Chapter 36Chapter 36

Vascular System in PlantsVascular System in Plants

Three ways water moves through root hairs

1) Apoplast: water moves through cell walls and never enter cells

2) Symplast: water moves from one cell to another through the cytoplasm

3) Transmembrane: water repeatedly moves back and forth from cell wall, crossing the membrane, and through the cytoplasm

apoplasticsymplastic

Pathway of Water Movement

1) Root hairs use osmosis to soak up water

2) Water travels via apoplast or symplast through the cortex until it reaches the endodermis that lines the vascular cylinder (stele).

3) Endodermis has a “casparian strip”, a selectively waxy layer, which BLOCKS the apoplast pathway, so water MUST move into the stele via symplast (to regulate what minerals in the water can enter the stele)

4) Xylem within the stele transports water to shoots

Transpiration

• Definition: the loss of water vapor from leaves and aerial parts of a plant via stomata

• Responsible for upward transport of water through the xylem due to:– pushing forces– pulling forces video

Pushing ForcesEndodermis accumlates ions in

stele, causing these effects:a) Ψw ______________ in steleb) Water moves ____ stele,

generating + pressure forcing water up the xylem (root pressure)

c) Guttation is a result of root pressure-low transpiration rate at night

-more water moves into stele than transpired

-water droplets forced out at end of leaves by morning

More solutes

-Ψs

-Ψw

Ψw= 0

decreases

into

Pulling Forces(transpiration, tension, cohesion

mechanism)Transpiration:

evaporation of water from stomata of leaves creates (-) pressure and develops tension

Bulk Flow:

Water moves up the xylem to replace the (-) pressure

Pulling Forces(transpiration, tension, cohesion

mechanism)

Cohesion:Water molecules are hydrogen bonded to each other due to its polarity

Water, therefore, moves up the xylem acting as one large polymer-like substance

How is transpiration controlled by plants?

There must be a compromise between photosynthesis and transpiration

The Great Debate!Pros and Cons of an Open Stomata

Pros:

Transpiration can occur for PS

Gas exchange for CR and PS can occur

Cons:

Plant risks dessication from excessive transpiration, leading to wilting

The Great Debate!Pros and Cons of a Closed

Stomata

Pros:

Minimal dessication and minimal wilting

Cons:

No transpiration for PS

No PS or CR can occur due to lack of gas exchange

Plants maintain a transpiration to PS ratio

Ratio (in grams)= water loss

CO2 assimilated for PS

C3 plants 600:1 ratio

C4 plants 300:1 ratio (greater CO2 assimilation rate)

Goal = maximize PS rate w/ minimal transpiration

When do plants open and close their stomata?

Guard Cells

High temperatures

[CO2] is low

Night

Day

guard cells close

guard cells open

guard cells close

guard cells open

How do guard cells open and close?

K+ is pumped into guard cells ψw ________ inside, and water moves into guard cells making them swell, turgid, and open

K+ is pumped out of guard cells ψw __________ inside, and water moves out of guard cells making them flacid and close

decreases increases

Translocation

Definition: the transport of carbohydrates (CHO) in plants from:

- the source (normally where CHO is produced)

to

- the sink (where CHO is used)

** storage organ (ex. bulb) can be a source or a sink

video

Translocation occurs in 2 steps

1) Sugar Loading

2) Pressure Flow

Sugar Loading

CHO are moved from source to sieve-tube members

a) Symplast pathwayor

b) Apoplast pathway - needs co-transport

mechanism to move CHO from apoplast to cytoplasm of sieve tube

symplastapoplast

ST members

H+H+H+H+

H+ ATPADP

Companion cell

1) CHO move from source to ST members by sugar loading mechanism

2) Ψ__________ in the phloem (STM), causing water from xylem to flow into the phloem

3) This increases pressure in the phloem. Increased pressure causes phloem sap to move to an area of less pressure

4) Carbohydrates move out towards sink

5) Ψ ____________ in phloem (STM) so water from phloem diffuses back out to xylem), decreasing pressure

Pressure Flowxylem

Phloem (STmembers)

source

sink

decreases

increases