Notes on Transpiration and Photosynthesis in Plants

Green Plants

• All animals, including humans, depend on plants for food and shelter.

• Plants synthesize their own food through the process of photosynthesis:

  • Uses sunlight energy to convert carbon dioxide and water into glucose.

  • Also releases oxygen into the atmosphere.

Chapter 2 - Transpiration

Learning Objectives

• Understand the concept of transpiration and its role in plants.

• Examine the presence and function of stomata in leaves.

• Explain the mechanism of stomatal opening and closing.

• Identify factors that affect transpiration rates.

• Investigate transpiration rates using Ganong's potometer.

Activity: Do Plants Sweat?

Materials Needed

• Well-watered potted plant

• String

• Scissors

• Plastic bag

Procedure

1. Cover the plant with a transparent plastic bag, secured at the stem's base.

2. Leave the plant in sunlight for two hours.

Questions to Consider

1. What were your observations after two hours?

2. If you place dry cobalt chloride paper inside the bag, what does a color change indicate?

3. How might the experiment differ if the plant had more leaves?

4. Design a control experiment.

5. How would adding another plastic bag alter the experiment?

What is Transpiration?

• Transpiration: The loss of excess water from plants in the form of water vapor.

• Occurs mainly through stomata, with some loss through the cuticle and lenticels:

  • Stomatal transpiration: 80-90% of water loss; mainly occurs during sunlight when stomata are open.

  • Cuticular transpiration: Direct diffusion of water vapor through cuticle; accounts for 5-10% of total water loss.

  • Lenticular transpiration: Water loss through lenticels on woody stems; remains open constantly.

Mechanism of Stomatal Transpiration

• Water absorbed by roots rises through the stem and enters the leaves through veins.

• In the leaves, spongy mesophyll cells lose water, which evaporates and saturates air in intercellular spaces.

• Water vapor then diffuses into sub-stomatal cavity and out through the stomata.

• Movement relies on diffusion from areas of high concentration to low concentration.

Anatomy of a Leaf

• Leaf structure includes:

  • Cuticle: Protective wax layer.

  • Epidermis: Upper and lower protective layers.

  • Mesophyll: Contains chloroplasts for photosynthesis.

  • Stomata: Tiny openings facilitating gas exchange.

Special Structures in Transpiration

• Stomata (singular: stoma):

  • Tiny pores on leaf's surface composed of guard cells.

  • Main functions:

  • Allow CO₂ intake for photosynthesis.

  • Expel oxygen and water vapor.

  • Typically, stomata are more abundant on the lower side of leaves.