Module 5 - Photosynthesis and the Chloroplast

heterotrophs

earliest living organisms, survive on nutrients from the environment

autotrophs

manufacture nutrients from CO2 and H2S,

Chemoautotrophs

use energy from inorganic molecules

photoautotrophs

use radiant energy to make organic compounds

photosynthesis

converts solar energy to chemical energy stored in carbohydrates. Low energy electrons are removed from a donor molecule

chloroplast

a cytoplasmic organelle located in mesophyll cells of leaves (around cell’s central vacuole) with a double membrane where photosynthesis takes place. Outer membrane contains porins and is permeable to large molecules. Inner membrane is impermeable, transporters are required to move molecules into organelle interior (stroma)

• 20-40 per cell

• arise by fission from preexisting chloroplast or proplastids (nonpigmented chloroplast precursor)

• Chloroplast DNA contains tRNAs, rRNAs, ribosomal proteins, or genes involved in photosyn

• membrane fluidity facilitates lateral diffusion of protein complexes through membrane during photosyn

stroma

interior of chloroplasts (within envelope, space outside of thylakoid), contain the chlorophyll molecules and protein complexes that comprise the energy-transducing machinery of the chloroplast and responsible for carbohydrate synthesis

• include small, ds-circular DNA molecules and prok-like ribosomes

thalykoids

part of internal membrane and is organized into flattened membrane bound sacs where light reactions occur, they are arranged in orderly stacks called grana

• space inside the thylakoid sac is the lumen

• membranes have HIGH protein content and a higher percentage of galactose-containing glycolipids

light-dependent reactions

the stage of photosynthesis where sunlight is absorbed, converting it to ATP and NADPH

light-independent reactions

stage of photosynthesis that uses the energy stored in ATP and NADPH to produce carbohydrates

photons

light particles

• energy of photon depends on wavelength of light

pigments

molecules that absorb light of particular wavelengths. Chlorophyll contains a porphyrin ring that absorbs light and a hydrophobic tail embedding it to the photosynthetic membrane

• Chlorophyll a - in all O2-producing photosyn organisms but missing from sulfur bacteria

• Chlorophyll b - in all higher plants and green algae

• Bacteriochlorophyll - only in green and purple bacteria

photosynthetic unit

~250-400 pigment molecules grouped in light-harvesting complex

• one member is the reaction-center chlorophyll

reaction center chlorophylls

transfers electrons to electron acceptor, a chlorophyll molecule in a photosynthetic unit (at center) in which electrons excited by light energy are passed to a primary acceptor molecule which is initial carrier of an electron transport chain

antenna pigments

do not participate directly in the conversion of light energy, instead they absorb light energy and transfer it to the reaction center chlorophyll

• absorb photons of varying wavelength and very rapidly transfer the energy (excitation energy) to the pigment molecule at the reaction center

photosystems

two large pigment-protein complexes that act together to raise electrons from H2O to NADP+ → described by Z scheme

• Photosystem II (PSII) boosts electrons from energy level below water to a midpoint

• Photosystem I (PSI) boosts electrons to a level above NADP+

• Electron flow: from water → PSII → PSI → NADP+

• When electrons leave PSI and PSII the reaction centers become positively charged and attract electrons and primary electrons acceptors become negatively charged

Reaction center core

a dimer of uniquely positioned chlorophyll molecules inside the photosystems

P680

PSII reaction center core, number refers to the wavelengths of strongest absorption

P700

PSI reaction center core, number refers to the wavelengths of strongest absorption

primary electron acceptor

elevated electrons are transferred from the reaction center chlorophyll to this, and then the flow through an ETS supplying the redox power to generate ATP and NADPH

light harvesting complex II (LHCII)

A complex of proteins associated with pigment molecules (including chlorophyll a, chlorophyll b, and carotenoids) that captures light energy and transfers it to reaction-center pigments in a photosystem.

pheophytin

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

plastiquinone

small protein that shuttles electrons from the primary e- acceptor to the cytochrome in photosystem II.

oxygen evolving complex

provides the oxidation potential to split the two H20 molecules and release four electrons. Protons produced are retained in the thylakoid lumen. O2 is produced as a waste product

cytochrome b6f

PQH2 moves through the bilayer and binds to this, which is a large multiprotein complex (this process translocates four protons to the lumen to add to the proton gradient)

plastocyanin

A small protein that shuttles electrons from photosystem II to photosystem I during photosynthesis.

noncyclic photophosphorylation

the movement of electrons during the formation of oxygen. named because ions move in a linear path

cyclic photophosphorylation

movement of electrons in the absence of PSII. Carried our by PSI independently of PSII. Thought to provide additional ATP required for carbohydrate synthesis

RuBP (ribulose 1,5-bisphosphate)

five carbon compound that immediately splits into two molecules of PGA when condensed with CO2

C3 plants

produce a three-carbon intermediate as the first compound during carbon dioxide fixation

• live in dry, hot climates

• closed stomata to avoid dehydration

• increase concentrations of O2 and decrease concentrations of CO2

• rubisco used O2———2-phosphoglycolate

• fixed CO2 release

Rubisco

main enzyme in C3 pathway that catalyzes the condensation of RuBP and the splitting of the six-carbon molecule. Most abundant protein on earth (because it is inefficient, will bind CO2 and O2)

• CO2 condensed w/ 5C compound (RuBP), to form a 6C molecule which then splits into two molecules of 3-phosphoglycerate (PGA)

Calvin cycle / C3 pathway

Includes the carboxylation of RuBP to form PGA, reduction of PGA to GAP using NADPH and ATP from light reactions, and the regeneration of RuBP. Produces 12 GAP for every 6 CO2 fixed, restores 6 RuBP for every 10 GAP rearranged, and consumes 12 NADPH and 18 ATP. Remaining two GAP are the payoff-- sugars that can be used for various biological functions

• fix CO2 and convert it to carbohydrates

• in cyanobacteria and all euk photosyn cells

• occurs in stroma

• Light-dependent reactions power the calvin cycle

  • ATP hydrolyzed to release energy

  • NADPH is oxidized to donate e- in reduction rxns

thioredoxin

with illumination, electrons flow from PSI to ferredoxin which passes electrons to a small, soluble, disulfide-containing protein called this

photorespiration

uptake of O2 and release of CO2. Rubisco catalyzes the attachment of both O2 and CO2 to RuBP. When oxygen levels are high, this catalysis is favored and CO2 that has been fixed is lost. Produces no ATP and leads to net loss of carbon and nitrogen, slowing plant growth

C4 pathway

mechanism to reduce uptake of O2 by Rubisco. Involves the production of phosphoenolpyruvate (PEP) which then combines with CO2 to produce 4-carbon compounds oxaloacetate or malate

• uses PEP carboxylase which has a high affinity for CO2 and does not react with O2

• CO2 is released and becomes highly concentrated around rubisco → mesophyll (CO2 fixation) and bundle-sheath cells (CO2 splitting by rubisco) minimize photorespiration

• usually tropical grasses → sugarcane, corn, and sorghum

• Cons: requires more ATP and NADPH, do not do well under cooler temps or high latitudes

• PEP-carboxylase has been introduced into C3 plants in hopes of generating CO2-concentration mech in maize-rice

CAM plants

carry out light reactions and CO2 fixation at different times of the day using the enzyme PEP carboxylase

• keep stomato closed during day and open at night

• at night open stomata and fix CO2 w/ pep carboxylase

• malate generated is mesophyll cells is transported into cell’s central vacuole

• during day malic acid is moved into cytoplasm where it is decarboxylated releasing CO2 and pyruvate → there it is fixed by rubisco under conditions of low O2 concentration

• cacti, orchids, pineapple, spanish moss, agave

carbon fixation

process that uses ATP and NADPH from light-dependent reactions to synthesize carbohydrates via carbon fixation

1st photoautotrophs did photosyn how?

H2S as source of electrons and a carbohydrate

CO2 + H2S → CH2O + 2S

Today: H2S is not as abundant so green sulfur bacteria are restricted to certain habitats like deep-sea vents or sulfur springs

Origin of Photosyn

2.7 - 2.4 bya → cyanobacteria took up residence inside mitochon-containing, non-photosyn, primitive euk

CO2 + H2O → CH2 + O2

Photosyn redox rxn

6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2

• Transfers e- from H2O to CO2

• O2 NOT derived from CO2 but breakdown of 2 H2O

• occurs in light-dependent and light-independent rxns

accessory pigments

• carotenoids produce orange colors; absorb in blue-green region

• act as secondary light collectors

• draw excess energy away from excited chlorophyll molecules and dissipate it as heat acting as a protective mechanism against photodamage

• also acts as an antioxidant → helps avoid formation of ultra-reactice free radicals

chromoplasts

family of plastids whose primary function is to be colorful

• why fruits change color as they ripen

• chloroplasts undergo structural and molecular changes to become chromoplasts

• pigments: carotene and lycopene (tomatoes)

plastid - gerontoplast

develops from chloroplast during senescence of plant foliage

plastid - amyloplast

specialized to synthesize and store starch

• potatoes

• potato turns green when exposed to sun for too long bc amyloplasts turn into chloroplasts

PSII → Plastoquinone

uses absorbed light energy to generate a proton gradient across the thylakoid membrane

• 1st step is absorption of light by an antenna pigment which mostly reside within a separate pigment protein complex, the light-harvesting complex II

• LHCII passes energy to inner-antennae molecules within PSII (P680 in chlorophyll a)

• Excited P680 (P680*) transfers energy to electron acceptor pheophytin (pheo; 1st e- acceptor)

• Pheo- is transferred to opp ends of thylakoid membrane where it passes e- to plastoquinone (PQ), occurs near stroma side of membrane

• D1/D2 proteins associated with reaction center

• PQ is lipid soluble similar to ubiquinone → acceptance of 2 e- from water and two protons from stroma reduces PQ to PQH2 (plastoquinol)

• PQH2 dissociates from D1 protein and diffuses into lipid bilayer

Flow of electrons from water to PSII

• Redox potential of P680+ pulls electrons from water (photolysis)

• formation of O2 requires 4 e- from H2O

• protons produced in photolysis are retained in thylakoid lumen

• oxygen produced is released as a waste product into the enviro

PSII to PSI

• prod of O2 leads to formation of two molecules of PQH2

• reduced PQH2 diffuses through thylakoid membrane binds to cytochrome b6f and releases p+ to lumen of thylakoid

• e- from cytochrome b6f are passed to another carrier, peripheral membrane plastocyanin (PC)

• PC transfers e- to P700+

PSI

• energy absorbed by antenna pigments of LHCI and passed to PSI reaction-center P700 (chlorophyll a dimer)

• (1) Transfer e- to A₀, the primary e- acceptor in PSI (P700+ and A₀-)

• (2-4) e- transfers to different iron-sulfur centers

• (5) e- transferred to ferredoxin, a small Fe-S protein in the stroma

• (6) reduction of NADP+ to NADPH is catalyzed by ferredoxin-NADP+ reductase

• (A) P700+ is reduced by an e- donated by plastocyanin

Summary of e- transport

• PSII generates strong oxidizing agent capable of producing O2 from 2H2O

• PSI generates a strong reducing agent capable of producing NADPH from NADP+

PSI/PSII contribution to proton gradient

e- transport produces a p+ gradient across the thylakoid membrane

(1) splitting of water in the lumen

(2) oxidation of PQH2 by cytochrome b6f which releases p+ to lumen

(3) reductions of NADP+ and PQ which removed p+ from the stroma

phosphorylation

• machinery for ATP syn in chloroplast is similar to mitochon

• ATP synthase consists of head (CF1) and a base (CF0)

• CF1 heads project outward into the stroma, keeping with the orientation of the proton gradient → contains catalytic site of enzyme

• CF0 spans membrane and mediates proton movement

C3 Pathway steps

1. Rubisco fixed CO2 by linkage to RuBP → prod rapidly splits into 2 molecules of PGA

2. 12 PGA molecules are phosphorylated via ATP hydrolysis to form 12 1,3-biphosphoglycerate (BPG) molecules

3. BPG is reduced by e- provided by NADPH to form 12 molecules of glyceraldehyde 3-phosphate (GAP)

4. 2GAPs are used in the synthesis of sucrose in the cytosol

5. The other 10 GAPs are converted into 6RuBP which can act as the acceptor for 6 more molecules of CO2 → regeneration of 6RuBPs requires the hydrolysis of 6 molecules of ATP

*NADPH and ATP used in calvin cycle represent 2 high-energy products of light-dependent reactions

• For every 6 molecules of CO2 fixed, 12 molecules of GAP are produced

  • GAP can be exported into cytosol and used to synthesize sucrose

  • GAP can remain in chloroplast where it is converted to starch

• sucrose is organic building block in plants (glucose)

• starch stored in leaves provides sugars at night (like glycogen)

• 12 NADPH and 18 ATP → large energy expenditure reflects fact CO2 is most highly oxidized form of C

Redox control

light dependent regulator of chloroplast metabolism

• Thioredoxin

  • Reduced form: activates calvin cycle enzymes → carbohydrate syn proceeds (SH+SH; light conditions)

  • Oxidized form: enzymes become inactive bc e- flow to thioredoxin stops (S-S; dark conditions)

Light → PSI → ferredoxin → FTR → thioredoxin → C# cycle enzyme activation

*ensures carbon fixation only occurs when light energy is available

photorespiration

series of reactions involving uptake of O2 and release of CO2

• accounts for loss of up to 50% of fixed CO2

• Rubisco can also bind O2 instead of CO2 → RUBP is converted into 1 molecule of 3-phosphoglycerate and 1 molecule of 2-phosphoglycolate (which can’t continue in C3 cycle and leads to CO2 release)

• photorespiration decreases photosyn efficiency

  • C4 and CAM plants increase CO2/O2 ratio around rubisco

  • less O2 binding → reduced photorespiration → improved CO2 fixation

Peroxisomes and photorespiration

• 2-phosphoglycolate is dephosphorylated and converted to glycolate

• glycolate is shuttled out of chloroplast into peroxisome

• In peroxisome glycolate is converted to glyoxylate which is then converted to glycine

• In the mitochon: 2 molecules of glycine form 1 molecule of serine, and one molecule of CO2 is released