Biology 102 Chapter 36
Transport in Plants
1. List four reasons why plants need water.
---photosynthesis
---transporting solutes
---cooling the plant
---developing the internal pressure that supports the plant body
2. Describe how water and minerals move from the soil into the
root and eventually the shoot. Include the following terms:
aquaporins, transport proteins, symplast, apoplast, Casparian
strip, and water potential.
---review osmosis; solute potential, pressure potential (turgor
pressure), water potential, pressure potential
---AQUAPORINS are membrane channels through which water can move
without interacting with hydrophobic part of bilayer
--allow for rapid movement of water
--permeability of aquaporins subject to change = change in
rate of osmosis across membrane
--ALWAYS PASSIVE
---TRANSPORT PROTEINS required for transport of mineral ions
--are charged and cannot move across plasma membrane unless
aided by transport proteins
---if [ ] of minerals in soil greater than in plant, facilitated
diffusion will work
---if more [ ] in plant, only way to move in from outside is by
active transport
---charge on ion also very important consideration
---combination of [ ] gradient and electrical gradient is called
electrochemical gradient
---plants use proton pump
--K+ moved in because of electrochemical gradient
--Cl- moves in; symport couples movement with diffusion
of H+
---mineral ions enter and move through plants in various ways
--bulk flow occurring, dissolved minerals carried with flow
--water moving slowing, minerals move by diffusion
--certain points (membranes encountered) = active transport
---movement of ions across membranes can also result in
movement of water
---water moves into root because root has more NEGATIVE water
potential than does the soil solution
---water moves from cortex of root to the stele (where vascular
tissues located) because stele has more negative water
potential than cortex
---water and minerals from soil pass through dermal and ground
tissue to stele via APOPLAST and SYMPLAST
---plant cells surrounded by cell walls that lie outside
their plasma membrane and intercellular spaces (spaces
between cells) are common in many tissues
--walls and intercellular spaces together constitute the
APOPLAST
---apoplast is continuous meshwork through which water and
dissolved substances can flow or diffuse WITHOUT ever having
to cross a membrane
--movement of materials through apoplast is unregulated
---remainder of plant body is the SYMPLAST
--is portion of plant body enclosed by membranes—the
continuous cytoplasm of living cells connected by
PLASMODESMATA
---selectively permeable membranes control access to symplast
--movement of water/dissolved substances tightly regulated
---water and minerals can move from soil through apoplast as
far as ENDODERMIS (inner layer of the cortex)
---endodermis distinguished from rest of ground tissue by
presence of CASPARIAN STRIPS
--these waxy, suberin-containing structures impregnate
endodermal cell wall and form belt surrounding endodermal
cells
--act as gasket that prevents water/ions from moving between
cells
---Casparian strips of endodermis completely separate apoplast
of cortex from apoplast of stele
---only way water and ions can enter stele is by passing through
cytoplasm of endodermal cells
---transport proteins in membranes of these cells determine
which mineral ions pass into stele and at what rate
---water and ions end up in the xylem where they constitute
the xylem sap
3. According to the transpiration-cohension-adhesion theory,
describe how xylem sap can be pulled upward in xylem vessels.
---transpiration pulls xylem sap upward
---cohesion of water transmits the upward-pull along the
entire length of xylem
--cohesion due to H bonds of water
---adhesion of water (by H bonds) to hydrophilic walls
of xylem cells also helps pull against gravity
---small diameter of vessels and tracheids important
to adhesion effect
---upward pull of sap causes tension (negative pressure) in
xylem, which decreases water potential and allows
passive flow of water from soil into stele
---transpiration pull can extend down to roots ONLY
through an unbroken chain of water molecules
4. Describe both the disadvantages and benefits of
transpiration.
---water vapor diffuses out of intercelluar spaces of leaf, by
way of the stomata to outside air
--water vapor [ ] greater inside leaf than outside
---results in tremendous water loss from the plant
--if transpiration exceeds delivery of water by xylem
leaves will begin to wilt as their cells lost turgor
pressure
---benefits include assisting in pull of xylem sap
--assists in mineral transfer from roots to shoots
--evaporative cooling reduces risk of leaf temperatures
become too high for enzymes to function
5. Explain how guard cells control the stomatal aperture and
how this, in turn, can affect photosynthetic rate and
transpiration.
---guard cells flank stomata and control stomatal diameter
by changing shape
--when turgid, they “buckle” due to radially-arranged
microfibrils and stomata “open”
--when flaccid, they sag and stomatal openings close
---change in turgor pressure of guard cells results from
reversible uptake and loss of K+
---stomata open, adequate amounts of CO2 available for
photosynthesis
--this increases water loss of the plant
--if adequate water available, no problem
---if shortage of water, guard cells will reduce stomatal
openings or close them completely
--this will save water for sure
--but it also stops photosynthesis
---very delicate balance and trade off
6. Describe the translocation of substances in the phloem using
the pressure flow model.
---translocation is the transport of products of
photosynthesis by phloem to rest of plant
---phloem sap moves through sieve tubes from a source
(production area) to a sink (use or storage area)
--sugar flows from source to sink
--direction of flow can change, depending on locations
of source and sink
---sugar produced at source must be loaded into sieve tube
members before it can be translocated to a sink
---sucrose is unloaded at the sink end of sieve tubes
---phloem flow too fast for just diffusion/cytoplasmic
streaming
---flow is by bulk flow (pressure-flow) mechanism
--build up of pressure at sink causes source-to-sink
flow
---at source end, phloem loading causes high solute []
---water potential decreases, so water flows into tubes
creating hydrostatic pressure
--greatest at the source end of the tube
---at sink end, water potential is lower outside the tube
due to unloading of sugar
--osmotic loss of water releases hydrostatic pressure
---xylem vessels recycle water from sink to the source