Transcript Topic 9 Plant Biologyx

Topic # 9: Plant Biology
I. Transport in the Xylem of plants
A. Transpiration
1. Transpiration is the inevitable consequence of
gas exchange in the leaf
2. Plant leaves are the primary organ of
photosynthesis
a. CO2 is vital to this process
b. O2 is produced as well
3. Exchange of CO2 and O2 must occur in order to
sustain photosynthesis
4. Absorption of CO2 is essential for
photosynthesis and the waxy cuticle of the top of
the leaf has low permeability to it
5. Pores in the epidermis are needed - stomata
I. Transport in the Xylem of plants
6. If stomata are open to absorb CO2, then water
will be lost from the leaf to the atmosphere
7. This is a problem
8. Transpiration: Loss of water vapor from the
leaves of a plant
9. Guard cells minimize water loss
10. Open and close based on the needs of the
plant
I. Transport in the Xylem of plants
B. Xylem structure helps withstand low pressure
1. The cohesive property of water and the
structure of the xylem vessels allow transport
under tension
2. Xylem vessels are long, continuous tubes with
thickened cell walls
3. Lignin is within the cell walls to help with this
thickening
3. Helps to strengthen the walls so that they will
not collapse under low pressure
4. Xylem vessels are formed from files of cells –
arranged end to end
5. When mature, the xylem cells are nonliving
I. Transport in the Xylem of plants
a. Water must move as a passive process
b. Water molecules are polar and the partial
negative oxygen atom in one molecule attracts
the hydrogen atom in another = cohesion
c. Water is also attracted to hydrophilic parts
of the cell walls of the xylem = adhesion
d. Water moves up the plant as a continuous
stream as the result of the interaction of
cohesive and adhesive forces
C. Tension in leaf cell walls maintains the
transpiration stream
1. The adhesive property of water and
evaporation generate tension forces in leaf cell walls
I. Transport in the Xylem of plants
2. When water evaporates from the surface of
the wall in a leaf, adhesion causes water to be
drawn through the cell wall from the nearest
available supply to replace the water lost by
evaporation
3. Even if the pressure in the xylem is low, the
force of adhesion between water and the cell
walls is enough to suck water out of the xylem
4. The low pressure generates a pulling force that
is transmitted through the water in the xylem
vessels
a. Down through the stems
b. All the way to the roots
I. Transport in the Xylem of plants
5. Transpiration pull
a. Strong enough to move water against the
force of gravity
b. Trees are kind of a big deal…they defy
gravity
6. It’s a passive process
a. The energy needed comes from the thermal
energy that causes the original evaporation
b. The pulling of the water in the xylem
depends on the cohesion between water
molecules
7. cavitation – low pressure in the xylem tubes
causing the liquid column to break
a. Usually doesn’t happen with water
I. Transport in the Xylem of plants
b. Even though water is a liquid, it can transmit
pulling forces the same way a solid length of
rope does
D. Active Transport of mineral ions in the roots
1. Active uptake of mineral ions in the roots
causes absorption of water by osmosis
2. Water is absorbed into root cells by osmosis
3. Solute concentration in root cells is greater
than that in water in the soil
4. The solutes in question are mineral ions
5. Mineral ion concentrations inside the root cells
can be 100X higher than those in the soil
 sounds like a little active transport to me
I. Transport in the Xylem of plants
6. There are separate pumps for each type of ion
needed by the plant
7. Active transport for mineral ions can only
happen if the ions come in contact with the
appropriate membrane protein
8. Mineral ions can also move in through
diffusion
and with mass flow when water carrying
the ions drains through the soil
9. Plant-Fungi relationship
a. Some ions move too slowly
b. Ions are bound to the surface of soil
particles
c. Certain plants have a relationship with a
fungus
I. Transport in the Xylem of plants
d. The fungus grows on the
surface of the roots
e. Thread-like hyphae grow out
into the soil and absorb mineral
ions from the surface of soil
particles
f. The ions are supplied to the
roots
g. Found in many trees,
members of the heather family
and orchids
Heather
Orchid
I. Transport in the Xylem of plants
E. Replacing losses from transpiration
1. Plants transport water from roots to leaves to
replace losses from transpiration
a. Water leaving through the stomata is
replaced by water from the xylem
b. Water in the xylem climbs the stem through
the pull of transpiration
i. Adhesion
ii. Cohesion
c. Water moves from the soil into the roots by
osmosis due to active transport of minerals
into the roots
d. Once water is in the root, it travels through
the xylem through cell walls (apoplast pathway)
and through the cytoplasm (symplast pathway)
II. Transport in the phloem of plants
A. Translocation occurs from source to sink
1. Plants transport organic compounds from
sources to sinks
2. Phloem tissue is found throughout plants
a. Stems
b. Roots
c. Leaves
3. Phloem is composed of sieve tubes
4. Sieve tubes are composed of columns of
specialized cells called sieve tube cells
5. Sieve tube cells are separated by sieve plates
II. Transport in the phloem of plants
6. Translocation – transport of organic
compounds throughout the plant
7. Links parts of the plant that need sugars and
amino acids to parts that have a surplus
8. Sometimes sinks turn into sources and vice
versa
a. Phloem can transport compounds in either
direction
b. Depends on fluid flow because of pressure
gradients
c. Energy is needed to generate the pressures
so it is an active process
Sources
Sinks
Photosynthetic Tissues
Roots that are growing or
absorbing mineral ions using
energy from cell respiration
Mature green leaves
Parts of the plant that are growing
or developing food stores:
Green Stems
Developing fruits
Storage tissues in germinating seeds
Developing seeds
Tap roots or tubers at the start of the growth
season
Growing leaves
Developing tap roots or tubers
II. Transport in the phloem of plants
B. Phloem Loading
1. Active transport is used to load organic
compounds into phloem sieve tubes at the source
2. Sucrose is the most prevalent solute in phloem
sap
3. Sucrose is a disaccharide that can flow through
the phloem without being metabolized in cell
respiration…like if it was glucose
4. Phloem loading
a. Apoplast route: sucrose transport proteins
actively transports sugar in from mesophyll
cells  cell walls of companion cells  sieve
cells
II. Transport in the phloem of plants
b. Concentration gradient of sucrose is
established by active transport
c. Uses ATP as an energy source to push H+ out
of the companion cells from surrounding
tissues
d. The build up of H+ flows down a
concentration gradient through a co-transport
protein
e. The energy released is used to carry sucrose
into the companion cell-sieve tube complex
b. Symplast route: sucrose travels between cells
through connections called plasmodesmata
(singular plasmodesma)
a. Once the sucrose enters the companion cell
II. Transport in the phloem of plants
it is converted to an oligosaccharide
b. Maintains the sucrose concentration
gradient
C. Pressure and water potential differences play a
role in translocation
1. Incompressibility of water allows transport by
hydrostatic pressure gradients
2. The build up of sucrose and other
carbohydrates draws water into the companion
cells through osmosis
3. The rigid cell wall combined with the
incompressibility of water result in a build-up of
pressure
II. Transport in the phloem of plants
4. Water will flow from this area of high pressure
to an area of low pressure
5. At the sink end, sucrose is withdrawn from the
phloem and either utilized as an energy source or
converted to starch
6. In either case, the loss of solute causes a
reduction in osmotic pressure
a. The water that carried the solute to the sink
is then drawn back into the transpiration
stream in the xylem
Stem in cross-section
Leaf in Cross Section
Root in Cross Section
III. Growth in plants
A. Growth in plants
1. Undifferentiated cells in the meristems of
plants allow indeterminate growth
2. Plants have indeterminate growth
a. The cells will continue to divide so long as
conditions are right
b. Many plant cells have the capacity to
generate whole plants (cuttings)
c. Cells are totipotent  sets plants apart
from animals
3. Meristem tissue
a. Composed of undifferentiated cells that are
undergoing active cell division
Shoot apical meristem
Developing flower bud on shoot
apical meristem
III. Growth in plants
b. Apical meristems: a type of primary
meristem found at the tips of stems and roots
i. The root apical meristem is responsible
for the growth of the root
ii. The shoot apical meristem is at the tip of
the stem
c. Lateral meristems: developed by many
dicotyledonous plants
B. Role of mitosis in stem extension and leaf
development
1. Mitosis and cell division in the shoot apex
provide cells needed for extension of the stem
and development of leaves
III. Growth in plants
2. Meristem cells are small and go through the
cell cycle repeatedly to produce more cells
3. Root apical meristem is responsible for the
growth of the root. period. Like, that’s it…roots beget roots
4. Shoot apical meristem is more complex
a. It sends off the cells needed for growth of
the stem
b. Also produces groups of cells that grow and
develop into leaves and flowers
c. With each cell division, one cell remains in
the meristem while the other increases in size
and differentiates as it is pushed away from
the
meristem region
III. Growth in plants
5. Each apical meristem can give rise to additional
meristems
a. Protoderm  gives rise to epidermis
b. Procambium  gives rise to vascular tissue
c. Ground meristem  gives rise to pith
6. Chemical influences also play a large role in
determining which type of specialized tissue
arises from unspecialized plant cells
7. Young leaves are produced at the sides of the
shoot apical meristem – they appear as small
bumps known as leaf primordia
 let’s look at my plant and find some!
III. Growth in plants
C. Plant hormones affect shoot growth
1. Plant hormones control growth in the shoot
apex
2. A hormone is a chemical message that is
produced and released in one part of an organism
to have an effect in another part of an organism
3. Auxins
a. Initiating growth of roots
b. Influencing the development of fruits
c. Regulating leaf development
d. IAA  indole-3-acetic acid is the most
abundant type of auxin and controls growth
of the shoot apex
III. Growth in plants
i. IAA promotes the elongation of cells in
stems
ii. IAA is synthesized in the apical meristem
and is transported down the stem to
stimulated growth
iii. At high concentrations, IAA can inhibit
growth
4. Axillary buds  shoots that form at the
junction or node of the stem and at the base of a
leaf
5. As the shoot meristem grows and forms
leaves, regions of meristem are left behind at the
node
III. Growth in plants
6. Growth at these regions are inhibited by auxin
 termed apical dominance
7. The further distant a node is from the shoot
apical meristem…
a. The lower the concentration of auxin
b. The less likely that growth in the axillary
bud
will be inhibited by by auxin
8. Cytokinins – hormones produced in the root
a. Promote axillary bud growth
b. The ratio of cytokinins and auxins determine
whether the axillary bud will develop
9. Gibberellins – another hormone group that
contribute to stem elongation
III. Growth in plants
D. Plant tropisms
1. Plants respond to the environments by
tropisms
2. Phototropism
a. Growth toward light
b. Auxin accumulates near the shady side of
the stem
c. Causing elongation of the cells on the shady
side
3. Gravitropism
a. Growth in response to gravitational force
E. Auxin influences gene expression
1. Auxin influences cell growth rates by changing
the pattern of gene expression
III. Growth in plants
2. The first stage in phototropism is the
absorption of light by photoreceptors
3. Phototropins have this role
a. They absorb light of a specific wavelength
b. Their conformation changes
c. They bind to receptors within the cell which
control the transcription of specific genes
d. The genes involved likely code for a group
of glycoproteins that transport the auxin from
cell to cell  PIN3 proteins
F. Intracellular pumps
1. Auxin efflux pumps can set up concentration
gradients of auxin in plant tissue
III. Growth in plants
2. The position and type of PIN3 proteins can be
varied to transport auxin to where growth is
needed
3. If phototropins in the tip detect a greater
intensity of light on one side than the other, auxin
will be transported laterally from the side with
the brighter light to the more shaded side
4. Gravitropism is also auxin dependent
a. Upward growth of shoots and downward
growth of roots
b. If a root is placed on its side, gravity causes
cellular organelles called statoliths to
accumulate on the lower side of cells
III. Growth in plants
c. This leads to the distribution of PIN3
transporter proteins that direct auxin
transport to the bottom of the cells
d. High concentration of auxin inhibit root cell
elongation so the top cells elongate at a higher
rate than the bottom cells
e. Causes the root to bend downward
f. The pattern of auxin effect is opposite in the
root as compared to its effect in the shoot
IV. Reproduction in plants
A. Flowering and gene expression
1. Flowering involves a change in gene expression
in the shoot apex
2. Vegetative phase  when a seed germinates,
the young plant grows roots, stems and leaves
a. This can last weeks, months or years
b. A trigger will cause the plant to change into
the reproductive phase
c. Happens when flowers are produced from
meristem instead of leaves
3. Flowers are reproductive organs for a plant
4. Temperature can play a role, but day length is
the main trigger
IV. Reproduction in plants
5. More precisely, the length of the dark period
6. Some plants are categorized as short-day
plants because they flower when the dark period
becomes longer than a critical length
 poinsettias
7. Other plants are long-day plants because they
flower during the long days of early summer
when nights are short
 red clover
8. Light plays a role in the production of either
inhibitors or activators of genes that control
flowering
9. phytochrome pigment
IV. Reproduction in plants
a. Long-day plants will transcribe the genes
that cause flowering when phytochrome is
active (FT gene)
b. The FT mRNA is then transported in the
phloem to the shoot apical meristem
c. There it is translated into FT protein
d. FT protein binds to a transcription factor
e. This leads to the activation of many
flowering genes which transform the leaf
meristem into a reproductive meristem
B. Photoperiods and flowering
1. The switch to flowering is a response to the
length of light and dark periods in many plants
IV. Reproduction in plants
2. Long-day plants flower in summer when the
nights are short
3. Short-day plants flower in the autumn when
the nights have become long enough
4. It’s the length of darkness that matters not the
length of daylight
5. The pigment that measures the length of dark
periods – called phytochrome – and can switch
from one form to another. PR and PFR
a. When PR absorbs red light (660nm) it is
converted to PFR
b. When PFR absorbs far-red light(730nm), it is
converted to PR
IV. Reproduction in plants
i. not of great importance bc sunlight
contains more wavelength of 660nm than
730nm
ii. In normal sunlight, phytochrome is
converted rapidly to PFR
c. However, PR is more stable than PFR, so in
darkness PFR very gradually changes into PR
6. Further experiments have shown that PFR is
the active form of phytochrome
7. receptor proteins are present in the cytoplasm
to which PFR but not Pr binds
a. In long-day plants, large enough amounts of
PFR remain at the end of short nights to bind
the receptor
IV. Reproduction in plants
b. Promotes transcription of genes needed for
flowering
c. In short-day plants, the receptor inhibits the
transcription of the genes needed for
flowering
when PFR binds to it
i. At the end of long nights, very little PFR
remains
ii. Inhibition fails
iii. Plant flowers
C. Mutualism between flowers and pollinators
1. Most flowering plants use mutualistic
relationships with pollinators in sexual
reproduction
Animal Pollinated Flower Diagram
IV. Reproduction in plants
2. Sexual reproduction in flowering plants
depends on the transfer of pollen from the
stamen to the stigma of another plant
3. Pollen is transferred via a number of strategies
a. Wind
b. Water
c. Animals
i. Birds
ii. Bats
iii. Insects  flies, butterflies and bees
4. Mutualism is a close association between two
organisms in which both organisms benefit from
the relationship
IV. Reproduction in plants
a. Pollinators gain food in the form of nectar
b. Plant gains a means of transfer of pollen to
another plant
C. Pollination, fertilization and seed dispersal
1. Success in plant reproduction depends on
pollination, fertilization and seed dispersal
2. Fertilization: after pollination
a. Actual joining of sperm with the egg
b. Each pollen grain on the stigma grows a
tube down the style to the ovary
c. The sperm swim down the tube to fertilize
the eggs
d. Fertilized egg develops into a seed and the
ovary develops into fruit
Bee with pollen sacs – cross pollinating
IV. Reproduction in plants
3. Seed dispersal
a. Seeds cannot move themselves
b. Seeds need to travel long distances away
from their parent plant
c. Reduces competition between offspring
and
parent plant
d. Helps to spread the species
e. Depends on the structure of the fruit
i. dry and explosive
ii. Fleshy and delicious
iii. Feathery or winged
iv. Covered in hooks
IV. Reproduction in plants
3. Seed dispersal
a. Seeds cannot move themselves
b. Seeds need to travel long distances away
from their parent plant
c. Reduces competition between offspring
and
parent plant
d. Helps to spread the species
e. Depends on the structure of the fruit
i. dry and explosive
ii. Fleshy and delicious
iii. Feathery or winged
iv. Covered in hooks
IV. Reproduction in plants
D. The structure of seeds
1. embryo root – becomes the root
2. embryo shoot – becomes the stem and leaves
3. cotyledons
a. Monocotyledon - one
b. Dicotyledon – two
c. Provides food for the seed while it
germinates
4. testa
a. Seed coat
b. Protects the seed
5. micropyle
a. Only part of the seed permeable to water
b. Looks like the seed’s belly button
IV. Reproduction in plants
E. Germination of seeds
1. The early growth of seeds before they sprout
leaves and begin photosynthesis
2. All seeds need water for germination
3. Water stimulates the release of gibberellins
4. Gibberellins stimulate the translation of
enzymes: amylase and maltase
5. Amylase breaks starch in the cotyledon down
to maltose
6. Maltase breaks maltose down into glucose
7. Glucose fuels cell respiration which produces
ATP
 seed can germinate and grow