Cellular Respiration and Metabolism: Study Notes for General Biology

Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is the process by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), and release waste products. It is a multi-step metabolic pathway essential for life.

• Definition: Cellular respiration is a set of metabolic reactions that take place in the cells of organisms to convert chemical energy from oxygen molecules or nutrients into ATP.

• Main Stages: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle (Krebs Cycle), and Electron Transport Chain.

• Location: Occurs in the cytoplasm (glycolysis) and mitochondria (other stages).

• Overall Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• Importance: Provides energy for cellular processes.

• Example: Muscle cells use ATP produced by cellular respiration for contraction.

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm. It breaks down glucose into pyruvate, producing ATP and NADH.

• Definition: Glycolysis is the metabolic pathway that converts glucose (C6H12O6) into pyruvate.

• Key Steps:

  1. Glucose is phosphorylated and split into two 3-carbon molecules.

  2. ATP is consumed in early steps, produced in later steps.

  3. NAD+ is reduced to NADH.

• Net Products: 2 ATP, 2 NADH, 2 pyruvate per glucose molecule.

• Equation:

$\text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \rightarrow 2\text{Pyruvate} + 2\text{NADH} + 2\text{ATP} + 2\text{H}_2\text{O}$

• Example: Red blood cells rely solely on glycolysis for ATP production.

Pyruvate Oxidation

Pyruvate produced in glycolysis is transported into the mitochondria and converted into acetyl-CoA, releasing CO2 and producing NADH.

• Definition: Pyruvate oxidation is the conversion of pyruvate into acetyl-CoA, a substrate for the citric acid cycle.

• Key Steps:

  1. Pyruvate is decarboxylated (CO2 released).

  2. NAD+ is reduced to NADH.

  3. Acetyl group is attached to Coenzyme A.

• Products per pyruvate: 1 NADH, 1 CO2, 1 acetyl-CoA.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA.

• Definition: The citric acid cycle is a central metabolic pathway that completes the breakdown of glucose derivatives.

• Key Steps:

  1. Acetyl-CoA combines with oxaloacetate to form citrate.

  2. Series of reactions regenerate oxaloacetate.

  3. Produces NADH, FADH2, ATP (or GTP), and CO2.

• Products per acetyl-CoA: 3 NADH, 1 FADH2, 1 ATP (or GTP), 2 CO2.

• Equation:

$\text{Acetyl-CoA} + 3\text{NAD}^+ + \text{FAD} + \text{ADP} + \text{P}_i + 2\text{H}_2\text{O} \rightarrow 2\text{CO}_2 + 3\text{NADH} + 3\text{H}^+ + \text{FADH}_2 + \text{ATP} + \text{CoA}$

• Example: The citric acid cycle is active in all aerobic tissues, such as heart and brain.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

The electron transport chain is a series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, producing water and driving ATP synthesis.

• Definition: The ETC is the final stage of cellular respiration, where most ATP is produced.

• Key Steps:

  1. Electrons from NADH and FADH2 pass through complexes I-IV.

  2. Protons are pumped into the intermembrane space, creating a gradient.

  3. ATP synthase uses the proton gradient to synthesize ATP from ADP and Pi.

• Equation:

$\text{NADH} + \text{H}^+ + 1/2\text{O}_2 \rightarrow \text{NAD}^+ + \text{H}_2\text{O}$

• ATP Yield: Approximately 34 ATP per glucose molecule (total yield from all stages is about 36-38 ATP).

• Example: Neurons require continuous ATP production via oxidative phosphorylation.

Fermentation

Types of Fermentation

Fermentation is an anaerobic process that allows glycolysis to continue in the absence of oxygen by regenerating NAD+.

• Lactic Acid Fermentation: Pyruvate is reduced to lactic acid. Occurs in muscle cells during intense exercise.

• Alcoholic Fermentation: Pyruvate is converted to ethanol and CO2. Occurs in yeast and some bacteria.

• Equation (Lactic Acid):

$\text{Pyruvate} + \text{NADH} \rightarrow \text{Lactate} + \text{NAD}^+$

• Equation (Alcoholic):

$\text{Pyruvate} \rightarrow \text{Ethanol} + \text{CO}_2$

• Importance: Allows ATP production when oxygen is limited.

• Example: Yeast cells ferment sugars to produce alcohol in brewing.

Regulation of Cellular Respiration

Control Points and Feedback Mechanisms

Cellular respiration is tightly regulated to meet the energy needs of the cell and prevent wasteful overproduction of ATP.

• Key Regulatory Enzymes: Hexokinase, phosphofructokinase, pyruvate dehydrogenase.

• Feedback Inhibition: High levels of ATP inhibit phosphofructokinase, slowing glycolysis.

• Allosteric Regulation: Enzymes are activated or inhibited by molecules binding at sites other than the active site.

• Example: When ATP is abundant, glycolysis slows; when ATP is low, glycolysis speeds up.

Comparison of Aerobic and Anaerobic Respiration

Key Differences

Aerobic and anaerobic respiration differ in their use of oxygen, ATP yield, and end products.

Feature	Aerobic Respiration	Anaerobic Respiration (Fermentation)
Oxygen Requirement	Required	Not required
ATP Yield (per glucose)	36-38 ATP	2 ATP
End Products	CO2, H2O	Lactic acid or ethanol, CO2
Location	Cytoplasm & mitochondria	Cytoplasm

ATP: The Energy Currency of the Cell

Structure and Function of ATP

ATP (adenosine triphosphate) is the primary energy carrier in all living organisms. It stores and supplies energy for many biochemical cellular processes.

• Structure: Composed of adenine, ribose, and three phosphate groups.

• Function: Hydrolysis of ATP releases energy for cellular work.

• Equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

• Example: ATP powers muscle contraction, active transport, and biosynthesis.

Mitochondria: The Powerhouse of the Cell

Structure and Function

Mitochondria are double-membraned organelles responsible for producing most of the cell's ATP through aerobic respiration.

• Structure: Outer membrane, inner membrane (with cristae), matrix.

• Function: Site of the citric acid cycle and electron transport chain.

• Example: Mitochondria are abundant in muscle cells due to high energy demand.

Additional info:

• Some diagrams and images in the original notes illustrate the steps of glycolysis, the citric acid cycle, and ATP synthesis, as well as regulatory mechanisms and comparisons between aerobic and anaerobic respiration.

• Key regulatory points and feedback mechanisms are highlighted for exam preparation.