PHOTOSYNTHESIS IN HIGHER PLANTS

Jancy Das

PGT (Biology)

JNV , Thrissur

KERALA

TOPICS

• Importance of photosynthesis
• Early experiments( Historical perspective.)
• Site of photosynthesis
• Light Harvesting Complexes( Photosystems)
• Process of Photosynthesis

Photochemical phase ( Light reactions)

Biosynthetic phase (Dark reaction)

The C₄ Pathway

Photorespiration

Factors affecting photosynthesis

Blackman’s aw of limiting factors

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IMPORTANCE OF PHOTOSYNTHESIS

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• It is the primary source of all food on earth.
• It is responsible for the release of Oxygen into the atmosphere.

Early experiments: historical perspective

Joseph Priestley(1770) observed that a candle burning in a closed space – a bell jar, soon gets extinguished and a mouse would soon suffocate in a closed space.

CONCLUSION

A burning candle or an animal that breathe the air, both somehow, damage the air. But when placed a mint plant in the same bell jar, found that the mouse stayed alive and the candle continued to burn.

Priestley hypothesised as follows: Plants restore to the air whatever breathing animals and burning candles remove

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• Showed that only the green part of the plants that

could release oxygen.

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• An elegant experiment with an aquatic plant showed that

in bright sunlight, small bubbles were formed around the green parts while in the dark they did not.

Later he identified these bubbles to be of oxygen and showed that it is only the green part of the plants that could release oxygen

. https://www.saps.org.uk/secondary/teaching-resources/190-using-cabomba-to-demonstrate-oxygen-evolution-in-the-process-of-photosynthesis-

Jan Ingenhouse ( 1730-1799)

• Julius von Sachs provided evidence for production of glucose when plants grow.

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• His later studies showed that the green substance in plants (chlorophyll) is located in special bodies (later called chloroplasts) within plant cells.

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• He found that the green parts in plants is

where glucose is made, and that the glucose is usually stored as starch.

Julius von Sachs( 1854)

Experiments).

• Using a prism he split light into its spectral components
• illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria.
• The bacteria were used to detect the sites of O₂

evolution.

OBSERVATION

• Bacteria accumulated mainly in the region of blue and red light of the split spectrum.
• A first action spectrum of photosynthesis was thus described.

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https://media.sciencephoto.com/image/k0034123/preview/K0034123-Bacteria_attracted_by_photosynthesis.mp4

T.W Engelmann

AEROBIC BACTERIA ATTRACTING TOWARS THE BLUE AND RED REGIONS (OXYGEN EVOLUTION MORE)

https://www.sciencephoto.com/media/471938/view/bacteria-attracted-by-photosynthesis

An action spectrum is a graph showing the effectiveness of different wavelengths (VIBGYOR) of light in stimulating the process of photosynthesis, where the response could be measured in terms of oxygen produced at different wavelengths of light

. An absorption spectrum is a graph representing the relative absorbance of different wavelengths of light by a pigment.

GRAPH SHOWING THE ACTION AND ABSORPTION SPECTRA OF DIFFERENT PIGMENTS

B

C

A

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For investigating a process such as photosynthesis that is activated by light, it important to establish the action spectrum for the process and to use this to identify the pigments involved

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• Photosynthesis occurs maximum in blue and red region of spectra.
• Photosynthesis is very little in green and yellow light, because these rays are reflected back from the leaf.

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SIGNIFICANCE OF ACTION SPECTRUM

Conducted experiments on purple and green bacteria,

demonstrated that photosynthesis is essentially

• a light-dependent reaction
• in which hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates.
• This can be expressed by:

Cornelius van Niel (1897-1985),

2H A + CO₂ 2A + CH₂O +H₂O

https://www.youtube.com/watch?v=SxHsFzouSbY

• In green plants H²o is the HYDROGEN donor,so evolution of oxygen is by the splitting of water

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• In Sulphur bacteria,H²S is the Hydrogen donor, and so the oxidation product is sulphur .This was later proved by using radioactive isotope of oxygen in water (H₂ ¹⁵O)

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Thus the correct equation for photosynthesis is represented as follows:

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6CO₂ + 12 H₂O C₆H₁₂O₆ +6H₂O + 6O₂

Conclusion -- Van Niels’s Experiments

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Membranous system consisting of grana, the

stroma lamellae, and the matrix stroma is present within the double walled envelope.

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The membrane system is responsible for trapping the light energy and also for the synthesis of ATP and NADPH.

In stroma, enzymatic reactions synthesise sugar, which in turn forms starch

SITE OF PHOTOSYNTHESIS

CHLOROPLAST

CHLOROPHYLL MOLECULE

PIGMENT INVOLVED IN PHOTOSYNTHESIS

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• Chlorophyll a

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• Chlorophyll b

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• Xanthophylls

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• Carotenoids

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accessory pigments.-absorb wide range of wavelength and transfer the energy to chl a

Is the main pigment in photosynthesis

PAPER CHROMATOGRAPHY

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Chromatographic separation of the leaf pigments

shows that the colour that we see in leaves is

not due to a single pigment but due to four

pigments: Chlorophyll a (bright or blue green

in the chromatogram), chlorophyll b (yellow

green), xanthophylls (yellow) and carotenoids

(yellow to yellow-orange).

https://youtu.be/6hT39B2o-7M

LIGHT HARVESTING COMPLEXES(PHOTOSYSTEMS)

The pigments are organised into two

discrete photochemical light harvesting

complexes (LHC) within the Photosystem I (PSI) and Photosystem II (PS II).

The LHC are made up of hundreds of pigment molecules bound to proteins.

PS I (P700)

Chl a---REACTION CENTRE

Chlb,carotenoids and Xanthophylls

PS II (P680)

Chl a---REACTION CENTRE

Ch lb,carotenoids and

Xanthophylls

PHOTON

PRIMARY ACCEPTOR

• Photochemical phase
• Occurs in the (Thylakoid) grana
• Directly depends on light

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• Biosynthetic phase
• Occurs in the stroma
• Enzyme catalysed steps

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PROCESS OF PHOTOSYNTHESIS

LIGHT REACTION

DARK REACTION

• Light absorption
• Photolysis of water
• Oxygen release
• Formation of ATP and NADPH

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• Utilisation of the power of ATP and NADPH
• Synthesis of

carbohydrate (Starch)

CYCLIC ELECTRON TRANSPORT

CYCLIC PHOTOPHOSPHORYLATION

1. When PS I ( P 700) is functional, the electron is circulated within the photosystem
2. Photophosphorylation occurs due to the cyclic flow of electrons.
3. A possible location for this to happen is in the Stroma lamellae as it lacks PS II and NADP reductase enzyme.
4. (Grana lamellae has both PS I and PS II )
5. Excited electron comes back to PS I itself
6. Only ATP Synthesis and no synthesis of NADPH + H⁺
7. Occurs only light of wavelength beyond 680nm is available.

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1. Absorption of photons by the reaction centre of Chl a (P 680)
2. Charge separation from the chl a ( Start of light reaction) by ejecting out electrons
3. Transport of electrons ejected to PSII—via electron acceptor Ferredoxin Plastoquinone cytochrome complex Plastocyanin P700
4. Movement of electrons is downhill ,in terms of an oxidation-reduction or redox potential
5. Splitting of water(Photolysis of Water
6. Synthesis of ATP
7. Simultaneous excitation of PS I with the reception of light of wave length of 700 nm
8. Transfer of electrons from PS I to another acceptor molecule
9. Reduction of NADP⁺ to NADPH + H ⁺ (by the hydrogen molecule)

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NON CYCLIC ELECTRON TRANSPORT (Z Scheme)

NON CYCLIC ELECTRON TRANSPORT (Z Scheme)

CHEMIOSMOSIS---MECHANISM OF ATP SYNTHESIS IN CHLOROPLAST

https://www.youtube.com/watch?v=G3Y2Ig3YTL0

C3 cycle and c4 pathway

CALVIN CYCLE (C₃ Cycle) DARK REACTION

RUBISCO

1. CARBOXYLATION
2. REDUCTION
3. REGENERATION

Carboxylation is the fixation of CO₂ into a stable organic intermediate.

Carboxylation is the most crucial step of the Calvin cycle where CO₂ is utilised for the carboxylation of RuBP.

This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA.

Since this enzyme also has an oxygenation

activity it would be more correct to call it RuBP carboxylase-oxygenase or RuBisCO.

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1. CARBOXYLATION

STEPS IN DARK REACTION( C3 cycle)

CALVIN CYCLE

• These are a series of reactions that lead to the formation

of glucose.

• The steps involve utilisation of 2molecules of ATP and two NADPH for reduction per CO₂ molecule fixed.

The fixation of six molecules of CO₂ and 6 turns of the cycle are required for the formation of one molecule of glucose from the pathway

• Regeneration of the CO₂ acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted.
• The regeneration

steps require one ATP for phosphorylation to form RuBP.

2. REDUCTION OF CO₂

3. REGENERATION OF RUBP

C₃ cycle

C₄ PATHWAY

An adaptation for plants growing under dry hot Environment (C₄ PlantS)

Leaves show‘ Kranz’ anatomy

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‘Kranz’ anatomy -large cells having large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces the vascular bundles of the C₄ plants are called bundle sheath cells

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‘Kranz’ anatomy

The basic pathway that results in the formation of sugars (Calvin Pathway) is common to the C₃ and C₄ plants

CALVIN PATHWAY

C₃ Plants in Mesophyll

C₄ Plants in the bundle sheath cell

C₄ PATHWAY

HATCH AND SLACK PATHWAY / C4 PATHWAY

• The primary CO₂ acceptor is a 3-carbon molecule phosphoenol

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pyruvate (PEP) and is present in the mesophyll cells.

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• The enzyme responsible for this fixation is PEP carboxylase or PEPcase.

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• It is important to register that the mesophyll cells lack RuBisCO enzyme

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• The C₄ acid OAA is formed in the mesophyll cells.

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• It then forms other 4-carbon compounds like malic acid or aspartic

acid in the mesophyll cells itself, which are transported to the bundle

sheath cells.

In Mesophyll cell

• In the bundle sheath cells these C₄acids are broken down

to release CO₂ and a 3-carbon molecule.(Pyruvic acid)

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• The 3-carbon molecule is transported back to the mesophyll where it is converted to PEP again, thus, completing the cycle.

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• The CO₂ released in the bundle sheath cells enters the C₃ or the Calvin pathway, a pathway common to all plants.

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• The bundle sheath cells are rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO), but lack PEPcase.

In Bundle sheath cell

HATCH AND SLACK PATHWAY / C4 PATHWAY

C₄ PATHWAY IN C₄ PLANTS

MESOPHYLL CELLS

• The initial acceptor of CO₂ is phosphoenol pyruvic acid or PEP,
• a 3-carbon compound.
• It combines with CO₂ in presence of an enzyme Phosphoenol pyruvate carboxylase (PEP carboxylase) and forms a C₄ acid, oxaloacetic acid (OAA).
• This fixation of CO₂ occurs in the cytosol of the mesophyll cells of the leaf.

BUNDLE SHEATH CELLS

• OAA then travels from mesophyll cells to the chloroplasts of bundle sheath cell
• where it releases the fixed CO₂. C₃ cycle operates within these cells and this
• CO₂ immediately combines with RuBP in C₃ cycle producing sugars.

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Events that occur

In C₄ plants OAA is the first stable product of this cycle which is 4 -carbon compound and hence the name C₄ pathway is given.

CARBOXYLATION

CARBOXYLATION

PHOTORESPIRATION

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Photorespiration (also known as the oxidative photosynthetic carbon cycle, or C₂ photosynthesis) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis.

RUBP 3PGA + PHOSPHOGLYCOLATE

RuBisCO oxygenates RuBP,

High temperature and oxygen concentration

RUBP

O₂

3 -PGA

SUGAR

3-PGA

PHOSPHOGLYCOLATE

+

GLYCOLATE

GLYCINE

2 glycine

Co2 + SERINE

SERINE

GLYCERATE

CHLOROPLAST

PEROXISOME

MITOCHONDRION

RUBISCO

PHOTORESPIRATION

How does the photorespiration pathway actually work?

follow the path of phosphoglycolate ……………

• Phosphoglycolate is first converted to glycolate inside of the chloroplast.
• Glycolate then travels to the peroxisome, where it is converted to the amino acid glycine.
• Glycine travels from the peroxisome to a mitochondrion. There, two glycine molecules (e.g., from two iterations of the pathway) are converted to serine, a three-carbon amino acid.
• Serine returns to the peroxisome, where it's converted to glycerate.
• In the chloroplast, glycerate is turned into 3-PGA and can thus enter the Calvin cycle.

FACTORS AFFECTING PHOTOSYNTHESIS

INTERNAL FACTORS

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1. Chlorophyll : The amount of chlorophyll present has a direct relationship with the rate of photosynthesis because this pigment is directly involved in trapping light energy responsible for the light reactions.

2. Leaf age and anatomy : Newly expanding leaves show gradual increase in rate of photosynthesis and the maximum is reached when the leaves achieve full size.Chloroplast functions decline as the leaves age. Rate of photosynthesis is influenced by variation in

(i) number, structure and distribution of stomata,

(ii)size and distribution of intercellular spaces (iii) relative proportion of palisade and spongy tissues and (iv) thickness of cuticle

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1. Carbon dioxide Concentration

Carbon dioxide is the major limiting factor for photosynthesis.

The concentration of CO₂ is very low in the atmosphere (between 0.03 and 0.04 per cent).

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Increase in concentration upto 0.05 per cent can cause an increase in CO₂ fixation rates; beyond this the levels can become damaging over longer periods The C₃ and C plants respond differently to CO₂ concentrations.

Light quality, light

intensity and the duration of exposure to light, affects

photosynthesis.

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At higher light intensities,

gradually the rate does not show further increase as other factors become limiting.

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2. Light

• The dark reactions being enzymatic are temperature.
• The C₄ plants respond to higher temperatures and show higher rate of photosynthesis while C₃ plants have a much lower temperature optimum.

Tropical plants have a

higher temperature optimum than the plants adapted to temperate

Temperature

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Water

• Reactant in photosynthesis
• Water stress causes the stomata to close
• reducing CO₂ availability
• reduces surface area of the leaves as they wilt and hence the metabolic activity too

states the following:

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If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value

: it is the factor which directly affects the process if its quantity is changed.

Blackman’s (1905) Law of Limiting Factors

Rate of Photosynthesis

As light intensity increases more energy is available and the rate of photosynthesis increases

Light intensity

LIGHT INTENSITY VS PHOTOSYNTHESIS

• (a) It was proposed by F. F. f Blackmann (1905).
• (b) It states that “if a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value; it is the factor that directly affects process as its quantity is changed”.
• ( c )The rate of photosynthesis is affected mainly by the concentration of CO₂ light intensity and the temperature.
• (d)As the light intensity is increased, the rate of photosynthesis increases proportionately until some other factor like CO₂ or temperature may become limiting.
• (e) Similarly, if the concentration of CO₂ is increased the rate of photosynthesis increases until light may become a limiting factor.

Law of limiting Factor: