Complex processes go on within a leaf

This past fall, the leaves falling from the many trees around my house inundated my yard and roof gutters. Many found their way into my garage and into my house itself. Needless to say, I was upset with this situation until I remembered that without leaves, life as we know it on this planet would not exist.

Green plants have few and relatively simple requirements for life: light, water, carbon dioxide and certain minerals. From these simple substances, green plants make the building blocks of carbohydrates, proteins, fats, nucleic aids and a host of other organic substances on which all animal and plant life depends. It is within the structure of the leaf that the extremely complex process of photosynthesis, which is responsible for this, takes place.

If we look at the internal structure of a leaf, we find loose and spongy tissues and bodies that contain the green-appearing pigment chlorophyll. Along the bottom of the leaf are various openings for gaseous exchange called stomata that are opened and closed by specialized guard cells. The key to the whole process of photosynthesis is chlorophyll, which has the outstanding ability to attract and hold energy derived from solar radiation. Solar radiation is the source of all energy required by living things.

This energy drives the reaction of the conversion of the raw materials of carbon dioxide (CO2) and water (H2O) that forms simple sugar and liberates free oxygen. Some of the energy is stored in the bonds of the sugar to be released later. The basic chemical reaction is the reduction of the CO2 by adding a hydrogen atom to form the basic atomic arrangement of sugar, CH2. This reduction of carbon dioxide takes place during the so- called the Light Phase of photosynthesis as it can only occur in the presence of sunlight.

During the night, another complex part of photosynthesis takes place, and is called the Dark Phase.. During this phase the reverse of the reaction in the Light Phase takes place in what is termed by biochemists as an oxidation reaction. The simple sugar (CH2O) reacts with oxygen (O2) to form the basic building blocks of carbon dioxide and water. The energy stored in the bonds holding the sugar together is liberated when the bonds are broken.

When we look farther into the anatomy of a leaf, we can observe what are called vascular bundles, a circulatory system that extends from the roots of the plants to the leaves, just like our blood vessels reach from the tip of our toes to our brain. And, like our circulatory system, the vascular tissue of higher plants consists of two major types of vessels.

One of these two,the xylem, develops from cells called tracheids and consists of hollow, drinking straw-like vessels that extend from the roots of the plant to the leaves. The vessels’ primary purpose is to transport water and dissolved minerals absorbed from the ground by root hairs to the scene of photosynthetic activity.

In close proximity to the xylem vessels are the phloem vessels. The phloem develops from cells called sieve tubes, and they also run throughout the plant. The primary purpose of the phloem vessels is to transport sugar manufactured in the leaves to various parts of the plant for storage to be broken down later for energy needed for growth. The close combination of the xylem and the phloem is called a vascular bundle. If we look on the underside of a leaf, we can see the extensive branching of the vascular bundles, which are otherwise called the veins of the leaf.

Terrestrial plants need a lot of water as a great deal is lost via evaporation or what is termed transpiration. Transpiration occurs mainly through the broad surfaces of the leaves and the many pores or stomata on their undersides. It has been demonstrated that a single corn plant growing in Kansas will transpire about 54 gallons or almost two barrels of water during a growing season. This is about 90 times the amount of water the corn plant would need for all purposes during a single season except to replace what is lost by transpiration.

The next time you sit down to a juicy steak, you might give a thought to where it came from. Photosynthesis formed the building blocks of the carbohydrates, proteins, and fats in the leaves of grass the bovine ate and were eventually reconstructed into your prime rib dinner.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

From the Nov.8-14, 2006, issue

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