LEAVES AND PHOTOSYNTHESIS

17th century Belgian physicist, Jan Baptista van Helmont, observed the growth of a willow tree and took various measurements in one of his scientific experiments. First he weighed the tree, then he weighed it a second time five years later, and saw that it was now 75 kilograms heavier. Yet, the soil in the pot in which the plant was grown lost only a few grams over the same time period. The physicist van Helmont revealed in this experiment that the soil in the pot was not the only reason for the growth of the willow tree. Since the plant had used only a very small part of the soil to grow, then it must have been receiving nutrition from somewhere else.34
This occurrence, which van Helmont attempted to discover in the 17th century, was photosynthesis, some stages of which are still not understood in our own time. In other words, plants' producing their own nutrition.
Plants do not just use the soil when producing their own nutrition. Besides the minerals in the soil, they also use water and the CO2 (carbon dioxide) in the atmosphere. They take these basic materials and process them in microscopic factories in their leaves, thereby carrying out photosynthesis. Before examining the various stages of photosynthesis, it will be useful to take a look at leaves, which play an important role in this process.

The General Structure of Leaves

When studied from either the point of view of general structure or of microbiology, it will be seen that leaves possess planned, very complex, and detailed systems to produce as much energy as possible. In order for leaves to produce energy they need to take heat and carbon-dioxide from outside. All the systems in leaves have been designed to take in these two things as easily as possible.

Let us first look at leaves' external structures.

The external surfaces of leaves are wide. This enables the exchange of gases (such processes as the absorption of carbon-dioxide and the release of oxygen, for instance) necessary for photosynthesis.
The leaves' flat and wide shape enables all the cells to be near to the surface. Thanks to this, the exchange of gases is made easier, and sunlight can reach all the cells which carry out photosynthesis. Let us imagine what would happen otherwise. If leaves were not flat, wide, and thin, but had any geometrical shape or any random and meaningless one, they would be able to carry out photosynthesis with only those regions directly in contact with the sun. This would mean that plants would not be able to produce enough energy and oxygen. The most important result of this for living things would certainly be the emergence of an energy shortage in the world.

The picture on the left shows the lesser celandine flower, which resembles a miniature radar station, as it tracks the sun across the sky. Like all other plants, it turns to follow the direction of the sun, so that it is better able to benefit from the sunlight. The sunflowers in the picture below change direction in line with the movement of the sun. Light-sensitive leaf cells immediately establish the direction and move towards the sun.

And the specially "planned" systems in leaves do not end there. The tissue of the leaf has another important property. Thanks to this, phototropism, or turning towards the light, takes place. This is the reason for plants' turning their leaves to the direction of the Sun, which can be easily observed in pot plants. In order to understand how these processes which are of vital importance, take place, we shall have to take a brief look at the physiological structure of the leaves.

If we look at a cross-section of a leaf, we will see a four-layered structure.

The first is the epidermis layer, which does not include chloroplasts. The role of the epidermis, which covers the top and bottom of the leaf, is to protect the leaf from external influences. The outermost part of the epidermis is covered with a protective and waterproof waxy layer, called the cuticle. When we look at the internal layers of the leaf, we see that it is generally made up of two layers of cells. Of these, cells rich in chloroplast stand in rows, with no gaps between them, making up the palisade layer, which forms the internal tissue. This is the layer which carries out photosynthesis. The spongy layer below this is the layer which enables respiration. There are air pockets between the layers of cells in this tissue. As we have seen, all these layers have very important tasks in the construction of the leaf. This kind of organization is of enormous importance from the point of view of photosynthesis, as it enables the leaf to spread and distribute light better. As well as this, the leaf's ability to carry out processes (such as respiration and photosynthesis) increases with the size of the leaf surface. For example, in dense tropical rainforests there is the tendency for large-leaved plants to grow. There are very important reasons for this. It is rather difficult for sunlight to reach all parts of plants equally in tropical rainforests, where the trees which make them up are all densely packed together and where it rains hard and often. This is what makes it necessary to increase the surface area of the leaf in order to catch the light. In those areas where the sunlight enters with difficulty, it is of vital importance for leaf surfaces to be large in order for plants to produce nutriment. Thanks to this feature, tropical plants are exposed to the sunlight in the most advantageous manner.

The general structures of plants living in tropical regions and in deserts are very different, as can be seen in this picture.

Small leaves, on the other hand, are found in dry, harsh climates, because under these climatic conditions the basic point of disadvantage is heat loss. And as the leaf surface becomes greater, water evaporation, and thus heat loss, increases. For this reason, the leaf surface, which catches the light, has been planned in the most economic way for the plant to conserve water. In desert environments the shrinking of leaves reaches exaggerated proportions. Cactus plants have thorns instead of leaves, for instance. In these plants photosynthesis is carried out by the fleshy stems themselves. The stem moreover, is where water is stored.
But that is not enough to control water loss on its own. Because no matter how small the leaf is, the presence of the minute pores in the epidermis called the stomata means that water loss continues. For this reason the existence of a mechanism to compensate for evaporation is essential. And plants do have a way of regulating too much evaporation. This is done by controlling the degree of openness of the stomata, either widening or constricting them as required.
Trying to capture light to carry out photosynthesis is not the leaves' only task. It is also important for them to take carbon-dioxide from the air and direct it to the areas where photosynthesis is carried out. Plants do this by means of the pores on their leaves.

The Stoma: A Flawless Design

These microscopic pores on the surface of leaves have the role of enabling the transfer of light and water and of taking the CO2 necessary for photosynthesis from the atmosphere. The stomata possess a structure which allows them to open or close as necessary. When they open, the oxygen and water vapour between the cells of the leaf are exchanged for the carbon-dioxide required for photosynthesis. In this way, surplus production is given off, and the required substances are absorbed to be made use of.
One of the interesting aspects of the stomata is that they are generally found on the underside of leaves. In this way the harmful effects of sunlight are reduced to a minimum. If the stomata, which give off the water in the plant, were on the tops of the leaves in great numbers, they would be exposed to sunlight for long periods. In such a situation, the stomata would continually be giving off the water in them because of continuous exposure to heat, in which case the plant would die of excessive water loss. Thanks to this special feature, the plant is prevented from being harmed by water loss.
The stomata are formed by sausage-shaped guard cells. Their concave structures permit the opening of the pores, which in turn allow the exchange of gases between the leaf and the atmosphere. The opening of the pores depends upon external conditions (light, heat, moisture, and carbon-dioxide levels) and the internal state of the plant, particularly its water levels. The pore's opening or closing regulates the exchange of gases and water.
There are very fine details in the structure of the pores, which have been designed with all external factors in mind. As we know, moisture levels, the degree of heat, gas levels, air pollution always change. Leaf pores possess structures which can adapt to all these changing conditions.
We can explain all of this with an example. In plants such as sugar cane and cornplant, which are exposed to heat and dry air for a long time, the pores stay completely or partially closed all day in order to conserve water. These plants need to absorb carbon-dioxide in the daytime for photosynthesis. Under normal conditions, the pores would have to remain as open as possible. But this is impossible. Because in that case the plant would continuously lose moisture from its pores and shortly die. For this reason, the pores need to remain closed.
But this problem, too, has been solved. Some plants, which live in hot climates, have a carbon dioxide pump which sucks the gas more efficiently out of the air into the leaf. These plants thus use chemical pumps to absorb carbon dioxide in their leaves, even if their pores are closed.35 If these pumps were absent for a time, the plant would be unable to produce any nutrients, because it could not take in any carbon-dioxide, and would therefore die. This is a sign that these complex chemical pumps could not have come about as the result of a series of coincidences over time. This system in plants can perform effectively only when all its components are together at once. For which reason there is no chance that the stomata could have evolved and emerged as the result of coincidences. The stomata, with their exceedingly special construction, have been planned, in other words created, to perform their tasks in the most sensitive manner possible.