Transcript Transport in Plants - Zanichelli online per la scuola

David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova
biologia.blu
Anatomia e fisiologia dei viventi S
Transport in Plants
How Do Plants Take Up Water and Solutes?
Terrestrial plants obtain water and mineral nutrients
from the soil.
Water is needed for
photosynthesis, for
transporting solutes, for
cooling the plant, and for
internal pressure for
support.
Plants lose large
quantities of water to
evaporation, which must
be replaced.
How Do Stomata Control the Loss of Water and the Uptake of CO2?
Leaf and stem epidermis has a waxy cuticle to
minimize water loss, but it also prevents gas
exchange.
Stomata, or pores in the leaf epidermis, allow CO2 to
enter by diffusion.
Guard cells control the opening and closing.
How Do Stomata Control the Loss of Water and the Uptake of CO2?
Stomata open and close rapidly
in response to changes in water.
Changes in pressure arise from
changes in K+ concentrations in
the guard cells.
How Are Water and Minerals Transported in the Xylem?
Transpiration: water vapor diffuses from the
intercellular spaces of the leaf to the outside air.
As water evaporates from walls of mesophyll cells,
surface tension of remaining water increases.
The increased tension (negative pressure potential)
draws more water into the walls from the cells.
Cohesion between water molecules in the column
prevents the column from breaking.
The Transpiration–Cohesion–Tension Mechanism
There is a continuous column
of water in the xylem, caused
by cohesion.
How Are Water and Minerals Transported in the Xylem?
The transpiration–cohesion–tension mechanism
accounts for water movement through the xylem.
Xylem vessels are dead and have no cell contents;
fused end to end, they form a long tubular “straw” of
lignified cell walls.
They provide both structural support and the rigidity
needed to maintain a gradient of pressure.
How Are Substances Translocated in the Phloem?
Movement of carbohydrates and other solutes through
the phloem is called translocation.
• Products of photosynthesis are called
photosynthates
• Content of the phloem is called phloem sap
How Are Substances Translocated in the Phloem?
Structure of phloem:
Sieve tube elements
meet end-to-end;
plasmodesmata in end
walls enlarge to form
sieve plates, and most of
the cell contents are lost.
Companion cells are
produced as daughter
cells along with the sieve
tube element when a
parent cell divides.
How Are Substances Translocated in the Phloem?
Photosynthates are translocated from sources to
sinks:
• Sources: organs that produce more sugars than
they require (e.g., leaf or storage root)
• Sinks: consume sugars for growth or storage
A given organ can be a sink at some times and a
source at others.
How Are Substances Translocated in the Phloem?
Phloem sap flows under positive pressure; bulk flow—
from regions of higher pressure potential to lower.
Two steps require energy:
•Transport of solutes from source into the sieve tube—
loading
•Removal of solutes at sinks—unloading
How Are Substances Translocated in the Phloem?
The pressure flow model of translocation:
What Nutrients Do Plants Require?
Mineral nutrients: inorganic elements, obtained
mostly from the soil.
Includes sulfur (S), phosphorus (P), magnesium (Mg),
iron (Fe), and calcium (Ca).
An essential element is one required for the plant to
complete its life cycle, and no other element can
substitute.
• Macronutrients—at least 1 g per kg of dry plant
matter is needed
• Micronutrients—less than 100 mg per kg is needed
Table 36.1
How Do Plants Acquire Nutrients?
Plants cannot change location to find nutrients (they
are sessile).
Except for CO2, plants’ nutrient supply is localized—
how can they find scarce nutrients?
Roots mine the soil for minerals and water as they
grow.
Plants must also deal with variation in micro
environments around them.
How Do Plants Acquire Nutrients?
Plants regulate nutrient uptake.
They have
specialized
transporters for
specific ions.
Genes for ion
transporters are
regulated at the
transcriptional
level by the
amount of
nutrient in cells.
How Does Soil Structure Affect Plants?
Soils contain both living and nonliving components.
The air spaces are important sources of oxygen for
plant roots.
How Does Soil Structure Affect Plants?
Most soils have recognizable layers or horizons, called
the soil profile.
• A horizon or topsoil:
layer from which plants
get nutrients;
• B horizon or subsoil:
accumulates materials
from the topsoil above
and parent rock below;
• C horizon or bedrock: is
in the process of breaking
down to form soil.
How Does Soil Structure Affect Plants?
Soil fertility: ability to support plant growth.
Partly determined by proportions of sand, silt, and
clay.
Sandy soils—mineral nutrients tend to leach out of
the topsoil and are carried to deeper horizons where
they are unavailable to plant roots.
Clay binds water and mineral ions and there is less
leaching; but the particles are tiny and pack tightly—no
air spaces.
Loam has an optimal mix of sand, silt, and clay, and
also organic matter. Best for agriculture.
How Does Soil Structure Affect Plants?
Soil organic matter or humus is used as a food source
by microbes that break down complex organic
molecules and release simpler molecules into the soil
solution.
Humus also provides air spaces that increase O2
availability to plant roots.
How Does Soil Structure Affect Plants?
Cation exchange.
Cation exchange capacity affects soil fertility.
How Do Fungi and Bacteria Increase Nutrient Uptake by Plant Roots?
Mycorrhizae are associations of
fungi with plant roots; occur in more
than 90% of terrestrial plants.
Some plants form associations with
nitrogen-fixing bacteria.
Legume plants form symbioses with
bacteria Rhizobium.
The nutrient most often in short
supply is nitrogen while the main
nutrient the plant gets is phosphorus.
How Do Carnivorous and Parasitic Plants Obtain a Balanced Diet?
Carnivorous plants
augment their nutrient
supply by capturing and
digesting insects.
Some plants are parasitic—
getting water, mineral
nutrients, or photosynthate
from the bodies of other
plants.
What Determines the Transition from the Vegetative to the Flowering
State?
Plants fall into three categories in terms of maturation
and flowering:
• Annuals complete life cycle in one year and have
little or no secondary growth
• Biennials take two years to complete the life cycle
Produce vegetative growth the first year and store
carbohydrates in underground roots (carrots) or
stems (onion).
• Perennials live three or more years
How Do Angiosperms Reproduce Sexually?
Most angiosperms reproduce sexually—producing the
genetic diversity that is the raw material for evolution.
It involves mitosis,
meiosis, and alternation of
haploid and diploid
generations.
How Do Angiosperms Reproduce Sexually?
The haploid gametophytes develop from haploid
spores in the flower:
Megagametophytes (female) are called embryo sacs;
develop in the ovules.
Microgametophytes (male) are called pollen grains;
develop in anthers on the stamens.
How Do Angiosperms Reproduce Sexually?
Microsporocytes undergo meiosis to produce 4
haploid microspores.
Each develops a cell wall and divides mitotically to
form 2 haploid cells in each pollen grain: the tube cell
forms the pollen tube that delivers the sperm to the
embryo sac.
After pollination the generative cell divides by mitosis
to form 2 sperm cells.
How Do Angiosperms Reproduce Sexually?
A megasporocyte undergoes meiosis to produce 4
haploid megaspores; 3 undergo apoptosis.
The surviving megaspore undergoes 3 mitotic
divisions with no cytokinesis to produce 8 haploid
nuclei. Cell wall formation leads to a gametophyte
(embryo sac) with 7 cells and 8 nuclei.
How Do Angiosperms Reproduce Sexually?
At one end of the gametophyte are 3 cells—egg and 2
synergids.
Synergids attract the pollen tube and receive the
sperm nuclei.
Three antipodal cells at the opposite end usually
degenerate.
The central cell has 2 polar nuclei.
Sexual Reproduction in Angiosperms (Part 1)
Sexual Reproduction in Angiosperms (Part 2)
How Do Angiosperms Reproduce Sexually?
Pollination: transfer of pollen from anther to stigma.
The evolution of pollen made it possible for male
gametes to reach the female gametophyte without
water.
This selective advantage allowed pollen-bearing
plants to colonize land.
In many species, wind transports
pollen.
Most angiosperms rely on animals
(insects, birds, and bats) to transfer
pollen.
How Do Angiosperms Reproduce Sexually?
Germination of the pollen grain involves uptake of
water from the stigma and growth of the pollen tube.
The pollen tube grows through the style to reach the
ovule.
Downward growth is guided
by a chemical signal
released by the synergids
in the ovule.
How Do Angiosperms Reproduce Sexually?
Double fertilization:
• One sperm cell fuses with the egg cell, forming a
diploid zygote
• The other sperm cell fuses with the two polar nuclei
in the central cell, forming a triploid (3n) cell
What Determines the Transition from the Vegetative to the Flowering
State?
Plants initiate flowering based on cues such as
photoperiod (day length) and temperature.
Short-day plants (SDPs) flower when the day is
shorter than a critical maximum.
Long-day plants (LDPs) flower when the day is
longer than a critical maximum.
Day Length and Flowering
What Determines the Transition from the Vegetative to the Flowering
State?
Photoperiodic plants actually measure length of night,
not day.
Experiments with cocklebur,
(SDP):
Day length was varied in
one group, night length in
another. The critical night
length was 9 hours.
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved