1. Transport in Plants

2. Transport in plants H2O & minerals Sugars Gas exchange
transport in xylem
transpiration
Water potential, adhesion & cohesion
Sugars
transport in phloem
bulk flow
Photosynthesis in leaves loads sucrose into phloem
Gas exchange
photosynthesis
CO2 in; O2 out
stomata
respiration
O2 in; CO2 out
roots exchange gases within air spaces in soil

3. Overview Sugars travel from leaves to roots
through phloem
Water and dissolved minerals travel from root to shoot
through xylem
It defies
gravity!

4. 1. Transport of Water and Minerals
Amount of water needed daily by plants is small compared to the amount that is lost through transpiration
Transpiration:
evaporation of water from plant surface
If water is not replaced, the plant will wilt and may die.

5. Hydrostatic pressure causes water to travel up tube
Water Potential
Water movement is governed by differences in water potential
The potential energy of water molecules
Solute concentration and pressure
Water moves from an area of higher water potential to lower water potential
High solute concentration = low water potential
Low solute concentration – high water potential PE
High Water Potential
Low Water Potential
Hydrostatic pressure causes water to travel up tube PE

6. The Process - Roots Minerals from the soil
Actively transported into the root hairs and start to accumulate
Increase solute concentration in root cells, decrease water potential
Water moves in through osmosis to xylem cells
High Water Potential
WATER
Low Water Potential
root hair
H2O

7. Hydrostatic pressure causes water to travel up xylem
As water enters the xylem, it forces fluid up the xylem due to hydrostatic root pressure
positive pressure
This pressure can only move fluid a short distance.
The most significant force moving the water and dissolved minerals in the xylem
The “pull” of water from transpiration
cohesion & adhesion
Hydrostatic pressure causes water to travel up xylem
Pull =
Negative
Pressure

8. Adhesion and Cohesion Water is a polar molecule
unequal sharing of electrons in the covalent bonds
oxygen atom has a stronger attraction for electrons then hydrogen
O becomes slightly negatively charged
H becomes slightly positively charged d– O H H d+ d+
H2O

9. Cohesion and Adhesion
Water molecules are attracted to one another and other materials
Cohesion
Due to: Hydrogen Bonds
Force of attraction between slightly “–” oxygen and slightly “+” hydrogen of adjacent water molecules
Adhesion
Attraction between water molecules and the side of xylem cells

10. Transpirational Pull Starts in Leaves
Evaporation of water through stomata
Lowers WP in the surrounding air spaces
Water moves from spongy cells (higher WP) to air spaces (lower WP)
Water in spongy cells exerts a pull on column of water molecules in the xylem all the way from the leaves to the roots (adhesion, cohesion)

11. LOW water potential
HIGH water potential

12. 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 of minerals to host plant

13. Mycorrhizae
The hyphae of mycorrhizal fungi extend into soil, where their large surface area and efficient absorption enable them to obtain mineral nutrients, even if these are in short supply or are relatively immobile. Mycorrhizal fungi seem to be particularly important for absorption of phosphorus, a poorly mobile element, and a proportion of the phosphate that they absorb has been shown to be passed to the plant.

14. 2. Transport of Sugars Photosynthesis: CO2 + H2O  C6H12O6 + O2
Storage form of sugar: Starch
Cannot be transported, must be broken down into smaller components
Transport form of sugar: Sucrose
Very sweet sap
Usable form of sugar: Glucose

15. Push and Pull
Water and minerals are mainly transported via transpiration
negative pressure or “pull”
Sucrose is mainly transported via:
positive pressure (hydrostatic pressure)
“push” force (+) pressure due to accumulation of water
“pull” force (-) pressure due to adhesion & cohesion

16. Companion cells ATP Cells that surround phloem
Contain a lot of mitochondria
Why?
A lot of active transport!
ATP

17. Transport of Sugars ATP Mass flow hypothesis Phloem loading in leaf
“source to sink” flow
Source = leaf, Sink = root
Phloem loading in leaf
active transport of sucrose into phloem
increased sucrose concentration decreases water potential
Water flows in from xylem cells
increase in pressure due to increase in water causes flow
Hydrostatic pressure
ATP
can flow 1m/hr
In contrast to the unidirectional transport of xylem sap from roots to leaves, the direction that phloem sap travels is variable. However, sieve tubes always carry sugars from a sugar source to a sugar sink. A sugar source is a plant organ that is a net producer of sugar, by photosynthesis or by breakdown of starch. Mature leaves are the primary sugar sources. A sugar sink is an organ that is a net consumer or storer of sugar. Growing roots, buds, stems, and fruits are sugar sinks. A storage organ, such as a tuber or a bulb, may be a source or a sink, depending on the season. When stockpiling carbohydrates in the summer, it is a sugar sink. After breaking dormancy in the spring, it is a source as its starch is broken down to sugar, which is carried to the growing tips of the plant.
A sugar sink usually receives sugar from the nearest sources. Upper leaves on a branch may send sugar to the growing shoot tip, whereas lower leaves export sugar to roots. A growing fruit may monopolize sugar sources around it. For each sieve tube, the direction of transport depends on the locations of the source and sink connected by that tube. Therefore, neighboring tubes may carry sap in opposite directions. Direction of flow may also vary by season or developmental stage of the plant.

18. ATP Phloem unloading into root cells
active transport of sucrose into root cells
Decreases pressure in bottom of plant
Sucrose will travel from high pressure near leaves to low pressure near roots
ATP

19. 3. Gas Exchange
What environmental conditions might impact transpiration of water?

20. Gas Exchange Regulation
Epidermal cell
Guard cell
Chloroplasts
Nucleus
In dry conditions
water leaves guard cells by osmosis
guard cells become flaccid
stomata close to prevent water loss
In humid conditions
water enters guard cells by osmosis
guard cells become turgid
stomata open to facilitate water flow
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Thickened inner
cell wall (rigid)
H2O
H2O
H2O
H2O
Stoma open
Stoma closed
water moves into guard cells
water moves out of guard cells

21. Control of transpiration
Balancing stomate function
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

22. Don’t get mad… Get answers!! Ask Questions!

23. Homework
Section 9.5 – pg. 326 #1-9
Read Transpiration Lab