Subtest II: Science Study Guide for the CSET Multiple Subjects Test

Life Sciences

The life sciences, in contrast with the physical sciences, study living systems and living beings. The main life sciences are biology, ecology, and genetics.

Cell Biology

Cell biology studies cells, the fundamental unit in living beings, and their properties, structure, function, organelles, interaction with the environment, and life cycle.

Levels of Organization in Living Things

Living beings are well organized and structured in a hierarchical manner that can be examined from the micro level to the macro level (smallest to largest).

Organelles—Organelles are smaller subunits inside the cell that perform defined functions, such as the production of energy (mitochondrion), genetic expression (nucleus), and protein transport (Golgi apparatus).

Cells—Cells are the smallest fundamental units that are able to perform nutrition, interaction, and reproduction by themselves (e.g., nervous cells, bacterial cells).

Tissues—Tissues are sets of cells with a single function (e.g., nervous tissue, heart chambers in humans, ovules in plants).

Organs—Organs are grouped tissues that form a functional unit (e.g., the stomach).

Organ systems—An organ system consists of several grouped organs that work together to achieve a specific function (e.g., the digestive system).

Structure and Function of Systems

In a system, a set of organs work together for a specific function. The most common systems in plants and animals are described below:

Nervous system—The function of the nervous system is to receive and process signals, exerting control and coordination over other organs to achieve a timely and effective interaction with the environment. The basic unit of the nervous system is the neuron.

Reproductive system—The reproductive system’s job is to form a new organism. The human male reproductive system forms male sexual cells (spermatozoa), and the human female reproductive system forms female sexual cells (ovules) and nourishes the new individual until birth.

Respiratory system—The function of the respiratory system is to exchange carbon dioxide and oxygen with the environment. In humans and other animals, the respiratory system consists of airways, lungs, and respiratory muscles that allow for movement of air inside and outside the body.

Circulatory system—The main function of the circulatory system is to transport nutrients and oxygen toward the cells and tissues, as well as to move waste substances from cells and tissues toward the lungs and kidneys. The components of the circulatory system in humans are the heart, blood, and blood vessels.

Digestive system—The digestive system transforms food into smaller molecules of nutrients that can be absorbed and used by the body for energy and the formation of cells. The main organs of the digestive system in humans are the pharynx, esophagus, stomach, and large and small intestines.

Chemistry and Living Things

Living beings are made of atoms and molecules, so knowing their chemistry is important to understand how living systems work.

The role of carbon—Life on Earth is based on carbon. Its capacity to bond with itself and with other atoms makes it the ideal element to join together into complex molecules that support life. Every major biomolecule (lipids, proteins, and carbohydrates) is made of carbon.

Water and salt—Water and salt are two compounds that are vital to life on Earth. Water is known as the universal solvent; 70% of the human body is composed of water, which helps to regulate body temperature, protect tissues, discard waste substances, and activate the metabolism. Salt, on the other hand, is important to controlling the amount of water in the body, moving water across cell membranes and transmitting nervous impulses.

DNA— Deoxyribonucleic acid (DNA) is the basis of genetics. It consists of a double strand of nucleotides, in which each nucleotide is formed by a sugar (deoxyribose), a phosphate group, and a nitrogen base (adenine, timine, cytosine, or guanine). It contains all the instructions for generating a living being and keeping its functions.

Photosynthesis—Photosynthesis is the main energy-converting process that occurs in living beings. In this process, chlorophyll-containing organisms like plants and bacteria capture energy from the Sun and transform it into chemical energy in the form of a carbohydrate. The general equation of photosynthesis is \(CO_2 + H_2 O + \text{ light } + \text{ chlorophyll } \rightarrow \text{ carbohydrate } + O_2\). Its importance goes beyond the energy transformation and includes the generation of oxygen, which is necessary for cellular respiration.

Ecology

Ecology is the science that studies the interaction between living beings and their environment. The environment is the combination of the non-living components, like climate and geology, with the living components, such as other organisms in the habitat. Ecology also studies the distribution and abundance of living beings as a result of such relationships.

Organisms

Organisms are individual plants, animals, or other life forms that are physiologically independent from other individuals. An organism can be a snail, a jellyfish, or a fungus, among countless other forms.

The role of cells—The role of cells in the study of organisms is important because the cell is the fundamental unit of living beings, and knowing the different types of cells and how they work will help the researcher understand the particular needs and behavior of each organism.

Characteristics—There are some common characteristics shared by many living organisms, most fundamentally that they complete a cycle composed of birth, growth, reproduction, and death. They have a metabolism, which is the series of chemical reactions that allow for the growth, maintenance, and repair of the cell. Living beings also maintain homeostasis, which is the self-regulation of conditions such as temperature, pH, and electrolyte concentration inside the organism. An important characteristic of organisms is that they can respond to stimuli in their surroundings, such as light, pressure, temperature, and the chemical composition.

Basic needs—To survive, living organisms need to fulfill certain basic needs, which include a habitat (place to grow, develop, and feed), light, water, oxygen, mineral salts, and reproduction.

Environmental alterations—Organisms can respond to their environment in order to meet their needs. They can undergo environmental accomodation, which is the modification of behavior to survive, or environmental adaptation, in which the behavioral or structural traits (e.g., change of fur color, migration) are genetically modified over time to prevent extinction.

Ecosystems

An ecosystem is the combination of organisms, the place where they live, and the ways they develop and interact.

Relationships—Organisms in an ecosystem can have different types of relationships with other organisms. Individuals of the same species group together in families (permanent associations) or gregarious associations (temporary associations) to feed, reproduce, or defend themselves.

Individuals of different species can have a predatory relationship, in which a predator feeds on its prey, a parasitic relationship, in which a parasite lives at the expense of its host while hurting it, a commensalistic relationship, in which only one organism benefits without harming the other, or a mutualistic relationship, in which the two individuals benefit each other. Parasitism, commensalism, and mutualism are forms of symbiosis.

Transfer of energy—All living beings need matter and energy to carry out their vital functions. Almost all of the energy used by living beings comes from the Sun. The energy that is lost in the form of heat cannot be used again, so energy flows in one single direction, from the lower trophic levels (e.g., plants) to the higher ones (e.g., predators). There is a loss of between 80% and 90% energy in each trophic level, so food chains are usually limited to four or five links.

Cycling of matter—Producers transform inorganic matter into organic by the process of photosynthesis. The organic matter then passes from one consumer to the other through trophic chains. When the consumers or the producers die or eliminate waste substances from their bodies, these substances return the mineral matter to the soil with the help of decomposers. That creates a cycling of matter that allows for the maintaining of natural equilibrium. Resources such as carbon and nitrogen also cycle in the environment and these cycles are important to life.

Resources—Ecosystems need resources (e.g., food, water, light) to keep their organisms alive. There are environmental factors that can change the availability of those resources that are divided into abiotic factors (e.g, gravity, atmospheric pressure, temperature, solar light, wind) and biotic factors (related to other organisms).

Response to change—When the environmental factors change, either naturally or because of human action, the delicate equilibrium in an ecosystem shifts to adapt to the new conditions. The species that cannot adapt to the changes will become extinct, and those that benefit from the changes will thrive.

Impacts on the Environment

Natural processes produce changes that are usually slow in an ecosystem. Examples of natural changes are changes in climate, droughts, diseases, or natural catastrophes (e.g., earthquakes, fires, volcanoes). Human activities produce rapid changes in ecosystems, and can negatively impact the environment. Examples are deforestation, introduction of foreign species into an ecosystem, chemical contamination, and light pollution. Positive actions that can help reduce the adverse impact of human activities on the environment are reforestation, efficient irrigation, treatment of wastewater, recycling, and using renewable sources of energy.

Genetics and Evolution

Genetics is the field of the life sciences that seeks to understand how traits are transmitted from one generation to the next.

Life Cycles

A life cycle is the series of successive steps that each living being goes through. It is composed of the following common steps: organisms are born, then they feed, grow, interact, reproduce, and die. Some organisms exhibit dramatic changes from one stage to another, such as metamorphosis and germination.

The following image shows a simple diagram of the life cycle of a frog:

Retrieved from: https://pixabay.com/illustrations/frog-tadpole-%C5%BEabka-758072/

Growth and Development

In the growth stage, the development of living beings depends on a series of factors. The most important is the genetic potential, which is a limiting factor: an organism cannot do what they are genetically incapable of doing. Organisms also depend on environmental factors, such as light, temperature, and food availability, which determine how well the organism can grow.

Reproduction

Reproduction is the biological process that allows for the creation of new organisms, and it is a common characteristic of all living beings. Here are some important concepts related to reproduction:

Sexual reproduction—Sexual reproduction requires the interaction of two individuals of different sexes, or one single individual that has both masculine and feminine sexual cells (hermaphrodite). The offspring will have the combination of the DNA of both parents.

Asexual reproduction—In asexual reproduction, one single organism is capable of originating new organisms that are exact copies of the parent. There is no exchange of genetic material (DNA).

Mitosis—Mitosis is the type of cellular division that results in the formation of two daughter cells with the same number of chromosomes as the parent cell.

Meiosis – Meiosis is a specialized form of cell division that results in the formation of reproductive cells, each containing half of the chromosome number of the parent. Reproductive cells combine in sexual reproduction to form a new individual.

Cell functions—Cells can be divided into two types: eukaryotic cells, in which the genetic material is found inside a membrane, forming the nucleus; and prokaryotic cells, in which the genetic material is dispersed in the cytoplasm of the cell. The functions of the cell include osmosis, cellular respiration, protein synthesis, replication, and, for cells containing chlorophyll, photosynthesis.

Replication—The genetic material of plants and animals (DNA) can be replicated (copied) thanks to the nature of the molecule. The existing strands serve as templates for new strands. It is semiconservative, meaning that after replication, there are two new molecules of DNA, each one composed of one strand of original DNA and one new complementary strand.

Variation

Genetic variation is a measure of the tendency of the genotypes of a population to differ. The individuals of any given species are not genetically identical. Genetic sources of variation are random mutations or genetic recombination during meiosis. Environmental sources of variation are external to the organism (e.g., radiation, or chemical substances called mutagenic agents).

Selection

Genetic variation is random. On the other hand, selection, the process in which species adapt to their environment, is not random. The individuals with the best phenotypes—the expression of the genotype—will be better suited to survive and will have greater reproductive success.

Natural—Natural selection is driven by the environment. For example, if food sources are further off the ground than usual, giraffes with longer necks are more likely to find more food, and hence will have greater reproductive success.

Artificial—Artificial selection is driven by humans. It is a technique of reproductive control in which people alter the genes of domestic or cultivated organisms through selective breeding. For example, the different breeds of domesticated dogs arose from the human need for specific tasks or aesthetic preferences.

Evolution

Evolution is a process that generated all living beings on Earth, by direct descent, from a single origin more than 3,800 million years ago. There is direct evidence of evolution, provided by paleontology, the science that studies the fossil remains of living beings. There is also indirect evidence, based on the establishment of inferences by studying present organisms. Indirect evidence is provided by sciences such as anatomy, embryology, biochemistry, philosophy, genetics, and zoology.

Darwin’s theory—The acceptance of evolution among the scientific community grew following the publication of English naturalist Charles Darwin’s book, On the Origin of Species, in 1859. His theory of evolution by natural selection was based on his observations of animals and plants during a five-year voyage in the HMS Beagle around the globe. It states that all living beings have evolved from a common ancestor, and the diversity that is observed in nature is the result of cumulative modifications over successive generations through the process of natural selection. You should be familiar with the four foundational principles of evolution: variation, inheritance, selection, and time.