Lesson 11: Cellular Respiration
1.Video Lesson
2.Objective
At the end of this lesson, you will be able to:-
- Define cellular respiration
- Describe the process of glycolysis.
- List the products formed at the end of glycolysis.
- Describe the structure of mitochondria.
- Explain how energy is harvested in aerobic respiration.
- Describe the different stages of aerobic respiration.
Brainstorming questions
- How do the various stages of cellular respiration (glycolysis, pyruvate oxidation, Krebs cycle, and oxidative phosphorylation) interconnect to optimize ATP production, and what regulatory mechanisms ensure that energy production meets cellular demands?
- How do different organisms adapt their cellular respiration processes to varying levels of oxygen availability, and what implications do these adaptations have for their metabolic efficiency and survival in diverse environments?
key words
- Cellular Respiration: The process by which cells convert glucose and oxygen into ATP (adenosine triphosphate), carbon dioxide, and water. This process involves several stages and occurs in both prokaryotic and eukaryotic cells.
- ATP (Adenosine Triphosphate): A high-energy molecule used by cells to store and transfer energy. Composed of an adenine base, a ribose sugar, and three phosphate groups. ATP is hydrolyzed to ADP (adenosine diphosphate) and a phosphate group, releasing energy.
- ADP (Adenosine Diphosphate): The product of ATP hydrolysis. It can be converted back into ATP through phosphorylation.
- AMP (Adenosine Monophosphate): The product of ADP hydrolysis. AMP can be recycled back into ADP or ATP.
- Coupled Reactions: Reactions where an exothermic reaction (releasing energy) drives an endothermic reaction (requiring energy). ATP hydrolysis is a common example, where the energy released from ATP hydrolysis is used for various cellular processes, such as muscle contraction.
- ATP Hydrolysis: The reaction where ATP is broken down into ADP and inorganic phosphate (Pi), releasing energy.
- Creatine Phosphate: A molecule that helps regenerate ATP in muscles by donating a phosphate group to ADP.
- Krebs Cycle: A series of enzyme-catalyzed reactions in the mitochondrial matrix that oxidize acetyl-CoA to produce ATP, NADH, FADH2, and CO2. It begins with the combination of acetyl-CoA and oxaloacetate to form citric acid.
- Acetyl-CoA: A molecule formed from the decarboxylation of pyruvate. It enters the Krebs cycle, combining with oxaloacetate.
- Oxaloacetate (OAA): A four-carbon molecule that combines with acetyl-CoA to form citric acid, starting the Krebs cycle.
- Citric Acid: A six-carbon compound formed in the first step of the Krebs cycle.
- Electron Transport Chain (ETC): A series of protein complexes and other molecules in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, creating a proton gradient that drives ATP synthesis.
- Oxidative Phosphorylation: The process of ATP production driven by the transfer of electrons through the electron transport chain to oxygen. It is responsible for most of the ATP generated during cellular respiration.
- Site I, Site II, Site III: Specific sites in the electron transport chain where energy is released and used to pump protons across the mitochondrial membrane, contributing to the formation of ATP.
- Phosphorylation: The addition of a phosphate group to a molecule, such as the formation of ATP from ADP and Pi.
- Decarboxylation: The removal of a carboxyl group from a molecule, as seen in the conversion of pyruvate to acetyl-CoA.
Cellular Respiration
Cellular respiration is the process by which cells generate energy from glucose in the form of ATP (Adenosine Triphosphate). The energy released during cellular respiration is temporarily captured in ATP molecules within the cell. ATP stores energy for future reactions or can be used immediately when the cell requires energy.
Animals convert the energy from food breakdown into ATP, while plants store energy from light during photosynthesis in ATP molecules. ATP is composed of an adenine base, a ribose sugar, and three phosphate groups linked by two high-energy phosphoanhydride bonds. When one phosphate group is removed through hydrolysis, energy is released, converting ATP to ADP. Similarly, energy is released when ADP loses a phosphate group to form AMP. AMP can be recycled back into ADP or ATP by forming new phosphoanhydride bonds to store energy again. Within the cell, AMP, ADP, and ATP are constantly interconverted as they participate in biological reactions
Coupled Reactions
Many biochemical reactions in which energy is given off- (is called exothermic), whereas many others reactions that require energy (are called endothermic). In order for both processes to be carried out efficiently, they must be “coupled”. Usually, a coupled reaction will involve ATP or some similar molecules. A coupled reaction is carried out when two reactions occur nearly simultaneously. The first reaction must be exothermic and that gives off energy. The second reaction is endothermic, which immediately uses the energy produced from the first reaction.
An example of a coupled reaction is the hydrolysis of ATP and the contraction of muscle tissue. Two proteins, actin and myosin, form a loose complex called actomyosin. When ATP is added to isolated actomyosin, the protein fibers contract. The hydrolysis of ATP releases energy which is used by muscles to contract. The coupled reaction is:
ATP+ H20 ADP+ P + energy
When the ATP is used up by the muscles, a further supply of energy is released from creatine phosphate. Another example of a coupled reaction is the hydrolysis of creatine phosphate to release energy which in turn is used for the formation of more ATP. The coupled reaction is:
- Creatine — P03 + H20 creatine H + HPQ4-3 + energy
- ADP+ HP04-3 +energy ATP+ H20
During periods of low muscular activity, the reactions are reversed to replenish the supplies of ATP and creatine phosphate. The energy for the formation of ATP is supplied by other metabolic reactions.
The site of cellular respiration
Glycolysis occurs in the cytosol of the cell and does not require oxygen, whereas the Krebs cycle and electron transport occur in the mitochondria and reqmres oxygen. Cellular respiration is carried out by both prokaryotic and eukaryotic cells. In prokaryotic cells, it is carried out in the cell cytoplasm, but eukaryotic cells it begins in the cytosol then is carried out in the mitochondria. In eukaryotes, the four stages of cellular respiration include
- Glycolysis
- Transition reaction (pyruvate oxidation)
- The krebs cycle (also known as the citric acid cycle)
- Oxidative phosphorylation (electron transport chain).
Glycolysis
Glycolysis begins cellular respiration by breaking glucose into two molecules of a three-carbon compound called pyruvate that show in (Figure 23).
The 10 steps of glycolysis can be grouped into three phases
The first phase three (steps 1-3) involves an energy investment. Two ATP molecules are hydrolyzed, and the phosphates from those ATP molecules are attached to glucose, which is converted to fructose-1,6- bisphosphate (Figure 23). The energy investment phase raises the free energy of glucose, thereby allowing later reactions to be exergonic
The cleavage phase (2nd) or (steps 4-5) breaks this six-carbon molecule into two molecules of glyceraldehyde-3-phosphate.
The energy liberation phase (3rd) or (steps 6-10) produces four ATP, two NADH, and two molecules of pyruvate. Because two molecules of ATP are used in the energy investment phase, the net yield is two molecules of ATP.
Stage II: Pyruvate oxidation (link reaction)
Pyruvate enter the next pathway, it must undergo several changes to become acetyl Coenzyme A. (The conversion of pyruvate to acetyl CoA is a three-step process (Figure 27). step
A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium. (Note: carbon dioxide is one carbon attached to two oxygen atoms and is one of the major end products of cellular respiration).
The hydroxyethyl group is oxidized to an acetyl group, and the electrons are picked up by NAD+, forming NADH (the reduced form of NAD+). The high- energy electrons from NADH will be used later by the cell to generate ATP for energy.
The enzyme-bound acetyl group 1s transferred to CoA, producing a molecule of acetyl CoA. This molecule of acetyl CoA is then further converted to be used in the next pathway of metabolism, or the citric acid cycle.
2pyruvate + 2NAD+ + 2 CoA –> 2 acetyl-CoA + 2NADH + 2H+ + 2C02
Stage III: Krebs cycle
The Krebs cycle itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (Figure 28). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle. After citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2, another energy- carrying compound. Carbon dioxide is also released as product of these reactions. The final step of the Krebs cycle regenerates OAA, the molecule that began the Krebs cycle. This molecule is needed for the next tum through the cycle. Two turns are needed because glycolysis produces two pyruvate molecules when it splits glucose.
Stage IV: Oxidative phosphorylation
It is the process in which ATP is formed from the transfer of electrons from NADH or FADH2 to O2 by a series of electron carriers. This process, which takes place in mitochondria, is the major source of ATP in aerobic organisms. Oxidative phosphorylation generates 26 out of the 30 molecules of ATP that are formed when glucose is completely oxidized to CO2 and H20.The three major steps in oxidative phosphorylation are:-
- oxidation-reduction reactions involving electron transfers between specialized proteins embedded in the inner mitochondrial membrane;
- The generation of a proton (H+) gradient across the inner mitochondrial membrane (which occurs simultaneously with step A.
- The synthesis of ATP using energy from the spontaneous diffusion of electrons down the proton gradient generated in step B.
Electron transfer occurs through a series of protein electron carriers, the final acceptor being O2 and the pathway is called the electron transport chain (ETC). The function of ETC is to facilitate the controlled release of free energy that was stored in reduced cofactors during catabolism.
as a result of the transfer of electrons from NADH or FADH2 to O2 by a series of electron carriers show in the figure above.
Energy is released when electrons are transported from higher energy NADH/FADH2 to lower energy O2. This energy is used to phsophorylate ADP. There are 3 sites of the chain that can give enough energy for ATP synthase. These sites are:
- Site I between FMN and Coenzyme Q at enzyme complex I.
- Site II between cyt b and cyt C1 at enzyme complex III
- Site III between cyt a and cyt a3 at enzyme complex IV
Energy generated by the transfer of electrons through the electron transport chain to O2 is used in the production of ATP, the overall process is known as oxidative phosphorylation.
Oxidative phosphorylation is responsible for 90% of the total ATP synthesis in the cell. Oxidative phosphorylation is the process in which ATP is formed