EXAMINING GOD'S UNIVERSE: Gas Exchange in Plants

In plants gas exchange takes place using three different designs, each suited for the part of the plant it is located on. Leaves use one system for gas exchange, stems and branches another, and roots yet another.

Let’s look a leaf first. The top of a Leaf has a waxy protective coating called the cuticle; this prevents passage of any gas through the top of the leaf. Whereas the bottom of the leaf does not have this surface, it is protected by the epidermis. So although the air can reach the outermost cells, it cannot penetrate to the cells inside the leaf.  

Inside the leaf is what is called the mesophyll. There are two types of mesophyll. The one that is most important in gas exchange is the spongy area of the mesophyll which provides a large area for gas exchange to take place. The shape of the cells allow for large open areas to form, so the leaf always has plenty of space for gas exchange. Since the spaces are fairly large, relative to the space needed for gas exchange, the process can go on without the need of any special transport systems for the gas. Also oxygen can diffuse through air almost 10,000 times quicker than through a liquid medium. So the mesophyll eliminates the need for a transport system, and also allows for quick transport of gasses.

But if there is air in the leaf wouldn’t that cause it to dry out? This is another problem that had to be solved. Since gas exchange cannot take place across a dry surface there has to be a way to allow air into the leaf without letting the moist interior. The humidity level inside the leaf is always near 100% meaning that the membranes of the cells are always moist, and ready for gas exchange.

The solution to the problem of letting the gas in and keeping the water in at the same time is the stoma.  The stoma allows gas to pass into the leaf and prevents water loss. To do this it must be able to open and close when needed. 

So the stoma has a purpose, to regulate gas and water levels in the leaf.  This means it needs to be switched on an off depending on the needs of the leaf. It does this by reacting to chemical stimuli. If the leaf is dry, the stoma will close to prevent more water loss, if the there is little CO² the stoma will open to allow more air to enter the leaf. If it is dark the stoma will also close because photosynthesis cannot take place in the dark.

The purpose of the stoma is to maintain the best conditions for gas exchange within the leaf. The stoma itself is composed of two guard cells. When the one side of the guard cells that faces the opening contracts, it bends the cell causing the stoma to open and allow the free exchange of gasses. But at the same time it also lets out water.

Because of the importance of water remaining inside the leaf there is a secondary system to prevent water loss. Most plants have fine hairs on the back of their leaves. These hairs create a thin barrier by inhibiting the movement of air across the leaf, thereby reducing evaporation. So we can see the integrated intelligent design of the leaf, several sub systems working together for the purpose of successful gas exchange. The mesophyll, stoma and hairs all work together to perform this task. Without these specialized cells, the plant would die.

There are some variations on this design. Pondweed for example has stomas on the top of the leaf rather than the bottom, which is in contact with the water. This also shows an intelligent alternative to the usual design.

Stems, Bark and roots also have their own systems for gas exchange. Stems and branches have a protective surface that does not allow for gas exchange with the exception of small groups of cells that scientists have named Lenticels. These occur along the surface of the bark or stem to allow air to enter into the plant.  Again without the Lenticels the plant would die.  Roots gather air from the soil. That is why moist, aerated soil is the best for plants. Loosely packed soil allows for the movement of air to the roots of the plant. The moistness of the soil provides the exchange surface needed.

For a plant to survive it must have gas exchange systems in all three of these areas. The integrated systems that provide solutions to the problem of gas exchange in a leaf are evidence of intelligent design.