Replacement Electrons From H 2 O example essay topic

Organisms Depend Upon PhotosynthesisA. Organisms Depend Upon Photosynthesis 1. Photosynthetic organisms (algae, plants and a few other organisms) serve as ultimate source of food for most life. 2. Photosynthesis transforms solar energy into chemical bond energy of carbohydrates. 3.

Most food chains start with photo synthesizers. Solar Radiation Key Discoveries of Photosynthetic Process Structure of Chloroplasts Function of Chloroplasts A. Solar Radiation 1. Solar radiation is described in terms of its energy content and its wavelength. 2. Photons are discrete packets of radiant energy that travel in waves. 3.

The electromagnetic spectrum is the range of types of solar radiation based on wavelength. a. Gamma rays have shortest wavelength. b. Radio waves have longest wavelength. c. Energy content of photons is inversely proportional to wavelength of particular type of radiation. 1. Short-wavelength ultraviolet radiation has photons of a higher energy content.

2. Long-wavelength infrared light has photons of lower energy content. 3. High-energy photons (e. g., those of ultraviolet radiation) are dangerous to cells because they can break down organic molecules by breaking chemical bonds. 4.

Low-energy photons (e. g., those of infrared radiation) do not damage cells because they do not break chemical bonds but merely increase vibrational energy. d. White light is made up of many different wavelengths; a prism separates them into a spectrum. 4. Only 42% of solar radiation that hits earth's atmosphere reaches surface; most is visible light. a. Higher energy wavelengths are screened out by ozone layer in upper atmosphere. b.

Lower energy wavelengths are screened out by water vapor and CO 2. c. Consequently, both the organic molecules within organisms are processes, such as vision and photosynthesis, are adapted to radiation that is most prevalent in the environment. 5. Earth's Energy-Balance sheet a. 42% of solar energy hitting atmosphere reaches earth surface; rest is reflected or heats atmosphere b. Only 2% of 42% is eventually used by plants; rest becomes heat. c.

Of this plant-intercepted energy, only 0.1 to 1.6% is incorporated into plant tissue. d. Of plant tissue, only 20% is eaten by herbivores; most of rest decays or is lost as heat. e. Of herbivore tissues, only 30% is eaten by carnivores. 6.

Photosynthetic pigments use primarily the visible light portion of the electromagnetic spectrum. a. Two major photosynthetic pigments are chlorophyll a and chlorophyll b. b. Both chlorophyll's absorb violet, blue, and red wavelengths best. c. Very little green light is absorbed; most is reflected back; this is why leaves appear green. d. Carotenoids are yellow-orange pigments which absorb light in violet, blue, and green regions. e. When chlorophyll in leaves breaks down in fall, the yellow-orange pigments show through.

7. Absorption and action spectrum a. A spectrophotometer measures the amount of light that passes through a sample of pigments. 1. As different wavelengths are passed through, some are absorbed.

2. Graph of percent of light absorbed at each wavelength is absorption spectrum. b. Action spectrum 1. Photosynthesis produces oxygen; production of oxygen is used to measure rate of photosynthesis. 2. Oxygen production and, therefore, photosynthetic activity is measured for plants under each specific wavelength; plotted on a graph, this produces an action spectrum.

3. Action spectrum resembles absorption spectrum; indicates chlorophyll's contribute to photosynthesis. B. Key Discoveries of Photosynthetic Process 1. The overall equation for photosynthesis is usually stated as carbon dioxide plus water forms carbohydrate d plus oxygen. 2. In 1930 C.B. van Niel showed that oxygen given off by photosynthesis comes from water and not from carbon dioxide.

The correct equation should then read: carbon dioxide plus water forms carbohydrate plus water plus oxygen. C. Structure of Chloroplasts 1. In chloroplasts, a double membrane encloses a fluid-filled space called the stroma; stroma contains enzyme-rich solution that reduces CO 2, converting it to an organic compound. 2. Even more internal membranes within stroma form flattened sacs called, which are sometimes organized into stacks called gran a. 3. Spaces within all are connected and form an inner compartment or space.

4. Chlorophyll and other pigments involved in absorption of solar energy are embedded within membranes; these pigments absorb solar energy, energize electrons prior to reduction of CO 2 in stroma. D. Function of Chloroplasts 1. In 1905, F.F. Blackman proposed two sets of reactions for photosynthesis. 2. Light-dependent reactions cannot take place unless light is present. a. Light-dependent reactions are the energy-capturing reactions. b.

Associated with light-absorbing molecules and electron transport systems of. c. They involve the splitting of water and the release of O 2. d. Low-energy electrons are removed from H 2 O; energized when membrane pigments absorb energy. e. Electrons move from chlorophyll a down electron transport system; produces ATP from ADP and P. f. Energized electrons are also taken up by NADP+, becoming NADPH. g.

NADPH temporarily holds energy in form of energized electrons that will fuel CO 2 reduction. 3. Light-independent Reactions a. These reactions take place in the stroma; can occur in either the light or the dark. b. The light-dependent reactions are synthesis reactions that use NADPH and ATP to reduce CO 2. Light-dependent Reactions Electrons Pathways ATP Production The Thylakoid Membrane.

Light-dependent Reactions 1. Occur in the membranes and require participation of two light-gathering units: photo system I (PS I) and photo system II (PS II). 2. A photo system is a photosynthetic unit comprised of a pigment complex and electron acceptor; solar energy is absorbed and high-energy electrons are generated. 3.

Each has a pigment complex composed of green chlorophyll a and chlorophyll b molecules and orange and yellow accessory pigments (e. g., carotenoid pigments). 4. Absorbed energy is passed from one pigment molecule to another until concentrated in reaction-center chlorophyll a. 5. Electrons in reaction-center chlorophyll a become excited; they escape to electron-acceptor molecule. B. Electrons Pathways 1. Cyclic electron pathway generated only ATP; non cyclic pathway results in both NADPH and ATP. a.

ATP production during photosynthesis is called since light is involved. b. This leads to cyclic and non cyclic. 2. Cyclic Electron Pathway a.

The cyclic electron pathway begins after PS I pigment complex absorbs solar energy. b. High-energy electrons leave PS I reaction-center chlorophyll a molecule but eventually return to it. c. Before they return, the electrons enter and travel down an electron transport system. 1. Electrons pass from a higher to a lower energy level. 2.

Energy released is stored in form of a hydrogen (H+) gradient. 3. When hydrogen ions flow down their electrochemical gradient through ATP complexes, ATP production occurs. d. Some photosynthetic bacteria utilize cyclic electron pathway only; pathway probably evolved early. e. It is possible that in plants, the cyclic flow of electrons is utilized only when CO 2 is in such limited supply that carbohydrate is not being produced. f. There is now no need for additional NADPH, which is produced only by the non cyclic electron pathway.

3. Non cyclic Electron Pathway a. During the non cyclic electron pathway, electrons move from H 2 O through PS II to PSI and then on to NADP+. b. The PS II pigment complex absorbs solar energy; high-energy electrons (e') leave the reaction-center chlorophyll a molecule. c. PS II takes replacement electrons from H 2 O, which splits, releasing O 2 and H+ ions: H 2 O 2 H+ + 2 e' + 1/2 O 2. d. Oxygen evolved from chloroplasts and plant as oxygen gas (O 2). e.

The H+ ions temporarily stay within the space. f. High-energy electrons that leave PS II are captured by an electron acceptor, which sends them to an electron transport system. g. As electrons pass from one carrier to next, energy to be used to produce ATP molecules is released and stored as a hydrogen (H+) gradient. h. As H+ flow down electrochemical gradient through ATP complexes, ATP synthesis occurs. i.

Low-energy electrons leaving the electron transport system enter PS I. j. PS I pigment complex absorbs solar energy; high-energy electrons leave reaction-center chlorophyll a and are captured by an electron acceptor. k. The electron acceptor passes them on to NADP+. l. NADP+ takes on an H+ to become NADPH: NADP+ + 2 e' + H+ NADPH. m. NADPH and ATP produced by non cyclic flow electrons in membrane are used by enzymes in stroma during light-independent reactions. C. ATP Production 1. The space acts as a reservoir for H+ ions; each time H 2 O is split, two H+ remain.

2. Electrons move carrier-to-carrier, giving up energy used to pump H+ from stroma into space. 3. Flow of H+ from high to low concentration across membrane provides energy to produce ATP from ADP + P by using an ATP enzyme. 4. This is called chemosmosis because ATP production is tied to an gradient. D. The Thylakoid Membrane 1.

PS II oxidizes H 2 O and produces O 2.2. The cytochrome complex transports electrons and pumps H+ ions into the space. 3. PS I is associated with an enzyme that reduces NADP+ to NADPH. 4. ATP complex has an H+ channel and ATP; it produces ATP.

Modes of PhotosynthesisA. Modes of Photosynthesis 1. In C 3, plants Calvin cycle fixes CO 2 directly; first molecule following CO 2 fixation is PGA, a C 3 molecule. 2.

C 4 leaves fix CO 2 by forming a C 4 molecule prior to the involvement of the Calvin cycle. 3. CAM plants fix CO 2 by forming C 4 molecule at night when stoma tes can open without loss of water. 4. C 4 Photosynthesis a.

In a C 4 plant, cells contain well-formed chloroplasts arranged in parallel layers. b. In C 4 plants, bundle sheath cells as well as the cells contain chloroplasts. c. In C 4 leaf, cells are arranged concentrically around the bundle sheath cells. d. C 3 plants use RuBP carboxylase to fix CO 2 to RuBP in; first detected molecule is PGA. e. C 4 plants use the enzyme PEP carboxylase (PET Case) to fix CO 2 to PEP ; end product to (a C 4 molecule). f. In C 4 plants, CO 2 is taken up in cells and ma late, a reduced form of, is pumped into the bundle-sheath cells; here CO 2 enters Calvin cycle. g.

In hot, dry climates, net photosynthetic rate of C 4 plants (e. g., corn) is 2-3 times that of C 4 plants. 5. Photorespiration a. In hot weather, stoma tes close to save water; CO 2 concentration decrease in leaves; O 2 increases. b. In C 3 plants, O 2 competes with CO 2 for the active site of RuBP carboxylase, resulting in production of only one molecule of PGA. c.

Called 'photo respiration's ince oxygen is taken up and CO 2 is produced; produces only one PGA. d. Photorespiration does not occur in C 4 leaves even when stoma tes are closed because CO 2 is delivered to Calvin cycle in bundle sheath cells. e. C 4 plants have advantage over C 3 plants: in hot and dry weather, photo respiration does not occur (e. g., bluegrass dominates lawns in early summer, crabgrass takes over in hot midsummer). 6. CAM Photosynthesis a. CAM (-acid metabolism) plants form a C 4 molecule at night when stoma tes can open without loss of water; found in succulent desert plants in family Crassulacaeae and other. b.

CAM plants use PEP Case to fix CO 2 by forming C 4 molecule stored in large vacuoles in. c. C 4 formed at night is broken down to CO 2 during the day and enters the Calvin cycle within the same cell, which now has NADPH and ATP available to it from the light-dependent reactions. d. CAM plants open stoma tes only at night, allowing CO 2 to enter photosynthesizing tissues; during the day, stoma tes are closed to conserve water and CO 2 cannot enter photosynthesizing tissues. e. Photosynthesis in a CM plant is minimal, due to limited amount of CO 2 fixed at night; does allow CAM plants to live under stressful conditions.