Available water is soil water between field capacity and the permanent wilting point. Water molecules having very slight positive charges at one end

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1 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.

moving from a region of higher concentration to a region of lower concentration. C.

2 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.

3 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

4 A simple osmometer, made by tying a differentially permeable membrane over the mouth of a thistle tube.

5 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.

6 A portion of a leaf of the water weed Elodea. A. Normal cells. B. Plasmolyzed cells. 100.

7 Black-eyed pea seeds before and after imbibition of water.

8 Pathway of water through a plant.

9 Capillarity in narrow tubes. The smaller the diameter of the tube, the greater the rise of the fluid.

10 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 massflow) 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.

11 The pressure-flow hypothesis.

12 An aphid feeding on a young stem of basswood (Tilia). A droplet of honeydew is emerging from the rear of the aphid. 10.

13 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.

14 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.

15 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

16 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.

18 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. Elements essential as building blocks for compounds synthesized by plants.

19 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.

20 N

21 METABOLISM Anabolism = building reactions (Photosynthesis, Citric acid cycle,etc,) Catabolism = breaking down compounds into simpler compounds molecules or atoms.(respiration,etc)

22 sunlight, water CO 2 Thylakoid membrane Light reactions ATP, NADPH Dark reactions Chloroplast stroma oxygen Fructose (carbohydrates)

23 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.

24 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.

25 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).

26 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.

27 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.

28 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.

29 A simplified summary of photosynthetic reactions.

30 Grana thylakoid Stroma lamella

31 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 CO 2 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.

32 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 CO 2 during the day. Regular photosynthesis occurs as the 4-carbon compounds that accumulate at night are converted back to carbon dioxide during the day.

33 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

34 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.

35 A portion of a cross section of a leaf of corn (Zea mays), a C4 plant with Kranz anatomy leaves.

36 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.

37 CAM Plants Chloroplast Cytosol

38 However, as indicated by its name, the enzyme RuBP carboxylase/oxygenase has the potential to fix both CO 2, through its carboxylase activity described by the Calvin cycle, and O 2, 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

39 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.

40 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.

Lecture 9: Photosynthesis

Lecture 9: Photosynthesis I. Characteristics of Light A. Light is composed of particles that travel as waves 1. Comprises a small part of the electromagnetic spectrum B. Radiation varies in wavelength