State that transpiration is the loss of water vapour from the plant through the stomata.

Topic 8.1 – Transport in Plants

Learning Objective

State that transpiration is the loss of water vapour from the plant through the stomata.

1. Vascular Tissue – Functions & Positions (Core)

• Xylem – transports water and dissolved mineral ions from the roots to the rest of the plant; also provides mechanical support.
• Phloem – transports organic nutrients (mainly sucrose, also amino acids) from sources (e.g., mature leaves) to sinks (e.g., roots, growing buds).

2. Water Uptake by Roots (Core)

Key structures

• Root hairs – epidermal extensions that increase surface area for water absorption.
• Endodermis (Casparian strip) – forces water to cross the plasma membrane, ensuring selective uptake.

Pathway of water (continuous column)

1. Root hair →
2. Cortex (apoplast → symplast) →
3. Endodermis (Casparian strip) →
4. Pericycle →
5. Xylem vessels →
6. Stem →
7. Leaf.

Investigation Idea – Dye Uptake

Place the cut end of a healthy young root in 0.5 % food‑colouring solution. After 30 min observe the coloured water in the stem and leaf. Record the distance travelled to illustrate the continuity of the water column.

3. Transpiration

3.1 Definition (Core)

Transpiration = the loss of water vapour from the plant to the atmosphere, mainly through the stomata on the underside of leaves.

3.2 Cohesion‑Tension Mechanism

• Water evaporates from moist cell walls of the mesophyll into internal air spaces and out through open stomata.
• Evaporation creates a negative pressure (tension) in the leaf air spaces.
• Because water molecules are strongly attracted to each other (cohesion) and to the walls of the xylem (adhesion), the tension is transmitted down the continuous water column, pulling water upward from the roots.
• The pull continues as long as the water potential in the soil is higher (less negative) than that in the leaf.

3.3 Importance of Transpiration (Core)

1. Generates the transpiration pull that drives upward movement of water and minerals.
2. Provides evaporative cooling, helping to maintain leaf temperature.
3. Maintains turgor pressure in cells, keeping leaves and young shoots rigid.
4. Facilitates the transport of nutrients to growing parts of the plant.

4. Factors Affecting the Rate of Transpiration (Core)

Quantitative Example (Optional)

Typical transpiration rates for a mature broad‑leaf plant: 0.5–2 g H₂O h⁻¹ per leaf under moderate conditions (25 °C, 50 % RH, 500 lux).

5. Practical Investigation – Measuring Transpiration Rate (Core)

1. Materials: potometer (or digital balance), potted plant, beaker of water, timer, ruler, aluminium foil (to shade leaves).
2. Method (Potometer):
   • Cut a shoot early in the morning, immediately place the cut end in water to prevent air entry.
   • Attach the shoot to the potometer, ensuring an airtight seal.
   • Fill the capillary tube with water, record the initial water level.
   • Expose the plant to the chosen condition (e.g., full light, high temperature, wind).
   • After a fixed time (e.g., 30 min) record the new water level.
   • Calculate ΔV (cm³) and express the rate as cm h⁻¹.
3. Alternative (Balance Method): Weigh the potted plant before and after a set period; the loss in mass equals the amount of water transpired.
4. Data to Record: condition, temperature, humidity, light intensity, ΔV (or mass loss), time.
5. Analysis: Compare rates under different conditions and relate findings to the factor table above.

6. Linking the Whole Water‑Movement System (Core)

• Root hairs absorb water → water moves through cortex → enters xylem via endodermis.
• Transpiration creates a tension that pulls the continuous water column upward (cohesion‑tension theory).
• Upward flow supplies minerals to leaves, maintains cell turgor, and drives cooling.

7. Suggested Diagrams (For Teacher Use)

1. Cross‑section of a dicot root or stem showing the relative positions of xylem (central, upward‑pointing vessels) and phloem (outside the xylem, downward‑pointing sieve tubes).
2. Pathway of water from root hairs to leaf (illustrating the steps listed in Section 2).
3. Leaf cross‑section with labelled stomata, palisade and spongy mesophyll, and arrows indicating water loss as vapour (for Section 3).

8. Quick Check (Core)

Question: What is the term for the loss of water vapour from a plant through the stomata?

Answer: Transpiration.