What is transpiration?

Loss of water vapour from leaves via stomata

What is the transpiration stream?

Movement of water from roots to leaves through xylem

What is cohesion?

Attraction between water molecules via hydrogen bonds

What is adhesion?

attraction between water molecules and xylem walls

What is tension in xylem?

Negative pressure created by transpiration pulling water up

What is the cohesion-tension theory?

Explains how water is pulled up the xylem due to transpiration and cohesion

Why are xylem vessels efficient?

Dead cells, no cytoplasm, continuous tubes → low resistance

Role of lignin in xylem?

Strengthens walls and prevents collapse under tension

What is water potential?

Measure of tendency of water to move

How does water enter roots?

Osmosis

What causes transpiration pull?

Evaporation of water from mesophyll cells

How does water move through the leaf?

Diffuses from mesophyll to air spaces, then out stomata

Why does water form a continuous column?

Cohesion between water molecules

How is the column maintained

Cohesion + adhesion prevent breaking

Explain the cohesion-tension theory

Water evaporates from mesophyll cells • Creates low water potential in leaf • Water enters leaf from xylem by osmosis • Cohesion holds water molecules together in a column • Transpiration creates tension pulling water up • Adhesion to xylem walls maintains column • Continuous flow from roots to leaves

What is translocation?

Movement of molecules (e.g. sucrose) in phloem

What are sources

Regions where sugars are produced

What are sinks

Regions where sugars are used/stored

What are sieve tube elements?

Living cells that transport sugars in phloem

What happens to water potential at source?

Decreases → water enters by osmosis