PHOTOSYNTHESIS

Chapter 10

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

LEAF STRUCTUREStomata (stoma) – microscopic pores that allow water, carbon dioxide and oxygen to move into/out of leafChloroplasts – organelle that performs photosynthesis

Found mainly in mesophyll – the tissue of the interior leafContain chlorophyll (green pigment)Stroma – dense fluid in chloroplastThylakoid membrane – inner membrane of chloroplastGrana (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 or Light Independent)

Figure 10.3 Tracking atoms through photosynthesis

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

LIGHTPhotons – discrete packets of light energyChlorophyll a – (blue-green)only pigment that is directly used in light reactionsChlorophyll b – (yellow-green) accessory pigmentCarotenoids - (yellow-orange)

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

PHOTOEXCITAIONWhen photons hit chlorophyll and other pigments, electrons are excited to an orbital of higher energyIn 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 REACTIONSPhotosystems:

Made of proteins and other molecules surrounding chlorophyll aContain a primary electron acceptor Photosystem I – P700Photosytem II – P680

Figure 10.11 How a photosystem harvests light

Require light to occurTwo pathways:

Noncyclic (predominant route)CyclicNoncyclic animationAnother animation

NONCYCLIC ELECTRON FLOW

Photosystem II absorbs lightTwo electrons excited and captured by primary electron acceptor“Hole” in photosystem II is filled by 2 electrons that come from the splitting of waterH2O 2H+ + ½ O2 + 2e-

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 1)

Oxygen is releasedExcited 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

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 2)

Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 3)

Photons excite 2 electrons from Photosystem I and are captured by its primary electron acceptorElectrons then move down another ETC to ferredoxin (Fd)Fd gives electrons to NADP+ (nicotinamide dinucleotide phosphate) making NADPHThe 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 4)

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

Figure 10.14 Cyclic electron flow

CYCLIC ELECTRON FLOW

Only Photosystem I is usedFd passes electrons back to Photosystem I via ETC

Some ATP madeNo NADPH madeNo oxygen released

Used when cell needs more ATP than NADPH

ETC

Food (chemical energy) to ATP (chemical energy) ATP synthasePumps H+ into intermembrane space

Light energy to ATP (chemical energy) ATP synthasePumps H+ into thylakoid space

MITOCHONDRIA

CHLOROPLAST

Figure 10.15 Comparison of chemiosmosis in mitochondria and chloroplasts

Figure 10.17 The Calvin cycle (Layer 1)

Figure 10.17 The Calvin cycle (Layer 2)

Figure 10.17 The Calvin cycle (Layer 3)

CALVIN CYCLEAlso called Dark Reactions because light is not needed; however products from light reactions are needed.Carbon Fixation – initial incorporation of carbon into organic moleculesCO2 attaches to a 5-carbon sugar called ribulose bisphosphate (RuBP)The enzyme that catalyzes this is called rubiscoCalvin cycle animation

Immediately splits into two 3-carbon molecules called 3-phosphoglycerate3-phosphoglycerate is phosphorylated by ATP (from light reactions) making 1,3-bisphosphoglycerate1,3-bisphosphoglycerate is reduced by taking electrons from NADPH making glyceraldehyde 3-phosphate (G3P)One G3P molecule leaves cycle to be used by plantThe remaining G3P’s are converted into RUBP in several steps and by getting phosphorylated by ATP

Recall, G3P is the sugar formed by splitting glucose in glycolysisG3P can be made into glucose, sucrose, cellulose etc. by plant

C3 PLANTS – have a problem

Examples : rice, wheat, and soy beansProblem - 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 muchPhotorespiration drains away as much as 50% of carbon fixed by Calvin Cycle in many plants.

C4 PLANTS – have a solutionExamples: sugarcane, corn and grassesLeaves contain bundle-sheath cells and mesophyll cellsBundle sheath surrounds veins of leaf (location of Calvin cycle)Mesophyll – between bundle and surface

In mesophyll cells: CO2 fixed to phosphoenolpyruvate (PEP)PEP carboxylase is the enzyme that does thisPEP carboxylase has higher affinity for CO2 than rubisco so less danger of O2 interfering

The fixed CO2 is then taken to Calvin cycle (in bundle-sheath) as part of a 4-carbon molecule (malate)Malate gives CO2 to Calvin cycle

Figure 10.18 C4 leaf anatomy and the C4 pathway

CAM PLANTS – have another solution(crassulacean acid metabolism)

Examples: succulent plants (pineapples and cacti etc.)Open stomata at night and close during dayAt night CO2 is fixed into organic acids in mesophyll and then taken to Calvin cycle (also in mesophyll) during day.

Figure 10.19 C4 and CAM photosynthesis compared

PHOTOSYNTHESIS FACTS

50% of organic material made is used by plant in respirationOrganic molecules often leave leaves as sucroseLarge amounts of cellulose are made (for cell walls)“And no process is more important than photosynthesis to the welfare of life on Earth.” (Campbell and Reece, 2005)

Figure 10.20 A review of photosynthesis