1. Bellwork: If a plant’s stomata close on a hot dry day, how could this affect the rate of photosynthesis?

2. Transport in plants
Section 23.5

3. What drives water transport in a plant?
Osmotic pressure
Not enough by itself…
Transpiration
Evaporation of water from leaves is the major driving force!
Water evaporates through stomata
Cell walls in leaf dry out
As cell walls contain cellulose, they draw water from deeper within the leaf
This pull extends to vascular tissue

4. How important is transpiration pull?
On a hot day, even a small tree may loose as much as 100 liter of water to transpiration
Hotter, drier and windier the day, the greater amount of water loss

5. How do cells pull water upwards?
Cohesion! Attraction between water molecule due to hydrogen bonding
Also adhesion – attraction between unlike molecules
Capillary action – tendency for water to rise in small tubes
Water is attracted both to the walls of the cell to and to other water molecules
The thinner the tube, the higher the water will rise

6. How is capillary action and water movement through the xylem related?
Water adheres very well to cellulose cell walls that make up xylem and tracheid
As water is removed from leaf, strong adhesion forces pull in more water from the xylem
This force can extend all the way down to the roots!
It is the combination of transpiration and capillary action that moves water through the xylem vessel of the plant

7. What about the phloem?
Leading explanation is the pressure flow hypothesis
Sugars are moved into sieve tube from surrounding tissues
Water follows by osmosis, creating pressure in the tube at the sugar source
If somewhere else needs sugars, they are pumped out of the tube at the required place
Due to osmosis, water will also leave at this point – reducing tube pressure, and creating a pressure driven flow of nutrients, from source to sink

8. Why is the pressure flow system important?
The pressure flow system allows plants to respond to changing seasons
During growing season – sugars are needed in ripening fruits, or storage in the roots
During spring, sugars are needed from the roots to power rapid plant growth
Sugars are needed to move in the phloem in both directions

9. Key points summary
What is the main driving force of the upward flow of water in a plant?
What hypothesis explains the movement of fluid through the phloem of a plant?
What are the roles of active and passive transport in the movement of nutrients in the phloem?