Cellular Respiration Quiz

Cell Respiration Overview

• Living cells need energy from outside sources to perform work, including assembling polymers, transporting materials across membranes, moving, and reproducing.
• Animals obtain energy by consuming other animals or photosynthetic organisms.
• Energy flows into an ecosystem as sunlight and leaves as heat.
• Chemical elements essential for life are recycled.
• Photosynthesis produces O₂ and organic molecules, which are used in cellular respiration.
• Cells use chemical energy stored in organic molecules to generate ATP, which powers cellular work.
• Catabolic pathways release stored energy by breaking down complex molecules.
• Electron transfer plays a role in catabolic pathways, which are central to cellular respiration.

Catabolic Pathways and ATP Production

• The breakdown of organic molecules is considered exergonic.
• Fermentation is an anaerobic process involving partial sugar degradation.
• Aerobic respiration requires oxygen, consumes organic molecules, and produces ATP.
• Anaerobic respiration is similar to aerobic respiration, but uses compounds other than oxygen as the electron acceptor.
• Cellular respiration encompasses both aerobic and anaerobic respiration, but typically refers to aerobic respiration.
• Carbohydrates, fats, and proteins are fuels, but glucose is often used as an example in tracing cellular respiration.
• The general formula for cellular respiration using glucose is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP + heat).

Redox Reactions

• Transferring electrons during chemical reactions releases energy stored in organic molecules.
• This energy is utilized to synthesize ATP.
• Redox reactions involve the transfer of electrons between reactants.
• Oxidation is the loss of electrons, while reduction is the gain of electrons. This is often remembered with the mnemonic "LEO the lion says GER".

The Principle of Redox

• Chemical reactions that transfer electrons between reactants are called redox reactions.
• During oxidation, a substance loses electrons.
• During reduction, a substance gains electrons, leading to a decrease in positive charge.

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain

• Cellular respiration involves the step-wise breakdown of glucose and other organic molecules.
• Electrons from organic compounds are often initially transferred to NAD+, a coenzyme.
• NAD+ acts as an oxidizing agent during cellular respiration.
• The reduced form of NAD+, NADH, stores energy that is used to synthesize ATP.

The Stages of Cellular Respiration: A Preview

• Harvesting energy from glucose involves three stages: -1. Glycolysis -2. The citric acid cycle, also known as the Krebs cycle. -3. Oxidative phosphorylation.

Glycolysis

• Glycolysis breaks down glucose into two pyruvate molecules in the cytosol.
• It consists of two major phases: energy investment and energy payoff phases.
• Glycolysis occurs regardless of the presence or absence of oxygen.

Oxidation of Pyruvate to Acetyl CoA

• Prior to entering the citric acid cycle, pyruvate is converted to acetyl CoA.
• This process links glycolysis to the citric acid cycle.
• This step involves a multienzyme complex that catalyzes three reactions: oxidation of pyruvate and release of CO₂, reduction of NAD+ to NADH, and combination of the remaining two-carbon fragment with coenzyme A to form acetyl CoA

The Citric Acid Cycle

• The citric acid cycle, or Krebs cycle, completes the breakdown of pyruvate to CO₂.
• The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH₂ per turn.
• The cycle comprises eight steps, each catalyzed by a specific enzyme.
• Citrate is formed by combining acetyl CoA with oxaloacetate.
• The cycle decomposes citrate back to oxaloacetate.
• NADH and FADH₂ carry electrons to the electron transport chain.

Oxidative Phosphorylation

• Oxidative phosphorylation is the process that generates most of the ATP in cellular respiration.
• It involves two major components: the electron transport chain and chemiosmosis.

The Pathway for Electron Transport

• The electron transport chain, located in the inner membrane (cristae) of the mitochondrion, consists of protein complexes.
• Electrons drop in free energy as they move through the chain and eventually combine with oxygen, forming water.
• Electron carriers alternate between reduced and oxidized states during electron transfer.

Chemiosmosis

• The energy released during electron transport is used to pump H⁺ from the mitochondrial matrix to the intermembrane space.
• H⁺ moves down its concentration gradient back to the matrix through ATP synthase.
• ATP synthase utilizes the flow of H⁺ to phosphorylate ADP to ATP.

An Accounting of ATP Production

• During cellular respiration, most energy flows in the order: glucose → NADH → electron transport chain → proton-motive force → ATP.
• Approximately 34% of the energy from glucose is transferred to ATP, yielding about 32 ATP.
• The rest of the energy is lost as heat.

Fermentation

• Fermentation enables cells to produce ATP without oxygen.
• In fermentation, glycolysis is followed by reactions that regenerate NAD+.
• Two common types are alcohol fermentation and lactic acid fermentation.
• In alcohol fermentation, pyruvate is converted to ethanol and CO₂.
• In lactic acid fermentation, pyruvate is reduced to lactate.
• Obligate anaerobes require either fermentation or anaerobic respiration.
• Facultative anaerobes can use either fermentation or cellular respiration depending on oxygen availability.

Evolutionary Significance of Glycolysis

• Glycolysis is a very ancient metabolic pathway.
• Early prokaryotes likely used glycolysis to produce ATP before oxygen was prevalent.
• It is the most widespread metabolic pathway on Earth.
• It occurs in the cytosol and does not require membrane-bound organelles, making it simpler.

Test your knowledge on cellular respiration, including glycolysis and the electron transport chain. This quiz covers the roles of NADH, FADH2, and ATP production, along with key processes and concepts associated with energy metabolism. Perfect for students in biology or biochemistry courses!