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Photosynthesis Respiration and Plant Growth

Photosynthesis is an amazing chemical process carried out by plants that provides the basic building block that is necessary for all living things on earth to survive. This is because the primary ‘waste product’, of photosynthesis is oxygen. Through photosynthesis, plants use light energy, and through a series of chemical reactions, change it into chemical energy in the form of glucose molecules which are the building blocks of plant cell walls and organs for filtering and pH adjustment for optimal plant photosynthesis and plant growth. By understanding photosynthesis and the factors that can increase or decrease the abilities of plants to conduct it, greater growth, flowering, and harvests can be achieved.

Chloroplast and Chlorophyll

All plants contain special cells called chloroplast. These cells are the site where a magic substance called chlorophyll is contained. Plants get their green color from chlorophyll which plays a primary role in the process of photosynthesis. There are three types of chlorophyll, the most important of which is called chlorophyll a, which contains pigments that can absorb blue violet and red light. Chlorophyll exists inside membranous sacs called thylakoids which are stacked inside the chloroplasts. Less than half of the solar energy reaching the earth can be used by plants for photosynthesis.

This light spectrum is from 400-700 nm and is termed Photosynthetically Active Radiation (PAR). This light is in the blue (400-500 nm) and red (600-700 nm) spectrum. When we look at a plant and see green, it is because the chlorophyll molecules in the plant absorb blue and red light and reflect other colors, resulting in the green color we see. Through experimentation, it has been determined that red light has a stronger impact on the rate of photosynthesis than the same intensity of blue light. Although light in the PAR range is the primary light used in photosynthesis other light spectrums are also important to plant growth.

The purpose of the chloroplast and chlorophyll is to make sugars and starches for plants, and the living creatures that eat them, to use for food energy. When solar energy hits a chloroplast, chlorophyll uses that energy to combine carbon dioxide (CO2) and water (H2O). These molecular reactions create sugar and oxygen (O2). A simple chemical formula for photosynthesis is: Six molecules of water plus six molecules of carbon dioxide, in the presence of sun light and chlorophyll, yields one molecule of glucose and six molecules of oxygen.

In the circle of life, green plants are eaten by herbivores which in turn are consumed by carnivores. Ultimately, the decomposers derive their nutrition from the dead plants and animals. Thus, all organisms are directly or indirectly dependent on green plants which synthesize food and release oxygen through photosynthesis, making it the most important life sustaining process of nature.

Light and Dark Reactions

The process of photosynthesis is divided into two main parts. The first part is called the light dependent reaction. This reaction happens when the light energy is captured and pushed into a chemical called ATP. The second part of the process is called the light independent reaction. This reaction happens when the ATP is used to make glucose. This process is referred to as the Calvin Cycle. The plants use the glucose produced from these reactions for growth of seeds, fruits, tubers, or other plant parts of economic value, such as leaves, roots, stems and flowers. Because crop productivity increases when photosynthesis occurs more rapidly, much research is directed toward improving this process.


When plants ‘burn’ the glucose produced by photosynthesis it results in a chemical process that is the opposite of photosynthesis. This process, where sugars react with oxygen to produce water and carbon dioxide, is called respiration. Respiration produces a great deal of energy that is needed by plants to grow and stay healthy. Excess sugars produced by photosynthesis that are not needed for respiration and growth are stored as starch which can then be converted back to sugars when needed during periods of low light.

Although light is an important factor for maximizing photosynthesis and in turn plant growth, all other growth factors both nutritional and environmental play a process and the promotion of optimum plant growth.    By understanding photosynthesis and the factors that can increase or decrease the abilities of plants to conduct it, greater growth, flowering, and harvests can be achieved.

Factors Affecting the Rate of Photosynthesis

It is important to understand and attempt to control as many factors as possible that may either increase or decrease photosynthetic processes in plants intended for harvest. In most instances it is the concentration of growth factors that either decrease or increase photosynthesis.

Light Intensity

The intensity of light is important for maximizing the rate of photosynthesis. The optimal light level for photosynthesis is 10,000 lux. Lux is a unit for measuring light intensity. Low light intensity lowers the rate of photosynthesis. However, after reaching an intensity of 10,000 lux, there is no increase in the rate. The rate of photosynthesis may in fact be lowered as chlorophyll is bleached from the chloroplast and the plant’s stomas are closed to slow down water lost through respiration.

For indoor and greenhouse growing a light meter should be used to determine how much light is reaching the plants. Useful light is decreased exponentially for each unit that artificial light is moved away from plants, but the heat generated by various types of lighting necessitate moving the lights a sufficient distance from the plants. Proper ventilation and use of fans can decrease the space needed between the plants and various types of lighting equipment.

Lights should always be installed with hanging mounts and other forms of adjustable mounts to allow the lights to easily be moved up as plants grow. Horizontal lights installed on the sides of plants can also be used to improve the light intensity reaching the lower branches of the plants. Light movers use small motors and tracks to move light above plants. This helps with even light coverage because it mimics the movement of the sun over the planets. It can also allow more plants to receive light from a given number and intensity of lights. Plants primarily use light from the red and blue wavelengths so good horticultural lights are designed to provide mostly light from these wavelengths in addition to full spectrum lighting.

Get the Best Seeds, Clones or Starter Plants

There are factors other than the intensity and wavelength of the light that reaches the plants that affect the rate of photosynthesis. These can be manipulated by the grower to achieve maximum speed of growth and larger yields in a shorter period. Although photosynthesis can be improved by maximizing growth factors, genetics plays a major role in the plants programmed ability to conduct photosynthesis. Most of a plant’s photosynthesis abilities and vigor are coded in seeds genetics, so it is important to start with the best seeds available.


Some areas have better natural humidity for growing specific plants than others. Humidity may also vary from season to season, and it can be changed for better or worse by indoor home climate control devises. Plants need the highest humidity during germination. This can be facilitated through proper seed starting techniques. Trial and error, as well as active scientific experimentation has shown that a relative humidity of 65% to 80% increases the growth rate of most greenhouse plants. Humidity below this level can result in decreased photosynthesis as plants develop thin leaves and close stomas to try to prevent excess loss of water through respiration and evaporation. Humidity above 80% will result in the plants having trouble disposing of toxic chemicals through evaporation and an increased probability of bud mold. Hygrometers should be used to measure relative humidity and various low-tech and high-tech means are available to adjust for optimal photosynthesis and plant growth.


If temperatures are too high or too low, enzymes stop working. Since both the stages of photosynthesis require enzyme activity, temperature has an affect on the rate of photosynthesis and corresponding plant growth. Select plants that are compatible with temperatures in the environment you can provide.

Carbon Dioxide

As discussed above, carbon dioxide is a gas that is essential for the light reactions in all plants that carry on photosynthesis. In the atmosphere, the concentration of carbon dioxide ranges from .03 to .04 %. However, it has been determined that 0.1% of carbon dioxide in the atmosphere increases the rate of photosynthesis significantly. Therefore, supplementing CO2 to the existing amount in the air will speed up photosynthesis and therefore, growth will occur faster. Experimentation has also shown that CO2 can help plants tolerate higher temperatures with little affect on the rate of photosynthesis.

Indoor and greenhouse growing provides for advantages in more easily manipulating growth factors and CO2 concentration is no exception. This is achieved in the greenhouses and other enclosed growing chambers by installing gas burners which liberate carbon dioxide as the gas burns. Crops grown under these conditions are found to be bigger and better yielding than their counterparts growing in natural conditions.


Although only a small portion of water absorbed by the plant is used in photosynthesis, a shortage of water does affect the rate photosynthesis occurs. Even a slight deficiency of water results in significant reduction in the crop yield. The lack of water not only limits the amount of water, but also the quantity of carbon dioxide available for photosynthesis. This is because in response to drying the leaves close their stomata to conserve water being lost as water vapor through them. Care should be taken to assure that both optimal oxygen and water are always available to the roots.

Chlorophyll Concentration

The concentration of chlorophyll affects the rate of reaction as they absorb the light energy without which the reactions cannot proceed. Several of the above growth factors and others can impact the amount of chlorophyll present in the chloroplasts. A lack of chlorophyll or deficiency of chlorophyll results in chlorosis or yellowing of leaves. This can occur due to mineral deficiency or disease. Lack of iron, magnesium, nitrogen, and light affect the formation of chlorophyll and thereby causes chlorosis. Care should be taken to assure plants have proper macro and micronutrients for maximum growth and flowering at all times.


It is more difficult to combat pollution when growing outdoors, but steps can be taken to curtail both water and air pollution indoors. Air filters on intake vents can help to purify the air in indoor growing environments. Limiting dust from settling on plants and occasional plant washing can help to keep stomas from being clogged which will reduce gas exchange necessary for optimal photosynthesis.  Various water pollutants are present in municipal and well water and water must be tested to determine its quality and need for filtering and pH adjustment for optimal plant photosynthesis and plant growth.

Dr. Christopher J. Kline is a master gardener and writer living in Paradise Valley Arizona.

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