PLANT PHOTOBIOLOGY

PLANT PHOTOBIOLOGY

• Studies the importance of light in the growth and development of green plants

• Light used as both Energy and Information

Ex: - Energy in Photosynthesis

- Information in Photomorphogenesis, Photoperiodism

The electromagnetic spectrum

Light is Electromagnetic energy that exists in two forms- Continuous and discrete particles

• “light” is narrow band of energy within the continuous

electromagnetic spectrum of radiation from sun• The band causes physiological response of vision in

humans-capable of stimulating the receptors in retina of eye

High energy Low energy

Light Energy can Interact with Molecules

• To be used by plants, needs to be absorbed and lead to photochemical reaction• First step in photochemical reaction is absorbance of light by a molecule (pigment)• Example: Photosynthesis• Pigments gives characteristics color of leaves, petals (attract pollinator), phytochrome for photomorphogenesis

WHAT HAPPENS WHEN A PIGMENT ABSORB LIGHT?

• Ground state• Excited state for nanosecond 10-9S• Absence of chemical reaction- must rid of excess energy

- 3 ways- Heat- Fluorescence (low energy- longer wavelength)- inductive resonance (Close donor acceptor)- Triplet state (spin of electrons- more stable

photooxidation of pigments)

Fluorescence of Chlorophyll

- UV radiation- causes ionization of molecules (ejection of electron)- UV-C (254 nm) induce thymine dimer-mutation- Little UV radiation reaches earth surface as absorbed by Ozone both UV-C and UV-B- Plants most sensitive to UV-B radiation are native to places

where UV-B fluxes are the lowest.

ULTRAVIOLET RADIATION

VISIBLE LIGHT RADIATION

- Fluence rate and spectral quality constantly changing on daily or seasonal basis- Convey information to plants- At midday (full sun) 2000 mmol m-2 s-1 and during sunsets 10 mmol m-2 s-1

Photoreceptors absorb light for use in physiological processes- Chlorophyll (responsible to harvest light energy for photosynthesis

- Phycobilins in red algae and cyanobacteriaPhytochrome (in green plants)

- Carotenoids for autumn color (stable, protect chlorophyll from Photo oxidation)- Cryptochrome (Blue and UV-A light, candidate is Flavin, HY4 to inhibit hypocotyl elongation)- Flavonoids (Anthocyanins) for flowers, soluble in water, found in vacuole. Sensitive to pH (red in acid, violet in neutral, blue in alkaline)- Betacyanins in beets

Chlorophyll molecule: Porphyrin head and hydrocarbon tail

- Head with tetrapyrrole-cyclic(N containing)- Tail with phytol (precursor-isoprene)- Magnesium ion in middle- No Mg++

no green chlorophyll (Pheophytin)- Four species of Chlorophyll a, b, c, and d.- Chl. a is principle in all higher plants- Formyl (-CHO) group substitute methyl (CH3) group in Chl. b in ring II.- Chl. c in diatoms, brown algae lacks tail- Chl. d in red algae replaces –CH=CH2 (divinyl group) in ring I with –O-CHO group(Formyl group).

CAROTENOIDS ACCOUNT FOR AUTUMN COLORS

- More stable than Chlorophyll- Mostly brilliant orange and yellow- Predominantly hydrocarbon, soluble in lipid- Found in specialized plastids called chromoplasts- Example: differs on structure

- carotenes (beta and alpha)- Lycopene (tomato)- xanthophylls (yellow oxygenated carotenoids)- lutenin (hydroxylated a-carotene)

- Beta-carotene: associated with protein, absorbs strongly in blue region- Quench singlet oxygen, triplet excited chlorophyll (protection)

Photosynthesis

- Provides food: 250 billion metric tons/year- Provides fuel for life- Produces oxygen- Helped life evolution by forming O3- General equation:6CO2 + 6H20 C6H12O6 + 6O2 - In Chloroplast

Where does Photosynthesis takes place?

- Chloroplasts of mesophyll cells

- Oxygen and Carbon-di-oxideenter/leave by stomata

- Water is brought by veins

Site of Reactions: Thylakoid membranes

- Chloroplast internal membrane- thylakoids contain pigment- Pigments absorb energy from light - Store energy in ATP and the electron acceptor molecule NADPH- In stroma, carbon dioxide is reduced to form carbohydrates (using ATP and NADPH).

How is light absorbed?

Light/heat

Pass energy

Leaves electron

Mechanical analogy of Photosynthesis reactions

The Z Scheme For Photosynthetic electron transfer

The locations of the ETC molecules

Cyclic Electron Transfer

- PSI involved- e- hops back between PSI and PSII - ATP is generated- No H2O is split- No O2 is released- Provides additional ATP for Calvin cycle- No NADPH produced

DARK REACTIONS: CARBON-FIXATION REACTIONS REDUCE CARBON DIOXIDE TO SIMPLE SUGARS

- ATP and NADPH on outside of thylakoid membrane- Energy in chemical bonds used to reduce CO2 to sugar- Called Calvin cycle (Nobel prize in 1961)

- Enzymes and reactions occur in the stroma of chloroplast.

- A molecule of CO2 is attached to ribulose 1,5-bisphosphate (RuBP, a 5 carbon sugar) by the enzyme Rubisco;

- The 6 carbon compound is immediately split into two 3 carbon compounds (PGA); called C3 photosynthesis; about 85% of plants use only this pathway, including wheat, soybeans and potatoes.

-After several more reactions requiring ATP and NADPH, a 3 carbon compound PGAL (phosphoglyceraldehyde) is produced and some PGAL is used to make glucose other PGAL is used to regenerate RuBP.

- 6 turns of Calvin cycle needed to produce 1 glucose.

- Rubisco is one of most important enzymes; needed to fix carbon dioxide.

CALVIN CYCLE

CALVIN CYCLE

Calvin cycle: Carbon-fixation reaction in stroma where ATP and NADPH produced in light reaction are used toReduce CO2 to simple 3-carbon molecules of PGAL

Occurs in C3 plants during hot summer weather; leaves close stomata to prevent loss of water vapor and then carbon dioxide levels in leaf fall; RuBP combines with oxygen instead of carbon dioxide.

produces 1 PGA instead of 2 for the Calvin cycle; this makes C3 plants less efficient in hot sunny weather.

Photorespiration

- Temperature- Loss of carbon- Scavenger of carbon- Photo-oxidative- Crop productivity

Light dependent PhotosynthesisATP/NADPH

Light Independent Photosynthesis (Dark reaction)Fix CO2 using ATP and NADPHproduce sugar

Respiration: Sugar is broken down (oxidized) to captureEnergy in sugar (produce ATP)

C4 Pathway:- Crabgrass, Sugarcane, corn- 1st product oxaloaccetate (C4)- Mesophyll cells, no calvin cycle- CO2 fixed twice: mesophyll, bundle-sheath- Spatially separated C4 and Calvin cycle- 10 times more CO2 in bundle-sheath- Photorespiration reduced-more efficient in humid weather

CAM PLANTS FIX CO2 AT NIGHT: REDUCEPHOTORESPIRATION

- Dessert plant- stomata closed, high photorespiration in C3. But Crassulaceae family use CAM (Crassulacean acid metabolism) pathway to reduce photorespiration- Use both C4 and Calvin but separated temporally.- At night C4 pathway, daylight Calvin cycle.- Cost (more ATP)/benefit (reduced photorespiration) determines adaptation

EXAMPLE OF CAM PLANTS

SNAKE PLANTS (Sansevieria) STONECARP (Crassula sp.)

ARTIFICIAL PHOTOSYNTHESIShttp://solarfuelshub.org/home

• Highly efficient, non‐biological, molecular‐level energy conversion “machines” • Generate fuels directly from

sunlight, water, and carbon dioxide • Inorganic photo‐catalytic methods to

split water or reduce carbon dioxid• Solar fuels generator

Components of Proposed Artificial Photosynthesis Process

• Light absorbsers, Catalyst, Membranes, Linkers

The GUS reporter system (GUS: beta-glucuronidase) is a reporter gene system, particularly useful in plant

molecular biology.

- Purpose is to analyze the activity of a promoter - Based on beta-glucuronidase, an enzyme from the bacterium E. coli - This enzyme, when incubated with some specific substrate (X-Gluc), develops into colored products- Over 6000 citations since 1st reported in 1987

In the C4 pathway, the Calvin cycle is:1. Absent2. Operating only during night3. Operating in specialized cells during day time4. Operates only when there is more CO2 avaialable

T / F : “light” is narrow band of energy within the continuous electromagnetic spectrum of radiation from sun

1. False

2. True

Identify the correct statement: 1. Light with lower wavelength have lower energy content2. Light with higher wavelength have higher energy content3. Light with higher wavelength have lower energy content4. Light with lower wavelength have less energy than that of higher wavelength

Identify the right answer: During cyclic Electron transfer1. PSII is involved2. Water is split3. ATP is generated4. NADPH is produced5. Oxygen is released

Thymine dimer formation is caused by exposure to1. UV-B radiation2. UV-C radiation3. UV-A radiation

CAM plants fix CO2 at night into1. Pyruvate2. Phosphoglyceraldehyde (PGAL)3. Malic acid4. Oxaloacetate

Identify the correct statement:1. Absorbsion of far-red light (730 nm) makes phytochrome inactivated2. Absorbsion of red light makes phytochrome inactivated3. Absorbtion of far-red light makes phytochrome activated

The end products of light reaction of photosynthesis are1. NADPH and ADP2. ATP and NAD3. ATP and NADPH4. GTP and NADPH

Calvin Cycle (Fixation of CO2 during photosynthesis) takes place in1. The thylakoid membrane of plant chloroplast2. The stroma of plant chloroplast3. The stroma lamellae of the chloroplast4. The inner membrane of the chloroplast

Chlorophyll c is like chlorophyll a except that1. A formyl group (-CHO) replaces the methyl group on ring II2. A formyl group (-O-CHO) replaces divinyl group (-CH=CH2) on ring I3. Lacks the phytol tail4. Do not contain the chelated magnesium ion