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Photosynthesis: light reactions & carbon fixation

PhotosynthesisGlobal Importance of Photosynthesis – Photosynthesis by green plants and algae provides nearly all

of the energy and organic carbon required by living organisms.

– Photosynthesis provides all of the oxygen required by aerobic organisms.

– The fossil fuels needed for human activity were produced by ancient photosynthetic organisms.

– Photosynthesis is the largest scale biosynthetic process on Earth.

Photosynthetic organisms remove approximately 200 billion tons of carbon, per year from the atmosphere. This is about 10% of the total CO2 in the atmosphere. In energy terms, this is roughly 0.1% of the incident visible sunlight that reaches the Earth every year. Virtually all of the CO2 "fixed" by plants is returned to the atmosphere by microbial, plant, and animal metabolism, and by biomass combustion.

The Two Phases of Photosynthesis

Light Reactions (Z Scheme)

– Photolysis (H2O → 2H+ + 1/2O2)– Photophosphorylation (ADP + P → ATP)– NADP Reduction (NADP + H+ → NADPH)

Dark Reactions (PCR,C3 or Calvin-Benson Cycle)

– Carbon Dioxide Fixation – (CO2 → C6H12O6)

Overall reaction– 6H2O + 6CO2 → C6H12O6+ 6O2

Some basic conceptson Radiation

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Radiation quality (or “color”, for visible light) is a function

of its wavelength (or frequency) distribution

The symbol “λ” is often used for wavelength

Note these two charts are arrayed in opposite directions –one by increasing wavelength / decreasing energy and the other by increasing frequency/ increasing energy

Radiation quantity is measured in one of three ways, depending on the application:

1. Quantum measurements (numbers of photons)2. Radiometric measurements (amount of energy)3. Photometric measurements (light intensity,

based on human perception)

The amount of radiation is expressed as fluence (also known as density; quantity per area), rate (also known as flux; quantity per time) or fluence rate (also known as flux density; amount per area per time)

Parameter Term Energy units

Quantum units

Quantity per area

fluence J m-2 μmol m-2

Quantity per time

rate(or flux)

J s-1 (watt) μmol s-1

Quantity per area per time

fluence rate (or flux density)

W m-2 μmol m-2 s-1

For studies of photosynthesis and photomorphogenesis, the quantity of radiation is usually measured in quantum units (quantum flux density; quantum fluence rate):

μmol m-2 s-1

PPFD = photosynthetically active photon flux density

PAR = photosynthetically active radiation (400-700 nm)

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The energy of a photon is proportional to its frequency and inversely proportional

to wavelength:

E = hν = hc/λEnergy per photon(joules)

Planck’s constant:6.63 x 10-34

joules s

Frequency (s-1)

Speed of light 3 X 108 m s-1

wavelength(in meters)

• visible = 400-700 nm, about 45% of incident insolation• solar IR = 700-5000 nm, about 46% of incident• UV = 190-400 nm, about 9% of incident

Spectral Quality

All of the irradiance that impinges on an object is either reflected, transmitted or absorbed

Emission = loss of energy that was absorbed.

Fluorescence = another form of energy loss after bsorption)

TRANSMITTED

ABSORBED

Reflectance, Transmission and Absorption

REFLECTED

Albedo = reflected / absorbed radiation.

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Pigments:Molecules that absorb radiation in the visible wavelengths.

The color of pigments results from the visible wavelengths that are not absorbed.

Absorbed light energy causes electrons in pigment molecules to jump to higher energy levels (excited)

Higher plants have two classes of pigments that are involved in energy capture and transfer for photosynthesis:

chlorophylls (a and b). •There is also a chlorophyll c in non-vascular plants)

carotenoids•there is a whole family of carotenoids; not all of them are involved in photosynthesis)

Other pigments that are not involved in photosynthetic energy capture but serve photoprotective or photosensoryroles:

anthocyanins and phytochrome,.

Chlorophylls and carotenoids are in chloroplasts

Anthocyaninsare in the vacuole

Phytochromein the nuclear envelope?

Chlorophyll b

Function follows structure: The “ring” structure, with alternating double bonds and Mg atom at the middle leads to “allowable” excited energy states at relatively low levels (most proteins absorb high-energy uv light; pigments absorb lower energy visible light because of the “resonance” resulting from the molecular structure). The ring is hydrophilic.The “phytol tail” is hydrophobic; it is lipid soluble (cell membranes are made of lipids)

Rings A,B,C and D form a “porphyrin ring”

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Carotenoids

The carotenoids include:

βcarotene, which is involved in light harvesting

and

Xanthophylls, which are involved in photoprotection. (Disruption of the photoprotection activity has been studied for development of possible herbicides)

Chlorophyll a

Chlorophyll b

Β-carotene

Phycoerythrin Chlorophyll c

Anthocyanins are water-soluble pigments that do not participate in the reactions of photosynthesis. These are the pigments that give “red” color to plants.

They strongly absorb uvradiation in addition to visible light and are thought to have two primary functions –attraction of insects and protection from uv radiation.

Chloroplasts are filled with thylakoidmembranes that form “stacks”of “grana”

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The membranes separate the inside “lumen”from the outside “stroma”. This structural

organization is critical to the functions involved in energy capture.

Most of the enzymes, pigments and electron carriers involved in light harvesting are imbedded in the thylakoid membranes

Note the lipid bi-layer structure of the membrane

OVERVIEW.

1. Light harvesting

3. Electron transport and photophosphorylation

4. Carbon fixation

5.Consumption;Transport

2. Water splitting(electron source)

The light reactions

The dark reactions

At this juncture there is a transformation from flow of photons to flow of electrons

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Energy from the excited state can be transferred to:

1. Chlorophyll fluorescence2. Heat (= “non photochemical quenching”)3. Photosynthesis (= “photochemical quenching)

of chlorophyll

of chlorophyll

The first step of photochemical electron transport (“photochemical quenching”) is the excitation of a chlorophyll reaction center,

which then passes an electron to an acceptor molecule (“A”)

This is where new electrons come into the cycle

The electrons from the first chlorophyll reaction center (PSII, or P680) are passed along a chain of acceptors until they reach another reaction center (PSI, or P700). Then this reaction center is then energetically “primed” to be excited by another photon absorption, and the electrons are finally passed along to other, higher energy acceptors (weakoxidants)

This is the “z-scheme”, or “photosynthetic electron transport”

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Another perspective on the same process …

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The electrons from the electron transport chain are ultimately accepted by NADP+, which reduces this molecule to NADPH (an important source of reducing power for carbon fixation) The splitting of water results

in an accumulation of protons in the thylakoidlumen. As electrons move along the electron transport chain, protons are also transported from the strmato the lumen. The result is an electrochemical gradient and across the membrane (lower pH in the lumen). Like a “battery”, this gradient is a form of potential energy, and is used to drive ATP production from ADP.

Photophosphorylation:adding a phosphate group onto adenosine diphosphate (ADP) to form ATP using energy from light.

Another perspective on the same process …

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A large class of herbicides operate by interrupting the photosynthetic electron transport chain:

Turning to C assimilation steps of photosynthesis

Carbon fixationAlso known as :• Dark reaction•PCR : Photosynthetic carbon reduction cycle•Calvin-Benson Cycle

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3 steps of the Calvin-Benson cycle:

Carboxylation

Reduction

Regeneration

Rubisco activase is an enzyme that converts Rubiscofrom an inactive to an active form

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One of the major problems facing terrestrial plants is dehydration.At times, solutions to this problem conflict with other metabolic processes, especially photosynthesis.The stomata are not only the major route for gas exchange (CO2 in and O2 out), but also for the evaporative loss of water.On hot, dry days plants close the stomata to conserve water, but this causes problems for photosynthesis.

Alternative mechanisms of carbon fixation Alternative mechanisms of carbon fixation have evolved in hot, arid climateshave evolved in hot, arid climates

In most plants (C3 plants) initial fixation of CO2occurs via rubisco and results in a three-carbon compound, 3-phosphoglycerate.– These plants include rice, wheat, and

soybeans.When their stomata are closed on a hot, dry day, CO2 levels drop as CO2 is consumed in the Calvin cycle.At the same time, O2 levels rise as the light reaction converts light to chemical energy.While rubisco normally accepts CO2, when the O2/CO2 ratio increases (on a hot, dry day with closed stomata), rubisco can add O2 to RuBP.

Photorespiration is initiated when RUBISCO acts as an oxygenase instead of a carboxylase:

Calvin cycle

Photorespiration

The phosphoglycolate is“recycled” to produce more3PGA, consuming energy and releasing more CO2

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Summary points of photorespiration:

1. The energy used to produce the RuBPacceptor is wasted

2. It takes even more energy to recycle the 2-carbon glycollate

3. High light, low CO2 and high O2 promote photorespiration

4. C3 plants can lose up to ¼ of their newly fixed carbon through photorespiration

5. Photorespiration may have a role in reducing photoinhibition and photooxidation

Certain plant species have evolved alternate modes of carbon fixation to minimize photorespiration.– C4 and CAM

The C4 plants fix CO2 first in a four-carbon compound.– Several thousand plants, including sugercane and

corn, use this pathway.In C4 plants, mesophyll cells incorporate CO2 into organic molecules.– The key enzyme, phosphoenolpyruvate

carboxylase, adds CO2 to phosphoenolpyruvate(PEP) to form oxaloacetetate.

– PEP carboxylase has a very high affinity for CO2and can fix CO2 efficiently when rubisco cannot -on hot, dry days with the stomata closed.

The mesophyll cells pump these four-carbon compounds into bundle-sheath cells.– The bundle sheath cells strip a carbon, as

CO2, from the four-carbon compound and return the three-carbon remainder to the mesophyll cells.

– The bundle sheath cells then uses rubisco to start the Calvin cycle with an abundant supply of CO2.

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Kranz anatomy

C4 plants have specialized “Kranz” anatomy that separates the initial carboxylation by PEP carboxylase from the Calvin cycle, and concentrates CO2 around Rubisco.

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Photorespiration does not occur in C4 plants because they concentrate CO2 around

RUBISCO. CO2 “out competes” O2 for the active sites on the RUBISCO enzyme

A second strategy to minimize photorespiration is found in succulent plants, cacti, pineapples, and several other plant families.– These plants, known as CAM plants for

crassulacean acid metabolism (CAM), open stomata during the night and close them during the day.

Temperatures are typically lower at night and humidity is higher.

– During the night, these plants fix CO2 into a variety of organic acids in mesophyll cells.

– During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO2 is released from the organic acids.

Both C4 and CAM plants add CO2 into organic intermediates before it enters the Calvin cycle.– In C4 plants, carbon fixation and the Calvin

cycle are spatially separated.– In CAM plants, carbon fixation and the Calvin

cycle are temporally separated.Both eventually use the Calvin cycle to incorporate light energy into the production of sugar.

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Assignment readingSYNTHESIS OF STARCH AND SUCROSE – Pages 162-168

Factors Affecting the Rate of Photosynthesis

Light quality (wavelength).

Light is absorbed by pigment molecules.

Chloroplasts contain pigments (chlorophyll a, chlorophyll b, and some carotenoids) that absorb mostly the red and blue portions of the visible wavelengths spectrum and reflect the green portion.

Blue and red wavelengths are more photosynthetically active than green. In green plants, light is collected primarily by chlorophylls, pigments that absorb light at a wavelength below 480 nm and between 550 and 700

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Factors Affecting the Rate of Photosynthesis

Blue and red wavelengths are more photosynthetically active than green.

Factors Affecting the Rate of Photosynthesis

Light intensity (the amount of incident light energy absorbed by the leaf).

The effect varies with different plants:

– Sun-loving plants: Some species require high light intensities to grow well (corn, potatoes, sugarcane).

– Shade-loving plants: Some plant species that do not grow well in high light intensities (dense shade of the forest floor, useful as house ornamentals).

– Intermediate: Other plant species grow well in moderately intense light.

Factors Affecting the Rate of Photosynthesis

Light intensity distinctly affects the size and shape of leaves:

– The leaves of a given plant species grow thinner and larger in area at the low light intensity than leaves grown at the high light intensity.

– The leaves of plants grown in high light intensities tend to be darker green than those grown in low light intensities.

Factors Affecting the Rate of Photosynthesis

Light intensity plants.

ThinnerThinner

DarkerDarker

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Factors Affecting the Rate of Photosynthesis

Light compensation point and light saturation point.

Light compensation point is a light intensity at which photosynthesis and respiration rates are equal and net gas exchange is zero (CO2 is still exchanged).

Light saturation point: the plant is said to be light saturated when further increases in light intensity increase photosynthesis little or not at all.

Factors Affecting the Rate of Photosynthesis

Light compensation point and light saturation point.

Light intensity increasingLight intensity increasing

More respiration than photosynthesisMore respiration than photosynthesis

More photosynthesis than respirationMore photosynthesis than respiration

Factors Affecting the Rate of Photosynthesis

Light compensation point of sun plants is higher than shade plants .

Factors Affecting the Rate of Photosynthesis

Light saturation point: the plant is said to be light saturated when further increases in light intensity increase photosynthesis little or not at all.

Light saturation point is variable and depends on CO2 concentration.

The light intensity at which saturation occurs increases as the CO2 concentration surrounding the plant raises.

At very high light intensities, the rate at which CO2 is available to the plant could limit the photosynthetic rate.

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Factors Affecting the Rate of Photosynthesis

Light saturation point.

Light intensity increasingLight intensity increasing

More respiration than photosynthesisMore respiration than photosynthesis

More photosynthesis than respirationMore photosynthesis than respiration

Light Saturation pointLight Saturation point

Factors Affecting the Rate of Photosynthesis

Light saturation point increases as the CO2 concentration surrounding the plant raises.

Factors Affecting the Rate of Photosynthesis

Carbon Dioxide

The carbon dioxide compensation point is the carbon dioxide concentration at equilibrium such that the amount of carbon dioxide evolved in respiration exactly equals the amount consumed in photosynthesis.

Increasing the CO2 concentration in a closed system, such as a sealed greenhouse to about 0.10 % approximately doubles the photosynthetic rate of certain crops.

Factors Affecting the Rate of Photosynthesis

Increasing the CO2 concentration in a closed system, such as a sealed greenhouse to about 0.10 % approximately doubles the photosynthetic rate of certain crops

– Applications of organic matter in the form of crop residues or green manure crops to the soil tends to increase CO2 levels in the atmosphere above the soil.

– In greenhouses, the use of horizontal airflow fans (HAFs) increases airflow around plants and exposes the leaves to a constant supply of CO2. The use of HAFs may eliminate the need for supplemental CO2 in most greenhouses.

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Factors Affecting the Rate of Photosynthesis

Carbon dioxide compensation.

Factors Affecting the Rate of Photosynthesis

Heat.

If light and CO2 are not limiting, the rate of photosynthetic activity approximately doubles for each 10 °C increase in temperature for many plant species in the temperate climates.

Temperature affects all biochemical reactions of photosynthesis, so it is not surprising that the responses to temperature are complex.

Factors Affecting the Rate of Photosynthesis

Heat.

Factors Affecting the Rate of Photosynthesis

Water.

Under conditions of drought (low soil moisture and hot, drying winds), plants often lose water through transpiration faster than their roots can absorb it. This causes:

– The stomata to close and the leaves to wilt temporarily.

– When this occurs, the exchange of CO2 and O2 is restricted.

– Resulting in a dramatic drop in photosynthesis.

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Factors Affecting the Rate of Photosynthesis

Water.

Excessive soil moisture sometimes creates an anaerobic condition (lack of oxygen) around the roots. This causes:

– Reducing root respiration and mineral uptake and transport of water and minerals to the leaves.

– Thus indirectly depressing photosynthesis in the leaves.

Factors Affecting the Rate of Photosynthesis

Water.

Factors Affecting the Rate of Photosynthesis

Plant development and the Source-Sink relationship.

When the leaf reaches full expansion, it is often called a source leaf because the carbohydrate synthesized in that leaf is in excess of local requirements and is exported to other parts of the plant that are actively growing. These sites of active growth and metabolism are often called sink tissues. (Roots and reproductive organs).

In many plant species, the photosynthesis rates in all the mature leaves begin to decline drastically when the plants flower and envelop fruits and seeds. (Proteins in the leaves, and specially photosynthetic enzyme proteins in the chloroplasts, are degraded to amino acids as the leaves begin the process called senescence. The amino acids in the leaves are transported to the developing seeds).