CHAPTER 30.5How Do Organic Compounds Move Through Plants?

AP BiologySpring 2011

CONDUCTING TUBES IN PHLOEM

Phloem: living vascular tissue with organized arrays of conducting tubes, fibers, and parenchyma cells

CONDUCTING TUBES IN PHLOEM

Sieve tube cells: alive at maturity and are interconnected side by side and end to end from the roots to the leaves

Companion cells: located next to sieve tubes and function to actively transport the products of photosynthesis into the sieve tubes

CONDUCTING TUBES IN PHLOEM

Carbohydrates are mainly stored as insoluble starch molecules that must be converted to more soluble carbohydrates Such as sucrose before being transported

throughout the plant

TRANSLOCATION

Translocation: transport of sucrose and other compounds through phloem

Movement of molecules through phloem is from sources to sinks Source: mostly leaves Sink: flowers and fruits

TRANSLOCATION

Observations of plant-sucking insects demonstrates that the sugary fluid in the phloem is under high pressure

TRANSLOCATION

Pressure flow theory: translocation depends on pressure gradients Solutes are loaded by active transport into the

phloem from a source (ex. Leaves) Water enters by osmosis due to increase in

solutes Pressure builds in sieve tubes pushing the

sucrose-laden fluid out of the source, into the stems, and on the sink (ex. Fruit)