Cellular Respiration and Fermentation Vocabulary

Introduction to Energy Release and Cellular Respiration

• Living organisms release energy primarily through the conversion of adenosine triphosphate ($ATP$) molecules into adenosine diphosphate ($ADP$) molecules.
• Cells must continually convert $ADP$ molecules back into $ATP$ molecules to maintain a steady energy supply.
• The specific process responsible for this continuous conversion is known as cellular respiration.

Distinction Between Breathing and Cellular Respiration

• Breathing is a physical process that allows animals and humans to come into contact with gases in the air.
• Cellular Respiration is a chemical process. It involves the release of energy from organic compounds (food) and the gradual conversion of that energy into a form stored within $ATP$ molecules.

Chemical Pathways and Glucose Processing

• Food serves as the raw material providing the energy necessary for bodily functions.
• Cells utilize food to synthesize new molecules required for carrying out life processes.
• Cells do not "burn" glucose; they slowly release energy from it and other food compounds through several distinct pathways.
• Glycolysis is the first pathway and releases a small amount of energy, resulting in a net gain of $2$ $ATP$.
• The presence or absence of oxygen determines the subsequent pathways:
  • If oxygen is present: Glycolysis is followed by the Krebs cycle and the Electron Transport Chain ($ETC$). These stages release a large amount of energy.
  • If oxygen is absent: Glycolysis is followed by either Alcoholic Fermentation or Lactic Acid Fermentation.

The Glycolysis Pathway: Cytosolic Energy Production

• Glycolysis takes place in the cytosol of the cytoplasm, located outside of the mitochondria.
• Definitions of cellular components:
  • Cytosol: The fluid surrounding the organelles, containing suspended molecules such as salts, sugars, amino acids, and enzymes.
  • Cytoplasm: The cytosol plus the organelles suspended within it (everything except the nucleus).
  • Cell Composition: Plasma membrane + Cytoplasm (Cytosol + Organelles) + Nucleus.
• Steps of Glycolysis:
  1. One molecule of glucose ($C_6H_{12}O_6$) is broken in half, producing two molecules of pyruvic acid, which is a $3$-carbon compound.
  2. An electron carrier, $NAD^+$ (nicotinamide adenine dinucleotide), accepts $4$ high-energy electrons. This transfers the energy stored in glucose to $2$ $NADH$ molecules and $2$ $H^+$ ions.
  3. Initially, $2$ $ATP$ molecules are used to start the process.
  4. Subsequently, $4$ $ADP$ are added, producing $4$ $ATP$ molecules.
  5. Result: A net gain of $2$ $ATP$ molecules.
• Characteristics of Glycolysis:
  • It is a fast process; cells can produce thousands of $ATP$ molecules in a few milliseconds.
  • It does not require oxygen.
  • Limitations: If a cell generates large amounts of $ATP$ via glycolysis alone, available $NAD^+$ molecules fill up with electrons. Without available $NAD^+$, glycolysis shuts down and $ATP$ production stops.

Evolutionary Context of Respiration

• Glycolysis is believed to have evolved before other stages of cellular respiration because the other stages require oxygen.
• Earth's atmosphere contained no oxygen when life first evolved approximately $3.5$ to $4$ billion years ago ($b.y.a.$).
• Early life relied on anaerobic respiration.
• Approximately $2$ to $3$ billion years ago, oxygen was gradually added to the atmosphere by early photosynthetic bacteria. This period is referred to as the ‘oxygen catastrophe,’ which led to the evolution of aerobic organisms.

Mitochondrial Structure

• Mitochondria possess two separate membranes: the inner membrane and the outer membrane.
• There are three internal compartments:
  1. Intermembrane space.
  2. Cristae space.
  3. Matrix.

Aerobic Respiration: Glycolysis, Krebs Cycle, and Electron Transport

• In the presence of oxygen (aerobic conditions), glycolysis is followed by the Krebs Cycle and the Electron Transport Chain.
• At the end of glycolysis, approximately $90\%$ of the chemical energy from glucose remains unused, locked in the high-energy electrons of pyruvic acid. Oxygen, the world's most powerful electron receptor, is required to extract this energy.
• The Chemical Equation for Cellular Respiration:     $6O_2 + C_6H_{12}O_6 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$     (Oxygen + Glucose $→$ Carbon Dioxide + Water + Energy)

The Krebs Cycle (Citric Acid Cycle)

• Named after Hans Krebs, a British biochemist who identified the cycle in $1937$.
• Process Details:
  • Pyruvic acid enters the mitochondrial matrix from the cytosol.
  • One carbon is removed, forming $CO_2$ (waste product).
  • $NAD^+$ is converted into $NADH$.
  • Coenzyme A ($CoA$) joins the remaining two carbons to form Acetyl-CoA.
  • Acetyl-CoA is added to a $4$-carbon compound to produce Citric acid (a $6$-carbon compound).
  • Citric acid is broken down into a $5$-carbon compound and then a $4$-carbon compound, releasing more $CO_2$.
  • High-energy electrons are captured by $NAD^+$ and $FAD$, forming $NADH$ and $FADH_2$.
  • One molecule of $ATP$ is produced per turn of the cycle.
• Total Yield per Molecule of Glucose (Two turns for two pyruvic acids):
  • $10$ $NADH$ (total, including $2$ from glycolysis).
  • $2$ $FADH_2$.
  • $4$ $ATP$ (total, including $2$ from glycolysis).

The Electron Transport Chain (ETC)

• High-energy electrons from the Krebs Cycle are passed to the $ETC$ by $NADH$ and $FADH_2$.
• The $ETC$ is located in the inner membrane of the mitochondria.
• Energy from transporting two high-energy electrons down the chain is used to pump $H^+$ ions across the membrane into the intermembrane space.
• This creates an electrochemical gradient (chemiosmotic gradient) where the intermembrane space is positively charged and the matrix side is negatively charged.
• $ATP$ synthase utilizes this gradient to convert $ADP$ into $ATP$.
• At the end of the chain, an enzyme combines high-energy electrons with $H^+$ ions and oxygen to form water ($H_2O$).

Energy Yield and Efficiency

• Absence of oxygen: Only $2$ $ATP$ molecules are produced (via glycolysis).
• Presence of oxygen:
  • $2$ net $ATP$ from glycolysis.
  • $36$ more $ATP$ from the Krebs Cycle and $ETC$.
  • Total: $38$ $ATP$ molecules per one molecule of glucose.
• Efficiency: These $38$ $ATP$ molecules represent approximately $38\%$ of the total energy available in glucose. The remaining $62\%$ is released as heat, which explains why bodies feel warmer after exercise and do not freeze in winter.

Anaerobic Respiration and Fermentation

• Fermentation occurs after glycolysis when oxygen is absent.
• Cells convert $NADH$ back to $NAD^+$ by passing high-energy electrons back to pyruvic acid, allowing glycolysis to continue producing $ATP$.
• The products of fermentation still contain chemical energy and are used in foods and fuels.

Alcoholic Fermentation

• Used by yeast and some microorganisms.
• Produces ethyl alcohol and carbon dioxide as waste.
• Equation: $pyruvic\,acid + NADH \rightarrow ethyl\,alcohol + CO_2 + NAD^+$

Lactic Acid Fermentation

• Occurs in many cells, including human muscle cells when they cannot supply enough oxygen during intense exercise.
• Regenerates $NAD^+$ so glycolysis can continue producing $ATP$.
• Equation: $pyruvic\,acid + NADH \rightarrow lactic\,acid + NAD^+$
• The buildup of lactic acid causes a painful burning sensation and muscle soreness. This is resolved by intaking oxygen.

Energy and Exercise Physiology

• Quick Energy:
  • Existing $ATP$ in muscles only lasts a few seconds.
  • $ATP$ from lactic acid fermentation lasts about $90$ seconds.
  • Panting heavily after a sprint is the body's way of taking in oxygen to clear the lactic acid byproduct.
• Long-Term Energy:
  • Exercise exceeding $90$ seconds requires cellular respiration for a continuous $ATP$ supply.
  • Cellular respiration releases energy slower than fermentation, requiring athletes to pace themselves.
  • The body stores energy as glycogen in muscles and tissues.
  • Glycogen stores typically last for $15$ to $20$ minutes of activity.
  • After glycogen is depleted, the body begins breaking down other molecules, such as fat, for energy. Significant fat burning typically begins after being active for $17$ to $22$ minutes.

Comparison of Aerobic and Anaerobic Respiration

• Aerobic Respiration Advantages: Releases significantly more energy (up to $38$ $ATP$ per glucose molecule).
• Anaerobic Respiration Advantages: Allows organisms to live in oxygen-poor environments and produces $ATP$ very quickly.

Interdependence of Photosynthesis and Cellular Respiration

• Comparison of Equations:
  • Cellular Respiration: $6O_2 + C_6H_{12}O_6 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$
  • Photosynthesis: $6CO_2 + 6H_2O + \text{energy (sunlight)} \rightarrow 6O_2 + C_6H_{12}O_6$
• Photosynthesis takes place in chloroplasts. It uses light energy to produce glucose and oxygen from carbon dioxide and water. Light energy is converted to chemical energy in glucose.
• Cellular Respiration takes place in mitochondria. It reacts glucose and oxygen to produce carbon dioxide, water, and $ATP$. Chemical energy in glucose is converted to chemical energy in $ATP$.

Questions & Discussion

Questions regarding yeast experiment setup:

• a. If the yeast is "trapped in the bottle without new air, why is the balloon inflating?
  • The balloon is inflating because the yeast is undergoing fermentation, which releases gas as a byproduct.
• b. Is this similar to what you have studied previously on aerobic respiration?
  • It differs because it represents an anaerobic process rather than aerobic respiration.
• c. What visible evidence shows that yeast is metabolizing sugar?
  • The inflation of the balloon and the production of gas bubbles within the liquid.
• d. What gas is likely inflating the balloon?
  • Carbon dioxide ($CO_2$).
• e. Why does this setup model an anaerobic condition for yeast after the available oxygen is used up?
  • Because the bottle is sealed by the balloon, preventing new air/oxygen from entering the system once the initial supply is exhausted.
• f. Why must the balloon fit tightly around the bottle neck with no air leak in the bottle-balloon setup?
  • To ensure that all gas produced is captured within the balloon and to prevent external oxygen from entering.
• g. How would a leak affect your interpretation of gas production and fermentation?
  • A leak would result in an underestimation of the rate of fermentation and gas production, as gas would escape rather than inflate the balloon.
• h. Based on the setup, what variables could be changed to test how fermentation rate is affected?
  • Variables could include the temperature of the water, the concentration of the sugar solution, or the type of yeast used.

Quick Review Questions:

• How many stages does cellular respiration have?
  • Three.
• What are the stages of cellular respiration?
  • Glycolysis, Krebs Cycle, and Electron Transport Chain.
• Where does glycolysis take place?
  • The cytosol of the cytoplasm.
• Where does the Krebs cycle take place?
  • In the matrix of the mitochondria.
• Where is the Electron Transport Chain located?
  • The inner membrane of the mitochondria.
• What do high-energy electrons help the cells build?
  • Molecules like glucose and $ATP$.
• What are the stage(s) of aerobic respiration?
  • Glycolysis, Krebs Cycle, and Electron Transport Chain.
• What are the stage(s) of anaerobic respiration?
  • Glycolysis followed by fermentation.
• What are the two types of fermentation?
  • Alcoholic fermentation and Lactic acid fermentation.
• Which fermentation process do humans use?
  • Lactic acid fermentation.