The ultimate source of energy for most life on Earth is the Sun. Photosynthesis is the overall process by which solar energy is used to convert molecules of water and carbon dioxide into chemical energy stored in simple sugars.

All green plants and some microorganisms are able to perform photosynthesis. During photosynthesis, the Sun’s energy is used to convert water and carbon dioxide into oxygen and glucose sugar.

The general equation for photosynthesis is:

The Importance of Photosynthesis

Without photosynthesis, most life on Earth would not exist. This vital process enables producers (autotrophs) to transform solar energy into chemical energy. The chemical energy is stored in the bonds of simple sugars until it is needed.

Cells need a constant supply of energy in order to carry out life processes. So, the ability to store and use energy is critical for all life on Earth.

Consumers (heterotrophs) are unable to transform sunlight into usable energy. Unlike producers, they cannot synthesize complex organic molecules from inorganic matter. Instead, they must consume other organisms that contain the complex molecules and their building blocks. Therefore, producers form the foundation of every food chain on Earth.

The process of Photosynthesis

The Sun is the source of almost all of the energy used by organisms to grow, survive, and reproduce. Producers capture and convert light energy into chemical energy through the process of photosynthesis.

In eukaryotic producers, photosynthesis takes place in chloroplasts, which are found in algal cells in addition to specialized plant cells. Chloroplasts contain a green pigment called chlorophyll that absorbs light energy.

The characteristic green color of leaves and stems is due to the abundance of chloroplasts present in their cells. During photosynthesis, the atoms in carbon dioxide and water rearrange to form a sugar called glucose. 

Scientists classify the chemical reactions of photosynthesis into two groups, they are:
1.    The light-dependent reactions(Light Reaction
2.    The light-independent reactions(Dark reactions)

In eukaryotic cells, these processes take place entirely inside chloroplasts. 

Light Dependent Reactions(Light Reactions)

The light-dependent reactions occur within the grana of chloroplasts, such as the one pictured below.

Pigment molecules, such as chlorophyll, inside the thylakoids of chloroplasts absorb light energy from the Sun. This causes the electrons inside the chlorophyll molecules to become excited to higher energy states. Then, the electrons move from one molecule to another inside the thylakoids, and energy is released with each move. The molecules along which the electrons are passed are together called the electron transport chain.

The energy released in the electron transport chain is stored in the chemical bonds of molecules called ATP (adenosine triphosphate). Cells then later use the energy stored in ATP when they remove one of the phosphate groups and form ADP (adenosine diphosphate).

In the light reactions, the free energy of sunlight is captured and then transferred to the carrier molecules ATP and NADPH. The diagram provides an overview of the process.

The NADPH’s, the protons, and the ATP’s made in the light-dependent reactions are later used to synthesize glucose during the dark reactions of photosynthesis. 

Note that no sugars are synthesized during the light-dependent reactions.

Light Independent Reactions(Dark Reactions)

The reactions of the Calvin cycle are also called the dark reactions of photosynthesis because they can take place when the plant is receiving sunlight or when the plant is not receiving sunlight. 

These reactions—powered by the ATP, NADPH, and proton produced by the light-dependent reactions—synthesize the organic sugar molecule glucose from inorganic carbon dioxide obtained from the atmosphere. 

Dark reactions take place in the stroma of chloroplasts.

The Calvin cycle uses ATP and NADPH, created in the light-dependent reactions of photosynthesis, to produce glucose and other carbohydrates in light-independent reactions.

The Steps of the Calvin Cycle:

  1. Carbon Fixation
  2. Reduction
  3. Regeneration

Carbon Fixation:

Carbon dioxide combines with five carbon compounds to form 3-phosphoglycerate (PGA), a three carbon molecule that is metabolic intermediate of light-independent reactions.


The chemical energy is stored in ATP and NADPH is transferred to 3-PGA to form the high energy molecule glyceraldehyde 3-phosphate (G3P). ATP supplies the phosphate group. NADPH supplies the hydrogen ions and electrons.


The remaining G3P molecules are converted into ribulose 1,5-bisphosphate (RuBP), a five carbon sugar that combines with CO2 in the first stage.

The reaction is speeded up by RuBP carboxylase (rubisco), an enzyme that catalyses the combination of carbon dioxide and RuBP.


Adenosine Triphosphate, or ATP, is an energy-storage molecule. It is the most important biological molecule that supplies energy to cells.

Components of ATP

Each molecule of ATP is composed of the following components.

  • one nitrogenous base (adenine)
  • one sugar (ribose)
  • three phosphate groups
NOTE: ADP (Adenosine Diphosphate) is a compound composed of one adenosine and two phosphate groups that occurs when the high energy phosphate bond is broken from ATP, resulting in the release of energy.    

The ATP-ADP Cycle

Cells acquire energy from food. Some of this energy is used to produce ATP, which serves as a kind of energy currency within the cell. The diagram below shows how the cell uses and recycles ATP.

The three phosphate groups found in ATP are linked by high-energy bonds.When energy is needed by the cell to perform cellular work, one of the phosphate bonds in ATP is broken, ADP (adenosine diphosphate) is formed, and energy is released. This process also releases a phosphate molecule (shown as P in the diagram) into the cytoplasm.

ATP → ADP + P + energy for cellular work

When a cell acquires excess energy from food, a phosphate group is reattached to an ADP molecule, and ATP is reformed.

ADP + P + energy from food → ATP

With its three phosphate groups, ATP carries much more energy than ADP. The process of recycling ADP back into ATP ensures that cells have a constant source of energy.

Cellular Respiration

During cellular respiration, organic molecules “burn” in the presence of oxygen, this releases energy in the form of ATP, carbon dioxide, and water. This process occurs in the mitochondria of eukaryotic cells. All living organisms perform some kind of cellular respiration, a chemical pathway in which organic molecules, such as glucose, are broken down to release energy in a quickly usable form (e.g., ATP). Generally speaking, cellular respiration is the opposite of photosynthesis. 

The Process Of Cellular Respiration

When organisms need energy for cellular processes, they break down glucose in a process known as cellular respiration.  All organisms, including producers, undergo cellular respiration. During the reaction, an energy molecule known as ATP (adenosine triphosphate) forms.

Cellular respiration occurs in the mitochondria of eukaryotic cells. There, glucose molecules break down in the presence of oxygen. 

This releases the stored energy in the glucose bonds, and cells use the energy to construct molecules of ATP. The reaction also produces carbon dioxide and water as waste products.

The chemical equation for cellular respiration is:

Cellular respiration can be divided into two basic categories: 

Aerobic (with O2) respiration

Anaerobic (without O2) respiration

Aerobic respiration 

Aerobic respiration uses oxygen (O2) as the final electron acceptor in the reaction pathway. Aerobic respiration is divided into 3 stages:


The first stage of cellular respiration, glycolysis, requires no oxygen, and it takes place in the cytoplasm. During glycolysis(a six-carbon glucose molecule is broken down with the aid of enzymes into two three-carbon pyruvic acid molecules). 

The process of glycolysis occurs in two main phases, they are:

  1. Energy Investment Phase
  2. Energy Payoff Phase

Energy Investment

Glycolysis is the anaerobic stage of cellular respiration. It occurs in the cytoplasm of the cell. During the process, a 6-carbon glucose molecule splits into two3-carbon pyruvate molecules. 

The first step in the process requires an energy input. The energy from two ATP molecules is used to bond two phosphate groups from ATP to the glucose molecule. The 6-carbonmolecule then splits into two 3-carbon G3P molecules.

 Energy Payoff

In the next step, a series of reactions produces energy in the form of ATP. First, a phosphate group is added to each of the 3-carbon G3P molecules. 

An electron carrier NAD+ accepts electrons and hydrogen ions from G3P to form NADH. NAD+ is similar to NADP+, the electron carrier in photosynthesis. 

Next, four ADP molecules accept the high-energy phosphates from the two 3-carbon molecules to form four ATP molecules. This reaction converts the 3-carbonmolecules to two pyruvate molecules.

It produces two molecules of ATP per molecule of glucose. Of the four molecules of ATP produced, two molecules are required to start the process. So, the net energy produced during glycolysis of a molecule of glucose is two molecules of ATP.

 Pyruvate is a key intermediate of cellular respiration because it represents the only branch point of the path of reactions. When oxygen is not present, or in any cell that lack mitochondria, the pyruvate is reduced to an end product. 

In human cells, this product is lactic acid. In certain yeast, the product is ethyl alcohol.

When oxygen is present, pyruvate is shunted into the mitochondria and enters the Krebs cycle, which is the second stage of cellular respiration. 

The Citric Acid Cycle (Kreb’s Cycle)

The first step of aerobic respiration is called the citric acid or Krebs cycle. During this cycle, the pyruvic acid formed in glycolysis travels to the mitochondria where it is chemically transformed in a series of steps. The Krebs cycle is the aerobic part of cellular respiration. It’s also called the citric acid cycle or the TCA (tricarboxylic acid) cycle. The Krebs cycle doesn’t directly require oxygen. But it requires products from the electron transport chain (ETC). The ETC can function only in the presence of oxygen. So, the Krebs cycle can only occur when oxygen is present.

Step 01:

The pyruvate molecules produced during glycolysis enter the mitochondrial matrix. Pyruvate first reacts with coenzyme A (CoA) to form a 2-carbon molecule, acetyl CoA. The reaction releases carbon dioxide. In a cyclic set of reactions, the pyruvate is completely oxidized into carbon dioxide. 

Step 02:

The acetyl coA binds with a 4-carbon compound to form citric acid, a 6-carbon compound.

Step 03:

The 6-carbon citric acid loses two hydrogen ions and high-energy electrons, which are taken up by NAD+ to form NADH. A carbon atom is removed in the form of carbon dioxide, leaving behind a 5-carbon molecule.

Step 04:

Second Oxidative Decarboxylation Reaction

The 5-carbon molecule again gives up hydrogen ions and high-energy electrons to NAD+ to form NADH. The last of the carbon atoms from the pyruvate molecule is removed in the form of carbon dioxide.

Step 05:

During the second oxidative dicarboxylic reaction, a simultaneous reaction produces one ATP molecule.

Step 06:

Oxidation of 4-Carbon Compound

The 4-carbon compound loses hydrogen to FAD, an electron carrier, which forms FADH2.

Step 07:

Cycle Ends

The oxidation rearranges the carbon atoms to form the original 4-carbon compound, which ends the cycle.

Step 08:

Energy Audit

Each glucose molecule generates two pyruvate molecules. So, one glucose molecule requires two “turns” of the Krebs cycle and produces a net yield of two ATP molecules. Carbon dioxide and water are eventually released. The hydrogen ions are accepted by NADH and FADH2, which then enter the electron transport chain.     Electron Transport Chain

Electron Transport Chain

The second step of aerobic respiration is the electron transport chain. Most of the ATP molecules are formed during this part of the cycle. 

The electron transport chain is a series of chemical reactions ending with water, carbon dioxide, and a net yield of up to 34 ATP molecules

NADH and FADH2 formed during the Krebs cycle transfer their high-energy electrons to proteins in the inner mitochondrial membrane. 

In the process, NAD+, FAD, and H+ions are released into the mitochondrial matrix, where they reenter the Krebs cycle.

 The electrons move through a series of proteins in the membrane. These proteins make up the electron transport chain. 

Electrons and hydrogen ions are transferred from one compound to the next, eventually reacting with oxygen to form water. Oxygen is the final electron acceptor in the ETC. It accepts the electrons and combines with H+ ions to form water.


The electron transport chain is a series of chemical reactions where electrons carried by electron carriers (NADH and FADH2) pass one molecule to another until they reach oxygen molecule to produce water.

The process of chemiosmosis causes H+ ions to flow back from the intermembrane space and into the matrix through ATP synthase channels.

As the H+ ions flow back, ATP synthase uses their energy to create ATP.

The electron transport chain makes 34 ATP molecules.Adding the ATP molecules created during glycolysis and the Krebs cycle, cellular respiration could produce a maximum of 38 ATP molecules. 

But this number is never quite reached because of other factors, such as membrane leakage and differences in efficiency of ATP synthase. 

Cellular respiration produces around 30 ATP molecules, but the actual number differs among individuals.

Anaerobic Respiration

Though aerobic respiration is much more efficient, making up to 36-38 total ATPs per glucose molecule, some organisms live in environments where oxygen is not abundant. These organisms undergo anaerobic respiration. Anaerobic respiration is a pathway that is very similar to aerobic respiration; it oxidizes all of the starting organic molecules into carbon dioxide and energy. Anaerobic respiration, however, uses an inorganic substance like nitrate or sulfate as the final electron acceptor instead of oxygen, allowing bacteria that use this pathway to survive in areas lacking O2.

Fermentation   Lactic Acid FermentationAnother alternative to aerobic respiration is fermentation. Like anaerobic respiration, fermentation is not as efficient as aerobic respiration and results in the formation of

far fewer ATP molecules, but some organisms depend on it when oxygen is not available.

Fermentation does not take place inside mitochondria. Instead, it uses components found in the cytoplasm and cell membranes of both prokaryotic and eukaryotic cells.

 Fermentation can have different end products depending on the type of organism performing the fermentation. There are two main fermentation processes, they are:

  1. Lactic Acid Fermentation
  2. Alcohol Fermentation
Lactic Acid Fermentation

In lactic acid fermentation, pyruvic acid formed during glycolysis is broken down to produce lactic acid and ATP.

glucose → pyruvic acid → lactic acid + ATP

An example of lactic acid fermentation occurs when the muscle cells of eukaryotic organisms are depleted of oxygen. Although this process produces energy, a buildup of lactic acid causes a burning, painful sensation.

Therefore, this process is limited, and muscle cells need oxygen for sustained activity. Lactic acid fermentation also occurs in bacteria, such as those used in the production of dairy products like cheese, buttermilk, and yogurt.

Alcohol Fermentation

In alcohol fermentation, pyruvic acid formed during glycolysis is broken down to produce alcohol, carbon dioxide, and ATP.

glucose → pyruvic acid → ethyl alcohol + carbon dioxide + ATP

Alcohol fermentation occurs in yeasts and some bacteria. While the ATP generated in this process is perhaps the most useful product to the cell itself, the by-products—carbon dioxide and ethyl alcohol—are very useful to humans. The food industry uses yeast cultures in the production of bread and alcoholic beverages, and the agricultural industry uses yeast to produce ethanol for fuel.

Summary of Cellular Respiration

Factors that Affect Photosynthesis and Respiration

Photosynthesis and respiration can be affected by a number of different factors. 

The factors that affect photosynthesis include:

  1. The amount of light available—photosynthesis occurs more quickly when there is more light available
  2. The color, intensity, and wavelength of light available—different light conditions will result in different rates of photosynthesis
  3. The amount of carbon dioxide available—photosynthesis generally occurs more quickly when more carbon dioxide is available
  4. temperature—since the process of photosynthesis is catalyzed by enzymes, photosynthesis will only occur efficiently at a narrow range of temperatures

The factors that affect cellular respiration include:

  1. The availability of glucose—cellular respiration generally occurs more quickly when there is more glucose available, as long as there is an excess of reactants (such as oxygen)
  2. The availability of reactants, such as oxygen—cellular respiration generally occurs more quickly when there is more oxygen available, as long as there is an excess of glucose
  3. Temperature—since the process of cellular respiration is catalyzed by enzymes, photosynthesis will only occur efficiently at a narrow range of temperatures

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