Post on 07-May-2018
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Chapter 36: Transport in Plants
• H2O & Minerals
o Transport in xylem
o Transpiration
� Evaporation, adhesion & cohesion
� Negative pressure.
• Sugars
o Transport in phloem.
o Bulk flow
� Calvin cycle in leaves loads sucrose into phloem.
� Positive pressure.
• Gas Exchange
o Photosynthesis
� CO2 in; O2 out
� Stomates
o Respiration
� O2 in; CO2 out
� Roots exchange gases within air spaces in soil.
Why does over-watering kill a plant?
Transport in Plants
• Physical forces drive transport at different scales.
o Cellular
� From environment into plant cells
� Transport of H2O & solutes into root hairs
o Short-distance transport
� From cell to cell
� Loading of sugar from photosynthetic leaves into phloem sieve tubes.
o Long-distance transport
� Transport in xylem & phloem throughout whole plant.
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Cellular Transport
• Active transport
o Solutes are moved into plant
cells via active transport.
o Central role of proton pumps.
� Chemiosmosis
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Short Distance (Cell-to-Cell) Transport
• Compartmentalized plant cells
o Cell wall
o Cell membrane
� Cytosol
o Vacuole
• Movement from Cell to Cell
o Move through cytosol
� Plasmodesmata junctions
connect cytosol of neighboring
cells.
� Symplast
o Move through cell wall
� Continuum of cell wall
connecting cell to cell.
� Apoplast
Routes from Cell to Cell
• Moving water & solutes between cells
o Transmembrane Route
� Repeated crossing of plasma membranes.
� Slowest route but offers more control.
o Symplast Route
� Move from cell to cell within cytosol
o Apoplast route
� Move through connected cell wall without crossing cell membrane.
� Fastest route but never enter cell.
Long Distance Transport
• Bulk flow
o Movement of fluid driven by pressure.
� Flow in xylem tracheids & vessels.
• Negative pressure
• Transpiration creates negative pressure pulling xylem sap upwards from
roots.
� Flow in phloem sieve tubes
• Positive pressure
• Loading of sugar from photosynthetic leaf cells generates high positive
pressure pushing phloem sap through tube.
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Movement of Water in Plants
• Water relation in plant cells is based on water potential.
o Osmosis through aquaporins.
� Transport proteins
o Water flows from high potential to low potential.
Water & Mineral Uptake by Roots
• Mineral uptake by root hairs.
o Dilute solution in soil.
o Active transport pumps.
o This concentrates solutes (~100x) in root cells.
• Water uptake by root hairs
o Flow from high H2O potential to low H2O potential.
o Creates root pressure.
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Route Water Takes Through Root
• Water uptake by root hairs
o A lot of flow can be through cell wall route
o Apoplasty
Controlling the Route of Water in Root
• Endodermis
o Cell layer surrounding vascular
cylinder of root.
o Lined with impervious Casparian
strip.
o Forces fluid through selective cell
membrane & into Symplast.
• Filtered & forced into xylem
vessels.
Mycorrhizae Increase Absorption
• Symbiotic relationship between fungi &
plant.
o Symbiotic fungi greatly increases surface area for absorption of water & minerals.
o Increases volume of soil reached by plant.
o Increases transport to host plant.
Rise of Water in a Tree by Bulk Flow
• Transpiration pull.
o Adhesion & cohesion.
� H bonding
o Brings water & minerals to shoot.
• Water potential
o High in soil → low in leaves
• Root pressure push
o Due to flow of H2O from soil to root cells.
o Upward push of xylem sap.
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Control of Transpiration
• Stomate function
o Always a compromise between photosynthesis & transpiration.
� Leaf may transpire more than its weight in water in a day…this loss must be
balanced with plant’s need for CO2 for photosynthesis.
• A corn plant transpires 125 L of water in a growing season.
Regulation of Stomates
• Microfibril Mechanism
o Guard cells attached at tips
o Microfibrils in cell walls
� Elongate causing cells to arch
open = open stomate
� Shorten = close when water is
lost
• Ion Mechanism
o Uptake of K+ ions by guard cells
� Proton pumps
� Water enters by osmosis
� Guard cells become turgid
o Loss of K+ ions by guard cells
� Water leaves by osmosis
� Guard cells become flaccid
• Other Cues
o Light trigger
� Blue-light receptor in plasma
membrane of guard cells
� Triggers ATP-powered
proton pumps causing K+
uptake
• Stomates open
o Depletion of CO2
� CO2 is depleted during photosynthesis (Calvin cycle)
o Circadian rhythm = internal “clock”
� Automatic 24-hour cycle
Transport of Sugars in Phloem
• Loading of sucrose into phloem.
o Flow through symplast via Plasmodesmata.
o Active cotransport of sucrose with H+ protons.
� Proton pumps
Pressure flow in Sieve Tubes (See: Page 753, Figure 36.18)
• Water potential gradient
o “source to sink” flow
� Direction of transport in phloem is variable
o Sucrose flows into phloem sieve tube decreasing
H2O potential
o Water flows in from xylem vessels
� Increase in pressure due to increase in H2O causes flow.
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Chapter 37: Plant Nutrition
Nutritional Needs
• Autotrophic does not mean autonomous
• Plants Need:
o Sun as an energy source
o Inorganic compounds as raw materials
� Water (H2O)
� CO2
� Minerals
Macronutrients
• Plants require these nutrients in relatively large amounts
o C, O, H, N, P, K, Ca, Mg, S
For What & from Where?
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Micronutrients
• Plants require in very small amounts.
o Primarily cofactors
Nutrient Deficiencies
• Lack of essential nutrients
o Exhibit specific symptoms
� Dependent on function of nutrient
� Dependent on solubility of nutrient
Magnesium Deficiency
• Symptoms
o Chlorosis = yellowing of leaves
o What is magnesium’s function?
o The chlorosis shows up in older leaves first, because plant moves Mg to newer leaves. Why?
Chlorophyll
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Water & Mineral Uptake
• Water uptake
o Plants cannot extract all water from soil, only free water
o Osmosis
• Cation uptake
o Cation uptake is aided by H+ secretion by root cells (proton pump)
o Active transport
The Role of Soils
• Plants are dependent on soil quality.
o Texture / structure
� Relative amounts of various sizes of soil particles
o Composition
� Organic & inorganic chemical components
� Fertility
Importance of Organic Matter
• Topsoil
o Most important to plant growth
o Rich in organic matter
� Humus
• Decomposing organic material
o Breakdown of dead organisms, feces, fallen leaves & other organic
refuse by bacteria & fungi
• Improves soil texture
• Reservoir of minerals
o Organisms
� 1 tsp. of topsoil has ~5 billion bacteria living with fungi, algae, protists, insects,
earthworms, nematodes.
• Not taking care of soil health has far-reaching, damaging consequences.
o 1920’s Dust Bowl
� Lack of soil conservation
• Growing wheat & raising cattle
o Land exposed to wind erosion & drought.
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Fertilizers
• “Organic” fertilizers
o Manure, compost, fishmeal
• “Chemical” fertilizers
o Commercially manufactured
o N-P-K (ex. 15-10-5)
� 15% nitrogen
� 10% phosphorus
� 5% potassium
Nitrogen Uptake
• Nitrates
o Plants can only take up nitrate
(NO3-)
• Nitrogen cycle by bacteria.
o Trace path of nitrogen fixation!
Soybean Root Nodules
• N fixation by Rhizobium bacteria.
o Symbiotic relationship with bean family (legumes).
Increasing Soil Fertility
• Cover crops
o Growing a field of plants just to plow them under
• Usually a legume crop.
• Taking care of soil’s health.
• Puts nitrogen back in soil