9.1 Notes

9.1.1 Transpiration

• Definition: Transpiration is the inevitable loss of water vapor from a plant, primarily through the stomata in the leaves. It's a consequence of gas exchange necessary for photosynthesis.

• Process: Water evaporates from the moist cell walls in the leaf. This creates a tension or pulling force.

• Stomata: Guard cells control the size of the stomatal opening, regulating water loss. They can adjust from fully open to closed.

• Inevitable: If plants absorb carbon dioxide, they will inevitably lose water.

• Factors affecting rate:

  • Temperature and humidity affect transpiration rate.

9.1.2 Evaporation

• Definition: Evaporation is the change of water from a liquid to a gas.

• Cooling effect: Evaporation requires energy to break hydrogen bonds, thus having a cooling effect.

• Water as a coolant: Water's high latent heat of vaporization makes it a good evaporative coolant, such as in sweating.

9.1.3 Transpiration Stream

• Definition: The transpiration stream is the continuous flow of water from the roots to the leaves via the xylem.

• Cohesion: Water molecules stick together due to hydrogen bonds (cohesion), allowing them to be pulled up the xylem.

• Adhesion: Water molecules are attracted to the hydrophilic walls of the xylem vessels (adhesion).

• Tension: Evaporation from leaves generates tension, which pulls water up the xylem.

• Transpiration Pull: The pulling force is strong enough to move water to the top of tall trees.

• Passive process: The process is passive, using thermal energy for water movement.

• Cavitation: Occasionally, the water column in xylem vessels can break (cavitation) due to pressure, but this is rare.

9.1.4 Root Uptake

• Osmosis: Water is absorbed by roots via osmosis, due to concentration gradients established by active transport of mineral ions.

• Active Transport: Protein pumps in the root cell membranes actively transport mineral ions.

• Mycorrhizae: Fungal hyphae can aid mineral ion absorption by increasing the surface area for absorption and delivering ions to roots. This is a mutualistic relationship because the plant also provides the fungi with sugars and nutrients.

9.1.5 Xylem Structure

• Xylem vessels: Xylem vessels are specialized tubes that transport water and minerals.

• Lignified walls: The walls of xylem vessels are strengthened with lignin.

• Continuous tubes: Xylem vessels form continuous, hollow tubes from roots to leaves.

• Adaptations for Tension: The structure of xylem withstands low pressure and tension.

9.1.6 Water Conservation

• Xerophytes: Plants adapted to dry environments (xerophytes) have strategies to reduce water loss.

  • Reduced leaves: They have small leaves or spines.

  • Thick cuticle: A thick cuticle on leaves reduces evaporation.

  • Stomata adaptations: Stomata may open at night when it is cooler.

  • CAM metabolism: Some use crassulacean acid metabolism (CAM) to fix CO2 at night, reducing water loss.

  • Water storage: Some plants have water storage structures in their leaves.

• Halophytes: Plants adapted to saline soils have adaptations such as structures for removing salt.

9.1.7 Plant Experiments

• Potometer: A potometer is used to measure water uptake in plants. It consists of a plant shoot, water reservoir, and a graduated capillary tube with an air bubble. The movement of the bubble is timed to measure transpiration rate.

• Investigating Transpiration: Experiments can be designed to test the effects of factors like temperature or humidity on transpiration rates.

• Osmosis experiments: Rigorous experimental design is needed to produce reliable results. Plant tissues can be bathed in hypotonic and hypertonic solutions. Measurements of mass change can be used to deduce the concentration of the solution that would be isotonic.

• Variables: In plant experiments, independent variables (such as light intensity or CO2 concentration) must be carefully controlled and varied with appropriate levels. Other variables should be kept constant to isolate the effect of the independent variable on the dependent variable.

• Plant Cuttings: Experiments can be designed to investigate factors affecting the rooting of stem cuttings.

• Germination: Experiments can be designed to test factors affecting germination. The type of seed, the amount of water, and presence or absence of other factors should be carefully controlled.