Biology 9700/13 — May/June 2011

Background Concept

Three distinct physical processes drive water movement through a plant, and each one is associated with a different part of the plant:

1. Osmosis — the net movement of water molecules across a partially permeable membrane from a region of higher (less negative) water potential to a region of lower (more negative) water potential. No energy from the plant is required. In the root, soil water has a higher water potential than the cytoplasm of root hair cells (because the cells contain dissolved solutes), so water enters the root by osmosis.

2. Cohesion and adhesion (the cohesion–tension theory) — once inside the xylem, water is pulled upwards as an unbroken column. Cohesion is the attraction between like water molecules (hydrogen bonds) and adhesion is the attraction between water molecules and the walls of the xylem vessels. The upward pull is generated by the loss of water from the leaves (transpiration), which creates a negative pressure (tension) in the xylem.

3. Transpiration — the loss of water vapour from the aerial parts of the plant, mainly through the stomata in the leaves. Water evaporates from the moist cell walls of the spongy mesophyll into the air spaces, then diffuses out via the stomata. This is the driving force for the whole transpiration stream because it generates the tension that pulls the water column up the xylem.

Understanding the Question

The diagram shows a maize plant with three labelled arrows indicating the stages of water movement:

• 1: water enters the root
• 2: water passes up the stem
• 3: water vapour is lost from leaves

The question asks us to match the three processes (cohesion and adhesion, transpiration, osmosis) to these three stages. The correct option is the one that places each process in the correct anatomical position.

Approach

Think about where each process happens. Cohesion–adhesion operates within the xylem in the stem; transpiration happens in the leaves (loss of vapour); osmosis happens at the root surface (water entering root hair cells). Then match these locations to the three numbered labels and read off the row that gives the correct numbering for each process.

Step-by-Step Reasoning

• Stage 1 — root: Water here crosses the plasma membrane of root hair cells down a water potential gradient. The only membrane-crossing movement of water is osmosis, so process 1 = osmosis.
• Stage 2 — stem: Water here moves up through the xylem vessels. The mechanism that sustains an unbroken column of water under tension is cohesion between water molecules and adhesion to the xylem walls, so process 2 = cohesion and adhesion.
• Stage 3 — leaves: Water here is lost as vapour through the stomata. This is the definition of transpiration, so process 3 = transpiration.

Reading the answer columns: cohesion and adhesion = 2, transpiration = 3, osmosis = 1 → row D.

Key Takeaways

• Osmosis occurs at the root–soil interface (water entry).
• Cohesion–adhesion is what keeps the water column continuous inside the xylem as it is pulled up the stem.
• Transpiration is the loss of water vapour from the leaves, and is the ultimate driving force for the whole stream.
• These three processes are sequential and causally linked: transpiration in the leaves pulls the column up via cohesion–adhesion, while osmosis at the root replaces the water that has been lost.

Common Mistakes

• Confusing transpiration (loss of water vapour from leaves) with the upward movement of water in the stem. Transpiration does not itself move water up the stem; the tension it creates, transmitted via cohesion–adhesion, does.
• Thinking that "water passes up the stem" is transpiration. It is not — the actual movement in the xylem is driven by cohesion–tension.
• Putting osmosis at the leaves or the stem. Osmosis requires a partially permeable membrane, which is only present at the root surface (or between cells along the symplast pathway).

Things to Be Careful About

• Read the question's column headers carefully — they list the three processes in a different order from the three stages. Make sure you are matching the correct process to the correct number, not the other way round.
• Cohesion–adhesion operates inside the xylem; it is not a feature of the root or the leaf surface.
• The question is about processes, not structures. Do not pick an option just because the word sounds right — match the mechanism to the location.