Water and Sugar Transport in Plants

1. What are the key differences between plant vs. animal circulatory systems?
2. What are the key concepts of water and sugar transport in plants?
3. What is the gradient that water moves along? and what direction does water move?
4. What is water’s potential energy in plants caused by? (two factors?)
5. Does water moves passively or actively? And what is the force driving water transport?
6. What is sugar transported? How is water involved in sugar transport?

Answers

2. water potential = solute potential + pressure potential. Cohesion, adhesion, root pressure vs. gravity. Cohesion-tension theory
3. Water moves from high to low water potential (along water potential gradient)
4. differences in solute concentration and physical pressure
5. passive. (without energy input, along water potential gradient). Transpiration at leaves causes negative pressure/ TENSION, which pulls water up from the roots through xylem
6. in phloem from source to sink. Active transport from source to phloem. Water from xylem osmosis (bc low solute potential in phloem near source) and causes pressure potential high near source. Water and sugar moves to low pressure potential near sink. Water goes back to xylem. (WATER CREATES PRESSURE GRADIENT driving movement of sucrose from source to sink.)

**Very small % of water used for photosynthesis. How do we know/

• Higher water potential  – higher water stress
• leaf expansion is very affected  by water stress
• photosythnesis is not that affected

What is a cause of transpiration in plants?

• transpiration = water loss, evaporation from leaf surface
• water and gas exchange when stomata open
• stomata opens because photosynthesis

How do plants regulate transpiration and water loss?

• How to reduce water loss through open stomata?
  • locate stomata on leaf under side (abaxial side)
    • Keep it cooler
    • shielded from direct sunlight (lower temperature)
    • If top (adaxial), direct sunlight incr
    • ease temperature and increases transpiration
• “invagination” pockets of epidermal layer protect and tuck in stomata
  • shield from wind -> wind increases transpiration
  • increases humidity   -> humidity decreases transpiration
• Trichomes (little hairs) collect moisture in stomata
  • think putting lid on a pot but lid has holes
• Thick waxy Cuticle
  • cover entire leaf with waxy. water impermeable. slows down water loss
• BIOCHEMICAL WAYS: up photosynthesis efficiency
  • more efficient Co2 uptake, get same fixation but less time open stomata
  • less time open stomata -> decreases transpiration
  • C4  – trap and release Co2 at different times.
  • CAM – open stomata at night to store co2, use co2 at day. trap and release at same time

 

WATER POTENTIAL IS A KEY CONCEPT. Describe how solute concentrations and turgor pressure control water potential and flow in plants

• water potential (psi) = potential energy of water at particular environment vs potential e of pure water
• water potential = solute (osmotic) potential + pressure potential
• water always flow from high to low water potential
• The more negative, the lower
• When water can diffuse but solute can’t, water moves towards higher solute concentration
• LOW Solute potential = high solute CONCENTRATION. confusing! Solute potential =
  • hypotonic surrounding? Low solute potential in cell
  • hypertonic surrounding? high solute potential in cell
  • isotonic
• Osmosis = diffusino of water across membrane
  • through aquaporins
• Osmosis can create a pressure potential

Explain potential of cell placed in pure water

• in-cell solute potential low relative to surrounding
• water moves into cell via osmosis
• uh oh, builds up pressure inside the cell
• animal cell may burst
• plant cell has cell wall that produces turgor pressure

U tube with different solute potential

• move to low solute potential
• unless pressure potential ( +) leads to no net movement of water
• Plant cells: solute potential and pressure potential balance out.
• Turgor pressure is really high!!! (as high as in bike tires)

What happens to plants when they lose turgor pressure?

 

CLICKER: A plant cell with solute potential -1.7 MPa and an internal turgor pressure (pressure potential) of +0.4 MPa is placed into a NaCl solution of -1.4 MPa and pressure potential of +0.1 MPa. The spontaneous net flow of water will be: No Net flow.

1. Calculate water potential inside the cell as a sum of solute potential and internal turgor pressure.
2. Calculate the water potential of the NaCl solution as a sum of solute potential and pressure potential.
3. Which water potential is greater?

Clicker 2: If plant cell solute potential is -1.3 MPa into solution of -0.5 solute p and +1 pressure potential, at equilibrium, what is the turgor pressure of the cell?

• The turgor pressure will be +0.9 MPa.
• Calculate the water potential of solution.
• At equilibrium, the water potential of solution and the cell must be equal.

Describe the strong water potential gradient between Soil, plant and atmosphere

• Low at top
• atmosphere, usually low, depends on humidity
• leaf, low when stomata open,  depends on transpiration rate
  • higher transpiration rates during day-time photosynthesis
  • When stomata open
• root, medium-high
• soil, low if dry, high if moist
• high at bottom

IMPORTANT CONCEPT: How does atmospheric humidity affect water potential? What is water potential at 100% humidity and how does this affect transpiration?

• humidity is water holding capacity compared to max water holding capacity
  • water holding capacity changes with pressure and temperature
  • At lower temperature, water holding capacity decreases. This is why we have so much rain in Vancouver. (Clouds rise up, temperatures colder high up, rain!)
• 100 % humidity = 0 water potential. there is no driving force for evaporation, transpiration.
  • in humid weather, your sweat doesn’t evaporate from your body and you feel gross

Predict relative plant transcription rates under different environmental conditions

• Looking at a graph of relative humidity, where do you expect maximum plant transpiration rates?
  • at lowest humidity, highest transpiration rate
• Clicker: in winter time, where would expect maximum transpiration rates?
  • Answer: B
  • The lowest humidity during the DAY time.
  • You need there to be sunlight for transpiration to occur
  • At the other points, even though humidity is lower, those are not during sunlight hours so no photosynthesis occurs, the number of stomata open at night is small, and not that much water loss.

What is the “plumbing” of the plant?

• Roots:
  • zone of maturation – root hairs
  • elongation zone
  • root cap
  • epidermis, cortex, endodermis
• Stomata serve as shut-off valve to the plumbing. All open or all close.
• Vascular tissue (central root tissue):
  • xylem – always one way, from soil to leaves
  • phloem – two directions, ground to leaves or leaves to ground
• DESCRIBE XYLEM – 2 types of water conducting cells
  • NON living (Dead)
  • Tracheids  – spindle shaped,
    • have pits – filter out small particles e.g bacteria
  • vessel elements – short, wide, perforations and pits

Describe the pathways of water movement in the root

• symplastic – through cells via plasmodesmata (intercellular connections)
  • pass through cell membranes
• apoplastic – around air spaces and cell walls until the Casparian strip
  • not pass through cell membranes
  • within porous cell walls

What physical forces drive water movement through the xylem over potentially long distances? 

1. Root pressure
   • low solute potential in vascular tissues  ( low water potential)
   • low water potential drives water uptake from soil
   • water uptake creates positive pressure
   • root pressure creates pressure potential even when transpiration not occurring
   • cause guttation   – root pressure push water out of leaf surfaces
2. Capillary action
   • water tends to form hydrogen bonds
   • cohesion  – water to water
   • adhesion  – water to non water
   • surface tension due to cohesion
3. BUT what is the force that prevents root pressure or capillary action from being strong enough to move water up to the top of the plant? Gravity.
4. Cohesion-tension theory
   • So what pulls up water? Surface tension.
   • What causes this surface tension? evaporation of water at leaf surface leads to NEGATIVE PRESSURE = TENSION. surface tension.
   • How is this tension transmitted? hydrogen bonding/ water cohesion
   • Analogy
     • evaporation = natural pull up
     • cohesion
     • gravity pull down
     • transpiration – tension = guy telling up
   • Evidence for cohesion-tension theory
     • leaf cutting. water withdraws (one up, one down) because you’ve disrupted cohesion
     • What happens if you cut the leaf at night with a small plant?
       • the force working on the water is gravity so direction for both water is down.
       • But in a small plant, in the lower half, the root pressure could be strong enough to push up.

IMPORTANT QUESTION: Given data on relative humidity, approximately when would you expect to see maximum transpiration rates in a cactus plant with CAM metabolism?

•  Plants with regular metabolism typically display maximum transpiration rates at the time of lowest relative humidity compared with most sunlight but plants that utilize CAM photosynthesis keep their stomata closed during the majority of the day to reduce transpiration and open their stomata at night for gas exchange. Therefore, the maximum transpiration rates for a CAM plant would occur at the points of lowest humidity during nighttime, as defined as after sundown and before sunrise. Based on the graph, I would predict that maximum transpiration rates for this cactus plant occurs approximately around 5 AM or  3 AM,the two points of lowest relative humidity during the night.
• Model response that received full marks from Connect: ” Maximum transpiration rates would occur during the hottest temperatures and lowest humidity (~ 2pm). Plants with CAM metabolism, such as the cactus plant in this question, keep their stomata closed during the daytime to avoid water loss. This stops transpiration in CAM plants during the hottest time of day but transpiration will occur during the night time (between 7pm and 5am) when CAM plants open their stomata. Therefore, in a cactus plant with CAM metabolism, maximum transpiration would occur at night with the lowest relative humidity. Maximum transpiration rates in a cactus plant occur between 12am and 5am.”

Cohesion –Tension theory

• cut a leaf
• gravity pulls water down
• tension pulls water up
• SUMMARY: negative pressure created by transpiration, this pressure transmitted by cohesion between water molecules

Tension and xylem contraction

• under negative pressure (suction), xylem contracts (diameter shrinks slightly)
• tree diameter shrinks more during transpiration/during the day because transpiration creates the negative pressure
• tree diameter expands during the night, when no transpiration
• What is the relationship between light intensity and xylem pressure?
  • no light- no pressure
  • when medium light intensity, negative pressure in xylem
  • when high light intensity, pressure more negative
  • With higher light intensity, pressure more negative. (“more” pressure?)
  • xylem pressure can serve as measure of photosynthesis
  • more pressure = greater transpiration rates= greater photosynthetic rates
• Recall CAM plants are special. This does not apply to CAM plants

WATER AND MINERAL TRANSPORT SUMMARY

• plant did NOT have expend energy to move that water!
  • So where is this energy from? Heat from the sun to evaporate water
• water moves from regions of high water to regions of low water potential
• Both pressure (including tension) and solute potential will affect how water moves.
• Differences in pressure potential  (cohesion and tension) is the main factor in pulling water up through a plant.
• Differences in pressure potential (cohesion and tension) is MAIN factor in pulling water up.

EXPLAIN SUGAR TRANSPORT

Sink?

• developing fruit, flower, roots
• Recall: roots need energy (from sugar) to pump ions in

CLICKER: In deciduous trees who lose their leaves in winter, what is the dominant direction of phloem sap flow in early spring?

• developing leaves not making enough sugar for themselves early in the spring season  = net sink
  • need more sugar to develop and grow
• flow up from roots to leaves

Coupling of xylem and phloem in vascular bundles

• packed very close together in vascular bundle

What are the two cell types in phloem? (just like how there are two cell types in xylem) and what are their roles in transport of sugar

• Sieve-tube membrane
  • separated by sieve plats
  • membrane-bound, sugar-conducting cell lacking nucleusand major organelles
  • sugar conducting cell
  • lacks nucleus and major organelles
• Sieve plate: between sieve-tube members
• Companion cell- smaller
  • responsible for all the metabolic activities in phloem
  • has nucleus and orgenlles
  • sugar unloading

How to move sugar from leaf to companion cell?

• membrane of companion cell adjacent to leaf cell
• active transport moves sucrose from leaf cell into companion cell
• (there is higher concentration of sugar in companion cell than in leaf cell)
• pH gradient  between interior and exterior of phloem cells – H+ (proton) pump
  • proton pump creates proton gradient (moves proton from inside companion to out companion)
  • symporter moves H and sucrose into companion passively (down proton gradient, against sucrose gradient)
  • IMPORTANT
  • energy has to be expended
• once in companion cells, sucrose builds up in sieve tube members via diffusion

CLICKER: How does loading sugars into sieve tubes affect the water potential in cells?

• increased water potential
  • By increasing solute concentration, solute potential is decreased so water potential is decreased.
  • when the solute potential decreases, water diffuses from xylem  into the phloem. This increase the pressure potential
• • So possible, net change in water potential is INCREASE

Why is it important that xylem and phloem next to each other?

• water flows from xylem to phloem
• creates turgor pressure

1. increase solute concentration. Active transport from leaf to companion, diffusion from companion to sieve tube.
2. uptake of water from xylem
3. increase in turgor pressure
4. overall increase in water potential
5. the water flows to areas of lower water potential, carrying sugars with it

(go back to slide 75)

What is the difference between active and passive sinks?

• metabolically active sink:
  • sucrose is consumed by metabolism  =  low concentration of sucrose in that cell = gradient
  • passive transport from sieve tube to leaf cell along gradient
  • cell is growing e.g. in growing leaves, expanding roots
• storage sink
  • If there’s a vacuole with active tranporter in tonoplast
  • tubers (roots)
  • storage to store up energy that plant can use in the spring time
  • But how does sugar moves in the root cell?
    • membrane bound transporter in tonoplast (vacuole membrane) uses ATP to move sugars into vacuole
    • This causes low concentration of sugar in the cytoplasm
    • So there is still a concentration gradient from phloem to root cell cytoplasm (still passive into root cell)
    • basically same as metabolically active except extra step of using energy to move sugars into vacuole

Explain the pressure-flow model

• most of water flowing upward in xylem
  • unidirectional
  • small fraction of water moves into phloem, then back up through xylem
  • water is main source of pressure in sieve tubes
  • high water potential near source cell
• vascular bundling

CLICKER: Where is lowest solute potential expect?

• Near source cell in phloem  – assuming that leaf is photo-synthetically active and producing sugars, then leaf is source cell.
• Remember to state your assumption during a test

SUMMARY OF SUGAR TRANSPORT

• sugar moves from source to sink
• some sugar transport require ATP to move sugar against concentration gradient e.g. into vacuole, pump protons so that sugar can diffuse into companion cell
• pressure potential drives water flow through phloem ( caused by water moving into the phloem)

Random: What causes maple syrup? xylem flow – NOt phloem. cool eh?

 

Principles of transport system in animal and plants

• Common theme: bulk flow is the way to transport large quantity efficiently to remote area
• But driving forces differ between animals and plants
  • Animals
    • breathing in lung driven by low pressure
    • blood circulation by cardiac muscle driven by high pressure
  • plants
    • xylem transport by cohesion-tension
    • phloem transport
• unique mechanisms to build forces
  • animals by muscular movement
  • plants by osmosis

 

Midterm studying

• resources available on connect:
  • clicker question
  • detailed study guides (super good)
  • sample midterm questions
• go to TA office hours because dr tortelle isn’t going to be here
  • especially Evan is the plant guy
  • don’t have to a specific question. even have a topic that you’re not confident on.
• Dr. tortelle is going on a ocean expedition (cool eh??) so we won’t be able to reach him for help.

Sample midterm questions

15. What accounts for day to day variability in mid-day trunk diameter?

• Transpiration causes negative pressure in the xylem, which causes the xylem to contract. Rates of transpiration depends on humidity and light intensity, which varies day to day. As a result, variability in humidity, light intensity and other environmental conditions affects transpiration which leads to the negative pressure in xylem to be higher some days and lower in some days. This causes the contraction of the xylem to be greater some days and lesser other days.
• Class answer: Changes in trunk diameter indicate changes in negative pressure in the xylem, which is driven by changes in the rates of evaporation/transpiration. These changes in evaporation are driven by changes in photosynthesis. The primary reason for differences in photosynthesis is differences in light available. The more light available, the longer stomata open. Differences in weather conditions e.g. humidity, temperature can lead to greater or lower evaporation rates while the stomata is open.

Question 7: CAM plants. measure concentration of c4, co2, rate of c fixation by rubisco, fraction stomata open. Draw diagram illustrate how these vary over a few day night cycles. justify and explain patterns

• C4 starts low at beginning of night, increases during the night as plant opens stomata to collect co2, co2 reacts to become c4. Decreases during the day when c4 is converted back to co2 for the plant to do photosynthesis  ( CAM plants open stomata during night. start beginning night of low c4, increases during the night. )
• Co2 decreases during the day as plant uses co2 for photosynthesis. Rate of Co2 may vary depending on photosynthetic rates (which depends on light availability which varies day to day)
  • During the night, CO2 is very low because reacts quickly to become C4
  • During day, C4 becomes Co2 (decarboxylation of C4) so get high concentration of Co2. (draw a hill that goes up and then goes down)
  • then Co2 drops back down because photosynthesis
• rate of C fixation is high during the day . Also varies day to day due to sunlight variability
  • Also draw hill!!  (goes up then goes down)
• stomata mostly open during the night, and mostly closed during the day.