Chapter 36: Resource Acquisition & Transport in Vascular Plants

Transcription

1 Chapter 36: Resource Acquisition & Transport in Vascular Plants 1. Overview of Transport in Plants 2. Transport of Water & Minerals 3. Transport of Sugars

2 1. Overview of Transport in Plants

3 H 2 O CO 2 O 2 Sugar Light Resources Needed by Plants H 2 O and minerals CO 2 O 2

4 Resources Needed by Plants CO 2 carbon source used during photosynthesis of sugars and other organic molecules O 2 required for the synthesis of ATP by aerobic respiration Sunlight source of energy for photosynthesis Water obtained primarily from the soil Minerals & other Nutrients obtained primarily from the soil

5 Leaf Arrangement (Phyllotaxy) Leaf arrangement and orientation evolved to: maximize light absorption reduce self shading (blocking light to lower leaves) avoid damage from intense light Ground area covered by plant Leaf area index represents % of ground area covered by plant: Plant A Leaf area = 40% of ground area (leaf area index = 0.4) Plant B Leaf area = 80% of ground area (leaf area index = 0.8) commonly >1 due to multiple layers of leaves plants self-prune structures that don t receive enough light

6 More on Leaf Arrangement Plants must balance light absorption and water loss: light absorption tends to correlate with water loss greater surface area for light absorption = greater surface area for water loss leaf shape & arrangement reflect a balance between the two: low light & high moisture = larger, horizontal leaves harsh light & low moisture = smaller, vertical leaves

7 3 Modes of Transport Apoplastic through extracellular spaces Symplastic through cytosol, plasmodesmata Transmembrane across multiple plasma membranes Cell wall Apoplastic route Cytosol Symplastic route Transmembrane route Plasmodesma Plasma membrane Key Apoplast Symplast

8 Solute Transport Across Plant Cell Membranes CYTOPLASM ATP EXTRACELLULAR FLUID Hydrogen ion S S Proton pump (a) and membrane potential H /sucrose cotransporter S Sucrose (neutral solute) Nitrate (b) and cotransport of neutral solutes K K K Potassium ion K /NO 3 cotransporter K K Ion channel K (c) and cotransport of ions (d) Ion channels

9 Transport Through Ion Channels plants have gated ion channels that, when opened, allow ions to flow down the electrochemical gradient K K K Potassium ion K K K K Ion channel Ion channels

10 The Role of in Cotransport ions are pumped by active transport to create an electrochemical gradient (membrane potential) CYTOPLASM ATP EXTRACELLULAR FLUID Hydrogen ion Proton pump and membrane potential

11 flow down its electrochemical gradient can be coupled to the active transport (movement from low to high conc.) of neutral solutes such as sugars S S S /sucrose cotransporter Sucrose (neutral solute) and cotransport of neutral solutes

12 or the active transport of ions such as nitrate (NO 3- ) Nitrate /NO 3 cotransporter and cotransport of ions

13 The Transport of Water Osmosis is the diffusion of water across a cell membrane. The net direction of osmosis (water movement) in plants depends on 2 factors: differences in the concentration of water & solutes across the membrane (water diffuses from high to low concentration) differences in pressure (water moves from high to low pressure) The combination of these 2 factors (concentration & pressure) is called water potential.

14 more on Water Potential Water potential (y) = the sum of solute potential (y S ) and pressure potential (y P ) Initial flaccid cell: ψ P = 0 ψ S = 0.7 Final plasmolyzed cell at osmotic equilibrium with its surroundings: ψ = 0.7 MPa ψ P = 0 ψ S = 0.9 ψ = 0.9 MPa (a) Initial conditions: cellular ψ > environmental ψ Environment 0.4 M sucrose solution: ψ P = 0 ψ S = 0.9 ψ = 0.9 MPa y = y S y P For pure water y S = 0, the more solutes the more negative the y S y P can be or in relation to atmospheric pressure Initial flaccid cell: ψ P = 0 ψ S = 0.7 Final turgid cell at osmotic equilibrium with its surroundings: ψ = 0.7 MPa ψ P = 0.7 ψ S = 0.7 ψ = 0 MPa (b) Initial conditions: cellular ψ < environmental ψ Environment Pure water: ψ P = 0 ψ S = 0 ψ = 0 MPa

15 Turgor Pressure in Plants The protoplast (interior part) of plant cells normally has a positive y P due to osmosis, a pressure called turgor pressure which keeps cells turgid (opposite of flaccid). Rate of osmosis is increased by aquaporins. Normal plant with turgid cells Wilted plant with flaccid cells In extracellular compartments such as xylem, y P is negative which aids in the movement of fluid up from the root system.

16 2. Transport of Water & Minerals in Xylem

17 From Root Hairs to Xylem Casparian strip Endodermal cell Pathway along apoplast Apoplastic route Symplastic route Transmembrane route 2 Symplastic route Pathway through symplast Plasma membrane Apoplastic route Root hair The endodermis: controlled entry to the vascular cylinder (stele) 1 Plasmodesmata 3 Epidermis 5 Casparian strip Endodermis Vascular cylinder Cortex (stele) Transport in the xylem Water moves upward in vascular cylinder Vessels (xylem)

18 Water & Mineral Uptake by Roots The transport of water, minerals and other nutrients via xylem vessels begins at the interface of the root tip & root hair epidermis and the surrounding soil. the root epidermal cells are permeable to the aqueous soil solution which freely passes along the cell walls (apoplastic route) to the root cortex once this material reaches the endodermis, water and desired solutes are transported across the endodermal cells to the vascular cylinder (stele) once in the stele, water and mineral nutrients enter the tracheids and vessel elements of the xylem as xylem sap to be transported throughout the plant

19 How is Xylem Moved Up? Xylem sap moves upward in the plant due to a combination of the following: ROOT PRESSURE (a minor factor) active transport of ions into the roots lowers the water potential resulting in water flowing in due to osmosis TRANSPIRATION (the major factor) loss of water through the stomata of leaves adhesion of water to xylem vessels & cohesion of water molecules to each other pull water up to replace water lost through transpiration

20 Source of Pull in Transpiration Diffusion of water vapor out of stomata starts the pull which creates a negative water potential drawing water up: Cuticle Upper epidermis 5 Water from xylem pulled into cells and air spaces. Xylem 4 Increased surface tension pulls water from cells and air spaces. Mesophyll Air space Microfibrils in cell wall of mesophyll cell 3 Air-water interface retreats. Lower epidermis Cuticle Stoma 2 Water vapor replaced from water film. 1 Water vapor diffuses outside via stomata. Microfibril (cross section) Water film Air-water interface

21 Water potential gradient Transpiration Outside air ψ = MPa Leaf ψ (air spaces) = 7.0 MPa Leaf ψ (cell walls) = 1.0 MPa Transpiration Xylem cells Xylem sap Mesophyll cells Stoma Water molecule Atmosphere Adhesion by hydrogen bonding Cell wall Trunk xylem ψ = 0.8 MPa Trunk xylem ψ = 0.6 MPa Soil ψ = 0.3 MPa Cohesion and adhesion in the xylem Cohesion by hydrogen bonding Water molecule Root hair Soil particle Water Water uptake from soil

22 Guard Cell Control of Stomata when guard cells are turgid, they bend and as a result open the stomata when guard cells are more flaccid the stomata are closed Guard cells turgid/ Stoma open Radially oriented cellulose microfibrils Cell wall Guard cells flaccid/ Stoma closed Vacuole Guard cell Changes in guard cell shape and stomatal opening and closing (surface view)

23 Regulation of Guard Cells Guard cell turgidity is controlled by K ions which move in response to changes in membrane potential due to active transport of : K Guard cells turgid/ Stoma open H 2 O pumping H out of guard cells H 2 O H 2 O H 2 O H 2 O Guard cells flaccid/ Stoma closed H 2 O H 2 O Role of potassium ions (K ) in stomatal opening and closing H 2 O H 2 O H 2 O pumped out of guard cells lowers the membrane potential (more negative) drawing K ions into the cell the intracellular increase in K lowers the water potential and water flows in Plants open stomata by pumping in response to light and low CO 2 (provided there is enough water)

24 3. Transport of Sugars

25 Sugar Translocation via Phloem The transport of photosynthetic products, a process called translocation, proceeds through phloem vessels in a direction opposite to that of xylem sap. photosynthetic products such as sucrose are produced in photosynthetic organs such as leaves they are transported in phloem sap to sites of sugar use or storage sugar sinks e.g., fruits, tubers, growing shoot and root tips the transfer of sugars to phloem sieve tube elements or companion cells occurs through both symblastic and apoplastic routes

26 Loading Sugars into Phloem Sieve Tube Elements Apoplast Symplast transport from apoplast to sieve tube element symplast involves cotransport with Mesophyll cell Cell walls (apoplast) Plasma membrane Companion (transfer) cell Sieve-tube element High concentration Proton pump Cotransporter S Plasmodesmata Mesophyll cell Bundlesheath cell Phloem parenchyma cell ATP Low concentration S Sucrose (a) Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube elements. (b) A chemiosmotic mechanism is responsible for the active transport of sucrose.

27 Bulk flow by negative pressure Bulk flow by positive pressure Bulk Flow of Phloem Sap unlike xylem sap, phloem sap flows toward sugar sinks due to positive pressure Vessel (xylem) H 2 O Sieve tube (phloem) 2 1 Source cell (leaf) Sucrose H 2 O 1 Loading of sugar decreases water potential sugar concentration decreases near the sugar sinks due to usage for energy or addition to polymers such as starch this results in a net diffusion of sugar and movement of water towards the sinks 4 H 2 O 3 Sink cell (storage root) Sucrose Uptake of water increases pressure Unloading of sugar and loss of water relieves the pressure Recycling of water