C1.3: Photosynthesis

Photosynthesis

process by which autotrophs (plants, algae, cyanobacteria) convert light energy to chemical energy (ATP)

chemical energy --> organic compounds (glucose) --> cellular structure or energy for cell respiration

primary source of chemical energy that supports life in ecosystems

*photosynthesis is anabolic synthesis is the reverse of cell respiration (catabolic breakdown)*

Photosynthesis Formula

6CO2 + 12H2O + sunlight ---> C6 H12 O6 + 6O2 + 6H2O

Photosynthesis 2 Main Steps

light dependent and light independent reactions

Light Spectrum

sun emits a visible region of radiation energy in which white light is about 400-700 nm

-colors represent different wavelengths and range from red (longest, 700 nm) to violet (shortest, 400 nm)

Photosynthetic Pigments

absorb specific wavelengths of light

-chlorophyll a is the main pigment, w/ chlorophyll b and carotenoids being accessory pigments

each have specific molecular structures that determine which wavelengths can be absorbed

-excited electrons within are used to transform light energy to chemical energy

Absorption Spectra

measurement of how different pigments absorb different wavelengths of light

-mostly blue then red wavelengths are absorbed best and green is reflected

Action Spectra

measurement of rate of photosynthesis at each wavelength of light

Similarity/Difference btwn Absorption vs Action Spectra

Differences:

-absorption spectra shows light absorbed, action spectra shows how they are used

Similarities:

-two spectra look similar bc absorbed light drives photosynthesis

-both indicate that blue/red light are the most effective while green is least

-both support the role of chlorophyll and accessory pigments in capturing energy

Chromatography

technique used to separate different types of pigments including xanthophylls and carotenes (carotenoid subgroups)

mixture is dissolved in a fluid (mobile phase) and passed through a static material (stationary phase)

-different pigments travel at different speeds, causing them to separate

Rf value can be calculated

-(distance traveled by solute)/(distance traveled by solvent)

Types of Chromatograph (2)

Paper chromatography:

-uses paper (cellulose) as stationary bed

Thin layer chromatography:

-uses a thin layer of adsorbent (silica gel), which runs faster and has better separation

Photosynthesis Limiting Factors

reaction rate will be determined by the factor at its minimum

photosynthesis depends on:

-temperature

-light intensity

-[CO2]

Temperature

impacts the frequency of enzyme-substrate collisions

-at lower temp, rxn rate slower is slower

-as temp increases, rxn rate increases due to increase kinetic energy

-above optimal temp rxn rate decreases due to enzyme denaturation

Light Intensity

required for photoactivation of chlorophyll to produce ATP

-as light increases, rxn rate increases due to increase number of pigments activated

-after point of saturation, rate will plateau

-different colors of light affect rxn rate differently

*measure by controlling distance of a light source and measure w/ lux meter*

[CO2]

CO2 is the main source of carbon with which glucose is made

-as [CO2] increases, rxn rate increases and more glucose produced

-after a point, rate will plateau due to saturation of enzymes

*measure by regulating [CO2] by using tablets of sodium bicarbonate, which dissociates in water to form CO2*

Measuring Photosynthesis (3)

CO2 uptake, O2 production, biomass increase

CO2 Uptake

place leaf tissue in enclosed space w/ water

-dissolved CO2 will interact w/ water to form bicarbonate and hydrogen ions, which increase acidity (decrease pH)

photosynthesis by leaf tissue will decrease [CO2] in solution, which will increase pH (more basic)

-measure CO2 levels using probe and data logger

O2 Production

Submerge leaf tissue in water-filled enclosure attached to sealed gas syringe

-measure O2 production by recording meniscus level in syringe

-can use appropriate probe/data logger

can also time how long submerged leaf discs rise to the surface

Biomass Increase

indirectly measure glucose production by weight

-plant tissue must be dehydrated to ensure that change in wight is due to organic and not water content

can also measure starch levels (how glucose is stored)

-identify starch via iodine staining

-quantify through titration and/or colorimeter

CO2 Enrichment Experiments

increase Co2 to a higher level than what is normally in fresh air

-fresh air contains about 400-450 ppm by volume

done to predict future photosynthetic rates and plant growth in response to human activity

-combustion of fossil fuels and deforestation increase [CO2] in atmosphere

-higher [CO2] typically increases plant growth, however excessive amounts can lead to damage

*either enclosed greenhouse experiments or Free Air CO2 Enrichment*

Enclosed Greenhouse Experiments

artificially increase CO2 in indoor greenhouses by using compressed gas tanks or fermentation buckets

-functions as a closed system, which allows for control of extraneous variables

however, conditions do not fully reflect natural environments and only plants that occupy small spaces can be measured

Free Air CO2 Enrichment (FACE)

involves placement of pipes that emit CO2 around the experiment area

-sensors monitor [CO2] and adjust its flow from pipes

represents open system that take into consideration natural conditions like rainfall and temperature fluctuations

-also measures CO2 enrichment on larger trees and considers impact of competition btwn plant species

one disadvantage is that certain factors (sun) cannot be controlled for

Chloroplasts

produce glucose from light energy

double membraned and contain thylakoids and stroma

Thylakoids

flattened structures arranged in stacks of grana

-large SA of thylakoid membranes ensures more space for: photosystems w/ chlorophyll to capture light and ETC and ATP synthase to produce more ATP and NADPH

Thylakoid Lumen

has a small volume of fluid to rapidly build a proton gradient (pump protons into thylakoid space then return to stroma

Stroma

fluid portion of the chloroplast; outside of the thylakoids

compartmentalization of enzymes/substrates occurs here

-localized [] of enzymes in their optimal conditions speed up the Calvin cycle

Light Dependent Reactions

light energy --> chemical energy (ATP)

-pigment molecules (chlorophyll) absorb light

-splits H2O via photolysis (to e- + O2 + H+)

e-, H+, ATP: are used, O2 is released as waste

Photosystems

arrays of protein-pigment complexes embedded in thylakoid membranes of chloroplasts and cyanobacteria (mesosomes here)

capture light and convert it to ATP

they contain:

-special chlorophyll a pigment in the reaction center

-accessory pigments like chlorophyll b, carotenoids, and xanthophylls in antenna complex

Special Chlorophyll a

located in the reaction center

-their electrons are excited and lost

-caught by primary electron acceptor

Antenna Complex

Part of a photosystem, containing an array of chlorophyll molecules and accessory pigments, that receives energy from light and directs the energy to a central reaction center during photosynthesis.

Types of Photosystems

photosystem II, absorption peaks at 680 nm (P680)

photosystem I, absorption peaks at 700 nm (P700)

Pigment Molecules

chlorophyll a, chlorophyll b, carotenoids

Chlorophyll a

primary pigment that directly participates in photoactivation of electrons

Chlorophyll b

absorbs additional wavelengths of light and funnels their energy to chlorophyll a

Carotenoids

absorb additional wavelengths and protect against photooxidation

Insufficiency of Single Pigment

one type of pigment limits light absorbing capacity

-an array ensures more photons are absorbed across different wavelengths, allows efficient energy transfer and prevents energy loss

diversity allows plants to adapt to different light conditions (ex shade plants have more chlorophyll b)

different pigments also protect against photooxidation

Absorption of Light

photons excite e- of accessory pigments in antenna complex

-as e- drop back down, energy is released

-energy is funneled to reaction center

-P680 (PSII) loses its e- to primary electron acceptor

Photolysis of Water

lost e- from PSII are replaced by e- from H2O

H2O --> H+ for chemiosmosis and O2 released as waste

photolysis of water generates H+ and e- used in light dependent reactions, but oxygen is a waste product

*photosynthesis is the only significant source of O2 in the known universe*

Changes in Atmospheric O2 Over Time

before photosynthetic organisms, any free oxygen available on Earch was captured and stored

-earth's oceans initially had high levels of dissolved iron

-when dissolved iron was converted into precipitates, O2 started to accumulate in atmosphere

*initial amounts of O2 may have been produced from abiotic processes or early primitive microbes*

Rise of atmospheric O2 critical to evolution of aerobically respiring organisms

-also led to development of the ozone layer which limits harmful radiation exposure

Electron Transport Chain

excited e- from PSII, sometimes PSI move through ETC embedded in thylakoid membrane via redox reactions

-e- release energy which pumps H+ from stroma into thylakoid lumen via carrier proteins

-increase [H+] creates proton motive force

Chemiosmosis

H+ in thylakoid lumen move down their gradient back to stroma through ATP synthase

-uses this energy to make ATP from ADP + Pi

-e- from PSII replaces those lost from PSI

Photophosphorylation

ATP production initiated by light

Non-Cyclic Photophosphorylation

involves both photosystems (II and I) and reduction of NADP

-e- from PSII go through ETC to make ATP

-e- from PSI are caught by NADP Reductase to form NADPH

-produces equal amounts of ATP and NADPH

Cyclic Photophosphorylation

involves only PSI and NO reduction of NADP

-e- from PSI go back to ETC

-produces ATP, but not NADPH or O2

done bc calvin cycle uses more ATP than NADPH (9:6)

-when NADPH levels rise, non-cyclic shifts to cyclic to produce more ATP

Reduction of NADP

processes in PSII repeat in PSI

-excited e- from PSI are caught by NADP reductase

-NADP + e- + H+ --> NADPH which is needed in calvin cycle along with ATP

Summary of Light Dependent Reactions

Occurs in Thylakoid lumen and membrane

Photoactivation: chlorophyll in PSII and PSI absorb light, e- become excited and those from PSII move down ETC, then replaced by photolysis of H2O

ETC uses this energy to pump H+ from stroma into thylakoid creating proton gradient, then chemiosmosis occurs to make ATP

e- from PSI move to NADP reductase, reducing NADP to NADPH, e- are replaced by PSII

Final Products: ATP and NADPH later used in calvin cycle

Light Independent Reactions

Calvin Cycle

Chemical energy (ATP) to organic compounds (glucose)

-ATP provides energy to affix carbon in anabolic reactions

-H+ is combined with CO2 to form C6H12O6

Light Independent Reaction General Process

Carbon fixation, Synthesis of TP, Regeneration of RuBP

Rubisco

RuBP Carboxylase, does carbon fixation

high [] are necessary bc it is slow and ineffective in low [CO2]

Rubisco joins circulating RuBP w/ entering CO2, produces unstable compound that breaks down into GP (glycerate-3-phosphate)

RuBP

rubulose bisphosphate, 5C

joins w/ CO2

Synthesis of Triose Phosphate

GP receives e- from NADPH and energy from ATP to become TP

Regeneration of RuBP

of the 6 TPs (3C) produced, one leaves the cycle

-therefore 2 cycles are necessary to make 1 glucose

remaining 5 TPs are phosphorylated by ATP to generate 3 RuBps

Interdependence of Light Dependent and Independent Reactions

both require products from the other in order to begin

-light dependent reaction requires NADP, which is made (unloaded) in the Calvin cycle

-lack of CO2 in calvin cycle will prevent production of NADPH by PSII in light dependent reaction

Calvin Cycle requires ATP and NADPH which are products of light dependent reactions

Synthesis of Organic Molecules

synthesis of 4 macromolecules happens via a variety of reactions in cytoplasm

-small fraction of TP that are not used to regenerate RuBP can be converted to form macromolecules

Most require additional assimilation of mineral nutrients

-proteins require nitrogen (and maybe sulfur) to form AAs

-Nucleic Acids require phosphorus and nitrogen to form nucleotides

-Certain lipids may also require additional components (phosphorus for phospholipids)