TOEFL mock tests

The following passage is an excerpt from an article about botany. Photosynthesis is the biochemical process by which plants, algae, and some bacteria convert light energy from the Sun into chemical energy stored in glucose molecules. This process is the foundation of nearly all life on Earth, as it provides the organic molecules and oxygen that sustain most living organisms. The overall equation for photosynthesis is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ (glucose) + 6O₂. In this equation, six molecules of carbon dioxide and six molecules of water, using energy from sunlight, are converted into one molecule of glucose and six molecules of oxygen. Photosynthesis occurs in specialized organelles called chloroplasts, which are found primarily in the cells of plant leaves. Chloroplasts contain a green pigment called chlorophyll, which absorbs light energy most efficiently in the blue and red wavelengths and reflects green light—this is why plants appear green. The process of photosynthesis occurs in two main stages: the light-dependent reactions and the light-independent reactions (also known as the Calvin cycle). In the light-dependent reactions, which take place in the thylakoid membranes of the chloroplast, chlorophyll absorbs light energy and uses it to split water molecules (a process called photolysis), releasing oxygen as a byproduct and generating energy-carrying molecules called ATP and NADPH. In the light-independent reactions (Calvin cycle), which take place in the stroma (the fluid-filled space) of the chloroplast, the ATP and NADPH produced in the light reactions are used to convert carbon dioxide from the atmosphere into glucose through a series of enzyme-catalyzed reactions. The Calvin cycle does not require light directly, but it depends on the ATP and NADPH produced by the light reactions, so it can only occur when light is available. The efficiency of photosynthesis is affected by several factors, including light intensity, carbon dioxide concentration, and temperature. In hot, dry conditions, plants may close their stomata (pores in the leaves) to conserve water, which also limits the intake of carbon dioxide and reduces the rate of photosynthesis. According to the passage, what is the role of ATP and NADPH in photosynthesis?

Choose the sentence that correctly uses a semicolon.

The following passage is an excerpt from a textbook on marine biology. Coral reefs are among the most diverse and productive ecosystems on Earth, often called the "rainforests of the sea." They are built by colonies of tiny animals called coral polyps, which secrete calcium carbonate (CaCO₃) skeletons that accumulate over thousands of years to form massive reef structures. Coral reefs occupy less than one percent of the ocean floor but support approximately 25 percent of all marine species, including fish, mollusks, crustaceans, and sea turtles. Corals have a symbiotic relationship with microscopic algae called zooxanthellae, which live within their tissues. The algae photosynthesize and provide the coral with nutrients and oxygen, while the coral provides the algae with carbon dioxide and a protected environment. This relationship is the foundation of reef productivity. However, this relationship is fragile: when water temperatures rise even 1–2°C above normal levels, corals become stressed and expel their zooxanthellae, causing coral bleaching — the coral turns white and, if the stress persists, may die from starvation. Other threats to coral reefs include ocean acidification (which makes it harder for corals to build their calcium carbonate skeletons), overfishing, pollution, and destructive fishing practices such as blast fishing. According to the passage, what causes coral bleaching?

Choose the option that best maintains subject-verb agreement in the sentence. "The collection of ancient artifacts, along with several accompanying manuscripts, ______ on display at the museum."

The following passage is an excerpt from a textbook on biology. DNA replication is the process by which a cell makes an exact copy of its DNA before cell division. This process is essential for the transmission of genetic information from one generation of cells to the next. DNA replication occurs during the S (synthesis) phase of the cell cycle. The process begins when the enzyme helicase unwinds and separates the two strands of the DNA double helix at a location called the origin of replication, forming a replication fork. Single-strand binding proteins stabilize the separated strands, preventing them from reannealing. The enzyme topoisomerase relieves the tension created by the unwinding by cutting and resealing the DNA backbone ahead of the replication fork. DNA polymerase, the main enzyme responsible for building new DNA strands, can only add nucleotides to the 3' end of an existing strand, meaning it synthesizes DNA only in the 5' to 3' direction. This creates an asymmetry: the leading strand is synthesized continuously toward the replication fork, while the lagging strand is synthesized discontinuously in short fragments called Okazaki fragments, which are later joined together by the enzyme DNA ligase. RNA primase synthesizes short RNA primers that provide the starting point for DNA polymerase. The result is two identical DNA molecules, each consisting of one original (parental) strand and one newly synthesized strand — a process known as semi-conservative replication, as demonstrated by Meselson and Stahl in 1958. According to the passage, why is the lagging strand synthesized discontinuously?

The following passage is an excerpt from a textbook on genetics. Epigenetics refers to heritable changes in gene expression that occur without alterations to the underlying DNA sequence. While the DNA sequence itself remains unchanged, epigenetic modifications affect how genes are read by cellular machinery. The two best-studied epigenetic mechanisms are DNA methylation and histone modification. DNA methylation involves the addition of a methyl group (CH₃) to the cytosine base of DNA, typically at CpG sites — positions where a cytosine nucleotide is followed by a guanine nucleotide. Generally, methylation of promoter regions silences gene expression by preventing transcription factors and RNA polymerase from accessing the DNA. Histone modification involves chemical changes to the histone proteins around which DNA is wound. DNA wraps around octamers of histone proteins (two each of H2A, H2B, H3, and H4) to form nucleosomes, the basic units of chromatin. Acetylation of histone tails — the addition of acetyl groups — typically loosens the chromatin structure, making genes more accessible for transcription and thereby increasing gene expression. Conversely, deacetylation tightens chromatin and represses transcription. The pattern of epigenetic marks on a cell's genome determines which genes are active or silent, and this pattern differs between cell types, explaining why a skin cell and a neuron — despite having identical DNA — perform entirely different functions. According to the passage, why do skin cells and neurons perform different functions despite having identical DNA?

Choose the option that best completes the sentence. "The ______ of the contract was disputed by both parties, leading to a lengthy legal battle."

The Renaissance was a cultural movement that profoundly affected European intellectual life in the early modern period. Beginning in Italy in the fourteenth century and spreading northward over the next two centuries, the Renaissance marked a revival of interest in the classical learning and values of ancient Greece and Rome. Key characteristics of the Renaissance included humanism, an intellectual movement that emphasized the study of classical texts, the potential of human achievement, and the value of individual expression. Artists such as Leonardo da Vinci, Michelangelo, and Raphael produced works that demonstrated mastery of techniques such as linear perspective, chiaroscuro (the use of light and shadow), and anatomical accuracy. The invention of the printing press by Johannes Gutenberg around 1440 played a crucial role in spreading Renaissance ideas, as books became more affordable and widely available. Which of the following is a defining characteristic of Renaissance humanism?