Lecture 6 Cellular Respiration and Metabolic Energy Pathways

The Fundamentals of Biological Energy

• The primary focus of cellular biology regarding energy is how cells obtain energy and what operations they perform with it.

• Two fundamental questions drive the study of biological energy:

  • Why do we eat?

  • Why do we breathe?

• Eating is necessary to consume calories, which are units of energy that fuel essential living functions.

• Breathing is necessary to intake oxygen ($O_2$), which is critical for metabolic processes.

• Cellular respiration is the specific process that utilizes both food (glucose) and oxygen to produce the energy required for life.

• The primary products of cellular respiration include:

  • Water ($H_2O$).

  • Carbon dioxide ($CO_2$).

  • Energy (the main goal of the process).

The Chemical Equations of Life

• Cellular respiration and photosynthesis are interconnected, cyclic chemical reactions where the products of one serve as the reactants for the other.

• The Equation for Photosynthesis:

  • $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

  • This represents six carbon dioxide molecules plus six water molecules yielding one glucose molecule and six oxygen molecules.

  • This process harvests light energy and stores it in the chemical bonds of glucose.

• The Equation for Cellular Respiration:

  • $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$

  • This represents one glucose molecule ($C_6H_{12}O_{12}$) plus six oxygen molecules yielding six carbon dioxide molecules, six water molecules, and energy.

• Relationships between the equations:

  • The reactants of photosynthesis ($CO_2$ and water) are the products of cellular respiration.

  • The reactants of cellular respiration (glucose and oxygen) are the products of photosynthesis.

  • Memorizing one equation effectively allows for the understanding of both, as they are essentially the reverse of one another.

The Role of Organelles and Endosymbiotic Theory

• Cellular respiration takes place in the mitochondria, often referred to as the "powerhouse of the cell" because of its energy-producing capabilities.

• Photosynthesis takes place in the chloroplast.

• Both mitochondria and chloroplasts are believed to have been gained by eukaryotic cells through the endosymbiotic theory.

• A common misconception is that plant cells only have chloroplasts while animal cells only have mitochondria. In reality:

  • Animal cells only have mitochondria.

  • Plant cells have both mitochondria and chloroplasts.

  • Plants are autotrophic, meaning they make their own food from the sun, whereas humans must consume food.

Physiological Transport of Oxygen and Carbon Dioxide

• At the multicellular level, oxygen is taken into the lungs and transported via the vascular system on red blood cells to every individual cell in the body.

• Every cell, including muscle cells and brain cells, requires oxygen to function; the absence of oxygen can cause the brain to cease functioning.

• Red blood cells pick up $CO_2$ as a waste product from the cells and transport it back to the lungs to be exhaled.

• $CO_2$ is a toxic waste product that must not be allowed to build up in the body.

• The sensation of the "need to breathe" (e.g., when underwater) is primarily triggered by a buildup of $CO_2$ and the resulting change in blood pH, rather than a lack of oxygen.

• Trees and forests are often described as the "lungs of the world" because they complement human physiology by taking in the $CO_2$ humans expel and releasing the oxygen humans inhale.

ATP: The Energy Currency of the Cell

• ATP (Adenosine Triphosphate) is the chemical energy currency of the cell that allows work to happen.

• ATP Hydrolysis:

  • ATP can be broken down into ADP (Adenosine Diphosphate) by breaking a phosphate bond, a process that releases energy to fuel cellular work.

• Examples of ATP usage:

  • Mechanical Work: Moving vesicles across microtubules within the cytoskeleton. Microtubules act as tracks, and proteins pull vesicles along them using ATP.

  • Transport Work: Pushing molecules against their chemical gradient across a semipermeable membrane (active transport).

  • Chemical Work: Powering various chemical reactions within the body.

Thermodynamics of the Human Body

• The average human requires approximately $2,200\,kcals$ of energy per day.

• Caloric distribution:

  • $75\%$ of daily caloric intake is used for homeostasis and the maintenance of individual cells (resting metabolic rate).

  • Only $25\%$ of daily caloric intake is typically used for physical activity.

• This distribution explains why diet is often more impactful than exercise alone for changing physique.

• Energy Consumption Statistics:

  • It requires approximately $10,000,000$ ATP molecules per second to power a single active muscle.

The Four Stages of Cellular Respiration

• The complete process of cellular respiration involves four distinct steps that yield a net total of approximately $32$ ATP molecules per glucose molecule:

  1. Glycolysis: Occurs in the cytoplasm (cytosol). Yields $2$ ATP.

  2. Acetyl CoA Production (Pyruvate Oxidation): Occurs within the mitochondria. Modifies pyruvic acid into Acetyl CoA and releases the first waste product of $CO_2$.

  3. Citric Acid Cycle (also known as the Krebs cycle or Kerrb cycle): Occurs in the mitochondrial matrix. Yields $2$ ATP and high-energy electron acceptors.

  4. Electron Transport Chain (ETC): Occurs on the inner mitochondrial membrane. Yields approximately $28$ ATP.

Detailed Breakdown of Stage 1: Glycolysis

• Glycolysis literally means "the splitting of sugar."

• It is a universal process performed by almost all living organisms, including plants, animals, and bacteria.

• Because it occurs in the cytoplasm and does not require membrane-bound organelles, prokaryotic cells (like bacteria and archaea) can perform glycolysis.

• The process involves $10$ chemical steps:

  • The first $5$ steps involve "energy investment."

  • The last $5$ steps involve "energy harvesting."

• Result: One glucose molecule is split into two molecules of pyruvic acid (pyruvate), producing a net gain of $2$ ATP.

• Glycolysis occurs in both aerobic (oxygen present) and anaerobic (oxygen absent) environments.

Detailed Breakdown of Stage 4: The Electron Transport Chain (ETC)

• The inner mitochondrial membrane has many infoldings to increase surface area, allowing for more ATP production.

• High-energy electron acceptors ($NADH$ and $FADH_2$) from the Citric Acid Cycle deposit electrons into the chain.

• This energy fuels transmembrane proteins to pump hydrogen ions ($H^+$) from a low concentration in the matrix to a high concentration in the intermembrane space (active transport).

• ATP Synthase:

  • A quaternary structured transmembrane protein consisting of alpha helixes and beta pleated sheets.

  • It acts as an enzyme that synthesizes ATP as hydrogen ions flow back down their concentration gradient.

  • An organism can produce up to $100$ ATP molecules per second through ATP synthase.

• Oxygen's Role:

  • Oxygen acts as the terminal (final) electron acceptor in the chain.

  • It combines with electrons and hydrogen ions to form water ($H_2O$), a product of respiration.

Alternative Metabolic Pathways and Food Sources

• While glucose is the most common energy source, the body can process other macromolecules through cellular respiration:

  • Carbohydrates: Broken down from polysaccharides into monosaccharides (sugars) via hydrolysis, entering at the start of glycolysis.

  • Fats (Lipids): Broken down into glycerol (enters glycolysis) and fatty acids (enter at the Acetyl CoA stage).

  • Proteins: Broken down into amino acids; different portions enter at glycolysis, Acetyl CoA production, or the Citric Acid Cycle.

Anaerobic Metabolism: Fermentation

• In the absence of oxygen (anaerobic conditions), cells can switch from cellular respiration to fermentation.

• Fermentation is much less efficient, yielding only the $2$ ATP produced during glycolysis.

• Lactic Acid Fermentation:

  • Occurs in human muscle cells when oxygen demand exceeds supply (e.g., during a marathon).

  • Pyruvate is converted to lactic acid.

  • The buildup of lactic acid (which has a low pH and high $H^+$ concentration) causes the "burning" sensation in muscles.

• Alcohol Fermentation:

  • Performed by yeast (a single-celled eukaryotic fungus in Kingdom Fungi).

  • Produces ethyl alcohol and $CO_2$ as byproducts.

  • In bread making, the $CO_2$ bubbles cause the dough to rise.

  • In brewing, it provides the alcohol and natural carbonation in beer (though industrial brewers often add extra $CO_2$).

• Products of Fermentation:

  • Cheese, yogurt, soy sauce, pickles, vinegar.

  • Industrial chemicals like acetone and isopropanol (nail polish remover).

  • Pickling preserves food because the acidic environment (low pH) prevents bacterial growth.

Summary of Inputs and Outputs

• Glycolysis: Input: Glucose; Output: Pyruvate, $2$ ATP. (Location: Cytosol).

• Acetyl CoA Production: Input: Pyruvate; Output: Acetyl CoA, $CO_2$. (Location: Mitochondria).

• Citric Acid Cycle: Input: Acetyl CoA; Output: $2$ ATP, $CO_2$, high-energy electron acceptors. (Location: Mitochondria).

• Electron Transport Chain: Input: Oxygen, high-energy electron acceptors; Output: Water ($H_2O$), ~$28$ ATP. (Location: Inner Mitochondrial Membrane).