Planet Earth and Its Environment A 5000-million year Journey

Biology in Focus, HSC CourseGlenda Childrawi, Margaret Robson and Stephanie Hollis

Maintaining a BalanceTopic 14: Transport of Nutrients in Plants1DOT POINTDescribe current theories about processes responsible for the movement of materials through plants in xylem and phloem tissueFunctionThe role of transport in plants is mainly to carry materials for photosynthesis to the cells and to move cell products away to other parts of the plant. In small plants, this may be achieved by diffusion and active transport, but in larger plants, specialised vascular tissue has developed to serve this transport function. cumbavac.org

FunctionThe vascular system in plants consists of vessels of xylem and phloem and the movement of materials from one part of the plant to another is known as translocation. b4-10y1-transport-in-plants.wikispaces.com

FunctionChemical substances that are needed for photosynthesis (such as water mineral ions) are carried by xylem tissue from the roots (the site of absorption) up to the leaves where they will be used for the manufacture of food (photosynthesis). Xylem tissue consists of xylem vessels, tracheids, fibres and parenchyma cells. bio1152.nicerweb.com

FunctionPhloem vessels are involved in the transport of organic nutrient products (particularly sugars, amino acids and plant hormones) to all parts of the plant. Movement occurs in two directions up towards the flowers and down to the roots. Phloem tissue consists of phloem fibres, phloem parenchyma, sieve cells and companion cells. alevelnotes.com

Theories about Xylem and PhloemExperimental evidence has shown the type of materials that move through xylem and phloem in plant stems and the directions in which they move, but the explanation of how this movement occurs in each is presented as a theory. Remember a theory is a scientists explanation of a phenomenon, based on observation. meaningfulmama.com

Theories about Xylem and PhloemThe theories of how movement of substances occurs in plants have been tested by examining whether their consequences (predictions) are borne out by observation and experimentation. They have modified over time, but the current most commonly accepted theories are:The transpiration stream theory (cohesion-adhesion-tension theory) of movement of water and mineral ions in xylemThe pressure flow theory (source-path-sink theory) of translocation of organic nutrients in phloem.mazecordblood.comThe Transpiration Stream TheoryThe transpiration stream in xylem occurs due to physical forces that result from water (and ions) being moved by passive transport. A column of water is sucked up the stem by the evaporative pull of transpiration and is known as the transpiration stream. powerplantccs.com

The Transpiration Stream TheoryOnce the water has been absorbed into the roots of plants (by osmosis) along with mineral ions (by diffusion and active transport), these substances move across the root into the xylem. A small amount of root pressure results from the continual influx of more water and ions, forcing the solution already present in the xylem upwards. mazecordblood.com

The Transpiration Stream TheoryMost of the upward movement in xylem seems to be as a result of the transpiration stream water is drawn up the xylem tubes to replace the loss of water from the leaves by transpiration. This is based on evidence gathered by biologists.mazecordblood.com

The Transpiration Stream TheoryEvidence:Xylem vessels are hollow and narrow, offering very little resistance to the flow of waterThe physical properties of water contribute to the formation of a continuous stream. Adhesive forces lead to capillarity (water rises up the bore of xylem) and cohesive forces (the attraction of water molecules to each other) which ensure that a continuous column of water that moves upwards is maintained in the xylem vesselssparknotes.com

The Transpiration Stream TheoryEvidence continued:A concentration gradient exists across the leaf:At the surface of the leaf, the osmotic pressure is high (water concentration is low) because water is continually being lost by evaporation through the stomata (transpiration) In the centre of the leaf the osmotic pressure is low (water concentration is high)bfwatercycle.blogspot.com

The Transpiration Stream TheoryThe flow of the transpiration stream can be explained by this diagram: mazecordblood.com

Handout diagramThe Pressure Flow TheoryTranslocation in phloem tissue moves products of photosynthesis (such as glucose, sucrose and amino acids) by active transport. Up to 90% of the dissolved substances in the sap of phloem is sucrose (common sugar). When sucrose reaches the cells, it may be converted back to glucose for respiration or starch for storage.home.earthlink.net

The Pressure Flow TheoryThe flow of minerals in phloem is an active process that requires energy. The mechanism of flow is driven by an osmotic pressure gradient, generated by differences in sugar and water concentrations. It involves the active loading of sugar into phloem at one end and then the active unloading from phloem into the surrounding tissues at the other end (the sink). wps.prenhall.com

The Pressure Flow TheoryThe loading of sugar into phloem at the source attracts water to flow in and the offloading at the sink causes the water to flow out of the phloem. This is why its called pressure flow.cas.muohio.edu

The Pressure Flow TheoryLoading at the SourceAmino acids, sucrose and other mineral nutrients are loaded into the phloem in the leaves. There are two theories as to how this happens:Symplastic loading sugars and other nutrients move in the cytoplasm from the mesophyll cells to the sieve elements through the plasmodesmata (strands of cytoplasm that pass through pits in the cell walls)fwcdscience.wikispaces.com

The Pressure Flow TheoryLoading at the Source2. Apoplastic loading sugars and nutrients move along a pathway through the cell walls until they reach the sieve element. They then cross the cell membrane to enter the phloem tube. These sugars pass into the sieve cell by active transport. ijanpromoters.blogspot.com

The Pressure Flow TheoryLoading at the SourceAs sugars enter the phloem, the phloem sap becomes more concentrated and so the osmotic pressure at the source end is high. This draws water into the phloem, from the adjacent xylem tissue by osmotic pressure. (blue arrows)

The Pressure Flow TheoryOffloading at the sinkMaterials flow to the sink. At the sink (for example roots, flowers or any part of the plant that needs nutrients), sugars and materials are removed from the phloem by active transport. (red arrows)

The Pressure Flow TheoryOffloading at the sinkAs sugars move out of the phloem, they draw water out with them (by osmosis). This results in a lower osmotic pressure due to the higher water concentration in the phloem at the sink region. cartage.org.lb

The Pressure Flow TheoryPressure Flow (along the path)This difference in osmotic pressure between the source and the sink in the phloem drives the phloem sap to flow. The direction of flow depends on where the sink areas (roots or flowers) of the plant are, in relation to the source (leaves).biologymad.com

The Pressure Flow TheoryPressure Flow (along the path)Water can move into the phloem by osmosis at any point along the gradient. The flow is continuous, because sucrose is continuous being added at one end and removed at the other. hexdome.com

Activity-Complete Transverse and Longitudinal Sections of Xylem and Phloem PRAC