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Available water is soil water between field capacity and the permanent
wilting point. Water molecules having very slight positive charges at one
end and very slight negative charges at the other end. Such molecules are
said to be polar.
Molecules and ions are in constant random motion and tend to
distribute themselves evenly in the space available to them. They move
from a region of higher concentration to a region of lower concentration
by simple diffusion along a diffusion gradient. Evenly distributed
molecules are in a state of equilibrium.
Osmosis is the diffusion of water through a semipermeable membrane.
It takes place in response to concentration differences of dissolved
substances.
Osmotic pressure or potential is the pressure required to prevent
osmosis from taking place. The pressure that develops in a cell as a result
of water entering it is called turgor. Water moves from a region of higher
water potential (osmotic potential and pressure potential combined) to a
region of lower water potential when osmosis is occurring.
Simple diffusion. A. A barrier separates two kinds of molecules. B. When the
barrier is removed, random movement of individual molecules results in both
kinds moving from a region of higher concentration to a region of lower
concentration. C. Eventually, equilibrium (even distribution) is reached. The rate
of diffusion gradually slows down as equilibrium is approached.
A. A turgid cell. Water has entered the cell by osmosis, and turgor pressure is
pushing the cell contents against the cell walls. B. Water has left the cell, and
turgor pressure has dropped, leaving the cell flaccid. The vacuole has disappeared.
×200.
A simple osmometer, made by tying a differentially permeable membrane over the
mouth of a thistle tube.
Plasmolysis is the shrinkage of the cytoplasm away from the cell
wall as a result of osmosis taking place when the water potential inside
the cell is greater than outside.
Imbibition is the attraction and adhesion of water molecules to the
internal surfaces of materials; it results in swelling and is the initial
step in the germination of seeds.
Active transport is the expenditure of energy by a cell that results
in molecules or ions entering or leaving the cell against a diffusion
gradient. Recent evidence suggests that this process involves an
enzyme complex and what has been referred to as a proton pump. The
pump involves the plasma membrane of plant and sodium ions.
The cohesion-tension theory postulates that water rises through
plants because of the adhesion of water molecules to the walls of the
capillary-conducting elements of the xylem, cohesion of the water
molecules, and tension on the water columns created by the pull
developed by transpiration.
Water that enters a plant passes through xylem and mostly
transpires into the atmosphere via stomata. Water retained by the
plant is used in photosynthesis and other metabolic activities.
A portion of a leaf of the water weed Elodea. A. Normal cells. B.
Plasmolyzed cells. ×100.
Black-eyed pea seeds before and after imbibition of water.
Pathway of water through a plant.
Capillarity in narrow tubes. The smaller the diameter of the tube, the greater the
rise of the fluid.
The translocation of food substances takes place in a water
solution, and according to the pressure-flow hypothesis, such
substances flow along concentration gradients between their sources
and sinks.
At present, the most widely accepted theory for movement of
substances in the phloem is called the pressure-flow (bulk or mass-
flow) hypothesis. According to this theory, food substances in
solution (organic solutes) flow from a source, where water enters by
osmosis (e.g., a food-storage tissue, such as the cortex of a root or
rhizome, or a food-producing tissue, such as the mesophyll tissue of a
leaf). The water exits at a sink, which is a place where food is
utilized, such as the growing tip of a stem or root. Food substances in
solution (organic solutes) are moved along concentration gradients
between sources and sinks.
One of the most important functions of water in the plant involves
the translocation (transportation) of food substances in solution by
the phloem, a process that has only recently come to be better
understood.
The pressure-flow hypothesis.
An aphid feeding on a young stem of basswood (Tilia). A droplet of “honeydew” is
emerging from the rear of the aphid. ×10.
Many of the studies that led to our present knowledge of the
subject used aphids (small, sucking insects) and organic compounds
designed as radioactive tracers. Most aphids feed on phloem by
inserting their tiny, tubelike mouthparts (stylets) through the leaf or
stem tissues until a sieve tube is reached and punctured.
Transpiration is regulated by humidity and the stomata, which
open and close through changes in turgor pressure of the guard cells.
These changes, which involve potassium ions, result from osmosis
and active transport between the guard cells and the adjacent
epidermal cells (subsidiary cells). Humidity plays an inverse but
direct role in transpiration rates: high humidity reduces
transpiration, and low humidity accelerates it.
In the absence of transpiration at night, the pressure in the xylem
elements builds to the point of forcing liquid water out of the
hydathodes in the leaves. Dew is water that is condensed from the
air. Guttation is the loss of water at night in liquid form through
hydathodes at the tips of leaf veins.
A. A small portion of the leaf epidermis of Wandering Jew (Zebrina sp.) with
several stomata interspersed among ordinary epidermal cells. Each stoma is
bordered by a pair of guard cells, and each guard cell is flanked to the outside by a
small epidermal cell called a subsidiary cell. ×100.
B. Left. An open stoma. The guard cells swell when turgor pressure in
them increases and the stoma opens as the thinner outer walls stretch more
than the thicker inner walls. Right. The stoma closes when the turgor
pressure in the guard cells decreases. ×400
Hydathode structure at the tip of a leaf vein through which water is
forced by root pressures. Root pressure forces liquid water out of
hydathodes, usually at night when transpiration is not occurring.
Furthermore, root pressure seems to drop to negligible amounts in
the summer, when the greatest amounts of water are moving through
the plant.
Droplets of guttation water at the tips of leaves of young barley plants.
Elements essential as building blocks for compounds synthesized by plants.
Growth phenomena are controlled by both internal and external means
and by chemical and physical forces in balance with one another. Besides
carbon, hydrogen, and oxygen, 15 other elements are essential to most
plants. When any of the essential elements are deficient in the plant,
characteristic deficiency symptoms appear.
The mineral elements are usually put into two categories: (1)
macronutrients, which are used by plants in greater amounts and
constitute from 0.5% to 3.0% of the dry weight of the plant; and (2)
micronutrients, which are needed by the plant in very small amounts, often
constituting only a few parts per million of the dry weight. The
macronutrients are carbon, hydrogen, oxygen, nitrogen, potassium,
calcium, phosphorus, magnesium, and sulfur, with the first four usually
making up about 99% of the nutrient total. Those elements remaining, the
micronutrients, are present in amounts ranging from bare traces.
Deficiency symptoms of Nitrogen are relatively uniform loss
of color in leaves (green to yellow), occurring first on the
oldest ones. Nitrogen is part of proteins, nucleic acids and
chlorophylls.
N
METABOLISM
Anabolism = building reactions
(Photosynthesis, Citric acid cycle,etc,)
Catabolism = breaking down compounds
into simpler compounds
molecules or atoms.(Respiration,etc)
sunlight, waterCO2
Fructose(carbohydrates)
Thylakoid membrane
Chloroplast stroma
oxygen
Light reactions
Dark reactions
ATP, NADPH
Fig. 10.2a
Enzymes catalyze reactions of metabolism. Many of these include
oxidation-reduction reactions. Oxidation is loss of electrons;
reduction is gain of electrons.
Photosynthesis is an anabolic process that combines carbon
dioxide and water in the presence of light with the aid of chlorophyll;
oxygen is a by-product. All life depends on photosynthesis, which
takes place in chloroplasts.
Carbon dioxide constitutes 0.038% of the atmosphere, but the
percentage has been rising in recent years. Increased carbon dioxide
levels have potential to elevate global temperatures through the
“greenhouse effect.”
Chlorophyll b and carotenoids (accessory pigments) are antenna
pigments that direct light energy to chlorophyll a. Photosynthetic
units containing chlorophylls and accessory pigments absorb units of
light energy, become excited, and pass this energy to acceptors
during the light-dependent reactions of photosynthesis.
Visible light that is passed through a prism is broken up into individual colors with
wavelengths ranging from 390 nanometers (violet) to 780 nanometers (red).
The structure of a molecule of chlorophyll a (Essential Pigment), the most
important of the pigments involved in photosynthesis. The boxlike ring
structure on the left, with magnesium and nitrogen inside, functions in
capturing light energy. The tail, which extends into the interior of a
thylakoid membrane, is insoluble in water; all chlorophyll molecules are,
however, fat soluble.
The absorption spectra of chlorophyll a, chlorophyll b, and a carotenoid. The
maximum absorption of the chlorophylls is in the blue and red wavelengths. The
maximum absorption of the carotenoids is in the blue-green to green parts of the
visible spectrum.
Less than 1% of all the water absorbed by plants is used in
photosynthesis; most of the remainder is transpired or incorporated
into cytoplasm, vacuoles, and other materials.
During the light-dependent reactions of photosynthesis, which occur
in thylakoid membranes of chloroplasts, water molecules are split, and
oxygen gas is released. Hydrogen ions and electrons are released from
water and transferred to produce NADPH and ATP.
The two types of photosynthetic units present in most chloroplasts are
photosystems I and II (PSI &PSII). The PSI present in stroma lamella and
PSII in granum. The events that take place in photosystem II come before
those of photosystem I. Each photosystem has a reaction-center molecule of
chlorophyll a (the most abundant pigment) that boosts electrons to a
higher energy level when it is excited by light energy.
Photosystem II boosts electron excitation to a level that, when it
encounters photosystem I, has the potential to reduce NADP to NADPH
through noncyclic electron flow. Photosystem I, by itself, can cycle
electrons for generation of ATP. Electron transport while the photosystems
are operating and proton movement across thylakoid membranes are both
involved in ATP production.
A simplified summary of photosynthetic reactions.
Stroma lamella
Grana thylakoid
The light-dependent reactions of photosynthesis, which occur
in more than one way. In noncyclic photophosphorylation,
involving photosystems I and II, which convert light energy to
biochemical energy in the form of ATP and NADPH, water
molecules are split, releasing electrons, protons, and oxygen
gas. The electrons are subsequently used to produce NADPH,
whereas the protons are used, in part, to enable production of
ATP. Oxygen gas is a by-product of this noncyclic
photophosphorylation, although aerobic organisms rely upon
this gas for respiration. The ATP and NADPH are used in the
carbon-fixing reactions that convert CO2 to sugars.
Only photosystem I is involved in cyclic photophosphorylation.
In this relatively simple system, electrons boosted from a
photosystem I reaction-center molecule are shunted back into
the reaction center via the electron transport system. ATP is
produced from ADP, but no NADPH or oxygen is produced.
Since rubisco catalyzes formation of the 3-carbon compound 3PGA as the
first isolated product in these light-independent reactions, plants
demonstrating this process are called C3 plants (3-carbon pathway).
The light-independent reactions occur through a series of reactions known
as the Calvin cycle, which takes place in the stroma of chloroplasts. In the first
step, carbon dioxide is combined with RuBP through catalytic action of the
enzyme rubisco to form two molecules of the 3-carbon compound, GA3P. The
ATP and NADPH from the light-dependent reactions furnish energy to
eventually convert GA3P to 6-carbon carbohydrates. This cycle also
regenerates RuBP to enable continued carbon fixation.
In the light-independent reactions of C4 plants (tropical grasses or arid
region plants), 4-carbon oxaloacetic acid is initially produced instead of 3-
carbon PGA. In the leaf mesophyll of C4 plants (4-carbon pathway), there are
large chloroplasts, which contain rubisco in the bundle sheaths, and small
chloroplasts in the mesophyll, which contain higher concentrations of PEP
carboxylase. C4 plants that facilitate the conversion of carbon dioxide to
carbohydrate at much lower concentrations than is possible in C3 plants.
CAM photosynthesis occurs in cacti and succulent plants whose stomata
are closed and admit little CO2 during the day. Regular photosynthesis occurs
as the 4-carbon compounds that accumulate at night are converted back to
carbon dioxide during the day.
The Calvin cycle
(light-independent
reactions) of
photosynthesis. The
cycle takes place in
the stroma of
chloroplasts,
where each step is
controlled by a
different kind of
enzyme. Carbon
dioxide molecules
from the air enter
the cycle one at a
time, making six
turns of the cycle
necessary to
produce one
molecule of a 6-
carbon sugar such
as Fructose
Organization of the thylakoid membrane showing the relative positions of photosystems
and protein complexes. Some hydrogen ions (protons) from the stroma are pumped into the
thylakoid space (lumen), producing a hydrogen gradient. ATP is produced when these
hydrogen ions and those from water flow from the thylakoid space into the stroma through
the ATP synthase complex.
A portion of a cross section of a leaf of corn (Zea mays), a C4 plant with Kranz anatomy leaves.
An illustration of the C4 photosynthesis pathway. Carbon dioxide is converted to
organic acids in mesophyll cells. After the acids move into bundle sheath cells, some
carbon dioxide is released and enters the Calvin cycle, where it becomes a 3-carbon
compound that moves back to a mesophyll cell; there it is converted to PEP, which
accepts carbon dioxide from the air.
CAM Plants
Cytosol
Chloroplast
However, as indicated by its name, the enzyme RuBP
carboxylase/oxygenase has the potential to fix both CO2, through its
carboxylase activity described by the Calvin cycle, and O2, through its
oxygenase activity. The oxygenase activity of rubisco enables C3 plants to
undergo a process called photorespiration.
Photorespiration requires cooperation among chloroplasts, peroxisomes,
and mitochondria to facilitate shuttling of intermediates along the
photorespiratory pathway. The products of photorespiration are the 2-carbon
phosphoglycolic acid, which is processed to some extent in peroxisomes and
eventually released as carbon dioxide in mitochondria, and the 3-carbon
phosphoglyceric acid that can reenter the Calvin cycle. No ATP is produced by
photorespiration.
Light that is too intense may change the way in which some of a cell’s
metabolism takes place. For example, higher light intensities and temperatures
may change the ratio of carbon dioxide to oxygen in the interiors of leaves,
which, in turn, may accelerate photorespiration.
Photorespiration is typically considered to be a wasteful process that uses
oxygen and releases carbon dioxide, although it may help some plants to
survive under adverse conditions. It differs from common aerobic respiration
in its chemical pathways.
Photooxidation, which involves the destruction (“bleaching”) of chlorophyll
by light, may also occur.
Effects of light on two forms of photosynthesis. Both forms of photosynthesis,
known as C3 and C4, respectively. A. In C3, the rate of photosynthesis will not
increase beyond a certain intensity of light. In C4 plants, when additional
carbon dioxide is available, photosynthetic rates undergo up to a 30% increase
in light intensity.
Effects of temperature on two forms of photosynthesis. Both forms of
photosynthesis, known as C3 and C4, respectively. B. In C3 plants, quantum yield
of photosynthesis decreases as temperatures increase, whereas in C4 plants, the
quantum yield of photosynthesis is not significantly affected by temperature
fluctuations between 10°C and 40°C.