Specific Learning Outcomes

1. Describe absorption of water and mineral salts in plants.
2. Explain translocation in plants.
3. Investigate factors affecting transpiration in plants.
4. Describe the significance of transpiration in sustainability of plant life.
5. Appreciate the role of transport in plants.

Overview

Transport in plants is the movement of water, mineral salts and manufactured food (sugars) within a plant. The main tissues involved are xylem (mainly for water and minerals) and phloem (mainly for organic food). Understanding transport helps explain how crops such as maize, tea and coffee grow well in Kenya’s different climates and how farmers manage water and nutrients.

1. Absorption of water and mineral salts

Roots absorb water and dissolved mineral ions from the soil. Most absorption takes place in the root hair zone because root hairs increase surface area.

• Water absorption: mainly by osmosis — water moves from soil (higher water potential) into root hair cells (lower water potential).
• Mineral absorption: often by active transport — plant cells use energy (ATP) to take up mineral ions (e.g., nitrate, phosphate) from low concentration in the soil into root cells.
• Pathways to xylem: apoplast (between cells), symplast (through cytoplasm via plasmodesmata), and across the endodermis (Casparian strip forces selective uptake).

2. Translocation (movement of food)

Translocation is the movement of manufactured food (mainly sucrose) from sources (e.g., leaves) to sinks (growing roots, developing seeds, fruits). This occurs in the phloem and is often explained by the pressure-flow (mass flow) hypothesis.

• Source to sink: sugars are loaded into phloem at the source (active transport), raising solute concentration. Water follows by osmosis, creating a high pressure that pushes sap toward sinks, where sugar is removed and pressure falls.
• Direction: phloem transport can be up or down depending on where sources and sinks are located (e.g., sugar moves down to roots, up to growing shoots).

3. Transpiration and factors affecting it

Transpiration is the loss of water vapour from plant surfaces, mainly through stomata in leaves. It creates a pull (transpiration stream) that aids the upward movement of water in xylem.

Main factors affecting transpiration

• Light: stomata open in light → transpiration increases.
• Temperature: higher temperature increases evaporation → higher transpiration.
• Wind: removes humid air around leaf → increases transpiration.
• Humidity: high humidity reduces the vapour gradient → lowers transpiration.
• Leaf surface area and cuticle: hairy leaves or thicker cuticle reduce transpiration.

Investigation: Factors affecting transpiration (simple practical)

Use this class practical to investigate one factor (light, wind, humidity or temperature). Example: light intensity.

1. Materials: 3 similar potted plants (maize or bean seedlings), weighing scale, plastic bags, fan or hairdryer (low), marker, stopwatch.
2. Method: Water plants equally. Weigh each plant + pot. Place one in bright sunlight, one in shade, and one in a dark cupboard. After 24 hours reweigh. Repeat for several days.
3. Record: mass loss = water lost by transpiration (assume soil evaporation reduced by covering soil with cling film).
4. Expected results: plant in bright light loses most water; dark cupboard loses least.
5. Conclusion: higher light increases transpiration (stomatal opening).

Safety note: supervise use of electrical equipment; avoid overheating plants.

4. Significance of transpiration and transport

• Transpiration helps draw water and dissolved minerals from the roots to leaves (transpiration pull), essential for photosynthesis.
• Evaporative cooling: transpiration prevents overheating of leaves (important in hot Kenyan days especially in lowland farms).
• Movement of minerals is vital for growth, development and yield — farmers supply fertilisers to replace lost minerals.
• Phloem transport distributes sugars for storage (e.g., sugarcane, tubers) and growth (seeds, fruits).

5. Appreciating the role of transport in plants

Good transport systems are essential for healthy crops. For example, during drought, reduced transpiration and xylem flow cause wilting; farmers in Kenya use irrigation and mulching to conserve soil moisture and support transport. Understanding transport helps you appreciate why soil management, watering and fertiliser timing matter.

Suggested Learning Experiences

• Practical: Measure transpiration using potted plants and weighing or a simple potometer (teacher-demonstration for potometer setup).
• Practical: Stomatal count — peel underside of leaf (e.g., geranium) or use clear nail polish technique, observe under microscope and relate stomatal density to transpiration.
• Field visit: Observe tea/coffee farms (where available) to see how leaf transpiration and irrigation are managed; note mulching and shade use.
• Group project: Model translocation using syrups and tubing to demonstrate source-sink flow and pressure differences.
• Investigative homework: Test how covering leaves with petroleum jelly affects transpiration and plant health (small scale, supervised).
• Use local examples: relate transport to nutrient uptake in maize after fertiliser application, or tea leaf quality in highland vs lowland conditions.

Check your understanding

1. Explain why a plant wilts when roots cannot take up water even though soil is wet.
2. Describe how sugars move from leaves to developing seeds.
3. Design a simple experiment to test the effect of wind on transpiration and state the control variables.
4. List three ways farmers can help maintain transport processes in crops during dry seasons.

Quick summary

Roots absorb water (osmosis) and minerals (active transport). Xylem moves water upward; phloem translocates sugars from sources to sinks. Transpiration drives water movement and is affected by light, temperature, wind and humidity. These transport processes are vital for growth, yield and survival of plants.

Teacher notes: Adapt experiments to local resources; ensure safety; link topics to local crops (maize, tea, coffee) to increase relevance for Kenyan learners.