Essentials of BiologySylvia S. Mader

Chapter 6Lecture Outline

Prepared by: Dr. Stephen EbbsSouthern Illinois University Carbondale

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

6.1 Overview of Photosynthesis

• Photosynthesis transforms solar energy into the chemical energy of carbohydrates.

• Photosynthetic organisms include plants, algae, and cyanobacteria.

• The products of photosynthesis provide both food and fuel (coal, wood) to humans.

Overview of Photosynthesis (cont.)

Flowering Plants as Photosynthesizers

• The green portions of plants, such as leaves, carry out photosynthesis, using carbon dioxide and water as substrates.

• Carbon dioxide enters leaves through openings called stomata.

• The carbon dioxide and water diffuse to the chloroplast, the site of photosynthesis.

Flowering Plants as Photosynthesizers (cont.)

• The structure of the chloroplasts is important to photosynthesis.– The chloroplast has a double membrane that

surrounds the liquid stroma.– The stroma contains numerous flat thylakoid

disks arranged in stacks called grana.– The chlorophyll pigments imbedded in the

thylakoid membranes absorb solar energy during photosynthesis.

Flowering Plants as Photosynthesizers (cont.)

The Photosynthetic Process

• The overall equation for photosynthesis can be written in this form.

CO2 + H2O (CH2O) + O2

oxidation

reduction

gain of hydrogen atoms

loss of hydrogen atoms

Solar energy

The Photosynthetic Process (cont.)

• The equation for photosynthesis can also be written in another form to show the formation of the product, glucose.

6 CO2 + 6 H2O C6H12O6 + 6 O2

Solar energy

Two Sets of Reactions

• Photosynthesis involved two sets of reactions.– The light reactions– The Calvin cycle reactions

• The light reactions involve several events.– Chlorophyll absorbs solar energy, which energizes

electrons.– ATP is produced using an electron transport chain.– NADP+, a coenzyme, accepts electrons to become

NADPH.

Two Sets of Reactions (cont.)

• The Calvin cycle reactions involve several events.– CO2 is taken up by one of the substrates in

the cycle. – ATP and NADPH from the light reactions

reduce CO2 to a carbohydrate.

Two Sets of Reactions (cont.)

6.2 Light Reactions

• Solar energy can be described in terms of its wavelength and energy content.

• While there are several forms of solar, or radiant energy, that strike the Earth’s atmosphere, visible light is the form of energy that gets through.

• The chloroplast pigments absorb solar energy during the light reactions.

Light Reactions (cont.)

Photosynthetic Pigments

• The two primary pigments used during photosynthesis are chlorophylls and carotenoids.

• Chlorophylls absorb violet, red, and blue wavelengths of visible light and reflect green light.

• Carotenoids absorb in the violet-blue-green range but reflect yellow-orange wavelengths.

• The carotenoids and other pigments become visible in the autumn as chlorophyll is degraded.

Photosynthetic Pigments (cont.)

The Electron Pathway of the Light Reactions

• The light reactions consist of an electron pathway that produces ATP and NADPH.

• The pathway uses two photosystems to complete the light reactions.

• A photosystem consists of several parts.– A pigment complex, or light antenna. – A special chlorophyll pigment, the reaction center.– Electron acceptor molecules.

The Electron Pathway of the Light Reactions (cont.)

• When photosystem II (PS II) absorbs solar energy, energized electrons are passed to electron acceptors.

• PS II splits a water molecule to recover the electrons passed to the electron acceptors.

• The electron acceptors send the energized electrons down an electron transport chain.

The Electron Pathway of the Light Reactions (cont.)

• As the electrons are passed down an electron transport chain, energy is released and stored in the form of a hydrogen ion (H+) gradient.

• This H+ gradient is used later in photosynthesis to produce ATP.

The Electron Pathway of the Light Reactions (cont.)

• When photosystem I (PS I) absorbs solar energy, energized electrons are passed to different electron acceptors.

• Electrons from the end of the electron transport chain replace the electrons from PS I.

• The electron acceptors pass the electrons to NADP+ to form NADPH.

The Electron Pathway of the Light Reactions (cont.)

Organization of the Thylakoid Membrane

• PS II, PS I, and the electron transport chain are located within the thylakoid membrane.

• Another component required for photosynthesis and ATP production is the ATP synthase complex.

Organization of the Thylakoid Membrane (cont.)

ATP Production

• During photosynthesis, the thylakoid space becomes an H+ reservoir.

• The H+ ions that fill this reservoir come from two sources.– The oxidation of water by PS II adds H+.– The flow of electrons through the electron

transport chain releases energy that pumps H+ into the thylakoid space.

ATP Production (cont.)

• As the H+ are released through an ATP synthase, the H+ flow down their concentration gradient and release energy.

• The ATP synthase couples that release of energy to the production of ATP.

NADPH Production

• Some enzymes require non-protein helpers to catalyze their chemical reaction.

• NADP+ is a coenzyme that accepts H+ from a substrate.

• NADPH is formed during the light reactions by accepting electrons from the electron transport pathway and then picks up H+.

6.3 Calvin Cycle Reactions

• The Calvin cycle is a series of reactions that continually produce a carbohydrate (glucose) from carbon dioxide during photosynthesis.

• The Calvin cycle has three steps.– Carbon dioxide fixation

– Carbon dioxide reduction

– Regeneration of ribulose-1,5-bisphosphate (RuBP)

6.3 Calvin Cycle Reactions (cont.)

Fixation of Carbon Dioxide

• During the first step of the Calvin cycle, CO2 from the air is attached (fixed) to RuBP.

• The enzyme for this reaction is RuBP carboxylase oxygenase (rubisco).

• Rubisco splits the resulting 6-carbon molecule to form two 3-carbon molecules.

Reduction of Carbon Dioxide

• Reduction of CO2 is a series of reactions that uses NADPH and ATP from the light reactions to form the carbohydrate.– NADPH provides electrons for the reduction.– ATP provides the energy.

Regeneration of RuBP

• The product of the Calvin cycle is glyceraldehyde-3-phosphate (G3P).

• About 1/6 of the G3P is used to make glucose.

• About 5/6 of the glucose is used to regenerate the RuBP required for the fixation of carbon dioxide.

The Importance of the Calvin Cycle

• The G3P molecules produced by plants can be used to make a wide variety of chemicals.

The Importance of the Calvin Cycle (cont.)

6.4 Other Types of Photosynthesis

• Plants have metabolically adapted photosynthesis to different climates.

• In areas with moderate temperature, plants carry out C3 photosynthesis, meaning that the first detectable molecule from the Calvin cycle is a 3-carbon compound.

6.4 Other Types of Photosynthesis (cont.)

6.4 Other Types of Photosynthesis (cont.)

• Plants in hot dry climates perform C4 photosynthesis, forming a 4-carbon compound.

• These other types of photosynthesis are necessary because O2 competes with CO2

for the binding site on rubisco, decreasing the efficiency of photosynthesis.

C4 Photosynthesis

• The anatomy of a C4 plant is different from that of a C3 plant.

• Although chloroplasts are found in both the mesophyll and bundle sheath cells, the Calvin cycle occurs primarily in the bundle sheath cells.

• CO2 taken in by the mesophyll cells is combined with a 3-carbon compound to form a 4-carbon compound (carbon fixation).

C4 Photosynthesis (cont.)

• The 4-carbon compound is shuttled to the bundle sheath cell, where it releases the CO2 into the Calvin cycle.

• This spatial separation minimizes the competition with O2.

• While more complex, C4 photosynthesis is more advantageous to plants in hot, dry climates.

C4 Photosynthesis (cont.)

CAM Photosynthesis

• Another type of photosynthesis is crassulacean acid metabolism (CAM), found commonly in desert plants.

• Similar to C4 photosynthesis, CAM plants separate CO2 fixation from the Calvin cycle reaction to minimize competition from O2.

• However CAM plants separate these events by time.– CO2 is fixed during the night.– The Calvin cycle reactions occur during the day.

CAM Photosynthesis (cont.)

• The primary advantage of CAM photosynthesis involves the conservation of water.

• When CAM plants open their stomata at night to obtain CO2, water loss is minimized.

CAM Photosynthesis (cont.)

Evolutionary Trends

• C4 plants most likely involved in areas with high light, high temperature, and low rainfall.

• C3 plants survive better than C4 plants in temperatures less than 25ºC.

• CAM plants compete well with both C3 and C4 plants, particularly in arid environments.