9700 · 7.2
Transport mechanisms
Water gets into a root by osmosis, crosses the root along cell walls or through the cells, is forced through a membrane at the endodermis, and is then pulled up the xylem by the water evaporating from the leaves. Sugar goes the other way: companion cells use energy to load sucrose into the phloem, water follows by osmosis, and the pressure that builds up pushes the sap to wherever sugar is needed.
Need to know
What you need to know
- **Apoplast pathway:** water moves through the continuous network of **cellulose** cell walls and the intercellular spaces, never crossing a membrane. Movement is by diffusion down the water potential gradient (and by mass flow when transpiration is fast), so it is quick; most water takes this route when transpiration rates are high. The walls of xylem vessels are lignified, and **lignin** is waterproof, so water that reaches the xylem stays in the lumen.
- **Symplast pathway:** water moves through the cytoplasm and vacuoles of cells and from cell to cell through the **plasmodesmata**. Every step crosses a partially permeable membrane, so the movement is osmosis, driven by the water potential gradient that the concentrated sap of inner cells maintains. It is slower than the apoplast route.
- **The endodermis and the Casparian strip:** each endodermal cell has a band of **suberin** in its radial and transverse walls — the Casparian strip — which is impermeable to water. The apoplast route is blocked here, so all water (and every dissolved ion) must cross a cell surface membrane to enter the symplast. The plant can therefore control which mineral ions reach the xylem, and the active pumping of ions into the xylem contributes to **root pressure**.
- Suberin deposits thicken as endodermal cells age, except in certain **passage cells**, through which water still passes freely. From the endodermis water moves into the xylem vessels through **pits** in their walls.
Explanation
How a plant moves water and sugar without a pump
- Water enters root hairs by osmosis, then crosses the cortex by the apoplast (cell walls) or the symplast (cytoplasm and plasmodesmata).
- The Casparian strip at the endodermis blocks the apoplast, so water must pass through a membrane before it reaches the xylem.
- Evaporation in the leaf creates tension; cohesion and adhesion carry that pull all the way down the xylem — the transpiration stream.
- Companion cells pump out H⁺ and cotransport sucrose in; water follows by osmosis, and sap moves by mass flow from source to sink.