Cell Respiration Lecture Notes

ATP as the Molecule that Distributes Energy within Cells

• ATP (Adenosine Triphosphate): A ribonucleotide composed of adenine, ribose, and three phosphate groups, serving as the primary energy currency within cells.

• Properties of ATP: Its properties, including readily hydrolyzable phosphate bonds and compatibility with enzyme binding sites, make it well-suited for use as the energy currency in cells.

• Structure: The structure of ATP as a ribonucleotide is important to understand.

  • Adenine: A nucleobase.

  • Ribose: A five-carbon sugar.

  • Triphosphate: A chain of three phosphate groups linked by phosphoanhydride bonds.

• Energy Storage: Energy is stored in the phosphate bonds of ATP, particularly the phosphoanhydride bonds linking the phosphate groups.

  • Hydrolysis of these bonds releases significant energy that can be harnessed for cellular work.

• Energy Release: Energy can be easily released from ATP through hydrolysis, where one or two phosphate groups are cleaved off, forming ADP (adenosine diphosphate) or AMP (adenosine monophosphate), respectively.

  • This released energy drives endergonic reactions and other cellular processes.

• Enzyme Attachment: Adenine and ribose components of ATP allow for its attachment to enzymes, facilitating its use in various reactions.

  • The adenine moiety allows ATP to bind specifically to certain enzymes.

Life Processes within Cells that ATP Supplies with Energy

• Biosynthesis: The assembly of organic polymers (macromolecules) requires ATP hydrolysis.

  • Anabolic reactions utilize ATP to construct complex molecules from simpler subunits.

  • Protein Synthesis: ATP provides the energy needed to form peptide bonds between amino acids.

  • DNA Replication: ATP is used to synthesize new DNA strands during cell division.

• Active Transport: ATP is essential for moving materials against their concentration gradients across cell membranes.

  • Vesicular transport (endocytosis/exocytosis) requires ATP to break and reform membranes.

  • Sodium-Potassium Pump: ATP powers the transport of sodium ions out of the cell and potassium ions into the cell, maintaining electrochemical gradients.

  • Endocytosis: ATP is required for the invagination of the cell membrane to engulf extracellular materials.

• Movement: The movement of cell components or the entire cell relies on ATP.

  • Chromosome segregation during mitosis and meiosis is an energy-dependent process.

  • Motor proteins like kinesin and dynein use ATP to move along microtubules, facilitating chromosome movement.

  • The contraction of muscle cells (via sarcomere shortening) involves ATP.

  • Myosin filaments use ATP to bind to actin filaments and generate the force needed for muscle contraction.

• Other Processes Requiring Energy:

  • Digestion

  • Muscle contraction

  • Cell division

  • Photosynthesis

  • Condensation and hydrolysis reactions

  • Endocytosis and exocytosis

  • Thinking

  • Cell respiration.

Energy Transfers During Interconversions Between ATP and ADP

• Coenzymes: Non-protein organic compounds that facilitate enzyme reactions.

  • They often carry chemical groups or electrons between different reactions.

• ATP as a Coenzyme: ATP functions as a coenzyme by adding chemical energy to reactions.

  • It donates phosphate groups to substrates, increasing their energy levels and reactivity.

• ATP Hydrolysis: ATP loses a phosphate group, releasing energy and becoming ADP (adenosine diphosphate).

  • The reaction: ATP + H2O \rightarrow ADP + Pi + Energy

• ADP as Low Energy: ADP is a low-energy form that must be replenished by adding a phosphate group to regenerate ATP.

  • This regeneration is crucial for maintaining a constant supply of ATP to meet the cell's energy demands.

• Energy Requirement: Energy is required to synthesize ATP from ADP and phosphate.

  • This energy typically comes from catabolic reactions, such as glucose oxidation during cell respiration.

• Energy Quantity: While the specific quantity of energy in kilojoules is not required, it's substantial enough for many cellular tasks.

  • Typically, the hydrolysis of one mole of ATP releases approximately 30.5 kJ/mol of energy.

Cell Respiration as a System for Producing ATP

• ATP Production: Cell respiration is a system within the cell for producing ATP using energy released from carbon compounds.

  • This process involves a series of catabolic reactions that break down organic molecules.

• Principal Substrates: Glucose and fatty acids are the main substrates (reactants) for cell respiration.

  • These molecules are rich in chemical energy stored in their bonds.

• Other Usable Compounds: A variety of organic compounds, including amino acids, can be used for energy.

  • These compounds are converted into intermediates that enter the cell respiration pathway.

• Process Overview: Carbon compounds are broken down to release energy, which is used to regenerate ATP from ADP.

  • The main stages include glycolysis, the Krebs cycle, and oxidative phosphorylation.

• Distinction from Gas Exchange: Cell respiration is distinct from gas exchange (breathing).

  • Cell respiration is a metabolic process occurring within cells, while gas exchange is the physical exchange of oxygen and carbon dioxide between an organism and its environment.

Glycolysis: Conversion of Glucose to Pyruvate

• Glycolysis: A series of stepwise reactions that convert glucose into pyruvate.

  • It occurs in the cytoplasm and does not require oxygen.

• Net Yield: Glycolysis results in a net yield of ATP and reduced NAD (NADH).

  • For each glucose molecule, 2 ATP and 2 NADH molecules are produced.

• Process Summary

  • Glucose is unstable

  • Uses 2 ATP molecules.

  • Glucose is phosphorylated using 2 ATP molecules, forming fructose-1,6-bisphosphate.

  • This phosphorylation destabilizes the glucose molecule, making it more reactive.

  • Fructose-1,6-bisphosphate is split into DHAP (dihydroxyacetone phosphate) and glyceraldehyde-3-phosphate.

  • This split results in two three-carbon molecules.

  • DHAP is converted into glyceraldehyde-3-phosphate.

  • This conversion ensures that all glucose molecules are processed through the same pathway.

  • Glyceraldehyde-3-phosphate goes through a series of reactions that produce 2 Pyruvate, 2 ATP and NADH

    • These reactions involve oxidation and phosphorylation steps to generate ATP and NADH.

• Fate of Pyruvate: The next step depends on the presence of oxygen.

  • In the presence of oxygen, pyruvate enters the mitochondria for further oxidation.

Anaerobic Respiration and Fermentation

• Anaerobic Respiration: In the absence of oxygen, glycolysis is followed by fermentation to regenerate NAD^+.

  • Fermentation allows glycolysis to continue by recycling NADH back to NAD^+.

• Fermentation: Fermentation does not produce any ATP.

  • Its sole purpose is to regenerate NAD^+.

• Lactic Acid Fermentation (Humans):

  • Glucose is converted to 2 pyruvate molecules via glycolysis, producing a net of 2 ATP and 2 NADH.

  • This process occurs in muscle cells during intense exercise when oxygen supply is limited.

  • Pyruvate is then converted to lactate, regenerating NAD^+.

    • Lactate accumulation can lead to muscle fatigue.

• Alcohol Fermentation (Yeast):

  • Glucose is converted to 2 pyruvate molecules via glycolysis, producing a net of 2 ATP and 2 NADH.

  • This process is used in the production of alcoholic beverages and bread.

  • Pyruvate is converted to acetaldehyde, releasing CO_2.

    • Acetaldehyde is then reduced by NADH to ethanol, regenerating NAD^+.

Variables Affecting the Rate of Cell Respiration

• Measurement of Cell Respiration Rate:

  • Measure ATP produced over time.

  • Measure carbon dioxide produced over time.

  • Measure glucose used up over time.

  • Measure oxygen used up over time.

  • Divide the measured quantity by time to obtain the rate of reaction.

• Factors Affecting Rate:

  • Enzyme Factors:

  • Temperature

    • Optimal temperatures promote enzyme activity, while extreme temperatures can denature enzymes.

    • pH

    • Each enzyme has an optimal pH range; deviations from this range can inhibit enzyme activity.

  • Substrate Factors:

    • Concentration of glucose

    • Higher glucose concentrations generally lead to higher respiration rates, up to a saturation point.

    • Concentration of oxygen

      • Oxygen is essential for aerobic respiration; limited oxygen availability can decrease the respiration rate.

  • Inhibitors:

    • Lower the rate of reaction

Measuring Cell Respiration

• Respirometer: An instrument used to measure the consumption of oxygen by a living specimen.

  • The specimen is sealed in a container with a substance to absorb CO_2.

  • This ensures that changes in gas volume are due to oxygen consumption only.

  • Changes in the water level of a manometer (tube) are tracked to determine oxygen consumption.