Photosynthesis

Photosynthesis is a process by which green plants and other organisms such as algae and some bacteria synthesize their own food in the presence of light

Historical perspective

Jan Baptista Van Helmont

Concluded that all the substance of the plant was produced from water and none from the soil

Joseph Priestley Showed that plants have the ability to take up CO2 from the atmosphere and release O2

Jan Ingenhousz Confirmed Priestley’s work; showed that sunlight is essential for photosynthesis, O2 is evolved during photosynthesis – this was demonstrated using aquatic plants;

Julius Von Sachs Provided evidence for the production of glucose

Theodore De Saussure

Showed that water is essential for photosynthesis

T.W. Engelmann First action spectrum (blue and red region) of photosynthesis was described

C.B. Van Niel Gave a simplifies equation for photosynthesis

n(CO2 + 2H2O) (CH2O)n + nH2O + nO2

T.W. Engelmann Experiment

C.B. Van Niel

H2 from a suitable oxidisable compound reduces CO2 to carbohydrates. H2O is the H2 donor and is oxidized to O2. In purple ad green sulphur bacteria H2S is the H2 donor and the oxidation product is sulphur or sulphate.

Inferred that O2 evolved by green plants comes from water not from CO2. This was proved by radio isotopic technique

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

Site for photosynthesis

Green plants – mostly in leaves to a lesser extends in green stems and floral parts

Specilized cells in leaves called mesophyll cells – these cells contain chloroplasts which are located at the outer margin with their broad surface parallel to the cell wall of mesophyll cells. This helps in easy diffusion of CO2

Chloroplasts in Onion Root Cells

Thylakoids contain pigment require for capturing solar energy to initiate photosynthesis

Pigment is a substance that absorbs light of different wavelength

Which part of the chloroplast contain the machinery for photochemical reactions of photosynthesis?

Chloroplast

Light Dependent Process, requires the direct energy of light to make energy carrier molecules that are used in the second process. The grana, the stroma lamellae trap light energy synthesis ATP and NADPH

The Light Independent Process, reactions are not directly light driven but are dependent on products of light reaction (ATP, NADPH) to form C-C covalent bonds of carbohydrates. This does not mean it occurs in darkness.

Photosynthesis is a two stage process

1. Light Reactions

2. Dark Reactions

Light reactions

Dark reactions

Chlorophyll absorbs light in the violet and blue wavelength and also in red region of the visible spectrum. This portion of the spectrum between 400 nm and 700 nm is referred to as PAR (photsynthetically active radiation)

Spectrum of sun light

Chlorophyll reflect the green light, hence, impart green colour to leaves

Light absorption properties of chlorophyll

Structure of chlorophyll

It is a large molecule composed of four 5 membered rings called pyrrole rings and a central core of magnesium. A side chain called phytol chain extends from one of the pyrrole ring. The long side chain is made of insoluble carbon and hydrogen atom which help to anchor the chlorophyll molecules with thylakoids

Molecular model of chlorophyll

Phytol

Phyrrol

Mg

Types of chlorophyll

In plants mostly there are two kinds – chlorophyll a and b. They are similar in their molecular structure except that the CH3 group in chlorophyll a is replaced by CHO group in chlorophyll b

Paper chromatography is used to separate leaf pigments

Four important pigments are:

1. Chlorophyll a (blue-green)

2. Chlorophyll b (yellow-green)

3. Xanthophyll (yellow)

4. Carotenoids (yellow-orange)

Different pigments in leaf

Absorption spectrum of pigments

Absorption spectrum

A curve obtained by plotting the amount of absorption of different wavelengths of light by a particular pigment

Action spectrum

A curve showing the rate of photosynthesis at different wavelength of light

What is light reaction?

1. Photochemical phase – light absorption

2. Water splitting

3. Oxygen release

4. Formation of ATP and NADPH

Photosystem are arrangements of chlorophyll and other pigments packed into thylakoids.

Many Prokaryotes have only one photosystem, Photosystem II (so numbered because, while it was most likely the first to evolve, it was the second one discovered).

Eukaryotes have Photosystem II plus Photosystem I.

Photosystem I uses chlorophyll a, in the form referred to as P700.

Photosystem II uses a form of chlorophyll a known as P680. Both "active" forms of chlorophyll a function in photosynthesis due to their association with proteins in the thylakoid membrane.

Photosystem is LHC – light harvesting complex

Photosystem

(antennae)

PS1 – The reaction centre, chlorophyll a, has absorption peak at 700 nm called P700

PS2 – The reaction centre, chlorophyll a, has absorption peak at 680 nm called P680

Reaction centres

Lamellae of the grana have both PSI and PSII

The Stroma lamellae lack PSII and NADP reductase enzyme

Cyclic photophosphorylation occurs only when light of wave length above 680 nm is available for excitation

Non-Cyclic Photo phosphorylation

Cyclic Photophosphorylation

Chemiosmotic hypothesis

1. Spliting of water molecules takes place on inner side of membrane, the H+ produced during this process accumulate within lumen of thylakoid

Chemiosmotic hypothesis

2. Electrons move through photosystems, protons are transported across the membrane (into lumen) by cytochrome complex which is a H+ carrier. Electrons are transported to the electron carrier present on the inner side of the membrane. The protons are released into the lumen.

H+ carrier

Chemiosmotic hypothesis

3. The NADP reductase enzyme is located on the stroma side of membrane. H+ are required for reduction of NADP+. The protons are removed from the stroma. Electrons are also required. They come from PSI.

Chemiosmotic hypothesis

4. These processes result in increased H+ concentration in lumen and decreases concentration in stroma. This creates a proton gradient. Proton gradient is important because the breakdown of this gradient leads to release of energy

Chemiosmotic hypothesis

5. The H+ move through the trans membrane Channel of ATPase to stroma. ATPase enzyme has two parts – F0 and F1. As the H+ pass through F0 and F1 complex, it releases enough energy to produce ATP. There is a conformation change in F1 which activates the enzyme.

F0

F1

Dark Reaction - Biosynthetic phase

Melvin Calvin, Ernest Orlando Lawrence Berkeley National Laboratory

Using carbon-14, and the new techniques of ion exchange, paper chromatography, and radio autography, Calvin and his many associates mapped the complete path of carbon in photosynthesis. The accomplishment brought him the Nobel prize in chemistry in 1961.

Phase 1: Carbon Fixation CO2 comes into the stroma

of the chloroplast. Rubisco catalyzes the bonding of CO2 to RuBP to create an

unstable 6-carbon molecule that instantly splits into two 3-carbon molecules of 3-PG.

Phase 2: Reduction ATP phosphorylates each 3-PG molecule and creates 1,3-bisphosphoglycerate.  This in turn results in the loss of the terminal phosphate group from ATP thus making ADP.

NADPH reduces 1,3-bisphosphoglycerate which causes the phosphate group to break off once again.  The molecule then picks up a proton (H+) from the medium to become glyceraldehyde-3-phosphate.  The broken off phosphate group also gains a proton to become H3PO4. 

NADPH is oxidized by this process and becomes NADP+.

Phase 3: Regeneration For every six molecules of G3P created five molecules continue on to phase 3 while one leaves to be used for organic compounds.  ATP is once again needed.  However, this time it phosphorylates G3P to regenerate RuBP after some rearrangement.

RuBPRubisco

2-Phosphoglycolate

3-PhosphoglycerateCO2

Calvin cycle

+

Photo respiration

When carbon dioxide levels decline below the threshold for RuBP carboxylase, RuBP is catalyzed with oxygen instead of carbon dioxide. The product of that reaction forms glycolic acid, a chemical that can be broken down by photorespiration, producing neither NADH nor ATP, in effect dismantling the Calvin Cycle.

O2

C4-Pathway

The C4 pathway is designed to efficiently fix CO2 at low

concentrations and plants that use this pathway are known as C4

plants. These plants fix CO2 into a four carbon compound (C4)

called oxaloacetate. This occurs in cells called mesophyll cells. Examples: Maize, Sugar cane, Pearl millet, Amaranth

C4-Plants

C4 plants require presence of two types of photosynthetic cells - Mesophyll cells and bundle sheath cells.

It contains dimorphic chloroplast. Chloroplast in mesophyll cells are granal and in bundle sheath

cells are agranal. Rubisco is present only in bundle sheath cells

C3 plant C4 plant (Kranz anatomy)

C4-Pathway

The C4 pathway is

designed to efficiently fix CO2 at low

concentrations and plants that use this pathway are known as C4 plants. These plants

fix CO2 into a four

carbon compound (C4)

called oxaloacetate. This occurs in cells called mesophyll cells.

C4-Pathway

1. CO2 is fixed to a three-

carbon compound called phosphoenolpyruvate to produce the four-carbon compound oxaloacetate. The enzyme catalyzing this reaction, PEP carboxylase, fixes CO2

very efficiently so the C4

plants don't need to to have their stomata open as much. The oxaloacetate is then converted to another four-carbon compound called malate in a step requiring the reducing power of NADPH.

C4-Pathway

2. The malate then exits the mesophyll cells and enters the chloroplasts of specialized cells called bundle sheath cells. Here the four-carbon malate is decarboxylated to produce CO2, a three-carbon

compound called pyruvate, and NADPH. The CO2

combines with ribulose bisphosphate and goes through the Calvin cycle.

C4-Pathway

4. The pyruvate re-enters the mesophyll cells, reacts with ATP, and is converted back to phosphoenolpyruvate, the starting compound of the C4 cycle.