Cellular Respiration: How Cells Harvest Energy

Posted on Aug 24, 2024 in Biology

Biology 112 – Chapter 6 Notes

How Cells Harvest Chemical Energy

I. Introduction to Cellular Respiration

A. Energy is Necessary for Life Processes
  1. Photosynthesis and cellular respiration provide energy for life.
    1. Energy in sunlight is used in photosynthesis to make glucose from CO2 and H2O with the release of O2.
    2. Other organisms, through cellular respiration, use the O2 and energy in sugar and release CO2 and H2O.
    3. Together, these two processes are responsible for the majority of life on Earth.
  2. Photosynthesis
    1. This takes sunlight, carbon dioxide (CO2), and water.
  3. Cellular Respiration
    1. It is the aerobic harvesting of energy from food molecules (sugars) by cells.
    2. It consumes oxygen and breaks down glucose to CO2 and water.
    3. In the process, it captures energy released in adenosine triphosphate (ATP).
    4. Chemicals, such as carbon, oxygen, and hydrogen, are recycled.
B. Breathing Supplies Oxygen to Cells and Removes Carbon Dioxide
  1. Breathing and cellular respiration are closely related.
    1. Breathing is necessary for the exchange of CO2 produced during cellular respiration for atmospheric O2.
    2. Cellular respiration uses O2 to help harvest energy from glucose and produces CO2 in the process.
C. Cellular Respiration Banks Energy in ATP Molecules
  1. Generating ATP for cellular work is the fundamental function of cellular respiration.
  2. Cellular respiration is an exergonic process that transfers energy from the bonds in glucose to ATP.
    1. The equation is: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATPs
    2. In this equation, glucose and oxygen yield carbon dioxide, water, and chemical-bond energy stored (banked) in ATPs.
  3. Cellular respiration can bank up to 38 ATPs for each glucose molecule.
    1. Other foods (organic molecules) can be used as a source of energy as well.
D. The Body Uses Energy from ATP for All of its Activities
  1. Our body requires a continuous supply of energy to:
    1. Keep the heart pumping
    2. Breathe
    3. Maintain body temperature
    4. Reproduce
  2. These and other life-sustaining activities use as much as 75% of the energy a person takes in as food in a normal day.
    1. Regardless of your activity, most of your cells are busy with cellular respiration, producing ATP just to maintain your body.
  3. The National Academy of Science estimates that the average adult needs to take in food that provides about 2,200 kcal per day.
    1. This is an estimate of the amount of energy a person burns in both maintenance and voluntary activity.
E. Cells Tap Energy from Electrons Falling from Organic Fuels to Oxygen
  1. The energy available to cells is contained in the covalent bonds of organic compounds.
  2. When the carbon-hydrogen bonds of glucose are broken, electrons are transferred to oxygen.
  3. Cellular respiration is the controlled breakdown of organic molecules.
    1. Energy is released in small amounts.
      1. This can be captured by a biological system and stored in ATP.
  4. The movement of electrons from one compound to another is an oxidation-reduction reaction or redox reaction.
    1. The loss of electrons from one substance is called oxidation.
    2. The gain of electrons by a substance is called reduction.
    3. Then, a molecule is said to be oxidized when it loses one or more electrons and reduced when it gains one or more electrons.
    4. Oxidation and reduction always go together.
    5. In cellular respiration:
      1. Glucose loses hydrogen atoms and is ultimately converted to CO2.
        1. It is oxidized.
      2. Simultaneously, oxygen gains hydrogen atoms and is converted to H2O.
  5. Enzymes are necessary to oxidize glucose and other foods.
    1. Dehydrogenase
      1. The enzyme that removes hydrogen from an organic molecule.
      2. Dehydrogenase requires a coenzyme called NAD+ (nicotinamide adenine dinucleotide) to shuttle electrons.
      3. NAD+ is a coenzyme that the cells make from the vitamin niacin.
      4. The transfer of electrons to NAD+ results in the formation of NADH, the reduced form of NAD+.
    2. There are other electron “carrier” molecules that function like NAD+.
      1. These electron carriers collectively are called the electron transport chain.
        1. As electrons are transported down the chain, ATP is generated.
F. An Overview of Cellular Respiration
  1. Cellular respiration consists of three stages:
    1. Glycolysis is stage 1.
    2. Citric acid cycle (= The Krebs Cycle) is stage 2.
    3. Oxidative phosphorylation is stage 3.
  2. Glycolysis
    1. Occurs outside the mitochondria.
      1. Occurs in the cell’s cytoplasm under anaerobic (without oxygen) conditions.
    2. Glycolysis begins respiration by breaking glucose, a six-carbon molecule, into two molecules of a three-carbon compound called pyruvate.
    3. This involves 9 enzyme-mediated steps.
      1. It takes two ATPs to drive glycolysis.
    4. In the process, two molecules of NAD+ are reduced to two molecules of NADH.
    5. At the same time, two molecules of ATP are produced by substrate-level phosphorylation.
      1. This involves an enzyme transferring a phosphate group from a substrate molecule directly to ADP.
        1. This ATP can be used immediately.
      2. The NADH produced must be transported through the electron transport chain to generate additional ATP.
    6. This is a universal energy-harvesting process of life.
      1. Some organisms can satisfy their energy needs through glycolysis alone.
      2. All cells can use glycolysis for the energy necessary for viability.
    7. Glycolysis produces:
      1. Four ATPs
      2. Two pyruvate molecules
    8. There is a net gain of two ATPs in glycolysis.
  3. Citric Acid Cycle (= The Krebs Cycle)
    1. The second stage.
    2. Occurs in the mitochondria within the mitochondrial matrix.
      1. Occurs under aerobic (in the presence of oxygen) conditions.
    3. The main function of this cycle, as well as glycolysis, is to supply the third stage of cellular respiration with electrons.
    4. Before the Citric acid cycle actually begins, the pyruvate molecules must undergo a preparatory step.
      1. This takes place in the mitochondria.
      2. The two pyruvate molecules that enter the mitochondria are converted into 2 Acetyl Coenzyme A (acetyl CoA) molecules.
        1. A high-energy fuel molecule for the citric acid cycle.
    5. This cycle produces from the two acetyl CoA molecules:
      1. Two ATPs by substrate-level phosphorylation
      2. Two FADH2 molecules
        1. This is another coenzyme electron carrier.
    6. The energy “banked” in NADH and FADH2 molecules is shuttled to the electron transport chain.
  4. Oxidative Phosphorylation
    1. The third stage of cellular respiration.
    2. Occurs in the cristae of the mitochondria.
    3. Oxidative phosphorylation involves electron transport and chemiosmosis and requires an adequate supply of oxygen.
      1. NADH and FADH2 and the inner membrane of the mitochondria are also involved.
      2. A H+ ion gradient formed from all of the redox reactions of glycolysis and the citric acid cycle.
        1. This provides energy for the synthesis of ATP.
    4. NADH and FADH2 shuttle electrons to the electron transport chain.
      1. A series of electron carrier molecules that shuttle electrons during the redox reactions.
        1. They release energy used to make ATP.
      2. It is located in the:
        1. Inner membrane of the mitochondria
        2. Thylakoid membrane of the chloroplast
    5. Most of the ATP generated in cellular respiration is generated in this stage.
    6. The electron transport chain passes electrons down the chain.
      1. It also pumps hydrogen ions across the inter-mitochondrial membrane.
        1. This results in a concentration gradient of hydrogen ions across the membrane.
      2. Chemiosmosis is the production of ATP using the energy of the hydrogen ion gradient across the membrane to phosphorylate ADP.
        1. The potential energy of the H+ concentration gradient is used to make ATP.
    7. By way of facilitated diffusion, hydrogen ions move down the concentration gradient and re-enter the matrix by using ATP synthase enzymes embedded in the cristae (=chemiosmosis).
    8. The total number of ATPs produced in this stage is 34.
    9. Without oxygen to function as the final electron acceptor in the electron transport chain:
      1. The cells may die from energy starvation.
  5. Review: Each molecule of glucose yields many molecules of ATP
    1. Recall that the energy payoff of cellular respiration involves:
      1. Glycolysis
      2. Alteration of pyruvate
      3. The citric acid cycle
      4. Oxidative phosphorylation
    2. The total yield of ATP molecules per glucose molecule has a theoretical maximum of about 38.
      1. This is about 40% of a glucose molecule’s potential energy.
      2. Additionally, water and CO2 are produced.
  6. Certain Poisons Interrupt Critical Events in Cellular Respiration
    1. One group of poisons blocks the electron transport chain.
      1. The poison binds with an electron carrier.
        1. This prevents the electrons from passing to the next carrier.
      2. Examples: Rotenone, cyanide, and carbon monoxide.
    2. A second type inhibits ATP synthase.
      1. This type blocks the passage of hydrogen ions through the channel in which ATP synthase works.
      2. Example: Oligomycin
        1. An antibiotic that is used to combat skin fungal infections.
    3. A third kind is referred to as uncouplers.
      1. These make the inner membrane of the mitochondria leaky to hydrogen ions.
        1. This eliminates the concentration gradient and stops the production of ATP.
      2. Example: Dinitrophenol
        1. It allows all steps of cellular respiration to run except chemiosmosis.
        2. This results in consuming fuel molecules even though almost all the energy is lost to heat.

G. Fermentation

  1. It is an anaerobic energy-generating process that is an alternative to cellular respiration.
    1. It takes advantage of glycolysis.
      1. It produces two ATP molecules and reduces NAD+ to NADH.
    2. It is an enzymatically controlled anaerobic breakdown of energy-rich compounds, such as carbohydrates (glucose) to:
      1. Carbon dioxide
      2. Alcohol
    3. The metabolic pathway that generates ATP during fermentation is glycolysis.
      1. Glycolysis uses no oxygen.
        1. It is an anaerobic process.
      2. Glycolysis generates a net gain of two ATPs.
        1. This oxidizes glucose to form:
        2. It reduces NAD+ to NADH.
      3. Two ATPs are enough to keep your muscles contracting when the need for ATP outpaces the delivery of oxygen by the bloodstream.
      4. Many microorganisms supply all their energy needs with the two ATPs generated in glycolysis.
      5. Fermentation provides an anaerobic step that recycles NADH back to NAD+.
    4. There are two types of fermentation:
      1. Lactic acid fermentation
      2. Alcoholic fermentation
    5. Lactic Acid Fermentation
      1. The conversion of pyruvate to lactate with no release of carbon dioxide.
        1. NADH is oxidized to NAD+ when pyruvate is reduced to lactate.
        2. Lactate builds up in the muscles during strenuous exercise.
          1. This contributes to muscle fatigue and soreness.
        3. Lactate is eventually carried to the liver via the bloodstream.
          1. Here it is converted back to pyruvate.
      2. This occurs in:
        1. Your muscle cells
        2. A few other types of cells
      3. This is used in the dairy industry to produce cheese and yogurt.
      4. Also used to turn soybeans into soy sauce and cabbage into sauerkraut.
    6. Alcoholic Fermentation
      1. The conversion of pyruvic acid produced by glycolysis to carbon dioxide and ethyl alcohol.
      2. Used for thousands of years for making:
        1. Wine
        2. Beer, ales, etc.
        3. Bread
      3. This is done by yeast.
        1. They normally use aerobic respiration to process their food.
        2. They are able to survive in anaerobic environments when there is plenty of glucose to keep glycolysis operating.
        3. When yeast are confined in a wine vat they will die when the alcohol reaches 14%.
    7. Yeast and many bacteria that live in stagnant ponds and deep in the soil are strict anaerobes.
      1. They require continuous anaerobic conditions.
        1. They are poisoned by oxygen.
      2. These organisms use glycolysis to satisfy their ATP needs.
    8. Facultative anaerobes
      1. Yeast and many bacteria.
      2. They are capable of making ATP by:
        1. Oxidative phosphorylation
      3. This depends on whether oxygen is present or not.

    II. Interconnections Between Molecular Breakdown and Synthesis

    A. Cells Use Many Kinds of Organic Molecules as Fuel for Cellular Respiration
    1. Glucose is considered to be the primary source of sugar for respiration and fermentation.
    2. However, there are actually three sources of molecules for the generation of ATP:
      1. Carbohydrates
        1. Primarily disaccharides.
      2. Proteins
        1. After conversion to amino acids.
      3. Fats
    B. Food Molecules Provide Raw Materials for Biosynthesis
    1. Biosynthesis
      1. The production of chemical compounds by living organisms through the use of ATP.
    2. Many metabolic pathways are involved in the biosynthesis of biological molecules.
      1. Not all food goes for the production of ATP.
      2. To survive, cells must be able to biosynthesize molecules that are not present in its foods.
      3. Cells can make molecules that are not present in food by using the intermediate compounds of:
        1. Glycolysis
        2. The citric acid cycle