Plant Transport PDF 2024/2025

Lecture 6 2024/2025 Transport in Plant It is clear from our previous studies for nutrition and digestion in living organisms that each living organism needs various substances that transport...

Lecture 6 2024/2025 Transport in Plant It is clear from our previous studies for nutrition and digestion in living organisms that each living organism needs various substances that transport (move) inside its body by different means. Transport in lower (primitive) plants  Lower (Primitive) plants (like algae) don’t need specialized transport tissues, as the raw materials (carbon dioxide, water and mineral salts) move with the products of photosynthesis from one cell to another by diffusion and active transport. Transport in higher plants  Gases (oxygen and carbon dioxide) move by diffusion.  Water, mineral salts and soluble products of photosynthesis are transported through specialized vascular tissues which are : 1) Xylem tissues (vessels and tracheids) - They translocate water and mineral salts that are absorbed from the soil through the root across its different tissues, till they reach the xylem vessels in root, then to the xylem of stem, then to the leaves where the photosynthesis process takes place. 2) Phloem tissues (sieve tubes) - They translocate the high-energy organic food substances (carbohydrates, fats and proteins) from the sites of their production (leaves) to the sites of their storage and consumption in the different tissues of plant (root, stem, fruits and seeds) through the sieve tubes in the phloem of leaf, stem and root. We should study the internal structure of stem to help us understanding the role of stem in the process of transporta on. Structure of the plant stem On examining a cross-section in a dicot plant stem under the microscope, it was found that it consists of a group of tissues, as shown in the following diagram : Structure of stem First Second Third Epidermis Cortex Vascular cylinder - Parenchyma cells. - Collenchyma cells. - Pericycle. - Parenchyma cells. - Vascular bundles. - Starch sheath. - Pith. - Medullary Phloem Cambium Xylem Sieve tubes Companion Parenchyma Vessels Tracheids Xylem cells cells parenchyma Dr / Maghraby 45 Lecture 6 2024/2025 Detailed T.S. in a dicot stem to show its internal structure First Epidermis It consists of:  One row of adjacent and barrel-shaped parenchyma cells, where the outer walls are covered by cuticle (layer of cutin). Second Cortex It consists of: 1) Collenchyma cells: - Description: they are several rows of cells which have thickened corners by - the deposition of cellulose and they may contain chloroplasts. - Functions:  Act as a mechanical tissue for the support of stem.  Take part in the photosynthesis process (in case of the presence of chloroplasts). 2) Parenchyma cells: - Description : they are several rows of parenchyma cells with plenty of intercellular spaces. - Function: aeration. 3) Starch sheath: - Description: the innermost row of cells in the cortex. - Function: storing and keeping the starch granules. Third Vascular cylinder It occupies a large space of the stem. It consists of : Pericycle Vascular bundles Pith Medullary rays Dr / Maghraby 46 Lecture 6 2024/2025 1) Pericycle Description: - A group of parenchyma cells alternates with groups of fibrous cells. - Each group of fibers faces a vascular bundle from outside. Function: makes the stem strong, erect and elastic. 2) Vascular bundles They are arranged in a cylinder, where each bundle is triangular in shape whose base is directed outward. It consists of: A. Phloem It represents the outer tissue (part) of the vascular bundle. Function: transporting organic food substances from the leaves to all the plant parts. of Structure: it consists of sieve tubes, companion cells and phloem parenchyma cells. Vascular bundle in Sieve tubes They appear in the longitudinal section as elongated cells which contain cytoplasmic strands without nuclei. They are separated from each other by cross-walls that areperforated by tiny pores through which the cytoplasmic strands extend from one tube to another, these perforated cross-walls are called "sieve plates L.S. in phloem T.S. in (barriers)". Companion cells They are living cells with a nucleus, where each cell of them is accompanied by a sieve tube. They contain a large number of ribosomes and mitochondria which enable them to organize the vital processes of sieve tubes. B. Cambium Description: it consists of one row or more of meristematic cells that are found between the phloem and xylem. Function: its cells divide to give a secondary phloem externally and a secondary xylem internally. C. Xylem It represents the internal tissue (part) of the vascular bundle. Functions:  Translocating water and solutes from the root to stem, then to the leaves.  Supporting the stem.  Structure it consists of vassle, tracheids and xylan parenchyma. Dr / Maghraby 47 Lecture 6 2024/2025 Vessels Do you know Structure: they consist of a chain of elongated Steps of the xylem cylindrical cells vessel formation: that are joined end to end. Steps of their formation: 1) At first, the transverse walls of the cylindrical cells have been completely dissolved, so the cells become one tube. The forms of thickening 2) The cellulose wall of cells is thickened by lignin in xylem vessels: Substance which is impermeable to water and solutes. 3) The protoplasmic contents of cells died, leaving a hollow vessel (tube). Numerous pits are scattered all over the vessel wall, where the primary wall is left without thickening to allow the passage of water from inside the vessel to its outside. Strands of lignin are laid down on the inner lining of the vessel with various forms, such as annular and spiral to support the vessel and prevent the collapse (curve) of its wall inwards. Tracheids They are more or less similar to vessels, whereas they appear in T.S. different in being:  Pentagonal or hexagonal form.  Pointed (tapered), as the two ends are not opened. They are similar to vessels in being pitted. Xylem parenchyma Description: rows of parenchyma cells that are present between the xylem vessels. Xylem components Notes : Xylem in the vascular bundles of stem is connected with the xylem of root and leaves. Also, the phloem of stem is connected with that of root and leaves. Therefore, there is a network of vessels that spreads all over the plant parts. 3) Pith Description: parenchyma cells occupy the centre of the stem. Function: storage. 4) Medullary rays Description: parenchyma cells extend among the vascular bundles. Function: join the cortex with the pith. Dr / Maghraby 48