BIOL 1011 Lecture Notes - Lecture 4: Sieve Tube Element, Water Potential, Seaweed

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Transport, Photosynthesis and Control Systems
January 22, 2015
Transport within plants: water potential = pressure potential + solute potential
Water spontaneously moves from regions of higher to regions of lower water
potential energy
In any region, water potential is the SUM of:
o Solute (osmotic) potential energy (0 or negative)
o Pressure potential energy (any value)
2 main tissues for transport:
o Xylem: sap: tracheids and sometimes vessel elements
o Phloem: sugar water: sieve-tube elements
Water flow in the xylem is a passive process that results in the ascent of sap. The
water potential is higher in roots than in the outside air. The Xylem is placed under
tension and a transpiration-cohesion-tension mechanism occurs to move the water
spontaneously from soil to leaves.
Phloem Transport: Pressure-flow Hypothesis
The sugar water solution is moved from the
source to a sink from bulk flow by both negative
and positive pressure.
Pumping sugar into the sieve tube element
decreases the solute potential, which then
decreases the water potential so water will from
into the phloem from the xylem.
Sugar is being removed by a sink cell which
establishes a water gradient so water will then
move into the xylem from the phloem.
Material in xylem will only move upward but
material in phloem can move in various
directions depending on water potential.
Photosynthesis
Importance:
Source of oxygen in atmosphere
o 50% terrestrial plants
o 50% marine phytoplankton and macroalgae
First step in moving energy into the living world; source of all energy in
ecosystems
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Two Stages of Photosynthesis:
1. Light Reactions
Light hits chlorophyll molecule
Electrons bounce to higher energy level and release a chlorophyll
molecule
The chl steals electrons from water
o This causes the water molecule to fall apart via photolysis
(releases oxygen)
o NADP+ + 2 electrons + H+ yields NADPH (reducing)
o *ADP + Pi yields ATP via photophosphorylation
OVERALL, light energy is converted first into chemical energy of ATP and
NADPH, and oxygen gas is released.
H20 + light + NADP+ + ADP + Pi O2 + NADPH + ATP
2. Calvin Cycle
Makes sugar as glucose
Supplies light reaction with NADP+ and ADP
CO2 + NADPH + ATP CH2O + NADP+ + ADP + Pi
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Document Summary

Transport within plants: water potential = pressure potential + solute potential: water spontaneously moves from regions of higher to regions of lower water potential energy. In any region, water potential is the sum of: solute (osmotic) potential energy (0 or negative, pressure potential energy (any value, 2 main tissues for transport, xylem: sap: tracheids and sometimes vessel elements, phloem: sugar water: sieve-tube elements. Water flow in the xylem is a passive process that results in the ascent of sap. The water potential is higher in roots than in the outside air. The xylem is placed under tension and a transpiration-cohesion-tension mechanism occurs to move the water spontaneously from soil to leaves. The sugar water solution is moved from the source to a sink from bulk flow by both negative and positive pressure. Pumping sugar into the sieve tube element decreases the solute potential, which then decreases the water potential so water will from into the phloem from the xylem.

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