Building It Up and Breaking It Down: Photosynthesis vs. Cellular Respiration

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Photosynthesis and cellular respiration are two biochemical processes that are essential to most life on Earth. Both of these processes involve multiple complex steps and many of the same molecules—oxygen (O₂), carbon dioxide (CO₂), water (H₂O), glucose (C₆H₁₂O₆), and adenosine triphosphate (ATP).

Today, we’ll briefly go over the main steps of photosynthesis and cellular respiration. We’ll explore their similarities and differences, and we'll also discuss how they interact with each other to create an “energy cycle” in living organisms.

What is photosynthesis?

Most plants are autotrophs, meaning they make their own food. Photosynthesis is the process these plants use to synthesize sugar molecules from sunlight, water, and carbon dioxide. During photosynthesis, plants release oxygen as a waste product.

Here is the basic chemical formula for photosynthesis: 

6CO₂ + 6H₂O + Sunlight → C₆H₁₂O₆ + 6O₂

Photosynthesis has two main series of reactions, which can (but don’t have to) take place simultaneously: light-dependent reactions and light-independent reactions.

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1. Light-dependent reactions

The light-dependent reactions make up the first few steps of photosynthesis. These reactions occur in the thylakoid membranes of the chloroplasts within plant cells. The goal of this series of reactions is to convert photons, or light energy (from the sun), into chemical energy. During the light-dependent reactions, the plant absorbs sunlight, breaks down water molecules, assembles the energy-storing molecules ATP and NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate, or NADP+), and releases oxygen as a waste product.

Photosynthesis: light-dependent reactions

The light-dependent reactions of photosynthesis go a little something like this. Sunlight hits a chlorophyll molecule in one of the thylakoid membranes, exciting an electron, which leaves the chlorophyll molecule. Carrier proteins move this electron along the thylakoid membrane. 

Chlorophyll is a pigment—a light-capturing molecule—that absorbs light from the sun. Chlorophyll can be found in structures called thylakoid membranes, which are located inside a plant cell’s chloroplasts. See those little stacks inside the chloroplast? Those are stacks of thylakoids, called grana (sing. granum).

The thylakoid membranes are located within the chloroplasts of plant cells. Image from VB Suite. 

The chlorophyll molecule—specifically chlorophyll a, in this case—is part of a complex called photosystem II. When the energy from sunlight excites an electron in chlorophyll a enough for it to leave and be passed on to another molecule, that departure leaves an “energy vacuum” in its wake. This vacuum is powerful enough that photosystem II splits a water molecule to restore the electron. Humans can't split water in a lab the same way plants can, so the light-dependent reactions of photosynthesis are truly remarkable and unique!

Photosystem II (highlighted in blue), water molecules being broken down, and electrons moving along to photosystem I. Image from VB Suite. 

Plants primarily get water from the soil. In vascular plants, tissue called xylem brings water from the roots to the leaves (the main site of photosynthesis).

Vascular tissues are located at the center of dicot roots. Image from VB Suite. 

Water molecules are composed of two hydrogen atoms and one oxygen atom. After a water molecule is broken down, its hydrogen ions are used to create ATP. These hydrogen ions help an enzyme called ATP synthase add another phosphate group to ADP (adenosine diphosphate). 

The oxygen atom from each disassembled water molecule joins up with another to form O₂ (oxygen gas), which is released as a waste product through openings in the leaves called stomata.

Stomata can be found on the upper and lower surfaces of monocot leaves. Image from VB Suite. 

The electron that has been moving along the thylakoid membrane eventually arrives at another chlorophyll-containing protein complex called photosystem I. At this point, it joins forces with another excited electron. An enzyme called NADP+ uses these electrons and a passing hydrogen ion to build the energy-carrying molecule NADPH. 

Photosystem I is highlighted in blue. Oxygen from the broken-down water molecules is released as O₂. With the help of the hydrogen ions and electrons, ADP will be converted to ADP and NADPH will be built. Image from VB Suite. 

Once the light-dependent reactions are complete, energy from sunlight has successfully been converted into chemical energy, which will be used in the next series of steps in photosynthesis—the light-independent reactions—to assemble sugar molecules.

2. Light-independent reactions (aka the Calvin cycle)

The next phase of photosynthesis is a series of reactions that don’t require light energy from the sun (photons). Therefore, they’re widely referred to as light-independent reactions or the Calvin cycle. (The old term “dark” reactions can be misleading, since light-independent reactions don’t have to take place in the absence of light, or at night—they just aren’t fueled by light like the light-dependent reactions.)

Photosynthesis: light-independent reactions

The goal of the light-independent reactions is to “fix” carbon from carbon dioxide into a form that can be used to build carbohydrates (sugars), such as glucose. 

An enzyme called RuBisCo combines a molecule of carbon dioxide with a molecule called ribulose biphosphate (RuBP), which contains five carbon atoms. The result is a 6-carbon intermediate (carboxylated RuBP), which is broken down into two 3-carbon molecules (3-phosphoglycerate).

With the help of ATP and NADPH, each 3-phosphoglycerate molecule gets a hydrogen atom, becoming glyceraldehyde-3-phosphate, or G3P.

Two molecules of G3P are used to make one molecule of glucose (which, if you recall, has six carbon atoms). Typically, one “instance” of the Calvin cycle uses six molecules of carbon dioxide at once, meaning that twelve G3P molecules are produced. Two of these are used to produce a molecule of glucose and the rest are recycled back into RuBP, so the cycle can continue.

What is cellular respiration? 

Humans, like other animals, are heterotrophs. We can’t make our own food via photosynthesis, so we have to eat other organisms to gain glucose, which powers the process of cellular respiration in our bodies. Cellular respiration is the process that breaks down glucose and produces ATP (a form of stored energy that cells use to carry out essential processes). 

Here is the basic chemical formula for cellular respiration:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + (approximately) 38 ATP

In organisms that carry out aerobic cellular respiration—that is, cellular respiration that uses oxygen—there are four main steps involved in breaking down glucose to produce ATP: glycolysis, pyruvate oxidation, the citric acid cycle (Krebs cycle), and oxidative phosphorylation. We have a more detailed blog post dedicated to cellular respiration, but we’ll also quickly go over each step of aerobic cellular respiration in the following sections.

1. Glycolysis

The first phase of cellular respiration, glycolysis, is the initial breakdown of glucose into pyruvate—one molecule of glucose produces two molecules of pyruvate. On its own, glycolysis doesn’t generate very much ATP. In fact, two ATP molecules are required to begin glycolysis in the first place. What’s really important about glycolysis in aerobic respiration is that it provides the material needed for the next step of cellular respiration: the citric acid cycle, also known as the Krebs cycle.

Cellular respiration: glycolysis

The results of glycolysis: 4 ATP, 2 pyruvate molecules, and 2 NADH. Image from VB Suite. 

Note: Since glycolysis doesn’t require oxygen, it’s also part of anaerobic cellular respiration. You can read more about metabolism in the absence of oxygen in this chapter from OpenStax Biology (2e). 

Glycolysis takes place in the cytoplasm of animal and plant cells, whereas the subsequent steps of cellular respiration take place in the mitochondria.

The cytoplasm contains cytosol, the jelly-like substance filling the inside of the cell. Image from VB Suite. 

2. Pyruvate oxidation

Before the citric acid cycle can begin in earnest, the pyruvate molecules produced during glycolysis lose their carboxyl groups and combine with coenzyme A to form acetyl-CoA. The carbon molecules that are removed during this process are released as carbon dioxide.

3. Citric acid cycle (Krebs cycle)

The citric acid cycle takes place twice per molecule of glucose that was broken down in the previous step—one “turn” of the citric acid cycle occurs for each molecule of acetyl-CoA.

During each of these two turns, the molecule of acetyl-CoA goes through a series of chemical reactions. The energy from these reactions (in the form of electrons) is captured in the “energy carrier” molecules NADH and FADH₂. Two more molecules of carbon dioxide and another molecule of ATP are also produced.

Cellular respiration: citric acid cycle

The results of the citric acid cycle: 2 ATP, 6 NADH, 2 FADH₂, and 4 CO₂ (waste product). Image from VB Suite. 

4. Oxidative phosphorylation

Oxidative phosphorylation, which includes the electron transport chain and chemiosmosis, is the part of aerobic cellular respiration that produces most of the ATP. The electron transport chain uses high-energy electrons from FADH₂ and NADH to pump hydrogen ions (H+) across the inner membrane of the mitochondrion, into the outer compartment. 

Mitochondria. The “membrane” label in this image refers to the outer membrane. The inner membrane is the yellow structure surrounding the matrix. Check out more AR models on the Biology Learn Site.

As a result of the electron transport chain, there are more positively charged ions on one side of the membrane than the other. As these ions travel back across the membrane to restore equilibrium, they pass through (and “power”) an enzyme called ATP synthase, which turns molecules of ADP into ATP by adding a third phosphate group. 

Cellular respiration: oxidative phosphorylation

The results of oxidative phosphorylation. So much ATP, and also some water (waste product)! Image from VB Suite. 

How are photosynthesis and cellular respiration connected? 

When I think about the connections between photosynthesis and cellular respiration, I can’t help but start singing “Circle of Life” from The Lion King in my head. Why? Because the products of photosynthesis are required for cellular respiration, and the products of cellular respiration can be used to power photosynthesis.

Putting the chemical formulas for these processes side-by-side shows this quite clearly:

Photosynthesis: 6CO₂ + 6H₂O + Sunlight → C₆H₁₂O₆ + 6O₂

Cellular Respiration: C₆H₁₂O₂ + 6O₂ → 6CO₂ + 6H₂O + (approximately) 38* ATP

*The number of ATP molecules produced can vary. 38 ATP is the theoretical maximum yield for the metabolism of one molecule of glucose.

The food that plants make (glucose) and the waste product from producing that food (O₂) give animals like us the materials we need to carry out aerobic cellular respiration. We breathe in the oxygen from the air and either eat plants or other animals—either way, plants and their delicious glucose are at the root of our food web. In return, humans and other organisms that carry out aerobic respiration put the waste products from this process (mainly CO₂) back into the atmosphere. 

Plants carry out both photosynthesis and cellular respiration. They make their own food, and then break down those glucose molecules later, generating ATP to power their cellular processes. 

Fun fact! Photosynthesis by microorganisms called cyanobacteria is what put oxygen into the Earth’s atmosphere in the first place. These organisms first produced oxygen between 2.7 and 2.8 billion years ago, and oxygen became a significant portion of the atmosphere by around 2.45 billion years ago. This paved the way for oxygen-breathing animals like us to evolve later.

Before we go, here’s a handy chart comparing photosynthesis and aerobic cellular respiration. Happy studying! 

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Additional Sources: 

• The Amoeba Sisters: Photosynthesis and the Teeny Tiny Pigment Pancakes
• Crash Course Biology: Photosynthesis
• Khan Academy: Cellular Respiration Review
• OpenStax Biology 2e: Metabolism without Oxygen
• Principles of Biology: An Overview of Cellular Respiration
• TEDed: Nature's Smallest Factory