An Overview of Photosynthesis

Photosynthesis: Using Light to Introduction Chapter 7 Make Food . Plants, algae, and certain prokaryotes

– convert light energy to chemical energy and – store the chemical energy in sugar, made from

– carbon dioxide and – water.

PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey

Figure 7.0_1 Introduction Chapter 7: Big Ideas

. Algae farms can be used to produce

– oils for biodiesel or The Light Reactions: – carbohydrates to generate ethanol. An Overview of Converting Solar Energy to Photosynthesis Chemical Energy

The Calvin Cycle: Photosynthesis Reviewed

Figure 7.0_2

AN OVERVIEW OF PHOTOSYNTHESIS

1 7.1 Autotrophs are the producers of the biosphere 7.1 Autotrophs are the producers of the biosphere

. Autotrophs . Photoautotrophs use the energy of light to produce organic molecules. – make their own food through the process of photosynthesis, . Chemoautotrophs are prokaryotes that use – sustain themselves, and inorganic chemicals as their energy source. – do not usually consume organic molecules derived from . Heterotrophs are consumers that feed on other organisms. – plants or – animals, or – decompose organic material.

7.1 Autotrophs are the producers of the biosphere Figure 7.1A-D

. Photosynthesis in plants – takes place in chloroplasts, – converts carbon dioxide and water into organic molecules, and – releases oxygen.

Figure 7.1A Figure 7.1B

2 Figure 7.1C Figure 7.1D

7.2 Photosynthesis occurs in chloroplasts in plant 7.2 Photosynthesis occurs in chloroplasts in plant cells cells

. Chloroplasts are the major sites of photosynthesis . Chloroplasts are concentrated in the cells of the in green plants. mesophyll, the green tissue in the interior of the leaf. . Chlorophyll . Stomata are tiny pores in the leaf that allow – is an important light-absorbing pigment in chloroplasts, – carbon dioxide to enter and – is responsible for the green color of plants, and – oxygen to exit. – plays a central role in converting solar energy to chemical energy. . Veins in the leaf deliver water absorbed by roots.

Figure 7.2 Figure 7.2_1

Leaf Leaf Cross Section

Mesophyll Vein Leaf Cross Leaf Section

CO2 Mesophyll O2 Vein Stoma Mesophyll Cell

Mesophyll Cell Chloroplast

Inner and outer membranes Granum

Thylakoid Thylakoid space Stroma CO 2 O2 Stoma Chloroplast

3 7.2 Photosynthesis occurs in chloroplasts in plant 7.2 Photosynthesis occurs in chloroplasts in plant cells cells

. Chloroplasts consist of an envelope of two . Thylakoids membranes, which – are often concentrated in stacks called grana and – enclose an inner compartment filled with a thick fluid – have an internal compartment called the thylakoid space, called stroma and which has functions analogous to the intermembrane space of a mitochondrion. – contain a system of interconnected membranous sacs called thylakoids. – Thylakoid membranes also house much of the machinery that converts light energy to chemical energy. . Chlorophyll molecules – are built into the thylakoid membrane and – capture light energy.

Figure 7.2_2 Figure 7.2_3

Chloroplast

Mesophyll Cell

Inner and outer membranes Granum

Thylakoid Chloroplast Thylakoid space Stroma

Figure 7.2_4 7.3 SCIENTIFIC DISCOVERY: Scientists traced the process of photosynthesis using isotopes

. Scientists have known since the 1800s that plants Granum produce O2. But does this oxygen come from carbon dioxide or water? Stroma – For many years, it was assumed that oxygen was

extracted from CO2 taken into the plant. – However, later research using a heavy isotope of oxygen, 18O, confirmed that oxygen produced by

photosynthesis comes from H2O.

4 Figure 7.3A 7.3 SCIENTIFIC DISCOVERY: Scientists traced the process of photosynthesis using isotopes

. Experiment 1: 6 CO2  12 H2O → C6H12O6  6 H2O  6 O2

. Experiment 2: 6 CO2  12 H2O → C6H12O6  6 H2O  6 O2

Figure 7.3B 7.4 Photosynthesis is a redox process, as is cellular respiration

. Photosynthesis, like respiration, is a redox (oxidation-reduction) process. Reactants: – CO2 becomes reduced to sugar as electrons along with hydrogen ions from water are added to it. – Water molecules are oxidized when they lose electrons along with hydrogen ions. Products:

Figure 7.4A 7.4 Photosynthesis is a redox process, as is cellular respiration

. Cellular respiration uses redox reactions to harvest the chemical energy stored in a glucose molecule.

Becomes reduced – This is accomplished by oxidizing the sugar and reducing O2 to H2O. – The electrons lose potential as they travel down the electron transport chain to O . Becomes oxidized 2 – In contrast, the food-producing redox reactions of photosynthesis require energy.

5 7.4 Photosynthesis is a redox process, as is Figure 7.4B cellular respiration

. In photosynthesis,

– light energy is captured by chlorophyll molecules to boost the energy of electrons, Becomes oxidized – light energy is converted to chemical energy, and – chemical energy is stored in the chemical bonds of sugars. Becomes reduced

7.5 Overview: The two stages of photosynthesis 7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH are linked by ATP and NADPH

. Photosynthesis occurs in two metabolic stages. 2. The second stage is the Calvin cycle, which occurs in the stroma of the chloroplast. 1. The light reactions occur in the thylakoid membranes. In these reactions – The Calvin cycle is a cyclic series of reactions that assembles sugar molecules using CO2 and the energy-rich – water is split, providing a source of electrons and giving off products of the light reactions. oxygen as a by-product, – During the Calvin cycle, CO2 is incorporated into organic – ATP is generated from ADP and a phosphate group, and compounds in a process called carbon fixation. – light energy is absorbed by the chlorophyll molecules to drive – After carbon fixation, enzymes of the cycle make sugars by the transfer of electrons and H+ from water to the electron further reducing the carbon compounds. acceptor NADP+ reducing it to NADPH. – The Calvin cycle is often called the dark reactions or light- – NADPH produced by the light reactions provides the independent reactions, because none of the steps requires electrons for reducing carbon in the Calvin cycle. light directly.

Figure 7.5_s1 Figure 7.5_s2

H2O H2O

Light Light

NADP+ NADP+ ADP ADP P P

Light Light Reactions Reactions (in thylakoids) (in thylakoids) ATP

NADPH

Chloroplast Chloroplast

6 Figure 7.5_s3

H2O CO2

Light

NADP+ THE LIGHT REACTIONS: ADP P CONVERTING SOLAR ENERGY

Calvin TO CHEMICAL ENERGY Light Cycle Reactions (in stroma) (in thylakoids) ATP

NADPH

Chloroplast

O2 Sugar

7.6 Visible radiation absorbed by pigments 7.6 Visible radiation absorbed by pigments drives the light reactions drives the light reactions

. Sunlight contains energy called electromagnetic . Light behaves as discrete packets of energy called energy or electromagnetic radiation. photons.

– Visible light is only a small part of the electromagnetic – A photon is a fixed quantity of light energy. spectrum, the full range of electromagnetic – The shorter the wavelength, the greater the energy. wavelengths. – Electromagnetic energy travels in waves, and the wavelength is the distance between the crests of two adjacent waves.

Figure 7.6A 7.6 Visible radiation absorbed by pigments Increasing energy drives the light reactions 105 nm 103 nm 1 nm 103 nm 106 nm 1 m 103 m . Pigments Gamma Micro- Radio X-rays UV Infrared rays waves waves – absorb light and – are built into the thylakoid membrane. . Plant pigments Visible light – absorb some wavelengths of light and – reflect or transmit other wavelengths. 380 400 500 600 700 750 . We see the color of the wavelengths that are Wavelength (nm) transmitted. For example, chlorophyll transmits

650 green wavelengths. nm Animation: Light and Pigments

7 Figure 7.6B Figure 7.6B_1

Light Reflected light

Chloroplast Absorbed light Thylakoid Transmitted light

7.6 Visible radiation absorbed by pigments 7.7 Photosystems capture solar energy drives the light reactions

. Chloroplasts contain several different pigments, . Pigments in chloroplasts absorb photons (capturing which absorb light of different wavelengths. solar power), which – Chlorophyll a absorbs blue-violet and red light and – increases the potential energy of the pigment’s reflects green. electrons and – Chlorophyll b absorbs blue and orange and reflects yellow-green. – sends the electrons into an unstable state. – Carotenoids – These unstable electrons – broaden the spectrum of colors that can drive photosynthesis and – drop back down to their “ground state,” and as they do, – provide photoprotection, absorbing and dissipating excessive – release their excess energy as heat. light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules.

Figure 7.7A Figure 7.7A_1

Excited state

Photon Heat of light

Photon (fluorescence)

Ground state

Chlorophyll molecule

8 Figure 7.7A_2 7.7 Photosystems capture solar energy Excited state

Photon Heat . Within a thylakoid membrane, chlorophyll and other of light pigment molecules – absorb photons and – transfer the energy to other pigment molecules. Photon (fluorescence) . In the thylakoid membrane, chlorophyll molecules are organized along with other pigments and Ground state proteins into photosystems.

Chlorophyll molecule

7.7 Photosystems capture solar energy Figure 7.7B Photosystem Light Light-harvesting Reaction-center . A photosystem consists of a number of light- complexes complex Primary electron harvesting complexes surrounding a reaction- acceptor center complex. . A light-harvesting complex contains various pigment molecules bound to proteins. . Collectively, the light-harvesting complexes function as a light-gathering antenna. Thylakoid membrane Thylakoid

Pigment Pair of Transfer molecules of energy chlorophyll a molecules

7.7 Photosystems capture solar energy 7.7 Photosystems capture solar energy

. The light energy is passed from molecule to . Two types of photosystems (photosystem I and molecule within the photosystem. photosystem II) cooperate in the light reactions. – Finally it reaches the reaction center where a primary . Each type of photosystem has a characteristic electron acceptor accepts these electrons and reaction center. consequently becomes reduced. – Photosystem II, which functions first, is called P680 – This solar-powered transfer of an electron from the because its pigment absorbs light with a wavelength of reaction-center pigment to the primary electron acceptor 680 nm. is the first step in the transformation of light energy to chemical energy in the light reactions. – Photosystem I, which functions second, is called P700 because it absorbs light with a wavelength of 700 nm.

9 7.8 Two photosystems connected by an electron Figure 7.8A transport chain generate ATP and NADPH

. In the light reactions, light energy is transformed   NADPH Electron transport chain Light NADP  H Light Provides energy for synthesis of ATP 6 into the chemical energy of ATP and NADPH. Photosystem II Photosystem I Stroma by chemiosmosis . To accomplish this, electrons are 1 Primary Primary acceptor acceptor – removed from water, 2

4 – passed from photosystem II to photosystem I, and 5 P680 P700

– accepted by NADP+, reducing it to NADPH. membrane Thylakoid Thylakoid 3 space

. Between the two photosystems, the electrons H2O 1  2 O2  2 H – move down an electron transport chain and – provide energy for the synthesis of ATP.

Figure 7.8A_1 Figure 7.8A_2

Electron transport chain Light Provides energy for  + synthesis of ATP Electron transport chain Light NADP  H NADPH Photosystem II by chemiosmosis Provides energy for Stroma synthesis of ATP Photosystem I 6 1 by chemiosmosis

Primary acceptor Primary 2 acceptor

4 5 P680 P700 Thylakoid membrane Thylakoid

Thylakoid 3 space

H2O 1  2 O2  2H

Figure 7.8B 7.8 Two photosystems connected by an electron transport chain generate ATP and NADPH ATP . The products of the light reactions are

– NADPH, NADPH – ATP, and – oxygen. Mill makes ATP

Photosystem II Photosystem I

10 7.9 Chemiosmosis powers ATP synthesis in the 7.9 Chemiosmosis powers ATP synthesis in the light reactions light reactions

. Interestingly, chemiosmosis is the mechanism that . In photophosphorylation, using the initial energy input from light, – is involved in oxidative phosphorylation in mitochondria and – the electron transport chain pumps H+ into the thylakoid space, and – generates ATP in chloroplasts. – the resulting concentration gradient drives H+ back . ATP is generated because the electron transport through ATP synthase, producing ATP. chain produces a concentration gradient of hydrogen ions across a membrane.

Figure 7.9 Figure 7.9_1

To Calvin Cycle H+ Light ADP P ATP Chloroplast Light

H+ NADP+ H+ NADPH To Calvin Cycle H+ + + ATP H H Light Light ADP P Stroma (low H+ H+ + + NADPH concentration) NADP H

H+ H+

Thylakoid + + membrane + H+ H H + H H2O + H 1 + H O 2 H + + + + + 2 2 H H H H H + + H H+ + H+ H + H + Electron H H2O H+ H 1 + Photosystem II transport chain Photosystem I ATP synthase O + 2 H + + + + Thylakoid space 2 2 H+ H H H H + (high H + Electron H+ H concentration) Photosystem II transport chain Photosystem I ATP synthase

7.9 Chemiosmosis powers ATP synthesis in the light reactions . How does photophosphorylation compare with THE CALVIN CYCLE: oxidative phosphorylation? REDUCING CO TO SUGAR – Mitochondria use oxidative phosphorylation to transfer 2 chemical energy from food into the chemical energy of ATP. – Chloroplasts use photophosphorylation to transfer light energy into the chemical energy of ATP.

11 7.10 ATP and NADPH power sugar synthesis in Figure 7.10A the Calvin cycle CO2 Input ATP . The Calvin cycle makes sugar within a chloroplast. NADPH . To produce sugar, the necessary ingredients are

– atmospheric CO2 and – ATP and NADPH generated by the light reactions. Calvin Cycle . The Calvin cycle uses these three ingredients to produce an energy-rich, three-carbon sugar called glyceraldehyde-3-phosphate (G3P).

. A plant cell may then use G3P to make glucose Output: G3P and other organic molecules.

Figure 7.10B_s1 7.10 ATP and NADPH power sugar synthesis in Step 1 Carbon fixation Input: 3 CO the Calvin cycle 2

. The steps of the Calvin cycle include Rubisco 1 3 P P 6 P – carbon fixation, RuBP 3-PGA – reduction,

Calvin – release of G3P, and Cycle – regeneration of the starting molecule ribulose bisphosphate (RuBP).

Figure 7.10B_s2 Figure 7.10B_s3 Step 1 Carbon fixation Input: 3 Step 1 Carbon fixation Input: 3

CO2 CO2

Rubisco Rubisco

1 1 3 P P 6 P 3 P P 6 P RuBP 3-PGA RuBP 3-PGA Step 2 Reduction 6 ATP Step 2 Reduction 6 ATP

6 ADP P 6 ADP P Calvin Calvin 2 2 Cycle Cycle 6 NADPH 6 NADPH

6 NADP Step 3 Release of one 6 NADP 6 P 6 P molecule of G3P 5 G3P P G3P G3P 3

Glucose Output: 1 P and other G3P compounds

12 Figure 7.10B_s4 Step 1 Carbon fixation Input: 3 7.11 EVOLUTION CONNECTION: Other CO 2 methods of carbon fixation have evolved in hot, dry climates Rubisco

1 3 P P 6 P . Most plants use CO2 directly from the air, and RuBP 3-PGA Step 2 Reduction 6 ATP carbon fixation occurs when the enzyme rubisco

3 ADP adds CO2 to RuBP. 6 ADP P Calvin 4 2 3 ATP Cycle 6 NADPH . Such plants are called C3 plants because the first product of carbon fixation is a three-carbon Step 3 Release of one 6 NADP 6 P compound, 3-PGA. molecule of G3P 5 P G3P G3P 3

7.11 EVOLUTION CONNECTION: Other 7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in methods of carbon fixation have evolved in hot, dry climates hot, dry climates

. In hot and dry weather, C3 plants . C4 plants have evolved a means of – close their stomata to reduce water loss but – carbon fixation that saves water during photosynthesis while – prevent CO2 from entering the leaf and O2 from leaving. – optimizing the Calvin cycle. – As O2 builds up in a leaf, rubisco adds O2 instead of CO to RuBP, and a two-carbon product of this reaction 2 . C4 plants are so named because they first fix CO2 is then broken down in the cell. into a four-carbon compound. – This process is called photorespiration because it occurs in the light, consumes O , and releases CO . . When the weather is hot and dry, C4 plants keep 2 2 their stomata mostly closed, thus conserving water. – But unlike cellular respiration, it uses ATP instead of producing it.

7.11 EVOLUTION CONNECTION: Other Figure 7.11 methods of carbon fixation have evolved in

hot, dry climates Mesophyll CO CO 2 2 cell Night

. Another adaptation to hot and dry environments 4-C compound 4-C compound has evolved in the CAM plants, such as Bundle- sheath CO2 CO2 pineapples and cacti. cell . CAM plants conserve water by opening their Calvin Calvin Cycle Cycle stomata and admitting CO2 only at night. . CO is fixed into a four-carbon compound, 2 3-C sugar 3-C sugar Day C4 plant CAM plant – which banks CO2 at night and Sugarcane Pineapple – releases it to the Calvin cycle during the day.

13 Figure 7.11_1 Figure 7.11_2

Mesophyll CO2 CO2 cell Night

4-C compound 4-C compound

Bundle-

sheath CO2 CO2 cell

Calvin Calvin Cycle Cycle

Sugarcane

3-C sugar 3-C sugar Day C4 plant CAM plant

Figure 7.11_3

PHOTOSYNTHESIS REVIEWED AND EXTENDED

Pineapple

7.12 Review: Photosynthesis uses light energy, carbon 7.12 Review: Photosynthesis uses light energy, carbon dioxide, and water to make organic molecules dioxide, and water to make organic molecules

. Most of the living world depends on the food- . About half of the carbohydrates made by making machinery of photosynthesis. photosynthesis are consumed as fuel for cellular respiration in the mitochondria of plant cells. . The chloroplast . Sugars also serve as the starting material for making – integrates the two stages of photosynthesis and other organic molecules, such as proteins, lipids, and

– makes sugar from CO2. cellulose. . Excess food made by plants is stockpiled as starch in roots, tubers, seeds, and fruits.

14 Figure 7.12 7.13 CONNECTION: Photosynthesis may

H2O CO2 moderate global climate change Chloroplast Light . The greenhouse effect operates on a global NADP scale. ADP P Light Reactions – Solar radiation includes visible light that penetrates the RuBP Photosystem II Calvin Earth’s atmosphere and warms the planet’s surface. Cycle 3-PGA Electron (in stroma) transport chain – Heat radiating from the warmed planet is absorbed by Thylakoids Photosystem I ATP Stroma gases in the atmosphere, which then reflects some of the heat back to Earth. NADPH G3P Cellular respiration – Without the warming of the greenhouse effect, the Earth Cellulose would be much colder and most life as we know it could Starch O 2 Sugars Other organic not exist. compounds

Figure 7.13A Figure 7.13B

Some heat energy escapes Sunlight into space

Atmosphere Radiant heat

trapped by CO2 and other gases

7.13 CONNECTION: Photosynthesis may 7.13 CONNECTION: Photosynthesis may moderate global climate change moderate global climate change

. The gases in the atmosphere that absorb heat . Increasing concentrations of greenhouse gases radiation are called greenhouse gases. These have been linked to global climate change (also include called global warming), a slow but steady rise in Earth’s surface temperature. – water vapor, . Since 1850, the atmospheric concentration of CO – carbon dioxide, and 2 has increased by about 40%, mostly due to the – methane. combustion of fossil fuels including – coal, – oil, and – gasoline.

15 7.13 CONNECTION: Photosynthesis may 7.13 CONNECTION: Photosynthesis may moderate global climate change moderate global climate change

. The predicted consequences of continued warming . Widespread deforestation has aggravated the include global warming problem by reducing an effective – melting of polar ice, CO2 sink.

– rising sea levels, . Global warming caused by increasing CO2 levels – extreme weather patterns, may be reduced by – droughts, – limiting deforestation, – increased extinction rates, and – reducing fossil fuel consumption, and – the spread of tropical diseases. – growing biofuel crops that remove CO2 from the atmosphere.

7.14 SCIENTIFIC DISCOVERY: Scientific study Figure 7.14A of Earth’s ozone layer has global significance

. Solar radiation converts O2 high in the atmosphere to ozone (O3), which shields organisms from damaging UV radiation. Southern tip of . Industrial chemicals called CFCs have caused South dangerous thinning of the ozone layer, but America international restrictions on CFC use are allowing a Antarctica slow recovery.

Figure 7.14B You should now be able to

1. Define autotrophs, heterotrophs, producers, and photoautotrophs. 2. Describe the structure of chloroplasts and their location in a leaf. 3. Explain how plants produce oxygen. 4. Describe the role of redox reactions in photosynthesis and cellular respiration. 5. Compare the reactants and products of the light reactions and the Calvin cycle.

16 You should now be able to You should now be able to

6. Describe the properties and functions of the 11. Compare the mechanisms that C3, C4, and CAM different photosynthetic pigments. plants use to obtain and use carbon dioxide. 7. Explain how photosystems capture solar energy. 12. Review the overall process of the light reactions and the Calvin cycle, noting the products, 8. Explain how the electron transport chain and reactants, and locations of every major step. chemiosmosis generate ATP, NADPH, and oxygen in the light reactions. 13. Describe the greenhouse effect. 9. Compare photophosphorylation and oxidative 14. Explain how the ozone layer forms, how human phosphorylation. activities have damaged it, and the consequences of the destruction of the ozone 10. Describe the reactants and products of the layer. Calvin cycle.

Figure 7.UN01 Figure 7.UN02

H2O CO2 Light

NADP Stroma Thylakoids ADP P Light energy Light Calvin 66CO H O C H O 6 2 2 6 12 6 O2 Reactions Cycle Carbon dioxide Water Oxygen gas ATP Photosynthesis Glucose

NADPH

Chloroplast

O2 Sugar

Figure 7.UN03 Figure 7.UN04

Photosynthesis converts includes both

(a) (b) (c) to in which in which Mitochondrion Chloroplast chemical energy light-excited Intermembrane  CO2 is fixed to H c. H2O is split electrons of space chlorophyll RuBP and and then

Membrane (d) are passed reduce (h) down NADP to using Matrix d. (f) to produce (e) a. producing b. e. sugar (g) (G3P) by chemiosmosis