BIOL 205 Lecture Notes - Lecture 14: Vascular Bundle, Turgor Pressure, Xylem
Document Summary
Live cells, no mitochondria and plasma membrane (requires little atp) Elongated cells with perforated cell walls (sieve plates) that are stacked end to end. Make long sieve tubes (phloem: companion cells. Cytoplasmic connection with sieve element (e. g. plasmodesmata) Loads organic compounds and provides energy to sieve elements. Large membrane surface area that facilitates transport. Transport pathway: mesophyll cells (leaf cells) produce sugar and transport to bundle sheath cells, and then to phloem parenchyma cells. Through plasmodesmata: sugar is loaded into companion cells. Can be both apoplastic and symplastic: large sugar and starch grains are hydrolyzed to sucrose before phloem loading. Sucrose-proton symport system that utilize a proton gradient, set up by proton. Symplastic (cross membrane) transport needs to break down sucrose into glucose and fructose: sugar diffuse from companion cells to sieve elements, sugar is loaded to sink organs through apoplastic or symplastic pathways.
