Greenhouse gases trap heat in the atmosphere. When fossil fuels burn, the carbon in the fuel combines with oxygen to form carbon dioxide. This gas is a major greenhouse gas because it efficiently absorbs infrared radiation emitted by Earth's surface and re-radiates it back downward, warming the lower atmosphere and surface. Even though some gases are more potent heat absorbers per molecule, carbon dioxide is released in much larger amounts from burning fuels, making it the dominant greenhouse gas from this process. The other options do not act as significant greenhouse gases in this context: oxygen and nitrogen are the main air components and don’t trap heat like CO2, and hydrogen isn’t present in the atmosphere in a form that contributes notably to the greenhouse effect.

Hydrogen ion concentration determines how acidic a solution is, and the term that expresses this is pH. It is defined as pH = -log10[H+], so it directly reflects the amount of hydrogen ions present. A low pH means a high hydrogen ion concentration, while a high pH means a lower hydrogen ion concentration. The other terms don’t describe hydrogen ion concentration specifically: molarity is a general measure of solute per liter, alkalinity is the capacity to neutralize acids, and pOH gauges hydroxide ion concentration (with pH and pOH adding to about 14 at room temperature).

Translation is the process being described. Here, ribosomes read the message carried by mRNA and use transfer RNA to bring in the correct amino acids. Each tRNA matches a codon on the mRNA with its anticodon, ensuring the amino acids are added in the right order. The ribosome then links these amino acids together with peptide bonds, forming a growing polypeptide chain that becomes a protein after folding. Transcription, by contrast, is the copying of DNA into messenger RNA and is not carried out by ribosomes. Replication copies DNA itself, and mutation refers to a change in the DNA sequence.

Cholesterol molecules embedded in the phospholipid bilayer regulate membrane fluidity and stability. By inserting between the fatty acid tails, they prevent the membrane from becoming too fluid at higher temperatures and from becoming too rigid at lower temperatures, which helps maintain integrity and controls permeability to small molecules. Carbohydrates on the membrane surface mainly serve roles in cell recognition and signaling rather than strengthening the bilayer. Nucleic acids are not components of the membrane, and while membrane proteins contribute to structure and function, they don’t primarily stiffen the bilayer. Therefore, cholesterol strengthens the cell membrane.