Gas Exchange and Water Movement in Plants Study Notes

Anatomy of the Leaf for Gas Exchange

• Internal Leaf Structure: The leaf is organized into distinct layers that facilitate the movement of gases and fluids:

  • Upper Epidermis: The outermost protective layer on the top of the leaf.

  • Palisade Layer: Located just below the upper epidermis, consisting of elongated cells where significant photosynthesis occurs.

  • Spongy Layer: Positioned beneath the palisade layer, characterized by loosely packed cells and air spaces to facilitate gas circulation.

  • Lower Epidermis: The protective layer on the bottom of the leaf, typically containing the majority of the stomata.

  • Guard Cells: Specialized cells found in the epidermis that surround and control the opening and closing of the stomata.

  • Stoma (plural: Stomata): Microscopic pores or openings in the leaf epidermis that allow for gas exchange.

The Mechanism of Gas Exchange and Respiration

• Diffusion Process: Air enters the leaf by diffusing through the stomata.

• Internal Circulation: Once inside, air circulates through the spaces between the spongy and palisade tissue cells.

• Carbon Dioxide ($CO_2$) Uptake: $CO_2$ moves down its concentration gradient. It first dissolves into the layer of water surrounding the internal cells before diffusing into the cells themselves for use in photosynthesis.

• Oxygen ($O_2$) Release: $O_2$ is produced as a byproduct of photosynthesis within the cells. It passes out of the cells into the internal air spaces and eventually exits the leaf through the stomata.

• Water Movement: Water enters the leaf via the xylem. It exits the leaf through the stomata in the form of water vapour.

Gas Exchange in Woody Plant Structures

• Barriers to Exchange: In woody plants and trees, layers of dead cork cells and waxy substances serve as protective barriers. While beneficial for protection, these layers prevent the direct diffusion of gases.

• Lenticels: These are specialized, lens-shaped openings that cut through the bark of woody stems and roots. Lenticels allow for the diffusion of air, enabling gas exchange in parts of the plant covered by bark.

Transpiration and Water Dynamics

• Definition of Transpiration: The process of water evaporation from the leaves of a plant.

• Water Loss Volume: Transpiration is a highly active process that can cause a plant to lose up to $99\%$ of the water it originally absorbed through its roots.

Regulation by Guard Cells and Turgidity

• Shape Modification: Guard cells regulate gas exchange and transpiration by changing their physical shape to open or close the stomatal pore.

• Impact on Photosynthesis:

  • When stomata are open, the rate of photosynthesis increases because $CO_2$ is permitted to enter the leaf and $O_2$ is allowed to exit.

  • The size of the stomatal opening directly controls the volume of gas exchange and the rate of transpiration.

• Turgor Pressure Mechanism:

  • The opening and closing of stomata are dictated by the amount of water present in the guard cells.

  • Opening: When water moves into the guard cells, it creates high internal water pressure known as Turgor Pressure. This pressure causes the guard cells to swell and curve, opening the stomatal pore.

  • Closing: As water levels decrease within the guard cells (often due to transpiration), the turgor pressure drops, causing the cells to become limp and the stomata to close.

• Microscopic Scale: Stomatal pores and guard cells operate at a minute scale, with specific structures like chloroplasts visible within the guard cells. A typical scale for these structures is approximately $20\,\mu m$.

  • Reference Citation: Image data sourced from "Plant Physiology," Eds: L. Taiz and E. Zeiger, 2nd edition, Sinauer Associates, Inc. Publisher, Sunderland MA, USA, p. 523.

Turgor Pressure and Cellular States

• Biological Function: Turgor pressure serves as a "skeleton" for the plant, providing structural support.

• Cellular Conditions:

  • Turgid: Cells with high turgor pressure that maintain a firm and rigid shape. This is the normal state for healthy plant cells.

  • Flaccid: Cells with less than high turgor pressure, appearing limp.

  • Plasmolysis: A critical state where cells have minimal turgor pressure. The cell membrane shrinks away from the cell wall, which often leads to cell death.

Osmotic Environments: Plant vs. Animal Cells

• Hypotonic Solution: A solution with a lower solute concentration than the cell.

  • Plant Cell: Becomes Turgid (normal/firm) as $H_2O$ enters.

  • Animal Cell: May become Lysed (burst) as excessive $H_2O$ enters.

• Isotonic Solution: A solution with a solute concentration equal to that of the cell.

  • Plant Cell: Becomes Flaccid (limp) as water movement ($H_2O$) is balanced with no net gain or loss.

  • Animal Cell: Remains Normal.

• Hypertonic Solution: A solution with a higher solute concentration than the cell.

  • Plant Cell: Becomes Plasmolyzed as $H_2O$ leaves the cell, causing the interior to shrink.

  • Animal Cell: Becomes Shriveled as $H_2O$ leaves the cell.