Photosynthetic Pigments

The University of the West Indies

Department of Biological and Chemical Sciences

BL 05B - Preliminary Biology II

Photosynthetic Pigments

All photosynthetic organisms contain one or more organic pigments capable of absorbing visible radiation, which will initiate the photochemical reactions of photosynthesis. The three major classes of pigments found in plants and algae are the chlorophylls, the carotenoids and the phycobilins. Carotenoids and phycobilins are called accessory pigments since the quanta (packets of light) absorbed by these pigments can be transferred to chlorophyll.

Chlorophylls

chlorophyll a - present in all higher plants and algae

chlorophyll b - present in all higher plants and green algae

chlorophyll c - diatoms and brown algae

chlorophyll d - red algae

(chlorophyll a is present in all photosynthetic organisms that evolve O₂.)

Chlorophyll molecules contain a porphyrin 'head' and a phytol 'tail'. The polar (water-soluble) head is made up of a tetrapyrrole ring and a magnesium ion complexed with the nitrogen atoms of the ring. The phytol tail extends into the lipid layer of the thylakoid membrane.

Carotenoids (carotenes and xanthophylls)

Carotenes:

"-carotene - higher plants and most algae

$-carotene - most plants & some algae

xanthophylls:

luteol, fucoxanthol and violaxanthol

Carotenoids contain a conjugated double bond system of the polyene type (C-C=C-C=C). Energy absorbed by carotenoids may be transferred to chlorophyll a for photosynthesis.

Phycobilins (found mostly in red and blur-green algae):

phycoerythrin

phycocyanin

allophycocyanin )

These are linear tertapyrroles structurally related to chlorophyll a but lack the phytol side chain and magnesium ion. The red algae have phycoerythrins that enable them to absorb light in the blue-green region of the spectrum which reaches deep-sea depths.

So higher plants and algae have various pigments, which allow them to capture the available solar radiation most efficiently. The relative abundance of the pigments depends on factors like the species, location of the plants and seasons.

Light energy is available in discrete packets called quanta. The longer the wavelength of radiation, the less energy that radiation contains. The visible spectrum is a small part of the electromagnetic spectrum, from about 400 nm (blue) to 700 nm (red).

When the pigments involved in photosynthesis are subjected to different wavelengths of light, they absorb some wavelengths more than others. A graph showing the degree of absorption of light by a pigment is referred to as the absorption spectrum for that pigment. Chlorophylls absorb strongly in the blue-violet and red regions of the spectrum (and not in the green region, hence leaves containing chlorophyll appear green), while carotenoids absorb in the blue and green regions. The colour of the carotenoids (yellow to orange and red) is usually masked by that of the chlorophylls, which are present in larger quantities.

A graph showing the degree to which different wavelengths affect photosynthesis is called the action spectrum for photosynthesis. The action spectrum for photosynthesis is closely correlated with the action spectra for chlorophylls a and b and the carotenoids. This suggests that these are the main pigments involved in harvesting light in photosynthesis.

Figure: The absorption spectrum for some pigments and the action spectrum for photosynthesis.

The Reactions of Photosynthesis

Figure: Variation in rate of photosynthesis with light intensity, temperature and carbon dioxide concentration.

A comparison of curves A & B shows that at low light intensity the rate of photosynthesis is the same at 15EC as at 25EC (look at the point on the graph labelled with the asterisk). At higher light intensities, the rate is much higher at 25EC. We know that photochemical reactions are independent of temperature so the influence of temperature indicates that photosynthesis must involve more than light absorption. In fact, the doubling of the rate observed with a 10EC rise in temperature suggests that photosynthesis has an enzymatic component. The photochemical reaction in photosynthesis is called the light-dependent (‘light) reaction while the non-photochemical process is called the light-independent (‘dark’) reaction. The ‘dark’ reaction is enzymatic and slower than the ‘light’ reaction, hence at high light intensities the rate of photosynthesis is entirely dependent upon the rate of the dark reaction. The light-independent (dark) reaction can proceed in both light and darkness.

Photosynthesis is therefore a two-stage reaction

A º B (light rxn)

B º C (dark rxn)

The Light-dependent Reaction

We say that photosynthesis is a photochemical reaction because light (photo) causes the chemical reaction to occur. Chemical reactions can be classified into many types, of which oxidation and reduction are two.

Substances are made up of molecules. Molecules are made up of atoms and atoms are made up of a nucleus containing protons (p) and neutrons (n) surrounded by one or more electrons (e). Some chemical reactions involve the transfer of electrons from one molecule to another. This transfer can be seen in terms of oxidation and reduction.

The loss of an electron by a molecule, atom or ion is referred to as oxidation. Molecules can also be oxidised by losing hydrogen atoms or gaining oxygen atoms. Reduction occurs by the gain of electron, gain of hydrogen or loss of oxygen atoms.

We know that photosynthesis is a photochemical reaction and that chlorophyll molecules are the photosynthetic pigments. What happens is that when a chlorophyll molecule absorbs light energy of the correct wavelength, the molecule loses an electron (oxidation) and is said to be photoactivated.
light

chlorophyll - electron º chlorophyll⁺

This is an oxidation reaction. The electron has acquired the energy from the light. In order for the activated chlorophyll to return to its normal state, an electron must be returned to it. This is exactly what happens. However, as the electron is returned something else happens - it loses the energy it had acquired. This energy is not wasted, but is used to make a high-energy chemical called adenosine triphosphate (ATP). ATP is the form of energy that hich all living cells use and is made from adenosine diphosphate (ADP) and inorganic phosphate (Pi):

ADP + Pi º ATP

This is also a phosphorylation reaction - the addition of phosphate to a compound.

So we have the conversion of light energy into chemical energy. For each electron returning to a chlorophyll molecule, one molecule of ATP is made. This is called cyclic phosphorylation because the electrons are lost from the chlorophyll molecule and while they are returning to it - a cyclic process - ADP is phosphorylated.

But that is not all...

It was shown that by separating chloroplasts from the rest of the cell’s organelles that illuminated chloroplasts also produced oxygen. The process outlined above does not produce oxygen, only ATP.

In other chlorophyll molecules, the electrons are not returned to the activated chlorophyll. The chlorophyll is returned to its normal state by electrons from water. Water is split to produce hydrogen ions and electrons and the oxygen is given off as a gas. This splitting of water in chloroplasts under the influence of light is called photolysis.

H₂O º hydrogen + electrons + oxygen gas

How was this discovered?

Many elements e.g. C. H. O. P and S, are found to have radioactive isotopes. The isotopes of an element are similar int heir chemistry to each other, e.g. ¹H, ²H; ¹²C, ¹⁴C, ¹⁶O; ¹⁸O. The difference between isotopes of an element is the number of neutrons found in their nuclei, e.g. ¹⁶O (‘normal’ oxygen) has 8 protons and 8 neutrons, while ¹⁸O has 8 protons and 10 neutrons. Radioactive elements give off radioactivity which can be detected and measured. Water is H₂O where the oxygen is ¹⁶O, but water can be made with ¹⁸O. By using ¹⁸O in water and then in CO₂ it was determined that the oxygen evolved by the chloroplasts came from water and not from carbon dioxide.

CO₂ + H₂¹⁸O º CH₂O + ¹⁸O₂

So water is split, the oxygen is given off as gas and the electrons are used to reduce activated chlorophyll. The hydrogens from the water are used to reduce a coenzyme called nicotinamide adenine dinucleotide phosphate. In fact, this is where the electrons that didn’t return to the activated chlorophyll go.

NADP⁺ + hydrogen + electrons º NADPH + H⁺

(hydrogen from water, electrons from chlorophyll)

To conclude, then, health green leaves on illumination split water, make energy (ATP), and reduced coenzyme (NADPH+H⁺), and give off oxygen gas (O₂). The ATP produced can be used to provide energy. The reduced coenzyme can give up its hydrogen to other atoms or molecules - it is termed a reducing agent.

On the internet:

http://www.blc.arizona.edu/courses/181gh/rick/photosynthesis/light_energy.html

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Last modified: August 27, 2004