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Botany for Gardeners: The Leaf – Photosynthesis

The leaves of the plants I grow in my garden add both color and texture but of course the plants don’t grow leaves for my benefit. Most plants have leaves to manufacture food so that they can grow and thrive. We also grow and thrive through this food manufacturing process because we enjoy the plants both in our garden and on our table as the food we eat. Understanding the process can improve the way we grow the plants for the maximum benefit of all.The food manufacturing process is called photosynthesis from the Greek words for “photo” meaning light and synthesis, meaning “putting together”. It’s a complex process involving many chemicals that we can forget but as the term tells us, light is all important. Light is an abundant form of energy that we, unfortunately, can not use for our energy needs. Plants have the ability to convert light energy into chemical energy that both we and the plants can use. The process is carried out by the chlorophyll in the leaves (or stems) and the product, a sugar called glucose, is carried to various parts of the plant where it is used or stored (think carrot or potato).

The leaf is especially adapted for photosynthesis. Most leaves are flat and expanded so that they expose the maximum surface area to the sun. Inside, the cells are arranged so that those containing chlorophyll are upper most and receive the most sunlight. Other cells are loosely packed and provide air spaces while others move water and food through the leaf. Special structures called stomata, usually located on the lower side of the leaf, allow gases (oxygen and carbon dioxide) to enter and leave the leaves. Here’s a diagram of a typical leaf.

Photosynthesis is a very complex process that involves many chemicals but the overall process of photosynthesis can be summarized in the equation:

Water + carbon dioxide in the presence of light and chlorophyll yields glucose + oxygen .

Here are the highlights of the process.

1. The process of photosynthesis takes place in the chloroplast (located in the palisade layer) of the leaf. The chlorophyll is necessary for the process but is not used up in it and can be used over an over again.

2. There are two distinct phases of the process; one requires light and chlorophyll the other does not.

3. In the first phase (light phase) water is broken down to form particles that can collect and carry the energy from the light. Oxygen molecules are also formed.

4. The second phase (carbon fixation) does not require light or chlorophyll and involves the joining of the energy from the first phase to molecules of carbon dioxide to form glucose.

Here’s the process in a diagrammatic form:

What is the importance of all this for the gardener? Look at the materials needed for the process: light, chlorophyll, water and carbon dioxide. Consider what can the gardener do to improve the supply of these.

1. Locating the plant in optimal light conditions is basic. Some plants need more light than others to carry on photosynthesis at the highest rate possible; a gardener needs to respect that and locate the plant accordingly.
2. Chlorophyll develops in the presence of light so its concentration in leaves is enhanced by the optimal amount of light. Thus, locating the plant so that it has the most light that it can use without being damaged will maximize chlorophyll content and add to the overall food that a plant can produce.
3. Most plants need about 1” water per week to grow well. Of course, there are exceptions; some plants are especially adapted to dry conditions but most do well in moist but well drained soil.
4. Carbon dioxide is abundant in the atmosphere and enters the leaves through microscopic pores, the stomata, that open during the day when photosynthesis is going on and close at night to reduce water loss. This loss of water can be significant and is a 2nd reason for making sure that the plant has a sufficient and consistent water supply.

The sugar (glucose) formed by photosynthesis not only keeps the plant growing but it aids in the production of the vegetables and fruits that provide you with vitamins, minerals, carbohydrates, fats, and proteins that you need for growth and activity. Plants form the base of most food chains because they can convert light energy into chemical energy that we can use, and animals including man can not do so. Considering that plants also remove carbon dioxide from the air and add oxygen, plants are amazing organisms.

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Recommended Reading:

From Seed to Plant
Trees: Their Natural History
Oh Say Can You Seed?