Resource Acquisition & Transport in Plants Chapter 36

Adaptations for Acquiring Resources  Land plants inhabit 2 worlds: Above ground – shoots acquire sunlight and CO 2 Below ground – roots acquire water and minerals  Early land plants absorbed water, minerals and CO 2 directly from or near a water source  As land plants increased in number, there was competition for resources  Plants developed specialized tissues to allow them to be successful in areas further from water

Shoot Architecture & Light Capture  Leaf size and structure accounts for much of the outward diversity in plant form  Largest leaves are found in tropical species, smallest in dry or cold environments  Photosynthesis is affected by leaf arrangement, orientation and height

Root Architecture and Acquisition of H 2 O and Minerals  The evolution of root branching enabled land plants to more effectively acquire water and nutrients from different types of soil while providing strong anchorage  The evolution of mutualistic associations between roots and fungi called mycorrhizae allowed plant to live in poorly developed soils

Pathways of Transport in Plants  Apoplast - everything external to the plasma membranes of living cells Cell walls Extracellular spaces Interior of vessel elements and tracheids  Symplast – entire mass of cytosol of the living cells in a plant Includes plasmodesmata and cytoplasmic channels that interconnect them

Transport in Plants  H 2 O & minerals transport in xylem transpiration o evaporation, adhesion & cohesion o negative pressure  Sugars transport in phloem bulk flow o Calvin cycle in leaves loads sucrose into phloem o positive pressure

 Gas exchange photosynthesis o CO 2 in; O 2 out o stomates respiration o O 2 in; CO 2 out o roots exchange gases within air spaces in soil

Ascent of Xylem Fluid Transpiration pull generated by leaf

Water & mineral absorption  Water absorption from soil o osmosis o aquaporins  Mineral absorption active transport proton pumps o active transport of H + H2OH2O root hair aquaporin proton pumps

Mineral Absorption  Proton pumps active transport of H + ions out of cell o chemiosmosis o H + gradient creates membrane potential o difference in charge o drives cation uptake creates gradient o cotransport of other solutes against their gradient

Water Flow Through Roots  Porous cell wall water can flow through cell wall route & not enter cells plant needs to force water into cells Casparian strip

Controlling the Route of Water in Root  Endodermis cell layer surrounding vascular cylinder of root lined with impermeable Casparian strip forces fluid through selective cell membrane o filtered & forced into xylem cells

Root Anatomy dicotmonocot

Mycorrhizae Increase Absorption  Symbiotic relationship between fungi & plant symbiotic fungi greatly increases surface area for absorption of water & minerals increases volume of soil reached by plant increases transport to host plant

Mycorrhizae

Transport of Sugars in Phloem  Loading of sucrose into phloem flow through cells via plasmodesmata proton pumps o cotransport of sucrose into cells down proton gradient

can flow 1m/hr Pressure Flow in Phloem  Mass flow hypothesis “source to sink” flow o direction of transport in phloem is dependent on plant’s needs  phloem loading active transport of sucrose into phloem increased sucrose concentration decreases H 2 O potential  water flows in from xylem cells increase in pressure due to increase in H 2 O causes flow

Chloroplasts Epidermal cell Nucleus Guard cell Thickened inner cell wall (rigid) Stoma openStoma closed H2OH2O water moves into guard cells H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O Control of Stomates  Uptake of K + ions by guard cells proton pumps water enters by osmosis guard cells become turgid  Loss of K + ions by guard cells water leaves by osmosis guard cells become flaccid K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ water moves out of guard cells

Control of Transpiration  Balancing stomate function always a compromise between photosynthesis & transpiration o leaf may transpire more than its weight in water in a day…this loss must be balanced with plant’s need for CO 2 for photosynthesis