Ch 5 – life depends on Photosynthesis

1. Ch 5 – life depends on Photosynthesis Photosynthesis is an important metabolic pathway in which producers use solar energy to convert it to chemical energy.

2. A. Photosynthesis builds carbohydrates out of CO2 and H2O Equation- 6CO2 + 6H2O +light  C6H12O6 + 6O2 It is an oxidation-reduction reaction(re-dox) Oxidation-electrons moved from H2O Reduction- electrons added to CO2 Photosynthesis is an endergonic reaction that requires an input of energy from the sun.

3. Where does glucose that’s made end up? -about half is used by plants for energy -some is locked up as cellulose in cell walls -extra is stored as starch Photosynthesis also provides energy, raw materials, and oxygen for other living things on earth. Photosynthesis keeps the earth alive.

4. The evolution of photosynthesis changed planet earth It is believed that the first organisms were heterotrophs and autotrophs evolved later. Autotrophs also produced the oxygen that became the ozone layer to allow life to move onto land.

5. 5.2 Sunlight is the energy source for photosynthesis Only a very small fraction of light from the sun reaches earth, and only about 1% of that energy is used to power all photosynthesis on earth. A. Visible light is part of the electromagnetic spectrum. All radiation consist of photons(discrete packets of kinetic energy. The shorter the wavelength the more energy.

6. Light that reaches earth is in three forms of radiation: UV light, visible light, infrared. UV is shortest wavelength. Visible is the middle length and we can see each wavelength as a different color. Infrared is the longest and not useful to organisms because it immediately converts to heat.

9. B. Pigments capture light energy Plant cells have several types of pigments to capture light energy. Chlorophyll a- green photosynthetic (plants, algae, and cyanobacteria) They usually have accessory pigments as well, such as chlorophyll b and carotenoids.

10. Different colors of pigments capture different wavelengths of light. Chlorophyll a and b absorb red and blue(reflect green). Carotenoids reflect red, orange or yellow. Only absorbed light is useful for photosynthesis.

11. 5.2 C. Chloroplasts are the sites of photosynthesis Photosynthesis occurs in leaves (broad surface for gas exchange) Water is absorbed by the roots and travels to the leaves Carbon dioxide and oxygen are exchanged with the atmosphere, but water is also lost through stomata

13. Most photosynthesis takes place in the mesophyll of the leaf (middle portion) There are abundant chloroplasts in the mesophyll. Each cell contains 40-200 chloroplasts.

14. Anatomy of a chloroplast • Two membranes enclose the stroma (gelatinous fluid that has ribosomes, DNA, and enzymes) • Grana are suspended in the stroma, each composed of 10-20 disk shaped thylakoids • Each thylakoid consists of a membrane studded with pigments and enclosing a volume called the thylakoid space.

16. In the thylakoid membrane, pigments and proteins that participate in photosynthesis are grouped into photosystems. • In each photosystem 300 chlorophyll molecules and 50 accessory pigments. • All of them absorb light, but only one chlorophyll a molecule uses the energy for photosynthesis

17. This molecule and its associated proteins is called the reaction center. All the other pigments are called antenna pigments because they act like a funnel for electrons directing them to the reaction center.

19. 5.3 Photosynthesis occurs in two stages • 1. Light reactions- convert solar energy to chemical energy • In the thylakoid membrane, two linked photosystems capture kinetic energy from photons and store it as potential energy in ATP and NADPH • 2. carbon reactions- ATP and NADPH are used to reduce carbon dioxide to glucose. This occurs in the stroma. They don’t use light directly but can happen any time of the day. “light independent” reactions.

21. 5.4 The light reactions • Plants would die without light because the light reactions begin photosynthesis. • In the thylakoid membranes, photosystems I and II specialized in slightly different wavelengths of light. • The two systems are connected by an electron transport chain. • One ETC results in ATP and one results in NADPH

23. A. Photosystem II produces ATP • Photosynthesis begins in photosystem II. • Light is transferred by antenna pigments to one chlorophyll a molecule that is the reaction center. • This creates potential energy when the chlorophyll a molecule ejects two “excited” electrons. • These electrons are grabbed by the first protein in the electron transport chain.

24. The reaction center replaces the two electrons by splitting water molecules producing oxygen gas and protons. • The chlorophyll a molecule replaces the electrons by splitting water. • As the electrons are passed down the ETC, the energy lost drives the active transport of protons from the stroma into the thylakoid space. This proton gradient is potential energy.

25. ATP synthase is an enzyme complex transforms this potential energy into ATP by adding phosphate to ADP. The energy is provided when a channel in ATP synthase allows protons trapped inside the thylakoid space to return to the stroma. The formation of ATP with the release of energy from the proton gradient is called chemiosmotic phosphorylation.

26. B. Photosystem 1 produces NADPH Photosystem I functions much like photosystem II, except that NADPH is produced instead of ATP. There is no proton gradient formed . This NADPH is the electron carrier that will reduce carbon dioxide in the carbon reactions.

29. 5.5 the carbon reactions produce carbohydrates Also called the Calvin cycle. Metabolic pathway that uses NADPH and ATP to assemble CO2 into three carbon molecules that are later assembled into glucose.

30. The first step of the Carbon cycle is carbon fixation- CO2 is combines with RuBP(five carbon sugar with 2 phosphate groups) Rubisco is the enzyme that catalyzes the initial reaction. The initial six carbon product immediately breaks down into two 3 carbon molecules called PGA. The cycle continues and converts PGA to PGAL.

31. PGAL leaves the calvin cycle and is made into glucose or sucrose, but some is converted back to RuBP to keep the cycle going.

32. The energy for the calvin cycle comes from ATP and NADPH produced in the light reactions. (the calvin cycle can’t happen without the energy from the light reactions) The calvin cycle can happen continuously(day or night)

33. 5.6 C3 plants use only Calvin cycle to fix carbon The calvin cycle is a C3 pathway because PGA is the first molecule produced. 95% of all plants are C3 plants. The weakness of photosynthesis is inefficiency. Photorespiration is a process that counteracts photosynthesis.

34. In photorespiration, the rubisco enzyme uses O2 as a substrate instead of CO2 which removes carbon from the calvin cycle(not good) Minimizing photorespiration requires getting rid of O2, but when plants have open stomata, they also lose water, it’s a delicate balance for plants that live in hot, dry climates.

35. 5.7 C4 and CAM pathways • C4 plants separate light reactions and carbon reactions into different cells • Light reactions and carbon fixation occur in mesophyll cells • CO2 combines with 3C molecule to form a 4 carbon compound called oxaloacetate which is then reduced to malate. • The malate moves to bundle sheath cells that surround leaf veins- this makes it less likely for the Rubisco to bind with oxygen

38. C4 plants require half as much water as C3 plants • Examples are sugar cane and corn- grow in hot, open environments. Only 1% of plants are C4

39. Crassulacean Acid Metabolism (CAM) • Happens in desert plants • Open stomata only at night to fix CO2 • Happens in the same cell • Mesophyll cells turn CO2 into malate and store it in a vacuole- during the day the stored malate is used for the calvin cycle • This reduces photorespiration by generating high concentrations of CO2 • 3-4% of plants use CAM including cacti and pineapples