Photosynthesis is to Chloroplasts as Cellular Respiration is to

Photosynthesis is to Chloroplasts as Cellular Respiration is to

Photosynthesis and cellular respiration are ii 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 (Chalf dozenH12Osix), 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?

Near plants are autotrophs, pregnant they make their own food. Photosynthesis is the process these plants use to synthesize sugar molecules from sunlight, h2o, and carbon dioxide. During photosynthesis, plants release oxygen equally a waste matter product.

Here is the bones chemic formula for photosynthesis:

+ 6H2O + Sunlight → C6H12Ohalf-dozen
+ 6O2

Photosynthesis has 2 chief series of reactions, which can (but don’t have to) accept place simultaneously: light-dependent reactions and light-independent reactions.

See how the 3D models in Visible Biology tin help students sympathize the basics of photosynthesis:

i. 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 serial of reactions is to convert photons, or light energy (from the sun), into chemical energy. During the low-cal-dependent reactions, the plant absorbs sunlight, breaks downward 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 production.

Photosynthesis: light-dependent reactions


Convert light energy into chemic free energy


Chloroplasts – thylakoid membranes


Sunlight, HtwoO, NADP+, ADP


(waste production)

The light-dependent reactions of photosynthesis become a niggling something like this. Sunlight hits a chlorophyll molecule in ane of the thylakoid membranes, exciting an electron, which leaves the chlorophyll molecule. Carrier proteins move this electron forth the thylakoid membrane.

Chlorophyll is a pigment—a lite-capturing molecule—that absorbs light from the sun. Chlorophyll can be constitute in structures chosen thylakoid membranes, which are located within 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 constitute cells. Image from Visible Biology

The chlorophyll molecule—specifically chlorophyll a, in this case—is function of a complex called photosystem II. When the free energy from sunlight excites an electron in chlorophyll a enough for it to get out and be passed on to another molecule, that departure leaves an “energy vacuum” in its wake. This vacuum is powerful enough that photosystem 2 splits a water molecule to restore the electron. Humans can’t split up h2o in a lab the same way plants can, and so the light-dependent reactions of photosynthesis are truly remarkable and unique!


Photosystem Ii (highlighted in blue), water molecules existence broken down, and electrons moving along to photosystem I. Prototype from Visible Biological science.

Plants primarily become water from the soil. In vascular plants, tissue called xylem brings h2o from the roots to the leaves (the main site of photosynthesis).

Vascular tissues are located at the center of dicot roots. Paradigm from Visible Biological science.

Water molecules are composed of 2 hydrogen atoms and one oxygen atom. After a water molecule is broken downward, its hydrogen ions are used to create ATP. These hydrogen ions assistance an enzyme called ATP synthase add another phosphate grouping to ADP (adenosine diphosphate).

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The oxygen atom from each disassembled h2o molecule joins upwardly with another to course O2
(oxygen gas), which is released equally a waste production through openings in the leaves called stomata.

Stomata tin be institute on the upper and lower surfaces of monocot leaves. Image from Visible Biology.

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


Photosystem I is highlighted in blue. Oxygen from the cleaved-downward water molecules is released as Oii. With the assistance of the hydrogen ions and electrons, ADP will be converted to ADP and NADPH will be built. Image from Visible Biology.

Once the light-dependent reactions are consummate, energy from sunlight has successfully been converted into chemical free energy, which will exist used in the next serial 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 serial of reactions that don’t require light free energy from the sun (photons). Therefore, they’re widely referred to equally light-independent reactions or the Calvin wheel. (The old term “dark” reactions can exist misleading, since lite-independent reactions don’t have to take identify in the absence of light, or at nighttime—they just aren’t fueled past lite similar the light-dependent reactions.)

Photosynthesis: calorie-free-contained reactions


Use stored chemical energy to “fix” COii
and create a product that can be converted into glucose


Chloroplasts – stroma





G3P (two G3P can be made into C6H12Ovi


The goal of the light-independent reactions is to “fix” carbon from carbon dioxide into a form that tin 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 v carbon atoms. The result is a 6-carbon intermediate (carboxylated RuBP), which is broken downward into two 3-carbon molecules (3-phosphoglycerate).

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

2 molecules of G3P are used to brand one molecule of glucose (which, if you recall, has six carbon atoms). Typically, one “instance” of the Calvin cycle uses vi molecules of carbon dioxide at in one case, 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, then the cycle tin can keep.

What is cellular respiration?

Humans, similar other animals, are heterotrophs. We can’t brand our ain food via photosynthesis, so nosotros have to swallow other organisms to proceeds glucose, which powers the process of cellular respiration in our bodies. Cellular respiration is the procedure that breaks down glucose and produces ATP (a form of stored energy that cells use to acquit out essential processes).

Here is the basic chemical formula for cellular respiration:

Chalf-dozenH12O6 + 6O2
→ 6COtwo
+ 6H2O + (approximately) 38 ATP

In organisms that carry out aerobic cellular respiration—that is, cellular respiration that uses oxygen—there are four chief steps involved in breaking downwardly glucose to produce ATP: glycolysis, pyruvate oxidation, the citric acrid bicycle (Krebs wheel), and oxidative phosphorylation. Nosotros have a more detailed blog post dedicated to
cellular respiration, but we’ll as well apace go over each step of aerobic cellular respiration in the following sections.

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1. Glycolysis

The first phase of cellular respiration, glycolysis, is the initial breakdown of glucose into pyruvate—one molecule of glucose produces ii 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 outset place. What’s really important about glycolysis in aerobic respiration is that it provides the cloth needed for the adjacent step of cellular respiration: the citric acid cycle, also known as the Krebs cycle.

Cellular respiration: glycolysis


Intermission down glucose into pyruvic acid (pyruvate)


Cytoplasm of cell


Chalf-dozenH12Ovi, ATP


ATP, Pyruvate (CthreeHfourO3), NADH


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

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

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

The cytoplasm contains cytosol, the jelly-similar substance filling the within of the cell. Epitome from Visible Biology.

2. Pyruvate oxidation

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

3. Citric acid cycle (Krebs bicycle)

The citric acid cycle takes place twice per molecule of glucose that was broken down in the previous step—1 “turn” of the citric acid bike occurs for each molecule of acetyl-CoA.

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

Cellular respiration: citric acid cycle


Capture energy from chemical reactions, produce a footling bit of ATP


Mitochondria – matrix


two Acetyl-CoA


(energy carriers), CO2


The results of the citric acid wheel: two ATP, half dozen NADH, 2 FADH2, and 4 COii
(waste product). Epitome from Visible Biology.

iv. Oxidative phosphorylation

Oxidative phosphorylation, which includes the electron transport concatenation 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” characterization in this image refers to the outer membrane. The inner membrane is the yellow structure surrounding the matrix. Bank check out more AR models on the Biology Larn Site.

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

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Cellular respiration: oxidative phosphorylation


Use stored energy from the citric acid cycle to power ATP synthase and generate ATP


Mitochondria – inner membrane



Oxygen—it is the terminal acceptor for “spent” electrons


Lots of ATP, H2O (waste production)


The results of oxidative phosphorylation. And so much ATP, and also some water (waste production)! Image from Visible Biology.

How are photosynthesis and cellular respiration connected?

When I remember about the connections between photosynthesis and cellular respiration, I can’t help merely kickoff singing “Circle of Life” from The Lion Rex in my caput. Why? Because the products of photosynthesis are required for cellular respiration, and the products of cellular respiration tin can be used to power photosynthesis.

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




+ 6H



+ Sunlight →







+ 6O


Cellular Respiration:







+ 6O




+ 6H



+ (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 nutrient that plants brand (glucose) and the waste product from producing that food (Otwo

) requite animals like the states 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 style, plants and their delicious glucose are at the root of our

food spider web
. In render, humans and other organisms that behave out aerobic respiration put the waste products from this process (mainly COtwo) back into the atmosphere.

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

Fun fact! Photosynthesis past

microorganisms called cyanobacteria

is what put oxygen into the World’south atmosphere in the get-go identify. These organisms first produced oxygen between ii.7 and ii.8 billion years agone, and oxygen became a significant portion of the temper by around 2.45 billion years ago. This paved the way for oxygen-breathing animals like us to evolve afterward.

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


Cellular Respiration (Aerobic)

Chemical equation

+ 6H2O → CviH12Ohalf dozen
+ 6O2

+ 6Oii
→ 6CO2
+ 6HtwoO + (approximately) 38 ATP


Carbon dioxide, water, sunlight

Glucose, Oxygen


1. Lite-dependent reactions

2. Light-independent reactions (Calvin cycle)
1. Glycolysis

2. Pyruvate oxidation
three. Citric acid wheel

four. Oxidative phosphorylation


Glucose, oxygen

ATP, carbon dioxide, water

Associated organelle



Office for the organism

Use light, water, and carbon dioxide to create food for the organism in the grade of sugar (glucose)

Utilise glucose to brand a form of free energy the organism can use in cellular processes (ATP)

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Additional Sources:

  • The Amoeba Sisters: Photosynthesis and the Teeny Tiny Paint Pancakes
  • Crash Class Biological science: 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

Photosynthesis is to Chloroplasts as Cellular Respiration is to