9.2 Transport in the phloem of plants Sponge: Set up Cornell Notes on pg. 39 Topic: 9.2 Transport in the phloem of plants Essential Question: What is the major difference between xylem and phloem? What is the major difference between xylem and phloem? Key Vocabulary: Translocation- Pressure flow hypothesis BIOZONE: 332-335 Text: 629,650-651

Applicaiton: Structure-function relationships of phloem tubes

Plant organic molecule movement Phloem: made up of living cells (unlike xylem) that transport their contents in various directions Occurs in ALL parts of the plants (not just the stems) Mostly sieve (siv) tube members Connected to one another by sieve plates to form sieve tubes Sieve plates have pores which allow the movement of water and dissolved organic molecules throughout the plant pore

Understandings: Plants transport organic compounds from sources to sinks

Plant organic molecule movement Movement is from a source to a sink Source: plant organ that is a net producer of sugar through photosynthesis Leaves are primary sugar sources Contain all the major organs for photosynthesis Sink: plant organ that uses or stores sugar Roots, buds, stems, seeds and fruits are all sugar sinks

Plant organic molecule movement It is possible for some structures to be both a source and a sink Bulb Tuber

Plant organic molecule movement Companion cells are connected to sieve tube members by plasmodesmata (Microscopic channels which enable transport and communication) Perform many of the genetic and metabolic functions of the sieve tube cells Supplies the ATP needed for active transport

Translocation Translocation: The movement of organic molecules in plants (products of photosynthesis) The organic molecules are dissolved in H²O and the solution is referred to as phloem sap Include: Mostly sugars (sucrose) Amino acids Plant hormones Small RNA molecules

Understandings: High concentrations of solutes in the phloem at the source lead to water uptake by osmosis

Translocation: Pressure Flow Hypothesis Sugar loading at source Creates high solute concentration in sieve tube (reducing H₂O) Causes osmosis from surrounding cells

Understandings: Incompressibility of water allows transport along hydrostatic pressure gradients Raised hydrostatic pressure causes the contents of the phloem to flow toward the sink

Translocation: Pressure Flow Hypothesis 2. H²O uptake causes hydrostatic pressure The positive pressure in the sieve tube results in a flow (bulk flow) of the phloem sap Adding solutes to a limited space filled with water increases pressure Two areas with different hydrostatic pressure produce a hydrostatic pressure gradient H₂O with its dissolved solutes will move from higher pressure area to lower pressure area

Translocation: Pressure Flow Hypothesis 3. Removal of sugar at the sink Pressure is diminished by the removal of the sugar into the sink Sugars are changed at the sink to starch Starch is insoluble and exerts no osmotic effect

Translocation: Pressure Flow Hypothesis 4. Xylem recycles H₂O The relatively pure H²O is carried by xylem from the sink back to the source

Understandings: Active transport is used to load organic compounds into phloem sieve tubes at the source

Translocation: Pressure Flow Hypothesis The loading of sugar at the sieve tube at the source and the removal of the sugar at the sink is accomplished by active transport Chemiosmotic process involving protein pumps Only the loading and unloading of the sugar requires energy The transport in the tube is passive

Crash Course: Vascular Plants (4m40s-11m53s) https://www.youtube.com/watch?v=h9oDTMXM7M8

Skill: Identification of xylem and phloem in microscope images of stem root

ID xylem and phloem in light micrographs Xylem cells are generally larger than phloem cells Phloem cells tend to be closer to the outside of the plant in stems and roots

Nature of Science: Developments in scientific research follow improvements in apparatus– experimental methods for measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes became available

Translocation The pressure that occurs within the phloem, as well as the composition of phloem sap, has been demonstrated using the stylets of aphids Phloem sap is nutrient rich But the only animals to consume it as the main part of their diet are insects belonging to a group called the Hemiptera Aphids Whitefly Mealybugs Psyllids

Translocation Aphids penetrate plant tissues to reach the phloem (p) using mouth parts called stylets (st) If the aphid is anaesthetized and the stylet is severed, phloem will continue to flow out of the stylet Both the rate of flow and the composition of the sap can be analyzed The closer the stylet is to the sink, the slower the rate at which the phloem sap will come out

Understandings: Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide

Analysis: Experiments using aphid stylets

Analysis: Experiments using aphid stylets a) (i) active transport of sugar (ii) create high solute concentration; water drawn in by osmosis; b) (i) no oligosaccharides at sucrose concentration below 0.25 mol dm-3; oligosaccharides concentration rises between 0.25 and 0.50 mol dm-3; no further increase above 0.50 mol dm-3; (ii) to reduce water loss from aphid/gut cells by osmosis; c) (i) poor source of amino acids, with many (especially essential amino acids) at a lower percentage in phloem sap that aphid proteins; (ii) plants synthesize amino acids for making plant proteins; plant and aphid proteins have different amino acid composition; d) (i) feed aphids on phloem sap containing antibiotics; test aphid growth rates/protein synthesis rates/amino acid contents; (ii) physiological problems have to be overcome; problem