Transcript Leaf structure - HRSBSTAFF Home Page

Plants
2.9, 8.3, 9.1, 9.2, 9.3
Recap
 What supplies are necessary for
photosynthesis?
 How does a plant obtain these supplies?
Leaf- Chloroplast
 Data question pg 398
Leaf- Pigments
 Absorption and Action
Spectra p133-134
 Action Spectrum = graph
showing rate of
photosynthesis at each
wavelength of light
 Absorption Spectra =
graph showing percent of
light absorbed at each
wavelength by a pigment
Leaf- Stomata
 Enable exchange of
gases
 Transpiration is the
consequence of gas
exchange in the leaf
 p404
Xylem and Phloem
 Xylem = transport of
water (p406-409)
 Phloem = transport of
“food” (p412-415)
Xylem
 The cohesive property of water and the
structure of xylem vessels allow transport
under tension.
 Xylem are long continuous tubes
 Formed from cells arranged end to end
 Cell walls between adjacent cells break down
 When mature, the cells are non-living
 What does this suggest about the kind of transport
possible?
 The walls are thickened and contain lignin
 Allows them to withstand low pressures
Xylem
 The adhesive properties of water and
evaporation generate tension forces in leaf
cell walls.
 When water evaporates from the surface of
the wall in a leaf, adhesion causes water to be
drawn from the nearest available source.
 The xylem in the leaf veins is this source
 Even if the pressure is low the force of adhesion will
suck water out of the xylem
 The low pressure creates a pulling force that is
transmitted down the stem to the roots
 Transpiration-pull is stronger than the force of gravity
 Pull depends on cohesive properties of water
 This a passive process for the plant driven by
the thermal energy of the sun
Roots
 Active uptake of mineral ions in the roots causes
absorption of water by osmosis
 Protein pumps in the plasma membrane of root cells
transport mineral ions into the root from the soil.
 Concentration of minerals can be 100x more inside the cell
 There are separate pumps for each type of ion.
 Mineral ions can only be absorbed if they make contact with
the right pump
 Some ions move very slowly
 Some plants develop mutualistic relationships with fungi to
overcome this problem
Water then moves to the xylem
through cell walls (apoplast pathway)
and through cytoplasm (symplast pathway).
Summary
 Plants transport water from
roots to leaves to replace losses
from transpiration.
 Water exits through stomata by
evaporation.
 Water climbs through xylem by
“transpiration-pull” and cohesion
and adhesion properties of water
 Water moves into roots by
osmosis due to mineral
concentration gradients
 Water moves to the xylem through
cell walls and cytoplasm.
Phloem
 Plants transport organic compounds from
sources to sinks
 Phloem is composed of sieve tubes
 Columns of specialized cells
 Cells are living but have reduced cytoplasm and no
nucleus
 Cells are separated by “sieve plates”
 Sieve tube cells are closely associated with
companion cells
 They share the same parent cell
 Companion cell has abundant mitochondria
 Companion cell performs many of the genetic and
metabolic functions to maintain the sieve cell
Why would these adaptations be helpful? See images on
page 416.
Phloem
 The transport of organic solutes is called
“translocation”
 Phloem links parts of the plant that need
sugars (and other solutes) with parts that
have extra
 Why would this be important?
 The tubes in phloem must transport in
both directions
 Why would this be important?
 See page 413 chart
Phloem
 Active transport is use to load
organic compounds into phloem
sieve tubes.
 Sucrose is the most common solute in
phloem sap.
 Is not readily available for metabolism and
is therefore good for transport
 The process of bringing sugars into the
phloem is called “loading”
 See page 413-414 for different mechanisms
 Compare these mechanisms
Phloem
 Incompressibility of water allows
transport by hydrostatic pressure
gradients.
 The build up of sucrose (and other
carbohydrates) draws water into the
companion cell through osmosis.
 Rigid cell walls and the incompressibility
of water builds up pressure.
 Water will flow from high pressure to
low pressure
Phloem
 At the sink end, sucrose is either
used as an energy source or stored
as starch.
 The loss of solute = reduced osmotic
pressure.
 Water moves back to the xylem
Identifying xylem and
phloem
 See page 420-421
Summary
 Compare xylem and phloem