Transport in Vascular Plants Chapter 36 By: Leah, Kim, CJ, and Renae
What is a Vascular Plant? Vascular plants have conducting, also referred to as vascular or liquefied, tissues that transport water, minerals, and photosynthetic materials throughout the plant’s roots, stems, and leaves. They differ from nonvascular plants, which do not have conducting tissues, and require water for fertilization. Other names for them include tracheophytes and higher plants. They make up the majority of plants found on the Earth today, with the exception of mosses and liverworts. Vascular Plant (Flower) Nonvascular Plant (Moss)
Overview: Pathways for Survival- For vascular plants…The evolutionary journey onto land involved the differentiation of the plant body into roots and shootsRoots absorb water and minerals from the soil.Shoots absorb light and CO2 for photosynthesis
Concept 36.1: Physical forces drive the transport of materials in plants over a range of distances • -Transport in vascular plants occurs on three scales… • Transport of water and solutes by individual cells, such as root hairs • Short-distance transport of substances from cell to cell at the levels of tissues and organs • Long-distance transport within xylem and phloem at the level of the whole plant
1 2 4 3 Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon for photosynthesis. Some O2produced by photosynthesis is used in cellular respiration. Sugars are produced by photosynthesis in the leaves. Transpiration, the loss of water from leaves (mostly through stomata), creates a force within leaves that pulls xylem sap upward. 6 5 7 Water and minerals are transported upward from roots to shoots as xylem sap. Roots absorb water and dissolved minerals from the soil. Roots exchange gases with the air spaces of soil, taking in O2 and discharging CO2. In cellular respiration, O2 supports the breakdown of sugars. • A variety of physical processes • Are involved in the different types of transport CO2 O2 Light H2O Sugar Sugars are transported as phloem sap to roots and other parts of the plant. O2 H2O CO2 Minerals Figure 36.2
Transport at the cellular level depends on the selective permeability of membranes ---The selective permeability of a plant cell’s plasma membrane controls the movement of solutes between the cell and the extracellular solution. • Specific transport proteins • ---enable plant cells to maintain an internal environment different from their surroundings
EXTRACELLULAR FLUID CYTOPLASM – + H+ + – ATP H+ – + H+ Proton pump generates membrane potential and H+ gradient. H+ H+ H+ – H+ + H+ – + Proton pumps in plant cells Create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work Contribute to a voltage known as a membrane potential
Water potential Is a measurement that combines the effects of solute concentration and pressure Determines the direction of movement of waterWaterFlows from regions of high water potential to regions of low water potentialBoth pressure and solute concentration affect water potential
Water and Minerals Ascend from Roots to Shoots Through the Xylem • Long distance transport of xylem sap • The sap flows upward from roots throughout the shoot system to veins that branch throughout each leaf. • Plants lose an astonishing amount of water by transpiration (evaporative loss of water from a plant). • Unless the transpired water is replaced by water transported up from the roots, the leaves will wilt and the plants will eventually die. • The upward flow of xylem sap also brings mineral nutrients to the shoot system.
Factors Affecting the Ascent of Xylem Sap • Xylem sap rises to heights of more than 100m in the tallest trees. • The sap is either pushed upward from the roots or pulled upward by the leaves.
Pushing Xylem: Roots Pressure • At night when transpiration is very low, root cells continue pumping mineral ions into the xylem of the vascular cylinder. • Endodermis helps prevent ions from leaking out • Water flows from the root cortex, generating root pressure (an upward push of xylem sap). • The root pressure sometimes causes more water to enter the leaves than is transpired, resulting in guttation (exudation of water droplets, caused by root pressure on certain plants).
Guttation fluid differs from dew, which is condensed moisture produced during transpiration. • In most plants, root pressure is a minor mechanism driving the ascent of xylem sap, at most forcing water upward only a few meters. • Many plants do not generate any root pressure. • Root pressure cannot keep pace with transpiration after sunrise. • For the most part, xylem sap is not pushed from below by root pressure but pulled upward by the leaves themselves.
Pulling Xylem Sap: The Transpiration-cohesion-Tension Mechanism • To move material upward, we can apply positive pressure from below or negative pressure from above. • The pressure of water is pulled upward by negative pressure in the xylem.
Transpiration Pull • Stomata, the microscope pores on the surface of a leaf, lead to a maze of internal airspaces that expose the mesophyll cells to the carbon dioxide they need for photosynthesis. • The air in these spaces is saturated with water vapor because it is in contact with the moist walls of the cells. • Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exists the leaf via the stomata when it is drier. • The leading hypothesis is that negative pressure that causes water to move up through the xylem develops at eh air-water interface in mesophll cell walls.
As more water is lost to the air, the air-water interface retreats deeper into the cell wall and becomes more curved. • The role of negative pressure fits with what you learned earlier about the water potiental equation because negative pressure lowers water potential. • The negative water potential of leaves provides the “pull” in transpiration pull.
Cohesion and Adhesion in the Ascent of Xylem Sap • Cohesion and adhesion facilitate this long distance transport • The cohesion of water due to hydrogen bonding makes it possible to pull a column of sap from above without the water molecules separating. • Water molecules exiting the xylem in the leaf tug on adjacent water molecules, and relayed molecule to molecule down the entire column of water in the xylem. • The upward pull on the sap creates tension within the xylem • Pressure will cause an elastic pipe to swell, but tension will pull the walls of the pipe inward.
The tension produced by transpirational pull lowers water potential in the root xylem to such an extent that water flows passively from the soil, across the root cortex, and into the vascular cylinder. • Transpirational pull can extend down to the roots only through an unbroken chain of water molecules. • In trees, root pressure cannot push water to the top, so a vessel with a water vapor pocket usually cannot function as a water pipe again. • Only the youngest, outermost secondary xylem transports water. • Root pressure enables small plants to refill embolized vessels in spring.
Xylem Sap Ascent by Bulk flow: A Review • In the long distance transport of water from roots to leaves by bulk flow, the movement of fluid is driven by a water potential difference at opposite ends of a conduct. • The water potential difference is generated at the leaf end by transpirational pull. • Water potential gradients drive the osmotic movement of water from cell to cell within root and leaf tissue. • Bulk flow depends only on pressure. • Another contrast is the bulk flow moves the whole solution, water plus minerals and any other solutes dissolved in the water. • The plant expends no energy to lift xylem sap by bulk flow. • The absorption of sunlight drives transpiration by causing water to evaporate from the moist walls of mesophyll cells. • The ascent of xylem sap is ultimately solar powered.
Stomata Help Regulate the Rate of Transpiration Large surface area of leaves is morphological adaption Enhances absorption of light needed to drive Photosynthesis Increase of water loss by the way of stomata Stomata – is the openings in leaves
Stomata Help Regulate the Rate of Transpiration Some evaporative water loss occurs when stomata are closed Prolonged drought – leaves begin to wilt as cells lose turgor pressure In cacti and other desert plants loss o water due to transpiration is greater threat than overheating Can tolerate high leaf tempuratures
Stomata – Major Pathways for Water Loss 90% of water a plant losses Each stomata surrounded by a pair of guard cells Control diameter of stoma Change shape widening/narrowing gap between two cells Stomata open when guard cells actively accumulate potassium ions form neighboring epidermal cells Close when lose potassium ions Potassium ions and water stored in vacuole
Stomata – Major Pathways for Water Loss Open during day Closed during night Light, depletion of CO2 within air spaces of leaf, and the plants internal “clock” - cause stomata to open during day 24 hour cycles of internal clock is called a circadian rhythm
Stomata – Major Pathways for Water Loss Environmental stress can cause stomata to close during day time In response to water deficiency abscisic acid is produced in the roots Signals guard cells to close stomata Passage of a cloud or the sun can affect rate of transpiration
Xerophyte Adaptions That Reduce Transpiration Xerophytes – plants that adapt to arid climates Have small, thick leaves, adaptation that limits water loss Adaptions – reflective leaves and hairy leaves Trap boundary layer of water
Translocation Xylem and phloem sap are both products from photosynthesis. Xylem sap flows from the roots to the leaves. Phloem is the exact opposite, it travels from the leaves to the rest of the parts of the plant Translocation- The transport of organic nutrients in the plant.
Sugar Source to Sugar Sink • The Sieve tubes always carry the sugars from a sugar source to a sugar sink. • Sugar Source- Is an organism that is a net producer of sugar, by photosynthesis or by breakdown of starch. • Mature leaves are the primary sugar source. • Sugar Sinks- Is an organism that is a net consumer of storer of sugar. • Growing nuts, buds, stems, and fruits are all sugar sinks.