1. CHAPTER 7 Photosynthesis: Using Light to Make Food Overview: Photosynthesis Light Reactions Calvin Cycle Review of photosynthesis & C3, C4, CAM plants Greenhouse effect Ozone Layer

2. Light is central to the life of a plant Photosynthesis is the most important chemical process on Earth –It provides food for virtually all organisms Plant cells convert light into chemical signals that affect a plant’s life cycle Life in the Sun All of the food consumed by humans can be traced back to photosynthetic plants

3. Light can influence the architecture of a plant –Plants that get adequate light are often bushy, with deep green leaves –Without enough light, plants become tall and spindly with small pale leaves Too much sunlight can damage a plant –Chloroplasts and carotenoids help to prevent such damage

4. Almost all plants are photosynthetic autotrophs (=self-feeders; they make their own food), as are some bacteria and protists THE BASICS OF PHOTOSYNTHESIS –They generate their own organic matter through photosynthesis All organisms that produce org molecules from inorg molecules using light: photoautotrophs

5. On land, plants such as oak trees and cacti are the predominant producers (produce their food supply)

6. In aquatic environments, algae and photosynthetic bacteria are the main food producers

7. Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water Overview of Photosynthesis

8. Photosynthesis Chloroplasts: Sites of Photosynthesis –Occurs in chloroplasts Chloroplasts: –Are found in the cells of the mesophyll (green tissue in the interior of the leaf) –Contain stroma, a thick fluid –Contain thylakoids, membranous sacs (The thylakoids contain chlorophyll - Chlorophyll is the green pigment that captures light for photosynthesis) –(grana- singular granum-, stacks of thylakoids)

9. Plants produce O2 gas by splitting water The O 2 liberated by photosynthesis is made from the oxygen in water

10. Water molecules are split apart and electrons and H + ions are removed, leaving O 2 gas –These electrons and H + ions are transferred to CO 2, producing sugar Photosynthesis is a redox process, as is cellular respiration

11. Photosynthesis is a two-step process – An Overview: Light Reaction & Calvin cycle The light reactions convert solar energy to chemical energy & produce O2 The Calvin cycle makes sugar from carbon dioxide Calvin cycle

12. Light Reactions

13. Sunlight is a type of energy called radiation, or electromagnetic energy THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY The full range of radiation is called the electro-magnetic spectrum Certain wavelengths of visible light drive the light reactions of photosynthesis

14. Chloroplasts absorb light energy and convert it to chemical energy Light behaves as photons, discrete packets of energy Chlorophyll molecules absorb photons –Electrons in the pigment (chlorophyll) gain energy

15. Chloroplasts contain several pigments Chloroplast Pigments –Chlorophyll a –Chlorophyll b –Carotenoids

16. Photosystems capture solar power Photosystems consist of many light-harvesting complexes (containing chlorophyll a, b, and carotenoid that function as a light-gathering antenna) surrounding a reaction center.

17. Reaction center: A protein complex that contains a chlorophyll a and a molecule called primary electron acceptor - a primary electron acceptor receives excited electrons from the reaction-center chlorophyll

18. Photosystems are 2 types that corporate in the light reaction; (photosystem II functions first) Photosystem I:Chlorophyll a of the reaction center is P700 (absorbs light with wavelength 700 nm) Photosystem II: Chlorophyll a of the reaction center is P680 (absorbs light with wavelength 680 nm)

19. Two types of photosystems cooperate in the light reactions Let’s see how these 2 systems work together to generate ATP and NADPH.

20. An electron transport chain –Connects the two photosystems –Releases energy that the chloroplast uses to make ATP

21. Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons The excited electrons are passed from the primary electron acceptor to electron transport chains –Their energy ends up in ATP and NADPH In the light reactions, electron transport chains generate ATP, NADPH, and O 2

22. Photosystem II regains electrons by splitting water, leaving O 2 gas as a by-product In photosystem I, electrons from the bottom of the cascade pass into its P700 chlorophyll

23. To Calvin Cycle 1/21/2 The light reactions in the thylakoid membrane

24. The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H + through that membrane –The flow of H + back through the membrane is harnessed by ATP synthase to make ATP –In the stroma, the H + ions combine with NADP + to form NADPH Chemiosmosis powers ATP synthesis in the light reactions

25. The production of ATP by chemiosmosis in photosynthesis

26. The Calvin Cycle

27. The Calvin cycle occurs in the chloroplast’s stroma –This is where carbon fixation takes place and sugar is manufactured ATP and NADPH power sugar synthesis in the Calvin cycle

28. The Calvin cycle –Functions like a sugar factory within a chloroplast –Regenerates the starting material with each turn THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE

29. The Calvin cycle constructs G3P using –carbon from atmospheric CO 2 –electrons and H + from NADPH –energy from ATP Energy-rich sugar is then converted into glucose

30. PHOTOSYNTHESIS REVIEWED AND EXTENDED

31. A summary of the chemical processes of photo- synthesis Review: Photosynthesis uses light energy to make food molecules

32. Many plants make more sugar than they need –The excess is stored in roots, tuber, and fruits –These are a major source of food for animals

33. Most plants are C 3 plants, which take CO 2 directly from the air and use it in the Calvin cycle –In these types of plants, stomata on the leaf surface close when the weather is hot –This causes a drop in CO 2 and an increase in O 2 in the leaf C 3 plants –Use CO 2 directly from the air –Are very common and widely distributed

34. C 4 plants –Close their stomata to save water during hot and dry weather –Can still carry out photosynthesis CAM plants –Open their stomata only at night to conserve water C 4 and CAM plants have special adaptations that save water

35. Some plants have special adaptations that enable them to save water –Special cells in C 4 plants—corn and sugarcane—incorporate CO 2 into a four-carbon molecule –This molecule can then donate CO 2 to the Calvin cycle

36. The CAM plants—pineapples, most cacti — employ a different mechanism –They open their stomata at night and make a four- carbon compound –It is used as a CO 2 source by the same cell during the day Night

37. PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE

38. Human activity is causing global warming; photosynthesis moderates it Sunlight Due to the increased burning of fossil fuels, atmospheric CO 2 is increasing –CO 2 warms Earth’s surface by trapping heat in the atmosphere –This is called the greenhouse effect

39. Old-growth forests How Photosynthesis Moderates the Greenhouse Effect –Are important for lumber –Are important for moderating world climates

40. Greenhouse gases are the most likely cause of global warming, a slow but steady rise in the Earth’s surface temperature –Destruction of forests may be increasing this effect Because photosynthesis removes CO 2 from the atmosphere, it moderates the greenhouse effect –Unfortunately, deforestation may cause a decline in global photosynthesis –It swaps O 2 for CO 2

41. The O 2 in the atmosphere results from photosynthesis –Solar radiation converts O 2 high in the atmosphere to ozone (O 3 ) –Ozone shields organisms on the Earth’s surface from the damaging effects of UV radiation

42. Industrial chemicals called CFCs have hastened ozone breakdown, causing dangerous thinning of the ozone layer Sunlight International restrictions on these chemicals are allowing recovery