Lecture Outline for April 4, 2005


Topic: water balance in plants

Chapter 22 section 22.1; chapter 6 section 6.5

Figures used in lecture: 4.4, 4.5, 4.9, 6.13, 6.14

Additional Resources:

            Responses of Organisms to the Environment – Plant Responses to Moisture and Drought


Please review the topics of diffusion and osmosis from Chapter 4 section 4.1.  These ideas will be important for our study of water balance.


Water Balance in Plants

Define/explain: osmotic stress, osmoregulation, guard cells, stomata, rubisco, C4 metabolism, CAM metabolism, PEP carboxylase


You should be able to:

1.  Describe the advantages and challenges plants faced as they moved from aquatic environments to terrestrial environments.

2.  Describe the dilemma plants encounter as they perform photosynthesis in hot, dry climates.

3.  Describe the alternative metabolic processes (C4 metabolism, CAM) some plants use to reduce the risk of desiccation.


Today’s Lecture (4-4-05)

Osmotic stress:





Transition to Land

1.  Advantages of living on land 



2.  Challenges of living on land 

Transporting water through the plant body

Establishing an upright body to compete for sunlight

Threat of desiccation

Transporting gametes without water



How to avoid desiccation (drying out):

1.  cuticle


2. Close stomata




There are alternative metabolic pathways-


1.  C4 Metabolism

- allows the accumulation of CO2

- incorporates CO2 into a 4-carbon molecule


2.  CAM Metabolism

- occurs in cactus and other plants adapted for very hot conditions

- allows accumulation of CO2 at night while stomata are open

- carbon fixation occurs in the day using stored CO2