High School Biology Study Guide for the STAAR test

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Evolution and Classification

Evolution is the study of how species change over time and how species are related to each other. This topic is addressed in 10 of the 54 questions on the test.

Evolutionary Theory

Evolutionary theory states that present life has developed gradually from organisms living in the past. All of the organisms alive today are descended from common ancestors. The theory explains, through scientific studies, both the present diversity of life and the common processes, such as inheritance through DNA, shared by all life.

Evidence of Ancestry

DNA sequencing, when available, provides definitive evidence of common ancestry among organisms. Other molecules, such as proteins, can indicate relationships between species, as well. For past organisms, evidence comes from the study of the anatomy of fossils. Structures that are shared through common ancestry are called homologies.

Scientific Explanations of Data

In the fossil record, organisms appear suddenly and often remain unchanged (in stasis) for long periods of time. They may show periods of rapid change into new species, or they may suddenly disappear from the record. Evolution provides scientific explanations for these changes.

Natural Selection

Natural selection produces changes in populations by affecting the survival of individuals with certain traits. For example, in a dark-colored environment, light-colored individuals will be preyed upon more often, producing a population of mostly dark-colored individuals.

Elements of Natural Selection

The principle of natural selection has four basic components:

  1. Organisms possess traits that are passed down to offspring.

  2. Organisms produce more offspring than can survive in any habitat.

  3. Organisms possessing traits that give an advantage to survival will survive more often and pass those traits to their offspring.

  4. Over time, the population will have more individuals with these favorable traits.

Adaptation and Diversity

Natural selection is the mechanism by which organisms develop adaptations to their environments. These adaptations help them to survive. Over time, as organisms adapt to a variety of conditions, new species may develop and increase the diversity in a region.

Other Mechanisms

Natural selection acts on variations that exist in a population. The variations can result from changes in DNA, such as mutation. Other processes that change the genes in a population are genetic drift, gene flow, and recombination.

Cell Complexity

One question in evolutionary theory is how the complex anatomy of eukaryotic cells with all of their organelles developed. One leading explanation is that the organelles were originally free-living bacteria that were ingested into larger cells and continued to function. This is supported by observations, such as certain organelles that have their own DNA.


Taxonomy is a system of classifying organisms based on shared characteristics and genetic relationship. Organisms that are closely related should be classified into the same group. Taxonomic classifications change as new discoveries are made, especially in the analysis of DNA.

Importance of a Standardized System

The Linnaean system classifies organisms in groups called taxa, ranging from kingdom down to species. Every organism has a scientific name in this system. The standardized system means that there is a consistent way to refer to an organism that all scientists will understand.

Categorizing Organisms

Organisms in the same taxa are related to each other. Scientists use dichotomous keys, along with knowledge of the characteristics of the taxa, to classify and identify organisms.

Group Characteristics

You should know the distinguishing characteristics of the kingdoms in classification. These taxa are archaea, bacteria, protists, fungi, plants, and animals. They are distinguished mainly at the cell level by the structure of the cells.

Processes and Systems

In multicellular organisms, cells are organized into systems of organs. These organs perform specialized functions, such as metabolism and energy conversion, that keep the organism alive. This topic is dealt with in 11 of the 54 questions on the test.

Molecules Involved

Carbohydrates and proteins play important roles in the metabolism of an organism. Carbohydrates are the basic fuel for metabolism. Proteins form enzymes that facilitate metabolism and are important components of regulatory molecules such as hormones.

Reactants and Products of Photosynthesis

The reactants for photosynthesis are water and carbon dioxide. The final products are sugar and oxygen. Energy from sunlight is needed for the reaction. During the reaction, sunlight energy is first converted to the chemical energy of ATP and NADPH. This chemical energy is then used to form sugar molecules from carbon dioxide.

Reactants and Products of Cellular Respiration

The reactants for cellular respiration are sugar and oxygen. The end products are carbon dioxide and water. The process also produces chemical energy in the form of ATP molecules. These molecules store energy and are transported to areas of the cell that need to use energy.

The Role of Enzymes

In biological systems, enzymes are a type of protein. Enzymes facilitate reactions to occur faster or at lower temperatures. Most steps of photosynthesis and cellular respiration are facilitated by enzymes. Without enzymes, these reactions would not occur under conditions that are safe for a cell.

Biological System Levels

Biological systems are organized into multiple levels. These levels exist both within organisms and among populations of various organisms.

In Animals

Although each system performs a function, systems must interact to provide the organism with its needs. For example, in animals, the digestive system and the circulatory system interact to provide nutrients throughout the body.

In Plants

Systems must interact in plants to ensure their survival. For example, the root system and the shoot system interact to provide the leaves with mineral nutrients from the soil.

Levels of Organization

Within organisms, the levels of organization from least to greatest complexity are cell, tissue, organ, and organ system. For groups of organisms, the levels from least to greatest complexity are individual, population, community, ecosystem, and biome.


Systems interact to achieve homeostasis by communicating with each other. These communications act as a feedback mechanism. Communications within plants are generally by hormones and within animals by both hormones and the nervous system.

Environmental Systems

Environmental systems consist of communities of organisms and their environment. The test addresses this topic in 11 of the 54 questions.


Biological systems act in a way to reach and maintain a state of balance or equilibrium. This is seen in the various feedback mechanisms that exist in the body to regulate factors such as body temperature, water balance, and calcium levels.

Response to External Factors

Organisms respond to changes in the environment. These changes can be interactions with other organisms or changes in conditions such as air quality or food availability.

Role of Microorganisms

Bacteria and fungi have both beneficial and detrimental effects. Many animals, including humans, are aided in their digestion by bacteria. Bacteria and fungi decompose and recycle minerals in ecosystems. However, other types of bacteria and fungi cause disease.

Effects of Ecological Succession

Ecological succession is the change in species in a given area over a period of time. This may occur due to environmental changes, such as a fire. The changes in species affect the environment, which in turn will lead to favorable conditions for new species to enter.


Interactions occur between individuals of the same species, between individuals of different species, and between organisms and the environment. These interactions can have either positive or negative effects on the organism.


Relationships among organisms include predation, parasitism, commensalism, mutualism, and competition. These are usually distinguished by whether one or both organisms in the relationship benefit or suffer because of the relationship.

Variations and Adaptations

Organisms possess adaptations and variations that are specific to the ecosystems they inhabit. Similar species in different ecosystems will show different adaptations; for example, foxes in deserts and foxes in the Arctic. Dissimilar species in the same ecosystem will have similar adaptations, such as white fur in arctic mammals.

Matter and Energy

Energy flows through ecosystems starting with producers, then through primary, secondary and tertiary consumers, and finally to decomposers. Each unit of energy flows through once and is finally lost as heat. Matter is continually recycled in ecosystems. These processes are modeled in food chains, food webs, and ecological pyramids.

Survival of the Species

The population level of each species is tuned to the level of the resources it depends on. If the levels of these resources changes, the population level of the species changes in response.

The Carbon and Nitrogen Cycles

Matter is recycled in ecosystems and on the planet as a whole. The carbon and nitrogen cycles are important examples of the cycling of matter. The amount of each type of matter is essentially constant on the planet. This matter continually moves between different forms but is neither created nor destroyed.


In a stable ecosystem, the relative populations of the various species remain fairly constant. This stability is a result of countless interactions between organisms and the environment. Any change that affects these interactions or the population of a single species will disrupt the stability and cause changes throughout the ecosystem.

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