Biology Sylvia S. Mader Michael Windelspecht Chapter 25 Flowering Plants: Nutrition and Transport Lecture Outline See separate FlexArt PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes. Copyright © McGraw-Hill Education. Permission required for reproduction or display. 1

25.3 Transport Mechanisms in Plants Biology, 9th ed, Sylvia Mader Chapter 26 25.3 Transport Mechanisms in Plants Plant Nutrition Vascular tissues transport water and nutrients. Xylem transports water and minerals. Two types of conducting cells Tracheids Vessel elements Water flows passively from an area of higher water potential to an area of lower water potential. Phloem transports organic materials. Conducting cells are sieve-tube members. They have companion cells to provide proteins. End walls are sieve plates. Plasmodesmata extend through sieve plates.

Plant Transport and Water Potential Biology, 9th ed, Sylvia Mader Chapter 26 Plant Nutrition Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Leaf intercellular spaces xylem phloem stoma O2 CO2 H2O O2 CO2 H2O sugar H2O Stem xylem phloem sugar Root H2O H2O xylem phloem

Transport Mechanisms in Plants Biology, 9th ed, Sylvia Mader Chapter 26 Transport Mechanisms in Plants Plant Nutrition Potential energy is stored energy. Water potential is the energy of water. Water moves passively from a region of higher potential to a region of lower potential. In terms of cells, two factors usually determine water potential: Water pressure across a membrane Solute concentration across a membrane

The Concept of Water Potential Biology, 9th ed, Sylvia Mader Chapter 26 The Concept of Water Potential Plant Nutrition Pressure potential is the effect that pressure has on water potential. Water moves across a membrane from the area of higher pressure to the area of lower pressure. The higher the water pressure, the higher the water potential. Pressure potential that increases due to osmosis is called turgor pressure. Osmotic potential takes into account the presence of solutes. Water tends to move from the area of lower solute concentration to the area of higher solute concentration. The lower the concentration of solutes (osmotic potential), the higher the water potential.

Water Potential and Turgor Pressure Biology, 9th ed, Sylvia Mader Chapter 26 Water Potential and Turgor Pressure Plant Nutrition Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. central vacuole Wilted central vacuole Turgid cell wall cell wall H2O enters the cell Extracellular fluid: water potential pressure potential osmotic potential Equal water potential inside and outside the cell higher Inside the cell: water potential pressure potential osmotic potential Pressure potential increases until the cell is turgid lower a. Plant cells need water. b. Plant cells are turgid.

Transport Mechanisms in Plants Biology, 9th ed, Sylvia Mader Chapter 26 Transport Mechanisms in Plants Plant Nutrition Water Transport Xylem vessels form an open pipeline. The vessel elements are separated by perforated plates. Water moves into and out of tracheids through pits. Water entering roots creates a positive pressure (root pressure). It pushes xylem sap upward. May be responsible for guttation Water forced out vein endings along edges of leaves

Root Pressure and Guttation Biology, 9th ed, Sylvia Mader Chapter 26 Root Pressure and Guttation Plant Nutrition

Transport Mechanisms in Plants Biology, 9th ed, Sylvia Mader Chapter 26 Transport Mechanisms in Plants Plant Nutrition Cohesion-tension model of xylem transport suggests a passive xylem transport. Cohesion is the tendency of water molecules to cling together. Adhesion is the ability of the polar water molecules to interact with molecules of vessel walls. A continuous water column moves passively upward due to transpiration.

Transport Mechanisms in Plants Biology, 9th ed, Sylvia Mader Chapter 26 Transport Mechanisms in Plants Plant Nutrition Leaves Transpiration causes water loss through stomata. Water molecules that evaporate are replaced by water molecules from leaf veins. Due to cohesion, transpiration exerts a pulling force (tension) drawing water through the xylem to the leaf cells. Waxy cuticle prevents water loss when stomata are closed. Stem Tension in xylem pulls the water column upward. Roots Water enters xylem passively by osmosis and is pulled upward due to tension in xylem.

Cohesion-Tension Model of Xylem Transport Biology, 9th ed, Sylvia Mader Cohesion-Tension Model of Xylem Transport Chapter 26 Plant Nutrition Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. mesophyll cells xylem in leaf vein Leaves • Transpiration creates tension. stoma • Tension pulls the water column upward from the roots to the leaves. intercellular space H2O cohesion by hydrogen bonding between water molecules adhesion due to polarity of water molecules H2O cell wall water molecule Stem • Cohesion makes water continuous. • Adhesion keeps water column in place. xylem H2O water molecule root hair H2O Roots • Water enters xylem at root. • Water column extends from leaves to the root. xylem

Transport Mechanisms in Plants Biology, 9th ed, Sylvia Mader Chapter 26 Transport Mechanisms in Plants Plant Nutrition Opening and Closing of Stomata: Each stoma in a leaf epidermis is bordered by guard cells. Increased turgor pressure in guard cells opens stoma. Active transport of K+ into guard cells causes water to enter by osmosis and stomata to open. H+ ions accumulate outside guard cells as K+ moves in. Opening and closing of stomata is regulated by light. ABA (abscisic acid) can also cause stomata to close.

Opening and Closing of Stomata Biology, 9th ed, Sylvia Mader Opening and Closing of Stomata Chapter 26 Plant Nutrition Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Open Stoma H2O H2O vacuole K+ guard cell stoma H+ K+ enters guard cells, and water follows. a. 25 µm Closed stoma H2O H2O K+ K+ exits guard cells, and water follows. 25 µm b. © Jeremy Burgess/SPL/Science Source

Transport Mechanisms in Plants Biology, 9th ed, Sylvia Mader Chapter 26 Transport Mechanisms in Plants Plant Nutrition Organic Nutrient Transport: Role of phloem Phloem transports sugar. Phloem sap doesn’t only move upward or downward as xylem does. Travels from source (sugar’s origin) to sink (sugars are unloaded) Girdling of tree below the level of leaves causes bark to swell just above the cut. Sugar accumulates in the swollen tissue. Radioactive tracer studies confirm that phloem transports organic nutrients. Phloem sap can be collected using aphids.

Biology, 9th ed, Sylvia Mader Acquiring Phloem Sap Chapter 26 Plant Nutrition Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. An aphid feeding on a plant stem b. Aphid stylet in place 25.19a: © M. H. Zimmermann/Harvard Forest, Harvard University; 25.19b: © Steven P. Lynch

Transport Mechanisms in Plants Biology, 9th ed, Sylvia Mader Chapter 26 Transport Mechanisms in Plants Plant Nutrition Pressure-Flow Model of Phloem Transport Sieve tubes form a continuous pathway for organic nutrient transport. Sieve-tube members are aligned end to end. Strands of plasmodesmata extend through sieve plates between sieve-tube members. Positive pressure drives the movement of sap in sieve tubes. Sucrose is actively transported into phloem at the leaves. Water follows by osmosis, creating positive pressure. The increase in pressure causes flow that moves water and sucrose from the source to the sink.

Pressure-Flow Model of Phloem Transport Biology, 9th ed, Sylvia Mader Chapter 26 Plant Nutrition Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. mesophyll cell of leaf Leaf phloem water xylem sugar Leaves • Leaves are the main source of sugar production. • Sugar (pink) is actively transported into sieve tubes. • Water (blue) follows by osmosis. xylem phloem Stems • Phloem contents flow from a source to a sink. • Xylem flows from the roots to the leaves. Roots • Sugar is stored in the sink. • Cells can use it for cellular respiration. • Water exits by osmosis and returns to the xylem. cortex cell of root xylem phloem Root