lec 13- plant water transport

Question 1

what bond holds water together?

Answer 1

hydrogen bonds

Question 2

how does heat cause water to evaporate

Answer 2

heat makes water molecules move and breaks the hydrogen bonds causing a phase change

Question 3

what is surface tension?

Answer 3

water does not like interacting with air and molecules with weak cohesion, making water want to shrink and have a smaller surface area. The energy required to increase the surface area is called surface tension.

Question 4

what causes evaporation?

Answer 4

temperature increase, large surface area of liquid water, when it is windy, and low humidity cause evaporation

Question 5

9 steps to water transport:

Answer 5

1. starts in the air around the plants. Water diffuses from high water concentration in plants to low concentration in atmosphere through the stomata, known as transpiration
2. water lost by transpiration is replaced by evaporation from the water film that coats mesophyll cell walls
3. surface tension causes the liquid-to-air interphase to be pulled
4. water moves from mesophyll cytoplasm to cell wall to replace lost water
5. water from the xylem vessel elements in leaf minor veins is pulled to replace lost water
6. the tension/negative pressure from many minor veins is extended to secondary , then primary veins and then into the stem
7. tension results in bulk flow of water in the low resistance xylem, water passes through one trachea to another by moving through pits and perforation plates
8. the tension from xylem in the root is stronger than living root cells causing a flow of water into the root system
9. living root cells accumulate solutes such as sugars and actively take up ions from the soil, which allow them to take up water from surrounding soil by osmosis, then water is drawn from roots to go up the xylem

Question 6

what is the cohesion-tension theory?

Answer 6

tension is generated by the loss of water from evaporation in leaves, the water column can withstand the tension because of the cohesion between water molecules generated by hydrogen bonds

Question 7

what formula test the cohesion-tension theory?

Answer 7

Yw = Ys + Yp + Yg

Yw = water potential
Ys = solute
Yp= pressure
Yg= gravity

Question 8

what does the modern cohesion-tension theory state?

Answer 8

that a continuous water potential gradient, from high to low, is required for water transport to occur. Highest in soil, than lower in roots, than leaves, and lowest in atmosphere

Question 9

can water withstand the large negative tensions generated by the xylem?

Answer 9

yes

Question 10

can the trachea/vessels withstand the inward forces exerted on them?

Answer 10

yes, cells have thick cellulose and lignin cell walls

Question 11

what is the level of water potential in the air?

Answer 11

low, the solute potential is 0

Question 12

when does air have the lowest water potential

Answer 12

when it is dry or warm

Question 13

what proof do biologists have for the cohesion-tension theory?

Answer 13

when the petiole is cut, it causes the water fluid in the xylem to withdraw inward via a pulling or suction caused by transpiration

Question 14

what tools can test for cohesion-tension theory?

Answer 14

-xylem pressure probe instantly measures changes in pressure in the xylem
-increase of light also causes reduced weight of plant because of higher transpiration rates caused by water loss
-scholander’s pressure bomb