Photosynthesis and cellular respiration space two biochemical procedures that are necessary to many life ~ above Earth. Both of these procedures involve multiple facility steps and also many the the very same molecules—oxygen (O2), carbon dioxide (CO2), water (H2O), glucose (C6H12O6), and adenosine tree phosphate (ATP).

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

What is photosynthesis?

Most plants space autotrophs, an interpretation they do their own food. Photosynthesis is the process these plants use to synthesize sugar molecules native sunlight, water, and carbon dioxide. During photosynthesis, plants relax oxygen together a waste product.

Here is the basic chemical formula for photosynthesis:

6CO2 + 6H2O + sunlight → C6H12O6 + 6O2

Photosynthesis has actually two main collection of reactions, which have the right to (but don’t have to) take location simultaneously: light-dependent reactions and also light-independent reactions.

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1. Light-dependent reactions

The light-dependent reactions comprise the first few steps the photosynthesis. These reactions take place in the optical membrane membranes the the chloroplast within plant cells. The goal of this collection of reaction is to convert photons, or light energy (from the sun), right into chemical energy. Throughout the light-dependent reactions, the plant absorbs sunlight, breaks down water molecules, assembles the energy-storing molecules ATP and NADPH (the reduced kind of nicotinamide adenine di nucleotide phosphate, or NADP), and releases oxygen as a waste product.

Photosynthesis: light-dependent reactions


Convert light energy into chemical energy


Chloroplasts - optical membrane membranes


Sunlight, H2O, NADP+, ADP


NADPH, ATP, O2 (waste product)

The light-dependent reactions of photosynthesis go a little something prefer this. Sunlight hits a chlorophyll molecule in among the thylakoid membranes, amazing an electron, which leaves the chlorophyll molecule. Transport proteins move this electron follow me the thylakoid membrane.

Chlorophyll is a pigment—a light-capturing molecule—that absorbs irradiate from the sun. Chlorophyll can be discovered in structures referred to as thylakoid membranes, i beg your pardon are located inside a tree cell’s chloroplasts. Check out those tiny stacks within the chloroplast? Those are stacks of thylakoids, referred to as grana (sing. Granum).

The optical membrane membranes are situated within the chloroplast of plant cells. Image from clearly shows Biology.

The chlorophyll molecule—specifically chlorophyll a, in this case—is component of a facility called photosystem II. As soon as the energy from sunshine excites one electron in chlorophyll a sufficient for it come leave and also be happen on to an additional molecule, that leave leaves one “energy vacuum” in the wake. This vacuum is an effective enough that photosystem II splits a water molecule to reclaim the electron. Human beings can"t break-up water in a laboratory the same method plants can, therefore the light-dependent reactions of photosynthesis space truly remarkable and also unique!

Plants primarily acquire water indigenous the soil. In vascular plants, tissue referred to as xylem bring water from the roots to the pipeline (the main site the photosynthesis).

Vascular tissues are situated at the center of dicot roots. Image from clearly shows Biology.

Water molecules space composed of 2 hydrogen atoms and also one oxygen atom. After ~ a water molecule is damaged down, the hydrogen ion are offered to create ATP. These hydrogen ions help an enzyme referred to as ATP synthase add another phosphate team to ADP (adenosine diphosphate).

The oxygen atom from every disassembled water molecule join up with an additional to type O2 (oxygen gas), i beg your pardon is released as a rubbish product with openings in the leaves dubbed stomata.

Stomata can be discovered on the upper and also lower surfaces of monocot leaves. Picture from clearly shows Biology.

The electron that has been moving along the thylakoid membrane eventually arrives at an additional chlorophyll-containing protein complicated called photosystem I. In ~ this point, the joins forces with another excited electron. An enzyme dubbed NADP+ offers these electrons and also a passing hydrogen ion to construct the energy-carrying molecule NADPH.

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

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

The next phase that photosynthesis is a series of reactions the don’t require light power from the sunlight (photons). Therefore, castle widely described as light-independent reaction or the Calvin cycle. (The old term “dark” reactions deserve to be misleading, since light-independent reactions don’t need to take location in the absence of light, or at night—they simply aren’t fueled by light favor the light-dependent reactions.)

Photosynthesis: light-independent reactions


Use save on computer chemical power to “fix” CO2 and also create a product that deserve to be converted into glucose


Chloroplasts - stroma




NADP+, ADP, G3P (two G3P have the right to be made into C6H12O6)

The goal of the light-independent reaction is to “fix” carbon indigenous carbon dioxide into a form that can be supplied to construct carbohydrates (sugars), such as glucose.

An enzyme referred to as RuBisCo combine a molecule that carbon dioxide through a molecule called ribulose biphosphate (RubP), which contains five carbon atoms. The an outcome is a 6-carbon molecule, i m sorry is damaged down right into two 3-carbon molecules (3-phosphoglycerate).

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

Two molecules of G3P are offered to do one molecule that glucose (which, if girlfriend recall, has six carbon atoms). Typically, one “instance” of the Calvin cycle offers six molecule of carbon dioxide at once, meaning that twelve G3P molecules are produced. Two of this are provided to produce a molecule of glucose and the rest are recycled earlier into RubP, for this reason the cycle deserve to continue.

What is to move respiration?

Humans, like various other animals, room heterotrophs. We can’t make our very own food via photosynthesis, so we have to eat various other organisms to obtain glucose, which strength the process of cellular respiration in ours bodies. To move respiration is the procedure that breaks down glucose and produces ATP (a type of stored energy that cells use to lug out crucial processes).

Here is the basic chemical formula because that cellular respiration:

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

In biology that bring out aerobic cellular respiration—that is, to move respiration that supplies oxygen—there room three main steps involved in breaking under glucose to create ATP: glycolysis, the citric acid cycle (Krebs cycle) and also the electron transport chain (ETC). We have a much more detailed blog post devoted to cellular respiration, but we’ll likewise quickly walk over each action of aerobic moving respiration in the adhering to sections.

1. Glycolysis

The very first phase of to move respiration, glycolysis, is the initial failure of glucose into pyruvate—one molecule of glucose produces 2 molecules the pyruvate. ~ above its own, glycolysis doesn’t generate very much ATP. In fact, 2 ATP molecule are forced to begin glycolysis in the first place. What’s really important around glycolysis in aerobic respiration is the it offers the product needed because that the next step of moving respiration: the citric acid cycle, likewise known together the Krebs cycle.

Cellular respiration: glycolysis


Break down glucose right into pyruvic acid (pyruvate)


Cytoplasm the cell


C6H12O6, ATP


ATP, Pyruvate (C3H4O3), NADH

Note: due to the fact that glycolysis doesn’t require oxygen, it’s also part of anaerobic moving respiration. You deserve to read an ext about line in the absence of oxygen in this thing from OpenStax biologic (2e).

Glycolysis takes place in the cytoplasm of animal and plant cells, conversely, the subsequent steps of to move respiration take ar in the mitochondria.

The cytoplasm is the jelly-like substance pour it until it is full the inside of the cell. Photo from clearly shows Biology.

2. Citric mountain cycle (Krebs cycle)

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

The citric mountain cycle takes ar twice every molecule of glucose that was damaged down in the ahead step—one “turn” of the citric acid cycle occurs because that each molecule the acetyl-CoA.

During each of these 2 turns, the molecule that acetyl-CoA goes v a collection of chemical reactions. The energy from this reactions (in the kind of electrons) is caught in the “energy carrier” molecules NADH and FADH2. Two much more molecules that carbon dioxide and also another molecule of ATP are additionally produced.

Cellular respiration: citric mountain cycle


Capture energy from chemistry reactions, produce a tiny bit that ATP


Mitochondria - matrix


2 Acetyl-CoA


ATP, NADH & FADH2 (energy carriers), CO2 (waste product)

3. Electron transport chain (ETC)

The electron move chain is the component of aerobic cellular respiration the produces most of the ATP. The electron deliver chain offers high-energy electron from FADH2 and also NADH come pump hydrogen ion (H+) across the inner membrane the the mitochondrion, into the outer compartment.


Mitochondria. The “membrane” brand in this image refers to the outer membrane. The within membrane is the yellow structure neighboring the matrix. Check out more AR models ~ above the Biology find out Site.

As a an outcome of the electron carry chain, over there are much more positively charged ions on one next of the membrane 보다 the other. Together these ions travel back throughout the membrane to gain back equilibrium, lock pass with (and “power”) an enzyme called ATP synthase, which turns molecules the ADP into ATP by including a 3rd phosphate group.

Cellular respiration: electron carry chain


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


Mitochondria - inner membrane



(Note: Oxygen’s role is important here—it is the last acceptor because that “spent” electrons!)


Lots that ATP, H2O (waste product)

How room photosynthesis and also cellular respiration connected?

When ns think around the connections between photosynthesis and cellular respiration, ns can’t assist but start singing “Circle the Life” indigenous The Lion King in my head. Why? due to the fact that the assets of photosynthesis are compelled for cellular respiration, and also the products of cellular respiration deserve to be supplied to strength photosynthesis.

Putting the chemical formulas because that these procedures side-by-side reflects this fairly clearly:

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

Cellular Respiration: C6H12O2 + 6O26CO2 + 6H2O + (approximately) 38* ATP

*The variety of ATP molecules produced can vary. 38 ATP is the theoretical preferably yield because that the management of one molecule the glucose.

The food the plants make (glucose) and the waste product from creating that food (O2) give animals like united state the products we require to carry out aerobic cellular respiration. We breathe in the oxygen from the air and either eat tree or other animals—either way, plants and their delicious glucose are at the source of ours food web. In return, humans and other biology that bring out aerobic respiration put the waste products from this procedure (mainly CO2) back into the atmosphere.

Plants bring out both photosynthesis and also cellular respiration. They make their own food, and also then failure those glucose molecules later, generating ATP to power their to move processes.

Fun fact! Photosynthesis through microorganisms dubbed cyanobacteria is what put oxygen right into the Earth’s environment in the very first place. These organisms very first produced oxygen between 2.7 and also 2.8 billion year ago, and also oxygen ended up being a far-ranging portion the the atmosphere by about 2.45 billion years ago. This paved the way for oxygen-breathing pets like united state to evolve later.

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Before us go, this is a comfortable chart to compare photosynthesis and also aerobic cellular respiration. Happy studying!


Cellular Respiration (Aerobic)

Chemical equation

6CO2 + 6H2O → C6H12O6 + 6O2

C6H12O2 + 6O2 → 6CO2 + 6H2O + (approximately) 38 ATP


Carbon dioxide, water, sunlight

Glucose, Oxygen


1. Light-dependent reactions2. Light-independent reaction (Calvin cycle)1. Glycolysis2. Krebs cycle3. Electron carry chain


Glucose, oxygen

ATP, carbon dioxide, water

Associated organelle



Function for the organism

Use light, water, and carbon dioxide to create food because that the organism in the form of street (glucose)

Use glucose to make a type of power the organism can use in cellular processes (ATP)

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