Notes on Fungi, Water Balance, and Photosynthesis

Water Balance in Mycelium

• Fungi engage in resource acquisition and maintain water balance through interaction with the environment.
• The entire mycelium (network of fungal threads) acts as a respiratory exchange surface for gas and water.
• Water is absorbed directly by the mycelium from moist environments, supporting metabolic activities.

Mechanisms for Maintaining Water Balance

• Mycelium can shift sugar transport and uptake of monomers (building blocks of larger molecules) to maintain water balance.
• Regions of the mycelium that import sugar (monomers) increase solute concentration, causing water to flow in, thus maintaining hydration.
• Cell division creates shared cytosol between cells, allowing water redistribution throughout the organism.
• Stagnant branches in the mycelium can still absorb water to support growing tips.
• External digestion requires moisture for enzyme function and monomer absorption, enabling the breakdown of polymers.

Fungal Structure Compared to Vascular Plants

• Fungi have a high surface area for interaction with the environment, facilitating nutrient and water exchange.
• Vascular plants have roots, xylem (carries water), and phloem (transports sugars) which allows for larger structures and reduced water loss.

Photosynthesis and Water Loss in Leaves

• Leaf structure features mesophyll cells surrounded by air pockets for carbon dioxide absorption.
• Stomata openings allow gas exchange but also lead to water loss, creating a balance that must be maintained.
• Water vapor concentration affects the leaf's internal humidity, making it critical for photosynthesis.

Photorespiration vs. Photosynthesis

• Photorespiration occurs when RuBisCO enzyme binds oxygen instead of carbon dioxide, reducing sugar yields and increasing energy costs.
• Situational triggers for photorespiration include hot and dry conditions, leading to stomatal closure.
• Stomata need to be open for carbon dioxide but closed to minimize water loss, leading to complex trade-offs.

CAM (Crassulacean Acid Metabolism) Plants

• Plants in extremely hot and dry environments may utilize CAM to regulate gas exchange more effectively.
• CAM plants open stomata at night to absorb carbon dioxide, storing it as an acid and using it during the day for photosynthesis, minimizing water loss.
• Example: A historical anecdote related to an observation from a researcher regarding the pH change of leaves illustrates CAM functioning.