process of cellular respiration

the krebs cycle

In the presence of oxygen, under aerobic conditions, pyruvate enters the mitochondria to proceed into the Krebs cycle. The second stage of cellular respiration is the transfer of the energy in pyruvate, which is the energy initially in glucose, into two energy carriers, NADH and FADH2 . A small amount of ATP is also made during this process. This process occurs in a continuous cycle, named after its discover, Hans Krebs. The Krebs cycle uses a 2-carbon molecule (acetyl-CoA) derived from pyruvate and produces carbon dioxide.


Stage one of cellular respiration is glycolysis. Glycolysis is the splitting, or lysis of glucose. Glycolysis converts the 6-carbon glucose into two 3-carbon pyruvate molecules. This process occurs in the cytoplasm of the cell, and it occurs in the presence or absence of oxygen. During glycolysis a small amount of NADH is made as are four ATP. Two ATP are used during this process, leaving a net gain of two ATP from glycolysis. The NADH temporarily holds energy, which will be used in stage three.

the chemical reaction

The overall chemical reaction for cellular respiration is one molecule of glucose (C6 H12 O6 ) and six molecules of oxygen (O2 ) yields six molecules of carbon dioxide (CO2 ) and six molecules of water (H2 O). Using chemical symbols the equation is represented as follows: C6 H12 O6 + 6O2 6CO2 + 6H2 O ATP is generated during the process. Though this equation may not seem that complicated, cellular respiration is a series of chemical reactions divided into three stages: glycolysis, the Krebs cycle, and the electron transport chain.

the products

What does the cell produce? The products of cellular respiration are carbon dioxide and water. Carbon dioxide is transported from your mitochondria out of your cell, to your red blood cells, and back to your lungs to be exhaled. ATP is generated in the process. When one molecule of glucose is broken down, it can be converted to a net total of 36 or 38 molecules of ATP. This only occurs in the presence of oxygen.

the electron transport chain

Stage three of cellular respiration is the use of NADH and FADH2 to generate ATP. This occurs in two parts. First, the NADH and FADH2 enter an electron transport chain, where their energy is used to pump, by active transport, protons (H+ ) into the intermembrane space of mitochondria. This establishes a proton gradient across the inner membrane. These protons then flow down their concentration gradient, moving back into the matrix by facilitated diffusion. During this process, ATP is made by adding inorganic phosphate to ADP. Most of the ATP produced during cellular respiration is made during this stage. For each glucose that starts cellular respiration, in the presence of oxygen (aerobic conditions), 36-38 ATP are generated. Without oxygen, under anaerobic conditions, much less (only two!) ATP are produced.

the reactants

What goes into the cell? Oxygen and glucose are both reactants of cellular respiration. Oxygen enters the body when an organism breathes. Glucose enters the body when an organism eats.

the process of cellular respiration

Cellular respiration is the process of extracting energy in the form of ATP from the glucose in the food you eat. How does cellular respiration happen inside of the cell? Cellular respiration is a three step process. Briefly: 1. In stage one, glucose is broken down in the cytoplasm of the cell in a process called glycolysis. 2. In stage two, the pyruvate molecules are transported into the mitochondria. The mitochondria are the organelles known as the energy "powerhouses" of the cells (Figure 1.1). In the mitochondria, the pyruvate, which have been converted into a 2-carbon molecule, enter the Krebs cycle. Notice that mitochondria have an inner membrane with many folds, called cristae. These cristae greatly increase the membrane surface area where many of the cellular respiration reactions take place. 3. In stage three, the energy in the energy carriers enters an electron transport chain. During this step, this energy is used to produce ATP. Oxygen is needed to help the process of turning glucose into ATP. The initial step releases just two molecules of ATP for each glucose. The later steps release much more ATP. Most of the reactions of cellular respira- tion are carried out in the mitochondria.


instructional diagrams

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cellular respiration occurs in the chloroplast.

a. true

-->  b. false

cellular respiration only occurs in the absence of oxygen.

a. true

-->  b. false

when is most of the atp generated?

a) during glycolysis

b) during the krebs cycle

-->  c) during the electron transport chain

d) during photosynthesis

how many atp are generated during glycolysis?

a) 0

b) 1

-->  c) 2

d) 4

when are most of the nadh and fadh2 generated?

a) during glycolysis

-->  b) during the krebs cycle

c) during the electron transport chain

d) during cellular respiration

what occurs during stage 3 of cellular respiration?

-->  a) nadh and fadh2 are used to generate atp.

b) pyruvate is produced

c) the energy in pyruvate is converted into nadh and fadh2.

d) sunlight is absorbed

how is atp made during the electron transport chain?

a) by active transport

-->  b) by facilitated diffusion

c) by anaerobic processes

d) by the krebs cycle

diagram questions

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