Respiration and Photosynthesis

Respiration and Photosynthesis

Respiration: Breaking Down Organic Molecules

• Respiration involves processes that break down organic molecules to release energy.

• These are exergonic processes.

Types of Respiration:

• Fermentation:

  • Partially degrades sugars.

  • Anaerobic (occurs without oxygen).

• Aerobic Respiration:

  • Consumes oxygen and organic molecules.

• Anaerobic Respiration:

  • Does not consume oxygen but consumes other compounds along with organic molecules.

ATP Production

• All types of respiration (fermentation, aerobic, and anaerobic) produce ATP.

• The term "respiration" often refers to aerobic respiration.

• Various molecules can be broken down, including carbohydrates, fats, and proteins, but the process is usually described with glucose as an example.

Glucose Breakdown (Aerobic Respiration)

• Glucose (a six-carbon molecule) and oxygen are broken down into carbon dioxide, water, and ATP.

Redox Reactions

• Redox reactions involve the transfer of electrons between reactants.

• Oxidation: A substance loses electrons.

• Reduction: A substance gains electrons.

• Oxidation and reduction always occur together.

• Example: Sodium is oxidized (loses an electron), and chloride is reduced (gains an electron) to form Na^+ and Cl^-.

Oxidation and Reduction in Respiration

• In respiration, glucose is oxidized (loses electrons), and oxygen is reduced (gains electrons).

Electron Carriers

• Organic molecules are broken down in a series of steps.

• Electrons are not directly passed from glucose to oxygen but are carried by intermediates called electron carriers or coenzymes.

• NAD (nicotinamide adenine dinucleotide) is one such coenzyme.

  • NAD takes electrons from glucose and ultimately passes them to oxygen.

• Electron carriers store energy released from glucose and are used to synthesize ATP.

Electron Transport Chain

• NADH (reduced form of NAD) passes electrons to the electron transport chain.

• Oxygen is the final electron acceptor in the chain, forming water.

• The electron transport chain consists of many molecules that accept and pass on electrons, leading to ATP production.

Cellular Respiration Processes

• Cellular respiration starts with glucose.

1. Glycolysis

• Glucose is broken down to pyruvate.

• Generates a small amount of ATP via substrate-level phosphorylation.

• Releases electrons carried by NADH.

2. Pyruvate Oxidation

• In eukaryotic cells, pyruvate moves into the mitochondrion.

• Pyruvate is broken down to acetyl CoA.

3. Citric Acid Cycle (Krebs Cycle)

• Occurs in the mitochondrion.

• Acetyl CoA is further broken down.

• Leads to ATP production via substrate-level phosphorylation.

• Releases electrons carried by NADH and FADH2 (another electron carrier).

4. Oxidative Phosphorylation

• Occurs via the electron transport chain.

• Electrons from NADH and FADH2 are used to produce a large amount of ATP.

• This step accounts for about 90% of the ATP generated by cellular respiration.

ATP Yield

• For each glucose molecule, a cell makes up to approximately 32 ATP molecules via aerobic respiration.

• Most ATP is generated during oxidative phosphorylation.

• A smaller amount (10%) is made during glycolysis and the citric acid cycle via substrate-level phosphorylation.

Substrate-Level vs. Oxidative Phosphorylation

Substrate-Level Phosphorylation

• ATP is made during the catabolism of an organic compound.

• An intermediate is phosphorylated, and the phosphate group is transferred to ADP to make ATP.

• Directly coupled to the catabolism of the organic molecule.

• Common in fermentation.

Oxidative Phosphorylation

• ATP production via electron transport and a proton motive force.

• Electron transport moves protons across the cell membrane, creating a charge (proton motive force).

• This force is used by an enzyme to make ATP.

• Photophosphorylation: Similar to oxidative phosphorylation but uses light as the energy source to generate the proton motive force.

The Electron Transport Chain and Chemiosmosis

• Glycolysis and the citric acid cycle generate electron carriers (NADH, FADH2).

• These carriers donate electrons to the electron transport chain.

• The electron transport chain generates a proton motive force, which drives ATP synthase to make ATP via oxidative phosphorylation.

Chemiosmosis

• Electron transfer causes proteins in the electron transport chain to pump protons across the membrane.

• Protons move from the mitochondrial matrix to the intermembrane space and then back into the matrix through ATP synthase.

• This movement drives ATP production from ADP.

• Chemiosmosis is universal in all organisms.

Components

• The electron transport chain has components that transfer electrons to oxygen.

• Proteins move protons across the membrane.

• ATP synthase makes ATP from ADP and inorganic phosphate (Pi).

• Electron transport chain and chemiosmosis by ATP synthase together are called oxidative phosphorylation.

Energy Flow

• Energy flows from glucose to electron carriers (NADH) to the electron transport chain to the proton motive force to ATP.

• About 34% of the energy in glucose is transferred to ATP during cellular respiration.

• This results in approximately 32 ATP molecules per glucose molecule.

Key Differences

• Substrate-level phosphorylation directly generates ATP through enzyme-catalyzed reactions.

• Oxidative phosphorylation generates ATP through the electron transport chain and chemiosmosis.

Anaerobic Respiration and Fermentation

• Most cellular respiration requires oxygen for ATP production.

• Without oxygen, the electron transport chain won't work in aerobic organisms.

• Glycolysis couples with anaerobic respiration or fermentation to produce ATP.

Anaerobic Respiration

• Uses electron transport chains but with a final electron acceptor other than oxygen (e.g., sulfate).

Fermentation

• Uses substrate-level phosphorylation instead of an electron transport chain.

• Many organisms that grow anaerobically rely solely on substrate-level phosphorylation.

Common Types of Fermentation

• Alcoholic Fermentation:

  • Pyruvate is converted to ethanol.

  • Produces CO2.

  • Used in brewing, winemaking, and baking.

• Lactic Acid Fermentation:

  • Pyruvate is reduced by NADH to form lactate.

  • No gas (CO2) produced.

  • Used by fungi and bacteria in cheese and yogurt production.

  • Human muscle cells use lactic acid fermentation when oxygen is scarce.

Fermentation Processes

• Alcoholic Fermentation: Glucose turns into pyruvate, which is converted to acetaldehyde, releasing CO2, and then to ethanol.

• Lactic Acid Fermentation: Pyruvate is converted directly to lactate, without CO2 release.

The Role of Oxygen

• Many organisms, including fish, plants, and humans, need oxygen.

• Plants perform cellular respiration and thus require oxygen, even though they produce oxygen in photosynthesis.

• Oxygen is vital for breaking down glucose to form ATP via cellular respiration.

ATP and Cellular Processes

• ATP (adenosine triphosphate) powers many cellular processes.

• It loses a phosphate to become ADP (adenosine diphosphate).

• In cellular respiration, enzymes add a phosphate to ADP to convert it back to ATP.

Cells Without Oxygen

• Types of bacteria, archaea, yeast, and muscle cells can function without oxygen.

• Some organisms use anaerobic respiration with a final electron acceptor other than oxygen.

• Others, like muscle cells, use fermentation.

Anaerobic Respiration

• Organisms can perform glycolysis, the Krebs cycle, and the electron transport chain, but use a different final electron acceptor (e.g., sulfate).

Fermentation Details

• Allows glycolysis to continue, making ATP without oxygen.

• Glycolysis converts glucose to pyruvate, yielding a net of two ATP molecules and two NADH.

• NADH is a coenzyme and electron carrier.

• NAD+ is reduced to NADH when it gains electrons (LEO - Lose Electrons Oxidation, GER - Gain Electrons Reduction).

• Fermentation regenerates NAD+ so glycolysis can continue.

• NADH gives its electrons to an electron acceptor (a derivative of pyruvate or pyruvate itself).

Alcoholic Fermentation Process

• Glycolysis yields two net ATP, two pyruvate, and two NADH.

• The two pyruvate molecules are used to produce carbon dioxide and two ethanol alcohol molecules with acetaldehyde as the intermediate electron acceptor that allows NADH to be oxidized to two NAD+.

Lactic Acid Fermentation Process

• Glycolysis yields two net ATP, two pyruvate, and two NADH.

• Two pyruvate molecules on the reactant side yield two lactate molecules, with pyruvate acting as the electron acceptor, allowing NADH to be oxidized to NAD+.

Comparison of Fermentation and Respiration

• Fermentation passes electrons to a product from glycolysis.

• It generates less ATP than aerobic respiration.

• Organisms using fermentation need to process more substrate to generate sufficient energy.

Key Differences Summary

• All use glycolysis as the initial step.

• Fermentation uses an organic molecule derived from glucose as the final electron acceptor.

• Respiration transfers electrons to an electron transport chain, with oxygen (aerobic) or another molecule (anaerobic) as the final electron acceptor.

• Cellular respiration produces significantly more ATP (32) than fermentation (2).

Facultative Anaerobes

• Many prokaryotes (bacteria and yeasts) can switch between aerobic and anaerobic metabolism.

• They use glycolysis to make pyruvate, which can then proceed through respiration (high ATP) or fermentation (low ATP) depending on oxygen availability.

• E. coli can switch between aerobic respiration and anaerobic fermentation based on oxygen presence.

Evolutionary Significance of Glycolysis

• Glycolysis arose very early in evolution when there was little or no oxygen available.

• Early prokaryotes likely used only glycolysis to generate ATP.

• It's an ancient and universal process.

Diverse Molecules Feeding into Respiration

• Proteins (amino acids) can be broken down to pyruvate, acetyl CoA, or citric acid cycle intermediates.

• Carbohydrates (sugars) can feed into glycolysis as glucose or glyceraldehyde-3-phosphate.

• Fats can be broken down into glycerol (feeding into glycolysis) or fatty acids (converted to acetyl CoA feeding into the citric acid cycle).