Photosynthesis and Cell Respiration

Step-by-Step Guide to Photosynthesis and Cellular Respiration for Beginners

What if the air you’re breathing right now suddenly vanished? Without oxygen, life would collapse in seconds. No plants, no animals, no humans—just silence. The truth is, oxygen wasn’t always part of Earth’s atmosphere. Billions of years ago, our planet was a barren, hostile place… until tiny microbes changed everything.

Through a groundbreaking process called photosynthesis, these early organisms learned how to capture sunlight and turn it into food, releasing oxygen as a “waste product.” Over millions of years, oxygen filled the atmosphere, transforming Earth into a planet bursting with life. Today, plants, algae, and certain bacteria still carry out photosynthesis, fueling nearly every ecosystem on Earth.

But sunlight alone isn’t enough to power you. Every breath you take, every muscle you move, every thought that sparks in your brain relies on cellular respiration—the process by which cells break down food molecules and release usable energy in the form of ATP (adenosine triphosphate). If photosynthesis is Earth’s energy bank, cellular respiration is how you withdraw and spend it.

That’s exactly what we’ll explore in this study guide. By the end, you’ll understand how a chain of energy that begins with sunlight billions of years away keeps you alive this very second—every step, every breath, every beat.

What You’ll Learn:

• How photosynthesis captures solar energy and creates oxygen
• How respiration converts food into ATP, your body’s universal fuel
• Why these two processes are inseparably linked in the cycle of life
• Overview of glycolysis, Krebs cycle, and oxidative phosphorylation

Key Takeaways

• Photosynthesis powers life on Earth: Plants, algae, and cyanobacteria convert sunlight, carbon dioxide, and water into glucose and oxygen, sustaining nearly all ecosystems.

• Two stages of photosynthesis:
  • Light-dependent reactions capture solar energy and produce ATP and NADPH.
  • Light-independent reactions (Calvin cycle) use that stored energy to build glucose.

• Chloroplasts drive photosynthesis: These organelles contain chlorophyll, which absorbs light energy and reflects green, explaining why most plants are green.
• Cellular respiration releases energy from food: Both plants and animals break down glucose in the presence of oxygen to produce ATP, the universal energy currency of cells.
• Three main stages of cellular respiration:
  • Glycolysis splits glucose into pyruvate, producing 2 ATP and 2 NADH.
  • Citric Acid Cycle (Krebs cycle) completes glucose oxidation, generating NADH, FADH₂, and a small amount of ATP.
  • Oxidative phosphorylation uses the electron transport chain and chemiosmosis to produce most of the cell’s ATP.
• The big picture: Photosynthesis and cellular respiration form a biological cycle—photosynthesis stores energy in glucose, while respiration releases that energy for life processes.

Our solar system wouldn’t exist without the Sun: its gravity is the glue that holds all the planets in orbit, and the energy it radiates sustains the layer of life that exists here on Earth. In fact, every organism on this planet gets nutrients from their environment, and all that energy in those nutrients originated almost 94 million miles (151 million km) away from the Sun. Photosynthesizing cells convert solar energy into energy-packed organic molecules. This process is called photosynthesis, and without it, we wouldn’t be here.

Photosynthesis. Using sunlight, plants, algae, and some bacteria make their own food—glucose (sugar)—from water and carbon dioxide, releasing oxygen as a byproduct.

Photosynthesis, which literally translates to “make light,” is the process by which plants, algae, and cyanobacteria capture the energy of the Sun and use it to make their own food from water and carbon dioxide. These days, most photosynthesizing organisms (with the exception of a few types of simple cyanobacteria) release oxygen as a waste byproduct, which keeps all us air-breathers alive on this planet. However, photosynthesis didn’t always work this way. The first cyanobacteria that evolved around 3.5-3.8 billion years ago developed a primitive form of anoxygenic photosynthesis that didn’t produce oxygen, fueled by molecules like hydrogen or iron instead of water as is used in modern oxygenic photosynthesis.

Great Oxidation Event. When oxygenic photosynthesis evolved, oxygen began accumulating in Earth’s atmosphere for the first time. This “Great Oxidation Event” transformed the planet—oxygenating surface oceans, enabling new life forms, and setting the stage for complex organisms to evolve.

In fact, photosynthesis has been around more than a billion years longer than our oxygen atmosphere—which makes sense because the rise of oxygenic photosynthetic organisms was the reason for Earth’s Great Oxidation Event (GOE) 2.2 billion years ago. Oxygen is now about 21% of the air we breathe, all thanks to photosynthesis. Let’s explore how it works.

How Photosynthesis Works

Photosynthetic cells require carbon dioxide and sunlight to make sugar molecules and oxygen (as a byproduct). The sugar initially made to store energy during photosynthesis is glucose, which can later be broken down during the process of cell respiration to generate ATP.

Photosynthesis and cellular respiration as a cycle. The glucose and oxygen produced during photosynthesis are used in cellular respiration to generate carbon dioxide and water—materials that feed back into photosynthesis. Cellular respiration also releases the chemical energy stored in glucose, converting it into usable energy in the form of ATP.

Photosynthesis in plants is achieved when they have access to water and carbon dioxide in the environment. Transpirational pull allows them to transport water into their bodies through their roots and, by the process of diffusion, capture carbon dioxide from the atmosphere through tiny pores in their leaves called stomata.

Water transport in plants. Water moves upward through xylem vessels by transpirational pull. As water evaporates from the stomata and diffuses out of the leaf, it pulls on the continuous column of water below. Cohesion between water molecules and adhesion to xylem walls keep this column intact, allowing water to rise from roots to leaves through capillary action.

CO₂ diffusion in plants. Plants exchange gases with the air by diffusion through stomata. In daylight, photosynthesis occurs, so CO₂ is used and oxygen is produced. Consequently, CO₂ diffuses into the leaf while oxygen diffuses out. At night, when photosynthesis stops, the stomata close. Guard cells control this opening and closing to balance gas exchange and water loss.

With the raw materials necessary to photosynthesize secured, organelles in the plant cells called chloroplasts capture light energy from the Sun. Chloroplasts are a type of pigment-containing organelle called a plastid which store energy and food and carry out photosynthesis during daylight hours.

Photosynthesis is primarily accomplished using a green pigment called chlorophyll, which absorbs red and blue light and reflects green light. This is why the photosynthesizing structures of a plant, like leaves and stems, appear green to our eyes. In autumn, environmental conditions initiate a series of processes that involve the degradation of chlorophyll, causing other pigments to take over—carotenoids that were present in the leaves become visible as yellow or orange, while anthocyanins, which may be newly produced, can give leaves red or purple colors.

Structure of the chloroplast. The chloroplast contains thylakoids, which are stacked into grana. Each thylakoid is bounded by a membrane embedded with protein complexes that contain chlorophyll. The spaces surrounding the grana are filled with a fluid called the stroma, which contains many enzymes involved in photosynthesis.

Chloroplasts themselves are encased in a double membrane called a chloroplast envelope, and each of these large organelles contains an internal membrane system called the thylakoid   membrane that forms long folds within the organelle. These stacks of thylakoid membranes, called grana, create a maze of connected compartments full of chlorophyll. Because of their double membrane, chloroplasts are much like mitochondria, which is unsurprising, as the purpose of both organelles is to generate ATP (adenosine triphosphate).

Light-dependent and light-independent reactions of photosynthesis. The light-dependent reactions take place in the thylakoid membranes, where sunlight powers the production of ATP and NADPH. These molecules then fuel the Calvin cycle in the stroma, where CO₂ is fixed into sugars. Together, both stages convert light energy into chemical energy stored in carbohydrates.

Photosynthesis takes place in two main stages: the light-dependent reactions and the light-independent reactions. In the light-dependent reactions—those that require sunlight—solar energy absorbed by chlorophyll is used to produce ATP and the electron carrier NADPH (nicotinamide adenine dinucleotide phosphate). These energy-rich molecules are stored for use in the next stage, while oxygen is released as a byproduct.

During the light-independent reactions, also known as the Calvin cycle, the ATP and NADPH generated earlier are used to build carbohydrate molecules from carbon dioxide. After transferring their energy, ATP becomes ADP and NADPH becomes NADP⁺, which return to the light-dependent reactions to be recharged. Hence, although the Calvin cycle doesn’t directly require light, it depends entirely on the energy and electron carriers produced by the light-dependent reactions.

Cellular Respiration

Organisms can’t recycle energy. Energy dissipates once it’s used, so living things require a constant input of new energy. Photosynthesizing organisms capture this energy from sunlight, while other living things obtain it from food in their environment. When nutrients are broken down, their stored chemical energy is released and used by cells to power essential tasks.

Much like firewood burning in a fire, cells “burn” sugar molecules by combining them with oxygen. This process, called cellular respiration, transforms chemical energy into usable work and produces carbon dioxide and water as byproducts. Because oxygen is required, this process is also known as aerobic respiration.

Overview of cellular respiration. Cellular respiration is the process of extracting energy from glucose using oxygen to produce ATP. It takes place in three key stages—glycolysis, the citric acid cycle, and oxidative phosphorylation—yielding carbon dioxide and water as byproducts. Glycolysis happens in the cytoplasm, whereas the latter two stages occur inside the mitochondria.

Cellular respiration occurs in all living cells, including the leaves and roots of plants. Unlike photosynthesis, it does not depend on light, so it takes place both during the day and at night.

In this process, cells break down glucose step by step, gradually capturing its stored energy. By the end, high-energy electrons from the original bonds have lost much of their energy, and these depleted electrons are transferred to the final electron acceptor, oxygen. The reactions occur in three main stages: glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation. Through these stages, cells generate ATP, the universal energy currency that powers life’s activities.

Technically, the term cellular respiration includes both aerobic and anaerobic processes although it often refers to aerobic respiration unless otherwise stated. In anaerobic respiration, cells still run the same general process, but use a different inorganic molecule as the final electron acceptor instead of oxygen.

Let’s now explore the three main stages of cellular respiration in more detail.

Glycolysis

Glycolysis takes place in the cytoplasm of the cell. The name literally means “sugar splitting,” and that is exactly what happens: one six-carbon molecule of glucose is broken down into two 3-carbon molecules of pyruvate. Along the way, the cell captures a modest but important amount of energy in the form of ATP and NADH (nicotinamide adenine dinucleotide, NAD⁺, in its reduced form). The overall yield of glycolysis is two molecules of ATP and two molecules of NADH for every molecule of glucose that enters the pathway.

Glycolysis. Glycolysis begins with an energy investment phase, which includes priming glucose to be split. The cleavage itself is also part of this phase. A payoff phase that involves oxidation and ATP formation follows, which generates a net yield of 2 ATP and 2 NADH.

Because it does not require oxygen, glycolysis is a universal process found in both aerobic and anaerobic organisms. It can be divided into two phases: the energy investment phase and the energy payoff phase.

• Energy Investment Phase. The first half of glycolysis consumes energy rather than producing it. Here, the cell uses two molecules of ATP to “prime” glucose for breakdown. In a series of reactions, glucose is ultimately broken down into two molecules of glyceraldehyde 3-phosphate (G3P), a 3-carbon molecule.

• Energy Payoff Phase. The second half of glycolysis more than repays the initial investment. Each G3P is oxidized, transferring a pair of electrons and a proton to NAD⁺, which forms NADH. At the same time, inorganic phosphate (Pi) is added, creating a high-energy intermediate. The phosphate groups are then transferred to ADP, producing ATP.
  
  Because there are two G3P molecules per glucose, this phase yields a total of four ATP and two NADH molecules, as well as two pyruvate molecules. After subtracting the two ATP used during the investment phase, the net result of glycolysis is two ATP, two NADH, and two pyruvate.

Oxidation of Pyruvate to Acetyl-CoA

Under aerobic conditions, the pyruvate produced by glycolysis undergoes further oxidation before entering the citric acid cycle. In eukaryotic cells, pyruvate is transported into the mitochondria. In prokaryotes, similar reactions occur in the cytoplasm and along the plasma membrane.

Pyruvate oxidation begins with a decarboxylation reaction, in which one of the three carbons of pyruvate is removed and released as carbon dioxide. The remaining two-carbon fragment, called an acetyl group, is then attached to coenzyme A to form acetyl-CoA.

Oxidation of pyruvate to acetyl-CoA. Pyruvate from glycolysis is transported into the mitochondrion, where it undergoes oxidative decarboxylation by the enzyme complex pyruvate dehydrogenase. In this process, one carbon atom is released as CO₂, NAD⁺ is reduced to NADH, and the remaining two-carbon acetyl group binds to coenzyme A, forming acetyl-CoA.

This process also forms NADH from NAD⁺, so each pyruvate molecule produces 1 acetyl-CoA, 1 NADH, and 1 CO₂. Since glycolysis generates two pyruvate per glucose, the complete oxidation of both produces 2 acetyl-CoA, 2 NADH, and 2 CO₂. The acetyl-CoA molecules then enter the citric acid cycle, where the oxidation of glucose-derived carbons is completed, and the reduced NADH molecules feed electrons into pathways that generate even more ATP.

If oxygen is absent or insufficient and cells cannot utilize an alternative electron acceptor, fermentation occurs instead of pyruvate oxidation to acetyl-CoA. Consequently, aerobic respiration does not proceed and the cell must rely solely on glycolysis to produce ATP.

Citric Acid Cycle (CAC)

The citric acid cycle, also called the Krebs cycle or tricarboxylic acid (TCA) cycle, completes the oxidation of glucose. It takes place in the mitochondrial matrix of eukaryotic cells and in the cytoplasm or plasma membrane of prokaryotes. The cycle begins when a 2-carbon acetyl group, carried by acetyl-CoA, combines with the 4-carbon molecule oxaloacetate to form citrate, a 6-carbon tricarboxylic acid. From there, citrate is progressively modified and oxidized, releasing two carbons as CO₂ and regenerating oxaloacetate so the cycle can begin again.

Although only a small amount of ATP (or GTP, guanosine triphosphate) is produced directly, the citric acid cycle is highly valuable because it captures most of the released energy in the form of high-energy electron carriers: NADH and FADH₂. These deliver electrons to the electron transport chain, where the majority of cellular ATP is ultimately synthesized. For each acetyl group that enters the cycle, the output is 3 NADH, 1 FADH₂, and 1 ATP (or its equivalent GTP), along with 2 molecules of CO₂. Since each glucose produces two acetyl-CoA molecules, a full turn of the cycle per glucose yields 6 NADH, 2 FADH₂, 2 ATP/GTP, and 4 CO₂.

Let’s look at the reactions that occur in a bit more detail.

Reactions in the CAC

Citric acid cycle. In the mitochondrial matrix, each acetyl-CoA from pyruvate oxidation enters the citric acid cycle. Through a series of enzyme-driven steps, CO₂ is released and the high-energy electron carriers NADH and FADH₂ are produced. Since two acetyl-CoA molecules come from each glucose, the cycle runs twice to complete glucose breakdown and supply energy carriers for the next stage of respiration.

Reaction 1: Condensation. The cycle begins when the acetyl group from acetyl-CoA irreversibly combines with oxaloacetate to form citrate. This step commits the acetyl carbons to the cycle. High ATP levels inhibit this reaction, ensuring that the cycle only runs when energy is needed.

Reactions 2: Isomerization. Citrate is rearranged into its isomer, isocitrate, through a two-step dehydration-hydration sequence. This repositioning of functional groups prepares the molecule for efficient oxidation.

Reaction 3: First Oxidation. Isocitrate undergoes oxidative decarboxylation, producing the 5-carbon α-ketoglutarate. In the process, NAD⁺ is reduced to NADH and one CO₂ is released.

Reaction 4: Second Oxidation. α-Ketoglutarate is further oxidized and decarboxylated, releasing a second CO₂. The remaining 4-carbon succinyl group attaches to CoA to form succinyl-CoA. Another NAD⁺ is reduced to NADH.

Reaction 5: Substrate-Level Phosphorylation. The high-energy bond of succinyl-CoA is broken, and the released energy drives the phosphorylation of GDP to GTP. Similar in structure and function to ATP, GTP can directly power the cell, or it can transfer a phosphate to ADP to form ATP. The bond-breaking of succinyl-CoA produced succinate, a 4-carbon molecule.

Reaction 6: Third Oxidation. Succinate is oxidized to fumarate. Unlike earlier steps, the free energy change is not enough to reduce NAD⁺, so FAD (flavin adenine dinucleotide) serves as the electron acceptor, producing FADH₂.

Reactions 7-8: Regeneration of Oxaloacetate. Fumarate is hydrated to malate, which is then oxidized back to oxaloacetate. This last oxidation reduces another NAD⁺ to NADH. Oxaloacetate is now ready to combine with another acetyl group and restart the cycle.

Oxidative Phosphorylation

Oxidative phosphorylation is the final stage of cellular respiration and the step where most of the cell’s ATP is produced. It combines two linked processes: the electron transport chain, which uses high-energy electrons to build a proton gradient, and chemiosmosis, which taps into that gradient to make ATP.

Oxidative phosphorylation. In the mitochondria, oxidative phosphorylation uses energy from electrons carried by NADH and FADH₂ to generate ATP. As electrons pass through the electron transport chain, their energy drives protons across the inner membrane, creating a proton gradient. Protons then flow back through ATP synthase during chemiosmosis, powering the conversion of ADP into ATP. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water.

Electron Transport Chain

Earlier in respiration, NADH and FADH₂ were formed as carriers of high-energy electrons. These molecules deliver their electrons to the electron transport chain, a series of proteins and carriers embedded in the inner mitochondrial membrane (or the plasma membrane in prokaryotes). As the electrons move from one component to the next, their energy is released in small steps. This energy is used to pump protons (H⁺) out of the mitochondrial matrix and into the intermembrane space, creating a proton gradient across the membrane.

Oxygen plays a critical role at the end of the chain. It acts as the final electron acceptor, combining with electrons and protons to form water. Without oxygen, the chain would stop, and the proton gradient could not be maintained.

Chemiosmosis

The proton gradient created by the electron transport chain stores both concentration and charge differences across the membrane. Protons naturally want to flow back into the matrix, but the membrane itself is relatively impermeable to ions. The only path available is through ATP synthase, a large enzyme complex that spans the membrane.

ATP synthase. ATP synthase functions like a tiny rotary motor embedded in the mitochondrial membrane. As protons flow down their concentration gradient, they drive the rotation of the enzyme’s rotor and stalk. This mechanical motion powers the enzyme to join ADP and inorganic phosphate, forming ATP—the cell’s main energy currency.

As protons flow through ATP synthase, the enzyme spins like a rotary motor. This mechanical motion provides the energy to bond ADP and inorganic phosphate (Pi) into ATP. Because this process depends on the diffusion of protons down their gradient, it is called chemiosmosis. In this way, the movement of protons is directly coupled to the production of large amounts of ATP.

Together, the electron transport chain and chemiosmosis convert the energy originally stored in glucose into a usable form of chemical energy. The proton gradient acts like an energy reservoir, and ATP synthase functions as the molecular machine that draws from it. The ATP produced during oxidative phosphorylation is then distributed throughout the cell, where it powers the wide range of energy-requiring processes that sustain life.

Let’s help you remember the key topics with these memory tricks or mnemonic. 

Inputs and Outputs of Photosynthesis

• “Cats that Want Light Gobble Oxygen” to remember the chemical equation:

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

Cats – Carbon dioxide

Want – Water

Light – Light

Gobble – Glucose (glucose is also considered the food of photosynthetic organisms!)

Oxygen – Oxygen

Stages of Photosynthesis

• Light-Dependent Reactions: NAPPS – NADPH and ATP Production Powered by Sunlight

The double P can also help you remember that NADPH—not NADH—is produced in light-dependent photosynthesis!

• Calvin Cycle (Light-Independent Reactions): “Can Plants Grow?” – CO₂ to Produce Glucose

Glycolysis

• “Glucose NAPS twice” – Glucose produces 2 NADH, 2 ATP, 2 pyruvate

Citric Acid Cycle

• “Cats In Kentucky Sell Small Furry Mice Often” – Citrate → Isocitrate → α-Ketoglutarate → Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate

Products of Citric Acid Cycle

• “1 Frappe, 1 Apple pie, 2 Cookies, 3 Nights—double it.” – 1 FADH₂, 1 ATP (or GTP), 2 CO₂, 3 NADH for one cycle

The “double it” indicates multiplying by 2 for the yield of a full cycle since one glucose molecule produces 2 molecules of acetyl-CoA, which both undergo the CAC: 2 FADH₂, 6 NADH, 2 ATP (or GTP), 4 CO₂.

Oxidative Phosphorylation

• Electron Transport Chain: “Electrons ride the chain, protons pumped to the intermembrane.” – As electrons move across the electron transport chain, protons are pumped out into the intermembrane space.
• Chemiosmosis: “Protons back in, synthase spins.” – As protons move back into the mitochondrial matrix, the ATP synthase spins and produces ATP.

Conclusion: The Cycle That Keeps You Alive

From the first cyanobacteria that transformed Earth’s atmosphere to the leaves outside your window and the cells inside your body, photosynthesis and cellular respiration are the twin engines of life.

Photosynthesis captures sunlight, builds food, and fills the air with oxygen. Cellular respiration takes that food and oxygen and turns them into ATP—the energy that powers every heartbeat, every step, and every thought. One process stores energy, the other spends it, and together they form the unbroken cycle that sustains all living things.

So the next time you take a deep breath or step into the sunshine, remember: you’re tapping into a chain of energy that began with the Sun and continues with every cell in your body.