Puerto Rico CRECE Science — Grade 10

Which organelle is primarily responsible for converting glucose into ATP during aerobic cellular respiration in eukaryotic cells?

• AMitochondrion
• BRibosome
• CGolgi apparatus
• DLysosome

Which set of structures is present in plant cells but absent in animal cells?

• ACell membrane and cytoplasm
• BCell wall and chloroplast
• CMitochondria and nucleus
• DRibosomes and endoplasmic reticulum

What are the primary chemical reactants required for photosynthesis to occur in green plants?

• AGlucose and oxygen
• BCarbon dioxide and water
• CCarbon dioxide and oxygen
• DGlucose and carbon dioxide

What happens when a substrate binds to the active site of a biological enzyme?

• AThe enzyme lowers the activation energy required for the chemical reaction to proceed
• BThe enzyme is permanently consumed and destroyed during the chemical reaction
• CThe enzyme raises the total enthalpy of the product molecules
• DThe substrate changes the chemical composition of the enzyme permanently

What is the primary biological purpose of mitosis in multicellular organism eukaryotic somatic cells?

• AGrowth, tissue repair, and asexual cell replacement
• BProduction of haploid gametes for sexual reproduction
• CReduction of chromosome number by half
• DGenerating genetic variation through crossing over

A student places freshwater Elodea plant leaf cells into a 10% saltwater solution. Under a microscope, the cytoplasm shrinks away from the cell wall. What osmotic mechanism causes this plasmolysis?

• AWater diffuses out of the cell because the external solution is hypertonic relative to th…
• BSalt molecules actively diffuse into the vacuole to raise internal osmotic pressure
• CWater enters the cell rapidly because the external solution is hypotonic
• DProteins in the plasma membrane actively pump water out of the cell against its concentra…

In aerobic cellular respiration, which stage takes place in the cytoplasm and does not directly require molecular oxygen (O2)?

• AGlycolysis
• BKrebs cycle (Citric Acid Cycle)
• CElectron transport chain
• DOxidative phosphorylation

When a student exercises under humid conditions in Ponce, bodily core temperature rises. Sweat glands secrete sweat, which evaporates to cool the skin and lower internal core temperature back toward 37°C. This physiological response is an example of:

• AA negative feedback mechanism maintaining homeostasis
• BA positive feedback loop amplifying internal disturbance
• CPassive diffusion of cellular waste products
• DAnaerobic metabolic compensation

When a human enzyme that operates optimally at 37°C is heated to 65°C, its catalytic reaction rate drops to zero. What structural alteration explains this loss of biological activity?

• AHigh thermal energy disrupts hydrogen and ionic bonds, denaturing the 3D tertiary structu…
• BExcess heat breaks covalent peptide bonds, converting the protein back into individual am…
• CHigh temperature converts the enzyme into a competitive lipid inhibitor
• DHeat causes the enzyme's mRNA template to degrade permanently inside the nucleus

How are phospholipid molecules arranged within a eukaryotic plasma membrane according to the fluid mosaic model?

• AHydrophilic phosphate heads face outward toward aqueous environments, while hydrophobic f…
• BHydrophobic phosphate heads face outward toward water, while hydrophilic fatty acid tails…
• CPhospholipids form a single rigid monolayer with embedded starch polymers
• DHydrophilic tails point outward into the extracellular fluid, while hydrophobic heads tou…

Adenosine triphosphate (ATP) supplies energy for cellular work such as muscle contraction and active transport. What chemical reaction releases this energy?

• AHydrolysis of the terminal phosphate group, yielding ADP, inorganic phosphate (Pi), and f…
• BCondensation of adenosine diphosphate with glucose to form starch
• COxidation of ribose sugar into carbon dioxide inside the nucleus
• DPhosphorylation of adenine nitrogenous base by RNA polymerase

During oxidative phosphorylation in mitochondria, how does the electron transport chain drive the mechanical synthesis of ATP by ATP synthase?

• AElectron transfers pump H+ ions into the intermembrane space, creating a proton gradient…
• BElectrons pass directly through ATP synthase channels, phosphorylating ADP via electrical…
• CPyruvate molecules bind to the rotor of ATP synthase, turning it via thermal expansion
• DOxygen gas pumps sodium ions into the matrix, generating an electrical osmotic current

The synthesis of glutamine from glutamate and ammonia is an endergonic reaction with a positive free energy change (Delta G = +14.2 kJ/mol). How do cellular enzymes enable this unfavorable reaction to proceed spontaneously?

• ABy coupling the endergonic reaction to ATP hydrolysis (Delta G = -30.5 kJ/mol), yielding…
• BBy raising cellular temperature above 100°C to force Delta G to become negative
• CBy converting the products directly into glucose monomers without consuming energy
• DBy removing all enzymes from the reaction pathway

Which characteristics are shared by all living organisms?

• AThey are made of cells, require a source of energy, respond to their environment, and rep…
• BThey all carry out photosynthesis and contain chlorophyll
• CThey all have a backbone and a closed circulatory system
• DThey all live on land and breathe atmospheric oxygen

Which structure allows a prokaryotic cell to be distinguished from a eukaryotic cell?

• AA nucleus enclosed by a membrane, which prokaryotes lack
• BA cell membrane, which only eukaryotes possess
• CRibosomes, which are found only in eukaryotes
• DGenetic material, which prokaryotes do not contain

What is the main role of the cell membrane in a living cell?

• ATo control which substances enter and leave the cell
• BTo store the cell's genetic instructions
• CTo assemble proteins from amino acids
• DTo provide the cell with its supply of chlorophyll

Which pair of human body systems works most directly together to deliver oxygen from the air to actively respiring muscle cells?

• AThe respiratory system and the circulatory system
• BThe digestive system and the skeletal system
• CThe endocrine system and the integumentary system
• DThe excretory system and the nervous system

Which organ system in the human body is chiefly responsible for filtering nitrogenous waste from the blood and regulating water balance?

• AThe excretory system, centred on the kidneys
• BThe lymphatic system, centred on the spleen
• CThe muscular system, centred on skeletal muscle
• DThe endocrine system, centred on the thyroid gland

Which statement describes the overall relationship between photosynthesis and cellular respiration?

• APhotosynthesis stores energy in glucose using carbon dioxide and water, while cellular re…
• BBoth processes release energy from glucose and produce oxygen as a product
• CPhotosynthesis occurs only at night, while cellular respiration occurs only during the day
• DCellular respiration occurs only in animals, while photosynthesis occurs only in plants

A red blood cell is placed in pure distilled water. What is the most likely outcome and why?

• AWater enters the cell by osmosis because the surrounding solution is hypotonic, and the c…
• BWater leaves the cell by osmosis because distilled water is hypertonic, and the cell shri…
• CNothing happens, because water cannot cross the plasma membrane
• DThe cell actively pumps water outward using ATP until equilibrium is reached

Which process is an example of active transport across a cell membrane?

• AA root cell absorbing mineral ions from soil where those ions are less concentrated than…
• BOxygen diffusing from the alveoli into the blood along its concentration gradient
• CWater moving into a plant cell by osmosis
• DCarbon dioxide leaving a muscle cell and entering the bloodstream

Which sequence correctly traces the path of a protein destined for secretion from a eukaryotic cell?

• ARibosome on the rough endoplasmic reticulum, then the Golgi apparatus, then a vesicle tha…
• BNucleus, then mitochondrion, then lysosome, then the cell membrane
• CGolgi apparatus, then ribosome, then the nucleus, then a vacuole
• DLysosome, then chloroplast, then the cell membrane

A biologist finds that a particular cell contains a cell wall, chloroplasts, and a large central vacuole. What is the most reasonable conclusion?

• AIt is a plant cell, because these three structures together are characteristic of plant c…
• BIt is an animal cell, because animal cells contain large central vacuoles
• CIt is a bacterial cell, because only prokaryotes have cell walls
• DIt is a fungal cell, because fungi carry out photosynthesis

During intense exercise, a student's breathing rate and heart rate both increase sharply. Which explanation best accounts for this coordinated response?

• AMuscle cells are respiring faster, so more oxygen must be delivered and more carbon dioxi…
• BMuscle cells stop respiring, so the body increases circulation to cool them
• CThe kidneys release oxygen into the blood, which forces the heart to beat faster
• DDigestion speeds up, which requires the lungs to expel more nitrogen

Why does a marathon runner's muscle produce lactate when oxygen supply cannot keep pace with demand?

• ACells continue glycolysis anaerobically, and reducing pyruvate to lactate regenerates the…
• BLactate is produced inside the mitochondria as the final product of the electron transpor…
• CLactate forms because muscle cells stop using glucose and begin breaking down DNA for ene…
• DLactate is produced only when oxygen is abundant, which is why it appears during exercise

A student investigates the effect of pH on the activity of the enzyme amylase. Activity rises from pH 5 to a maximum near pH 7 and then falls sharply by pH 9. What is the best interpretation?

• AThe enzyme has an optimum pH, and conditions far from it alter the shape of the active si…
• BThe enzyme is consumed as a reactant, so activity declines once it is used up
• CThe substrate becomes a catalyst above pH 7, which is why the reaction slows
• DThe enzyme works best at extreme pH values and is inactive near neutrality

Photosynthesis and cellular respiration are often described as coupled processes that cycle both matter and energy. Which analysis of energy flow through this pair is correct?

• AEnergy enters as light and leaves largely as heat, so the same energy is not recycled eve…
• BBoth matter and energy are perfectly recycled, so no external energy input is required
• CEnergy is recycled while carbon atoms are permanently consumed and never reused
• DNeither matter nor energy moves between the two processes, since they occur in different…

A drug blocks the protein pumps that maintain a high concentration of sodium ions outside nerve cells. Which consequence would be expected first?

• AThe concentration gradient across the membrane would decay, so the cell would lose its ab…
• BThe cell wall would rupture, releasing the cytoplasm into the surrounding tissue
• CPhotosynthesis in the nerve cell would stop immediately
• DThe nucleus would begin transcribing sodium directly into RNA

Which cellular feature distinguishes organisms belonging to Kingdom Fungi from organisms belonging to Kingdom Plantae?

• AFungal cell walls contain chitin and fungi lack chloroplasts, whereas plant cell walls co…
• BFungi possess a true nucleus, whereas plants are prokaryotic organisms
• CPlant cells lack a cell membrane, whereas fungal cells are surrounded by a thick peptidog…
• DFungi produce seeds for reproduction, whereas all plants reproduce exclusively via binary…

Red mangroves (*Rhizophora mangle*) line coastal estuaries in Puerto Rico. How do their specialized stilt roots protect coastal communities during tropical storms?

• ABy trapping coastal sediment, dissipating storm surge wave energy, and preventing shoreli…
• BBy absorbing saltwater and releasing pure freshwater into local municipal water supplies
• CBy increasing ocean surface currents to accelerate storm movement out to sea
• DBy lowering atmospheric humidity around coastal towns

During Prophase I of meiosis, non-sister chromatids of homologous chromosome pairs align closely and exchange segment fragments. What is the evolutionary significance of this crossing-over process?

• AIt generates novel combinations of maternal and paternal alleles on chromosomes, enhancin…
• BIt doubles the chromosome number in gametes to ensure fertilization success
• CIt prevents gene mutations from occurring during DNA replication
• DIt ensures that daughter cells are genetically identical to the parent cell

Puerto Mosquito in Vieques is famous for its vibrant bioluminescence produced by microscopic dinoflagellates (*Pyrodinium bahamense*). What chemical reaction produces this cold blue light when water is agitated?

• AEnzymatic oxidation of luciferin substrate catalyzed by luciferase enzyme
• BNuclear decay of radioactive potassium isotopes in mangrove mud
• CPhosphorescent emission of absorbed solar ultraviolet radiation
• DThermite combustion of dissolved sodium metal in seawater

In the scientific name *Eleutherodactylus coqui*, what does the first word indicate?

• AThe genus to which the species belongs
• BThe family in which the species is placed
• CThe common name used locally
• DThe kingdom of the organism

Which list places the Linnaean taxonomic ranks in order from the broadest to the most specific?

• AKingdom, phylum, class, order, family, genus, species
• BSpecies, genus, family, kingdom, class, order, phylum
• CClass, kingdom, phylum, order, species, family, genus
• DGenus, species, order, class, family, phylum, kingdom

Which pair of plant tissues transports water and dissolved food through a flowering plant?

• AXylem carries water and minerals upward, while phloem carries dissolved sugars throughout…
• BPhloem carries water upward, while xylem carries sugars downward
• CEpidermis carries water, while cuticle carries sugars
• DBoth xylem and phloem carry only water, while sugars diffuse through the air spaces

What is the main function of stomata in the leaves of a plant?

• ATo allow gas exchange, admitting carbon dioxide and releasing oxygen and water vapour
• BTo anchor the leaf firmly to the stem
• CTo absorb mineral nutrients directly from the soil
• DTo produce the pigment chlorophyll used in photosynthesis

Which statement correctly contrasts asexual and sexual reproduction?

• AAsexual reproduction produces offspring genetically identical to the parent, while sexual…
• BAsexual reproduction always requires two parents, while sexual reproduction requires only…
• CSexual reproduction produces identical clones, while asexual reproduction produces geneti…
• DOnly asexual reproduction occurs in plants, and only sexual reproduction occurs in animals

A student uses a dichotomous key to identify an unknown insect. What is the defining feature of this kind of key?

• AAt each step the user chooses between two contrasting statements, narrowing the possibili…
• BThe user compares the specimen with photographs until one matches by overall appearance
• CThe user measures the specimen and looks the mass up in a table of species
• DThe user records the specimen's habitat and assigns it to the most common local species

Modern classification recognizes three domains of life. Which grouping is correct?

• ABacteria and Archaea contain prokaryotes, while Eukarya contains all organisms whose cell…
• BBacteria, Archaea, and Eukarya all contain only prokaryotic organisms
• CEukarya contains only animals, while plants and fungi belong to Archaea
• DArchaea contains only viruses, which are the simplest domain of life

In a flowering plant, which structures are directly involved in the transfer and receipt of pollen?

• AThe anther produces and releases pollen, and the stigma receives it
• BThe sepal produces pollen, and the petal receives it
• CThe ovary produces pollen, and the anther receives it
• DThe root produces pollen, and the stem receives it

Ferns reproduce using spores, while mango trees reproduce using seeds. Which comparison of the two structures is accurate?

• AA seed contains an embryo together with a stored food supply and a protective coat, where…
• BA spore contains an embryo and stored food, while a seed is a single cell
• CBoth spores and seeds are produced only after fertilization by a pollen grain
• DSeeds and spores are identical structures given different names in different plant groups

A butterfly passes through egg, larva, pupa, and adult stages, while a grasshopper hatches as a nymph resembling a small wingless adult. How are these developmental patterns classified?

• AThe butterfly shows complete metamorphosis and the grasshopper shows incomplete metamorph…
• BThe butterfly shows incomplete metamorphosis and the grasshopper shows complete metamorph…
• CBoth insects show complete metamorphosis because both hatch from eggs
• DNeither insect undergoes metamorphosis, since both change size as they grow

Which set of characteristics distinguishes amphibians from reptiles?

• AAmphibians typically have moist, permeable skin and most depend on water or humid conditi…
• BAmphibians have dry, scaly skin, while reptiles have moist, permeable skin
• CAmphibians are endothermic, while reptiles are ectothermic
• DAmphibians reproduce only asexually, while reptiles reproduce only sexually

Why must gametes be produced by meiosis rather than by mitosis in a sexually reproducing organism?

• AMeiosis halves the chromosome number, so fusion of two gametes restores the normal diploi…
• BMeiosis doubles the chromosome number, which the offspring then halves during development
• CMitosis cannot occur in reproductive organs, so meiosis is the only option available
• DMeiosis produces identical cells, which is required for fertilization to succeed

Which comparison of angiosperms and gymnosperms is correct?

• AAngiosperm seeds develop enclosed within a fruit derived from the ovary, while gymnosperm…
• BAngiosperm seeds are exposed on cones, while gymnosperm seeds are enclosed in fruits
• COnly gymnosperms produce flowers, and only angiosperms produce cones
• DNeither group produces seeds; both reproduce exclusively by spores

Many marine fish release large numbers of eggs into open water, while sea turtles bury a smaller number of shelled eggs on a beach. Which analysis of this contrast is best supported?

• AExternal fertilization in water exposes gametes and embryos to heavy losses, so producing…
• BExternal fertilization guarantees a higher survival rate per egg, so fewer eggs would be…
• CShelled eggs on land face greater predation than eggs released into open water, so turtle…
• DThe number of eggs an organism produces is unrelated to how and where fertilization occurs