Photosynthesis: What, Where, How and Why?

by | Oct 17, 2023 | blogs, Grow Lights | 0 comments


What is photosynthesis?  For many, it’s something we’ve learned or heard about, but the details have escaped us over the years.  So, we thought it might be helpful to review the salient points and terminology of this amazing process for you.

Some biochemistry is inevitable, but we’ll keep it simple and manageable. Here are the topics:

  • What is Photosynthesis?
  • Where does Photosynthesis happen?
  • How does Photosynthesis work?
  • Why is Photosynthesis important?

By the end of this article, we hope you’ll be able to summarize photosynthesis like a pro.


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Fig 1

What Is Photosynthesis?

As you may know, Photosynthesis is a process by which plants use energy from light to produce “sugars”.   

The plant eventually uses these sugars as energy, needed for many metabolic purposes, like stem growth, leaf production, flowering, and fruiting.


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Fig 2

Where does Photosynthesis take place?

For green plants, Photosynthesis will occur in the “green” parts of the plant, namely the leaves.

Please take a look at figure 4.


  1. Leaves are made of plant cells encased by an outer boundary.
  2. Within the leaf cell are smaller bean-like objects called Chloroplasts.
  3. Within the Chloroplast are stacked pancake-like objects called Thylakoids.
  4. Thylakoids have an outer membrane – within this membrane, the first part of Photosynthesis occurs.

Fig 3  (Albert Melu on Unsplash)

Fig 4 – Where photosynthesis takes place.

How does Photosynthesis work?

You can think of Photosynthesis as a small factory that makes sugar. A factory needs inputs like energy and materials.

In our photosynthesis factory, a leaf uses energy from light, H2O (water), and C02 (Carbon Dioxide). 

And the outputs are sugar and Oxygen.  Sugar goes back into the plant for metabolic purposes.  The Oxygen is expelled as a vital waste product that becomes the air we breathe.

Fig 5 – Photosynthesis is like a factory

Fig 6 – Photosynthesis Inputs and Outputs

Tale of two factories

Actually, two factories make up Photosynthesis – Light Reactions and Calvin Cycle.

We will first cover the Light Reactions in the Thylakoid membrane.  The goal of this factory is to harness light energy into a more usable form.


Fig 7 – The two factories of Photosynthesis.

Light Reactions Factory – INPUTS

In Figure 9, You can see the light coming into our system.  You can also see Water input into the system.  And you can see Oxygen exiting the system.

Fig 8 – The Thylakoid

Fig 9 – Light Reactions Inputs

Light Reactions – Accumulating Energy

In figure 11 (red circles), energy from light is converted to H+ and e- in a process called Electron Transport.  They are the energies or electricity that are accumulated throughout the processing.

Also in Figure 11 (blue circles), are the outputs from the process is ATP and NADPH.  They are temporary stores of energy (batteries) used for the second stage of photosynthesis.  So, this first part of Photosynthesis aims to build up energy stores to be used in the 2nd factory.

Fig 10 – The Thylakoid

Fig 11 – Light Reactions – Energy accumulation


The 2nd factory of Photosynthesis – The Calvin Cycle

The 2nd factory or stage is where we synthesize  sugar – it is called the Calvin Cycle, named after the renowned biochemist Melvin Calvin.

In Figure 13, you can see the ATP and NADPH from the Light Reaction process used as input into the Calvin Cycle (letters E and D).

Also note that Carbon Dioxide is also used in the Calvin Cycle (letter F). The final product is the sugar 3 phosphoglycerate (letter G).

Note that the Calvin Cycle occurs immediately outside the Thylakoid in the spaces of the Chloroplast stroma.

Fig 12 – Melvin Calvin

Fig 13 – Tale of two factories – Calvin cycle. (2023, September 5). In Wikipedia.

What about Chlorophyll?

We still need to talk about Chlorophyll, the essential molecule responsible for absorbing light energy.

Chlorophyll is a pigment molecule, so called because it reflects a color making leaves look green.  The other colors are absorbed, particularly in the blue and red areas.

Fig 14 – Chlorophyll Molecule

Fig 15 – Grow Lights

If we go back to the Light Reaction production line in the Thylakoid membrane, you will notice a PSII and a PSI module.  This is where Chlorophyll resides.

PSII modules contain mostly a type of chlorophyll that absorbs blue light, and PSI contains a chlorophyll that absorbs more red light.  

It’s no wonder that grow-lights exhibit blue and red colors – it’s because they are targeting the blue-absorbing chlorophyll in PSII, and the red-absorbing chlorophyll in PSI.

Fig 16 – Chlorophyll in PSII and PSI

What is Light Energy?

Light contains energy that powers all Photosynthetic activity.  When you think about light energy, you can look at it from two perspectives – light as particles and light as waves – it is the dual nature of light.


Fig 17 – Spectrum from Spectrometer

Light as particles

When you think about light as a particle, you can imagine droplets of light falling on a leaf.  These droplets are called photons.

Fig 20 – Light as particles

If you increase the brightness or intensity of the light, the number of photons increase, and the energy increases.

Fig 18 – Brightness or Intensity

Light as waves

When you think about light as waves, you should think about colors. And we refer to color in terms of wavelengths.

Higher frequency wavelengths tend toward the blue spectrum and have more energy, while lower frequency wavelengths tend toward the red and have less energy.

Fig 21 – Color, wavelength and energy

A Photon has an associated wavelength

Each photon of light will have an associated wavelength.  So, you will have blue photons, red photons, green photons, yellow photons etc.  Blue photons will have more energy than red photons.

PSII will absorb blue photons as a very high energy source to produce energy-packed NADPH and ATP.  Most leaves above the canopy receive adequate amounts of blue light for photosynthesis.

However, leaves underneath the canopy can still participate in photosynthesis because some blue and red light is still penetrating the leaves.  In fact, the Chlorophyll-a color range is wider than Chlorophyll-b, to allow for as much light absorption in the shade as possible.

Fig 22 – Photons and wavelengths

Using a Spectral PAR Meter to measure light energy.

You can use a Spectral PAR meter to see the light energy from the grow lights or the sun.

In Figure 23, you can see a spectral diagram that shows the wavelengths of photons and their intensities.

We see an abundance of high-energy blue photons that PSII can use – however, the blue color is not aligned with optimal chlorophyll absorption.  You can tell by the spectrum’s shape, that it is an LED grow light –  if it is a “Dynamic” LED light, colors must be adjusted.


LED light color and Photosynthesis alignment

Fig 23 – Photons and wavelengths


Photosynthesis is a process plants use to make sugar, a vital element used for many metabolic functions.  This process is buried deep in the plant’s cell, in a place called the Thylakoid membrane.  It is like a factory production line with inputs (Light, Water, CO2) and outputs (Oxygen and Sugar).

There are two stages to photosynthesis – Light Reaction and Calvin Cycle – the Light Reaction stage harnesses energy from light and transforms it into a more usable form (ATP and NADPH).

The Calvin cycle uses ATP and NADPH to synthesize the sugar for later metabolic processes such as plant growth, flowering, and fruiting.

Photosynthesis is one of the marvels of nature, and having a good grasp of the basics gives farmers the confidence to tweak the parameters and improve farming productivity.

But Photosynthesis is only half the grow-light story – In the next post, we will show how plants use light to grow, flower, and fruit.



Fig 24 – Photosynthesis

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