1. AP Biology
Ch. 10
Photosynthesis

3. Photosynthesis
Transforms solar energy trapped by chloroplasts into chemical bond energy stored in sugar and other organic molecules.
Directly or indirectly supplies energy to most living organisms
Uses energy-poor molecules, CO2 and H2O to make energy-rich molecules such as glucose

5. Autotrophs (Producers)
Organisms get organic molecules used for energy by one of two ways: autotrophic nutrition or heterotrophic nutrition.
Autotrophs synthesize organic molecules from inorganic raw materials.
Photoautotrophs use light as an energy source to make food. Ex) plants, algae, and some prokaryotes
Chemoautotrophs use the oxidation of inorganic chemicals, such as sulfur or ammonia, to make organic molecules. Ex) some bacteria living on the ocean floor near volcanic vents.

7. Purple Sulfur Bacteria

8. Heterotrophs (Consumers)
Nutrition is acquired from organic molecules produced by other organisms
Examples are animals that eat plants (herbivores), other animals (carnivores), or both (omnivores).
Some heterotrophs decompose and feed on organic litter (detritivores)

9. Chloroplasts Site of photosynthesis in plants, mainly in the leaves
Chlorophyll is the green pigment in chloroplasts that gives a leaf its color, and that absorbs light energy used to drive photosynthesis.
Chloroplasts are concentrated in cells of mesophyll, green tissue in the leaf’s interior.
Carbon dioxide enters and leaves the leaf through stomata
Water absorbed by roots is transported to the leaves through vascular bundles.

11. Chloroplasts: structure
Divided into three functional compartments by a system of membranes:
1) Intermembrane space- separates the two layers of the double membrane surrounding the chloroplast.
2) Thylakoid space- thylakoids are stacks of membranes within the chloroplast, contain chlorophyll, location of light reactions
3) Stroma- viscous fluid outside the thylakoids, location of the Calvin cycle (converts CO2 to sugar)

12. Tracing atoms through photosynthesis

13. Overview of Photosynthesis: cooperation of light reactions and Calvin cycle

14. Pathways of Photosynthesis: Light reactions
Convert light energy to chemical bond energy in ATP and NADPH (NADP+ is reduced).
Occurs in the thylakoid membranes of chloroplasts
Oxygen given off as by-product from the splitting of water. H+ passed on to the Calvin cycle, joining CO2 to from sugar.
ATP is generated by photophosphorylation.

15. Properties of Light
Sunlight is electromagnetic energy, having wavelike and particle-like properties.
The visible range of light is the radiation that drives photosynthesis.
Blue and red, the two wavelengths most effectively absorbed by chlorophyll, are the colors most useful as energy for photosynthesis.

17. Photosynthetic Pigments: Light receptors
Light may be reflected, transmitted, or absorbed when is contacts matter.
Pigments are substances that absorb different wavelengths of light: wavelengths absorbed disappear, while the color seen is reflected.
For example, leaves look green because chlorophyll absorbs red and blue light, but transmits and reflects green light.

19. Absorption vs. Action spectrum
Each pigment has a characteristic absorption spectrum, or pattern of wavelengths that it absorbs. It is expressed as a graph of absorption vs. wavelength.
Absorption spectrums for a pigment are determined using a spectrophotometer.
Action spectrums profile the relative effectiveness of different wavelengths of visible light for photosynthesis. Graph is wavelength vs. rate of photosynthesis.

21. Pigments of Photosynthesis
Chlorophyll a- participates directly in the light reactions, accessory pigments can absorb and transfer energy to chlorophyll a
Accessory pigments- include chlorophyll b (yellow-green pigment), and carotenoids (yellow and orange pigments)
Accessory pigments expand the range of wavelengths of light available for photosynthesis

23. Photoexcitation of Chlorophyll
When pigments absorb photons, the absorbed photons boost one of the pigment molecule’s electrons in its lowest energy state (ground state) to an orbital of higher potential energy (excited state)
The excited state is unstable, so excited electrons fall back to the ground state, releasing heat or by fluorescing.

24. Photoexcitation of chlorophyll, cont.
Pigment molecules in the thylakoid membrane do not fluoresce because the primary electron acceptor molecules trap excited state electrons that have absorbed photons.
Isolated chlorophyll fluoresces in the red part of the spectrum and dissipates heat.

26. Photosystems
Chlorophyll in the chloroplast is organized with proteins into photosystems.
A photosystem has a light-gathering “antenna complex” consisting of chlorophyll a, chlorophyll b, and carotenoid molecules.
When light energy hits the chloroplast, energy is transferred to chlorophyll a in the reaction center.
The solar-powered transfer of electrons from chlorophyll a to the primary electron acceptor is the first step of the light reactions. It is a redox reaction.

28. Photosystems I and II
The thylakoid membrane has two types of photosystems that cooperate in the reactions of photosynthesis.
Photosystem II occurs first, and has chlorophyll P680 (absorbs at 680 nm wavelengths of light, red).
Photosystem I has chlorophyll P700, which absorbs best at 700 nm (far red part of the spectrum)

29. Noncyclic electron flow
Light drives the synthesis of NADPH and ATP by energizing the two photosystems in the thylakoid membranes of chloroplasts.
During photosynthesis, there are two possible routes for electron flow: cyclic and non-cyclic electron flow (non-cyclic is most common)

30. Noncyclic electron flow, cont.
Photosystem II absorbs light, excites e- to a higher level, chlorophyll becomes oxidized
Water is split; oxygen is released
Each photoexcited electron passes from photosystem II to photosystem I by an electron transport chain. Electron carriers are plastoquinone (Pq) and a cytochrome complex.

31. As electrons move down the chain, ATP is formed by noncyclic photophosphorylation.
Primary electron acceptor of Photosystem I passes photoexcited electrons to a second electron transport chain, producing NADPH.
ATP and NADPH will be used in the Calvin cycle to make sugar.

38. Cyclic electron flow
Under certain conditions, photoexcited electrons take an alternate path called cyclic electron flow.
Cyclic electron flow uses Photosystem I, but not Photosystem II.
ATP is produced by cyclic photophosphorylation, but no oxygen or NADPH.
Noncyclic electron flow produces ATP and NADPH in equal amounts, but the Calvin cyclic requires more ATP.
Cyclic electron flow makes up the difference.

40. Chemiosmosis: Chloroplasts vs. Mitochondria (similarities)
Chloroplasts and mitochondria both generate ATP by chemiosmosis.
Both have an electron transport chain assembled in a membrane
Both have an ATP synthase complex built into the same membrane that couples the diffusion of H+ down their gradient to the phosphorylation of ADP.
The ATP synthase complexes of both organelles are similar.

41. Chemiosmosis: Chloroplast vs. Mitochondria (differences)
Mitochondria transfer chemical energy from food molecules to ATP, while chloroplasts transform light energy into chemical energy.
Spatial organization of chemiosmosis differs in mitochondria and chloroplasts:
The inner membrane of the mitochondrion pumps protons from the mitochondrial matrix out to the intermembrane space, which is a reservoir of protons that power ATP synthase.
The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space. ATP is produced as protons diffuse from the thylakoid compartment back to the stroma through ATP synthase. ATP forms in the stroma where it drives sugar synthesis during the Calvin cycle.

43. Summary of Light Reactions
Noncyclic electron flow pushes electrons from water ( low P.E.) to NADPH (high P.E.)
Water is split by photosystem II on the side of the membrane facing the thylakoid space.
The diffusion of H+ from the thylakoid space to the stroma (along the H+ gradient) powers ATP synthase.
Light-driven reactions across the membrane store chemical energy in NADPH and ATP, which shuttle energy to the Calvin cycle.

45. Melvin Calvin

46. Calvin cycle
Carbon enters the Calvin cycle in the form of CO2 and leaves in the form of sugar.
It consumes ATP as an energy source, and NADPH as the reducing agent to add high energy electrons to form sugar.
The Calvin cycle produces a 3-C sugar, glyceraldehyde-3-phosphate (G3P). The cycle must take place 3 times, fixing three molecules of CO2, to make one molecule of G3P.

47. Calvin cycle: Phase I Carbon fixation
Each CO2 is incorporated by attaching to a 5-C sugar, ribulose bisphosphate (RuBP). Rubisco is the enzyme that catalyzes this step.
A 6-C intermediate forms that immediately splits into two molecules of 3-phosphoglycerate.

49. Calvin cycle: Phase 2 Reduction
Endergonic reduction is a 2-step process that couples ATP hydrolysis with the reduction of 3-phosphoglycerate to glyceraldehyde phosphate (G3P).
For every three CO2 molecules that enter the Calvin cycle, six G3P molecules are produced, only one of which can be counted as net gain.
The other five G3P molecules are recycled to regenerate molecules of RuBP.

51. Calvin cycle: Phase III Regeneration of CO2 acceptor (RuBP)
A complex series of reactions rearranges the carbon skeletons of five G3P molecules into three RuBP molecules.
3 ATP molecules are used, RuBP is now prepared to receive CO2 again, and the cycle continues.

53. Energy totals
To make one G3P molecule, the Calvin cycle consumes 9 ATPs and 6 molecules of NADPH.
G3P is the starting material for the synthesis of glucose and other carbohydrates.
The Calvin cycle uses 18 ATPs and 12 molecules of NADPH to produce one molecule of glucose.
The light reactions and Calvin cycle are both required to make sugar from CO2.

54. Carbon fixation: Alternative methods in hot, dry climates
In most plants, the fixation of carbon via rubisco results in a 3-carbon compound. These are called C3 plants. Examples are rice, wheat, and soybeans.
On hot, dry days, these plants close their stomata to conserve water. CO2 coming into the plant decreases.
Rubisco can also bind oxygen, in place of CO2. O2 enters the Calvin cycle. This is photorespiration.

55. Photorespiration
This process is called photorespiration because it occurs in light (photo), and uses oxygen (respiration).
However, photorespiration does not generate ATP or make glucose.
Photorespiration decreases photosynthetic output by removing organic material from the Calvin cycle.
Perhaps photorespiration is an evolutionary “relic” from a time when there was more CO2 in the atmosphere than O2, and rubisco did not have to differentiate between them.

56. C4 Plants
C4 plants have adapted to hot, dry climates by producing a 4-carbon compound as its first product.
Sugar cane and corn are examples of C4 plants.
There two types of photosynthetic cells: Bundle-sheath cells and mesophyll cells.
The Calvin cycle is confined to chloroplasts of the bundle-sheath cells, but CO2 can be incorporated into organic compounds in the mesophyll cell to be used later in the Calvin cycle.

57. C4 plants, cont.
CO2 is added to phosphoenolpyruvate (PEP) by the enzyme PEP carboxylase.
Compared to rubisco, PEP carboxylase has a higher affinity for CO2 and can fix it under conditions (hot and dry) when rubisco cannot.
The four carbon products are then exported to the bundle-sheath cells through plasmodesmata.
C4 photosynthesis minimizes photorespiration, and enhances sugar production.

59. CAM plants
A second adaptation to arid conditions in succulent (water-storing) plants is crassulacean acid metabolism (CAM).
Cacti and pineapples are examples of CAM plants.
The stomata are closed during the day, but open at night to collect CO2.
Carbon fixation into organic acids occurs at night. These molecules are stored in the mesophyll cell.
The Calvin cycle operates during the day when the light reactions can provide ATP and NADPH to make sugar from the stored CO2.

62. Review of Photosynthesis
Light reactions convert light energy to the chemical energy of ATP and NADPH.
Pigments and protein molecules that carry out the light reactions are found in the thylakoid membranes and include two photosystems and electron transport chains.
The light reactions split water and release oxygen to the atmosphere.

63. Review of photosynthesis, cont.
The Calvin cycle takes place in the stroma of the chloroplast, and uses ATP and NADPH to convert CO2 to carbohydrates (glucose).
The direct product of the Calvin cycle is G3P. Enzymes in the chloroplast convert this molecule into a diverse group of other organic compounds.
The Calvin cycle returns ADP and NADP+ to the light reactions.