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Chapter 12: Transport Processes, Cross connection: Phloem is part of Long…
Chapter 12: Transport Processes
Concepts
Transport Processes
Short-Distance Transport
Few cell diameter distance or shorter
Transfer of basic nutrients between cells
Long-Distance Transport
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between cells not close together
Not absolutely essential to construction of a large plants
Isolation Mechanisms
inhibit movement of substances
Epidermis
cutin-lined
keep water in shoot
Casparian strip
prevent diffusion of minerals from one part of root to another
Essential for useful transport
Diffusion, Osmosis, and Active Transport
Diffusion
simplest method
due to random movement of particles in a solution
Diffusion through a membrane called osmosis
Types of Membranes
Freely Permeable
allow all solutes to diffuse through them
little biological significance
Completely permeable
do not allow anything to pass through
occur as isolation barriers
Differentially or selectively permeable membranes
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allow only certain substances to pass through
all lipid/protein cell membranes
hydrophobic molecules diffuse easily
hydrophilic molecules move through protein channels
called aquaporins
Active Transport
use molecular pumps
ATP forces molecules across membrane
ATP splits into ADP and phosphate
molecule is carried across membrane
Intracellular transport
vesicles migrate through cytoplasm and fuse with organelles
during fusion membranes merge with vesicles
Water Potential
Cells and Water Movement
If placed in pure water
animal cells burst (lysis)
plant cells cannot burst due to walls
incipient plasmolysis
protoplast loses water and slightly pulls away from wall
may continue to lose water and pull completely away from wall
known as plasmolyzed
chemical potential in botany
pressure potential
effect that pressure has on water pontential
measured in megapascals or bars
osmotic potential
effect that solutes have on water potential
related to number of particles present in a solution
matric potential
water's adhesion to nondissolved structures
Short-Distance Intercellular Transport
Motor Cells
similar to guard cells
can accumulate or expel potassium
Transfer Cells
walls smooth on outer surface
numerous finger-like and ridge-like outgrowths on inner surface
found in areas where rapid short-distance transport is expected
Guard Cells
potassium is pumped into guard cells
guard cells swell
stoma open
potassium is pumped out of guard cells
stoma close
Symplast
all protoplasm of one plant
Apoplast
wall and intercellular spaces
Long-Distance Transport: Phloem
pressure flow hypothesis
important driving forces
Molecular pumps
Active transport
sources
where water and nutrients are transported from
sugars actively transported to sieve elements in some species
Other species use polymer trap mechanism
deliver to sinks
STM/CC complex
sieve tube member
companion cell
mass transfer
amount of sugar/nutrients transported by phloem per hour
specific mass transfer
mass transfer divided by cross-sectional area of phloem
Two mechanisms seal broken sieve tube elements
P-protein
forms a P-protein plug
present in all eudicot and some monocots
Absent in conifers and some monocots
callose
only stays in solution when under pressure
carried with P-protein
Long-Distance Transport: Xylem
Properties of Water
cohesive
any force acting on one molecule acts on all neighboring ones
adhesive
molecule interacting with many other substances
Water Transport Through Xylem
cohesion-tension hypothesis
stomatal pores open
water loss unavoidable
transstomatal transpiration
water loss through stoma
transcuticular transpiration
water lost through cuticle
cativation
hydrogen bond broken and water column breaks
molecules above shoot upward
molecules below rush downward
spaces between called embolism
Control of Water Transport by Guard Cells
Open stoma
water is being lost
closed stoma
water is being conserved
Cross connection: Phloem is part of Long distance transport
Cross connection: Xylem used in long distance transport
Cross Connection: Guard cells use selectively permeable membranes for potassium pumping.
