BIOL 130 - Ch. 10 Photosynthesis

b-Carotene

Accessory pigments - Extend range - Protection!

3. Regeneration phase

Glyceraldehyde-3-phosphate to Ribulose bisphosphate The rest of the G3P keeps the cycle going by serving as the substrate for the third phase in the cycle: reactions that use additional ATP in the regeneration of RuBP. 5 G3P + 3 ATP = 3 RuBP

Chlorophylls a and b

Head(ring structure that absorbs light) Tail(anchors chlorophyll in thylakoid membrane)

Cam plants

In CAM Plants, Carbon Fixation Occurs at Night and the Calvin Cycle Occurs during the Day. stored in central vacuole DIFFERENT TIME...not different place (C4 plants)

LIGHT REACTIONS

The light reactions occur in the thylakoids, and use light energy to drive the processes ● The light reactions have two functions: □ Produce ATP via chemiosmosis □ Reduce NADP+ to NADPH. ● Electrons are excited by sunlight, and moved through an electron transport chain. ● The proton gradient powers ATP synthase to synthesize ATP via photophosphorylation.

Photosynthesis SLOW

fate of G3P depends on rate of photosynthesis • G3P to glucose = gluconeogenesis• Glucose + fructose = sucrose (in cytosol) • Water soluble • Transported to other parts of plant for: - Fuel for respiration- growth

Light energy

from the sun is radiated in discrete packets called photons. ● Photons can be perceived as fluctuations in the electromagnetic field.

Rubisco

has 16 subunits and a total of 8 active sites. Most abundant enzyme in leaf tissue and on Earth! Can react with CO2 AND O2...VERY SLOW. Rubisco's active sites can interact with CO2 or O2

Photosynthesis

has two stages: □ Light reactions (photo) - ATP is produced in chloroplasts via chemiosmosis □ Calvin cycle (synthesis) - CO2 is turned into the 3-carbon sugar G3P

Cyclic electron flow

is an alternate path that electrons may take which uses photosystem I, but not photosystem II ● Electrons are passed from ferrodoxin back to the cytochrome complex □ Cyclic flow produces ATP, but no NADPH

Photosystem II

is the start of linear electron flow □ P680 - the reaction-center chlorophyll pair of photosystem II that transfers electrons to primary electron acceptor. ● Water is split supplying electrons to replace those transferred by P680 □ H+ is pumped into thylakoid space ● The primary electron acceptor gives electrons to plastiquinone (PQ) the start of the electron transport chain. ● PQ gives the electrons to a cytochrome complex (cytochrome/b6-f complex) that pumps protons into thylakoid space ● Electrons are passed to plastocyanin (PC), then to reaction center in photosystem I

Oxygenic photosynthesis

making O2

Photosystem II

max capture of 680nm light - Chlorophyll a = P680 Feeds Excited Electrons to an Electron Transport Chain. When an excited electron leaves the chlorophyll in the reaction center of photosystem ii, the electron is accepted by pheophytin, transferred to plastoquinone (pQ), and then stepped down in energy along an electron transport chain.

Photosystem I

max capture of 700nm light - Chlorophyll a = P700 In thylakoid membrane Produces NADPH. When excited electrons leave the chlorophyll molecule in the reaction centerof photosystem i, they pass through a series of iron- and sulfur- containing proteins until they are accepted by ferredoxin. in an enzyme-catalyzed reaction, the reduced form of ferredoxin reacts with naDp+ to produce naDph.

splitting water

only place it can occur is in photosystem II, chlorophyll a in the reaction center is very electronegative along with an enzyme that transfers e- from H2O to chlorophyll a

Chlorophyll b

plants and green algae have this in addition to chlorophyll a

ferredoxin

receives electrons from photosystem I and passes them to NADP reductase

plastoquinone (pQ)

shuttles electrons from pheophytin across the thylakoid membrane to a cytochrome complex.

Granum

stack of thylakoids

The electromagnetic spectrum

the full range of electromagnetic radiation. □ Electromagnetic radiation is measured by wavelength and frequency ● Visible light accounts for only a small fragment of the spectrum every color except for green is involved in photosynthesis 425-450nm (blue) and then from 600-700nm (red) Blues have more energy then reds

Chloroplast

the organelle of photosynthesis, is the site of light absorption, and the subsequent reactions. ● Chloroplasts are found in high density in the tissue inside leaves, called the mesophyll

Photosystem I

uses P700 chlorophyll pair to transfer electrons to electron carriers □ Electrons from PC replace those transferred ● Electron carriers pass electrons along, eventually to ferrodoxin ● Ferrodoxin transfers electrons to NADP reductase, which reduces NADP+ to NADPH

Splitting water

very endergonic because Enzymes that do this are in PSII When PSII becomes oxidized: VERY electronegative - rxn to split water becomes exergonic -Oxygenic photosynthesis "making O2" -Anoxygenic photosynthesis (use other electron donors- ex. H2S) "making other"

Calvin Cycle

• 3 CO2 in fixation phase: RUBISCO enzyme • 6ATP and 6 NADPH in reduction phase - 2 ATP and 2 NADPH per CO2 • 3 ATP in regeneration phase - 1 ATP per CO2 • All takes place in STROMA - Ribulose bisphosphate=RUBISCO=> 3 phosphoglycerate =>Glyceraldehyde 3 phosphate The fixation phase is when CO2 is fixed to RuBP by rubisco to form 3-phosphoglycerate (3PGA). The reduction phase uses ATP to phosphorylate the carbons and NADPH to reduce them with high-energy electrons to form G3P. The regeneration phase uses more ATP to convert some of the G3P to RuBP to continue the cycle.

Photosynthesis

• Light reactions: Light + 12 H2O + 12 NADP+ + 18 ADP + 18 Pi → 6 O2 + 12 NADPH + 18 ATP • Calvin cycle: 12 NADPH + 18 ATP + 6 CO2 → C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi + 6 H2O Need 6 turns of the Calvin cycle to get 2 G3P molecs (2 x 3C = 6C molecule)

Photosynthesis is FAST

• Lots of sucrose so glucose polymerized toform....? STARCH (in chloroplast) - stored leaves, roots • Starch broken down at night in leaves to make sucrose for respiration. • Chloroplasts provide sugars day and night!

How Is Carbon Dioxide Reduced to Produce Sugars?

• The Calvin cycle starts when rubisco catalyzes the fixation of CO2 to a five-carbon compound called ribulose bisphosphate (RuBP). The six-carbon compound that results immediately splits to form two molecules of 3-phosphoglycerate (3PGA), which are then phosphorylated by ATP and reduced by NADPH to produce glycer- aldehyde-3-phosphate (G3P). Some G3P is used to synthesize other organic molecules, like glu- cose; ATP phosphorylates the rest in a series of reactions to regen- erate RuBP so the cycle can continue. Rubisco catalyzes the addition of oxygen as well as carbon dioxide to RuBP. The reaction with oxygen leads to a loss of fixed CO2 and ATP through a process called photorespiration. In C4 plants and CAM plants, CO2 is initially fixed to four-carbon compounds, then released to fuel the Calvin cycle. This increases CO2 levels in plant tissues and reduces the effect of photorespiration when stomata are closed.

Chloroplast anatomy

□ Double membrane □ Stroma - the fluid in the chloroplast, surrounding the internal structures □ Thylakoids are the membrane bound compartments in which the light reactions occur

Anoxygenic photosynthesis

(use other electron donors- ex. H2S) "making other"

Mesophyll cells (C4)

3-C compound + CO2 =PEP carboxylase=> 4-C organic acids

2. Reduction phase

3-phosphoglycerate to glyceraldehyde- 3-phosphate (G3P) The 3PGA is phosphorylated by ATP and then reduced by electrons from NADPH. The product is the phosphorylated three-carbon sugar glyceraldehyde- 3-phosphate (G3P). Some of the G3P that is synthesized is drawn off to produce other organic molecules, like the six- carbon sugar glucose. 6 3PGA + 6 ATP + 6 NADH => 5 G3P (to step 3) and 1 G3P yield

Photosynthesis

6CO2 + 6H2O = light => C6H12O6 + 6O2 CO2 and H2O involved in separate reactionsO2 from H2OCO2 to sugarsAbsorb light energy and transform to chemical energy!!! CO2 builds sugar H2O harvest electrons forming O2

Thylakoids

A flattened membrane sac inside the chloroplast, used to convert light energy to chemical energy.

Carotenoids

ABSORB: blue and green light TRANSMIT: yellow, orange, or red light

chlorophylls

ABSORB: violet-to-blue and red light TRANSMIT: green light

Other types of plants fix CO2 to a 4C compound

Big point: TO CONCENTRATE CO2 so will dophotosynthesis...C4 plants can do C3 reactions too.They occur in different parts of the leaves.

CAM PLANTS

CAM (crassulacean acid metabolism) plants are similar to C4 plants ● The C4 pathway is physically separated from the Calvin cycle, CAM pathway is separated by time, only occurring at night. □ Unlike most plants, CAM plants keep their stomata closed during the day, and open them at night.

Calvin Cycle

Carbon Dioxide Is Reduced in the Calvin Cycle. the number of reactants and products resulting from three turns of the cycle are shown. Of the six G3ps that are generated during the reduction phase, one is used in the synthesis of other molecules, such as glucose, and the other five are used to regenerate ruBp. the three ruBps that are regenerated participate in fixation reactions for additional turns of the cycle.

Photosystem II

Electron Transport between Photosystem II and the Cytochrome Complex. plastoquinone (pQ) carries electrons from photosystem ii along with protons from the stroma. the cytochrome complex oxidizes plastoquinone, releasing the protons in the thylakoid lumen that drive atp synthesis. LOW PH IN THE THYLAKOID LUMEN

FLUORESCENCE and/or HEAT

Electron drops back down to lower energy level and emits fluorescence and/or heat. Chlorophyll molecule

REDUCTION/OXIDATION

Electron is transferred to a new compound. Electron acceptor e- Reaction center Chlorophyll a only

Cyclic flow

Electrons cycle back to Photosystem I reaction center. Only involves Photosystem I. Produces ATP but not NADPH. No oxygen is released. Cyclic Electron Flow Leads to ATP Production. Cyclic electron flow is an alternative to the Z scheme. instead of being donated to naDp+, electrons are returned to plastoquinone (pQ) and cycle between photosystem i and the etC, resulting in the production of additional atp via photophosphorylation.

RESONANCE ENERGY TRANSFER

Energy in electron is transferred to nearby pigment. Chlorophyll and β-Carotene molecules in antenna complex

Rubisco enzyme (1. Fixation phase)

Fixation of carbon dioxide Ribulose bisphosphate combines with 3CO2 to form 3-phosphoglycerate The Calvin cycle begins when CO2 reacts with RuBP. This phase fixes carbon and produces two mol- ecules of 3PGA, which is a three-carbon organic acid. 3 RuBP + 3 CO2 => 6 3PGA

C4 and C3 pathways

In different PLACES (a) C4 plant Leaf surface Mesophyll cells contain PEP carboxylase Bundle-sheath cells plasmodesmata MORE CO2 than O2 in bundle-sheath cells: rubisco will do photosynthesis in C4 Plants, Carbon Fixation and the Calvin Cycle Occur in Different Cell Types. (a) the carbon-fixing enzyme pep carboxylase is located in mesophyll cells, while rubisco is in bundle- sheath cells. (b) CO2 is fixed to the three-carbon compound pep by pep carboxylase, forming a four-carbon organic acid. a CO2 molecule can be released from the organic acid to feed the Calvin cycle.

Pheophytin

In photosystem II, a molecule that accepts excited electrons from a reaction center chlorophyll and passes them to an electron transport chain.

The Discovery of Photosystems I and II

In photosystem II, excited electrons are transferred to plasto- quinone at the start of an electron transport chain. The redox reactions in the ETC are used to generate a proton-motive force that drives the synthesis of ATP. Electrons taken from photo- system II are replaced by splitting water, releasing oxygen and protons. In photosystem I, excited electrons are passed to ferredoxin. In an enzyme-catalyzed reaction, the reduced form of ferredoxin passes electrons to NADP+, forming NADPH. The Z scheme connects photosystems II and I. Electrons excited by light in photosystem II are passed through the ETC, picked up by plastocyanin, and transferred to oxidized pigments in the photo- system I reaction center. These electrons are again excited by light in photosystem I and subsequently used to reduce NADP+ • Electrons from photosystem I may occasionally be passed back to plastoquinone instead of NADP+. This cyclic flow of electrons between the photosystem I and the ETC boosts ATP supplies.

Guard cells

Leaves have waxy coating, control opening What if it is a hot, dry day? Stomata close...avoid evaporation. What gas gets used up?What gas builds up? Leads to photorespiration Leaf Cells Obtain Carbon Dioxide through Stomata. Guard cells + Pore = Stoma

Z-scheme model

Links Photosystems II and I. the Z scheme proposes that electrons from water are first excited by photosystem ii to generate atp and then excited again by photosystem i to reduce naDp+ to naDph. how photosystems I and II interact in linear electron flow □ The Z represents the change in potential energy of electrons as they move through the redox reactions

NONCYCLIC FLOW

PE change over reactions opposite of respiration: reduced final acceptor has higher PE than the original electron donor (H2O) Electrons Are Passed from Water to NADP+ in a Linear Pathway. in the thylakoid membrane, photosystem ii uses light to excite electrons taken from water and pass them through an ETC including plastoquinone (pQ), the cytochrome complex, and plastocyanin (pC). the ETC produces a proton-motive force that is used to make atp. photosystem i excites electrons from pC and passes them on to ferredoxin to reduce naDp+ to naDph.

photosynthesis

Photo- light dependent reactions in thylakoid Synthesis = making sugars in stroma • Water is oxidized • Carbon dioxide is reduced

HowDoPigmentsCapture Light Energy?

Pigment molecules capture light energy by exciting electrons after a photon is absorbed. Each pigment absorbs photons of particular wavelengths. After a pigment molecule absorbs a photon, the excitation energy is quickly released as fluorescence and heat, heat alone, resonance energy that excites another pigment, or it is transferred as an excited electron to reduce an electron acceptor. Pigments organized into antenna complexes transfer absorbed light energy via resonance to the reaction center, where an excited electron is transferred to an electron acceptor.

pheophytin

Primary electron acceptor

Normal conditions (rubisco)

Reaction with carbon dioxide during photosynthesis: used in Calvin cycle CO2 + RuBP => 3-PGA

Hot, dry conditions (Rubisco)

Reaction with oxygen during photorespiration: O2 + RuBP → 3-PGA + 2-PGA + CO2 (uses ATP) Prdts used in cell signaling, developmentProtective function?

Overview of Photosynthesis

Reactions that prdc O2 gas occur only in presence of light - don't need CO2. - 2 separate reactions• Light used to get electrons from H2O = light reactions - Series of redox reactions that take high energy electrons from H20 - absorb energy from photons - reduce NADP+ to NADPH and make ATP • Converts CO2 to sugars = Calvin cycle- Uses products of light reactions to do this!- Doesn't happen if you don't have those products

Bundle Sheath cells (C3)

RuBP + CO2 =Rubisco=> 2 3-phosphoglycerate (3-C sugar)

C4 plants

The C4 pathway is an evolutionary adaptation that allows plants to concentrate CO2 for photosynthesis. ● C4 plants carry out the C4 pathway in the mesophyll cells. ● The Calvin cycle occurs exclusively in bundle-sheathe cells.

Calvin Cycle

The Calvin cycle (dark reactions, light independent) occurs in the stroma of chloroplasts. ● It takes in CO2, and builds it into G3P, using ATP and NADPH. ● It takes three complete turns of the cycle to build one G3P.The Calvin cycle has three phases:1. Carbon Fixation - rubisco combines RuBP with CO2 2. Reduction - G3P is formed □ For every CO2, 2 ATP and 2 NADPH are expended 3. Regeneration of RuBP □ 1 ATP is consumed for every CO2Overview of the Calvin cycle: 3 CO2 => 1 G3P 9 ATP => 9 ADP 6 NADPH => 6 NADP+

Stroma

The fluid of the chloroplast surrounding the thylakoid membrane; involved in the synthesis of organic molecules from carbon dioxide and water.

Photosynthesis Harnesses Sunlight to Make Carbohydrate

The light-capturing reactions of photosynthesis occur in internal membranes of the chloroplast that are organized into structures called thylakoids that stack to form grana. The Calvin cycle takes place in a fluid portion of the chloroplast called the stroma. The CO2-reduction reactions of photosynthesis depend on the products of the light-capturing reactions: ATP and NADPH.

Photophosphorylation

The process of generating ATP from ADP and phosphate by means of a proton-motive force generated by the thylakoid membrane of the chloroplast during the light reactions of photosynthesis.

Four Fates for Excited Electrons

When sunlight promotes electrons in pigments to a high-energy state, four things can happen: they can fluoresce, release heat, pass energy to a nearby pigment via resonance, or transfer the electron to an electron acceptor. FLUORESCENCE and/or HEAT Electron drops back down to lower energy level and emits fluorescence and/or heat. Chlorophyll molecule or RESONANCE ENERGY TRANSFER Energy in electron is transferred to nearby pigment. Chlorophyll and β-Carotene molecules in antenna complex or REDUCTION/OXIDATION Electron is transferred to a new compound. Electron acceptor e- Reaction center Chlorophyll a only Endergonic-without light Exergonic with light

Red Photons

excite electrons to a high-energy state

Photosynthetic pigments

absorb light energy, exciting their electrons □ Chlorophyll a - the main pigment of photosynthesis □ Accessory pigments - chlorophyll b and carotenoids broaden the spectrum of light that can be used □ Carotenoids provide photoprotection, protection from damaging frequencies of light ● Pigments have characteristic absorption spectrums. ● The plot of photosynthetic activity against wavelength of light is called the action spectrum.

Chlorophyll a

all plants, algae and cyanobacteria contain this only one that can transfer electron

Blue photons

excite electrons to an even higher energy state

In plants

cells that photosynthesize typically have 40-50 chloroplasts

2 major classes of pigments

chlorophylls carotenoids

Photosystems

complexes of proteins, photopigments, and organic molecules embedded in the thylakoid membrane. ● Light-harvesting complex (antenna complex) - system of photopigments and proteins ● Reaction-center complex - specialized pair of chlorophyll molecules that transfer e- to e- acceptor □ Primary electron acceptor (pheophytin)

PEP carboxylase

concentrates CO2 in a different form for later use or in a different place.

PHOTORESPIRATION

consumes O2 and produces CO2 ● Rubisco is capable of binding O2 and incorporating it into the Calvin cycle, resulting in the loss carbon from the cycle. □ This can happen when leaves CO2 concentrations drop and O2 concentrations increase ● Water, O2, and CO2 move in and out of plants through pores on their leaves called stomata □ Plants close their stomata when it's hot and sunny, in order to conserve water ● Photorespiration decreases photosynthetic output ● Photorespiration may provide some form of protection from excess light (delete this, or deemphasize)

Plants at night

do cellular respiration at night just like 'we' do all the time! at night CO2 is very high, highest right at the end of the night before dawn because everyone has been respirating

Chloroplast

double-membrane organelle that captures light energy and converts it to chemical energy through photosynthesis

NADP+ reductase

enzyme that transfers a proton and two electrons from ferredoxin to NADP+, forming NADPH (2 ELECTRONS are required).