Photosynthesis, Respiration & Transpiration 

The three plant processes of photosynthesis, respiration and transpiration are the most fundamental to plant growth and development. Photosynthesis is the process by which green plants utilize light to manufacture their own food. Plant respiration is the next step. Respiration is where the plant metabolizes food created during photosynthesis and uses the energy obtained to carry out life processes. Lastly, transpiration is responsible for the movement of water, and thus nutrients, throughout the different parts of the plant. Let’s take these simple explanations up a notch!


The equation for photosynthesis = [carbon dioxide + water + light energy = glucose +oxygen]. During photosynthesis, carbon dioxide from the air and water from the soil react with the sun’s energy in subcellular structures called chloroplasts. The result is the creation of sugars, starches and carbohydrates (aka photosynthates), and the release of oxygen.

Photosynthesis is directly dependent on three limiting factors- light, water, and carbon dioxide. The concept of a limiting factor is based on the Liebig’s Law of the Minimum. Liebig’s Law states that growth is controlled by the scarcest resource, not by the total amount of resources available.  In other words, if any of the three -light, water or carbon dioxide- is limited, photosynthesis will be impeded along with plant vigor and growth.

In an indoor growing environment, carbon dioxide levels can easily become a limiting factor for plant growth. Supplemental carbon dioxide is needed to improve photosynthesis in these grow systems.


Respiration is the conversion of photosynthates back into energy for the metabolic processes that control growth and life processes. The equation looks like this [glucose + oxygen = energy + carbon dioxide + water]. Since oxygen is required for respiration, an excess of water (soil saturation) will negatively affect the plant’s ability to convert food into energy.


Transpiration is the loss of water vapor through leaf stomata. Thus, it is transpiration that triggers the uptake of water in the roots and the transportation of water (and nutrients) throughout the plant.

Approximately 90% of the water that enters a plant is used in transpiration and the remaining 10% is used during photosynthesis and respiration.

There are three indispensable roles that transpiration plays within plants. Its first role is mineral uptake where water serves as both the solvent and the avenue of transport. Minerals are taken up by the roots and transported by the xylem throughout the plant.

The second role of transpiration is cooling. Water vapor is released from the leave’s stomata, essentially the plant is perspiring. Additionally, cool water from the root zone is pulled up through the plant which adds to the cooling effect of ‘plant perspiration’.

Finally, transpiration is indispensable for a plant’s utilization of water to maintain turgor pressure within its cells. Turgor pressure is responsible for a variety of plant functions. Without turgor pressure forcing plants to remain rigid and erect, they would be unable to support themselves.

Turgidity is just as important to the root as it is in the shoots where it serves as the force that pushes roots through the soil allowing them to mine the soil for life-sustaining substances. In addition, turgidity is responsible for regulating stomata’s guard cells, which control when the stomata are opened to release water vapor or uptake carbon dioxide.