Photosynthesis

1. Photosynthesis The conversion of light energy into chemical energy stored in organic compounds.

2. Biochemical pathway involves a complex series of chemical reactions in which the product of one reaction is consumed in the next reaction. • Chlorophyll occurs in all photosynthetic eukaryotic cells and is considered to be essential for photosynthesis of the type carried out by plants. • It functions in the capture of light energy by either directly or indirectly absorbing or receiving it in the form of high energy electrons from the accessory pigments.

3. Light Energy Photosynthesis Organic Compounds + Oxygen Autotrophs Autotrophs and heterotrophs Carbon Dioxide + Water Cellular Respiration

4. Light Absorption in Chloroplasts • Light Reactions • Absorption of light in chloroplasts. • A cell of a plant may have as many as 50 chloroplasts. • Chloroplasts • Surrounded by a pair of membranes. • Inside the inner membrane are flattened disks called thylakoids. • Thylakoids form stacks called grana. • A solution called stroma surrounds the thylakoids. • Stroma contains the enzymes responsible for the dark reactions of photosynthesis.

5. Grana • Each grana is composed structurally of layers of protein molecules alternating with layers of chlorophyll, carotenes, and other pigments, and special types of lipids containing galactose or sulfur but only one fatty acid. • The surface active lipids are believed to be absorbed between the layers and serve in stabilizing the lamellae composed of one of the alternate layers of protein and pigments. • This lamellar structure is important in permitting the transfer of energy captured from the sun.

6. Light and Pigments • Light from the Sun is composed of many colors. • The array of colors is called the visible spectrum. • ROYGBIV • Light travels through space as waves of energy. • Light waves are measured in terms of their wavelength. • A pigment is a compound that absorbs a specific band of wavelengths. • By absorbing certain colors, a pigment reflects the remaining colors of the spectrum.

7. Chlorophyll • Chlorophylls are a variety of pigments located in the membrane of the thylakoids. • The two most common types of chlorophyll are chlorophyll a and chlorophyll b. • Chlorophyll a • Absorbs less blue light. • Absorbs more red light. • Directly involved in the light reactions of photosynthesis

8. Chlorophyll • Chlorophyll b • Absorbs more blue light • Absorbs less red light. • Assists chlorophyll a in capturing light energy. Therefore it is called an accessory pigment. • Other compounds found in the thylakoid membrane • Yellow, Orange, and Brown Carotenoids. • Carotenoids serve as accessory pigments. • By absorbing colors that chlorophyll a cannot absorb, the accessory pigments convert the remaining energy. • Chlorophyll c or d take the place of Chlorophyll b in some plants.

9. Carotenoids • Red, orange, or yellow, fat-soluble pigments found in almost all chloroplasts. • Carotenoids that do not contain oxygen are called carotenes. They are deep orange in color. • Those that contain oxygen are called xanthophylls and are yellowish in color. • Carotenoids are bound to proteins within the lamellae of the chloroplast.

10. Carotenoids • In the green leaves, the color of the carotenoids is masked by the much more abundant chlorophylls. • In some tissues , such as the ripe tomato or the petals of flowers, the carotenoids predominate. • The carotenoids function in absorbing light not usable by the chlorophylls and in transferring the absorbed energy to chlorophyll a.

11. Phytochromes • A pigment that is a light sensitive, blue phytochrome. • They play a fundamental role in allowing the plant to detect whether it is in a light or dark environment.

12. Electron Transport • Photosystem is a cluster of a few hundred pigment molecules • Light reactions begin when accessory pigment molecules in both photosystems absorb light. • Those molecules acquire energy from the light wave. • The energy is passed to other molecules until it reaches chlorophyll a molecules.

13. Electron Transport • Step One: • Light energy forces electrons to enter a higher energy level in the two chlorophyll a molecules of photosystem II. • These electrons are said to be excited.

14. Electron Transport • Step Two • The excited electrons have enough energy to leave chlorophyll a molecules. • Because they have lost electrons, the chlorophyll a molecules have undergone an oxidation reaction. • The electrons are accepted by a molecule in the thylakoid membrane called the primary electron acceptor.

15. Electron Transport • Step Three • The primary electron acceptor then donates the electron to the first of a series of molecules located in the thylakoid membrane. • This series of molecules is called an electron transport chain. • As the electrons move from molecule to molecule in the electron transport chain they lose energy. • The energy they lose is harnessed to move proteins into the thylakoid.

16. Electron Transport • Step Four • At the same time light is absorbed by photosystem II it is also absorbed by photosystem I. • Electrons move from a pair of chlorophyll a molecules in photosystem I to another primary electron acceptor. • The electrons that are lost by these chlorophyll a molecules are replaced by electrons that have passed through the electron transport chain from photosystem II.

17. Electron Transport • Step Five • The primary electron acceptor of photosystem I donates electrons to a different electron transport chain. • This chain brings the electrons to the side of the thylakoid membrane that faces the stroma. • There the electrons combine with a proton and NADP+. • NADP+ is an organic molecule that accepts electrons during redox reactions. • This reaction causes NADP+ to be reduced to NADPH.

18. Electron Transport • If the electrons from photosystem II were not replaced, both electron transport chains would stop and photosynthesis would not occur. • The replacement electrons are provided by water molecules. • An enzyme inside the thylakoid splits the water molecules into protons, electrons, and oxygen. • For every two water molecules that are split, four electrons become available to replace those lost by chlorophyll molecules in photosystem II.

19. Electron Transport • The protons that are produced are left inside the thylakoid, while the oxygen diffuses out of the chloroplast and can then leave the plant. • Oxygen is not needed for photosynthesis to occur. • Oxygen is a byproduct of the light reactions and is essential for cellular respiration in most organisms.

20. Chemiosmosis • Chemiosmosis is a process in chloroplasts and mitochondria in which the movement of protons down their concentration gradient across a membrane is coupled to the synthesis of ATP. • Adenosine Triphosphate (ATP) is a molecule present in all living cells and acts as an energy source for metabolic processes.

21. Chemiosmosis • Some protons are produced from the breakdown of water molecules inside the thylakoid. • Other protons are pumped from the stroma to the interior of the thylakoid. • The energy to pump those protons is supplied by the excited electrons as they pass along the electron transport chain of photosystem II.

22. Chemiosmosis • These actions act to build up a concentration gradient of protons. • The concentration is higher inside the thylakoid than in the stroma. • The concentration gradient of protons represents potential energy. • That energy is harnessed by a protein called ATP synthase which is located in the thylakoid membrane.

23. Chemiosmosis • ATP synthase makes ATP by adding a phosphate group to adenosine diphosphate (ADP). • The energy that drives this reaction is provided by the movement of protons from inside of the thylakoid to the stroma. • ATP synthase converts the potential energy of the proton concentration gradient into chemical energy stored in ATP.

24. Chemiosmosis • ATP synthase is a multifunctional protein. • By allowing protons to cross the thylakoid membrane, ATP synthase functions as a carrier protein. • By catalyzing the synthesis of ATP from ADP, ATP synthase functions as an enzyme.

25. The Calvin Cycle • The Calvin Cycle is a biochemical pathway that produces organic compounds, using energy stored in ATP and NADPH during light reactions. • Carbon atoms from CO2 are bonded into organic compounds and is called carbon fixation. • The Calvin Cycle occurs within the stroma of the chloroplast.

26. The Calvin Cycle • Step One • CO2 diffuses into the stoma from the surrounding cytosol. • An enzyme combines a CO2 molecule with a five-carbon carbohydrate called RuBP. • The product is a six-carbon molecule that splits immediately into a pair of three-carbon molecules known as PGA.

27. The Calvin Cycle • Step Two • PGA is converted into another three-carbon molecule called PGAL in a two part process. • Each PGA molecule receives a phosphate group from a molecule of ATP. • The resulting group then receives a proton from NADPH and releases a phosphate group thereby producing PGAL. • In addition ADP, NADP, and phosphate are produced where they are then used again in the light reactions to synthesize additional molecules of ATP and NADPH.

28. The Calvin Cycle • Step 3 • Most of the PGAL is converted back into RuBP in a complicated series of reactions. • This requires a phosphate group from another molecule of ATP, which is changed into ADP. • By regenerating RuBP that was consumed in step one, the reactions of step three allow the Calvin Cycle to continue operating. • Some PGAL molecules are not converted into RuBP. Instead they leave the Calvin Cycle to be used by the plant to make other organic compounds.

29. Alternate Pathways • Plants that fix carbon exclusively through the Calvin Cycle are known as C3 plants because of the three-carbon compound PGA that is initially formed. • Other plant species fix carbon through alternative pathways and then release it to enter the Calvin Cycle. • These plants are generally found in hot, dry climates where plants can rapidly lose water to the air. • Most water loss is through small pores called stomata which are usually located on the underside of the leaf.

30. Alternate Pathways • Plants can partially close their stomata when the air is hot and dry thereby reducing water loss. • Stomata are also the major path for CO2 to enter and O2 to leave a plant. • When the stomata are partially closed, the level of CO2 in the plant falls as CO2 is consumed in the Calvin Cycle. • At the same time the level of O2 in the plant rises as the light reactions split water and generate O2.

31. Alternate Pathways • Both of these conditions, low CO2 levels and high O2 levels, inhibit carbon fixation by the Calvin Cycle. • Some plants have evolved a way of dealing with this problem using alternative pathways for carbon fixation.

32. The C4 Pathway • During the hottest part of the day, the C4 plants have their stomata partially closed. • Certain cells in a C4 plants have an enzyme that can fix CO2 into four-carbon compounds even when the CO2 level is low and the O2 level is high. • These compounds are then transported to other cells where the CO2 is released and enters the Calvin Cycle.

33. The C4 Pathway • C4 plants include corn, sugar cane, and crabgrass. • C4 plants lose only about half as much water as C3 plants when producing the same amount of carbohydrate.

34. The CAM Pathway • Some plants close their stomata during the day and open them at night opposite of most plants • Such plants fix carbon through a pathway called CAM. • At night CAM plants take in CO2 and fix it into a variety of organic compounds. • During the day CO2 is released from those compounds and enters the Calvin Cycle.

35. The CAM Pathway • CAM plants grow fairly slow compared to other plants. • CAM plants use less water than either C3 or C4 plants.

36. Rate of Photosynthesis • The rate of photosynthesis increases and then reaches a plateau as light intensity or CO2 concentration increases. • Below a certain temperature, the rate of photosynthesis increases as the temperature increases. • Above that temperature, the rate of photosynthesis decreases as temperature increases.

37. Harvesting Light • Photosystem I, shown here looking from the top, contains an electron transfer chain, colored here in bright colors, at the center of each of the three subunits. • Each one is surrounded by a dense ring of chlorophyll and carotenoid molecules that act as antennas. • In this picture, the protein is transparent so that only the cofactors are seen. These antenna molecules each absorb light and transfer energy to their neighbors. • Rapidly, all of the energy funnels into the three reaction centers, where is captured to create activated electrons.

38. This picture shows the electron transfer chain at the center, drawn in spacefilling spheres. • Two special chlorophyll molecules, residues 1140 and 1239, are also shown in spheres and colored green. • These two chlorophyll molecules act as a bridge between the reaction center in the middle and the many molecules in the surrounding antenna. • The many antenna cofactors are shown here in bond representation with small spheres for the magnesium ions at the center of each chlorophyll