What is phloem and its structure

am Puls Biologie 5, textbook

134 Conductive tissues distribute water, nutrients and nutrients in the plant. Basic concept Structure and function: Tube-like structures are well suited for the long-distance transport of substances. (Compare the structure of the sieve tube cells with the trachea of ​​insects!) Exercise E 1 The arrangement of the leaf veins is an important distinguishing feature: seeded leaf veins always have parallel leaf veins. In dicotyledons they are arranged like a net. Find five monocotyledon and dicotyledonous plants in your area and bring one of each leaf to the class for comparison. Determine beforehand which plants you are dealing with. In the shoot, too, one-and two-seeded leaves differ in terms of their vascular bundles. Compare corresponding shoot cross-sections. The resulting carbohydrates can be stored on site. This happens in the form of starch in the chloroplasts. The majority, however, is distributed in the plant as sucrose dissolved in water. Depending on the season, different organs are supplied: In spring it is mainly buds and young leaves, later the flowers, that need the sugar for their growth. Ripening fruits require a lot of sugar - especially when they taste sweet and should be eaten (see p. 121). In autumn, the carbohydrates are stored in organs of persistence. These can be roots, tubers or bark cells from tree trunks. The distribution of sucrose in the plant is done by guiding elements (k Fig. 23). While the water is mainly transported in the tracheids and trachea of ​​the wooden part (xylems), sugar and amino acids in the bast part of the vascular bundle (phloem) are brought to the places where they are needed. The phloem consists of "tubes" of cells, the transverse walls of which are broken through by large pores. In contrast to tracheids and trachea, these sieve tubular cells are alive. The transport of substances in them is therefore correspondingly slower, because the cytoplasm is much more viscous than the liquid in the guiding elements of the xylem, which consists mainly of water. Phloem (outside) and xylem (inside) together form the vascular bundles. You can see them particularly well in the leaves as leaf veins. In bedspamers, they are surrounded by a vascular bundle sheath, the thick-walled cells of which ensure stability and limit the exchange of substances with the rest of the leaf tissue. In the stem of dicotyledonous plants, the xylem and phloem are separated by a meristem, here called the cambium (k Fig. 24). Its cells are able to divide throughout life and thus enable secondary growth in thickness. You can see annual rings on many felled trees. They come about because the growth in thickness takes place unevenly: In spring and early summer, a lot of water is required for the leaves to shoot and grow. The vessels are therefore wider. The need for water decreases in autumn. The cambium then releases smaller cells with thicker walls inwards, which increase the stability of the strain. From the middle of the trunk to the bark, the medullary rays run, which serve to supply the trunk. The sugar is distributed over the phloem Structure and function Leaf cell Sieve plate Sucrose Root cell Sieve tube of the phloem Vessel of the xylem Mineral water Fig. 23: Mass transport. In the guiding elements, substances are transported along a concentration gradient (phloem) or due to the transpiration flow (xylem). In both cases, loading and unloading takes place actively through transport molecules in the cell membrane. This requires energy (see Chapter 2.1). e a b d c… and water moves through the cells and cell walls of the vascular bundles back into the xylem. A leaf cell loads the sieve tube of the phloem with sucrose ... ... whereby water flows osmotically from the leaf cells and the xylem. This creates a pressure that allows the phloem sap to flow in the direction of the consumer cell. Sucrose is fed into a consumer cell, e.g. B. a root cell, discharged ... Fig. 24: Sprout cross-section of mono- and dicotyledonous plants. Xylem Phloem Cambium Epidermis Monocot plant dicotyledonous plant Xylem Phloem For testing purposes only - property of the publisher öbv

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