PHOTOSYNTHESIS Chapter 10

BASIC VOCABULARY Autotrophs – producers; make their own “food” Heterotrophs – consumers; cannot make own food

LEAF STRUCTURE Stomata (stoma) – microscopic pores that allow water, carbon dioxide and oxygen to move into/out of leaf Chloroplasts – organelle that performs photosynthesis Found mainly in mesophyll – the tissue of the interior leaf Contain chlorophyll (green pigment) Stroma – dense fluid in chloroplast Thylakoid membrane – inner membrane of chloroplast Grana (granum) – stacks of thylakoid membrane

Figure 10.2 Focusing in on the location of photosynthesis in a plant

PHOTOSYNTHESIS SUMMARY 6CO2 + 6H20 + light energy C6H12O6 + 6O2 Oxygen comes from water, not CO2 Two parts: Light Reactions The Calvin Cycle (Dark Reactions)

Figure 10.3 Tracking atoms through photosynthesis

Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle

LIGHT Photons – discrete packets of light energy Chlorophyll a – (blue-green)only pigment that is directly used in light reactions Chlorophyll b – (yellow-green) accessory pigment Carotenoids - (yellow-orange) accessory pigment

Figure 10.6 Why leaves are green: interaction of light with chloroplasts

Figure 10.8 Evidence that chloroplast pigments participate in photosynthesis: absorption and action spectra for photosynthesis in an alga Aerobic bacteria gather where there is more oxygen along this algal filament, which was exposed to different colors of light.

PHOTOEXCITAION When photons hit chlorophyll and other pigments, electrons are excited to an orbital of higher energy In solution when the excited electrons fall, they give off energy (a photon) and fluoresce

Figure 10.9 Location and structure of chlorophyll molecules in plants

LIGHT REACTIONS Photosystems: Made of proteins and other molecules surrounding chlorophyll a Contain a primary electron acceptor Photosystem I – P700 Photosytem II – P680

Figure 10.11 How a photosystem harvests light

Noncyclic (predominant route) Cyclic Require light to occur Two pathways: Noncyclic (predominant route) Cyclic Noncyclic animation Another animation

NONCYCLIC ELECTRON FLOW Photosystem II absorbs light Two electrons excited and captured by primary electron acceptor “Hole” in photosystem II is filled by 2 electrons that come from the splitting of water H2O 2H+ + ½ O2 + 2e-

Oxygen is released Excited electrons pass from primary electron acceptor down an electron transport chain to photosystem I (filling its “hole”) ATP is made by photophosphorylation as electrons fall down ETC

Photons excite 2 electrons from Photosystem I and are captured by its primary electron acceptor Electrons then move down another ETC to ferredoxin (Fd) Fd gives electrons to NADP+ (nicotinamide dinucleotide phosphate) making NADPH The enzyme that helps this transfer of e- is called NADP+ reductase

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 5)

Figure 10.13 A mechanical analogy for the light reactions

Fig. 10-17- Photosystems in Thylakoid Membrane STROMA (low H+ concentration) Cytochrome complex Photosystem II Photosystem I 4 H+ Light NADP+ reductase Light Fd 3 NADP+ + H+ Pq NADPH e– Pc e– 2 H2O 1 1/2 O2 THYLAKOID SPACE (high H+ concentration) +2 H+ 4 H+ To Calvin Cycle Figure 10.17 The light reactions and chemiosmosis: the organization of the thylakoid membrane Thylakoid membrane ATP synthase STROMA (low H+ concentration) ADP + ATP P i H+

Figure 10.14 Cyclic electron flow

CYCLIC ELECTRON FLOW Only Photosystem I is used Fd passes electrons back to Photosystem I via ETC ATP made No NADPH made No oxygen released Used when cell needs more ATP than NADPH

Figure 10.17 The Calvin cycle (Layer 3)

CALVIN CYCLE Also called Dark Reactions because light is not needed; however products from light reactions are needed. Carbon Fixation – initial incorporation of carbon into organic molecules CO2 attaches to a 5-carbon sugar called ribulose bisphosphate (RuBP) The enzyme that catalyzes this is called rubisco Calvin cycle animation

One G3P molecule leaves cycle to be used by plant Immediately splits into two 3-carbon molecules called 3-phosphoglycerate 3-phosphoglycerate is phosphorylated by ATP (from light reactions) making 1,3-bisphosphoglycerate 1,3-bisphosphoglycerate is reduced by taking electrons from NADPH making glyceraldehyde 3-phosphate (G3P) One G3P molecule leaves cycle to be used by plant The remaining G3P’s are converted into RUBP in several steps and by getting phosphorylated by ATP 3 phosphoglycerate and 1-3 bisphosphoglycerate from glycolysis

Figure 10.17 The Calvin cycle (Layer 3)

C3 PLANTS – have a problem Examples : rice, wheat, and soy beans Problem - produce less food when stomata are closed during hot days because low CO2 starves Calvin Cycle and rubisco can accept O2 instead of CO2 High oxygen levels = O2 passed to RUBP (not CO2) and Calvin cycle stops When this oxygen made product splits, it makes a molecule that is broken down by releasing CO2

This process is called photorespiration. Occurs during daylight (photo) Uses O2 and makes CO2 (respiration) NO ATP made (unlike respiration) and NO food made Early earth had low O2 so this would not have mattered as much Photorespiration drains away as much as 50% of carbon fixed by Calvin Cycle in many plants.

Alternative methods of Carbon Fixation- the solution to the previous problem C4- Plants 2 photosynthetic cells- bundle-sheath & mesophyll PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll new 4C molecule releases CO2 in bundle sheath cell CAM Plants open stomata during night close during day (crassulacean acid metabolism) cacti, pineapples, etc.

Figure 10.18 C4 leaf anatomy and the C4 pathway Malate and oxaloacetate from Krebs

Figure 10.19 C4 and CAM photosynthesis compared

Figure 10.20 A review of photosynthesis