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Praxis Elem CKT Science study materials
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Praxis Elem CKT Science practice questions

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Question 1Science Content Knowledge: Life Science

A student observes a thin layer of onion skin under a microscope. Which structure controls which materials enter and leave these onion cells?

  1. The cell membrane Correct answer.
  2. The nucleus
  3. The cell wall
  4. The cytoplasm

Explanation

The cell membrane is a thin, selectively permeable layer that determines which substances move into and out of a cell. In plant cells the cell wall lies outside the membrane and provides support, but water and dissolved materials pass through it freely. The nucleus stores genetic material, and the cytoplasm is the jelly-like material in which the organelles are suspended.

Question 2Science Content Knowledge: Life Science

A green plant is sealed in a clear container with carbon dioxide and water and placed in bright light. Which change in the air inside the container shows that photosynthesis is taking place?

  1. Oxygen is used up and carbon dioxide increases
  2. Carbon dioxide is used up and oxygen increases Correct answer.
  3. Water vapor increases and nitrogen gas disappears
  4. Sugar builds up and carbon dioxide increases

Explanation

In photosynthesis, chloroplasts use light energy to combine carbon dioxide and water into sugar, and oxygen is released as a byproduct. A sealed container therefore shows falling carbon dioxide and rising oxygen when photosynthesis outpaces respiration. Oxygen being consumed and carbon dioxide rising describes cellular respiration, not photosynthesis.

Question 3Science Content Knowledge: Life Science

In a grassland food web, grasshoppers eat grass, mice eat grasshoppers, and hawks eat mice. A disease sharply reduces the grasshopper population. What is the most likely short-term effect on this food web?

  1. The mouse population will decrease and the grass population will increase. Correct answer.
  2. The mouse population will increase and the grass population will increase.
  3. The hawk population will increase and the grass population will decrease.
  4. The grass population will decrease and the hawk population will increase.

Explanation

Grasshoppers are the mice's food supply, so fewer grasshoppers means less food for mice and their numbers should fall. Fewer grasshoppers also means less grass is being eaten, so the grass population should grow. These effects then tend to push the hawk population down, not up.

Question 4Science Content Knowledge: Life Science

A desert plant has thick, waxy leaves and a shallow, wide root system. How do these features most likely help the plant survive?

  1. They allow the plant to store large amounts of sugar for the winter.
  2. They reduce water loss and take up water quickly after rain. Correct answer.
  3. They let the plant absorb sunlight more efficiently than other plants do.
  4. They help the plant attract insects for pollination.

Explanation

A thick waxy leaf surface limits evaporation, which conserves water in a dry habitat. Shallow roots spread widely near the surface so the plant can absorb sparse rainfall rapidly before it soaks away or evaporates. Together these are classic structural adaptations to limited water, not to light capture, food storage, or pollination.

Question 5Science Content Knowledge: Life Science

A bean seed is planted and later produces new seeds of its own. Which sequence correctly describes the main stages in the life cycle of the bean plant?

  1. Seed, mature plant with flowers, seedling, seed
  2. Seedling, seed, mature plant with roots, seed
  3. Mature plant, seed, seedling, flowers, seed
  4. Seed, seedling, mature plant with flowers, seed Correct answer.

Explanation

A seed germinates into a seedling, which grows into a mature plant that forms flowers. Pollination and fertilization in the flowers lead to fruit and new seeds, completing the cycle. The other sequences place seedling growth or seed formation out of order.

Question 6Science Content Knowledge: Life Science

Which statement best describes how the respiratory and circulatory systems work together in the human body?

  1. Oxygen enters the blood in the lungs and travels to cells. Correct answer.
  2. Air is warmed and moistened in the nose before it reaches the lungs.
  3. Waste is filtered by the kidneys and carried to cells by blood.
  4. Food is digested in the stomach and carried to cells by blood.

Explanation

The respiratory system exchanges gases at the lungs, adding oxygen to the blood and removing carbon dioxide. The circulatory system then pumps that oxygen-rich blood through vessels to body cells, where the oxygen is used to release energy from food. The two systems are linked by the blood that flows through the lungs.

Question 7Science Content Knowledge: Life Science

In a certain plant, purple flowers (P) are dominant to white flowers (p). Two heterozygous purple-flowered plants are crossed. What fraction of the offspring is expected to have white flowers?

  1. One half of the offspring
  2. Three quarters of the offspring
  3. All of the offspring
  4. One quarter of the offspring Correct answer.

Explanation

Each heterozygous parent has the genotype Pp and can pass on either P or p, giving the offspring genotypes PP, Pp, Pp, and pp in equal proportions. Only the pp genotype expresses the recessive white trait, which is one of four boxes in the Punnett square. The three remaining offspring carry at least one P and show purple flowers.

Question 8Science Content Knowledge: Life Science

Which statement best explains why a food chain rarely has more than four or five trophic levels?

  1. Decomposers remove energy from the chain before it reaches the top.
  2. Energy is lost as heat at each level, so little reaches top consumers. Correct answer.
  3. Predators at higher levels need less energy than the animals they eat.
  4. Too much energy is stored at each level for the higher consumers to use.

Explanation

At each trophic level most of the energy taken in is used for metabolism and released as heat, and only about ten percent is passed to the next level. After several transfers the remaining energy is too small to support another population of consumers. This steady loss, not excess storage, limits the length of food chains.

Question 9Science Content Knowledge: Life Science

A student claims that a plant's leaves are the main site of photosynthesis. Which observation best supports this claim?

  1. A plant kept in complete darkness for a week makes no new sugar
  2. A plant's leaves are broad, flat, and thin
  3. Removing a plant's leaves stops sugar production even in light Correct answer.
  4. A plant's leaves turn yellow when the soil lacks nitrogen

Explanation

To show that leaves are the main site, evidence must link leaves specifically to sugar production while other conditions are adequate. If a plant with water and light makes no sugar once its leaves are removed, the leaves are implicated as the necessary site. The other observations support a need for light, a need for nitrogen, or a structural description rather than location of photosynthesis.

Question 10Science Content Knowledge: Life Science

Bacteria and fungi break down dead leaves on a forest floor. How does this activity most benefit the living plants?

  1. It releases oxygen that the plants use to make sugar
  2. It provides energy that plants capture during photosynthesis
  3. It releases nitrogen and other nutrients into the soil Correct answer.
  4. It removes harmful carbon dioxide from the soil

Explanation

Decomposers break down dead organic matter and release mineral nutrients such as nitrogen and phosphorus into the soil. Plants absorb these nutrients through their roots and use them to build proteins and other molecules. The energy released by decomposers is used by the decomposers themselves; plants obtain energy from sunlight, not from soil.

Question 11Science Content Knowledge: Life Science

A student learns that muscle cells contain far more mitochondria than skin cells do. Which function of mitochondria best explains this difference?

  1. They release usable energy from food molecules through cellular respiration. Correct answer.
  2. They store the genetic instructions used to build muscle fibers.
  3. They package proteins so they can be exported from the cell.
  4. They control which substances enter and leave the cell.

Explanation

Mitochondria are the site of cellular respiration, the process that releases energy from food molecules in a form the cell can use. Muscle cells contract repeatedly and therefore need large amounts of that energy, which explains their high number of mitochondria.

Question 12Science Content Knowledge: Life Science

A butterfly develops through complete metamorphosis. Which sequence correctly orders its life stages?

  1. egg → nymph → adult
  2. egg → larva → adult → pupa
  3. larva → egg → pupa → adult
  4. egg → larva → pupa → adult Correct answer.

Explanation

Complete metamorphosis has four distinct stages: egg, larva, pupa, and adult. The larva looks very different from the adult, and the body is rebuilt during the pupal stage.

Question 13Science Content Knowledge: Life Science

A class lists traits of a pet dog. Which listed trait is inherited rather than learned?

  1. Sitting on command
  2. Coming when its name is called
  3. Walking calmly on a leash
  4. Coat color Correct answer.

Explanation

Inherited traits are passed from parents to offspring through genes, and coat color is determined by genetic material the puppy receives from its parents. Sitting, coming when called, and leash walking are all behaviors shaped by training and experience.

Question 14Science Content Knowledge: Life Science

In a population of pea plants, two plants both produce purple flowers, but one has genotype PP and the other has genotype Pp. Which statement best explains why both plants show the same phenotype?

  1. Each plant carries two recessive alleles for flower color.
  2. The dominant allele masks the recessive allele in both plants. Correct answer.
  3. The two plants produce gametes with identical allele combinations.
  4. Purple flower color in these plants is caused by the environment rather than by genes.

Explanation

A dominant allele is expressed whenever it is present, so a plant with PP and a plant with Pp both show purple flowers. The recessive allele appears in the phenotype only when two copies are present, as in pp.

Question 15Science Content Knowledge: Life Science

A patient takes an antibiotic exactly as prescribed, but a few bacteria survive and multiply. Months later, the same antibiotic no longer controls the infection. Which explanation is most consistent with natural selection?

  1. The antibiotic taught individual bacteria how to resist the medicine.
  2. Exposure to the medicine caused every bacterium to become stronger over time.
  3. The patient's immune system created bacteria that could resist the antibiotic.
  4. Some bacteria already carried a resistance trait, survived, and passed that trait to their offspring. Correct answer.

Explanation

Natural selection acts on variation already present in a population. Bacteria with traits that help them survive the antibiotic reproduce and pass those traits to offspring, so the resistant form becomes more common.

Question 16Science Content Knowledge: Life Science

A pond can normally support about 200 frogs. A long drought shrinks the pond and reduces the number of insects living there. What is the most likely effect on the frog population?

  1. The frogs will produce extra offspring to replace the insects they can no longer eat.
  2. The carrying capacity will rise because fewer frogs are competing for space.
  3. The frogs will immediately evolve into a new species adapted to dry conditions.
  4. The carrying capacity will fall, and the population will likely decline until it matches the new limit. Correct answer.

Explanation

Carrying capacity is the largest population an environment can support given its resources. When food and water become scarce, the habitat supports fewer frogs, so the population shrinks toward the lower limit.

Question 17Science Content Knowledge: Life Science

After a meal, nutrients absorbed through the small intestine must reach muscle cells in the leg. Which statement best describes how those nutrients travel?

  1. Nutrients are carried by air moving through the respiratory system to the muscles.
  2. Nutrients travel through nerve fibers directly to the muscle cells that need them.
  3. Nutrients move through the bones of the skeletal system to reach the leg muscles.
  4. Nutrients pass into the blood and are carried by the circulatory system to cells throughout the body. Correct answer.

Explanation

The small intestine absorbs nutrients into the bloodstream, and the circulatory system delivers that blood to tissues throughout the body. Digestion and circulation therefore work together to supply cells with the materials they need.

Question 18Science Content Knowledge: Life Science

A single cell that grows very large may not survive, because materials cannot enter and leave fast enough. Which statement best explains this limit?

  1. Volume grows faster than surface area, so the membrane cannot exchange enough materials for the whole cell. Correct answer.
  2. A larger cell stops needing nutrients because it no longer grows or divides.
  3. A larger cell has too few mitochondria to produce the energy it needs.
  4. A larger cell has a thicker membrane that blocks nearly all materials from entering.

Explanation

As a cell enlarges, its volume increases much faster than its surface area. The cell membrane is the surface through which materials move, so a very large cell cannot exchange enough oxygen and nutrients to support its whole volume.

Question 19Science Content Knowledge: Life Science

During vigorous exercise on a hot day, a person's body temperature begins to rise. Which response is part of a feedback mechanism that helps return body temperature toward normal?

  1. Sweat glands release sweat, and evaporation cools the skin. Correct answer.
  2. Blood vessels in the skin narrow and shivering begins.
  3. The heart rate slows sharply to reduce blood flow to the muscles.
  4. The body retains extra water to conserve heat.

Explanation

Sweating is a cooling response triggered when body temperature rises, and evaporation of sweat removes heat from the skin. This is negative feedback because the response counteracts the original change and brings temperature back toward the normal range.

Question 20Science Content Knowledge: Life Science

Which statement correctly describes the transport roles of xylem and phloem in a vascular plant?

  1. Xylem carries gases between leaves, while phloem carries water throughout the plant.
  2. Xylem and phloem both move only water upward through the stem.
  3. Xylem moves sugars down to the roots, while phloem moves water up to the leaves.
  4. Xylem moves water and minerals up from the roots, while phloem moves sugars from the leaves to other plant parts. Correct answer.

Explanation

Xylem vessels transport water and dissolved minerals absorbed by the roots upward to the rest of the plant. Phloem tissue carries sugars produced in the leaves during photosynthesis to roots, stems, and growing tissues.

Question 21Science Content Knowledge: Life Science

A student is comparing cell structures. Which structure is the main site of protein synthesis in a cell?

  1. Ribosome Correct answer.
  2. Mitochondrion
  3. Lysosome
  4. Golgi apparatus

Explanation

Ribosomes are the organelles that assemble amino acids into proteins using instructions carried by mRNA. Mitochondria release energy, the Golgi apparatus modifies and packages proteins, and lysosomes contain digestive enzymes.

Question 22Science Content Knowledge: Life Science

Which human body system is mainly responsible for breaking food into nutrients small enough to be absorbed into the blood?

  1. Nervous system
  2. Respiratory system
  3. Digestive system Correct answer.
  4. Circulatory system

Explanation

The digestive system breaks food into smaller nutrient molecules and absorbs them across the wall of the small intestine into the blood. The respiratory system exchanges gases, the circulatory system transports materials, and the nervous system coordinates signals.

Question 23Science Content Knowledge: Life Science

In a flowering plant, which structure produces the male gametes that are later delivered to the stigma?

  1. Anther Correct answer.
  2. Stigma
  3. Sepal
  4. Ovary

Explanation

The anther is the part of the stamen that produces pollen grains, which contain the male gametes. The stigma receives pollen, the ovary contains ovules, and sepals protect the developing flower bud.

Question 24Science Content Knowledge: Life Science

In rabbits, brown fur (B) is dominant to white fur (b). A heterozygous brown rabbit (Bb) is crossed with a white rabbit (bb). What fraction of the offspring is expected to have white fur?

  1. 1/4
  2. 0
  3. 3/4
  4. 1/2 Correct answer.

Explanation

A Bb rabbit produces B and b gametes, while a bb rabbit produces only b gametes. The offspring are half Bb (brown) and half bb (white), so 1/2 are expected to be white.

Question 25Science Content Knowledge: Life Science

A population of beetles lives on green leaves. A mutation produces some dark green beetles and some pale yellow beetles. Birds easily see and eat the pale yellow beetles against the green leaves. Over many generations, which outcome is most likely?

  1. Dark green beetles will become more common because they are harder for birds to see. Correct answer.
  2. Pale yellow beetles will become more common because birds prefer dark green beetles.
  3. Both color forms will disappear because birds eat every beetle they see.
  4. The beetles will stop reproducing until the leaves change color.

Explanation

Beetles whose color blends with the green leaves are less likely to be seen and eaten by birds, so they survive and reproduce more often. Over generations, the allele for dark green color increases in frequency because it is favored by selection.

Question 26Science Content Knowledge: Life Science

In a food chain, about 10 percent of the energy at one trophic level is transferred to the next level. If the producers in an ecosystem contain 20,000 units of energy, how much energy is available to the primary consumers?

  1. 2,000 units Correct answer.
  2. 20,000 units
  3. 10,000 units
  4. 200 units

Explanation

Energy transfer between trophic levels is inefficient, and in this chain only 10 percent of producer energy reaches primary consumers. Ten percent of 20,000 units is 2,000 units.

Question 27Science Content Knowledge: Life Science

A freshwater paramecium is placed in a drop of saltwater. The saltwater has a higher solute concentration than the paramecium's cytoplasm. What will most likely happen to the paramecium?

  1. It will remain exactly the same size.
  2. It will lose water and shrink. Correct answer.
  3. It will gain water and burst.
  4. It will absorb salt until its cytoplasm becomes pure salt.

Explanation

Water moves by osmosis from an area of lower solute concentration to an area of higher solute concentration across a selectively permeable membrane. Because the saltwater outside has more solute, water leaves the paramecium and the cell shrinks.

Question 28Science Content Knowledge: Life Science

During vigorous exercise, a person's breathing rate increases. Which statement best explains this response?

  1. The body needs to digest food more quickly.
  2. The body needs to produce more hormones for sleep.
  3. The body needs to take in more oxygen and remove more carbon dioxide. Correct answer.
  4. The body needs to slow the heart to save energy.

Explanation

Exercise increases cellular respiration in muscles, which uses more oxygen and produces more carbon dioxide. Increasing breathing rate helps bring in oxygen and expel carbon dioxide to maintain homeostasis.

Question 29Science Content Knowledge: Life Science

In snapdragons, flower color shows incomplete dominance. The allele for red flowers (R) and the allele for white flowers (r) produce pink flowers when both are present (Rr). If a red-flowered plant (RR) is crossed with a white-flowered plant (rr), what phenotypes will the offspring have?

  1. All pink Correct answer.
  2. Half red and half white
  3. All white
  4. All red

Explanation

In incomplete dominance, the heterozygote has an intermediate phenotype because neither allele completely masks the other. A cross between RR and rr produces all Rr offspring, which are pink.

Question 30Science Content Knowledge: Life Science

A yeast population is placed in a flask with a limited supply of nutrients. At first the population grows rapidly, but then the growth rate slows and the population size levels off. Which term best describes this pattern?

  1. Exponential growth with no limit
  2. Logistic growth with a carrying capacity Correct answer.
  3. Decline to extinction
  4. Linear growth at a constant rate

Explanation

Logistic growth occurs when a population grows rapidly at first but then slows as resources become limited, leveling off at the carrying capacity. The yeast population levels off because nutrients are used up and wastes accumulate.

Question 31Science Content Knowledge: Life Science

During a science investigation, a student places a healthy water plant in a clear beaker of water and keeps the beaker in bright light. Small bubbles form on the leaves. Which gas makes up most of the gas in these bubbles?

  1. Hydrogen
  2. Nitrogen
  3. Carbon dioxide
  4. Oxygen Correct answer.

Explanation

In photosynthesis, the plant uses light energy to combine carbon dioxide and water into glucose. Oxygen is released as a by-product, so the bubbles are mostly oxygen.

Question 32Science Content Knowledge: Life Science

Students add fruit scraps and grass clippings to a compost bin. After several weeks, the material has broken down into dark, soil-like matter. Which organisms are mainly responsible for this breakdown?

  1. Hawks and owls
  2. Bacteria and fungi Correct answer.
  3. Grasshoppers and beetles
  4. Ferns and mosses

Explanation

Bacteria and fungi are decomposers. They break down dead plant material and release nutrients back into the environment.

Question 33Science Content Knowledge: Life Science

A student is sorting animals into groups based on shared traits. Which group contains only animals that are vertebrates?

  1. Beetle, crab, spider
  2. Earthworm, snail, jellyfish
  3. Starfish, clam, ant
  4. Frog, snake, robin Correct answer.

Explanation

Vertebrates are animals with a backbone. Frogs, snakes, and robins all have backbones, while the animals in the other groups are invertebrates.

Question 34Science Content Knowledge: Life Science

A student chews a plain cracker for about a minute and notices that it begins to taste sweet. Which process best explains the sweeter taste?

  1. Bile in the mouth changes starch into fat.
  2. Stomach acid immediately digests the cracker.
  3. Chewing removes all flavor molecules from the cracker.
  4. Salivary enzymes break starch into sugars. Correct answer.

Explanation

Saliva contains enzymes, such as amylase, that begin digesting starch into simpler sugars. These sugars taste sweet.

Question 35Science Content Knowledge: Life Science

A lichen is a partnership between a fungus and an alga. The alga makes food by photosynthesis, while the fungus provides water and a protected place to live. Which term best describes this relationship?

  1. Commensalism
  2. Competition
  3. Mutualism Correct answer.
  4. Parasitism

Explanation

Mutualism is a relationship in which both organisms benefit. The alga gains shelter and water, and the fungus gains food.

Question 36Science Content Knowledge: Life Science

In a human body cell, where are most genes located?

  1. On the ribosomes
  2. In the cell membrane
  3. On chromosomes in the nucleus Correct answer.
  4. In the cytoplasm

Explanation

Genes are segments of DNA. In eukaryotic cells, most DNA is packaged into chromosomes inside the nucleus.

Question 37Science Content Knowledge: Life Science

A fast-growing plant from another continent is introduced into a grassland. It spreads widely and crowds out many native plants. Which statement best explains why this plant is so successful in its new habitat?

  1. It has few natural predators or competitors in the new habitat. Correct answer.
  2. It requires more nutrients than the native plants.
  3. It is eaten by many of the local insects.
  4. It produces fewer seeds than the native plants.

Explanation

An introduced species often spreads rapidly when it has no natural enemies in its new habitat. Without predators or competitors to limit it, it can outgrow native plants.

Question 38Science Content Knowledge: Life Science

A potted plant is placed on a windowsill. After several days, the stem has bent toward the window. Which plant response does this illustrate?

  1. Hydrotropism
  2. Phototropism Correct answer.
  3. Thigmotropism
  4. Geotropism

Explanation

Phototropism is a plant's growth response to light. The stem bends toward the light so the leaves can capture more energy for photosynthesis.

Question 39Science Content Knowledge: Life Science

A muscle cell and a skin cell in the same person have different shapes and functions, but they contain the same DNA. Which statement best explains how these cells become different?

  1. Different genes are turned on in each type of cell. Correct answer.
  2. Each cell has lost the genes that it does not use.
  3. Only the muscle cell contains DNA.
  4. Each cell type contains a different set of chromosomes.

Explanation

Cell differentiation occurs when different genes are expressed, or turned on, in different cells. The DNA sequence is the same, but different proteins are made.

Question 40Science Content Knowledge: Life Science

In a closed ecosystem, owls hunt mice. For several years, mice are plentiful. Then the mouse population drops sharply. Which change is most likely to occur in the owl population next?

  1. The owl population will decrease because less food is available. Correct answer.
  2. The owl population will increase because there are fewer mice to compete for space.
  3. The owl population will grow because mice are easier to catch.
  4. The owl population will remain exactly the same.

Explanation

Owls depend on mice for food. When prey becomes scarce, owls may starve, leave the area, or raise fewer young, so the predator population tends to fall after the prey population falls.

Question 41Science Content Knowledge: Life Science

Which organelle is the main site of cellular respiration, the process that releases usable energy from glucose?

  1. Chloroplast
  2. Large central vacuole
  3. Mitochondrion Correct answer.
  4. Cell wall

Explanation

Mitochondria are the main site of cellular respiration. In this process, the cell uses oxygen to break down glucose and release usable energy, which is stored briefly in ATP. Chloroplasts carry out photosynthesis, while vacuoles store materials and cell walls provide support.

Question 42Science Content Knowledge: Life Science

Which human body system includes the heart, blood vessels, and blood and moves oxygen and nutrients throughout the body?

  1. The circulatory system Correct answer.
  2. The excretory system
  3. The respiratory system
  4. The skeletal system

Explanation

The circulatory system consists of the heart, blood vessels, and blood. It transports oxygen, nutrients, wastes, and hormones throughout the body. The respiratory system exchanges gases, the skeletal system supports the body, and the excretory system removes wastes.

Question 43Science Content Knowledge: Life Science

Grasses, mosses, and trees use sunlight to make their own food. In an ecosystem, these organisms are best described as

  1. producers Correct answer.
  2. predators
  3. decomposers
  4. consumers

Explanation

Producers are organisms that make their own food, usually by photosynthesis. Grasses, mosses, and trees use sunlight to produce glucose, forming the base of food webs. Decomposers break down dead material, consumers eat other organisms, and predators hunt prey.

Question 44Science Content Knowledge: Life Science

A desert plant such as a cactus has a thick waxy covering on its stems. Which function does this covering most directly serve?

  1. It stores water inside the stem.
  2. It limits water loss. Correct answer.
  3. It traps heat for the plant.
  4. It attracts pollinators.

Explanation

A thick waxy covering, or cuticle, forms a waterproof barrier on the plant surface. This barrier greatly reduces evaporation of water from the stem. The cactus stem itself stores water, but the wax does not store water or attract pollinators.

Question 45Science Content Knowledge: Life Science

Cystic fibrosis is caused by an autosomal recessive allele. Two parents without the condition have a child with cystic fibrosis. What is the probability that their next child will have cystic fibrosis?

  1. 100 percent
  2. 0 percent
  3. 25 percent Correct answer.
  4. 50 percent

Explanation

Both parents must be carriers of the recessive allele because they do not have cystic fibrosis but have an affected child. A cross between two carriers gives a 25 percent chance of an affected child for each pregnancy. The outcome for one child does not change the probability for the next child.

Question 46Science Content Knowledge: Life Science

Some bacteria in a patient survive a course of antibiotics because they carry a resistance allele. These bacteria reproduce. Which process best explains why the population later becomes mostly resistant?

  1. Natural selection favors bacteria with the resistance allele. Correct answer.
  2. All bacteria become resistant at the same time.
  3. The antibiotics teach each bacterium to become resistant.
  4. The patient's immune system produces resistant bacteria.

Explanation

The bacteria vary in their traits, and some already carry an allele that resists the antibiotic. The antibiotic kills susceptible bacteria, so resistant bacteria survive and reproduce. Their offspring inherit the resistance allele, and the population shifts toward resistance over time.

Question 47Science Content Knowledge: Life Science

On a hot day, sweat evaporates from a person's skin. This helps cool the body because

  1. evaporation adds water vapor to the air.
  2. sweat carries salt out of the body.
  3. evaporation removes heat from the skin. Correct answer.
  4. sweat blocks sunlight from reaching the skin.

Explanation

As sweat evaporates, it changes from a liquid to a gas. This process absorbs heat from the skin, so the skin loses heat and the body cools. Carrying salts away or adding vapor to the air does not directly remove heat from the body.

Question 48Science Content Knowledge: Life Science

A human body cell has 46 chromosomes, while a gamete has 23. Which statement best explains why a gamete has half the chromosome number?

  1. Sister chromatids separate in meiosis II, halving the chromosome number.
  2. Homologous chromosomes separate in meiosis I, leaving one set per cell. Correct answer.
  3. DNA is not copied before meiosis begins.
  4. Gametes are made by mitosis from body cells.

Explanation

Meiosis begins with a cell that has two sets of chromosomes. During meiosis I, homologous chromosomes pair and then separate, so each daughter cell receives one chromosome from each pair. Meiosis II separates sister chromatids, but the chromosome number is already halved.

Question 49Science Content Knowledge: Life Science

Fertilizer containing nitrogen and phosphorus runs into a pond. Soon, algae grow thickly on the surface, then die. Bacteria break down the dead algae, and many fish die. Which sequence best explains the fish deaths?

  1. Nutrients raise sunlight levels, which makes water colder.
  2. Nutrients cause algae to release toxins that directly kill fish.
  3. Nutrients fuel algae; decomposers use oxygen, so oxygen drops. Correct answer.
  4. Nutrients make fish grow too large, and bacteria block gills.

Explanation

Fertilizer adds nitrogen and phosphorus, which are limiting nutrients for algae. The algae grow rapidly, then die, and bacteria decompose them. Decomposition consumes dissolved oxygen, so oxygen levels fall and fish may suffocate.

Question 50Science Content Knowledge: Life Science

Which combination of characteristics is shared by all arthropods?

  1. A notochord and gills
  2. A backbone and lungs
  3. An exoskeleton and jointed appendages Correct answer.
  4. Warm blood and fur

Explanation

Arthropods are invertebrates with an external skeleton and jointed appendages. Insects, spiders, and crustaceans are all arthropods. A backbone, notochord, fur, and lungs are traits of other animal groups, not all arthropods.