BIOL 341 Lecture Notes - Lecture 11: Symporter, Cw Complex, Starch
Document Summary
Sugars and other organic materials often have a long distance to travel. Measured velocities of transfer far exceed rate of diffusion: ave = 1m/hr (diffusion ave = 1m/32yr!) Generally considered to be a passive process. Bundle sheath- compact cells surrounding vascular tissue. Lose their nuclei and tonoplast during development: no microfilaments/ microtubules, golgi, ribosomes. Retain mitochondria, plastids, smooth er (ser): are living cells. Sieve plate pores: angiosperms: open channels (low resistance, gymnosperms: covered by ser. Note peripheral position of cellular components; unobstructed sieve plate pores. The cellular components are distributed along the walls of the sieve tube elements where they offer less resistance to mass flow. Different from tracheid elements which are lack plasma membrane, have lignified secondary walls, and are dead at maturity. Sieve tubes are under high turgor pressure: when tubes are damaged = large loss of energy-expensive sap.