respiration

Autotrophs vs. Heterotrophs

1. Autotrophs: Produce their own organic molecules (e.g., photoautotrophs).
2. Heterotrophs: Live on organic compounds produced by other organisms.

Chemical Equation for Cellular Respiration

C{6}H{12}O{6} + 6O{2} \rightarrow 6H{2}O + 6CO{2} + ATP + \text{heat}

This is an exergonic reaction that releases energy.

Redox Reactions: Oxidation and Reduction

• Oxidation: The loss of electrons (e^{-}) or hydrogen atoms (OiL: Oxidation is Loss).
• Reduction: The gain of electrons (e^{-}) (RiG: Reduction is Gain).

NAD+

An electron carrier that accepts 2e^{-} and 1H^{+} to become NADH. This reaction is reversible and is used to shuttle energy-rich electrons to the Electron Transport Chain (ETC).

Final Electron Acceptors

1. Aerobic respiration: Oxygen (O_{2}).
2. Anaerobic respiration: An inorganic molecule (not O_{2}).
3. Fermentation: An organic molecule.

Substrate-Level vs. Oxidative Phosphorylation

• Substrate-level phosphorylation: Transferring a phosphate directly to ADP from another molecule.
• Oxidative phosphorylation: Using ATP synthase and a proton (H^{+}) gradient to make ATP.

Glycolysis

A 10-step biochemical pathway in the cytoplasm that splits glucose (6C) into two pyruvates (3C).

• Net Gain: 2\,ATP, 2\,NADH, and 2\,H_{2}O.
• Oxygen required?: No (Anaerobic).

Pyruvate Oxidation

In the presence of O_{2}, pyruvate is converted to Acetyl CoA in the mitochondrial matrix.

• Products: 1\,CO_{2}, 1\,NADH, and 1\,Acetyl\,CoA (per pyruvate molecule).

Citric Acid Cycle (Krebs Cycle)

Occurs in the mitochondrial matrix. Acetyl CoA combines with oxaloacetate to form Citrate.

• Yield per glucose (2 turns): 2\,ATP, 6\,NADH, 2\,FADH{2}, and 4\,CO{2}.

Electron Transport Chain (ETC)

A series of membrane-bound carriers in the inner mitochondrial membrane. It uses electrons from NADH and FADH_{2} to pump protons (H^{+}) into the intermembrane space, creating a gradient.

Chemiosmosis

The process where the H^{+} gradient (proton-motive force) drives protons back across the membrane through ATP synthase to produce ATP.

Theoretical vs. Actual Energy Yield

• Theoretical: 36 \text{--} 38\,ATP per glucose.
• Actual: Approximately 30\,ATP per glucose (due to "leaky" membranes and other uses for the proton gradient).

Fermentation Types

1. Alcohol Fermentation: Pyruvate is converted to ethanol and CO_{2} (e.g., yeast).
2. Lactic Acid Fermentation: Pyruvate is converted to lactate (e.g., human muscle cells).

Phosphofructokinase

An allosteric enzyme that acts as the primary regulator of glycolysis.

• Inhibited by: ATP and Citrate.
• Stimulated by: AMP.

Autotrophs, Heterotrophs, and Energy Flow

1. Autotrophs: Produce organic molecules from inorganic sources (photoautotrophs/chemoautotrophs). 2. Heterotrophs: Live on organic compounds produced by others. Energy flows into ecosystems as sunlight, is transformed into chemical energy by autotrophs, and leaves as heat.

Chemical Equation for Cellular Respiration

C{6}H{12}O{6} + 6O{2} \rightarrow 6H{2}O + 6CO{2} + ATP + \text{heat}. Respiration is an exergonic reaction that releases energy used to perform chemical, transport, and mechanical work.

Redox Reactions: Oxidation and Reduction

Oxidation is the loss of electrons (e^{-}) or hydrogen atoms (OiL: Oxidation is Loss). Reduction is the gain of electrons (e^{-}) (RiG: Reduction is Gain). These are often dehydrogenations, where lost electrons are accompanied by protons (H^{+}).

NAD+ Role in Respiration

A coenzyme and electron carrier that accepts 2e^{-} and 1H^{+} to become NADH (storing energy). It shuttles electrons from food to the Electron Transport Chain (ETC).

Final Electron Acceptors

1. Aerobic respiration: Oxygen (O_{2}). 2. Anaerobic respiration: An inorganic molecule such as sulfate or nitrate. 3. Fermentation: An organic molecule.

Substrate-Level vs. Oxidative Phosphorylation

Substrate-level phosphorylation: A kinase enzyme transfers a phosphate group directly from a substrate to ADP. Oxidative phosphorylation: Uses ATP synthase and energy from a proton (H^{+}) gradient to generate ATP.

Stage 1: Glycolysis

Occurs in the cytosol and splits glucose (6C) into two pyruvates (3C). It is an ancient, anaerobic 10-step pathway. Net Gain: 2\,ATP (via substrate-level phosphorylation), 2\,NADH, and 2\,H_{2}O.

Stage 2: Pyruvate Oxidation

In the presence of O{2}, pyruvate is converted to Acetyl CoA (2C) in the mitochondrial matrix by the enzyme pyruvate dehydrogenase. Products per pyruvate: 1\,CO{2}, 1\,NADH, and 1\,Acetyl\,CoA.

Stage 3: Citric Acid Cycle (Krebs Cycle)

A 9-step biochemical pathway in the mitochondrial matrix. Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Yield per glucose (2 turns): 2\,ATP, 6\,NADH, 2\,FADH{2}, and 4\,CO{2}.

Stage 4: Electron Transport Chain (ETC)

A collection of molecules in the inner mitochondrial membrane that alternate between reduced and oxidized states. It uses energy from electrons to pump protons (H^{+}) into the intermembrane space, creating a gradient.

Chemiosmosis and ATP Synthesis

The proton-motive force (proton gradient) drives H^{+} through ATP synthase, which couples the diffusion of protons back into the matrix with the synthesis of ATP.

Theoretical vs. Actual energy Yield

Theoretical: 36\text{--}38\,ATP per glucose (38 for bacteria, 36 for eukaryotes). Actual: \approx 30\,ATP per glucose due to leaky inner membranes and gradient use for other work.

Fermentation Types and Concepts

Anaerobic process that regenerates NAD^{+} to keep glycolysis running. 1. Alcohol Fermentation: Produces ethanol and CO_{2}. 2. Lactic Acid Fermentation: Produces lactate. Facultative anaerobes (like muscle cells) can switch between respiration and fermentation.

Phosphofructokinase Regulation

The primary allosteric regulator of glycolysis. It is inhibited by ATP and Citrate, and stimulated by AMP (signaling low energy).

Alternative Fuel Sources

Carbohydrates, fats, and proteins can all be used for energy. Their monomers enter cellular respiration at different points