AP biology Question?

Question

Xylem cells are dead when functional. Why must phloem cells be alive when functional?

Answer 1

Phloem transports dissolved sugars using a combination of diffusion and osmotic pressure. These require cytoplasm, not an empty tube.

Answer 2

Phloem must be alive because it's transport is energetic and requires ATP and living cell membranes. Sugars are pumped into the phloem and kept there by the cell membrane barriers. Even then the sugar tends to leak out slowly so it is actively pumped back in to keep it flowing. All this handling requires the cells of the phloem to be alive. However mature phloem cells are very limited, like red blood cells in animals they don't have a nucleus. In fact, mature phloem tubes are "almost" dead in that they require the action of a companion cell (a normal plant cell) to provide it with the essentials of life.

Xylem transport can be done with "dead" tissue because its driving force comes from water transpiration. Water is drawn under tension out the leaves drawing more up from he roots. Its transport also requires no intact membranes only a strong structure that can be provided by cell walls alone.

Answer 3

phloem cells need to be alive to transport food against a gradient. This is active transport

Xylem cells are doing passive transport. no energy is required to move water from one place to another. water always flow down a gradient.

pretty sure this is it