Which explains how the Calvin cycle and the Krebs cycle differ?

A. The Calvin cycle uses ATP to make glucose.

B. The Krebs cycle uses ADP to make glucose.

C. Only the Calvin cycle occurs in plant cells.

D. The Krebs cycle only repeats in animal cells.

Diagram of the Calvin cycle
Diagram of the Calvin cycle

The correct answer is A. The Calvin cycle uses ATP to make glucose.

The Calvin cycle and Krebs cycle both require energy molecules in order to complete their reactions. However, the Calvin cycle uses some ATP to make glucose, while the Kreb’s cycle produces some ATP in the process of glucose modifications.

Photosynthesis occurs in organisms such as some protists (unicellular living organisms), and plants. The process involves using light energy from the sun to form sugar through a series of enzyme-mediated cellular reactions.

The first part of the process is the light reactions in which energy, ATP, is generated. This is then used in the next stage, the Calvin cycle, to form sugar. In eukaryotic cells, photosynthetic reactions occur in the chloroplasts.

Cellular respiration is a process that occurs in all living organisms and is the way that energy is released from organic molecules.  Some energy is generated throughout the process but the most ATP formation actually occurs in the electron transport chain.

The other reactions occur before this stage and include glycolysis and the Kreb’s cycle. It is important to realize that glucose is not made at any stage during respiration, and in fact, the reverse is what occurs.


Photosynthesis has the general equation of 6 CO2 + 6 H2O → C6H12O6 + 6 O2. The process actually consists of two general stages during which various enzymes are involved.

The reactions in eukaryotic cells occur in organelles known as chloroplasts. The first series of reactions take place in a series of internal membranes known as thylakoids.

These structures are disc-shaped and contain chlorophyll pigment molecules. The presence of the pigment is what gives plant stems and leaves the green color that we see.

The atoms of carbon, hydrogen, and oxygen are taken from the raw materials that enter the plant and are used to form the 6-carbon sugar, glucose. Water is taken in and split in a process known as photolysis, and carbon dioxide is taken in and used in the dark reactions.

Light reactions

These first reactions do need light and use the solar radiation to generate energy for use in the Calvin cycle. Two photosystems are involved in which there are electron transport chains.

Chlorophyll pigments in the thylakoids capture light energy and use this to excite an electron in each case. The electron moves to a higher energy level and is accepted by a molecule. It then is passed along a chain providing the energy for ATP synthesis to occur.

The Calvin cycle

This process is also known as the dark reactions since light is not a requirement for these reactions to occur. The aim of the Calvin cycle is to fix carbon to form glucose, and similar to the Kreb’s cycle, the starting substance is actually regenerated. This process takes place in the stroma of the chloroplasts.

The ATP and NAPHthat are made during the light reactions stage of photosynthesis are used in the cycle to provide energy for forming the sugar. Carbon dioxide enters the plant through pores called stomata.

This then provides the carbon which is used in the dark reactions to produce ribulose 1,5-biphosphate (RuBP). This molecule becomes converted to other substances by means of different enzymes until finally a 6-carbon glucose is formed.

Cellular respiration

This process is the reverse of photosynthesis and occurs in all forms of life. The general formula for respiration in the presence of oxygen is C6H12O6 + 6O2 → 6CO2 + 6H20 + ATP.  This is a catabolic process in which sugars are actually broken down and in the process, ATP is formed.

In photosynthesis, oxygen is produced while in aerobic respiration oxygen is used and in fact, is needed. Some organisms can also undergo anaerobic respiration in which sugar is broken down in the absence of oxygen. Some energy molecules are formed but not as many as occurs in the respiration when oxygen is present.

Glycolysis is the first stage in all types of cellular respiration. This process uses some energy from ATP to split a glucose molecule through a series of reactions that occur in the cytoplasm. The end product of the reactions is a pyruvate is formed.

If oxygen is present then the next set of reactions will take place in the mitochondrion of the cell. If no oxygen is present then anaerobic respiration, also known as fermentation will occur in the cytoplasm of the cell.

The Kreb’s cycle

This stage occurs in the matrix of the mitochondrion. The pyruvate molecule from glycolysis is converted to acetyl CoA which then enters the mitochondrial matrix and into the cycle. These molecules then combine with oxaloacetate to form a 6-carbon citrate molecule.

Various reactions occur during which decarboxylation happens. This is the process in which carbons are stripped from the molecule and released as carbon dioxide.

Isocitrate is the first intermediate formed; then an alpha-ketoglutarate is formed. During this stage, NADH2 is also formed by the addition of hydrogen to NAD+.

The alpha-ketoglutarate is then modified to generate carbon succinyl CoA and more NADH2 and carbon dioxide are produced. The next molecule formed is succinate which is then converted to fumarate.

At this stage, FADH2 is produced from FAD+. This is also an energy-carrying molecule that will be used in the electron transport chain. Fumarate becomes modified to form malate.

Malate is then converted to oxaloacetate, and at the same time, additional NADH2 is formed. This oxaloacetate is then returned to the start of the cycle at which time it can join with more acetyl CoA. The entire process can begin again.

Electron transport chain

The electron carriers NADH2 and FADH2 enter the electron transport chain that occurs in the inner membrane, the cristae of the mitochondrion. ATP is formed as electrons move down the chain attracted by oxygen at the end.


  1. Editors of Encyclopedia Britannica (2018). Chloroplast. Retrieved from Encyclopedia Britannica.
  2. Editors of Encyclopedia Britannica (2018). Cellular respiration. Retrieved from Encyclopedia Britannica.
  3. Editors of Encyclopedia Britannica (2018). Tricarboxylic acid cycle. Retrieved from Encyclopedia Britannica.
  4. H Lambers, JA Bassham (2018). Photosynthesis. Retrieved from Encyclopedia Britannica.
  5. RH Raven, RF Evert, SE Eichhorn (1987). Biology of plants, 4th edition. New York: USA, Worth Publishers.


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