Chapter 5

1. Chapter 5 Making SugarPhotosynthesis 0

2. BioFlix: Photosynthesis

3. Photosynthesis Photosynthesis is the process by which plants and other microorganisms trap light energy from the sun. Photosynthesis takes Carbon dioxide and combines it with water using light energy to produce glucose and oxygen. Photosynthesis is done by plants and some bacteria, but not by animals or fungi.

4. The Chloroplast: The machine of Photosynthesis • Plastids: group of organelles that perform many functions • synthesis, storage and export • storage plastids for sugar = amyloplasts • plastids with bright red and yellow pigments = chromoplasts • plastids for photosynthesis = chloroplast • also known as green plastids

5. The Chloroplast: The machine of Photosynthesis • like mitochondria – plastids are comprised of an outer and inner membrane • the inner membrane of the chloroplast is extensively folded to increase surface area for the enzymes of photosynthesis • these folded membranes are called thylakoid membranes • a stack of thylakoid membranes = granum • plus an inner fluid = stroma • also have ribosomes and DNA

6. Photosynthesis • 6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O • Light Reactions: light + water = O2 • Stroma Reactions - Calvin Cycle: CO2 + ATP + NADPH = sugar H2O CO2 Light NADP+ ADP + P i CALVIN CYCLE LIGHT REACTIONS ATP NADPH Chloroplast [CH2O] (sugar) O2

7. Photosynthesis • 6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O • requires the reduction of carbon – converting it into carbohydrate • reduction – chemical process whereby a molecule accepts electrons • for each molecule of CO2 - photosynthesis will require 4 electrons and a good source of energy to reduce the carbon • electrons come from water • energy comes from light • water and light do not act directly on CO2 • rather they create the intermediates ATP and NAPDH via light-dependent reactions • the ATP and NADPH then interact with CO2 in the stroma reactions to produce carbohydrates (i.e. sugars)

8. Light 1 m (109 nm) 10–3 nm 103 nm 106 nm 10–5 nm 103 m 1 nm • light is a small segment of the electromagnetic radiation spectrum • from gamma rays to radio waves • the radiation can be thought of as a set of waves or as a set of energized particles called photons • each wave has a specific wavelength and photons with specific energy levels • in photosynthesis – specialized pigments are present to absorb wavelengths of radiation in the visible range Gamma rays Micro- waves Radio waves X-rays Infrared UV Visible light 650 750 nm 500 550 600 700 450 380 Shorter wavelength Longer wavelength Higher energy Lower energy

9. Photosynthetic Pigments: The Light Receptors Light Reflected light • Pigments are substances that absorb visible light • they absorb the photons of light • different pigments absorb different wavelengths of light • wavelengths that are not absorbed are reflected or transmitted • Leaves appear green because chlorophyll reflects and transmits green light • the pigments of photosynthesis are located in the chloroplast – in the thylakoid membranes Chloroplast • photosynthetic pigments: chlorophylls & carotenoids • chlorophyll a & chlorophyll b • transfer absorbed light energy to electrons that then enter chemical reactions Absorbed light Granum Transmitted light

10. Chlorophyll: The Light Receptor Light • photosynthetic pigments: chlorophylls • chlorophyll a & chlorophyll b • transfer absorbed light energy to electrons that then enter chemical reactions • as these chlorophylls absorb light energy – electrons are “excited” up into a higher energy electron shell • as they return to where they belong (i.e. their “ground state” – they release the absorbed energy • this energy will be used in photosynthesis Reflected light Chloroplast Absorbed light Granum Transmitted light

11. Animation: Photosynthesis Click “Go to Animation / Click “Play”

12. Photosynthesis - The Light Reactions Light reactions (thylakoids): Produces oxygen, ATP and NADPH comprised of Photosystem II, then Photosystem I

13. Thylakoid Photosystem STROMA Photon Light-harvesting complexes Reaction center Primary electron acceptor e– Thylakoid membrane Special chlorophyll a molecules Pigment molecules Transfer of energy THYLAKOID SPACE (INTERIOR OF THYLAKOID) Photosystems • embedded in the thylakoid membranes are protein complexes called Photosystems (named in the order of their discovery NOT their function) • photosystem I – occurs after PSII • photosystem II • each Photosystem is made up of : • 1. light harvesting complex • 2. reaction center • light harvesting complexes: proteins and chlorophyll b molecules that surround a reaction center • reaction center: pair of chlorophyll a molecules that will absorb light energy • absorbed light energizes these two photosystems and induces a flow of electrons through these photosystems • flow is known as the light reactions – requires Sunlight

14. Light Reactions of Photosynthesis • 1. a photon of light strikes the light-harvesting complex • the energy of this photon is relayed to the reaction center of PSII • the electrons of chlorophyll a are excited to a higher energy state – 2 chlorophylls, 2 excited electrons • 2. excited electrons are captured by a primary electron acceptor in the reaction center • considered a photosynthetic pigment • 3. AT THE SAME TIME: water is split into two H+, two electrons and an oxygen atom • these 2 electrons are transferred to the chlorophyll a of PSII to replace the electrons it has lostto the primary electron acceptor • oxygen atoms combine to form O2 Light Harvesting Complex Primary acceptor e– H2O 2 H+ Reaction Center + O2 1/2 Energy of electrons e– e– Light chlorophyll a pigments absorbing light energy Photosystem II (PS II)

15. Light Reactions of Photosynthesis • while light is being absorbed by the proteins and pigments of photosystem II – they are also being absorbed by the proteins and pigments that make up the light harvesting complex of Photosystem I • so both photosystems run at the same time • they are connected to one another through an Electron Transport Chain Primary acceptor e– chlorophyll a Light pigments absorbing light energy Photosystem I (PS I)

16. Linking the two Photosystems: The Electron Transport Chain • each excited electron passes from the primary electron acceptor of PSII to a cofactor called Pq • then are transferred onto a protein complex called a cytochrome complex • then transferred onto another cofactor called Pc • these electrons ends up replacing the electrons that were excited up to the primary acceptor of PSI Electron Transport chain Primary acceptor Primary acceptor Electron transport chain Fd e– Pq e– e– e– NADP+ H2O Cytochrome complex 2 H+ + 2 H+ NADP+ reductase + O2 NADPH 1/2 Pc e– + H+ chlorophyll a Energy of electrons e– Light chlorophyll a Light ATP Photosystem I (PS I) Photosystem II (PS II)

17. Here’s the cool thing • as the electrons are transferred through this ETC – they power the synthesis of ATP • ATP synthase just like in mitochondria Primary acceptor Primary acceptor Electron transport chain e– Pq e– H2O Cytochrome complex 2 H+ + O2 1/2 Pc e– chlorophyll a Energy of electrons e– Light chlorophyll a ATP Photosystem I (PS I) Photosystem II (PS II)

18. Mitochondrion Chloroplast CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE Diffusion H+ Thylakoid space Intermembrane space Electron transport chain Membrane ATP synthase Key Stroma Matrix Higher [H+] Lower [H+] ADP + P i ATP H+

19. Here’s the cool thing • remember how light energy also excites electrons in PSI up to their primary acceptor?? • this creates a “hole” in the chlorophylls of PSI – we have to replace them • what do we use? • we use the electrons that have just passed through the ETC Electron Transport chain Primary acceptor Primary acceptor Electron transport chain Fd e– Pq e– e– e– NADP+ H2O Cytochrome complex 2 H+ + 2 H+ NADP+ reductase + O2 NADPH 1/2 Pc e– + H+ Energy of electrons P700 e– Light P680 Light ATP Photosystem I (PS I) Photosystem II (PS II)

20. Another Electron Transport Chain • what happens to the excited electrons in PSI? • they run through a second ETC • electrons are passed to a cofactor called ferredoxin (Fd) and then ultimately onto an enzyme called NADP+reductase • NADP+ reductase takes TWO electrons from Fd and passes them to NADP+ reducing it to NADPH Electron Transport chain Primary acceptor Primary acceptor Electron transport chain Fd e– Pq e– e– e– NADP+ H2O Cytochrome complex 2 H+ + 2 H+ NADP+ reductase + O2 NADPH 1/2 Pc e– + H+ Energy of electrons P700 e– Light P680 Light ATP Photosystem I (PS I) Photosystem II (PS II)

21. H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH O2 [CH2O] (sugar) STROMA (Low H+ concentration) Cytochrome complex Photosystem I Photosystem II Light NADP+ reductase Light 2 H+ NADP+ + 2H+ Fd NADPH + H+ Pq Pc H2O O2 1/2 THYLAKOID SPACE (High H+ concentration) 2 H+ +2 H+ To Calvin cycle Thylakoid membrane ATP synthase STROMA (Low H+ concentration) ADP + ATP P i H+

22. How’s this instead? e– ATP e– e– NADPH – requires 2 electrons e– e– e– Mill makes ATP Photon e– Photon Photosystem II Photosystem I

23. What now? The Calvin Cycle Series of reactions that occur in the stroma to produce glucose using ATP AND NADPH CO2 gets incorporated into a 5-carbon molecule called RuBP – to make a 6 carbon sugar that will be split to form 2 molecules of a 3 carbon sugar called G3P this needs energy and electrons energy comes from ATP hydrolysis the electrons come from NAPDH the enzyme that incorporates the CO2 is called Rubisco -most abundant protein on Earth??

24. What now? The Calvin Cycle a total of SIX G3P molecules are required for the plant to run its Calvin cycle 3 CO2 molecules + 3 RuBP molecules = 6 G3P molecules for every 6 G3P molecules made – ONE is converted into sugar and other carbs by other parts of the plants the other 5 G3Ps are used again by the Calvin cycle

25. Animation: Leaves: The Site of Photosynthesis Click “Go to Animation” / Click “Play”

26. BioFlix: Carbon Cycle

27. Carbon cycles between living organisms, the atmosphere, bodies of water, and the soil. humans contribute to the flow of carbon through their burning of fossil fuels fossil fuels are stored carbon most common form = coal – fossilized carbon found in sedimentary rock forms when decomposing plant matter is subject to extremes temps and pressures How does Photosynthesis figure into the flow of carbon?

28. The Flow of Carbon Measurements of carbon dioxide from Antarctic ice cores shows that it has been increasing in the atmosphere over the past 50 years. • Data from ice cores show present levels of carbon dioxide are the highest in the last 400,000 years. • Temperature and amount of carbon dioxide are correlated

29. The Greenhouse Effect Global warming is the progressive increase of Earth’s average temperature. Effects of global warming: Rise in sea levels Global melting of glaciers Loss of habitat for temperature-sensitive species More severe storms

30. The Greenhouse Effect Greenhouse effect is the mixture of carbon dioxide and other greenhouse gases in the atmosphere. Sunlight warms the surface of the Earth. Most of the warmth radiates into space. Some of the warmth is absorbed by gases in the atmosphere, making the Earth warmer. These gases are what keeps the earth a warm planet, but in excess these gases cause global warming.

31. How Global Warming Might Reduce Photosynthesis Stomata = openings in leaves for entrance of gases Guard cells regulate stomata openings Transpiration: movement of water out of a plant through stomata Stomata open: plenty of carbon dioxide, loss of water Stomata closed: conserves water, limits photosynthesis when the stomata are closed – the plant cells undergo photorespiration. during photorespiration oxygen is used instead of carbon dioxide to react with RuBP. this leads to plants releasing carbon dioxide instead of oxygen.

32. 5.4 How Global Warming Might Reduce Photosynthesis Deforestation: clearing of forests for farming and human settlements 25% of carbon dioxide added to the atmosphere comes from cutting and burning forests in the tropics. Replanting helps decrease carbon dioxide levels: young trees have faster net photosynthetic rates than older trees.