Unit 04 · Water & Nutrient Transport
Water moves through a plant without a pump, and understanding how is the heart of this unit. It begins with water potential — the gradient that decides which way water flows — then follows the transpiration stream up the xylem, held together by the cohesion-tension the leaves create as they lose water. You study how guard cells open and close stomata to balance CO₂ intake against water loss, how the phloem moves sugar by pressure-flow, and which mineral elements a plant must pull from the soil. Mastery means you can explain how a tall tree lifts water to its crown and why roots, not leaves, do the absorbing.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| Water potential | Thinks water simply moves “up” with no driving force. | Defines water potential but cannot predict flow direction from it. | Uses water potential to predict which way water moves between soil, root, stem, leaf, and air. |
| Transpiration & cohesion-tension | Believes the plant pushes water up from the roots. | Knows leaves lose water but not how that pulls the column upward. | Explains how transpiration at the leaves creates tension that pulls a cohesive water column up the xylem — roots absorb while leaves pull. |
| Stomatal control | Sees stomata as fixed holes that never change. | Knows guard cells open stomata but not what triggers it. | Explains how guard cells swell and shrink to open and close stomata, balancing CO₂ intake against water loss. |
| Phloem translocation & mineral nutrition | Confuses phloem transport with the xylem stream. | Knows phloem carries sugar but not the pressure-flow mechanism. | Explains pressure-flow from source to sink and names the essential mineral elements and their roles. |
| Lab technique (transpiration & the potometer) | Cannot seal the potometer or gets no measurable uptake. | Takes readings but ignores temperature, humidity, or air-flow effects. | Sets up a potometer, measures water uptake under changing conditions, and links the rate back to transpiration. |
| Integration (cross-domain) | Treats the science as isolated facts; makes no cross-domain connection. | Names a link to history, reading, or writing but cannot defend why it matters. | Connects the unit to its anchor across History · Reading · Writing (plus chosen electives) and defends why the connection matters. |
“The water column is pulled up, not pushed — as the leaves transpire, the tension reaches all the way down the xylem, and cohesion keeps the column from breaking. When I put a fan on the plant the potometer uptake sped up, because faster evaporation pulls harder. The roots are doing the absorbing; the leaves are doing the pulling.”
“The roots push the water up to the leaves. Stomata are just holes. The potometer number went up when I did the thing.”
You demonstrate this unit through a transpiration lab with the potometer and capillary-transport investigations, explaining the water potential and forces behind every reading aloud — not a multiple-choice test. A criterion counts as mastered only when your measurements match the mechanism and you can justify the plant biology behind it. Mastery is demonstrated, not awarded.
A 5-page clipboard packet — unit overview, key terms, the mastery rubric, anchor examples, and a score sheet you can print and grade against.