TRANSPORT1.7 Transport of Substances in Plants

Transport substances to all parts of plantWater for photosynthesis and cellular metabolismMineral ions for healthy growth and developmentOrganic food materials for cellular metabolism or storage Problem Small TSA/V ratioLong distances OvercomeInternal system of tubes = VASCULAR SYSTEM

The vascular system involved in transporting substances in plantsexcept movement of gases (diffusion) Consist of 2 types of tissues: xylem and phloemwhich function as two separate transport system.

Monocot plantleaf:stem:root:

Dicot plant

Dicot stem

Dicot roots

Dicot root

XylemTransports water and dissolved mineral ions from the roots to the upper parts of the plantAgainst gravitational pullGives mechanical support to woody plants

Xylem adaptation Consists of 4 types of cells: Xylem vessels Long, hollow, continuous tubeDead cell, no protoplasm Thickened by lignin (patterns: pits, spiral, ring)Numerous pits (holes), end walls open TracheidsDead cells, hollow Longer and smaller than xylem vesselsPointed end and pitted, end walls breaks down in pits Parenchyma food storageFibres - support

PhloemTransports organic substances (e.g. sucrose, amino acids) synthesised mainly in the leaves to other parts of plantUsed in cellular metabolism Growth (shoot, root, buds, fruits)Respiration Stored in in storage organ e.g. tuber

Phloem adaptation Consists of Sieve tubesLiving cells (nucleus disintegrates during maturation, thin layer of cytoplasm, small mitochondria) Cylindrical, elongated, end-to-endEnd walls perforated to form sieve platesCompanion cells (transport food into tubes)Shorter than sieve tubesLiving cells (with nucleus, many mitochondria)Adjacent to and support the function of sieve tubeParenchyma food storageFibres - support

Plasmodesma : microscopic channel

Transport of substances:a) Food (Translocation)The movement of sucrose and other organic materials within the plant body Primarily through phloem (sieve tube)Drives by concentration (pressure) gradientsImportance: Plants survivalStore and convert sucrose to other sugars

Bark RingingRemoval of a ring of tissue external to the xylem from around the trunk of a woody plant.Removal of phloem

Observation Inference Tissue above the ring swells up Tissue below the ring withers Tissue removed is phloemDisrupt flow of food to rootAccumulation causes swelling at the upper partInsufficient food cause the lower part to wiltThe upper part of plants lives normallyXylem is not removed Flow of water is not disruptedThe upper part do not wilt

Transport of substances: b) Water and minerals Water by osmosis Minerals by active transportAided by three factors: Root pressure Capillary action Cohesion and adhesion Transpiration pull

1) Root PressureThe process that forces the water absorbed from the soil to move through the roots and up the stem of a plant Caused by osmosis of water and active transport of mineral ions into the root xylem

The mechanism: Cell sap of root hair cells is hypertonic to soil waterWater enter by osmosisCell sap is diluted, hypotonic to adjacent cellsWater moves to the next cells until crosses cortex and endodermisCreates root pressure to push water into xylemHelped by the active transport of minerals into xylem that increase osmotic pressure

Guttation In small plants, water may be pushed out of special pores at leaf (hydathodes) Usually seen early morning

2) Capillary action Chemical forces that move water as a continuous column rather than as individual molecules. Combination of 2 forces due to hydrogen bonding: cohesion & adhesionThe forces can be overcome by gravity.

Cohesion and AdhesionCohesion: attraction between water molecules, prevents break of water column Adhesion: attraction between water molecules and xylem walls, prevents slipping of molecules

3) Transpiration pull When water evaporates, it creates an empty space which is filled continuously. As the water moves, more water is absorbed to replace the evaporated water (transpiration pull)

Conceptualise!

Transpiration The process of water loss (as water vapour) from plants to the atmosphere due to evaporation (99% of water intake)Through the stomata in leaves (90%)Through cuticle (5%) or lenticels (5%) Importance Creates transpiration pull Provide cooling effect and turgor pressureMaintain osmotic pressure

The process:Water absorbed by roots to mesophyl cellsHeat from Sun cause water to evaporate to air spacesWater vapour diffuse through stomata (traspiration)

Regulation by stomata:Open During dayPhotosynthesis produce glucoseK+ pumped into guard cells Accumulation of glucose and K+ increase osmotic pressureWater enters by osmosis

Guard cells become turgid, curved outwards, stoma opensTranspiration occurs

Regulation by stomata:CloseAt night/darkNo photosynthesis, no glucose producedK+ moves out to epidermal cells Osmotic pressure decrease Water diffuses out by osmosis

Guard cells become flaccid, stoma closes Transpiration stops

Factors affecting transpiration rate:Air MovementMore air movement, easier evaporation Transpiration rate Air movement

Factors affecting transpiration rate: TemperatureHigher temperature, higher rate of transpiration Transpiration rate Temperature

Factors affecting transpiration rate: Light intensity Higher intensity, higher rate of transpirationLight intensityTimeTranspiration rate Light intensityTranspiration rate

Factors affecting transpiration rate: Relative humidityHigher surrounding humidity, lower rate of transpirationTranspiration rate Relative humidity