Photosynthesis and carbon fixation

What kind of reaction is photosynthesis

Redox

First major step of photosynthesis

Oxidation of oxygen in water molecules in the presence of light

First major step of photosynthesis equation

2H2O → O2 + 4H+

Second major step of photosynthesis

Hydrogen ions reduce the carbon in carbon dioxide

Second major step of photosynthesis equation

4H+ + CO2 → (CH2O) + H20

net reaction of photosynthesis equation

CO2 + H2O → (CH2O) + O2

Chloroplast

Plastid which contains chlorophyll and within which photosynthesis takes place. Double membraned and contains thylakoids.

Thylakoids

Bounded by pigmented membranes on which the light reactions of photosynthesis takes place. Bathed in the stroma.

granal stacks

the way thylakoids are arranged. they are joined together by stromal lamella

Photosystems present in all O2 evolving organisms

Photosystem I and photosystem II

What wavelength does PSI absorb at?

700nm

What wavelength does PSII absorb at?

680nm

Where does PSI predominantly take place?

Stromal lamella

Where does PSII take place

Granal stacks

What does one light harvesting complex (LHC) typically contain?

8 molecules chlorophyll a, 7 molecules chlorophyll b, 5-7 molecules carotenoid

Once excited by light, a chlorophyll electron can return to its ground state...

by heat, by fluorescence, or by resonance energy transfer

resonance energy transfer

no protons move but their energy does

Thylakoid electron transport chain in PSII

Passes electrons in the linear electron transport chain to make NADPH and generates proton gradients for ATP

Thylakoid electron transport chain in PSI

Passes electrons to ferredoxin (Fdx). Fdx can choose to pass to NADP+ to make NADPH or recycle back into a transport chain.

products of photosynthetic electron transport

NADPH and a proton motive forced

What is the proton motive forced used for in photosynthesis

To make ATP

What does cyclic electron transport involve?

PSI, plastoquinone, cyctochrome b6 f complex, plastocyanin

what do reduced electrons get transferred to in cyclic electron transport

reduced ferredoxin donates electrons to plastoquinone

NDH complex

present in the thylakoid membrane. Similar to mitochondrial complex I but electron donor is reduced ferredoxin instead of NADH.

When is the NDH pathway especially prominent

During oxidative stress

Ferredoxine-quinone oxidoreductase involving pathway

depends on a complex of two thylakoid proteins - PGRL1 and PGR5.

PGRL1

accepts electrons from ferredoxin and contains redox active cysteine residues and an Fe-containing cofactor that is necessary for quinone reduction

What does the presence of the cyclic electron transport with the option of two pathways do for the plant?

gives the plant flexibility in controlling the light-dependant production of ATP and NADPH

Reasons to move to cyclic electron flow: High light intensity

Downstream calvin cycle uses mostly ATP and NADPH builds up

Reasons to move to cyclic electron flow: CO2 limitation

On hot, dry days, stomata close to save water which prevents CO2 from entering. without CO2 the calvin cycle can't run and NADPH builds up

Reasons to move to cyclic electron flow: Induction phase

When a plant moves from shade into bright sunlight the Calvin cycke enzymes take a few minutes to reach full activity but the light reactions start instantly. NADPH builds up.

Why is it a problem if NADPH builds up?

plant is unable to dissipate sunlight energy down the electron transport chain

How does cyclic flow act as a safety valve?

Allows the plant to transfer energy into a flexible protein gradient and enables the light harvesting complexes to dissipate excess energy as heat.

Problem if too much sunlight builds up

Reactive oxygen species build up and this destroys photosynthetic proteins (photoinhibition)

Non-photochemical quenching

Drop in lumen pH to around 4 activates Violaxanthin De-epoxidase (VDE). VDE converts the pigment violaxanthin into zeaxanthin. up to 80% of absorbed protons can be vented as heat during peak sunlight to protect the delicate photosynthetic machinery.

3 components of the calvin cycle

Carbon fixation, reduction, regeneration of the starting molecule

Carbon fixation (carbon cycle)

CO2 combines with the 5-carbon acceptor RuBP. An unstable six-carbon compound is initially formed that splits into two molecules of a 3-carbon compound, 3 phosphoglycerate . This is catalysed by rubisco.

RuBP (in carbon fixation)

The acceptor molecule

3-PGS in carbon fixation

the product

Rubisco in carbon fixation

The enzyme

What does rubsico do beside fixing CO2?

Oxygenase activity which depends on affinities for and relative concentrations of CO2 and O2. The reaction is generally thought of as "unwanted".

Affinity of rubisco for CO2 compared to O2

about 100 times more

Organelles required to recover fixed carbon from phosphoglycate via photorespiration

Chloroplasts, Peroxisomes, Mitochondria

Photorespiration increases when...

Temperature increases

How does increasing temperature effect the stomata?

Plants close stomata in warmer temperatures so less CO2 enters. O2 is unable to leave and builds up. The ratio of O2: CO2 increases so there is more photorespiration

Stomata

Where CO2 enters and O2 and water leave

Affinity of rubisco for O2 at higher temperatures

Higher

Why plants have "wasteful" photorespiration?

Evolved in ancient bacteria before the buildup of O2 in the atmosphere.

Where is the C4 pathway seen

Tropical species such as sugarcane and maize

Where is the Crassulacean acid metabolism pathway seen?

cacti and succulents

C4 cycle

CO2 diffuses into the leaf via stomata of the mesophyll cells. CO2 is converted to bicarbonate by carbonic anhydrase which is then fixed to the C4 acid oxaloacetic acid by phosphoenol pyruvate carboxylase. PEP carboxylase is O2 insensitive. OAA is transferred deeper into the leaf into bundle sheath cells. Here, C4 is decarboxylated, releasing CO2 to the active site of rubisco where it is fixed into PGA by the calvin cycle. C3 acid is shuttled back to mesophyll cells and converted from pyruvate to phosphoenol pyruvate by pyruvate orthophosphate dikinase. this uses both phosphate groups from ATP

When does C4 occur

nightime

When does C3 occur

During the day

What do CAM plants do at night?

open their stomata. PEPC fixes CO2 from the atmosphere into oxaloacetic acid. NAD-malate dehydrogenase converts the OAA into malate and the malate accrues in the vacuole over the course of the night.

What happens to CAM plants when the sun rises in the morning?

Stomata shut, malate is decarboxylated by NAD(P)- malic enzyme, and carbon dioxide is fixed by rubisco and triose phosphates are made.

How does CAM use extra ATP?

1. To pump malate into the vacuole against a concentration gradient 2. when converting the pyruvate back into malate.

What converts pyruvate back into malate?

pyruvate-phosphate-dikinase (PPDK)

Reduction in the calvin cycle

needs both energy and reductant input. Using 6 ATP molecules and 6 NADPH molecules to convert the 3-PGA molecules into G3P.

Regeneration of RuBP

ribulose 5-P to RuBP is catalysed by phosophoribulose kinase and this uses ATP.

Sucrose properties

Chemically stable, energy dense, excellent for long distance transport around the plant and storage

PEP carboxylase

it is the enzyme that transiently fixes CO2 in C4 photosynthesis. Know the substrate and product. Understand where (mesophyll cells) and why (to be decarboxylated in the bundle sheath cells to inc CO2 conc around Rubisco, thus reduced its oxygenase activity, and need for photorespiration) it is located.

thylakoid safety valve

it exists as a mechanism to protect light-sensitive machinery.

Light

Electromagnetic radiation which has “wave” properties. Measured by the distance between the wave peaks.

Shorter wavelengths have…

More energy

Primary photosynthetic pigment in higher plants

chlorophyll a and b

Flavonoids and Carotenoids

Have antioxidant roles and suppress damaged photochemical reactions. Absorb light which they can feed into light harvesting complexes.

Photosynthetic pigments

Very hydrophobic molecules localised exclusively within the thylakoid membranes of the cholorplasts

Action spectrum

Graph of the photosynthetic rate plotted against light wavelength.

What light wavelengths does maximum photosynthesis take place under?

Red (~440-470nm) and blue (620-670nm) light

The absorption spectrum

The range of wavelengths absorbed by the photosynthetic pigments.

Absorption spectrum is broadened by a range of chromophores:

Chlorophylls, carotenoids, phycobilins

Energy of a photon is inversely proportional to…

Its wavelength (E = hc/wavelength)

Photon

Light particle. Carries a quantum of light

Energy of a photon is proportional to…

Its frequency (E= hf)

Plastoquinone

Redox sensor in the ETC.

Is photosynthesis exergonic or endergonic?

Endergonic

Reduced plastoquinone

Plastoquinol

What does reduced plastoquinone activate?

Serine/threonine protein kinase. This phosphorylates a mobile light harvesting complex and directs it to PSI.

What does the plastoquinone-controlled stoichiometrical adjustment do?

Improves the quantum efficiency of photosynthesis by enabling the plant to respond to changing light environments

What happens to LHCII molecules after phosphorylation?

They become negatively charged. Electrostatic repulsion displaces LHCs from the hydrophobic core of the granal stacks to the less hydrophobic stromal membrane region. Leads to partial unstacking of grana. LHC can now associate with PSI.