Question 1

How does water move up a tree against gravity?

Accepted Answer

Water moves up a tree due to transpiration pull, not against gravity. Water evaporates from leaf surfaces, creating a negative pressure (tension) that pulls water up the xylem. Cohesion between water molecules keeps the column intact, and adhesion to xylem walls helps prevent collapse. This process is passive and does not require energy from the plant.

Question 2

What is the difference between xylem and phloem?

Accepted Answer

Xylem transports water and minerals from roots to shoots, and is composed of dead, hollow cells with lignified walls. Phloem transports sugars and other organic compounds from sources to sinks, and consists of living sieve tube elements and companion cells. Xylem flow is driven by transpiration pull (passive), while phloem flow is driven by active loading and pressure differences (mass flow).

Question 3

Why do plants wilt on hot days?

Accepted Answer

On hot days, transpiration rate increases due to higher temperatures and lower humidity, causing water loss faster than absorption. This leads to a water deficit in leaves, reducing turgor pressure in cells, and the plant wilts. Wilting reduces leaf surface area, decreasing transpiration and helping conserve water.

Question 4

How is sucrose loaded into the phloem?

Accepted Answer

Sucrose is loaded into phloem sieve tubes via active transport. In source tissues (e.g., leaves), sucrose is produced during photosynthesis and moved into companion cells, then into sieve tubes through plasmodesmata. This process requires ATP and involves co-transport with protons (H+). The accumulation of sucrose lowers water potential, causing water to enter by osmosis, increasing hydrostatic pressure.

Question 5

What is the cohesion-tension theory?

Accepted Answer

The cohesion-tension theory explains water transport in xylem. Water molecules cohere (stick together) via hydrogen bonds, forming a continuous column. Transpiration from leaves creates tension (negative pressure) that pulls the column upward. Adhesion of water to xylem walls also helps. The theory is supported by evidence such as the continuity of water columns and the ability to measure negative pressures in xylem.

Question 6

Can phloem transport occur in both directions?

Accepted Answer

Phloem transport is unidirectional in a given sieve tube at any one time, flowing from source to sink. However, the direction can change if the source-sink relationship changes (e.g., a storage organ becomes a source in spring). Different sieve tubes can transport in opposite directions simultaneously, but within a single tube, flow is one-way due to pressure gradients.

Transport in plants

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Examiner Tips for Transport in plants

Common Mistakes in Transport in plants

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