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Photosynthesis & Respiration

By Sunny Datko

Plant physiology (phys`i*ol”o*gy) is the study of the functions and processes occurring in plants and the ways that plants control or regulate internal functions. It examines the biological and chemical processes of individual plant cells, and how the plant responds to conditions in the environment.  In essence, plant physiology is a study of the plant way of life, which includes various aspects of the plant’s lifestyle and survival including photosynthesis and respiration. These aspects are important to understand when you are trying to dial in the correct environmental and nutritional factors for your plants.  The better you understand the basic processes that your plants go through during their plant life cycle, the better you can provide for their needs accordingly.


Almost all life on our planet directly depends on photosynthesis (from the Greek photo-, “light”, and synthesis, “putting together”). Photosynthesis is the process by which plant life converts light energy into chemical energy and stores it as sugar.

The essence of this process is that energy from the sun is used to split water into its constituents of hydrogen and oxygen. The hydrogen is used to convert carbon dioxide into more useful sugars, namely glucose.  These sugars then become the food used in cellular respiration.  The oxygen is released into the atmosphere as a by-product.  This is the source of the air we breathe.  The equation for this process is listed below:

photosynthesis formula

The family of molecules responsible for photosynthesis is collectively known as photosynthetic pigments. Of these, the chlorophylls are the major contributors of this group.  Chlorophyll is a green pigment found in almost all plants and algae.  It is located in the chloroplasts, where photosynthesis takes place.  Its name is derived from the Greek words chloros (“green”) and phyllon (“leaf”).

Chlorophyll looks green because it absorbs red and blue light, making these colors unavailable to be seen by our eyes. It is the green light that is NOT absorbed that finally reaches our eyes, making chlorophyll appear green. However, energy from the red and blue light is able to be used for photosynthesis. The green light we can see is not/cannot be absorbed by the plant, and thus cannot be used to do photosynthesis. So if a green LED flashlight turns on in an otherwise totally dark grow room, the plants still believe it’s dark since they can’t see that part of the spectrum. This allows growers to safely manage their flowering garden during the dark cycle.

photosynthesis & temperature graphThe process of photosynthesis is directly dependent on the supply of water, light, and carbon dioxide.  Limiting any one of these factors (carbon dioxide, water, or light) can limit photosynthesis regardless of the availability of the other factors.  This factor will always prevent the rate of photosynthesis from rising above a certain level even if other conditions needed for photosynthesis are improved.  This limiting factor will control the maximum possible rate of the photosynthetic reaction.

The rate of photosynthesis is also somewhat temperature dependent.  For example, with tomatoes, when temperatures rise above 96°F (35°C), the rate of food used by respiration rises above the rate of which food is manufactured by photosynthesis.  Plant growth comes to a stop and the tomatoes lose their sweetness. Most other plants are similar.


Respiration is another fundamental process of living organisms. Before we proceed, it is important to compare two definitions of respiration. Many of you may consider respiration the process of breathing air in and out of your body. Mammals like yourself pull air into their lungs by contraction of the diaphragm and exhale the air as the diaphragm relaxes. The inhalation and exhalation of air is one valid definition of respiration.

Biologists often use another definition of respiration to describe what happens to some of the components of the air, particularly CO2 and O2, at the cellular level. Cellular respiration is defined as the process by which cells release energy from organic compounds.

Once the energy that was in sunlight is changed into chemical energy (glucose) by photosynthesis, an organism has to transform the chemical energy into a form that can be used by the organism.  The plants use oxygen to break down the stored glucose molecules to release energy molecules known as ATP (Adenosine triphosphate).  ATP works like a rechargeable battery.  When the bond between the second and third phosphate is broken, it releases energy and a phosphate, turning ATP into ADP (Adenosine diphosphate).  Plants can then use this available energy to perform different metabolic processes.  In many cases, the energy is used to reattach the phosphate molecule to the ADP, turning it back into ATP.  Then the cycle of bond breaking and bond making begins all over again, alternately releasing and storing energy, as needed.

Simply stated, this equation means that oxygen combines with glucose to break molecular bonds, releasing the energy (in the form of ATP) contained in those bonds. In addition to the energy released, the products of the reaction are carbon dioxide and water.  The main results are organic compounds broken down to simpler compounds, with some energy beformcoming available for use in other metabolic steps.

Chemically, cellular respiration is the exact opposite of photosynthesis. While photosynthesis occurs only in some cells, respiration occurs in all cells, and is described by the following equation:

cellular respiration equationThe process of respiration is similar to the oxidation that occurs as wood is burned, producing heat.  When compounds combine with oxygen, the process is often referred to as “burning”, for example, athlete’s “burn” energy (sugars) as they exercise.  The harder they exercise, the more sugars they burn so the more oxygen they need.  That is why at full speed, they are breathing very fast.  Athletes take up oxygen through their lungs.  Plants take up oxygen through the stomata in their leaves and through their roots.

Again, respiration is the burning of sugars for energy to grow and to do the internal “work” of living.  It is very important to understand that both plants and animals (including microorganisms) need oxygen for respiration.  This is why overly wet or saturated soils are detrimental to root growth and function, as well as the decomposition processes carried out by microorganisms in the soil.

The same principles regarding limiting factors are valid for both photosynthesis and respiration.

Table 1: Comparison of Photosynthesis & Respiration

Photosynthesis Respiration
Produces sugars from light energyStores energy

Only occurs in cells with chloroplast

Releases oxygen

Uses water

Uses carbon dioxide

Requires light

Burns sugars for energyReleases energy

Occurs in most cells

Uses oxygen

Produces water

Produces carbon dioxide

Occurs in dark and light

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