Plant Photosynthesis Explained: Sun, Water, and Carbon Dioxide (CO2) Key Requirements for Plant Growth and Energy


Plant photosynthesis is the process of how sunlight, water, and carbon dioxide converge to create the energy that plants need. From food production to energy transfer. Plants are the primary carriers of photosynthesis. Still, other autotrophs can perform this process.

All life on Earth depends on photosynthesis, and organisms can be grouped based on their direct or indirect requirements for this process. Among these, plants, algae, phytoplankton, and other autotrophic organisms directly engage in photosynthesis to generate their nourishment and support various biological activities.

What Is Photosynthesis?

Photosynthesis is the process of converting light energy into chemical energy (food). The term is of Greek origin, derived from the words phos, which means light, and synthesis, which means combining.

Plants are the primary carriers of photosynthesis. Still, other autotrophs can perform this process. For those wondering, autotrophic organisms are creatures that can make their food using sunlight and inorganic nutrients according to the science textbook Stream Ecology by professor J. David Allan.

Aside from green plants, algae, and some microorganisms, like cyanobacteria and phytoplankton, can perform this biological process.

As you might have guessed, photosynthesis isn’t a one-step procedure. It’s a cycle consisting of four primary steps. These are light absorption, electron transfer, ATP production, and carbon fixation.

The first three reactions are light-dependent. The goal is to produce enough energy to synthesize the carbohydrates. Once that happens, the plant starts fixing carbon to make the food. Unlike the former reactions, this step is light-independent.

All these chemical processes occur in the chloroplast, an organelle found in the leaves. It contains pigments that help absorb light to kick-start these reactions.

What Is the Photosynthesis Equation?

As you know, plants use sunlight to make food. Still, this isn’t the only ingredient necessary for photosynthesis. Water and carbon dioxide go into the mix to create carbohydrates (food). Not only that, but oxygen is also released as a by-product.

To sum up all these steps, scientists represent photosynthesis with the following equation:

  • 6CO2 + 6H2O + light energy → C6H12O6 + 6O2

In some cases, light energy is represented by the arrow without writing it. Regardless, you’ll always find six carbon dioxide molecules as well as water in the reactants. Once the chlorophyll pigment in the chloroplasts absorbs enough sunlight, photosynthesis begins. 

As a result, the reactants undergo a series of steps to produce one sugar molecule and six oxygen molecules.

What Do Plants Need for Photosynthesis?

Besides chlorophyll, three other things are needed for photosynthesis. These are light to fuel the reactions, water to provide electrons, and carbon dioxide to make sugars.

Let’s discuss photosynthesis ingredients in further detail!

  1. Light

Before we jump into the role of light, let’s discuss its characteristics. As you might know, light is electromagnetic radiation. This process happens when atoms absorb energy and heat up. As a result, electrons jump from their normal energy level to a higher one.

The problem is that this excess energy makes the molecule unstable. To return to their original state, electrons lose this energy in the form of photons (light). Why is this important?

Well, that’s because the emitted light produced from this process has a dual nature— a wave-like and a particle-like character. The former helps radiation travel through space, while the latter is responsible for exciting electrons.

The question is this: what does sunlight do in photosynthesis?

As you know, chloroplasts contain chlorophyll. This green pigment is responsible for absorbing sunlight. Still, other pigments contribute to this process.

Once absorption takes place, electrons start jumping to a higher energy level and bounce back to their original state, releasing energy.

Now, this process can happen in several ways:

  1. Electrons can lose the energy as heat or re-emit it as light. These pathways are a form of wasted energy—the plant doesn’t make use of them.
  2. Both the energy and the excited electrons transfer from the pigment to a neighboring molecule. And that brings us to the next essential ingredient—water.
  3. Water

Following light absorption, pigments become excited, releasing photons (energy). The question is this: how do plants make use of the photons?

That’s when water comes to the rescue. It helps solve two problems. For one, it replaces the electrons lost by plant pigments. Additionally, it supplies the plant with hydrogen ions. 

These ions, alongside the excited electrons, enter an energy transfer chain, producing ATP and NADPH molecules (chemical energy). As you might have guessed, these molecules further react to produce sugars. 

This process takes place in two reaction centers in plant pigments. These are known as photosystem I and photosystem II.

Scientists Govindjee and Rajni Govindjee from University of Illinois describe this flow of energy using the Z-scheme. Here’s a brief explanation of its steps:

  1. P680, a special chlorophyll in photosystem II (PSII), absorbs light and becomes excited.
  2. To become stable, P680 loses an electron (negative charge) and becomes P680+.
  3. Water gets split through photolysis, providing electrons to P680+.
  4. The energized electron released from PSII is passed along a series of protein carriers like a relay race.
  5. During this trip, positive hydrogen molecules (H+) are pumped across a membrane, creating a proton gradient.
  6. PSI joins the party by grabbing the electron from the relay race, activating another special chlorophyll pigment, P700.
  7. Naturally, P700 loses this extra electron through a series of protein carriers until it reaches the NADP molecule, a co-enzyme.
  8. Combined with the proton released from PSII, NADP becomes NADPH, the molecule that fuels the reactions of building sugars.
  9. The released proton also powers ATP molecule production, providing more energy to the plant.
  10. Carbon Dioxide

Now, you might wonder: why do plants perform all this hassle to make ATP and NADPH molecules? That’s where carbon dioxide gas steps in.

You see, inside the chloroplasts, a mini-factory called the stroma is buzzing with activity. ATP, NADPH, and CO2 enter a cycle to form glucose as the final product.

Unlike light and water reactions, this stage of photosynthesis isn’t light-dependent. For the dark reaction, plant cells require different components. These include:

  • Energy: ATP, a product of the light-dependent reaction, powers certain molecules in the cycle by losing a phosphate group.
  • Reducing agent: NADPH donates electrons and hydrogen to bind with CO2.
  • Enzymes: Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is the primary enzyme in the dark reaction cycle. It facilitates the production of glyceraldehyde 3-phosphate (G3P), a three-carbon sugar used to make glucose.

What Is the Byproduct of Photosynthesis?

Oxygen is the primary by-product of photosynthesis. Based on the science journal Plant Physiology authors Karl-Josef Dietz, Ismail Turkan and Anja Krieger-Liszkay mention during water splitting (photolysis), oxygen molecules are released. The problem is that these species are highly reactive. To become stable, they react with cell proteins, lipids, and more, damaging them.

For that reason, plants get rid of this by-product through two mechanisms: diffusion or respiration.

During electron-transfer reactions, carrier proteins within the chloroplast membrane snag the oxygen. They then escort these molecules toward the cell wall. Through simple diffusion, this gas leaves the plant and exits through the stomata.

Still, not all oxygen is kicked outside the cells. Plants make use of this gas during the respiration process.

Why Is Photosynthesis Important for Plants?

Several reasons make photosynthesis an essential process for plants. For one, this magical process produces carbohydrates, an energy source for plants. Not only that, but many biological products are synthesized during photosynthesis.

And if that wasn’t enough, this process helps fight climate change. This directly benefits green life, as it ensures their growing conditions remain unchanged.

What Is Involved in the Photosynthesis Process?

Aside from the three essential ingredients—light, water, and carbon dioxide—plants require other components to perform photosynthesis. These include chloroplast and pigments. Let’s discuss them in further detail!

  1. Chloroplast

Sure, chloroplasts contain pigments, which are essential for light absorption and photosynthesis. Still, there’s more to these organelles than just the pigments.

Here’s a brief explanation of chloroplast components and their role in this biological process:

  • Envelop: It consists of an inner and outer lipid bilayer. It’s semi-permeable, regulating the transportation of several molecules, including glucose, oxygen, and carbon dioxide.
  • Stroma: It’s a protein-rich fluid that surrounds all the internal components of the chloroplast. Contains enzymes that carry out the dark reactions of photosynthesis.
  • Thylakoids: These are chlorophyll-carrying membranous sacs. It carries out the light-dependent reactions.
  • Grana: It’s a stack of 10-20 thylakoids and makes the site where light energy is converted to chemical energy.
  1. Pigments

While plant pigments can absorb sunlight, they’re not equal. Each color can absorb a particular wavelength. As you know, visible light is a spectrum that consists of seven colors—remember that glass prism experiment in high school?

Each color has a particular wavelength. Sure, visible light ranges from 400 nm to 700 nm. Still, no pigment can cover the entire spectrum—it only absorbs a specific range.

The question is this: what pigments are involved in photosynthesis? Generally, three primary pigments take part in this process. These include:

  • Carotenoids: These are red, orange, and yellow pigments. They exist at the lipid membrane and absorb between 400nm and 540 nm.
  • Chlorophyll b: This green pigment absorbs wavelengths between 430 nm and 642 nm. Its primary function is to pass light to chlorophyll a.
  • Chlorophyll a: The most abundant pigment in plants. Its range is between 372 nm and 642 nm.

How Do Plants Get Energy From Photosynthesis?

Photosynthesis converts sunlight, water, and carbon dioxide into sugars, like glucose. These molecules serve as primary fuel sources. They’re the powerhouse, driving essential processes like respiration, growth, and reproduction.

Aside from that, plants convert glucose into starch to store it as food. Since only chlorophyll-containing plants can perform photosynthesis, other cells are completely dependent on stored carbohydrates to function.

What’s more, during photosynthesis, plants produce ATP and NADPH molecules. Both components serve as energy carriers, playing many roles in various biological processes according to the science journal Nature Communications (Article number: 3238) by Shey-Li Lim, Chia Pao Voon, Xiaoqian Guan, Per Gardeström and Boon Leong Lim.

What Factors Affect Plant Photosynthesis?

Before a plant can produce food, it requires three primary components: water, light, and carbon dioxide. Still, that doesn’t mean they’ll carry out the process efficiently when these essentials are present. 

That’s because several factors, like light intensity, water and carbon dioxide concentration, as well as temperature, affect the photosynthetic rate. Here’s a brief explanation of each:

  1. Light Intensity

Since radiation is responsible for exciting electrons, it’s no surprise that light intensity impacts photosynthesis. 

For those wondering, the former measures the energy transmitted per unit area. The brighter the light is, the more photons it carries. Naturally, this excites more electrons, increasing the rate of photosynthesis.

That said, too much sunlight can damage the leaves, burning them. The trick is to provide optimal light conditions for each species.

  1. Temperature

This one might come as a surprise. After all, temperature isn’t a component of photosynthesis. Interestingly, that’s far from the truth. 

As you know, several enzymes play a role in light and dark reactions. The problem is that these chemicals require optimal temperatures to work. Cold conditions slow down enzyme movements. 

As the temperature increases gradually, molecule movement, or collision, increases. As a result, enzymes react with substrates quickly. Of course, this facilitates photosynthesis.

Still, high temperatures above 68ºF can be too much of a good thing. That’s because enzymes are proteins. Consequently, they’re heat-sensitive. Temperatures exceeding 104ºF can damage these molecules and deactivate them.

  1. Carbon Dioxide and Water Concentration

Carbon dioxide and water are raw materials for building plant food. Consequently, as the concentration of CO2 increases, the photosynthetic rate increases.

According to Erik S. Runkle from Michigan State University this effect continues until CO2 reaches a saturating concentration of around 1000 ppm. At this point, the rate won’t change.

Likewise, water availability increases photosynthesis. Not only does limited water slow down light-dependent reactions, but it can also affect CO2 intake. That’s because plants close the stomata to reduce water vapor loss during drought.

What Energy Is Created in the Photosynthesis Process?

According to the law of conservation of energy, photosynthesis doesn’t produce a new form of energy. That’s because it’s conserved—you can’t create or destroy energy. 

During photosynthesis, solar energy is harvested and converted to glucose, a form of chemical energy. 

Plants metabolize this sugar molecule through glycolysis, oxidizing it to produce ATP and pyruvate. The latter molecule enters another cycle in the mitochondria, producing more ATP.

Not to mention, light reactions also generate this energy-carrying molecule—all of which are essential for many biological processes.

What are C3 and C4 Photosynthesis?

As mentioned earlier, carbon dioxide enters a dark reaction cycle to produce sugars. This process is known as the Clavin cycle or C3 photosynthesis. It has three primary steps:

  • Carboxylation: CO2 is fixed by carboxylating RuBP, a 5-carbon molecule, to form two molecules of  3-phosphoglycerate (3-PGA). RuBPCO enzyme facilitates this step.
  • Reduction: During this step, 3-PGA is reduced to form glyceraldehyde-3 phosphate (GAP), a 3-carbon molecule used to make glucose. ATP and NADPH provide energy for this reaction.
  • Regeneration: This complex process requires ATP to produce glucose. Other molecules are recycled to produce RuBP, which helps fixate more CO2.

Around 95% of plants use the C3 pathway. Interestingly, some tropical plants contain a special structure in the leaves known as Kranz’s anatomy.

These plants developed a different pathway, C4 photosynthesis, to reduce water vapor loss while letting CO2 in through the stomata.

Unlike C3, the C4 cycle produces a 4-carbon acid—oxaloacetate. This chemical is then reduced, forming malate, which is used to make soluble sugars.

What Is the Respiration Cycle for Photosynthesis?

As you know, photosynthesis releases oxygen as a byproduct. This gas can either exit the plant or be used in respiration.

For those wondering, cellular respiration is the opposite of photosynthesis. Produced sugars are broken down in the presence of oxygen, releasing water, CO2, and ATP. 

This process takes place in the mitochondria and consists of a series of cycles. These are glycolysis, the citric acid cycle (TCA), and oxidative phosphorylation.

Cellular respiration is essential as it converts carbohydrates to energy (ATP), which is necessary for plant growth and other biological functions, including photosynthesis.

How Does Photosynthesis Help All Types of Plants Grow?

Photosynthesis is the most important biological process for plants. It helps all types of plants produce their food and provides energy for growth. Here are some of the benefits of this reaction:

  • Energy production: Sugars are the primary fuel in plants. It plays a role in growth, reproduction, stress response, and more! Without carbohydrates, plants become stunted and eventually wither away.
  • Synthesis of essential components: Aside from fueling, sugars help build structural components like cellulose. Additionally, plant cells can convert carbohydrates and store them as fat or combine them with nitrates to form amino acids (building blocks of proteins).
  • Climate regulation: CO2 gas is responsible for the greenhouse effect, which causes the earth to become warmer. Plants can fight off climate change and protect the environment by filtering excess CO2 from the atmosphere through photosynthesis.

How Does Photosynthesis Help With the Overall Energy Transfer?

Photosynthesis is the primary source of energy for all life forms on Earth. It helps maintain energy transfer in ecosystems. The process starts by converting solar energy into chemical energy.

Plants make use of the latter to grow and reproduce. Herbivores, aka plant eaters, consume these greens. As a result, they transfer the captured solar energy up the food chain. Carnivores then feed on these animals, further transferring the energy.

Who Needs Photosynthesis?

All life on Earth needs photosynthesis. You can classify these organisms into groups based on their direct or indirect needs.

Plants, algae, phytoplankton, and other autotrophic organisms rely on this process directly. They use it to make their food and fuel different biological functions.

Herbivores, carnivores, and omnivores indirectly depend on this process, as it ensures the food web remains stable.

Additionally, decomposers also benefit from photosynthesis indirectly. These microorganisms feed on dead organic matter, returning nutrients to the soil.

Aside from food and energy, living organisms need photosynthesis byproducts for respiration.