Question

1. Fermentation vs. aerobic respiration – why organisms or specific tissues would need to utilize anaerobic respiration?

2. Citric acid: where does it occur and what is produced/harvested from this cycle? (know the electron carriers)

3. Signal-based targeting: where does it begin, what types of proteins does it produce?

Answer 1

1. All the organisms respire either aerobically or anaerobically in order to release energy to fuel their living processes. Aerobic respiration uses glucose and oxygen whereas anaerobic respiration uses only glucose.

Most of the organisms cannot respire without oxygen but some organisms and specific tissues can continue to respire if the oxygen runs out. Such organisms and tissues need to utilize anaerobic respiration.

For example some plants and some fungi like yeast need anaerobic respiration in the absence of oxygen as its preferable to release less energy but remain alive. In yeast cells glucose is converted to Co2 and ethanol and releases energy to remain alive.

Glucose --------> ethanol + Co2 + energy released.

Muscle of human respire anaerobically for short periods of time in absence of oxygen, even though the process is relatively inefficient but saves life. The glucose in muscle gets converted into lactic acid and releases energy.

Glucose -----------> Lactic acid + energy released

2. Citric acid cycle refers to the first molecule that forms during the cycle's reactions - citrate or in its protonated form, citric acid.

In Eukaryotes the citric acid cycle takes place in the matrix of mitochondria. In prokaryotes it takes place in the cytoplasm.

One turn of citric acid cycle releases two Co2 molecules and produces three NADH, one FADH2 and one ATP or GTP. The cycle goes around twice for each molecule of glucose that enters cellular respiration as there are two pyruvates and thus two acetyl CoAs are produced per glucose.

3. Signal based targeting is targeting the specific proteins to the different parts of a cell. Cells have various shipping systems, one among them is signal based targeting in which the proteins have an amino acid sequence called a signal peptide. The targeting begins in the cytosol through ribosomes. Proteins that are destined through signal based targeting is mainly into endoplasmic reticulum. These proteins have a signal peptide sequence which is a series of hydrophobic amino acids usually found in beginning at N terminus of protein. When this sequence sticks out if ribosome its recognized by a protein complex called Signal Recognition Particle (SRP) which takes ribosome to the ER.

This type of targeting can also be done into non endomembrane organelles of cell like mitochondria, chloroplasts, Peroxisome and nucleus through their specific targeting signals. All these organelles proteins are targeted through signal based targeting.