Transcript ecology and evolution

Mr. Lajos Papp
The British International School, Budapest
2014/2015
Plant structure
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/
P/PlantTissues.html
Stem: epidermis, cortex, vascular bundle (xylem,
cambium, phloem), pith.
Epidermis: surface of the stem made of a number of
layers often with a waxy cuticle to reduce water loss.
Cortex: forming a cylinder of tissue around the outer
edge of the stem. Often contains cells with secondary
thickening in the cell walls which provides additional
support.
Vascular
bundle:
cambium tissue.
contains
xylem,
phloem
and
Xylem: a longitudinal set of tubes that conduct water
from the roots upward through the stem to the leaves.
Phloem: transports sap through the plant tissue in a
number of possible directions.
Cambium: produces the secondary xylem and phloem
through cell division in the vertical plane.
Pith: composed of thin walled cells.
Leaf: upper epidermis, palisade mesophyll, xylem,
phloem, spongy mesophyll, stoma, guard cells, lower
epidermis.
Upper epidermis: a flattened layer of cell that forms
the surface of the leaf and makes the cuticle.
Palisade mesophyll: the main photosynthetic region of
the leaf.
Vascular bundle: contains the transport system (xylem,
phloem) and vascular meristem tissue (vascular
cambium).
Spongy mesophyll: contains spaces that allow the
movement of gases and water through the leaf tissue.
Lower epidermis: bottom surface layer of tissues
which contains the guard cells that form each stoma.
9.1 Transport in the xylem of plants
http://www.biologymad.com/PlantTransport/PlantTrans
port.htm
The loss of water vapour from the leaves and stems of
plants is called transpiration.
Transpiration is the inevitable consequence of gas
exchange in the leaf.
Leaf structure: stomata, guard cells.
Gas exchange: oxygen, carbon dioxide.
Plants transport water from the roots to the leaves to
replace losses from transpiration.
Roots: osmosis due to the active transport of minerals.
Water travels through cell walls and cytoplasm to the
xylem.
Xylem: pull of transpiration, forces of adhesion and
cohesion.
The cohesive property of water and the structure of the
xylem vessels allow transport under tension.
Cohesion: hydrogen bond between water molecules.
Xylem vessel: nonliving, thickened walls, rigid
structure, withstand low pressure.
The adhesive property of water and evaporation
generate tension forces in leaf cell walls.
Transpiration-pull is strong enough to move water
upwards, against gravity. The pulling of water upwards
in xylem vessels depends on the cohesion that exists
between water molecules.
Active uptake of mineral ions in the roots causes
absorption of water by osmosis.
The concentrations of mineral ions in the root are higher
than those in the soil.
Active transport, protein pumps.
Applications
Adaptations of plants in deserts and in saline soils for
water conservation.
Xerophytes.
Xerophytes are plants that are able to live in dry environments.
reduced leaves: reducing the leaf surfaces reduces the amount
of water loss.
a thickened waxy cuticle will reduce water loss through the
cuticle even further.
a reduced number of stomata will reduce water loss but also
the amount of gaseous exchange and hence the amount of
photosynthesis.
stomata may be found in pits and surrounded by
"hairs” Both of these measures will reduce the air flow
past the pore. Any water vapour that has diffused out
through the pore will therefore stay near it and reduce
the concentration gradient.
having deep roots may allow the plant to reach water
deep in the soil. Some plants have extensive superficial
root systems to take maximum advantage of (rare)
rainfall and collect water where soil surface layers /
areas near the plant are dry.
water storage tissue, such as found in the stems of
cacti, will help the plant with survival through a long
dry period.
being a small plant, growing near the ground will also
reduce water loss (effects of wind are avoided).
some plants will germinate, grow and flower in the
wet season, producing seeds before the beginning of the
dry season. The plant will die (reduced life cycle) but
the seeds will survive (dry periods in a resistant form)
and start growing in the next wet season.
Applications
Models of water transport in xylem using simple
apparatus including blotting or filter paper, porous pots
and capillary tubing.
Skills
Drawing the structure of primary xylem vessels in
sections of stems based on microscope images.
Skills
Measurement of transpiration rates using potometers.
Skills
Design an experiment to test hypothesis about the effect
of temperature or humidity on transpiration rates.
https://www.youtube.com/watch?v=oUr1P9RZnEU
9.2 Transport in the phloem of plants
Plants transport organic compounds from sources to
sinks.
The transport of organic solutes in a plant is called
translocation.
sources
sinks
Photosynthetic tissues:
Roots
• mature green leaves
absorbing mineral ions using
• green stems
Storage
organs
are
growing
or
energy from cell respiration.
that
are
unloading their stores:
• storage tissues in germinating seeds
• tap roots or tubers at the start of the
growth season
that
Parts of the plant that are
growing
or
developing
stores:
• developing fruits
• developing seeds
• growing leaves
• developing tap roots or tubers
food
Active transport is used to load organic compounds into
phloem sieve tubes at the source.
Apoplast route: through cell walls, transport protein.
Symplast route: through plasmodesmata.
Incompressibility of water allows transport along
hydrostatic pressure gradients.
High concentrations of solutes in the phloem at the
source lead to water uptake by osmosis.
Raised hydrostatic pressure causes the contents of the
phloem to flow towards sinks.
Low pressure near the sink. Sucrose withdrawal.
Water flows from high to low.
Application
Structure-function relationship of phloem sieve tubes.
Functions:
loading,
transport,
unloading
carbohydrates.
Structure of phloem: sieve tubes, tube cells.
of
Companion cells carry out metabolic functions to keep
the sieve tube cells alive.
Active transport of sucrose: mitochondria.
Infolding in the companion cell: loading capacity,
apoplastic route.
Plasmodesmata: movement of materials between cells.
Active transport proteins.
Rigid cell wall of the sieve elements.
Perforated cell walls between sieve elements.
Skills
Identification of xylem and phloem in microscope
images of stem and root.
Xylem cells are generally larger.
Phloem cells tend to be closer to the outside.
Skills
Analysis of data from experiments measuring phloem
transport rates using aphid stylets and radioactively-
labelled carbon dioxide. Worksheets.
9.3 Growth in plants
Undifferentiated cells in the meristem of plants allow
indeterminate growth. Cells continue to divide
indefinitely.
Meristems are composed of undifferentiated cells.
Primary meristems: tips of stems and roots (apical
meristems).
Many dicotyledenous plants develop lateral meristems.
Mitosis and cell division in the shoot apex provide cells
needed for extension of the stem and development of
leaves.
Shoot apical meristem: growth of the stem, cells grow
and develop into leaves and flowers.
Plant hormones control growth in the shoot apex.
Auxin is synthesized in the apical meristem of the shoot
and is transported down the stem. It promotes the
elongation of cells in stem.
Plant shoots respond to the environment by tropism.
Directional growth responses to directional external
stimuli are called tropism.
Phototropism.
Gravitropism.
Auxin efflux pumps can set up concentration gradients
of auxin in plant tissue.
In the shoot, auxin promotes elongation but in the root
auxin inhibits shoot elongation.
Auxin influences cell growth rates by changing the
pattern of gene expression.
Phototropism is an adaptation response, through
which plants grow towards the light. It involves light
perception and asymmetric distribution of the plant
hormone auxin.
Light perception initiates auxin redistribution that leads
to directional growth. Light polarizes the cellular
localization of the auxin efflux carrier PIN3 proteins,
resulting
in
changes
differential growth.
in
auxin
distribution
and
Application
Micropropagation of plants using tissue from the shoot
apex, nutrient agar gels and growth hormones.
Micropropagation produces large numbers of identical
plants.
Sterilization, cut into pieces (explants).
Growth media with plant hormones. Auxin, cytokinin.
Cytokinin: promotes cell division in roots, shoots.
Ratio.
Roots and shoots are developed, transferred to soil.
Application
Use of micropropagation for rapid bulking up of new
varieties, production of virus-free strains of existing
varieties and propagation of orchids and other rare
species.
9.4 Reproduction in plants
Flowering involves a change in gene expression in
shoot apex.
Vegetative phase, reproductive phase.
Flowers are produced by the shoot apical meristem.
The length of the dark period is the main trigger.
Long-day plants, short dark period.
Short-day plants, long dark period.
The switch to flowering is a response to the length of
light and dark periods in many plants.
Flowering Cues
Plants have to coordinate the production of flowers to
coincide with the best reproductive opportunities. The
photoperiod is the most reliable indicator on 'time' of
year.
The photoperiod: the period of day light in relation to
dark (night). In northerly and southern regions this
photoperiod is a reliable indication of the time of year
and therefore one of the most reliable indicators of the
seasonal changes.
Short and Long day Plants
Short day plants (SDP) typically flower in the spring or
autumn when the length of day is short.
Long day plants (LDP) typically flower during the
summer months of longer photoperiod.
Critical Night Length
The important factor determining flowering is the length
of night rather than the length of day.
SDP have a critical long night. The length of night has
to exceed a particular length before there will be
flowering.
LDP have a critical short night. The length of night must
be shorter than a critical length before there will be
flowering.
Phytochrome System
The receptor of photoperiod is located within the leaf. The
chemical
nature
of
the
receptor
is
the
molecule
phytochrome. Phytochrome can be converted from one
form to another by different types of light.
Flowering in LDP
Long day plants flower when the night period is short.
In day light the Pr is converted to PFR. During periods
when the day light period is long but critically the dark
period is short, PFR does not have long to breakdown in
the dark.
Consequently there remains a higher concentration of
PFR. In LDP, high PFR concentration is the trigger for
flowering.
Long-day plants
Active phytochrome PFR → binds to receptor protein →
transcription of FT gene → FT mRNA to shoot apical
meristem → FT protein → binds to transcription factor →
activation of many flowering genes → leaf-producing apical
meristem turns to a reproductive meristem
Flowering in SDP
Short day plants flower when the night period is long. In
day light, Phytochrome Red (PR) is converted to
Phytochrome Far Red (PFR). The conversion requires a
brief exposure to white or red light. In the dark, PFR is
slowly converted back to PR.
A long night means that there is a long time for the
conversion. Under short day conditions at the end of the
night period the concentration of PFR is low. In SDP,
low PFR concentration is the trigger for flowering.
Short-day plants
Active phytochrome PFR bound to receptor protein →
inhibition of transcription of FT gene
After a long night → little PFR remains → inhibition fails →
activation of many flowering genes → leaf-producing apical
meristem turns to a reproductive meristem
Skills
Drawing of half-views of animal-pollinated flowers.
Success in plant reproduction depends on pollination,
fertilization and seed dispersal.
Pollination: is the transfer of pollen from an anther to a
stigma.
Fertilization: a zygote is formed by the fusion of a male
gamete (in the pollen grain) with a female gamete inside the
ovule.
Seed dispersal: the seeds provide the plants with a way
to spread out and grow in new places. This is important
because if the seeds are not dispersed, many
germinating seedlings will grow very close to the parent
plant. This results in competition between the seedlings
as well as with the parent plant.
Most flowering plants use mutualistic relationships with
pollinators in sexual reproduction.
Pollinators including: birds, bats and insects.
Mutualism: both organisms benefit from the relationship.
Pollinators: gain food (nectar).
Plant: gains a means to transfer pollen.
Application
Methods used to induce short-day plants to flower out
of season.
Growers can manipulate the length of the days and
nights to force flowering.
Skills
Drawing internal structure of seeds.
a) Testa protects the plant embryo and the cotyledon
food stores.
b) Radicle is the embryonic root.
c) Plumule is the embryonic stem.
c) Plumule is the embryonic stem.
d) Cotyledons contain food store for the seed.
e) Micropyle is a hole in the testa through which water
can enter the seed prior to germination.
Skills
Design of experiments to test hypotheses about factors
affecting germination.
Every seed needs a combination of oxygen for aerobic
respiration, water to metabolically activate the cells,
temperature for optimal function of enzymes.