When we're talking about photosynthesis and cellular respiration, we can't just separate them. They are different, but still connected through an important relationship; also, both of them are crucial biochemical processes for every life on Earth.

Before we jump right into their relationship,, maybe it's better for us to learn first — what is photosynthesis? And what is cellular respiration?

What is photosynthesis?

Photo: Jeremy Zero (Unsplash)

Have you ever wondered how plants eat their food? Here's the answer: photosynthesis. Plants make their own food; they use sunlight, water, and carbon dioxide to synthesize sugar molecules and release oxygen into the air. As a result, they can generate energy sources and store them as food.

However, easy as it sounds, plants actually need to go through several steps of the process. First off, they take carbon dioxide and water in from the air and soil. When the water is oxidized in the plant cell, it loses electrons — and so carbon dioxide is reduced as well.

The water is then transformed into oxygen; meanwhile, as for the carbon dioxide, it will turn into glucose — which is resulted in oxygen released into the air. After those long steps, finally comes the main outcome: the food, which is in the form of energy sources stored in glucose molecules.

Photosynthesis has two kinds of reactions, namely light-dependent reactions and light-independent reactions. So what are they, exactly? Take a look below!

1. Light-dependent reactions

Process of light dependent reactions.
Photo: Wikipedia

As the name suggests, this type of reaction requires a steady amount of sunlight to perform the process. Taking place in the thylakoid membrane, the process absorbs energy sources from light waves and converts them into ATP and NADPH.

2. Light-independent reactions (Calvin cycle)

Process of light-independent reactions.
Photo: Lumen Learning

This reaction takes place in the stroma, which is located between the thylakoid and chloroplast membrane. This reaction is the exact opposite of light-dependent reactions; it doesn't need to be exposed to the sunlight to operate.

In this stage, the energy from ATP and NADPH will then be used to build carbohydrate molecules from carbon dioxide — such as glucose.

What is cellular respiration?

Different types of green plants.
Photo: Jackie DiLorenzo (Unsplash)

After the plants get their glucose, they will use this stored energy later to transfer food — that process is called cellular respiration. Scientifically, it is a process that breaks down glucose and produces ATP — stored energy used to carry out crucial processes.

As for organisms that perform aerobic cellular respiration, they need to go through three main steps to produce ATP:

1. Glycolysis

Glycolysis breaks down glucose into pyruvate. To make it simpler, one molecule of glucose can generate two molecules of pyruvate. However, during this process, there will not be much ATP produced. During its aerobic respiration, glycolysis will provide materials required for the next step: ATP, pyruvate, and NADH.

2. Citric acid cycle (Krebs cycle)

Here, pyruvate will lose its carboxyl groups and join coenzyme A to produce acetyl-CoA. During this process, the carbon molecules will be removed; instead, they will be released as carbon dioxide.

The acetyl-CoA will go through chemical reactions and produce energy, which later will be captured in NADH and FADH2 — the energy carriers. Moreover, this process also has ATP and CO2 as the output.

3. Electron transport chain (ETC)

After receiving NADH and FADH2, this process will make use of the high-energy ions contained in those two molecules to pump hydrogen ions across the mitochondrion's inner membrane into the outer compartment. For that reason, one side of the membrane will contain more positively charged ions.

While the ions restore equilibrium, they will pass through ATP synthase — which turns ADP into ATP. Now that the process is done, ETC will produce two things: lots of ATP and water!

Now... what's the relationship between the two?

Instead of comparing those two processes as two different things, we can say that they both complement each other. Actually, if you see their formulas, it's basically just reversed:

Photosynthesis and cellular respiration equation.
Photo: Cincinnati Public Schools

While photosynthesis produces products required for cellular respiration, the case is also similar to cellular respiration; the products of cellular respiration can be used to start photosynthesis.

Also, both of them play a huge role in keeping the carbon cycle goes on. While cellular respiration produces carbon dioxide for the environment, photosynthesis pulls it out of the atmosphere.

During photosynthesis and cellular respiration, there will be a constant exchange of carbon dioxide and oxygen — which of course, helps our environment healthy by keeping the levels of oxygen and carbon dioxide stable.

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