BIOLOGY

Units and Measurements | Scientific Method | Applying Science | Cells and Cellular Transport |
Chemistry of Biological Molecules | Nucleic Acids and Cell Division | Genetics, Heredity, and Biotechnology |
Diversity and Evolution | Interactions in the Environment | Sample Tests

Units and Measurements

The SI units of measurements are used when performing calculations related to scientific investigations. SI units were adapted from the metric system. The base units are meter (m), gram (g), and second (s) to measure length, mass, and time. Volume, density, and temperature are measurements frequently used in the laboratory.

The English system of measurement is used in the United States. The foot (ft), the pound (lb), and the second (s) are the standard measurements of length, weight, and time.

Download the English and Metric Systems chart.

Mass is the measure of the amount of matter in an object. Its standard SI unit is the gram.

Volume is the amount of space occupied by an object. Volume is found in different ways depending on the shape and state of the matter. For a regularly shaped object, volume is found by multiplying the length times the height times the width of the object (V=l x h x w). The units used for volume are cubic centimeters or milliliters. One cubic centimeter is equal to one milliliter.

Density (D) is the mass (m) of an object divided by its volume (V). Since density is a characteristic material property, the density of two objects made of the same material will always be the same even if the objects have different masses.

Ratio is a unitless quantity that compares one measurement relative to another. Ratios are often expressed as two numbers, separated by a colon. When two changing quantities exhibit a constant ratio, the quantities are proportional.

Temperature measures how hot or cold something is. Measurements for temperature are taken in degrees. While Celsius is often used, the SI unit for temperature is Kelvin. The English unit is Fahrenheit. To convert from one unit to another, use the following formulas where C is degrees Celsius; F is degrees Fahrenheit; and K is Kelvin:

Scientific Method

A hypothesis is an educated guess. It is a way of forming an opinion about how or why something happens based on patterns observed over time. A hypothesis can be developed through inductive or deductive reasoning. Inductive reasoning is the ability to draw from a set of knowledge and experiences to make a general explanation. Inductive reasoning is used to identify patterns. Deductive reasoning relies on applying reasoning from a general principle to a specific case. A hypothesis formed from deductive reasoning is a prediction that will be proved or disproved by experimentation.

A scientific experiment should either prove or disprove a hypothesis. The experiment must be set up to examine only one variable (condition) at a time. There are three types of variables:

• Independent variables (manipulated variables) - the factors that are changers or manipulated during the experiment. These variables are the ones that the experiment is trying to test.
• Dependent variable (responding variable) - this is usually the factor that the experimenter is measuring or counting. The dependent variable is the one that changes in response to the independent variable.
• Control variables - all the other factors in the experiment. These are the things the experimenter attempts to control. Control variables are kept constant during the course of the experiment.

Data is gathered from observations and measurements taken during scientific experiments. Qualitative data are the observations made with your senses (i.e. color, texture, taste, smell). This information does not have a numerical value. Quantitative data are the measurements (i.e. lengths, weights, masses, volumes, time, temperature,etc.) Qualitative data is subjective. Quantitative data is more objective.

A prediction is a forecast of the possible result of events. Scientists can use the knowledge they have gained from observation and experimentation to make predictions about seemingly unrelated events.

A conclusion is a judgment based on observation and experimentation. The conclusion should be a logical statement based on the results of the experiment. Conclusions can be used to develop inferences and models. An inference involves using the conclusion as a starting point in inductive reasoning. A model is a mathematical description of an event.

Applying Science

Scientists often monitor their research through the peer-review process. Peer review subjects the research to independent review by qualified experts in that area of science.

A clinical trial is a type of experiment where products are tested on animals or people. A clinical trial only occurs after the product has been well-researched and tested in the laboratory. Clinical trial results help the makers of the product decide if the product can be sold to the public.

Scientific research is often connected with economic or political issues that can cause bias in scientists' interpretation of the data. Biased data has been interpreted to suit the goals of the interpreter. However, bias can be avoided when scientists conduct blind trials where the researchers do not know who has received real medicine and who has received a placebo.

Unknown factors can affect an experiment despite scientists' best efforts to establish control. During data interpretation, scientists decide what each piece of experimental data means. Incorrect or biased data interpretation will generate an incorrect conclusion.

Cells and Cellular Transport

All organisms (living things) share the following characteristics: cells, sensitivity (response to stimuli), growth, homeostasis (stable internal environment), reproduction, metabolism (transformation and use of energy), and adaptation.

Download the life processes chart.

A cell is the structural and functional unit of all organisms. Some cells function using organelles (small, specialized cellular subunits separated from the rest of the cell by a membrane), while other cells do not have organelles.

The cell theory states that:

• All living things are made of cells.
• All cells come from other living cells of the same kind.
• Cells are the basic unit of of all living things.

The cells has three basic parts: the cell wall or membrane, cytoplasm, and genetic material. The cell wall or membrane is the boundary surrounding the cell. The cytoplasm is the watery, jelly-like part of the cell that contains salts, minerals, and the cell organelles. The genetic material is the area of the cell where the DNA (deoxyribonucleic acid) is stored and it regulates all the cellular activities.

There are two basic types of cells: eukaryotic and prokaryotic. A eukaryotic cell has a nucleus surrounded by a nuclear membrane. Plant and animal cells are both eukaryotic. A prokaryotic cell does not have a true nucleus. Bacteria are prokaryotic.

Download the parts of the eukaryotic cell chart.

Organisms that only have one cell are unicellular. The single cell carries out all life functions on its own.

Multicellular organisms have many cells that work together to carry out life processes. The cells group together and divide the labor. Tissues are groups of cells that perform the same function. Organs are composed of several types of tissues. An organ system is a group of organs working together for a particular function.

A solution is a liquid mixture of solute dissolved in solvent. The interior of a cell is also a solution. The cytoplasm (the solvent) contains a variety of substances, like salt and minerals (the solutes). Too much or too little of any solute in the cytoplasm can cause damage to the cell.

Homeostasis is the process of maintaining a balance of solutes within a cell. Hormones are chemical messengers that regulate some body functions in multicellular organisms. Hormones help maintain homeostasis.

The cell membrane regulates the movement of materials into and out of the cell. Since the cell membrane is semi-permeable, only certain substances can move through it.

In passive transport, molecules move spontaneously through the cell membrane from area of higher concentration to areas of lower concentration. These molecules move with the concentration gradient. The three types of passive transport are diffusion, facilitated diffusion, and osmosis.

Diffusion is the process by which a substance moves directly through the cell membrane. Facilitated diffusion involves the help of a carrier protein to move a substance from one side of the cell wall to the other. Osmosis is the movement of water from an area of high water concentration to an area of low water concentration through a semi-permeable membrane.

In active transport, substances move from an area of low concentration to an area of high concentration. Exocytosis is a form of active transport that removes materials from the cell. Endocytosis is a form of active transport that brings materials into the cell without passing through the cell membrane.

Chemistry of Biological Molecules

An element is a type of matter composed of only one kind of atom which cannot be broken. Six elements are commonly found in living cells: sulfur, phosphorous, oxygen, nitrogen, carbon, and hydrogen. These elements make up ninety-nine percent of all living tissue and combine to form molecules that help cells function.

Carbon is in all living things and in the remains of all living things. Molecules that contain carbon are called organic molecules while inorganic molecules do not contain carbon. Water is the most important inorganic molecule for living things. Cellular reactions take place in water.

Biomolecules are molecules produced by living cells. Carbohydrates, lipids, proteins, and nucleic acid are the four types of biomolecules. These biomolecules are organic molecules and polymers (compounds formed by the combination of simpler molecules known as monomers).

Carbohydrates are often called sugars and act as an energy source. Lipids are fats that have several functions, but are most well known for storing energy. Proteins (which consist of long chains of amino acids known as polypeptide) have many different biological functions. Most organisms must have protein to survive. Nucleic acids are found in the nucleus of a cell and form the backbone of DNA and RNA. DNA (deoxyribonucleic acid) directs the activities of the cell. RNA (ribonucleic acid) is involved in protein synthesis.

Each chemical reaction in a cell requires energy. ATP (adenosine triphosphate) is a molecule that serves as the chemical energy supply for all cell.. When chemical bonds are formed, energy is stored. When chemical bonds are broken, energy is released. Free energy is the energy available to do work. It is stored in the chemical bonds of molecules.

A catalyst is a substance that speeds up a chemical reaction without being chemically changed by the reaction. Activation energy is the amount of energy required in order for reactant molecules to begin a chemical reaction. Catalysts decrease the amount of activation energy required for a reaction to occur. Human bodies use catalysts called enzymes to break down food and convert it to energy. A cofactor is a substance, such as a metal ion or coenzyme, that must be associated with an enzyme for the substance to function. Important cofactors in photosynthesis and cellular respiration are NADP+ (nicotinamide adenine dinucleotide phosphate) and NAD+ (nicotinamide adenine dinucleotide).

Vitamins are organic biomolecules that function as cofactors and coenzymes in enzymatic reactions. Minerals are elements including calcium, potassium, sodium, and magnesium that are required by organisms. Both vitamins and minerals are essential to the survival of organisms.

Organisms must have food in order to live. Consumers are organisms that obtain their food from other living things. They ingest and digest food and then excrete waste.

Water is essential to life. Important characteristics of water include cohesion, adhesion, and high specific heat. Cohesion is the intermolecular attraction between molecules. It is the reason water droplets form into spheres. Surface tension is the tendency of a liquid to minimize the area of its surface by contracting. As a result of high surface tension, many light objects float on water. Adhesion is the ability of water to stick to other hydrophilic (water-loving) molecules. Specific heat is the amount of energy it takes to change one gram of water by one degree Celsius. The high specific heat of water means that it takes a lot of energy to change the temperature of water.

Photosynthesis is the process of converting carbon dioxide, water, and light energy into oxygen and high energy sugar molecules. Plants, algae, and some bacteria use sugar molecules produced during photosynthesis to make complex carbohydrates such as starch or cellulose for food. The process of photosynthesis consists of two basic stages: light-dependent reactions (also known as the Calvin cycle) and light-independent reactions.

In the light-dependent phase, sunlight hits the leaf of the the plant and it absorbed by the pigments in the leaf. The main pigment used in photosynthesis is a green pigment called chlorophyll. The end-products of light-dependent reactions are ATP, oxygen and NADPH. Light-independent reactions use the ATP as an energy source while the oxygen is released into the atmosphere. Carbon and NADPH are used to form glucose.

Cellular respiration is the process of breaking down food molecules to release energy. Plants, algae, animals, and some bacteria use cellular respiration when breaking down food molecules. There are two basic types of cellular respiration: aerobic and anaerobic. Aerobic respiration occurs in the presence of oxygen. Anaerobic respiration (fermentation) is the process by which sugars break down in the absence of oxygen.

Chemosynthesis is the process by which inorganic chemicals are broken down to release energy. Only bacteria are able to carry out chemosynthesis.

Photosynthesis and chemosynthesis are used for energy storage while cellular respiration is used to release energy. Cellular respiration takes place in all eukaryotic cells and some prokaryotic cells. Chemosynthesis takes place only in prokaryotic cells.

Nucleic Acids and Cell Division

DNA is the genetic basis of life. It carries the code for all the genes of an organism, in turn creating the proteins that perform all the work of living organism. DNA is also the template for future generations.

Genes are pieces of the DNA molecule that code for specific proteins. The process of making genes into proteins is called protein synthesis. In order to manufacture proteins:

• The DNA code of the gene segment must be copied in the nucleus of the cell.
• The copied code must then be carried from the nucleus into the cytoplasm and finally to a ribosome, where it is translated into an appropriate protein.
• The protein is then assembled from the code and released from the ribosome.

The steps above are carried out by RNA, the molecule used to translate the code from the DNA molecule into protein. There are many types of RNA, and messenger, ribosomal, and transfer RNA are all involved in protein synthesis.

The first step of protein synthesis is the manufacture of a specific kind of RNA called messenger RNA (mRNA). The copying process is called transcription. Translation is the step in protein synthesis where mRNA is decoded (translated) and a corresponding polypeptide is formed.

Cells must divide in order for an organism to grow, reproduce, and repair itself. Multicellular organisms contain reproductive (sex) cells and somatic (body) cells. Both cells contain DNA that is stored in the nucleus. The DNA combines with proteins to form genetic material called chromatin. When a cell divides, the chromatin coils and condenses to form chromosomes. Reproductive cells have a single set (haploid number) of chromosomes while somatic cells have two sets (diploid number) of chromosomes.

Replication is the process through which DNA copies itself. When the cell divides, the chromosomes are distributed between the newly produced cells.

The cell cycle is the sequence of stages through which a cell passes between one cell division and the next. Most of the cell cycle is spent in interphase which consists of three parts: G1, S, and G2. During G1, the cell grows in size. During S, replication of the DNA containing the genetic material occurs. During G2, the cell prepares for mitosis by replicating organelles and increasing the amount of cytoplasm.

With the exception of the reproductive cells, all the cells in the body are somatic cells. Somatic cells under mitosis, a type of cell division that generates two daughter cells with the identical components of the mother cell. Mitosis is the mechanism for asexual reproduction, which only requires one parent. The stages of mitosis are:

• Prophase - The nucleus of the cell organizes the chromatic material into thread-like structures called chromosomes. The centriole, in animal cells only, divides and moves to teach end of the cell. Spindles form between the centrioles.
• Metaphase - The chromosomes attached at the center, or centromeres, line up on the spindle at the center of the cell.
• Anaphase - Chromosomes separate at the center, and the spindles pull them toward either end of the cell. A nuclear membrane forms around the chromosomes as they disorganize.
• Telophase - Chromatic again forms from the chromosomes, and a cell membrane begins to grow across the center between the two new nuclei.

Cytokinesis is the division of the cell cytoplasm. It usually follows mitosis. Cytokinesis begins during the telophase of mitosis and finalizes the production of the two new daughter cells.

Meiosis is the type of cell division necessary for sexual reproduction. Meiosis produces four reproductive cells called gametes. These cells contain half the number of chromosomes from the mother cell and the chromosomes are not identical.

Before meiosis begins, each pair of chromosomes replicates while the cell is in its resting place (interphase). During meiosis I (the first phase of cell division), each set of replicated chromosomes lines up with homologous chromosomes (matched pairs of chromosomes). Homologous chromosomes are not identical and when they separate, the cell splits into two daughter cells, each containing one pair of homologous chromosomes. Interkinesis is the resting period before meiosis II (the second phase of cell division).

During meiosis II, the two daughter cells divide again without replicating the chromosomes. The result is four gametes, each of which have half the number of chromosomes of the mother cell. In males, all four gametes produce a long whip-like tail. In females, one gamete forms an egg cell. The other three gametes disintegrate.

In asexual reproduction, the offspring produced are always genetically identical to the parent. In sexual reproduction, there is potential for genetic variability. Cell differentiation is the production of cells that are different from themselves.

Mutations are mistakes or misconnections in the duplication of the chromatin material. Some mutations are harmful to an organism while others are beneficial. Gene mutations are mistakes that affect individual genes on a chromosome. Chromosomal mutations are mistakes that affect the whole chromosome. The are four major categories of chromosomal mutations:

• Duplication mutations occur when a chromosome segment attaches to a homologous chromosome that has not lost the complementary segment. One chromosome will then carry two copies of one gene, or a set of genes.
• Deletion mutations occur when a chromosome segment breaks off and does not reattach itself. When cell division is complete, the cell will lack the genes carried by the segment that broke off.
• Inversion mutations occur when a segment of chromosome breaks off and then reattaches itself to the original chromosome, but backwards.
• Translocation mutations occur when a chromosome segment attaches itself to a nonhomologous chromosome.

Mutations in somatic cells only affect the tissues of the organism. Mutations in reproductive cells may be transmitted to the gametes and passed on to future descendants of the organism.

Genetics, Heredity, and Biotechnology

Genes determine hereditary characteristics since they carry traits that can pass form one generation to the next. Alleles are different molecular forms of a gene. Each parent passes on one allele for each trait to the offspring. The genotype is the combination of alleles inherited from the parents.

The dominant gene is the trait that will most likely express itself. In order for the expression of the recessive gene to occur, both alleles must be the recessive ones. The phenotype is the physical expression of the traits, but it does not necessarily reveal the combination of alleles.

If an individual inherits two of the same alleles for a characteristic, the individual is homozygous.

Gregor Mendel, the "father of genetics", grew and bred pea plants in order to observe genetic traits. He discovered the principle of dominance which states that some forms of a gene or trait are dominate over other traits, which are called recessive. He also developed the principle of segregation which states that when forming sex cells, the paired alleles separate so that each egg or sperm only carries one form of the allele. Mendel's principle of independent assortment states that each pair of alleles segregates independently during the formation of the egg or sperm.

Sex chromosomes are the chromosomes responsible for determining the sex of an organism. Males have the genotype XY and females have the genotype XX. In a male, the X chromosome comes from his mother and the Y chromosome comes from his father. In a female, one X comes from her mother and the other X comes from her father.

Incomplete dominance is the situation when one trait is not completely dominant over the other. Instead, the two traits blend together. When both traits contribute to the phenotype of the offspring of an organism, the trait is co-dominant. Multiple alleles are different molecular forms of a gene that can interact in different ways. Polygenic traits are the result of the interaction of multiple genes.

Biotechnology is the commercial application of biological products. Processes that manipulate DNA include recombinant DNA technology, genetic engineering of agricultural crops, gene therapy, cloning, and DNA fingerprinting.

Recombinant DNA revolves are the concept of protein synthesis. Changing DNA can produce a different protein. The DNA of one organism is cut into pieces. A piece that produces a desired protein is inserted into another organism's DNA. The organism with the new piece of DNA will then produce the desired protein.

Recombinant technology can potentially improve agricultural products. However, there is concern that genetically modified foods may be harmful to human health. Additionally, genetic pollution can occur through the cross-pollination of genetically modified and non-genetically modified plants by wind, birds, and insects.

Gene therapy is used to help cure diseases. The idea behind gene therapy is that if a defective protein is replaced with a good one, then the disease caused by the defective protein can be eliminated. Stem cells are cells found in the human body that have yet to become a specialized type of cell. Stem cells can become any type of cell or tissue and can potentially be used to treat diseases.

Cloning is the creation of genetically identical organisms. Possible benefits of cloning include the creation of tissues for transplantation that would not be rejected by their host and the creation of therapeutic proteins like antibodies.

DNA fingerprinting is the identification of a person using his of her DNA. DNA fingerprinting is performed by cutting DNA with enzymes and separating the fragments through electrophoresis. Electrophoresis uses electrical charges to separate pieces of molecules based on both size and charge.

The genome is an organism's complete set of DNA. The Human Genome Project (HGP) - established in 1990 and completed in 2003 - sought to identify all human genes and determine all of the base pair sequences in all human chromosomes.

Diversity and Evolution

Biodiversity is the variation among organisms. It includes ecosystem diversity, species diversity, and genetic diversity. Ecosystem diversity is the variety of ecosystems available. An ecosystem is a community of organism and their environment. Species diversity is the number of different species of organisms. Genetic diversity distinguishes among individuals with a species.

According to the theory of evolution, new species develop from preexisting species over long periods of time. However, organisms within the same species may evolve differently over time or remain unchanged. Adaptation is a change in structure or function that allows an organism to be more successful. Extinction is the condition in which there are no living representatives of an organism.

Taxonomy is the practice of classifying organisms into an ordered structure. The primary tool in classifying an organism is its evolutionary relationship to other organisms. Scientists also use similarities in physical structures, embryo development, genetic information, reproductive strategies, nutritional strategies, and biochemical similarities to classify organisms.

Homologous structures develop from a common ancestor and are similar in shape, but have different functions. Vestigal organs are structures that are no longer used or have greatly decreased in importance.

Taxonomy divides organisms into seven categories that start out broadly and become more specific: kingdom, phylum, class, order, family, genus, and species. Species is the most specific category. Organisms of the same species are grouped together due to their ability to breed and produce fertile offspring.

Botanist Carl Linnaeus devised the system of binomial nomenclature to classify organisms. Binomial nomenclature gives organisms a two-part name that labels the species. Written in Latin, the binomial name consists of the generic name (genus) and the specific epithet (species). Linneaus' classification system has some limitations. Although classification is based on evolutionary theory, it does not reflect the idea that evolutionary processes are continual. Species are not fixed. Variation also exists among individuals within a species.

A fossil is the recognizable remains or body impressions of an organism that lived in the past. Although fossils mostly come from organisms that are now extinct, one exception is the living fossil. Living fossils are organisms that have remained essentially unchanged from earlier geologic times. The fossil record is the total number of fossils that have been discovered, as well as the information derived from them about the evolution of Earth. The fossil record is not complete; scientists agree that there are periods of time when organisms either seem to disappear or to evolve rapidly and without explanation.

Naturalist Charles Darwin proposed the idea of natural selection in 1859. Natural selection states that organisms best suited to the environment are the ones most likely to survive and reproduce. In his book The Origin of Species Darwin observed:

• Resources are limited in all environments. This leads to competition among living organisms as they fight to get what they need for survival.
• Most organisms have more offspring than the environment can support.
• There is a natural variation (difference in a trait between organisms) within a population.
• Natural selection is always taking place. Organisms with the most desirable traits are selected to survive. Fitness is the ability of an organism to live, survive, and reproduce in that environment. Not all individual animals have the same fitness.

Modern synthesis is the merging of Darwinian ideas along with modern knowledge about genetics. There are four important facets in modern evolutionary thought:

• There are several mechanisms responsible for the evolution of organisms. One of the most important is genetic drift (random change in genes), which occurs through natural selection.
• Characteristics that are inherited are carried by genes, and natural variation within a population is the result of several alleles working together.
• Micro evolution is the process that is responsible for the variations that exist within a species, or a change in the allele frequency. Macro evolution is evolution that occurs between species.
• Speciation (formation of new species) is generally due to gradual genetic changes, and large scale evolution is the result of a lot of small scale evolution.

Modern synthesis recognizes that evolution can be seen as the change in gene frequencies of a population.

Gene flow is the change in the occurrence of genes in a population. It occurs during emigration (when an individual leaves a population) or immigration (when a new individual joins a population). Emigration increases the isolation of a species and quickly reduces the differences in the population. Immigration can result in short-tem genetic diversification, but over the long term, the population absorbs the new characteristics of the immigrating individuals.

Genetic drift provides random changes in the occurrence of genes through chance events. Genetic drift can also occur during bottlenecking when a large population is reduced to a few individuals, and the genes of subsequent generations become very similar.

Interactions in the Environment

An ecosystem is the interdependence of plant and animal communities and the physical environment in which they live. The biosphere is the zone around the earth that contains self-sustaining ecosystems composed of biotic and abiotic factors. Biotic factors include all living things, such as birds, insects, trees, and flowers. Abiotic factors are those components of the ecosystem that are not living, but are integral in determining the number and types of organisms that are present (i.e. soil, water, temperature, light).

Biomes are large land areas characterized by a dominant form of plant life and climate type that make up large ecosystems. Organisms that live in biomes have adapted to the climate of the geographic region. The tundra, the coniferous forest, the deciduous forest, the grasslands, the tropical rain forest, and the deserts are biomes.

Aquatic ecosystems depend on different factors such as the amount of light, oxygen, and the salinity (salt) level of the water. Aquatic ecosystems include marine areas, freshwater areas, and estuaries, all of which are determined by the salinity of the area.

Download the ecosystems chart.

A community is a collection of the different biotic factors in a particular ecosystem. Each member of a community has its own habitat, a dwelling place where an organism seeks food and shelter. A community of living things is composed of populations, which are made up of the individual species in a community.

Each organism in an ecosystem interrelates with the other members in one of three ways: symbiosis, competition, or predation.

A symbiotic relationship is a long-term association between two members of a community in which one or both parties benefit. There are three types of symbiotic relationships:

• Commensalism is a relationship in which one member benefits, and the other is unaffected.
• Mutualism is a relationship that is beneficial to both organism.
• Parasitism is a relationship that benefits one organism (the parasite), but harms the other (the host).

Competition occurs when two or more organisms seek the same resource that is in limited supply. Intraspecific competition occurs between members of the same species while interspecific competition occurs between members of different species.

A predator is an organism that feeds on other living things. The organism it feeds on is the prey. Predation helps ecosystems maintain an ecological balance.

Population dynamics is the study of important characteristics of populations including the growth rate, density, and distribution of a population.

The growth rate of a population is the change in population size per unit time. Organisms can enter the population through birth or immigration. Organisms can leave the population through death or emigration. The growth rate can be zero, positive, or negative. Exponential growth (sometimes called a J-shaped curve) occurs when the population growth starts out slowly and then increases rapidly as the number of reproducing individuals increase.

Density is the number of organism per unit area. The distribution of a population refers to the pattern of where the organisms live. Random distribution occurs when there is no set pattern of individuals within the ecosystem. Random distribution is rare in nature. Clumped distribution occurs when individuals are found in close-knit groups, usually located near a resource. Even distribution occurs when a set pattern or even spacing is seen between individuals.

Carrying capacity is the number of individuals the environment can support in a given area. When the population size exceeds the carrying capacity, the number of births will decrease and the number of deaths will increase. Logistic growth (sometimes called an S-shaped curve) refers to this type of growth curve where the population eventually returns back to the carrying capacity.

A limiting factor is anything in a population that restricts the population size. Density-dependent factors and density-independent factors are the two main types of limiting factors. Density-dependent factors are phenomenon (competition, disease, and predation), which only become limiting factors when a population in a given area reaches a certain size. Density-independent factors are limiting no matter the size of the population (i.e. unusual weather, natural disasters, seasonal cycles).

Ecological succession is the series of changes an ecosystem goes through over time. Succession happens when one community slowly replaces another as the environment changes. Primary succession occurs in areas that are barren of life because of a complete lack of soil. Secondary succession occurs in habitats where the community of living things has been partially or completely destroyed.

Producers (autotrophs) are members of an ecosystem that used abiotic factors to obtain and store energy. Consumers are members of the ecosystem that depend on other members for food. There are three types of consumers:

• Herbivores - animals that eat only plants.
• Omnivores - animals that eat both plant and other animals.
• Carnivores - animals that eat only other animals.

Saprophytes are organisms that obtain food from dead organisms or from the waste products of living organisms.

Decomposers are members of the ecosystem that live on dead or decaying organisms and reduce them to their simplest forms. Decomposers include fungi and bacteria.

Heterotrophs are consumers and decomposers that use biotic (living) factors to obtain energy because they cannot make it themselves.

A trophic level is the position occupied by an organism in a food chain. Producers comprise the first level of the food chain. Primary consumers make up the second trophic level, secondary consumers occupy the third trophic level, and tertiary consumers are at the top of the food chain. Tertiary consumers are generally omnivores or carnivores.

The process of recycling substances necessary for life is called the nutrient cycle, which consists of the carbon cycle, the nitrogen cycle, the phosphorous cycle, and the water cycle.

The carbon cycle is the cycling of carbon between carbon dioxide and organic molecules. Carbon passes from one organism to another through food chains. It returns to Earth through respiration, excretion, or decomposition after the death of organisms. Fossil fuels develop from decomposing organic matter over long periods of time. When fossil fuels burn, carbon dioxide returns to the atmosphere.

The nitrogen cycle transforms nitrogen (the most abundant atmospheric gas) into ammonia, nitrite, and finally nitrate. Nitrogen fixation is the conversion of nitrogen gas into nitrate by several types of bacteria. Nitrogen is converted into ammonia by bacteria called nitrogen fixers. Nitrifying bacteria convert ammonia into nitrite and finally into nitrate. The bacteria live on the roots of legumes (pea and bean plans) and increase the amount of usable nitrogen in the soil.

The water cycle circulates freshwater between the atmosphere and the geosphere. Precipitation falls to Earth and ends up in lakes, rivers, and oceans through precipitation itself or through runoff. The sun drives evaporation which sends water vapor into the atmosphere from bodies of water. Respiration from people and animals and transpiration from plants also send water vapor back into the atmosphere to form clouds. Eventually clouds form precipitation, continuing the water cycle.

Heat travels to the surface of Earth from the sun. The planet absorbs some of the energy and some of it is reflected back into the atmosphere. Gases in the atmosphere trap the reflected energy. The natural phenomenon of trapping heat within the Earth's atmosphere is called the greenhouse effect.

Human activity is increasing the level of greenhouse gasses which means that more heat will be trapped in the atmosphere. This may result in a permanent increase in the Earth's temperature, which is commonly referred to as global warming. A slow global warming trend will affect the oceans, land surfaces, and all living things.

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