Chapter 6: Photosynthesis

1. Chapter 6: Photosynthesis Life Depends on Photosynthesis Light ultimately provides the energy that powers nearly all life. Photosynthesis enables plants, algae, and certain microorganisms to harness light energy and convert it to chemical energy

2. Photosynthesis • is the process that plants undergo in order to transform solar energy into chemical energy stored in the bonds of sugar molecules. • The overall process of photosynthesis is usually represented by this equation: • 6 CO2 + 6 H20 + sunlight -----> C6H12O6 + 6 O2

3. Energy from the sun, with carbon dioxide and water- is used to make inorganic compounds into organic compounds: oxygen is a waste product!

4. http://missbzscience.wikispaces.com/file/view/photosynthesis.jpg/231477052/photosynthesis.jpghttp://missbzscience.wikispaces.com/file/view/photosynthesis.jpg/231477052/photosynthesis.jpg

5. There are 2 parts to photosynthesis • Light reactions– Light energy is absorbed form the sun and is converted to chemical energy- temporarily stored in the bonds of ATP and NADPH • Calvin cycle– organic compounds are formed using CO2 (now using the chemical energy stored from the light reactions)

6. Capturing Light Energy • The light reactions • begin with the absorption of light in- Chloroplasts • organelles found in the cells of plants, some bacteria, and algae. • Inside chloroplasts are Thylakoids, a system of membranes inside the chloroplast that look like flattened sacs

7. Light and Pigments • White light from the sun is composed of an array of colors called the visiblespectrum. • Pigments absorb certain colors of light and reflect or transmit the other colors. • Chlorophyll a & b • Carotenoids

8. The sun emits energy at a range of wavelengths: the visible spectrum is a small part of that range.

9. Chloroplast Pigments • Located in membranes of the thylakoids of chloroplasts are several pigments, including chlorophylls (chlorophyll aand chlorophyll b) and carotenoids.

10. Why Chlorophyll looks green. • Since a pigment is any substance that absorbs light, the color of the pigment comes from the wavelengths of light reflected (in other words, those not absorbed). • a green pigment absorbs other wavelengths of visible light, except green, which it reflects. http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookps.html

11. How light is absorbed

12. What happens in the Light Reactions? • Two linked photosystemscapture light energy and store it in the chemical bonds of ATP and NADPH. • These reactions cannot proceed without light.

13. Light Energy into Chemical Energy • Photosystems- In the thykaloid membranes of chloroplasts- are the clusters of pigment molecules that harvest light energy for photosynthesis • There are 2 photosystems: • Photosystem II • Photosystem I The 2 photosystems have similar pigments but different jobs in the chloroplast:

15. Photosystems II & I: • Light Energy is absorbed by chlorophyll a molecules. • “Excited electrons”in this higher energy level have enough energy to leave the chlorophyll a molecules. • the primary electron acceptor donates the electrons to the electron transport chain. • NADPH is produced. (now Energy is stored as chemical energy!)

16. http://online.santarosa.edu/homepage/cgalt/BIO10-Stuff/Ch07-Photosynthesishttp://online.santarosa.edu/homepage/cgalt/BIO10-Stuff/Ch07-Photosynthesis

17. Water is needed: • The electrons are replaced by breaking down water • The Hydrogen is used to replace the H+ and the e- used in the light reactions • Oxygen is a waste product.

18. Making ATP in Light Reactions • An important part of the light reactions is the synthesis of ATP. • Chemiosmosisis the movement of protons through ATP synthase (an enzyme) & then into the stroma (This causes a concentration gradient. It releases energy, which is used to produce ATP.) • Stroma-the solution that surrounds the thykaloid membrane in chloroplasts.

19. II. Calvin Cycle(The dark reactions) Carbon Fixation: • The ATP and NADPH produced in the light reactions drive the second stage of photosynthesis, the Calvin cycle. • In the Calvin cycle, CO2 is incorporated into organic compounds, a process called carbon fixation.

20. Calvin Cycle– 2ndstage of photosynthesis: • The reactions of the Calvin cycle use the energy stored in the light reactions to reduce carbon dioxide to carbohydrates. • Takes place in the stroma of the chloroplast, can occur without the presence of sunlight. • In the Calvin Cycle, carbon molecules from CO2 are fixed into glucose (C6H12O2).

21. The Calvin cycle is the most common way that plants fix carbon • Occurs in the stromaof the chloroplast • Is a series of enzyme-assisted chemical reactions that produces a three-carbon sugar calledG3P • Some G3P sugars are used to make organic compounds, (energy is stored for later use.) • Some G3P is converted to a five-carbon sugar(RuBP) to keep the cycle going.

23. The Calvin Cycle Reactions: http://library.thinkquest.org/C004535/calvin_cycle.html

24. The Calvin Cycle reactions explained in detail: • A five-carbon sugar (ribulosebisphosphate, or RuBP) is the acceptor that binds CO2 dissolved in the stroma. This process, called CO2 fixation, is catalyzed by the enzyme RuBP carboxylase, forming an unstable six-carbon molecule. • This molecule quickly breaks down to give two molecules of the three-carbon 3-phosphoglycerate (3PG), also called phosphoglyceric acid (PGA). • Two 3PG molecules are converted into glyceraldehyde 3-phosphate (G3P, phosphoglyceraldehyde, PGAL) molecules, a three-carbon sugar phosphate, by adding a high-energy phosphate group from ATP, then breaking the phosphate bond and adding hydrogen from NADH + H+. • Three turns of the cycle, using three molecules of CO2, produces six molecules of G3P. However, only one of the six molecules exits the cycle as an output, while the remaining five enter a complex process that regenerates more RuBP to continue the cycle. Two molecules of G3P, produced by a total of six turns of the cycle, combine to form one molecule of glucose. http://library.thinkquest.org/C004535/calvin_cycle.html

25. Why is crab grass green even when there is no rain? • Some plants like crabgrass, corn & sugar cane have a modification to conserve water. These plants capture CO2 in a different waywith an enzyme that produces oxaloacetate, which contains four carbons. (These plants are called C4 plants.) This is why crabgrass can stay green and keep growing when all the rest of your grass is dried up and brown.

26. Alternatives to Calvin Cycle “Dark” Paths • The C4 Pathway • Some plants that evolved in hot, dry climates fix carbon through the C4 pathway. These plants have their stomata partially closed during the hottest part of the day. • The CAM Pathway • Some plants in hot, dry climates fix carbon through the CAM pathway. These plants carry out carbon fixation at night and the Calvin cycle during the day to minimize water loss

27. Summary photosynthesis

28. References: • http://www.citruscollege.edu/lc/archive/biology/Pages/Chapter06-Rabitoy.aspx • http://library.thinkquest.org/C004535/calvin_cycle.html