Download Transport in Plants

UNIT 7
Chapter 36: Transport in Plants
Chapter 37: Plant Nutrition
Chapter 38: Plant Reproduction
Types of Transport
 Transport of water and minerals occurs on
three levels:
 In/out of individual cells
 Short distance
 Long distance
 Differences in water potential drive transport of
water in plant cells
 = s + p
 Pure water, s = 0
 Addition of solutes
decreases s
 Water moves from
areas of high water
potential to areas of
low water potential
 Water potential
impacts uptake and
loss of water
 Flaccid cell
 Cell loses water
 Cell will eventually
plasmolyze
 Turgid cell
 Cell
gains/maintains
water
Turgid
plant
Absorption of Water & Minerals
 Root hairs and mycorrhizae increase surface
area and enhance absorption
 Water and minerals transported = xylem sap
 Ascent of xylem sap depends mainly on
transpiration
 Xylem sap not “pushed” by pressure in roots,
but “pulled” up
 Force created by transpiration and
cohesion/adhesion of water molecules extends
down shoot, into roots and even into soil
 Water
potential
gradient
drives
water up
 Air in the
xylem,
cavitation,
breaks the
chain
Control of Transpiration
 A plant can transpire more than its weight in
water every day
 Flow in xylem can reach 75cm/min
 Guard cells control the size of the stomata and
balance the plant’s water needs and loss
 Transpiration-to-photosynthesis is amount of
water loss per gram of CO2 fixed
 Many plants ~600:1 – 600g of water lost per 1g
of CO2 fixed
 C4 plants (ex. corn) ~300:1
 Transpiration also results in evaporative
cooling, which can cool leaves 10-15°C
 Prevents denaturation of enzymes and
disruption of metabolism
 Each stoma is flanked by a pair of guard cells
suspended by other epidermal cells over an air
space
 Water into guard cells = turgid  stoma open
 Water out of guard cells = flaccid  stoma
closed
 K+ plays an important role in osmosis in guard
cells
 Presence of K+ ions lowers s and water flows
in or out based on where s is lower
 There are three cues that initiate the opening
of stomata in the morning
 1. Blue-light receptors promote active uptake of
K+ into guard cells
 Photosynthesis provides ATP
 2. Depletion of CO2 as photosynthesis begins
 3. Internal “clock” within guard cells that cycles
on a 24-hour basis – circadian rhythm
Xerophytes
 Xerophytes are plants that are well adapted to
arid (very dry) environments
 CAM plants are xerophytes (family
Crassulaceae)
 A number of anatomical adaptations exist
to reduce water loss
 Concentration of stomata on lower surface of leaf
 Presence of trichomes
 Placement of stomata within “crypts”
Phloem Transport
 Phloem sap is typically moved from sugar
sources to sugar sinks
 Sources: sugar being produced by photosynthesis,
esp. in mature leaves
 Sinks: growing parts of the plant, fruits
 Sinks usually receive sugar from the sources closest to
them
 Can be in any direction – even against pull of gravity
 Pressure flow is the primary force behind the
translocation of phloem sap
 Phloem sap
flows from
source to sink
at about 1m/hr
 Flow is fastest
near sources
 Sugar
concentration
is highest
 Water is
recycled
because of
differences in

END
Nitrogen Requirements
• 80% of the atmosphere is nitrogen (N2),
but plants can still suffer deficiencies
• Plants can only use nitrogen in certain
forms
• Ammonium (NH4+) or nitrate (NO3-) ions
• Bacteria in the soil metabolize unusable
forms of nitrogen
• Nitrogen fixation makes nitrogen available
• Nitrogen fixing bacteria convert N2 into NH4+
and ammonifying bacteria convert
decomposing organic material into NH4+
• Nitrifying bacteria convert NH4+ into NO3-
• ALL (eukaryotic) life on earth depends
on nitrogen fixation
Parasitic & Carnivorous Plants
• Some plants supplement or replace
their photosynthesis by taking
advantage of other plants
• ex. Indian pipe
• Epiphytes are autotrophic
plants, but they simply live on
other plants
• Not truly parasitic
• ex. some mosses and ferns
• Carnivorous plants supplement their
nutrition by digesting animals
• Typically found in areas with poor soil
conditions
• Use photosynthesis for carbohydrates, but get
some nitrogen and minerals from animals
• Modified leaves trap animals and secrete
enzymes
END
Flower Structure: Review
Pollination
• Pollination is the attachment of pollen to a
flower’s stigma
• Pollen released is carried by wind or animals
• Pollen grain produces a pollen tube
• Grows through style, into ovary and discharges
sperm
• Zygote gives rise to an embryo
• Ovule develops into a seed, ovary develops
into a fruit containing seed(s)
Plant Reproduction Terminology
• Plant biologists distinguish between
complete flowers, those having all four
organs, and incomplete flowers, those
lacking one or more of the whorls
• A bisexual flower (“perfect flower”) is
equipped with both stamens and carpals
• A unisexual flower is missing either stamens
or carpels
• A monoecious plant has male and female
flowers on the same individual plant
• ex. corn
• A dioecious species has male flowers and
female flowers on separate plants
• ex. date palms
Prevention of Self-Fertilization
• Some flowers self-fertilize, but most
angiosperms have mechanisms that make
this difficult
• Barriers prevent self-fertilization to maintain
genetic variety
• In some species
stamens and carpels
mature at different
times
• May be arranged so
that it is unlikely that an
animal pollinator could
transfer pollen from the
anthers to the stigma
of the same flower
• Most common anti-selfing mechanism is
self-incompatibility
• Ability to reject its own pollen
• Biochemical block prevents fertilization
• Self-incompatibility systems are analogous
to the immune response of animals
• Difference is that the animal immune system
rejects non-self, self-incompatibility in plants
is a rejection of self
• Based on genes for self-incompatibility,
called S-genes, with as many as 50 different
alleles in a single population
• If a pollen grain and the carpel’s stigma have
matching alleles at the S-locus, pollen grain
fails to initiate or complete the pollen tube
Double Fertilization
• Pollen grain lands on stigma, absorbs
moisture and begins producing a pollen tube
• Directed by a chemical attractant, the pollen
tube enters the ovary through the micropyle
and discharges two sperm within the embryo
sac
• Both sperm fuse with nuclei in the embryo
sac
• One sperm fertilizes the egg to form the
zygote
• Other sperm combines with the two polar
nuclei to form a triploid nucleus in the
central cell
• Gives rise to the endosperm, a food-storing
tissue of the seed
Fig. 38.9
• After double fertilization, the ovule develops
into a seed, and the ovary develops into a
fruit enclosing the seed(s)
• As the embryo develops, the seed stockpiles
proteins, oils, and starch
Fruit Development
• As the seeds are developing from ovules, the
ovary of the flower is developing into a fruit
• Pollination triggers hormonal changes that
cause the ovary to begin its transformation
into a fruit
• If a flower has not been pollinated, fruit usually
do not develop, and the entire flower withers
and falls away
• The wall of the ovary becomes the
pericarp, the thickened wall of the fruit
• In some angiosperms, other floral parts
contribute to what we call a fruit
Seeds & Seed Germination
• As a seed matures, it dehydrates and enters
a dormancy phase
• Extremely low metabolic rate, suspension of
growth and development
• Conditions required to break dormancy
• Some seeds germinate as soon as they are in
a suitable environment
• Remains dormant until some specific
environmental cue causes them to break
dormancy
• Seeds of many desert plant germinate only
after a substantial rainfall, ensuring enough
water
• Where natural fires are common, many
seeds require intense heat to break
dormancy
• Other seeds require a chemical attack or
physical abrasion
• ex. through an animal’s digestive tract before
they can germinate
• Germination of seeds depends on
imbibition, the uptake of water due to the
low water potential of the dry seed
• Expanding seed ruptures its seed coat and
triggers metabolic changes in the embryo
that enable it to resume growth
• Enzymes begin
digesting the storage
materials of endosperm and the nutrients are
transferred to the
growing regions of
the embryo
END