Transcript Table of Contents - Milan Area Schools

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Uptake and Movement of Water and Solutes
• Water enters the plant through osmosis, but the
uptake of minerals requires transport proteins
 Why does water move into the root from the soil?

Moves into due to water potential difference.
Figure 36.1 The Pathways of Water and Solutes in the Plant
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Uptake and Movement of Water and Solutes
• The force exerted by the rigid cell wall as water
enters is called the pressure potential, or turgor
pressure.
• Water enters a plant cell until the pressure
potential exactly balances the solute potential.
The cell is then called turgid.
 Keeps plant cells firm and plants don’t wilt.
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Uptake and Movement of Water and Solutes
• Mineral ions generally require transport proteins in
order to cross membranes.
 Can be passive or active.

Proton pump helps get ions in by facilitated
diffusion.
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Transport of Water and Minerals in the Xylem
• In the xylem, water and minerals constitute the
xylem sap.
 What causes the xylem sap to move upward?
 transpiration–cohesion–tension
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Transport of Water and Minerals in the Xylem
• The transpiration–cohesion–tension mechanism:
• The concentration of water vapor is higher inside the
leaf than outside, so water diffuses out of the leaf
through the stomata. This process is called
transpiration.
• This creates a tension that draws water from the
xylem.
• The removal of water from the veins, in turn,
establishes tension on the entire volume of water in
the xylem, so the column is drawn up from the roots.
Figure 36.8 The Transpiration–Cohesion–Tension Mechanism
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Transport of Water and Minerals in the Xylem
• Mineral ions in the xylem sap rise passively with
the solution.
• Transpiration also contributes to the plant’s
temperature regulation, cooling plants in hot
environments.
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Transpiration and the Stomata
Guard cells control the opening and closing of the
stomata.
• Most plants open their stomata only when the light
is intense enough to maintain photosynthesis.
• Stomata also close if too much water is being lost.
Figure 36.11 Stomata (Part 1)
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Transpiration and the Stomata
• Opening closing and of the stomata are regulated
by controlling K+ concentrations in the guard cells.
 Light activates a proton pump => K moves
inside.
 Water moves inside to lower water potential.
 Guard cells open due to shape change.
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Translocation of Substances in the Phloem
• Sugars, amino acids, some minerals, and other
solutes are transported in phloem and move from
sources to sinks.
• Transport often proceeds in both directions— both
up and down the stem simultaneously.
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Translocation of Substances in the Phloem
• Sieve tube cells at the source have a greater
sucrose concentration that surrounding cells, so
water enters by osmosis. This causes greater
pressure potential at the source, so that the sap
moves by bulk flow towards the sink.
• At the sink, sucrose is unloaded by active
transport, maintaining the solute and water
potential gradients.
• This is called the pressure flow model.
Figure 36.14 The Pressure Flow Model
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36
The Acquisition of Nutrients
• Plants are sessile organisms. Nutrients and
energy must be brought to them in some way.
• A plant can extend itself by growing. The roots
obtain most of the mineral nutrients needed.
• The essential elements for plants were identified
by growing plants hydroponically, or without soil.
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Table 37.1 Mineral Elements Required by Plants (Part 1)
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Table 37.1 Mineral Elements Required by Plants (Part 2)
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Soils and Plants
• Soils are complex mixtures of living and nonliving
components, including bacteria, fungi,
earthworms and other animals, particles of rock,
clay, water, dissolved minerals, air spaces, and
dead organic matter.
• The type of soil in a given area depends on the
type of rock from which it forms and how it is
broken down.
• Rocks are broken down by mechanical
weathering, or physical breakdown; and
chemical weathering.
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Soils and Plants
• The availability of nutrient ions is influenced by
soil pH. pH 6.5 is optimal for most crops.
• In the process of liming, compounds such as
calcium carbonate, calcium hydroxide, or
magnesium carbonate are added to acidic soil to
raise the pH.
• The pH of soil can be lowered by adding sulfur,
which soil bacteria convert to sulfuric acid.
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Nitrogen Fixation
• Most nitrogen fixation is done by bacteria.
• Cyanobacteria are the principle nitrogen fixers in
aquatic ecosystems.
• Some nitrogen fixers live in close association with
plant roots in a mutualistic relationship.
 Rhizobium fix nitrogen only in close
association with the roots of legumes.
 These bacteria infect plant roots, causing the
roots to develop nodules
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Figure 37.5 Root Nodules
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Nitrogen Fixation
• Three things are required for fixation:
 A strong reducing agent to transfer the
hydrogen atoms to N2.
 Energy, supplied by ATP.
 The enzyme nitrogenase.
 Nitrogenase is strongly inhibited by O2.
 Many nitrogen fixers are anaerobes. But
others, such as Rhizobium, are not.
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Nitrogen Fixation
• Another type of symbiosis in which plants depend
on another organism for their nutrition is that of
mycorrhizae, the root-fungus association.
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Nitrogen Fixation
• The nitrogen cycle includes the process of
nitrogen fixation, nitrification, nitrate reduction,
and denitrification.
• Soil bacteria called nitrifiers oxidize NH3 to nitrite
(NO2–) and nitrate ions (NO3–) in a process called
nitrification.
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Nitrogen Fixation
• Nitrate reduction is carried out by plants using
their own enzymes, and reduces nitrate back to
ammonia. The ammonia is used to produce amino
acids.
• Animals can not reduce nitrate, and depend on
plants for reduced nitrogen compounds.
• Bacteria called denitrifiers return the nitrogen from
animal wastes and dead organisms back to N2
gas in a process called denitrification.
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Figure 37.8 The Nitrogen Cycle
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Carnivorous and Heterotrophic Plants
• Some plants that grow in acidic, nitrogen-poor
environments trap and digest insects to help
augment nitrogen and phosphorus supplies.
• These carnivorous plants include sundews, Venus
flytraps, and pitcher plants.
• These plants have adaptations to capture small
animals and digest the proteins.
• Carnivorous plants can survive without feeding on
insects, but they grow much faster in their natural
habitats when they succeed in capturing insects.
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Figure 37.9 Carnivorous Plants