Changing Colors of Leaves

Nature is a great chemist. It is playing with chemical pigments to present vivid colors. Even a single leaf is a piece of art. There are many classes of pigments present inside it but their amount and times of breakdown and synthesis decides the resultant color. The different colors are on display during autumn season. The leaves begin to look less and less green. They can take yellow, orange and red hues depending upon the ratios of the amounts of different pigments present in the leaves.

Most important pigment in the leaves is the chlorophyll. It is this pigment which imparts the green color to the leaf. It’s amount is dictated by the warmth and amount of sunlight the plant receives. It’s presence is the indication that plant is alive and carrying out photosynthesis to convert carbondioxide and water into sugars and oxygen. Sugars contain energy from the sun which is harvested by tree or plant during photosynthesis.


What happens when it is not sunny. We see a kaleidoscope of different colors in leaves. There are yellow, orange and red hues. These colors are attributed to other pigments present inside the leaves. These were there throughout the life of the leaf but there colors were masked by the strong green color.

In the autumn, when sunlight is not available in plenty, the production of chlorophyll is halted. On the other hand, the chlorophyll present begins to breakdown. At this time, color contribution from other pigments begin to show up.

Chlorophyll is a type of chlorine with magnesium as the central metallic ion. There are 4 nitrogen atoms which are Lewis bases and thus trap the positively charged magnesium ion. Chlorophyll is synthesized in the warm and sunny conditions by the plants. It’s green color dominates the color in the leaves. During autumn, the sunlight is not fully available and hence the production of chlorophyll halts and since it is not required the already present chlorophyll in the leaves begins to breakdown and hence result is the decrease in green color of the leaves.

Carotenoids and flavonoids are pigments which are always present in the leaves but there color is masked by the green color of the chlorophyll. When during autumn, the chlorophyll begins to breakdown, the color of these two classes of compounds begins to show up.

Xanthophylls which are oxygenated carotenoids are responsible for the yellow color of leaves. They do not require light for synthesis, so that xanthophylls are present in all young leaves as well as in etiolated leaves.


A class of carotenoids known as beta carotene is responsible for the orange color in leaves. It absorb light of green and blue wavelengths and reflects red and yellow wavelengths light thus causing the orange color in leaves during autumn. Beta-carotene are also responsible for this color in carrots. They begin to degrade at the same time as chlorophyll but at a slower rate thus showing up the orange color gradually.


There is another class of compounds called anthocyanins which begins to get synthesized in the mature leaves due to the high amount of sugars in them. These are red in color. These are thought to prolong the falling of leaves.



Our Body is a Wonder Machine

Photosynthesis is the process by which plants make two essential things on which the very survival of animals is hinged. These are namely a sugar called Glucose and oxygen. In the beginning oxygen was a poison to many microbes. But since it was a question of survive or perish, slowly they adapted to respiration using oxygen. Those which could not change receded to great depth where oxygen cannot reach.

Glucose is a like a charged battery which stores energy. Where does this energy come from? Obviously it is the Sun on which the life on the Earth is based upon. Plants use carbon dioxide and water and a mediator called chlorophyll to make glucose and oxygen. Anyone familiar with thermodynamics knows that this reaction is not favorable as its overall Gibbs free energy is positive. Second law of thermodynamics requires that only those reactions are spontaneous for which this energy change is negative. In simple words the energy of products should be lower than that of reactants. Here the energy content of the products is higher by 2880 Kilo Joules per mole. It is the Sun who provides this energy and plants store it in the glucose.

Many of the reactions that take place in living organisms require a source of free energy to drive them. The immediate source of this energy in heterotrophic organisms, which include animals, fungi, and most bacteria, is the sugar glucose. Now reverse reaction that is the oxidation of glucose to carbon dioxide and water takes place when animals consume glucose and oxygen. Thus 2880 KJ/mole energy is liberated.

Of course it would not do to simply “burn” the glucose in the normal way; the energy change would be wasted as heat, and rather too quickly for the well-being of the organism! Effective utilization of this free energy requires a means of capturing it from the glucose and then releasing it in small amounts when and where it is needed. This is accomplished by breaking down the glucose in a series of a dozen or more steps in which the energy liberated in each stage is captured by an “energy carrier” molecule, of which the most important is adenosine diphosphate, known shortly as ADP. At each step in the breakdown of glucose, an ADP molecule reacts with inorganic phosphate and changes into adenosine triphosphate ATP

The 30 kJ mol–1 of free energy stored in each ATP molecule is released when the molecule travels to a site where it is needed and loses one of its phosphate groups, yielding inorganic phosphate and ADP, which eventually finds its way back the site of glucose metabolism for recycling back into ATP. The complete breakdown of one molecule of glucose is coupled with the production of 38 molecules of ATP according to the overall reaction

For each mole of glucose metabolized, 38 × (30 kJ) = 1140 kJ of free energy is captured as ATP, representing an energy efficiency of 1140/2880 = 0.4. That is, 40% of the free energy obtainable from the oxidation of glucose is made available to drive other metabolic processes. The rest is liberated as heat.

Thinking Brain hogs more Energy

Though human brain accounts for less than 2% by weight, it is a energy hog. About 20% of all the energy used by our body is consumed by our brain. This is because brain cells are always at work. Even while you sleep, much of your brain is busy managing your body’s physiological and biochemical operations. Brain uses neurotransmitters for sending and receiving the commands and messages to the organs of the body. Like the head honcho of a company, it is always thinking to run the body trouble free and thus constantly consumes more energy. Not only that, the energy requirements of different parts of the brain may differ depending on the nature of work they do.

So it logical to assume that the activities which require more thinking may require the brain to consume more energy. Researcher Tim Forrester, from Cannyminds website, explained: “‘Our brains require 0.1 calories every minute simply to survive. ‘When we do something challenging such as a puzzle or a quiz we can burn through 1.5 calories every minute”.  The brain extracts sugar three-quarters of sugar glucose, available calories and a fifth of oxygen from the blood to create neurotransmitters.So doing difficult crosswords or challenging Sudokus means your brain will crave more glucose and more calories too, added Mr Forrester. This means that if you spent two hours doing puzzles, you would have used 180 calories – which is more than are contained in a creme egg (173) or a bag of Hula Hoops (175), and only slightly less than in a pint of beer (182). Brain extracts energy from glucose only because it is easy to burn unlike the fats which take much longer to breakdown and provide the energy. So although doing puzzles and solving Soduko can burn carbohydrate energy but still you require exercise and work to melt away the extra fat.

Artificial Leaf

Nature is so grand that the structure of leaf which is a basic unit for converting absorbed CO2 and H2O into glucose is beyond full comprehension. The human beings break down every phenomena in nature into smaller manageable parts and understand it and then integrate the results. This is due to the evolution of our brain in a way that left hemisphere dominates. This hemisphere is the logical unit of the brain and analyses the problems by breaking them into modules. It is unable to see the things in the holistic manner. Only the artists use the right brain more and they see the scene as one single entity.

What do the plants do for the food? They use glucose to run the various processes in their lives. The inputs for making glucose include the CO2 and H2O and sunlight with the help of a special pigment called chlorophyll. It is this chemical which gives the leaves their green color. When the glucose is formed, the sunlight is stored in it in the form of chemical energy. Human beings take up the glucose as food. The enzyme which is called alpha amylase breaks down the starch (higher form of glucose) into CO2 and H2O again and releases the stored energy which is utilized by our bodies. This cycle goes on and on this cycle depends the existence of animals and plants. The end products of one’s actions are the starting materials for the other. This shows that we owe our existence to the Sun. Without the Sun life is impossible on the earth. Hindus and Zoroastrian people worship the Sun. When the Vedas and Zend Avesta was written, they may not be knowing the chemistry but even then intuitively they chose the Sun for worship.

The news has broken out from MIT about a professor who has been able to make a synthetic leaf. It does not synthesize the glucose but it breaks down the water into its individual components namely hydrogen and oxygen. The hydrogen so generated is a fuel, the cleanest of the fuels because when it is burned, it again forms the water vapors and releases energy. So the leaf can be more appropriately called a fuel cell. The water as such is a very stable molecule and it requires energy to break it into its components. Conventionally, electrolysis is used to break down the water and this requires energy. The professor must have used the metals which acts as catalysts and makes the reactions possible and lowers cost. He says that his leaf is 10 times more prolific then the natural leaf. But its working life is about 45 minutes.

Let us hope that energy problems are solved and our dependence on the exhaustible natural energy sources like fossil fuels decreases and these isolated inventions do not remain confined to the laboratory but become commercially viable. To read more about the leaf click this link.