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

Transport in plants Section 23.5

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

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

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

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

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

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

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?