LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- compare xylem and phloem
- trace water through a plant
- explain transpiration and its factors
- describe translocation from sources to sinks
THE BIG IDEA
How xylem, phloem, roots and leaves move water, minerals and assimilates.
This chapter builds each process from its structures, inputs and outputs. Follow the connections rather than memorising isolated facts, then use precise biological terms when explaining cause and effect.
SECTION 01
Xylem, phloem and water uptake
Xylem carries water and mineral ions upwards and supports the plant. Phloem translocates sucrose and amino acids between sources and sinks.
DETAILED EXPLANATION
- Xylem vessels are dead, hollow continuous tubes with no cross walls and lignin-thickened walls.
- Root hairs provide a large area for water uptake by osmosis and ion uptake, often by active transport.
- Water follows root hair → cortex → xylem → mesophyll.
- A coloured stain can reveal the xylem pathway through stem and leaves.
SECTION 02
Transpiration
Water evaporates from moist mesophyll surfaces, enters leaf air spaces and diffuses through stomata as vapour. This loss creates a pull that draws a continuous water column upward through xylem.
DETAILED EXPLANATION
- Cohesion between water molecules keeps the column together.
- Higher temperature and faster wind usually increase transpiration; higher humidity reduces the water-vapour gradient and lowers it.
- Many stomata and a large internal air-space surface increase potential loss.
- If loss exceeds uptake, cells lose turgor and the plant wilts.
SECTION 03
Translocation
Translocation moves sucrose and amino acids in phloem. Movement can be upward or downward depending on source and sink locations.
DETAILED EXPLANATION
- A source releases assimilates, such as a photosynthesising leaf or mobilising storage organ.
- A sink uses or stores assimilates, such as a root tip, fruit or tuber.
- The same organ can change role: a tuber stores sucrose while growing, then becomes a source during sprouting.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Xylem transports water and supports.
- Phloem moves sucrose and amino acids.
- Transpiration pull lifts a cohesive water column.
- Environmental factors alter water loss.
- Sources release and sinks use or store assimilates.