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
Light is Electromagnetic energy that exists in two formsContinuous and discrete particles
High energy
Low energy
The electromagnetic spectrum
• “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
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
ULTRAVIOLET RADIATION
- 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.
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 cyanobacteria
Phytochrome (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)
CALVIN CYCLE
-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: Carbon-fixation reaction in stroma where ATP and NADPH produced in light reaction are used to
Reduce CO2 to simple 3-carbon molecules of PGAL
Photorespiration
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.
-Temperature
-Loss of carbon
-Scavenger of carbon
-Photo-oxidative
-Crop productivity
Light dependent Photosynthesis
ATP/NADPH
Light Independent Photosynthesis (Dark reaction)
Fix CO2 using ATP and NADPH
produce sugar
Respiration: Sugar is broken down (oxidized) to capture
Energy 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: REDUCE
PHOTORESPIRATION
- 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 PHOTOSYNTHESIS
http://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.
Absent
2.
Operating only during night
3.
Operating in specialized cells during day time
4. 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 content
2. Light with higher wavelength have higher energy content
3. Light with higher wavelength have lower energy content
4. Light with lower wavelength have less energy than that of higher wavelength
Identify the right answer: During cyclic Electron transfer
1. PSII is involved
2. Water is split
3. ATP is generated
4. NADPH is produced
5. Oxygen is released
Thymine dimer formation is caused by exposure to
1.
UV-B radiation
2.
UV-C radiation
3.
UV-A radiation
CAM plants fix CO2 at night into
1. Pyruvate
2. Phosphoglyceraldehyde (PGAL)
3. Malic acid
4. Oxaloacetate
Identify the correct statement:
1. Absorbsion of far-red light (730 nm) makes phytochrome inactivated
2. Absorbsion of red light makes phytochrome inactivated
3. Absorbtion of far-red light makes phytochrome activated
The end products of light reaction of photosynthesis are
1. NADPH and ADP
2. ATP and NAD
3. ATP and NADPH
4. GTP and NADPH
Calvin Cycle (Fixation of CO2 during photosynthesis) takes place in
1. The thylakoid membrane of plant chloroplast
2. The stroma of plant chloroplast
3. The stroma lamellae of the chloroplast
4. The inner membrane of the chloroplast
Chlorophyll c is like chlorophyll a except that
1. A formyl group (-CHO) replaces the methyl group on ring II
2. A formyl group (-O-CHO) replaces divinyl group (-CH=CH2) on ring I
3. Lacks the phytol tail
4. Do not contain the chelated magnesium ion