Physiology Oct. 9th

Gases move down their partial pressure gradient (C1 to C2), a fundamental principle governed by diffusion.
The partial pressure gradient is essential for gas exchange between the atmosphere and plant tissues, as well as within tissues.
Gas movement rate is quantitatively described by Fick's Law of Diffusion: Rate = -D \frac{A \Delta P}{\Delta x}
- Where D is the diffusion coefficient (a constant for a given gas and medium),
- A is the area available for diffusion,
- \Delta P is the partial pressure gradient,
- and \Delta x is the diffusion distance.
Various factors affect gas exchange:
- Area: Larger surface areas facilitate greater exchange.
- Constants: Specific diffusion coefficients for gases like CO2 and H_2O in air and water.
- Molecular weights of gases involved: Lighter gases generally diffuse faster.

This guide discusses a generalized cross-section of a C3 leaf.
C3 metabolism is the default metabolism in plants, where CO2 is initially fixed into a three-carbon compound.
Other types: C4 and CAM plants have evolved mechanisms to concentrate CO2, improving water use efficiency:
- C4 plants spatially separate initial CO2 fixation (in mesophyll cells) from the Calvin Cycle (in bundle sheath cells).
- CAM plants temporally separate CO2 uptake (at night) from fixation (during the day), allowing stomata to be closed during hot daytime conditions.

Stomata are pores, primarily on the lower leaf surface, for gas and water exchange between plants and the atmosphere.
The upper leaf surface (illuminated side) generally lacks pores due to direct sun exposure, which would lead to excessive water loss.
- Mainly does not contain photolytic cell types; thus, minimal photosynthesis occurs here.
Mesophyll cells:
- Two main types in C3 leaves:
- Palisade mesophyll cells: Densely packed and columnar, located just beneath the upper epidermis, optimized for light absorption and primary site of photosynthesis.
- Spongy mesophyll cells: More loosely arranged with ample air spaces, facilitating efficient gas diffusion throughout the leaf.

Air spaces in the spongy mesophyll are nearly saturated with water vapor (relative humidity around 97% to 99%).
Water vapor moves from high humidity inside the leaf (high partial pressure) to lower humidity in the atmosphere (lower partial pressure), driving transpiration.

Atmospheric CO2 concentration is low (approximately 0.04%, or 400 ppm).
To facilitate carbon fixation, plants maintain even lower CO2 levels in air spaces:
- CO2 is continuously consumed during the dark reactions (Calvin Cycle) of photosynthesis, maintaining a steep concentration gradient.
- The low internal CO2 levels create a partial pressure gradient for diffusion from the atmosphere into the leaf.
Air spaces near stomatal pores are continuous with the atmosphere, enhancing gas diffusion pathways to the photosynthetically active cells.

Plants continuously transpire and photosynthesize, driving water movement from roots to leaves and out into the atmosphere.
Resistance factors for water evaporation and diffusion:
- Size and density of stomatal pores: Smaller and fewer pores reduce transpiration by increasing diffusion resistance.
- Boundary layer: A layer of stagnant air immediately surrounding the leaf surface that acts as a barrier to gas exchange. It thickens under still conditions, reducing the rate of water vapor diffusion out of the stomata and thus decreasing transpiration efficiency.
- Cuticular resistance: The waxy cuticle on the leaf surface significantly reduces water loss, although some minor diffusion can still occur.

In C3 plants, the ratio is approximately:
- 400 molecules of water transpired for every 1 molecule of CO2 taken up. This high ratio reflects the trade-off between CO2 acquisition and water conservation.
C4 plants have higher water use efficiency; they transpire fewer water molecules to collect the same amount of CO2 due to their CO2 concentrating mechanism.

Under a microscope, stomata consist of two specialized guard cells that surround the stomatal pore and regulate its opening and closing.
Guard cells expand by absorbing water due to changes in turgor pressure, causing them to bow outwards and open the stomata for gas exchange.
When in a flaccid state (low water potential/turgor), the guard cells relax, causing the stomata to close.

Light primarily impacts guard cells, specifically blue light which triggers phototropins (blue light receptors) located in the guard cell plasma membrane.
Upon activation by blue light:
- Plasma membrane H+-ATPase proton pumps are activated, consuming ATP to actively pump protons (H+) outside the cell, into the apoplast. This action creates an electrochemical gradient and leads to hyperpolarization of the guard cell membrane.
- This hyperpolarization triggers the opening of voltage-gated potassium (K+) channels, allowing potassium ions to flow rapidly into guard cells, moving down their electrochemical gradient.
- Concurrently, anion channels (e.g., chloride channels) are inhibited from efflux, or specific anion uptake mechanisms are activated, also increasing chloride (Cl−) levels inside the cell.
- Together, this massive influx of K+ (and Cl−) significantly lowers the water potential within the guard cells. Water then osmotically moves into the cells from surrounding epidermal cells, causing vacuoles to expand, increasing turgor pressure, which leads to stomatal opening.

Stomata do not simply open and close at set times; their behavior is dynamic and precisely regulated:
- Influenced by light levels (blue light for opening, general light intensity affects photosynthesis).
- CO2 availability: Low internal CO2 levels promote stomatal opening, while high CO2 concentrations trigger stomatal closure to conserve water.
- Water stress: Conditions of drought or water scarcity induce stomatal closure to retain moisture, often mediated by the plant hormone abscisic acid (ABA), which signals water deficit and promotes ion efflux from guard cells.

Some aquatic plants develop Aerenchyma structures (tubular formations of air-filled tissues) that facilitate gas exchange between submerged tissues and above-water tissues.
- Aerenchyma provide mechanical support, buoyancy, and reduce resistance to gas diffusion in fluid environments where oxygen availability is often limited.
- Gas exchange within aerenchyma is primarily through diffusion, although in some cases, pressure-driven convection currents (ventilated systems) may assist, especially in larger plants.

Under waterlogging conditions, roots are deprived of oxygen. Plants may form larger hollow tubes within their stems and roots via programmed cell death (lysis of cells) to link submerged tissues to the atmosphere.
- This adaptation maintains gas exchange by diffusion, ensuring oxygen supply to roots and allowing CO2 removal, even when roots are saturated with water and soil oxygen is scarce.