HOW SUCROSE IS TRANSPORTED IN PLANTS TSAMWISI FORGET B1128107 PRESENTATION BZ004 TRANSPORT SYSTEM HOW SUCROSE IS TRANSPORTED IN PLANTS

INTRODUCTION Sugars, which are formed by the plant during photosynthesis, are an essential component of plant nutrition. Like water, sugar (usually in the form of sucrose, though glucose is the original photosynthetic product) is carried throughout the parts of the plant by the vascular system. Phloem, the vascular tissue responsible for transporting organic nutrients around the plant body, carries dissolved sugars from the leaves (their site of production) or storage sites to other parts of the plant that require nutrients. Within the phloem, sugars travel from areas of high osmotic concentration and high water pressure, called sources, to regions of low osmotic concentration and low water pressure, called sinks. (Osmotic concentration refers the concentration of solutes, or sugars in this case; where the concentration of solutes is highest, so is the osmotic concentration).

Sources The nutrient-rich regions that supply sugars for the rest of the plant are called the sources. Sources include the leaves, where sugar is generated through photosynthesis. When they are high in supplies, the nutrient storage areas, such as the roots and stems, can also function as sources. In the sources, sugar is moved into the phloem by active transport, in which the movement of substances across cell membranes requires energy expenditure on the part of the cell

Sinks Sinks are areas in need of nutrients, such as growing tissues. When they are low in supply, storage areas such as the roots and stems cane function as sinks. The contents of the phloem tubes flow from the sources to these sinks, where the sugar molecules are taken out of the phloem by active transport

Pressure Flow The mechanism by which sugars are transported through the phloem, from sources to sinks, is called pressure flow. At the sources (usually the leaves), sugar molecules are moved into the sieve elements (phloem cells) through active transport. Water follows the sugar molecules into the sieve elements through osmosis (since water passively diffuses into regions of higher solute concentration). This water creates turgor pressure in the sieve elements, which forces the sugars and fluids down the phloem tubes toward the sinks. At the sinks, the sugars are actively removed from the phloem and water follows osmotically, so that conditions of high water potential and low turgor pressure are created, driving the pressure flow process

Phloem transport sucrose Most carbohydrates manufactured in and other green parts are distributed through the phloem to the rest of the plant . this process is responsible for the availability of suitable carbohydrates building blocking in roots and other actively growing regions of the plant. Carbohydrates concentrated in storage organs such as tubers , often in the form of starch ,are also converted into transportable molecules such as and moved through the phloem. The glucose is used to make sucrose , which is transported in the phloem. This shows that the sucrose move up and down of the phloem

Shoot(sink) leaf (source) root (sink)

Transport of water by osmosis Active transport

SUCROSE IN LEAVES

MUNCH’MASS FLOW  However, Munch’s Mass flow or pressure flow theory enjoyed the support of many concomitant observations.  The mass flow theory is mainly based on the extreme concentration gradient between the source and the sink.  During translocation organic solutes enter from the mesophyll cells into sieve tubes; with the entry of food material high DPD gradient develops.  As a result of this, water from the surrounding xylem cells enters into sieve tube cells, thus causing high degree of hydrostatic pressure.  This pressure is the motive force for the movement of food materials enmass towards the least concentration region.  This mass or pressure flow envisages unidirectional movement of materials

Munch’s mass flow

HYDROPONICS

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