Photosynthesis

Question 1

Photosystem

Answer 1

Consists of multiple antenna systems and their pigments (packed on thylakoid membrane proteins), surrounding a reaction centre.

Question 2

Where does excitation energy pass?

Answer 2

From pigments absorbing short wavelengths to those absorbing longer wavelengths, and ends up in the reaction centre pigments

Question 3

Antenna complex

Answer 3

Light harvesting complex
Internal antenna within the PS proper and an external antenna made of protein complexes termed light-harvesting complexes (LHCs). These contain a variety of compounds whose function is to absorb photons and transmit captured energy to reaction centres. Plants used chlorophyll a, b and carotenoids. Algae don’t use cb but have other complexes specialised for harvesting red light

Question 4

Reaction centre

Answer 4

Converts light -> chemical energy
Excited chlorophyll molecule is the reducing agent and there is a acceptor molecule which is the first in a chain of electron carriers on the thylakoid membrane - electron transport, a series of redox reactions. Final electron acceptor is NADP+

Question 5

Engelmann’s famous action spectrum measurements

Answer 5

Spectrum of light onto spiral chloroplast. Oxygen-Seeking bacteria introduced and collected in region where chlorophyll pigments absorb. More at blue and red.

Question 6

Auxiliary pigments

Answer 6

e.g. carotenoids give extra colour to plants

Question 7

Similarities/differences of photosynthesis to oxidative phosphorylation

Answer 7

Occurs across a membrane
Involves electron transport
Generates a proton gradients across a membrane used to drive ATP synthesis
Many components in the cell are similar
Occur in cellular organelles
Photosynthesis uses energy of light photons to generate redox potentials and oxidative phosphorylation uses chemical reactions

Question 8

Cyclic and linear photosynthesis

Answer 8

Cyclic = ATP only - PSI only - released energy stored and can are used to form ATP
Linear = ATP and NADH - must oxidise water to oxygen (sulphides to sulphur or hydrogen to protons in bacteria) to balance out the production of a reduced cofactor. Oxidised chlorophyll a must be reduced back to its starting state by an inorganic substrate such as hydrogen sulphide. Allows cytochrome bc1 complex to use the electrons it obtains from QH2 to reduce NAD+ to NADH

Question 9

Emerson’s enhancement effect upon rate of photosynthesis

Answer 9

Photosynthesis works best with both far-red light and red light than each alone

Question 10

PSII

Answer 10

Light energy oxidised water -> O2 + H_ and electrons

Reaction centre has chlorophyll a molecules P680

Question 11

PSI

Answer 11

Light energy reduces NADP+ to NADPH

Reaction centre has chlorophyll a molecules P700

Question 12

Q cycle

Answer 12

Pumps an additional proton from inside to out.
QB + 2e- + H+ -> QH2 diffused through membrane to an exofacial site on the cytochrome bc1 complex. Oxidised back to Q, releasing the 2 protons which exit the exterior or the cell.

Question 13

Cytochrome bc1 complex

Answer 13

Transfers a soluble cytochrome in the periplasmic space on the exterior of the cell. Reduced cytochrome is oxidised by the chlorophyll a+ in the reaction centre, returing the system it its original state.

Question 14

Z scheme

Answer 14

Flow of electrons through carriers in PSII and PSI from H2O to NADP+

Question 15

Protein complexes transferring electrons

Answer 15

PSII - 2 hydrophobic plastoquinones - accept electrons from
Cytochrome b6f - electrons pass through when protons are transported into the thylakoid lumen
PSI - NADP+ reduced, using 3 Fe-S centres and ferreoxin as electron carriers
ATP synthase - protons move down electrochemical gradient passing through an ATP synthase and forming ATP

Question 16

Plastoquinone and plastocyanin

Answer 16

Carry electrons between PSII and PSI

Question 17

How is light energy used in chloroplasts?

Answer 17

By thylakoid-based photosystems to oxidise water, yielding O2 and generating reduced ferredoxin, NADPH and ATP
In the storm, ATP and NADPH drive the fixation of atmospheric CO2 and proaction of carbon skeletons for growth and development

Question 18

CO2 fixation

Answer 18

CO2 is reduced to carbohydrates - enzymes in the storm use energy in ATP and NADPH to reduce CO2. Must take place in light as Production of ATP and NADPH is light-dependent.

Question 19

How was the Calvin cycle worked out?

Answer 19

Calvin and Benson used 14C radioisotope to determine the sequence of relations by exposing chlorella to it, then extracting the organic compound and separating by paper chromatography

Question 20

Calvin cycle

Answer 20

Fixation of CO2 (rubisco catalyses carboxylation of ribulose-1,5-bisphosphate by CO2, yielding 3-phosphoglycerate), reduction of 3PG to G3P (3-phosphoglycerate is transformed to triose phosphates using ATP and NADPH) and regeneration of RuBP (5/6 triode phosphates are used to regenerate ribulose 1,5-bisphosphate). Stimulated by light - photons pumped from storm into thylakoids increase pH which favours the activation of rubisco. Electron flow from PSI reduces disulphide bonds to activate calvin cycle enzymes

Question 21

Rubisco

Answer 21

An oxygenase and a carboxylase - can add O2 to RuBP instead of CO2, reducing the amount of CO2 converted to carbohydrates, may limit plant growth.
RuBP + O2 -> 3PG + phosphoglucolate (2C)
Has 10x more affinity for CO2
CO2 is both an activator and substrate for rubisco.
Rubisco activase complements the CO2-dependent activation of rubisco by lowering the inhibition caused by sugar phosphates.

Question 22

Photorespiration

Answer 22

Takes place in light and consumes O2, releases CO2
In the least, if O2 concentrations high, photorespiration occurs. If CO2 conc high, CO2 is fixed.
More likely at high temps when stomata are closed.

Question 23

Phosphoglycolate

Answer 23

Forms glycolate, moved into peroxisomes and is converted into glycine, which diffused into mitochondria. 2 glycine are converted into glycerine + CO2.

Question 24

Light controls enzymes…

Answer 24

Light controls the activity of rubisco activase and 4 enxumes of the calvin-benson cycle via the ferrodoxin-thioredoxin system and by changes in mg2+ and pH. Changes in light intensity regulate the formation of supramolecular complexes of enzymes controlling the activity of phosphoribulokinase and G-3-P dehydrogenase

Question 25

Inorganic carbon-concentrating mechanisms

Answer 25

Increase CO2 conc around active site of rubisco. so reducing the rate of photorespiration

Question 26

CAM - Crassulacena acid metabolism

Answer 26

CAM photosynthesis functions to capture atmospheric CO2 and scavenge respiratory CO2 in aid environments
Generally associated with anatomical features that minimise water loss. Initial capture of CO2 and its final incorporation into carbon skeletons temporally separated. Genetics and environmental factors determine CAM expression.

Question 27

Changes in a typical CAM plant during night and day

Answer 27

Affect rate of uptake of atmospheric CO2, malic acid content and the degree of stomatal opening

Question 28

Obligate CAM

Answer 28

Up to 99% of CO2 assimilation occurs during night, therefore, supporting the hypothesis that CAM is adaptive because it allows CO2 fixation during the part if the day with a lower evaporative demand, making life in water-limited environments possible

Question 29

Facultative CAM and CAM-cycling plants

Answer 29

Drought-induced dark CO2 fixation may only be a small proportion of C3 CO2 assimilation in watered plants and may only occur a few days a year

Question 30

Cam-Cycling

Answer 30

With daytime CO2 fixation, but no nocturnal stomatal opening

Question 31

CAM-idling

Answer 31

With stomatal closure during the entire day and night in severely stressed plants. A low dark CO2 fixation and/or recycling or respiratory CO2 may help maintain a positive carbon balance under stress

Question 32

C4 carbon cycle

Answer 32

Fixes atmospheric CO2 into carbon skeletons in one compartment and releases CO2 in another to increase CO2 concentration of rubric for refixing via the Calvin-Benson cycle
Reduces photorespiration and water loss in hot, dry climates

Question 33

C4 cycle

Answer 33

Involves five stages in 2 different compartments, with phosphoenolpyruvate carboxylase, not rubisco, catalysing the primary carboxylation. Operation must be driven by diffusion gradients within a single cell as well as gradients between mesophyll and bundle sheath cells
Concentrates CO2 in single cells. Dimorphic choloroplasts located in different cytoplasmic compartments have photosynthetic functions analogous to mesophyll and bundle sheath cells in Krantz NAD-malic enzyme type 4 plants.

Question 34

Light regulates the activity of C4 cycle enzymes

Answer 34

NADP-malate dehydrogenase
PEPCase
Pyruvate-phosphate dikinase

Question 35

C4/C3 plants

Answer 35

C4 plants evolved multiple times from C3 plants, as they adapted to diverse environmental conditions causing CO2 to be limiting for photosynthesis
C4 photosynthesis can be accomplished within a single photosynthetic cell with dimorphic chloroplasts
Two novel means of separating the C4 function and dimorphic chloroplasts within the photosynthetic cells

Question 36

Light saturation point

Answer 36

Maximum rate of photosynthesis

Question 37

Light compensation point

Answer 37

Where net CO2 uptake = 0

Question 38

How can strong light (high PPFD) damage leaves?

Answer 38

Production of free radicals (ROS)
Reversible - photoinhbition - dynamic temporarily diverse excess light absorption to hear but maintains maximal photosynthetic rate
Irreversible - photo-oxidation

Question 39

Protection against ROS

Answer 39

Leaf movements, a range of molecular mechanisms, including the xanthophylls cycle, enzymatic destruction of ROS and rapid destruction and repair of D1 protein in PSII

Question 40

Shade vs sun leaves…

Answer 40

Have a limited capacity for damage repair and are easily photo-oxidised, whereas sun leaves have a high repair capacity (unless stressed) do not not usually become photo-oxidised
Different response curves - shade plant lower light saturation point but lower light compensation point = adaptive

Question 41

The xanthophyll cycle

Answer 41

Dissipates excess light energy to avoid damaging the photosynthetic apparatus; chloroplast movements may also limit light absorption

Question 42

Plant acclimation to different light regimes

Answer 42

Structural adaptations e.g. inclination of leaf/curve upright - more irradiance
Leaves grow much thicker in sun
Much energy from sunlight absorbed by leaf is dissipated as heat
Under elevated CO2 concs, stomatal aperture is smaller and hence leaf temps are higher due to lower transpirational cooling. Different diurnal courses of CO2 uptakes by individual leaves in a vertical succession of 6cm thick layers of leaves up an oak canopy

Question 43

Photosynthetic responses to temperature

Answer 43

Optimal photosynthetic temps have strong genetic and environmental components.
Temperature sensitivity curves allow us to determine a temp range where enzymatic events are stimulated, a range for optimal photosynthesis, a range where destructive events occur

Question 44

Quantum yield

Answer 44

Strongly dependent upon temp in C3 plants, but nearly independent of temp in C4 plants due to photorespiration
Reduced quantum yield and increased photorespiration leads to difference in the photosynthetic capacities of C3 and C4 plants at different latitudes

Question 45

Adaptive response to excessive light - Zeaxanthin/violaxathin

Answer 45

Highly quenched state of PSII is associated with the carotenoid zeaxanthin, the unquenched state with violaxanthin - the enzymes interconverts through an intermediate (anteraxathin) in response to light intensity.
Diurnal changes in xanthophyll content as a function of irradiance - violaxanthin lowest in most light and vice versa with zeaxanthin and antheraxanthin.
More Z+A in sun plants, in winter and low N

Question 46

Chloroplast positioning

Answer 46

Important in response to high light energy - plants grow best when chloroplast accumulation is outside burst cel. After UVB exposure, flavonoid-deficient and hydroxycinnamic acid deficient plants grew much less than wild type.