Ecophys Midterm

Where does photosynthesis occur?

Within the chloroplast

Chloroplast

specialized cell with a double membrane, contains thylakoids (pigments)

What is inside the thylakoid?

Lumen

What is outside the thylakoid?

Stroma

Two steps of light reactions

1. Absorption of photons in thylakoids creates energy

2. Energy used to split H2O and create NADPH and ATP

Third step of dark reaction

NADPH and ATP used in carbon-reduction cycle

Step 1: Absorption of photons

Photons intercepted by chlorophyll (chl) embedded in membrane, clustered in light harvesting complexes around reaction centers - PSI and PSII. Absorption raises electron to excited state, moves through chl molecule (PSII → PSI)

Three places energy can go during absorption of photons

1. Photochemistry (passed to another chl or rxn center)

2. dissipated as heat

3. fluorescence (photon is reemitted at longer wavelength)

Step 2: Electron transport

1. PSII uses excitation energy to split H2O- creates O2 proton and electron

2. electron passed to plastoquinone (PQ) - reduces PQ to PQH2

3. PQH2 passes to cytochrome complex- reduces plastocyanin, building electron gradient

4. Plastocyanin oxidized by PSI - additional redox potential from different photon

5. Energy used to reduce ferrodoxin

Ferrodoxin

Reduces NADP+ to NADPH (linear electron transport), transfers electrons to O2 → H2O formation, reduces PQ - increasing proton motive

where does most energy go during electron transport?

goes to reducing NADP+, goes to other pathways if no NADP+

Step 3: Carbon reduction

Occurs in stroma, creation of sugar using ATP and NADPH from light reactions. Phosphoglyceric acid (PGA) is created, reduced to triose-phosphate using energy from light reaction.

Substrates used in carbon reduction

RuBP and CO2

Enzyme used in carbon reduction

Rubisco

Rubisco is not faithful to…

CO2. Will also catalyze with oxygen, resulting in a loss of energy.

Photorespiration

Consumes O2, releases CO2. Increases with high photosynthetic rates and low CO2 availability.

Why is oxygenation bad?

Carboxylation results in 2 PGA molecules, whereas oxygenation results in 1 PGA and 1 C2 molecule

Photosynthesis rates are governed by…

supply and demand of CO2 to chloroplast

What is CO2 supply limited by?

diffusion of CO2 from atmosphere → sites of carboxylation in stroma

what is CO2 demand controlled by?

The structure and chemistry of the chloroplast, environmental factors, and demand for photosynthates

What is movement of CO2 driven by?

Diffusion (high concentration → low concentration)

Ca

CO2 concentration in atmosphere

Ci

CO2 concentration inside leaf

Cc

CO2 concentration at site of carboxylation

CO2 diffusion order

Ca > Ci > Cc

Fick’s first law

diffusion rate is the limitation for photosynthesis. The diffusion rate is proportionate to the concentration gradient - difference between Ca and Ci.

Diffusion rate =

gradient/resistance

Anet

rate of CO2 assimilation, = rate of diffusion

Anet =

(Ca-Ci)/rc (resistance to CO2 transport)

Conductance (G) =

1/R (resistance)

CO2 transport is often measured via…

water vapor transport

two components of water vapor (Rw)

boundary layer (Ra) and stomatal (Rs)

boundary layer

thin area of still air around leaf with higher humidity than the atmosphere

what affects the boundary layer?

wind speed and leaf morphology

What is the flaw in using water vapor to calculate CO2 transport?

Most water vapor is not coming from the chloroplast, but the mesophyll. This calculation does not account for movement from Ci to Cc (mesophyll conductance).

Where is Anet at very low CO2 levels? Why?

Below zero - due to photorespiration and dark respiration

Where is the CO2 compensation point? (gamma)

Where Anet and CO2 are equal

How does Anet increase with CO2?

Quickly- RuBP is plentiful

What is Anet limited by before it levels off?

Availability of CO2

What is Anet limited by after it levels off?

RuBP

What is the rate of Anet increase dependent on?

The speed of the CR cycle (affects ATP and NADPH)

What is the slow increase at the top of an A-Ci curve due to?

decreased photorespiration (more CO2 around)

Stable isotopes

Have more neutrons than protons, don’t decay, act chemically identical to each other.

Fractionation

Discrimination against 1 isotope- can be against lighter or heavier isotope

How can stable isotopes be used?

Compare to universal standard (PDB), use mass photospectrometry. Plants will always be negative compared to standard. Differences in deltas can estimate photosynthetic performance.

Fractionation in photosynthesis

Occurs during various photosynthetic steps- Rubisco at 30% prefers light CO2 (12CO2). Positive fractionation numbers result in more 12CO2 uptake.

Typical 𝝳13C for C3 plants

27-30 %0

Typical 𝝳13C for C4 plants

14.4 %0

CO2 compensation

point where there is enough CO2 for dark respiration

Variables of light response

photoperiod, direction of light, spectral quality, ultraviolet/infrared radiation, level of irradiance (intensity) - biggest factor

Photosynthetically active radiation (PAR)

the spectral range of solar radiation that plants use to drive photosynthesis. very low → stress, very high → damage to photosynthetic apparatuses. Either way → low photosynthesis

adaptation

genotypes or species (across generations)

Acclimation

phenotypes (within lifetime)

More vertical leaves →

decrease excessive radiation and provide more light to lower leaves

90% of PAR is absorbed by…

sun leaves (esp. red >700nm)

Why have lower leaves?

Sunflecks - short spells of high irradiance due to fluttering of leaves, movement of branches, changing sun angle

Light response curves

A net (x-axis) vs PAR levels/irradiance (y-axis)

A-Ci curve

Anet (y-axis) vs CO2 concentration (x-axis)

Light compensation point (LCP)

Point where Anet is zero

What limits Anet before the light response curve levels off?

the ETC

Sun leaves have higher…

Amax

Do sun leaves have thicker or thinner leaves?

Thicker- generally have thicker palisade cells- more space for chloroplast.

Do shade leaves have thicker or thinner leaves?

Thinner- more spongy mesophyll, air spaces increase path length for light to transmit. Fewer chloroplast per unit area, can also increase mesophyll conductance of CO2.

Differences in chloroplast between sun and shade leaves

Shade leaves have less stroma, greater density of thylakoid membranes. More stroma = more rubisco and more stroma exposed to thylakoid membranes (side that contains cytochromes and ATPase)

Lower ratio of chl a:chl b in shade →

more blue light in upper canopy

which plants transition to a plateau faster in light response curves?

Shade adapted/acclimated - due to photoinhibition

Photoinhibition

Traffic jams in photosynthesis (often at CR cycle) → excess excitation can occur when CR becomes limiting → can cause damage to PSII. Mechanisms for repair exist, but can get overwhelmed. Photoinhibition occurs when damage outweighs the repair abilities of a plant. Can be measured with fluorescence - energy comes off of CR cycle as heat or fluorescence.

Dynamic inhibition

Short term reduction in yield

Chronic or sustained inhibition

More serious damage

Regulation of photosynthetic rates

Traffic jams in CR cycle, CO2 availability, build up of triose P/depletion of inorganic phosphorous (Pi) in stroma

Products of photosynthesis

Most triose P not used to regenerate RuBP is exported to the cytosol in exchange for Pi through a gate protein. Sucrose is synthesized from triose P, which can be used in source leaf or exported to sinks via the phloem.

What regulates transport?

Concentration of Pi in cytosol. High concentration = high rates, low rates = build up of triose P in chloroplast.

What slows the CR cycle?

low rates of Pi import into the chloroplast- needed for formation of ATP

What happens if [Pi] in chloroplast drops?

Alternate pathway activates to create starch

Starch

neutral compound, storage compound for all plants, requires small amounts of ATP to break down later to transport as hexose, formation releases Pi in chloroplast

Feedback inhibition

Traffic jam- happens when sucrose levels get high in cytosol. Lots of intermediates, Pi bound to intermediates in cytosol. Causes decline in Pi in chloroplast → insufficient ATP, intermediates for RuBP regeneration are less available.

Long term regulation

Feedback inhibition is short term- occurs at cellular level. Long-term regulation occurs in response to high [hexose], downregulates expression of photosynthetic genes, decreases investment of nitrogen when not needed.

Plants exchange water vapor for…

CO2

Transpiration (E) is regulated by…

gs (stomatal conductance)

How does transpiration (E) change with an increase in stomatal conductance?

increases linearly

How does A-Ci change with an increase in stomatal conductance?

Increases- downward curve

How does Ci change with an increase in stomatal conductance?

increases- downward curve

How does WUE change with an increase in stomatal conductance?

decreases- upward curve

WUE

water use efficiency- exchange rate of water for CO2. Assimilation/transpiration: mol CO2/mol H2O/second

Soil water stress

WUE is higher at low gs- plants close stomata in response to lower water availability in soil.

Opening/closing of stomata is regulated by…

Abscisic acid (ABA) - hormone synthesized in roots in contact with dry soil, transported to leaves

Responses to dry air

stomata close when air is extremely dry

Vapor pressure deficit (VPD)

difference between how much moisture air can hold vs what it actually holds. Warm air can hold more moisture than cold → hot droughts are more intense

Large VPDs create…

large gradients between leaf and atmosphere

dry air →

feedforward response- preventing water loss at leaf level before relative water content (RWC) of plant decreases

dry soil →

feedback response - plants are already experiencing low water availability in soil

Feedback and feedforward responses keep —— and —— consistent

E and RWC- can cause closure of stomata midday and reduce Anet

Water stress limites ——- more than photosynthesis

leaf growth - changes source:sink dynamics

Nonstructural carbohydrates (NSCs)

accumulate in leaves under stress- sugars, fructans, glucose, starch

High [sugars] leads to…

feedback inhibition

Why is Pi critical?

TPT (gate protein) needs Pi to work. Pi is also needed inside the chloroplast for the ETC

Carbon-Isotope Fractionation and WUE

Plants with higher WUE have lower fractionation

Do plants under water stress have lower or higher WUE?

higher WUE

What are 2 of the most important nutrients for plants? (also tend to be most limited)

nitrogen and phosphorous

nearly half of all nitrogen in leaves are within…

chloroplasts