photosynthesis and carbon balance

why are plants considered ‘green’

chlorophyll is the main pigment in most leaves.

it reflects green light

identify the general light wavelengths absorbed by different pigments

chlorophyll (green pigment)- absorbs blue and red and reflects green

carotenoids (yellow & orange)- absorbs blue and blue-green and reflects yellow/orange

anthocyanins (red/purple)- absorbs blue, blue-green, and green and reflect red/purple

photosynthesis requires two sets of reactions

1. light reactions

• involves chlorophyll

• requires light energy

• creates energy molecules

2. dark reactions

• involves rubisco

• requires CO2

• creates carbohydrates

both occur within chloroplasts

part 1: light reactions

chlorophyll embedded within thylakoid membranes

thylakoid

internal structures stacked to form a granum

chlorophyll absorbs light

absorb high energy photons

plants have two types of chlorophyll (a&b) to maximize light absorbance

chlorophyll in thylakoids

chlorophylls a & b arranged in an antenna complex to gather and focus light energy

reaction center

where photons are absorbed and energy transferred to central pair of chlorophyll a pigments

activated chlorophyll

light energy is funnelled to chlorophyll a in reaction center

boosted to a higher energy level causing it to ionize

photosystem II

generates oxygen gas

type of antenna complex

reaction center responds to 680nm (red light) to lose the e-

electron transport chain

energized electron from reaction center of PS II is transferred to a chain of electrons acceptors in thylakoid membrane

replaced by splitting water

power ATP synthesize to create ATP: ADP+Pi = ATP

final receptor molecule is plastocyanin

photosystem I

refers to 700nm + red light (slightly lower energy)

replaces electron from electron transport chain by plastocyanin NOT from water

second electron transport chain

power creation of NADPH (energy molecule)- NADP+ + H+= NADPH

final receptor molecule is ferrodoxin

how is oxygen released from plants

exits leaf passively via stomata

where is oxygen generated during photosynthesis

comes from water, not from carbon dioxide

energized chlorophyll a & b molecules in PS I and PS II can

transmit energy to another chlorophyll molecule

lose an electron

release energy- usually if PS I & PS II became overloaded

part 2: dark reactions

carbohydrate-producing reactions occur in the stroma

rubisco

main enzyme producing carbohydrates

fixing inorganic CO2 and turns into organic C

three phases of dark reactions in photosynthesis (calvin-benson cycle)

carboxylation, reduction, regeneration

carboxylation

where CO2 is joined to RuBP

forms 2 molecules of 3-PGA- organic form of carbon

reduction

3-PGA is reduced to form G3P

requires energy from ATP & NADPH

regeneration

G3P produced RuBP

RuBP re-enters cycle

formation of sugar

2 G3P molecules used to produce one 6-carbon sugar

3 CO2 needed to generate one G3P that can exit the cycle- will go on to create sugar

how does CO2 enter plants

enters leaf passively via stomata

what happens if there is too much light in the photosynthesis/reaction

water-splitting enzyme and electron transport chains can become out of sync

photoinhibition

slows down photosynthesis

excess light effects

oxygen becomes electron acceptor- O2

highly reactive oxygen species

extremely potent and can cause damage

destroy membrane

photooxidation

no ATP or NADPH

what happens if there is too much heat in photosynthesis/reaction

rate of photosynthesis increases to thermal optimum

decreases above certain temps

how does both cold and heat dec photosynthesis

cold: increasing kinetic energy

heat: protein denaturation

how can oxygen impact photosynthesis

rubisco can catalyze reactions using O2 instead of CO2

prefers CO2- evolved under low oxygen conditions

higher enzyme slows catalytic speed

photorespiration

rubisco catalyzing reactions using O2

wasteful, no sugars are produced

energy required to free molecules in this pathway

releases CO2

what impacts photorespiration

drought, heat, salt stress increase

results in water stress & closed stomata- high amounts of O2 generated from photosynthesis need to be released & CO2 needs to be replenished

what increases likelihood of photorespiration

as CO2 is consumed and O2 concentrations greatly increase

daily changes in photosynthesis

often decrease during afternoon

highest in the morning

seasonal changes in photosynthesis

maximum photosynthetic rate of a leaf declines as it ages

carbon balance

equalizing gain of organic carbon with any loss of carbon

balancing gain in carbon from photosynthesis against loss from respiration and photorespiration

respiration

involves converting chemical energy into usable energy and releases CO2

converting organic carbon into inorganic carbon

carbon balance in plants

photosynthesis > respiration- growth and carbon storage are possible

photosynthesis = respiration- photosynthesis only meets plant respiration needs

photosynthesis < respiration- photosynthesis is insufficient and growth stops

maintenance respiration

energy stores in sugar molecules is released to maintain cell metabolism

growth respiration

additional energy stored is released to support tissue growth

relationship between photosynthesis and maintenance respiration

photosynthesis must be higher than maintenance respiration to support any plant growth

relationship between photosynthetic capacity and respiration

inc in respiration from young tissue growth = dec in photosynthetic capacity in young leaves

inc in respiration from controlled senescence = dec in photosynthetic capacity in senescing leaves

influence of light on carbon intake

photosynthesis measured via PAR- net carbon gain relative to light quality

photosynthesis measured via PPFD- net carbon gain relative to light quantity

bright light = high PPFD

dim light = low PPFD

PAR

photosynthetically active radiation

PPFD

photosynthetic photon flux density

identify the different components of a light response curve

light saturation point- minimum PPFD to reach max photosynthesis

light compensation point- minimum PPFD where photosynthesis = respiration

what can a light response curve tell us about a plant

how much light a plant needs, how well it uses light, how productive it can be, and how much energy it loses at night

effects on light on tree carbon balance

sun leaves- high PPFD

shade leaves- low PPFD

sun vs shade leaves- light compensation point

sun leaves:

higher, greater maintenance costs

more cells, chlorophyll, proteins

shade leaves:

lower, lower maintenance costs

fewer cells, chlorophyll, proteins

sun vs shade leaves- light saturation point

sun leaves:

higher, greater photosynthesis

thicker palisade, more thylakoids and chlorophyll

shade leaves:

lower, lower photosynthesis

thinner palisade, fewer thylakoids and chlorophyll