Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 6 Lecture Slides

6.1 An Overview of Photosynthesis All of the energy used by almost all living cells ultimately comes from the sun  plants, algae, and some bacteria capture the sunlight energy by a process called photosynthesis  only about 1% of the available energy in sunlight is captured

6.1 An Overview of Photosynthesis The leaf cells of plants contain chloroplasts  the chloroplast contains internal membranes called thylakoids  the thylakoids are stacked together in columns called grana  the stroma is a semiliquid substance that surrounds the thylakoids

6.1 An Overview of Photosynthesis Embedded in the thylakoid membrane are photosystems that contain pigments  the primary pigment molecule in most photosystems is chlorophyll  the pigments act as an antenna to capture energy from sunlight  individual chlorophyll pigments pass the captured energy between them

6.1 An Overview of Photosynthesis Photosynthesis takes places in three stages 1.capturing energy from sunlight 2.using the captured energy to produce ATP and NADPH 3.using the ATP and NADPH to make carbohydrates from CO 2 in the atmosphere

6.1 An Overview of Photosynthesis The overall reaction for photosynthesis may be summarized by this equation 6 CO H 2 O + light  C 6 H 12 O H 2 O + 6 O 2 The process of photosynthesis is divided into two types of reactions  light-dependent reactions take place only in the presence of light and produce ATP and NADPH  light-independent reactions do not need light to occur and result in the formation of organic molecules more commonly known as the Calvin cycle

Overview of Photosynthesis, part 1

Overview of Photosynthesis, part 2

6.2 How Plants Capture Energy from Sunlight Light is comprised of packets of energy called photons  sunlight has photons of varying energy levels the possible range of energy levels is represented by the electromagnetic spectrum Pigments are molecules that absorb light energy  the pigment in human eyes only absorbs photons of intermediate energy levels

Figure 6.1 Photons of different energy: the electromagnetic spectrum

6.2 How Plants Capture Energy from Sunlight The main pigment in plants is chlorophyll chlorophyll absorbs light at the end of the visible spectrum, mainly blue and red light  two versions of chlorophyll are present in plants chlorophyll a chlorophyll b

6.2 How Plants Capture Energy from Sunlight In plants, the light-dependent reactions occur on the thylakoid membranes within chloroplasts  pigments and protein molecules occur as complexes called photosystems  light energy is first captured by any one of the chlorophyll pigments in a photosystem  the energy is passed along to other pigments until it reaches the reaction center chlorophyll molecule  the reaction center then releases an excited electron, which is then transferred to an electron acceptor  the excited electron that is lost is then replaced by an electron donor

Figure 6.2 How a photosystem works

6.2 How Plants Capture Energy from Sunlight The light-dependent reactions in plants and algae use two photosystems  Photosystem II captures a photon of light and releases an excited electron to the electron transport system (ETS) the ETS then produces ATP  Photosystem I absorbs another photon of light and releases an excited electron to another ETS this ETS produces NADPH

Figure 6.3 Plants use two photosystems

Animation: Cyclic and Noncyclic Photophosphorylation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at

6.3 How Photosystems Convert Light to Chemical Energy Photosystem II  its reaction center consists of more than ten transmembrane proteins  this is surrounded by an antenna complex of pigments that funnel captured photons to the reaction center  the reaction center yields an excited electron to the primary electron acceptor  water is split to provide replacement electrons to the reaction center, resulting in the production of O 2

6.3 How Photosystems Convert Light to Chemical Energy The electron transport system (ETS) receives the excited electron from the electron acceptor  the ETS is comprised of proteins that are embedded in the thylakoid membrane  one of these proteins acts as a proton pump to move a proton from the stroma into the thylakoid space  at the end of the ETS, the electron is passed to the reaction center of photosystem I

6.3 How Photosystems Convert Light to Chemical Energy As a result of the proton pump of the ETS, a large concentration of protons builds up in the thylakoid space  the thylakoid membrane is impermeable to protons  protons can only re-enter the stroma by traveling through a protein channel called ATP synthase  as protons diffuse back down their concentration gradient and pass through ATP synthase, ADP is phosphorylated into ATP in a process called chemiosmosis

Figure 6.4 The photosynthetic electrons are used to produce ATP and NADPH

Animation: Proton Pump Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at

6.3 How Photosystems Convert Light to Chemical Energy Photosystem I  receives the electron from the first ETS and absorbs of another photon of light  the reaction center yields an excited electron to another ETS, that uses the electrons (and hydrogen ions) to form NADPH because this removes a proton from the stroma, the production of NADPH also aids in establishing the proton gradient for chemiosmosis to occur

Animation: Photosynthetic Electron Transport and ATP Synthesis Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at

6.4 Building New Molecules The synthesis of organic molecules from CO 2 employs the Calvin cycle  these reactions are also known as C 3 photosynthesis  The Calvin cycle must “turn” 6 times in order to form a new glucose molecule  the products of the light-dependent reactions are used ATP energy drives the cycle NADPH provides hydrogen atoms

Essential Biological Process 6A: The Calvin Cycle

Animation: How the Calvin Cycle Works Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at

6.5 Photorespiration: Putting the Brakes on Photosynthesis Many plants have trouble carrying out C 3 photosynthesis when it is hot  plants close openings in their leaves, called stomata (singular, stoma), in order to prevent water loss  the closed stoma also prevent gas exchange  O 2 levels build up inside the leaves while the concentrations of CO 2 fall the enzyme rubisco, which carries out the first step in the Calvin cycle, binds O 2 instead of CO 2 this process is called photorespiration and short-circuits photosynthesis

Figure 6.6 Plant response in hot weather

6.5 Photorespiration: Putting the Brakes on Photosynthesis Some plants have adapted to hot climates by performing C 4 photosynthesis  these C 4 plants include sugarcane, corn, and many grasses  they fix carbon first in the mesophyll cells and then regenerate CO 2 in bundle-sheath cells CO 2 then enters the Calvin cycle in bundle-sheath cells

Figure 6.7 Carbon fixation in C 4 plants

6.5 Photorespiration: Putting the Brakes on Photosynthesis Another strategy to avoid a reduction in photosynthesis in hot weather occurs in many succulent (water-storing) plants, such as cacti and pineapples  these plants undergo crassulacean acid metabolism (CAM) photosynthesis occurs via the C 4 pathway at night and the C 3 pathway during the day

Inquiry & Analysis What does the increase in chlorophyll a levels say about numbers of phytoplankton? What general statement can be made regarding the effect of iron seeding on phytoplankton levels? Does Iron Limit Phytoplankton Growth?