Correct Answer: A

Rationale: The correct answer is Glycolysis. Glycolysis is the initial step in the conversion of glucose to pyruvate. During glycolysis, glucose is broken down into pyruvate through a series of enzymatic reactions.
Choice B, the Krebs cycle, occurs after glycolysis in aerobic cellular respiration.
Choice C, the Electron transport chain, is the final step in aerobic respiration where the majority of ATP is produced.
Choice D, Aerobic respiration, is a broader term that encompasses glycolysis, the Krebs cycle, and the electron transport chain, but it is not the specific first step in the conversion of glucose to pyruvate.

Correct Answer: B

Rationale: The correct answer is the cell membrane. The cell membrane is responsible for protecting the cell, facilitating communication with the external environment, and regulating the passage of substances into and out of the cell. The nucleus (
Choice
A) is the control center of the cell and houses the genetic material but is not primarily involved in these functions. The endoplasmic reticulum (
Choice
C) is involved in protein synthesis and lipid metabolism, not primarily in protection or communication. The cytoplasm (
Choice
D) is the gel-like substance that fills the cell but does not specifically contribute to protection, communication, or substance passage.

Correct Answer: D

Rationale: The correct answer is 'meiosis I.' During meiosis I, the chromosome number is reduced from diploid to haploid through two rounds of division.
Choice A ('S phase') is incorrect as the S phase is part of the cell cycle where DNA is replicated.
Choice B ('interphase') is also incorrect as interphase is the phase where the cell prepares for division.
Choice C ('mitosis') is incorrect as mitosis is a process where a diploid cell divides to produce two identical diploid daughter cells, maintaining the chromosome number.

Correct Answer: A

Rationale: Phospholipids are indeed a crucial component of the plasma membrane. They have a unique structure with hydrophilic heads and hydrophobic tails, which allows them to form the lipid bilayer of the membrane. Steroids, sugars, and amino acids are not primary components of the plasma membrane. Steroids are a different type of lipid, sugars are often found in glycoproteins on the membrane surface, and amino acids are the building blocks of proteins, some of which are membrane proteins, but not the membrane itself.

Correct Answer: A

Rationale: The correct answer is A. DNA plays a crucial role in metabolic activities by controlling the synthesis of enzymes. Enzymes are the biological catalysts that regulate and facilitate metabolic reactions within the cell.
Choice B is incorrect because DNA is not involved in stabilizing the cell wall; that role is usually associated with other components like the cell membrane.
Choice C is incorrect as cell division is primarily regulated by different processes and molecules, not directly by DNA.
Choice D is incorrect because DNA is not related to preventing the diffusion of nutrients; instead, it is involved in coding for proteins that aid in various cellular functions.