PV Gap

Gloria Said:

If photosynthesis is operating day and night, will this prevent cellular respiration from operating?

We Answered:

The simple answer to your question is -- Yes, continuous exposure to an energetic enough light source and adequate supply of water and carbon dioxide WILL prohibit cellular respiration from taking place. The reason for this is found in the processes that take place during both photosynthesis and cellular respiration.

First, photosynthesis takes place in two phases 1) photophosphorylation (which needs light to take place) and 2) carbon fixation (Calvin cycle – which can occur in the light or dark). During photophosphorylation, energy in the form of light is captured and stored as NaDPH and/or ATP. This is also the stage that utilizes the H+ proton from water molecules and releases oxygen as a by-product. During the Calvin cycle, the carbon in carbon dioxide is “fixed” or assimilated into other products and some ATP is utilized to for the building blocks for the plant, such as glucose. As I said before, as long at a photosynthetic plant has light, water, and CO2 (and other trace minerals of course), photosynthesis will continue.

So where does cellular respiration come it, and do plants even do it? The answer is, yes, they do use cellular respiration and they do it when they need an alternate energy source when light is not present. Keep in mind that cellular respiration is something akin to the opposite of photosynthesis and its purpose is to supply energy in the form of ATP by breaking down things like glucose. This provides the needed maintenance energy at the expense of stored energy in the glucose.

So, why would constant exposure to light keep a plant from respiring? The answer is – the plant has a constant source of ATP through the photophosphorylation phase of photosynthesis that there is no need to break down its stored “fuel” to provide maintenance energy. In fact, if enough light is provided, the resulting photosynthesis will provide enough energy to meet maintenance needs and allow generation of building blocks and stored fuel.

As for the second part of your question, there are many reasons that a plant in natural light for only daylight hours might grow better than one continuously lit artificially. Most likely though, this has to do with the availability of the necessary “ingredients” for growth mentioned above. Especially of interest here is the INTENSITY of the light rather than the duration. However, even if the intensity is high enough, if the other ingredients are lacking relative to the natural environment the plant will still suffer.

Hope that clarifies the issue for everyone.

Christy Said:

can a grow light heat a small greenhouse?

We Answered:

It depends on the size of the grow light (wattage) and the climate. Certainly you might be able to store enough power from a solar panel to power the light, but the greenhouse should be able to simply use sunlight during the day. You might want to consider getting a small electric heater for temperature control for winter.