Chapter 7 Outline and Terms

7.1. Sunlight Provides Solar Energy (p. 114)

A. Organisms Depend Upon Photosynthesis

1. Photosynthesis uses sunlight as a source of energy to produce carbohydrates.

2. Photosynthetic organisms (algae, plants and a few other organisms) serve as ultimate source of food for most life.

3. Most food chains start with photosynthesizers.

4. Photosynthesis has produced most of the oxygen in the atmosphere of our planet.

B. 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. (Fig. 7.1a)

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.

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 CO2.

c. Consequently, both the organic molecules within organisms and processes, such as vision and photosynthesis, are adapted to radiation that is most prevalent in the environment.

5. 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 chlorophylls absorb violet, blue, and red wavelengths best. (Fig. 7.1)

c. Very little green light is absorbed; most is reflected back and is why leaves appear green.

d. Carotinoids 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.

6. 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 the absorption spectrum. (Fig. 7.1)

b. Action spectrum

1) Photosynthesis produces oxygen; production of oxygen 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 chlorophylls contribute to photosynthesis.

7. 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.

7.2. Photosynthesis Occurs in Chloroplasts (p. 116)

A. Key Discoveries of Photosynthetic Process

1. The overall equation for photosynthesis is usually stated as carbon dioxide plus water goes to carbohydrate plus oxygen.

2. In 1930 C. B. van Niel showed that the oxygen given off by photosynthesis comes from water and not from carbon dioxide. In order to balance the equation, the equation should then read as: carbon dioxide plus water goes to carbohydrate plus water plus oxygen.

B. Chloroplasts Have Two Parts (Fig. 7.2)

1. In chloroplasts, a double membrane encloses a fluid-filled space called the stroma; stroma contains enzyme-rich solution that reduces CO2, converting it to an organic compound.

2. Even more internal membranes within stroma form flattened sacs called thylakoids, which are sometimes organized into stacks called grana. (Fig. 7.2) [transp. 43]

3. Spaces within all thylakoids are connected, forming inner compartment or thylakoid space.

4. Chlorophylls and other pigments involved in absorption of solar energy are embedded within thylakoid membranes; these pigments absorb solar energy, energize electrons prior to reduction of CO2 in stroma.

C. Photosynthesis Has Two Sets of Reactions

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 thylakoids.

c. They involve the splitting of water and the release of O2.

d. Low-energy electrons are removed from H2O; energized when thylakoid membrane pigments absorb energy.

e. Electrons move from chlorophyll a down electron transport system; produce ATP from ADP and .

f. Energized electrons are also taken up by NADP+, becoming NADPH.

g. NADPH temporarily holds energy in form of energized electrons that will fuel CO2 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-independent reactions are synthesis reactions that use NADPH and ATP to reduce CO2.

c. CO2 from air is fixed by substrate of the Calvin cycle, a series of reactions producing carbohydrate.

7.3. Solar Energy Is Captured (p. 118)

A. Light-dependent Reactions

1. Occur in the thylakoid membranes and require participation of two light-gathering units: photosystem I (PS I) and photosystem II (PS II).

2. A photosystem is a photosynthetic unit comprised of a pigment complex and electron acceptor; solar energy is absorbed and high-energy electrons are generated.

3. Each photosystem 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 chlorophylla. (Fig. 7.3) [transp. 44]

5. Electrons in reaction-center chlorophyll a become excited; they escape to electron-acceptor molecule.

B. Electrons Have Two Pathways

1. 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) Some carrier molecules are cytochrome molecules 3/4 called cytochrome system in chloroplasts.

2) Electrons pass from a higher to a lower energy level.

3) Energy released is stored in form of a hydrogen (H+) gradient.

4) When hydrogen ions flow down their electrochemical gradient through ATP synthase 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 CO2 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 noncyclic electron pathway.

2. Noncyclic Electron Pathway

a. During the noncyclic electron pathway, electrons move from H2O through PS II to PS I and then on to NADP+. (Fig. 7.4) [transp. 45]

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 H2O, which splits, releasing O2 and H+ ions: H2O 2 H+ + 2 e- + 1/2 O2.

d. Oxygen evolves from chloroplasts and plant as oxygen gas (O2).

e. The H+ ions temporarily stay within the thylakoid 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 synthase complexes, chemiosmotic 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 noncyclic flow electrons in thylakoid membrane are used by enzymes in stroma during light-independent reactions.

C. ATP Production

1. The thylakoid space acts as a reservoir for H+ ions; each time H2O is split, two H+ remain.

2. Electrons move carrier-to-carrier, giving up energy used to pump H+ from stroma into thylakoid space.

3. Flow of H+ from high to low concentration across thylakoid membrane provides energy to produce ATP from ADP + by using an ATP synthase enzyme; this is called chemiosmosis.

D. The Thylakoid Membrane (Fig. 7.5) [transp. 46]

1. PS II oxidizes H2O and produces O2.

2. The cytochrome complex transports electrons and pumps H+ ions into the thylakoid space.

3. PS I is associated with an enzyme that reduces NADP+ to NADPH.

4. ATP synthase complex has an H+ channel and ATP synthase; it produces ATP.

7.4. Carbohydrate Is Synthesized (p. 121)

A. Light-independent Reactions

1. The second stage of photosynthesis; light is not directly required.

2. Require CO2, which enters through leaf and NADPH and ATP, which have been produced by light-dependent reactions.

3. PS I initiates regulatory mechanism by which enzymes of the light-independent reactions are turned on.

4. NADPH and ATP are used to reduce CO2: CO2 becomes CH2O within a carbohydrate molecule.

5. The reduction of CO2 occurs in the stroma of a chloroplast by series of reactions called the Calvin cycle. (Fig. 7.6)

B. The Importance of PGAL (Fig. 7.7)

1. PGAL (glyceraldehyde-3-phosphate) is product of Calvin cycle; is converted to many organic molecules.

2. Glucose phosphate is product of PGAL metabolism; important source of sucrose, starch, cellulose.

3. Hydrocarbon skeleton of PGAL is used to form fatty acids and glycerol of plant oils, and amino acids.

C. The Calvin Cycle Has Three Stages

1. Fixing Carbon Dioxide (Fig. 7.8) [transp. 47]

a. CO2 fixation is the attachment of CO2 to an organic compound.

b. RuBP (ribulose bisphosphate) is a five-carbon molecule that combines with carbon dioxide.

c. Enzyme RuBP carboxylase speeds reaction; is 20-50% protein in chloroplasts.

2. Reducing PGA

a. Six-carbon molecule immediately breaks down, forms two PGA (3-phosphoglycerate[C3]) molecules.

b. Each of two PGA molecules undergoes reduction to PGAL in two steps.

c. Light-dependent reactions provide NADPH (electrons) and ATP (energy) to reduce PGA to PGAL.

3. Regenerating RuBP

a. Every three turns of Calvin cycle, five molecules of PGAL are used to re-form three molecules of RuBP.

b. Every three turns of Calvin cycle, there is net gain of one PGAL molecule; five PGAL regenerate RuBP.

c. First molecule identified by Calvin was PGA [C3], a three-carbon product; Calvin cycle is also known as C3 cycle.

D. Photosynthesis Takes Other Routes

1. In C3, plants Calvin cycle fixes CO2 directly; first molecule following CO2 fixation is PGA, a C3 molecule.

2. C4 leaves fix CO2 by forming a C4 molecule prior to the involvement of the Calvin cycle.

3. CAM plants fix CO2 by forming C4 molecule at night when stomates can open without loss of water.

4. C4 Plants Flourish When It Is Hot and Dry

a. In a C3 plant, mesophyll cells contain well-formed chloroplasts arranged in parallel layers.

b. In C4 plants, bundle sheath cells as well as the mesophyll cells contain chloroplasts.

c. In C4 leaf, mesophyll cells are arranged concentrically around the bundle sheath cells.

d. C3 plants use RuBP carboxylase to fix CO2 to RuBP in mesophyll; first detected molecule is PGA.

e. C4 plants use the enzyme PEP carboxylase (PEPCase) to fix CO2 to PEP (phosphoenolpyruvate); end product is oxaloacetate (a C4 molecule).

f. In C4 plants, CO2 is taken up in mesophyll cells and a reduced form of oxaloacetate is pumped into the bundle-sheath cells; here CO2 enters Calvin cycle. (Fig. 7.9)

g. In hot, dry climates, net photosynthetic rate of C4 plants (e.g., corn) is 2-3 times that of C4 plants.

5. Photorespiration

a. In hot weather, stomates close to save water; CO2 concentration decreases in leaves; O2 increases.

b. In C3 plants, O2 competes with CO2 for the active site of rubisco, resulting in production of only one molecule of PGA.

c. RuBP carboxylase (also called rubisCO) has both carboxylase and oxygenase activity.

d. Called "photorespiration" since oxygen is taken up and CO2 is produced; produces only one PGA.

e. Photorespiration does not occur in C4 leaves even when stomates are closed because CO2 is delivered to Calvin cycle in bundle sheath cells.

f. C4 plants have advantage over C3 plants: in hot and dry weather, photorespiration does not occur (e.g., bluegrass dominates lawns in early summer, crabgrass takes over in hot midsummer).

6. CAM Plants Have an Alternative Way

a. CAM (crassulacean-acid metabolism) plants form a C4 molecule at night when stomates can open without loss of water; found in succulent desert plants.

b. CAM plants use PEPCase to fix CO2 by forming C4 molecule stored in large vacuoles in mesophyll.

c. CAM plants open stomates only at night, allowing CO2 to enter photosynthesizing tissues; during the day, stomates are closed to conserve water and CO2 cannot enter photosynthesizing tissues.

d. C4 formed at night is broken down to CO2 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.

e. Photosynthesis in a CAM plant is minimal, due to limited amount of CO2 fixed at night; does allow CAM plants to live under stressful conditions.


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