TOEFL Complete the Words : Biology (Intermediate)

Complete passage

Living organisms share largely similar genetic material; however, cells differ dramatically in structure and function. This diversity arises not from changes in DNA sequences themselves, but from the regulation of gene expression. Certain genes are activated or suppressed depending on cellular context, developmental stage, and environmental conditions. Recent research has emphasized the role of epigenetic mechanisms, which modify how genes are expressed without altering the underlying DNA code. Chemical tags such as methyl groups can bind to DNA or associated proteins, thereby inhibiting transcription. These modifications may persist through cell division, allowing cells to maintain specialized functions over time. As a result, gene regulation is now understood as a dynamic process shaped by both genetic instructions and environmental influence.

Although most cells in a living organism contain nearly identical DNA, they don’t all behave in the same way. This passage explains that the wide variety of cell types comes from differences in how genes are used, not from differences in the genetic code itself. In other words, cells become different because certain genes are turned on or off depending on their role and environment.

The text then focuses on gene expression, which refers to the process by which genetic information is used to produce proteins. Some genes are activated when their products are needed, while others are suppressed when their activity would be unnecessary or harmful. Factors such as the type of cell, the stage of development, and surrounding environmental conditions influence which genes are expressed at a given time.

A major idea introduced in the passage is epigenetics. Epigenetic mechanisms change gene activity without altering the DNA sequence itself. Instead of rewriting genetic instructions, chemical markers attach to DNA or related proteins and affect how easily genes can be transcribed. For example, the addition of methyl groups can block transcription, preventing certain genes from being expressed.

These chemical modifications can remain in place as cells divide, which helps cells retain their specialized functions over time. Because of this persistence, gene regulation is no longer seen as a fixed process. The passage emphasizes that it is dynamic, shaped by an ongoing interaction between inherited genetic instructions and external influences from the environment.

Complete passage

Living organisms survive by maintaining internal stability despite external change. This ability, known as homeostasis, allows biological systems to regulate variables such as temperature, pH, and fluid balance within narrow limits. When these conditions deviate from optimal levels, feedback mechanisms are activated to restore equilibrium. Such regulatory processes rely on precise signaling between cells and organs. Disruptions in these systems can lead to disease, illustrating that homeostasis is not a passive state but an active process that requires constant adjustment in response to environmental and physiological demands.

Homeostasis is one of the topics appears in TOEFL biology passages, especially those discussing how organisms respond to environmental stress. In simple terms, homeostasis refers to the body’s ability to maintain stable internal conditions.

These internal conditions, often called variables, include factors that can change inside the body, such as body temperature or the salt concentration of the blood. The body constantly monitors these variables to prevent them from moving too far away from their optimal levels.

When homeostatic regulation does not function properly, internal balance is disrupted, which can lead to various diseases or health problems.

Complete passage

To protect the body from harmful microorganisms, complex defense mechanisms have evolved over time. The immune system can detect invading pathogens and differentiate between harmful substances and the body’s own cells. When a threat is identified, immune responses are activated to eliminate the pathogen and limit potential damage. Some immune reactions occur rapidly and nonspecifically, while others develop more slowly and target specific threats. If immune regulation fails, the system may mistakenly attack healthy tissues, leading to autoimmune disorders.

Some TOEFL biology passages focus on how the body protects itself from disease, and the immune system is a central part of that discussion. At its core, the immune system works by recognizing harmful organisms such as bacteria and viruses and responding to them before they cause serious damage.

An important point in this passage is the immune system’s ability to tell what belongs to the body and what doesn’t. This ability matters because immune cells must attack invading pathogens without harming healthy tissue. Without such selective recognition, the body would not be able to defend itself effectively.

The passage also draws attention to the fact that immune responses do not all operate in the same way. Some responses act quickly and provide broad protection, while others develop more slowly and target specific threats. This difference explains why certain infections are controlled almost immediately, whereas others take time for the body to fight off.