AP Biology CHAPTER 10

- The early atmosphere lacked oxygen. From the 1 st plants, it took _____ years to produce oxygen. We now enjoy approximately _____ % oxygen in the atmosphere.  This made aerobic respiration possible.  This formed the ozone layer (O 3 ), which protects us from harmful solar radiation

 Photosynthesis is a REDOX reaction  Respiration is an exergonic RXN (NRG released from oxidation of sugar)  Photosynthesis is an endergonic RXN (NRG needed to reduce CO 2 )  Light = NRG (boost potential energy of electrons)  Water is split & electrons are transferred to CO 2 reducing it to sugar

Photosynthesis has 2 stages  LIGHT REACTION (light dependent reaction): convert light energy to chemical bond energy in ATP and NADPH  Occurs on the thylakoids  NADP+  NADPH  Oxygen is a byproduct  Generates ATP  CALVIN CYCLE (light independent reaction) – take carbon dioxide and REDUCE it to carbs/organic compounds  Occurs in the stroma  Carbon fixation  Does not require light directly  NADPH provides the reducing power  APT provides the chemical energy

 AUTOTROPHS = self-feeders  Photoautotrophs – use light  Chemoautotrophs- use inorganic substances such as sulfur or ammonia as an energy source  “producers”  Heterotrophs = other feeders  Consumers  Chemoautotroph = autotrophs that get their energy from chemicals Chloroplasts are Mostly in the mesophyll

CHLOROPLAST

Cross section of a leaf

Light can be  Reflected (how you see things)  Transmitted (passed through)  Absorbed (changed from light energy to another form) Pigments – substances that absorb visible light. They absorb different wavelengths. * Each pigment has a characteristic absorption spectrum which can be determined by a spectrophotometer.

PIGMENTS IN PLANTS  Chlorophyll a molecules can participate directly in the light RXN; accessory pigments help by transferring energy to chlorophyll a  Chlorophyll b – green-yellow pigment  Carotenoids – yellow, orange, and/or pink  Anthocynanin – Reds, purples and blues  Xanthophylls - yellows

Visible Spectrum  Wavelength is the distance between the crests of electromagnetic waves.  Visible light is detectable by the human eye ( nm)  Light behaves as if it consists of particles called photons.  Sun radiates the full specturm of electromagnetic energy

Absorption Spectrum chlorophyll a - "team captain" chlorophyll b - accessory (antenna) pigments Carotenoids etc - accessory (antenna) pigments

When chlorophyll absorbs a photon, one of its electrons is boosted to a higher energy state. higher energy state higher energy state Energy is captured in a chemical bond.

LIGHT REACTION  Occur in the thylakoid membrane  Reduce NADP+  NADPH  Give off O2 as a by-product  Generate ATP (photophosphorylation)

LIGHT DEPENDENT REACTION  ON BOARD

PHOTOSYSTEMS Pigments are assembled into photosystems in the thylakoid membrane (light collecting units). Each are composed of  1. antenna complex: several hundred pigments each with different absorption spectra – they absorb photons from a wide rage of light  2. reaction-center chlorophyll: one of many chlorophyll a molecules in each complex can actually transfer an excited electron to start the light reaction. These pigments are located in the reaction center.  3. primary electron acceptor : traps high energy e- released from the reaction center. This energy is stored as ATP and NADPH

2 Photosystems (PS) in photosynthesis  PS II comes first – absorbs best at 680 (aka p680)  PS I comes second – absorbs best at 700 (aka p700)  p680 and p700 are identical chlorophyll a molecules but each is associated with a different protein.

Noncyclic flow (NOT A CYCLE)  Occurs in the thylakoid membrane  Passes e- from water to NADP+ (photolysis)  Produces ATP by noncyclic photophosphorylation  Produces NADPH  Produces O 2

NONCYCLIC e- flow 1.At PSII =Water is split with the help of sunlight (which excites the e-); oxygen is given off as waste, e- are carried by a primary electron acceptor to the electron transport chain 2.The e- are passed down protein carriers (in doing do provides energy for chemiosmotic synthesis of ATP). 3.At PS I = the sunlight excites the e- again! The e- get shot up to another primary acceptor. 4.The e- are passed down another ETC and with the help of NADP+ reducase NADP+ picks up 2 H+ and becomes NADPH+H+

CYCLIC ELECTRON FLOW  e- leave chlorophyll a at the reaction center return to the reaction center.  Photons are absorbed by PSI (p700) releases high energy e- to the primary e- acceptor which passes them to the cycle.  Absorption of two photons of light sends a second pair of e- through the cycle  FUNCTION: to produce additional ATP without the generation of NADPH or evolving oxygen.

Chemiosmosis  Coupeling of exergonic e- flow down ETC to endergonic ATP production by creation of an electrochemical proton gradient across the membrane.

ETC in Mitochondria vs Chloroplast  Mitochondria:  Transfer chem. Energy from food to ATP. High energy e- pass down chain are extracted by the oxidation of food molecules  Inner mitochondrial membrane pumps protons from matrix out of the intermembrane space  Chloroplast:  change light energy into chemical energy. Photosystems capture light energy to drive electrons to the top of the chain.  Pumps protons from stroma into the lumen as a reservoir. ATP forms in the stroma where it drives sugar synthesis during the calvin cycle.

 Occurs in the stroma  Similar to the Krebs Cycle – starting material is regenerated  C enters as CO 2 and leaves as carbohydrates  ATP (chemical energy) and NADPH (reducing power) are energy sources  Calvin cycle produces 3-C sugar (G-3-P)

1. Carbon dioxide enters thru stomates) and bonds to RuBP (enzyme action) 2. ATP and NADPH (from light reaction) are unstable and must be used quickly! They are used to form molecules of PGAL. PGAL – regenerates RuBP for light ind. to continue. ATP input. (PGAL is also G3P or glyceraldehyde 3-phosphate) PGAL is used to produce glucose, which is stable and can be stored!

C4 plants  Incorporate carbon dioxide into 4-C compounds.  Corn, sugarcane and important agricultural grasses  Leaf anatomy of C4 plants spacially segregates the calvin cycle from the initial corporation of CO2 into organic compounds.

ALTERNATIVE MECHANISIMS OF CARBON FIXATION  C4 Plants: HOT ARID CLIMATES  Calvin cycle in most plants produces 3-PGA as the first intermediate - these are called C3 plants because first intermediate has 3 carbons (rice, wheat and soybeans)  C4 plants produce 4-C compounds initially. (ex. corn, sugarcane and grasses)  STEP 1: CO 2 added to (PEP) to form oxaloacetate – a four carbon product. In comparison to RuBP – PEP has a higher affinity to CO 2 and none for O 2. This can fix CO 2 efficiently under hot, dry conditions that cause the stomata to close and O 2 concentration to rise.  STEP 2: After CO 2 fixed by the mesophyll cells they convert oxaloacetate to another 4-C compound (usually malate)  STEP 3 Mesophyll cells export the 4-C products through plasmodesmata to bundle-sheath cells.

CAM Plants

 CAM Plants : VERY ARID CONDITIONS - NIGHT  Plants open their stomata mostly at night and closes them during the day.  + Conserves water, - but doesn’t allow CO 2 in…..  CO 2 taken in at night and incorporated into organic acids. Carbon fixation is called crassulacean acid metabolism (CAM)  Acids stored  Day – light reaction runs as normal and acids release CO 2 and calvin cycle runs.

CAM Plants  Crassulacean Acid Metabolism  occurs mainly in Crassulacean species (and other succulent plants).  The chemical reaction of the carbon dioxide accumulation is similar to that of C4 plants but here are carbon dioxide fixation and its assimilation not separated spatially but in time  arid regions  uptake of carbon dioxide during the night  The prefixed carbon dioxide is stored in the vacuoles as malate, and is used during the daytime for photosynthesis.