Page 2 Science Study Guide for the TASC

Biological Science Ideas

Structure and Function of Living Things

Amino Acids

Most biologically important molecules are made of carbon, oxygen, and hydrogen atoms. Biological molecules are classified based on what type of atoms they contain and how these atoms are arranged. Amino acids contain an amino group () and a carboxyl group (a carbon-oxygen double bond plus ). Amino acids are the building blocks of proteins.


Proteins are an important class of macromolecules used by every living organism. Proteins are built from basic units called amino acids. The type and order of amino acids will determine the structure and function of the protein. In turn, the sequence of amino acids is encoded in DNA. DNA has basic units called nucleotides. A sequence of 3 nucleotides (codon) specifies a particular amino acid that will be incorporated into the protein. A sequence of these triplets will encode the amino acids for an entire protein.

Multicellular Organisms

Multicellular organisms are organized into systems of specialized cells that perform the specific functions needed to keep the organism alive. The systems have components, usually organs, that interact and depend on each other and on other systems to perform their functions. For example, proper functioning of the intestines depends on the stomach digesting food properly, as well as depending on the nervous and endocrine systems for correct signals.


Homeostasis refers to the ability of an organism to maintain constant conditions. If these conditions change, the organism activates processes to return to the proper levels. For example, if the body temperature drops, muscles will shiver to generate heat. If it is too warm, perspiration will occur to help lower the body temperature. The animal may also change its behavior to facilitate these changes, such as moving between shady and sunny areas.


Mitosis is the process of cell division and results in 2 identical cells. Genetic material is duplicated so that a complete set of chromosomes is passed to both cells. In this way, the organism can replace dead cells and grow. Cells can develop specialized functions, different from the cell that produced them, through a process of differentiation. Differentiation occurs as specific genes are activated and deactivated in the cell. For example, all the different types of blood cells are differentiated from hematopoietic stem cells that have undergone mitosis.


Photosynthesis is the process used by plants and some other organisms to convert the energy in sunlight into chemical energy. The chemical energy is stored in the chemical bonds of carbohydrate molecules. The energy from sunlight is used to convert the reactants of water () and carbon dioxide () into the products of sugar molecules () and oxygen (). Oxygen is released as a by-product. Photosynthesis occurs in the chloroplasts of eukaryotic cells.

Cellular Respiration

In cellular respiration, the chemical energy in the bonds of carbohydrate molecules is released and captured to be used in the maintenance of the cell. The sugar molecules (), oxygen (), and water () enter a series of reactions where energy from the bonds in the sugar molecule is captured by smaller molecules and delivered to sites where energy is required. In the process, carbon dioxide () and more water () are released as by-products.


Carrying Capacity

The carrying capacity is defined for a given population of organisms for a specific area. It is the population level of that organism that can be supported by that environment. The carrying capacity can change upwards or downwards depending on conditions such as food availability, room for territory, and the presence of competing species.

Biodiversity and Populations

Biodiversity is a measure of the variety and abundance of different species in an ecosystem. A high level of biodiversity is considered to be an important contributor to the stability of an ecosystem. Biodiversity can be affected by any factor that promotes or inhibits a variety of species. For example, more microclimates may enhance diversity by favoring a variety of species. An invasive species may overrun an area, deplete resources needed for many other species, and decrease biodiversity.

Aerobic and Anaerobic Conditions

Anaerobic respiration occurs in the absence of oxygen. It was the original form of respiration among Earth’s first bacteria. Although anaerobic respiration may not be noticed in today’s oxygen-rich atmosphere, many bacteria still use these process and provide important links in global nutrient cycles. Examples of these cycles are the conversion of nitrates to nitrogen and the production of methane.

Carbon Cycles

Photosynthesis and cellular respiration are major flows in the global carbon cycle. Photosynthesis removes carbon dioxide from the atmosphere and “fixes” the carbon in the form of plant structures (e.g., carbohydrate synthesis). Cellular respiration returns the carbon dioxide to the atmosphere when the plant matter decays.

Changing Ecosystem Conditions

The variety of species and the population levels of each one in an ecosystem are the result of many interactions among these species and among each species and the environmental conditions. In a stable ecosystem, these populations remain fairly steady. However, if there is a change, such as removal of a species, the composition of the ecosystem can change radically. This has been seen in studies where the “keystone species” was removed from ecosystems.

Human Impact on the Environment

Humans can reduce impacts on the global environment by increasing their understanding of ecological processes and interactions. Actions such as burning of fossil fuels has changed the carbon cycle by adding more carbon dioxide to the air. Humans can act to reduce these trends and to engage in other activities such as promoting forest growth, which would help to remove some of the excess carbon dioxide.

Survival in an Ecosystem

The survival of a species in an ecosystem depends on the ability of individuals of the species to reproduce. Survival and reproduction can be enhanced by both individual and group behaviors. An individual that is more aggressive or inquisitive may survive better and reproduce more. Group behavior such as living as part of a herd can increase survivorship by providing better protection. Group behaviors often demonstrate interesting exceptions to the general rule of competition.


Genetics is the study of how biological information is inherited from one generation to the next. It tells us how the information is encoded and how the information is transformed into the structures and functions of the organism. Genetics also studies variations within a species and what causes these variations.

DNA and Chromosomes

The chromosomes of a cell contain DNA. DNA is a molecule that encodes the instructions to make all the proteins in cells. These proteins perform the functions of cells and can make up important structures. These structures and functions are visualized as various traits of an organism. DNA can be thought of as a blueprint for the cells and instructions for every organism.

Genetic Variations

Genetic variations are the basis of change in a species and can allowing it to adapt to its environment. As DNA is replicated and passed on to new generations, changes can occur in the DNA sequence causing variation in the species. This can occur during meiosis as copies of DNA are made in the reproductive cells. Errors can also be introduced when DNA is replicated during cell division or when DNA is damaged by an environmental factor.

Statistics and Probability in Genetics

Principles of genetics predict the distribution of genes and traits in a population. Different genetic processes will cause different distributions. Statistics can be used to test an observed set of traits in a population against various genetic models to see which genetic process is most likely responsible. Examples of this are dominant and recessive genes.

Unity and Diversity

Ancestry and Biological Evolution

Modern DNA analysis has given us the most direct way to establish relationships and show trends in evolution. The same DNA sequences can be seen in a variety of organisms and shows a relationship between them. Other clues to similar ancestry include anatomical structures and patterns of development in embryos.

Natural Selection

The argument for natural selection is based on 4 observations. First, all organisms produce far more offspring than can survive in a given environment. Second, the traits that help them survive are variable among the individuals. Third, these traits can be passed to offspring. Fourth, traits that favor survival will allow that individual to reproduce more, passing those favorable traits on in greater numbers to the next generation.


Changes in the environment will allow individuals that are better adapted to those changes to survive and pass on that trait. The population will change as this trait becomes more common. This was demonstrated by the researchers, Rosemary and Peter Grant, who examined the beak sizes of Galapagos finches and compared them to the available food sources. Beak size determines what food the birds can access.

Traits and Environmental Changes

Some species are generalists and can adapt to environmental changes and can increase after a change. Others are specialists and may not be able to adapt to extreme changes and disappear from an area. A drastic change may cause favorable conditions that allow a new species to move into an area. An organism’s traits are what allow it to adapt (or not adapt) to environmental changes.

Human Impact on Biodiversity

Human actions can impact biodiversity, usually decreasing it as an unintended effect. Human-induced changes can occur too rapidly and species don’t have time to adapt. Humans can intentionally or accidentally introduce new species to an area. These new species can overrun an area because there are no natural checks and balances on it. Humans can also intentionally restore habitats and improve biodiversity.