MAGICAL PIGMENT CHLOROPHYLL

CHLOROPHYLL THE MAGICAL PIGMENT THAT FEEDS THE WORLD AND SUSTAINS US WITH OXYGEN

Photosynthesis is critical for the existence of vast majority of life on earth as primary food and oxygen producers, photosynthetic organisms form the base of the FOOD CHAIN and are consumed directly or indirectly by all higher life forms. Over and above all the OXYGEN in the atmosphere is the result of photosynthesis. If photosynthesis stops, it would lead to shortage of food on earth, most of the life forms would disappear and earth’s atmosphere eventually would become devoid of gaseous oxygen.

The site of the photosynthesis is the chloroplasts. Chloroplasts are found in the cells of all green tissues are concentrated in the mesophyll cells of the leaves. Chlorophyll the primary pigment used for photosynthesis absorbs the blue and red part of the sun light and reflects back the green part of the sunlight spectrum. This gives the leaves the green colour.

Photosynthesis consists of both light dependent and light independent reaction centres. The light reaction takes place in the thylakoid. There the water is oxidized and the oxygen we breath is released. The electrons freed from water are transferred to ATP and NADPH is used to ix carbon dioxide from atmosphere. The products of this reaction are sugar molecules fatty acids and amino acids. The dark reaction takes place in the stroma the aqueous fluid surrounding the stacks of thylakoids and in the cytoplasm.

LIGHT AND DARK REACTIONS

STRUCTURE OF CHLOROPLAST

Chloroplasts are found in the mesophyll cells of plant leaves. They are organelles. Any membrane bound cell is called an organelle. So, chloroplasts are organelles. They have two membranes. There is an outer membrane and an inner membrane with an intermembrane space. (See the following diagrams)

There are two distinct regions present inside a chloroplast 1. GRANA 2. STROMA

Grana is made up of stacks of disc shaped structures known as THYLAKOIDS. The stack is called LAMELLAE. The grana of the chloroplast contain chlorophyll pigments, which are the functional units of chloroplasts.

STROMA is the homogenous liquid similar to the cytoplasm of cells. Stroma contains the grana various enzymes, DNA, and ribosomes. STROMA LAMELLAE functions as a connecting bridge between grana or stacks of thylakoids.

The chloroplast structure includes

1.Inner and outer membranes with intermembrane space

2. Thylakoids. It is a collection of membranous sacs. It is also called LAMELLAE. The chlorophyll pigments are found in the thylakoid membrane. It is at this site the light dependent reactions of photosynthesis take place. The thylakoids are arranged in stacks like coins and is called GRANA and each granum contains around 10 to 20 thylakoids

3. Stroma, it is a colourless alkaline aqueous protein rich fluid present within the inner membrane surrounding the grana

4. Grana, stacks of thylakoids. It is here the conversion of light energy into chemical energy takes place

THYLAKOID

The word thylakoid comes from the Greek word Thylakos meaning sac or pouch. Thus, thylakoid means “sac like” or “pouch like”.

Hence thylakoids are membrane bound compartments inside chloroplasts and cyanobacteria. They are the site of light dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen (space). Chloroplast thylakoids form stacks of discs referred to as GRANA (SINGULAR GRANUM). Grana is connected by intergranal or stromal thylakoids which joins granum stacks together as a single functional compartment.

In thylakoid membrane, chlorophyll pigments are found in packets called QUANTASOMES. Each quanta some contains 230 to 250 chlorophyll molecules

In the chloroplast with the chlorophyll molecules the food we eat and oxygen we breath are produced (refer the equation above)

The word chlorophyll was coined in the year 1818 by two French pharmacists, Pierre Joseph Pelletier and Joseph Bienaime Caventou. The word chlorophyll originates from two Greek words, CHLOROS which means GREEN and PHYLLON which means leaf.

About 0.423 billion years ago VASCULAR PLANTS appeared on the earth. They are called vascular plants because of their water conducting system called XYLEM and food conducting system called PHLOEM. They were able to grow tall to capture more sun light. They were able to draw water and nutrients from soil. They are the forefathers of modern day economically important plants.

Plant chloroplasts are large organelles (5 to 10 micro meter long). They are bounded by double membrane called the chloroplast envelope. In addition to the outer and inner membranes chloroplasts have a third inner membrane called the thylakoid membrane.

Stroma is the liquid material found througout the cavity of the chloroplast. The function of stroma is to give volume around the different structures inside the chloroplast for protection. The stroma is where the light independent reaction of photosynthesis also called the carbon cycle takes place.

Chloroplasts are endosymbiotic organelles derived from cyanobacteria (blue green algae). The thylakoid membranes inside the chloroplast are the site of the light dependent reaction of photosynthesis.

Mesophyll is internal ground tissue located between the upper and lower epidermis of leaves. Mesophyll is composed of TWO kinds of tissues. The palisade parenchyma (like pillars) containing large amount of chloroplast and spongy parenchyma, a lower layer of spherical or ovoid cells

RICHARD MARTIN WILLSTATER a German organic chemist whose study of plant pigments including chlorophyll won him the 1915 Nobel Prize in chemistry. He showed that the chlorophyll of different plant species is the same but is a mixture of two different types. Chlorophyll “a” and chlorophyll “b” (See the following diagram), produced them in pure form and showed that MAGNESIUM is an essential central part of chlorophyll structure and pointed out chlorophyll’s relationship with hemoglobin in red blood cells.

CHLOROPHYLL IS A CHELATE, where the central metal ion is MAGNESIUM. The magnesium ion is bonded to a larger organic hydrogenic molecule called PORPHYRIN.  Porphyrin molecule is composed of carbon, oxygen, hydrogen, nitrogen. The haemoglobin of blood is similar to chlorophyl except that the central metal i on is IRON

Magnesium is so important to photosynthesis. If there are magnesium deficiency plants will remove magnesium ions from the lower leaves and translocate to the upper leaves where the upper leaves need the magnesium ion most.  That is why magnesium deficiency shows up as interveinal chlorosis in the lower leaves first. The veins stay green but the leaf tissue turn yellow from the lack of magnesium from the roots

MAGNESIUM IS CRITICAL FOR PLANT TO USE CARBON DIOXIDE

Magnesium is required to activate two key enzymes, for the efficient functioning of chlorophyll. The first enzyme is called RUBP CARBOXYLASE. During photosynthesis the plant takes up carbon dioxide through the stomatal openings of the leaves and combines it with water from the roots to make simple sugars with oxygen and water vapour as by products.  RUBP CARBOXYLASE is the enzyme that assimilates the carbon so adequate. The second one is the thylakoid stacking enzyme. magnesium is critical for a plant to use carbon dioxide in the air.

MAGNESIUM IS RESPONSIBLE FOR THE TRANSPORT OF SUGARS

Magnesium at the PHLOEM vessels activates the enzyme that loads sugars into the phloem vessels. Phloem transports sugars from the leaf downwards throughout the plant. The transported sugars are then stored in the root’s fruits and grains. Without phloem loading the sugars would stay in he leaves locked up as starch in the chloroplast. So, magnesium is not only responsible for the production of sugars and also responsible for the transport of sugars.

ARTIFICIAL SYNTHESIS OF CHLOROPHYLL

Robert Burns Woodward of Harvard University was able to artificially synthesis chlorophyll in 1960. For this achievement Woodward was awarded the Nobel Prize in 1965 in chemistry.

The following diagram shows the various uses of synthesized chlorophyll.

Professor K.A.Balasubramanian

Ph.D. (IARI-New Delhi), Ph.D. (Imperial College London), DIC (Imperial College London)

Dallas, Texas, USA

 May 29, 2024