# Transport in Plants This section outlines the mechanisms by which plants absorb water and minerals from the soil, transport them to the leaves, and distribute the organic solutes produced during photosynthesis throughout the plant body. ## Movement of Water: Water Potential The movement of water in plants is governed by water potential ($\psi_w$), which is influenced by two main components: solute potential ($\psi_s$) and pressure potential ($\psi_p$). **Equation:** $$\psi_w = \psi_s + \psi_p$$ - **Water Potential ($\psi_w$):** - The measure of the free energy of water in a system. - Water always moves from an area of *higher* $\psi_w$ to an area of *lower* $\psi_w$. - The water potential of pure water is 0 MPa. All solutions have a negative water potential. - **Solute Potential ($\psi_s$):** - Also called *osmotic potential*. - It represents the effect of dissolved solutes on water potential. - The addition of solutes *reduces* the free energy of water, making the solute potential negative. - The more solutes present, the more negative the $\psi_s$. - **Pressure Potential ($\psi_p$):** - The physical pressure exerted on water. - In plant cells, this is the turgor pressure exerted by the cell contents against the cell wall. - It is usually positive and increases the water potential. For water to move from the soil to the atmosphere through the plant, the following gradient must exist: $\psi_{w\ \text{soil}} > \psi_{w\ \text{root}} > \psi_{w\ \text{stem}} > \psi_{w\ \text{leaf}} > \psi_{w\ \text{atmosphere}}$ ### Uptake of Water by Roots Water enters root hairs via osmosis and travels to the xylem through three distinct pathways. 1. **Apoplast Pathway:** - Water moves through the continuous system of adjacent cell walls and intercellular spaces. - This pathway is blocked at the endodermis by the **Casparian strip** (a waxy band of suberin and lignin), forcing water to enter the cytoplasm (symplast) to be regulated before entering the xylem. 2. **Symplast Pathway:** - Water moves through the interconnected cytoplasm (protoplasts) of adjacent cells. - Cells are connected by channels called **plasmodesmata**. - This pathway allows for selective transport across cell membranes. - For more on cell connections, see . 3. **Vacuolar Pathway:** - Water moves from vacuole to vacuole of neighboring cells, crossing the cell membrane and the tonoplast (vacuolar membrane) by osmosis. ### Structure and Function of Vascular Tissues | Tissue | Function | Components | | :----- | :------------------------------------------------------------------------- | :------------------------------------------------------------------------------------------------------------ | | **Xylem** | Conducts water and dissolved minerals from roots to the rest of the plant; provides mechanical support. | **Tracheids** (dead), **Vessel elements** (dead), **Xylem fibres** (dead), **Xylem parenchyma** (living). | | **Phloem** | Translocates organic solutes (food, e.g., sucrose) from source to sink. | **Sieve tube elements** (living), **Companion cells** (living), **Phloem parenchyma** (living), **Phloem fibres** (dead). | ### Ascent of Sap The upward movement of water and minerals from the roots to the leaves through the xylem is called the ascent of sap. The most accepted explanation is the **TACT Theory**. - **T - Transpiration pull:** Evaporation of water from leaf surfaces creates a negative pressure potential (tension) that pulls the water column upwards. - **A - Adhesion:** The attraction between water molecules and the polar cellulose walls of the xylem vessels helps counteract gravity. - **C - Cohesion:** The attraction between water molecules (due to hydrogen bonds) creates a continuous, unbroken column of water from roots to leaves. - **T - Tension:** The force created by cohesion that holds the water molecules together in a continuous string. ### Opening and Closing of Stomata Stomata (pores on the leaf surface) are flanked by two **guard cells**. Their opening and closing are regulated by changes in the turgidity of these cells. 1. **Starch-Sugar Interconversion Theory:** - *Daytime:* Photosynthesis produces sugar, lowering the water potential in guard cells. Water enters via osmosis, cells become turgid, and the stoma opens. - *Nighttime:* Sugar is converted to starch, raising the water potential. Water leaves, cells become flaccid, and the stoma closes. 2. **K⁺ ions Influx/Efflux Theory (Modern View):** - *Daytime (Light):* Light stimulates the active transport of **K⁺ ions into** guard cells. This lowers the water potential, causing water to enter via osmosis. The cells swell and the stoma opens. - *Nighttime (Dark):* **K⁺ ions are actively pumped out** of the guard cells. Water potential increases, water leaves, and the cells become flaccid, closing the stoma. ### Translocation of Organic Solutes This process is explained by the **Pressure Flow (Mass Flow) Theory**. - **Source:** An area of food production (e.g., leaves) or supply (e.g., storage root). - **Sink:** An area of food utilization (e.g., growing tips) or storage (e.g., fruit). **Mechanism:** 1. **Loading at the Source:** Companion cells actively load sucrose into the sieve tube elements of the phloem. This is a form of active transport. See . 2. **Pressure Buildup:** The high solute concentration in the phloem lowers its water potential. Water moves from the adjacent xylem into the phloem by osmosis, creating high hydrostatic pressure. 3. **Mass Flow:** This high pressure pushes the phloem sap (sugary solution) along the sieve tube towards a sink. 4. **Unloading at the Sink:** At the sink, sucrose is actively unloaded. The solute concentration in the phloem decreases, causing water to move back into the xylem.