Multicellularity and Transport in Plants

Constraints: Multicellular organisms face a small surface area to volume ratio and large distances between internal cells and the environment. They cannot rely on direct diffusion and require specialized transport systems for nutrients, gases, and waste.
Evolutionary Timeline: Land plants emerged approximately $450-500\text{ mya}$ , with vascular plants appearing around $400\text{ mya}$ .
Vascular Benefits: The evolution of tracheids and woody tissue provided transport and rigid structural support, allowing for massive growth.
Examples of Scale:
- Hyperion: A California redwood and the tallest living organism ( $115.9\,m$ ).
- General Sherman: A giant sequoia and the largest living organism by volume.

Water and Minerals: Acquired from soil via roots. Water is essential for photosynthesis, solute transport, cooling through transpiration, and structural support via turgor pressure.
Mineral Nutrients: Macronutrients ( $Nitrogen$ , $Phosphorus$ ) and micronutrients ( $Iron$ ) are required for growth and organic compound synthesis.
Sugars: Photosynthetic tissue (source) produces sucrose, which is transported in the phloem to non-photosynthetic tissue (sink), such as roots.

Water Potential ( $WP$ ): Water always moves across selectively permeable membranes toward regions of lower (more negative) water potential.
Formula: $WP = WPS + WPP$
- $WPS$ (Solute potential): Increasing solute concentration lowers $WP$ .
- $WPP$ (Pressure potential): Increasing internal pressure (turgor pressure) raises $WP$ .
Cell State: Low turgor results in a flaccid cell (wilting), while high turgor makes a cell turgid.

Apoplast: Rapid, unregulated movement through interconnected cell walls and intercellular spaces.
Symplast: Slow, regulated movement through the cytoplasm via plasmodesmata.
Casparian Strip: Located at the endodermis, this diffusion barrier forces apoplastic water into the symplast, enabling selective solute uptake to protect the plant.

Structure: Composed of dead vessel elements and tracheids with secondary walls reinforced by lignin.
Mechanism:
1. Transpiration: Water vapor evaporates from stomata, creating negative surface tension in mesophyll cells.
2. Tension: This tension pulls water from leaf veins into the apoplast.
3. Cohesion and Adhesion: Water molecules stick together via hydrogen bonds (cohesion) and stick to xylem walls (adhesion), forming a continuous column.
Bulk Flow: Driven by a pressure potential gradient: Soil ( $-0.3\,MPa$ ) > Root ( $-0.6\,MPa$ ) > Stem ( $-0.8\,MPa$ ) > Leaf ( $-1.0\,MPa$ ) > Atmosphere ( $-10.0$ to $-100.0\,MPa$ ).

Structure: Sieve tube elements (living cells lacking nuclei/ribosomes) and companion cells (life-support via plasmodesmata).
Mechanism:
1. Loading: Sucrose is actively transported (requires $ATP$ ) into sieve tubes.
2. Osmosis: The negative $WP$ in the phloem draws water from the xylem.
3. Hydrostatic Pressure: Increased $WPP$ forces the sap to flow from source to sink.
4. Unloading: Sucrose enters sink cells; water follows back out via osmosis, maintaining the pressure gradient.

Tallest Trees: Discussed the height limits of the tallest trees ( $\approx 115\,m$ ) regarding the physical limits of water transport against gravity ( $Gravity = -0.01\,MPa$ per meter) versus the competitive advantage of height for light access.
Practice Exam Question: Identification of vascular tissue cells (A-E) and their roles in mass flow, specifically distinguishing between xylem vessels, sieve tube elements, and companion cells.