Base your answers to questions 1 through 5 on the information below and on your knowledge of biology.

Carbon! Where Does It Come From? Where Does It Go?

On Earth, carbon compounds are found in the oceans, atmosphere, and living organisms, as well as stored in rocks and sediments. Earth and its atmosphere can be considered a closed system. The amount of carbon in different locations within Earth’s system is always changing.

Sea otters help maintain the carbon balance in their ecosystem. Otters eat sea urchins. Eating sea urchins is important, as sea urchins are herbivores that can destroy a kelp forest. Kelp are large autotrophic algae that grow much faster than most plants. When kelp die, they sink into the deep ocean. The low oxygen conditions of the sea floor cause decomposition to be slow or incomplete.

Scientists calculated the carbon pool (how much carbon kelp stores) with and without sea otters, as shown in the model below.

Increased atmospheric carbon dioxide has been linked to changes within marine ecosystems. When $CO_2$ combines with water, it produces carbonic acid, lowering the water’s pH. A pH of less than 7.8 can interfere with the ability of some marine organisms to make shells and skeletons. These organisms include corals, mussels, plankton, seastars, and sea urchins.

The graph below shows the relationship between atmospheric $CO_2$ concentration and the pH of seawater.

The following diagram shows some information about the cycling of carbon.

The model below shows the equation for how sea urchins make their shells.

$Ca^{2+}$ + $CO_3^{2-}$ → $CaCO_3$

Calcium ions and carbonate ions produce calcium carbonate.

Base your answers to questions 6 through 10 on the information below and on your knowledge of biology.

Drinking Water Is Just The Beginning!

The amount of water taken in must be in balance with the amount lost. The urinary system is involved in maintaining the salt and water balances within the body.

Human Urinary System

Each kidney is composed of about a million waste-filtering structures known as nephrons. Water is reabsorbed through some parts of the nephron, such as the tubule. The model below shows the structure of a nephron in two organisms.

Another function of the kidney is to aid in regulating the number of red blood cells. The kidneys produce a protein known as Erythropoietin (EPO) that stimulates the increased production of red blood cells.

The graph below shows the results of a study. Participants were exposed to different conditions prior to time zero, then EPO levels were measured over a 4.5-hour time period.

In addition to their other functions, red blood cells (RBCs) have the capacity to carry water. The surface of the cell contains structures called aquaporins, which transport water across cell membranes. Due to their unique cell structure, RBCs are able to expand up to 74% or shrink up to 40% compared to the original cell size.

RBCs typically have a biconcave disk shape. A scientific study examined the effect of changing the RBC shape and membrane flexibility on its ability to transfer oxygen. For this study, when the RBC membrane was less flexible, the oxygen transfer capacity decreased by 18%. When the RBC was more flexible, the oxygen carrying capacity increased by 21%. When RBCs take on water their membrane initially becomes more flexible.

Base your answers to questions 11 through 16 on the information below and on your knowledge of biology.

Heads or Tails?

In the mid 1990s, people across several states were finding large numbers of frogs and other amphibians with extra limbs. Possible explanations regarding the cause of these abnormalities ranged from UV radiation, chemical contaminants in the water, parasites, or even airborne substances.

Pacific Chorus Frog with Extra Legs

After further research, scientists discovered that these deformities in frogs were not caused by genetic mutations. The actual cause was a parasitic flatworm called Ribeiroia. Ribeiroia completes a complex life cycle by inhabiting several hosts. This life cycle is summarized in the diagram below.

Hox genes are an important group of regulatory genes that help determine the body plan and head-to-tail orientation of animals in their early stages of development. High concentrations of retinoic acid have been found to influence the activity of Hox genes.

The diagram below shows how proteins produced by the activated Hox genes attach to DNA sequences that act as molecular switches to turn large numbers of different genes on.

Scientists have found that when the Ribeiroia parasite enters the frog tadpole, it burrows into the limb bud which develops into the frog’s leg. The levels of a chemical called retinoic acid increase rapidly in the limb bud of the tadpoles due to the parasitic infection.

Hox genes are also found in arthropods. The diagrams below show some information about the Hox genes and body segmentation in a fruit fly. The body plans for other arthropod species are also shown. The differing shades of gray indicate the Hox genes responsible for each body segment’s development.

Hox genes are also present in mammals and other vertebrates to produce specific body parts in the correct orientation. Specific Hox genes from a mouse and fruit fly can be interchanged.

Base your answers to questions 17 through 21 on the information below and on your knowledge of biology.

Organisms of the Tibetan Plateau

The yak is an herbivore that lives in the high altitude of the Tibetan Plateau, located in the Himalayan Mountains. They inhabit the Tibetan Plateau at elevations between 3,000 and 5,000 meters. Yaks have a large heart and lungs, as well as a specialized hemoglobin in the cells of their blood that enables them to extract more oxygen from the air.

A gene called EPAS1 is involved in allowing animals to respond to a low oxygen environment. Scientists studied this gene in yaks. They found that yaks having a certain allele of the gene had a greater amount of hemoglobin, which transports oxygen through the body. This allele has a small change in nucleic acid sequence from other alleles of the EPAS1 gene found in yak populations.

Plants that yaks eat grow under stressful conditions. Decreased atmospheric pressure results in changes in the concentrations of gases, as represented below.

The pika is another herbivorous mammal species that inhabits the Tibetan Plateau. Pika move quickly and spend much of their time foraging for food and keeping a lookout for predators. They are small (5–9 inches long), live in systems of underground tunnels that they dig and maintain, and lack a large heart and large lungs.

The photograph and graph below show some information about the Tibetan Plateau.

Other organisms that are part of the Tibetan Plateau ecosystem include carnivores such as wolves, eagles, and snow leopards.

The model below shows some approximate biomass at each trophic level of the Tibetan Plateau ecosystem.

Base your answers to questions 28 through 32 on the information and diagrams below and on your knowledge of biology.

Limb Evolution

Tetrapods include all animals with backbones that have four limbs that end with digits (fingers and toes). Some tetrapods, such as whales and snakes, do not have four obvious limbs but are included because they have a four-limbed ancestor.

The front limbs of tetrapods are believed to have evolved from the pectoral fins of an ancestral bony fish.

The diagram summarizes some of the current structural and fossil information regarding front limb evolution in some living animal species and some extinct water-dwelling animal species.

Evolution of Tetrapod Front Limbs

Scientists are searching for transitional fossils to provide evidence that land-living tetrapods evolved from bony fish. This missing species is represented by Species X on the Evolution of Tetrapods Front Limbs diagram. In 2004, fossilized remains of a possible contender were discovered in Canada. It was named Tiktaalik. Tiktaalik was a large, fish-like organism that lived about 385 million years ago, when the seas were crowded with many species of fish.

It is believed that Tiktaalik lived in shallow, warm water. During this time period the first plants were taking over the land and creeping insects and spiders flourished. Tiktaalik could spot prey on the land and in the water using its eyes, located on top of its head. It could also use its very strong front fins to chase and catch prey on the shore. Although it was a large animal, it is likely that Tiktaalik was also the prey for even larger predatory fish, such as the gigantic Rhizodus, which had two massive fangs located at the front of its jaw.

The diagram below shows some information about possible extinct organisms.

Base your answers to questions 33 through 37 on the information below and on your knowledge of biology.

Does It Matter?

The carbon used by plants is moved between living organisms, the minerals in the soil, the hydrosphere, and the atmosphere through processes in the carbon cycle.

Matter Conversions

Plants rearrange matter to produce other needed compounds. The model below shows some of the compounds that plants synthesize. Boxes X, Y, and Z represent element(s) used to make these compounds.

Base your answers to questions 38 through 42 on the information below and on your knowledge of biology.

Keystone Species: Black-Tailed Prairie Dogs

The black-tailed prairie dog is a keystone species because it maintains the complex web of relationships in North America’s central grassland ecosystems. They feed primarily on plants that are high in moisture and nutrients. As they eat the plants, they drop leaf clippings, which add nutrients to the soil. They construct burrows, which when abandoned can provide homes to rattlesnakes, burrowing owls, and insects. Prairie dogs are the primary prey for many organisms, including the black-footed ferret, one of the rarest and most endangered animals in North America.

The prairie dog population of North America’s central grassland is on a steady decline. The most significant threats facing prairie dogs are the conversion of rangeland to cropland, urban development, hunting, and use of poison because they are considered a pest to local farmers and ranchers.

The graph below shows some data collected in a study of 17 prairie dog colonies in Nebraska.

A disease known as the Sylvatic Plague is caused by a bacterium that is carried by fleas on rats. The disease targets small mammals, including the prairie dog. This disease entered the western United States as a consequence of the shipping industry and has been spreading eastward. The graph below shows changes in the prairie dog populations observed in two different states during the 1990s.

Various methods of controlling this plague are being investigated. Two methods that have been found to be effective are described in the table below.

The location of the study is near residential areas and open ranges that are used to graze cattle and serve as habitats for wild animals. Researchers have been asked to make recommendations regarding which strategy would be the best to use to protect the prairie dog populations from the plague without negatively impacting the nearby areas.

Base your answers to questions 43 through 48 on the information below and on your knowledge of biology.

Nature or Nurture?

During one winter in the 20th century, the Netherlands experienced a severe famine (a shortage of food). Some of the women that were in the early stages of pregnancy during the famine gave birth to children that surprisingly had average or even above-average birth weights, considering the poor nutrition of the mothers.

Scientists determined that the children whose mothers were in early pregnancy during the famine (children of the famine), had more obesity and chronic health problems in adulthood compared to their siblings that did not have this exposure. The children of the famine had changes in expression of some of their genes. One of these genes, known as IGF2 (insulin-like growth factor 2), codes for a hormone.

The codon chart below can be used to determine the amino acids that are coded for by a DNA sequence.

A portion of the IGF2 DNA sequence is included in the table below.

In a cell, DNA interacts with different molecules. It also interacts with methyl groups in a process known as methylation.

The IGF2 gene codes for a hormone that promotes fetal growth. The children of the famine had less methylation of the IGF2 gene than is typical in other children.

Researchers tracked the health of the children of the famine and their offspring for many years. The research findings showed similar results in their offspring.

Besides famine during pregnancy, other environmental factors such as smoking nicotine can cause demethylation (removal of methyl groups) of portions of DNA responsible for cell division. The result of this demethylation is shown in the model below.