentration of sucrose is higher at the sink than in the phloem.
If the sink is an area of active growth, such as a new leaf or a reproductive structure, then the sucrose concentration in the sink cells is usually lower than in the phloem sieve-tube elements because the sink sucrose is rapidly metabolized for growth. If the sink is an area of storage where sugar is converted to starch, such as a root or bulb, then the sugar concentration in the sink is usually lower than in the phloem sieve-tube elements because the sink sucrose is rapidly converted to starch for storage.
But if the sink is an area of storage where the sugar is stored as sucrose, such as a sugar beet or sugar cane, then the sink may have a higher concentration of sugar than the phloem sieve-tube cells.
In this situation, active transport by a proton-sucrose antiporter is used to transport sugar from the companion cells into storage vacuoles in the storage cells. Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements. This reduces the water potential, which causes water to enter the phloem from the xylem.
The resulting positive pressure forces the sucrose-water mixture down toward the roots, where sucrose is unloaded. Transpiration causes water to return to the leaves through the xylem vessels. This video beginning at provides a more detailed discussion of the pressure flow hypothesis:. It should be clear that movement of sugars in phloem relies on the movement of water in phloem. But there are some important differences in the mechanisms of fluid movement in these two different vascular tissues:.
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