The Phases of Photosynthesis: Light and Dark Reactions in Your Cannabis Plants

Photosynthesis has two main phases: the light-dependent reactions, where chlorophyll captures light and turns it into chemical energy (ATP and NADPH), and the light-independent "dark" phase, or Calvin cycle, where that energy turns CO2 into sugar. It's how plants, cannabis included, convert sunlight, carbon dioxide and water into usable energy and oxygen. It drives plant growth and helps keep Earth's atmosphere in balance.

In cannabis, photosynthesis plays out in several stages, each one setting off specific biochemical processes the plant needs to grow healthy and to produce the compounds growers care about, like cannabinoids and terpenes.

What is photosynthesis?

Photosynthesis is the process plants, cannabis included, rely on to turn sunlight, water and carbon dioxide into usable energy and oxygen.

It's made up of several key steps that take place in leaf cells, inside the chloroplasts, the structures that handle photosynthesis. Here's a quick rundown of each one:

1. Capturing light: Chlorophyll pigments in the chloroplasts absorb sunlight, and that energy is used to split water molecules into oxygen, electrons and protons.
2. Making ATP and NADPH: The electrons and protons released in the previous step are used to make adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), two molecules that store and carry energy.
3. The Calvin-Benson cycle: Carbon dioxide from the air bonds to a five-carbon compound called ribulose-1,5-bisphosphate (RuBP), forming three-carbon molecules called 3-phosphoglycerate (3-PGA). Those get turned into carbohydrates like glucose, which the plant uses for energy and as raw material for growth.
4. Regenerating RuBP: To keep the Calvin-Benson cycle running, some of the 3-PGA is used to rebuild RuBP so the cycle can start over.

Photosynthesis never stops while there's light, and plants can't survive without it: it supplies the energy they need to grow and develop. It doesn't just benefit plants, either. By releasing oxygen and capturing carbon, it also helps the environment and slows down climate change.

Photosynthesis from root to leaf: what are the stages?

Photosynthesis is fundamental to plant life: it's how water, carbon dioxide and light become usable energy. A simple way to picture the whole process in a plant is in four stages: absorption, circulation, photosynthesis and feeding.

1. Absorption: The plant takes up water and minerals from the soil through its roots. They're essential to photosynthesis because they supply the building blocks for making energy.
2. Circulation: The absorbed nutrients, water included, travel through the plant's vascular system up to the leaves, where most photosynthesis happens. This transport is what gets nutrients where the plant needs them.
3. Photosynthesis: This is the heart of the process, where the plant uses light energy to turn carbon dioxide and water into sugars and oxygen. It's split into two main phases:
   • Light phase: Light energy is converted into chemical energy in the form of ATP and NADPH. Those energy molecules are then used in the next phase.
   • Dark phase: Sugars are built from carbon dioxide using the energy stored in ATP and NADPH. Even though it's called the "dark phase," it doesn't happen in the dark; it just doesn't depend directly on light.
4. Feeding and growth: Finally, the plant uses what photosynthesis produced to feed itself and grow. Sugars and other nutrients serve as fuel and as building material for new cells and new growth.

What is the light phase of photosynthesis?

The light phase is the first crucial step in turning solar energy into chemical energy. Light is absorbed by structures called photosystems, made of chlorophyll and proteins, which sit inside the chloroplasts of plant cells. As the name says, this phase can only happen with light, natural or artificial, like your grow lights.

Photosystems I and II are the stars of the light phase. They capture light and use its energy to kick off a transfer of electrons along a chain of acceptors, generating a sort of electric current inside the chloroplast. Picture electrons hopping from one molecule to the next down a transport chain.

One of the most important parts of the light phase is the splitting of water (photolysis), which happens in photosystem II. Light energy excites the electrons in the water molecule, breaking it into oxygen, protons (H+) and electrons. The released oxygen pairs up with another oxygen atom to form atmospheric oxygen (O2), the same stuff we breathe!

Why does cannabis need this phase?

The light energy absorbed by chlorophyll is passed to electrons, which then enter the electron transport chain, where adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH) are made.

These molecules are essential for the next phase of photosynthesis, the Calvin cycle, where the plant makes the first sugars it'll use for energy and as raw material for growth.

In short, the light phase is a vital step in photosynthesis, because it converts solar energy into forms of chemical energy the plant can use to feed itself and grow.

What is the dark phase of photosynthesis?

The dark phase of photosynthesis takes place in the stroma (the fluid inside the chloroplasts). That's where the Calvin cycle runs, the key process for making organic compounds, mainly carbohydrates or sugars, from carbon dioxide (CO2) in the air.

Here, ribulose bisphosphate reacts with CO2 from the air, helped along by an enzyme called RuBisCO, to form 3-phosphoglycerate molecules. These are then converted into 1,3-bisphosphoglycerate and finally into glyceraldehyde-3-phosphate (G3P), using the ATP and NADPH made in the light phase.

Some of that G3P goes into making glucose and other carbohydrates, while the rest is recycled to regenerate ribulose bisphosphate, closing the Calvin cycle. This process is essential to the plant because it supplies the compounds it needs to grow and develop.

What's the point of the dark phase?

From there, the products of the dark phase feed a series of chemical reactions that build starch, carbohydrates and other organic compounds essential to the plant's structure and function. For example, sugars are sent down to the roots, where they're stored as starch, an energy reserve.

In short, the dark phase of photosynthesis is complex but vital, because it lets plants turn CO2 from the air into the organic compounds they use to grow and develop.

Photosynthesis: essential for life on Earth

Photosynthesis is fundamental to life on Earth, not just for plants but for animals and fungi too.

It's responsible for the oxygen in Earth's atmosphere and is by far the main way living things turn mineral elements into organic matter. Without photosynthesis, life on this planet as we know it wouldn't be possible.

Why photosynthesis matters for life on Earth

Photosynthesis in land plants, algae and certain bacteria is crucial to keeping the atmosphere rich in oxygen. That oxygen is vital for the breathing of nearly every living thing, humans included.

Photosynthesis is also what builds the base of the food chain. Plants and other photosynthetic organisms make sugars and other organic compounds that feed herbivores, which in turn feed carnivores.