Plant Transport

IGCSE Edexcel Biology
2.51–2.58 Xylem, phloem, transpiration and mineral ion uptake
Key Concepts: Plants transport water and mineral ions upwards in xylem, and sucrose and amino acids in phloem (translocation). Transpiration is the loss of water vapour from leaves. Water enters roots via root hair cells by osmosis; mineral ions enter by active transport.

Section A — Xylem and Phloem

1. Complete the table comparing xylem and phloem. [6]
Feature Xylem Phloem
Substance(s) transported
Direction of movement
Cells alive or dead?
2. Describe the pathway of water from the soil to a leaf cell (the transpiration stream), naming each structure in order. [4]
3. State one structural feature of xylem vessels that makes them suitable for their function, and explain why. [2]

Section B — Transpiration

4. State three factors that increase the rate of transpiration and explain why each has this effect. [6]
5. Describe how a potometer is used to measure the rate of transpiration. [3]
6. Explain why transpiration rate is much lower at night than during the day. [2]

Section C — Root Hair Cells and Mineral Uptake

7. Describe two structural features of root hair cells that make them well adapted for absorbing water, and explain how each helps. [4]
8. Explain why water enters root hair cells by osmosis rather than active transport. [2]
9. Explain why mineral ions (e.g. nitrate) must be absorbed by active transport. [3]
10. A student places a plant in a sealed plastic bag in the light. After several hours, water droplets appear on the inside of the bag. Explain where this water came from. [3]

Total marks: 35

Mark Scheme

1. Xylem: water and mineral ions / upward from roots to leaves / dead (hollow lignified cells); Phloem: sucrose and amino acids / up and down (both directions, translocation) / alive [6 — 2 per row]
2. Water enters root hair cells by osmosis [1]; moves through root cortex cells by osmosis [1]; enters xylem vessels [1]; travels up stem in xylem to leaves [1] [4]
3. Any one: hollow tubes with no end walls for uninterrupted flow [1+1]; lignified walls for strength/support [1+1]; no living cell contents = clear channel for water [1+1] [2]
4. Any three, 2 marks each: Light (stomata open, more water lost by evaporation) [2]; Temperature (increases kinetic energy of water molecules, evaporation faster) [2]; Wind speed (removes water vapour at leaf surface, maintains steep concentration gradient) [2]; Low humidity (steeper concentration gradient between leaf and air, faster diffusion) [2] [6]
5. A shoot is placed in the potometer under water (to prevent air bubbles) [1]; an air bubble is introduced and moves along a capillary tube [1]; the distance the bubble moves per unit time gives the rate of transpiration [1] [3]
6. At night, stomata close (no photosynthesis requiring CO₂ uptake) [1]; with stomata closed, water vapour cannot diffuse out of the leaf, so transpiration effectively stops [1] [2]
7. Any two: long hair-like extension [1] — greatly increases surface area for absorption [1]; thin cell wall [1] — short diffusion distance for rapid absorption [1] [4]
8. Water moves down its water potential gradient [1]; the soil solution has a higher water potential than the root hair cell cytoplasm, so water moves in by osmosis without energy needed [1] [2]
9. Mineral ions are in low concentration in the soil [1]; the concentration inside root hair cells is higher than in the soil [1]; so active transport uses energy (ATP) from respiration to move ions against the concentration gradient into the cell [1] [3]
10. Water is absorbed by the roots and travels up the xylem [1]; it evaporates from leaf cells and diffuses out through stomata as water vapour (transpiration) [1]; this water vapour condenses on the inside of the cool bag [1] [3]