Science Study Guide for the NLN PAX

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There are 60 questions on the science section of the National League for Nursing (NLN) Pre-Admission Examination (PAX), and you will have 40 minutes to complete them. These questions cover the scientific domains of:

  • biology
  • human anatomy and physiology
  • chemistry
  • physics

All questions on the NLN PAX are typical multiple-choice questions with four answer choices.


Biology is the study of living things. Its scope ranges from the molecules that make up cells to the complex interactions between living things and their environments.

Cell Structure and Function

The cell is the basic unit of life. Cells vary in complexity, but all cells have cytoplasm (the fluid inside the cell), DNA (deoxyribonucleic acid, the molecule that contains directions for making proteins), ribosomes (the structures that translate the code in DNA to make proteins), and a plasma membrane (the barrier that surrounds the cell). The plasma membrane is selectively permeable, or semipermeable, which means that it only allows certain materials to enter the cell. The space between cells contains fluid known as interstitial fluid.

The Cell

The structures that make up a cell are known as organelles (“little organs”). They each carry out an important cellular process. Organelles involved in energy acquisition are chloroplasts and mitochondria. Chloroplasts are found in plants and other organisms that can carry out photosynthesis. They capture solar energy and use it to make food in the form of glucose. Glucose is broken down in the mitochondria, releasing energy in the form of adenosine triphosphate (ATP). Organisms such as plants, bacteria, and fungi have a cell wall that supports and protects the cell. Other organelles include the vacuole, which stores water, food, or waste, and the lysosome, which breaks down waste.

The Nucleus

Complex cells contain a nucleus that stores DNA. The code in DNA is used to make proteins that carry out various functions. The endoplasmic reticulum (ER) typically surrounds the nucleus, and helps in protein synthesis. Proteins then travel to the Golgi complex, which packages and modifies proteins to be used either inside or outside the cell.

Cell Division

Cells divide to create more cells for growth, tissue repair, or reproduction. Each new cell needs a complete copy of the original DNA, since that contains all the directions for making new cell parts.

Mitosis is the division of nuclear material that produces two identical “daughter” cells from an original “parent” cell. In preparation for mitosis, the parent cell makes a copy of its DNA. The DNA then condenses from chromatin (loosely coiled strands) into chromosomes (tightly wrapped and organized). The chromosomes are separated from each other so that each cell has a copy. During meiosis, the chromosomes divide twice to create sex cells that have only half the original DNA of the parent cell. During fertilization, two sex cells will combine to make a zygote, the first cell of a new organism.

Organization Levels

Some organisms exist as a single cell, which must carry out all of the functions of life. In multicellular organisms, cells often differentiate and become specialized in carrying out specific tasks. The cells of multicellular organisms are organized into various levels.


A tissue is a bundle of cells that perform a similar function. Humans have four main tissue types:

  • muscle tissue—This tissue is composed of flexible cells that contract and relax to cause movement. The three types of muscle tissue are cardiac, skeletal, and smooth.

  • nervous tissue—This tissue is made up of neurons and is found in the brain, spinal cord, and other nerves.

  • connective tissue—This tissue serves to connect, anchor, and support other tissues found throughout the body.

  • epithelial tissue—This tissue is made of stacked layers of cells and has a role in protection, absorption, and secretion.


A group of tissues that perform a particular function make up an organ. Examples of organs in the human body include the heart, stomach, brain, uterus, and femur. Each of these is composed of various tissue types that work together to help that organ carry out its job.

Organ System

Organs are typically part of an organ system, a larger network of organs that all work together to perform a specific function. The heart, for example, is part of the circulatory system, which delivers nutrients and removes waste products from all of the body’s tissues. The femur is part of the skeletal system, which supports and protects the body and works with the muscular system to produce movement.


All of the organ systems of the body combine to create the organism, an individual living thing. Complex organisms, such as humans, are organized into levels as described in this section. Other organisms, such as fungi, algae, and sponges, may only have specialized cells or tissues that carry out their life functions.

Evolution: Evidence and Theories

The theory of evolution is one of the pillars of the science of biology. Evolution is a theory that explains how life on Earth has changed throughout its 3.5 billion-year history. It brings together evidence from many scientific disciplines to piece together an understanding of this complex puzzle.

Theory of Evolution

The evidence supporting the theory of evolution is wide ranging, and it wouldn’t be possible to cover it all here. The following is among the most commonly cited examples:

  • fossil record—Fossils are preserved impressions of once-living things. Fossils provided the first clues that life on Earth is not static. Scientists can determine the date of fossils and use them to compare how structures of living things changed over time. However, most organisms from Earth’s past did not make it into the fossil record.

  • biogeography—This is the study of how plants and animals are distributed worldwide. Generally, species that are closely related inhabit similar geographical regions. Species that are separated by geographical barriers such as mountain ranges or rivers evolve separately.

  • comparative embryology—This is the study of the pre-birth development of organisms. Species that are more closely related undergo similar stages of development. For example, different vertebrates (organisms with a backbone) will have a more similar development to each other than to an invertebrate such as a starfish.

  • comparative anatomy—This is the study of similarities in physical structure among organisms. Organisms with similar bone structure, body plans, and other features are more closely related.

  • molecular biology—This involves comparing the chemical structures of the molecules that make up living things. This can include DNA sequences, protein sequences, and other biological molecules.

Natural Selection

The theory of natural selection was first developed by Charles Darwin. Natural selection is based on the idea that not all organisms can survive in an environment, and the ones with traits that help them overcome their environmental pressures are more likely to survive and therefore reproduce. These organisms then pass down their favorable traits to their offspring, who in turn will have an advantage in the struggle for survival.


The favorable traits that help organisms survive in their environments are known as adaptations. They can be physical traits, such as fur color, sharp claws, or a streamlined body. They can also be behavioral, such as migration, hibernation, or social interactions.

Classification of Organisms

Classification helps scientists organize the vast variety of life on Earth and analyze the relationships among living things.


Taxonomy is the scientific discipline of naming and categorizing species based on similar characteristics. Organisms are classified based on cellular structure, mode of nutrition, biochemistry, and other characteristics. All life is divided into six kingdoms:

  • Archaebacteria—These unicellular bacteria live in extreme environments.

  • Eubacteria—These unicellular bacteria are commonplace, and their cell wall has a different chemical structure from Archaebacteria.

  • Protista—Either unicellular or multicellular, Protista cells are more complex than bacteria and they are common in aquatic environments.

  • Fungi—These are multicellular decomposers such as yeast, mold, and mushrooms.

  • Plantae—These multicellular organisms can make food using sunlight.

  • Animalia—These multicellular organisms can move and must eat to survive.

Classification Levels

Living things are sorted into categories that become more and more specific. From broadest to most specific, the categories are:

  • domain
  • kingdom
  • phylum
  • class
  • order
  • family
  • genus
  • species

Each organism is given a unique scientific name using binomial nomenclature, which comes from its genus and species levels of classification. For instance, the binomial nomenclature for humans is Homo sapiens, with our genus being Homo and our species being H sapiens.

Diffusion and Osmosis

One important function in cells is the passage of materials in and out of the cell. This occurs through the cell membrane, which is the flexible barrier surrounding the cell. It has special components that aid in this essential function.

Methods of Transport

The transport of materials into and out of the cell can be differentiated based on whether the cell uses energy to move materials. In passive transport, materials cross the membrane without the use of energy. Diffusion and osmosis are types of passive transport. In active transport, the cells must use energy to move substances through the membrane.


Diffusion is the movement of substances from high concentration to low concentration. They continue diffusing until the concentration on both sides of the membrane is the same. Some substances can pass directly through the membrane itself. Some substances, such as glucose, must use special passageways in the membrane.


Osmosis is the diffusion of water. The movement of water through the membrane depends on the concentration of solutes inside and outside of the membrane. Terms related to differences in concentration include:

  • isotonic—Concentration of solutes inside and outside the cell are equal. In this case water will move into and out of the membrane equally, and there is no net movement of water.

  • hypertonic—Concentration outside the cell is greater than inside the cell. Water will move out of the cell until isotonic conditions are reached.

  • hypotonic—Concentration outside the cell is less than inside the cell. Water will move into the cell until isotonic conditions are reached.


In general terms, filtration is the passage of certain substances through a barrier under pressure. In cells, filtration occurs when substances pass through the cell membrane due to pressure from an outside source. This is a type of passive transport. An example in humans would be kidney cells filtering toxins from blood, aided by the pressure applied by the cardiovascular system.


Ecology is the study of the interactions of organisms with each other and their nonliving environment.

Food Chain

Food chains describe the one-way flow of energy through producers and consumers in an environment. Autotrophs are producers that make their own food using energy from the sun. Heterotrophs are organisms that must consume other living things for energy. A decomposer is a type of heterotroph that breaks down waste products. Predators kill and eat other organisms, and prey are the organisms that get eaten.

Food Web

Food webs describe all of the interconnected food chains in a particular environment. They are a more thorough picture of how organisms are related through feeding relationships, and they help scientists predict how changes in the health of one population of organisms may affect the others.

Trophic Levels

Trophic levels describe the steps in a food chain, from producer to consumer. From lowest to highest, these are the levels:

  • producer—This is the autotroph that makes food and provides energy for the rest of the food chain.

  • primary consumer—This is the first level of consumers that eat the producers. Herbivores that only eat plants are always primary consumers. Omnivores that eat plants or animals sometimes act as primary consumers.

  • secondary consumer—These are the organisms that eat the primary consumers. Carnivores that only eat meat belong to this and subsequent trophic levels.

  • tertiary consumer—These eat the secondary consumers.


Symbiosis is a close relationship between two different species. There are three categories of symbiotic relationships. Mutualism is beneficial to both organisms involved. Commensalism is beneficial to one organism and has no effect on the other. Parasitism is beneficial to one organism and harms the other.

Biosphere and Ecosystems

Ecosystems are made up of the biotic (living) and abiotic (nonliving) components of the environment. Large ecosystems with similar climate and types of living things are known as biomes. Land biomes are classified by their main climate features and/or dominant plant life. Forest biomes include tropical rainforest (warm and humid with high biodiversity), deciduous forest (broadleaf trees that lose their leaves seasonally), and coniferous forest (evergreen trees with needle-like leaves that withstand the cold).

A desert is a biome that receives less than 25 centimeters of rainfall per year. A tundra is a desert found in polar regions. Grassland biomes receive more rainfall than deserts, but not enough to support large stands of trees. Tropical grasslands are found near the equator, and temperate grasslands are found between the equator and poles. All life on Earth as well as the abiotic systems that support it are known as the biosphere.


There are many different types of plants, but all of them are multicellular autotrophs that are capable of sexual reproduction. Plants evolved from water-based organisms to land-based organisms with the help of a cuticle, a waxy coating that prevents them from drying out. Their leaves have stomata, which are specialized structures that can open and close to exchange gases. Plants vary in complexity, from mosses and ferns to the more complex gymnosperms (woody plants that carry seeds in cones) and angiosperms (flowering plants).


Plants create food using energy from sunlight through the process of photosynthesis. The organelle in the plant cell where this takes place is the chloroplast. Plants take in carbon dioxide and water, then use solar energy to transform them into glucose (food) and oxygen. Chlorophyll is the green pigment that absorbs solar energy for this process to occur.

Plant Parts

The diagram below details the reproductive structures in a typical angiosperm. Angiosperms rely on pollinators such as insects or birds to transfer pollen between plants for genetic variation. The brightly colored petals and sweet nectar serve to attract pollinators.

The male structure of the flower is the stamen, which consists of a stalk known as the filament with an anther at the end that makes pollen. The female structure is the pistil. At the top of the pistil is the stigma, a sticky surface to which pollen can stick. The pollen grains travel down the style, which connects to the ovary. The ovules inside the ovary will be fertilized by the pollen grains and will develop into seeds. In flowering plants, those seeds are surrounded by a fruit to protect it, attract animals that will help distribute it, and provide nourishment for the seedling when it grows. Before the flower blooms, it is protected by a covering known as a sepal.

1 Flower Parts.png

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Genetics is the study of how traits are inherited and expressed. In this section, you will learn about some of the patterns and rules of inheritance, as well as the cellular mechanism that turns a sequence of DNA into a physical trait.

Basic Genetic Theory

A gene is a section of DNA that codes for a specific trait. One version of a gene is known as an allele. Each organism has two copies of each gene, one inherited from the mother and one from the father. If the two alleles are identical, they are homozygous for that trait. If they are different, they are heterozygous. A person’s genotype describes the set of alleles they inherited (homozygous or heterozygous), and the phenotype is the physical trait that is expressed.


Some alleles are dominant, which means it only takes one allele for the trait to be expressed. If black fur color is dominant to white fur color in rabbits, a heterozygous rabbit with one allele for black and one for white will have a phenotype of black fur. The dominant trait overpowers the recessive trait.

Sometimes two alleles can be codominant, which means they are both expressed equally in the heterozygous individual. In the example with the rabbits, if black and white fur are codominant, a rabbit with one of each allele will be black and white. Genes that are located on autosomes, chromosomes 1 to 22 (all but the sex chromosomes), will follow these patterns of inheritance. Sex-linked traits are those associated with the sex chromosomes. They are inherited and expressed differently in males and females because they have a different combination of chromosomes: XX for females and XY for males.


During meiosis, the process that creates reproductive cells known as gametes, the two copies of each gene an individual carries are separated from each other. This is known as segregation, and it means that a person can only pass down one copy of each gene to their offspring.

Independent Assortment

As segregation occurs during meiosis, the chromosomes (bundles of DNA) separate from each other in a random fashion. This means that the chances of inheriting one trait has no influence on inheriting another. This is true for genes on separate chromosomes, but genes that are close together on the same chromosomes are said to be linked. They will often be inherited together, even if they are unrelated, due to their proximity to each other.

Deoxyribonucleic Acid (DNA)

DNA is the molecule that contains instructions for building living things. It is this molecule that is inherited from one generation to the next. DNA is a long molecule made of building blocks called nucleotides. Each nucleotide consists of a phosphate group, a deoxyribose sugar, and one of four nitrogenous bases: adenine, thymine, cytosine, and guanine. These nucleotides are linked together to create a shape known as a double helix. DNA is found in every cell. When a cell is preparing to divide, the DNA must be copied in a process called replication so that the new cell will have a complete copy of the DNA.

Ribonucleic Acid (RNA)

The instructions in DNA are used to create proteins that build the cell and carry out its functions. This is known as protein synthesis, which involves these steps:

  1. transcription—A gene sequence on the DNA strand is copied into RNA, which is made of nucleotides like DNA but is single-stranded and contains the base uracil instead of thymine. RNA is also much shorter because it contains information for one gene at a time, not the entire genome. The type of RNA created in transcription is messenger RNA (mRNA). The mRNA travels outside the nucleus into the cytoplasm until it reaches a ribosome, the cellular structure that creates proteins.

  2. translation—The gene sequence in the mRNA is decoded into a sequence of amino acids (the building blocks of proteins). Another type of RNA, transfer RNA (tRNA), is involved in this process. It reads the code in the mRNA three bases at a time and brings a corresponding amino acid to the ribosome to create a chain that will become a protein.

Research Procedures

Research and experimentation are important parts of any scientific discipline. Scientists gain insight into phenomena by performing controlled experiments. A controlled experiment isolates one cause to determine its effect. Factors that can change in an experiment are known as variables. The variable isolated or applied by the scientist is the independent variable, and the dependent variable is the one that changes as a result. All other variables are kept unchanged; these are the constants. The results of the experiment are known as data.


Often, special tools or instruments are needed to make observations of scientific phenomena. One essential tool in the field of biology is the microscope. There are two main types of microscopes.

  • light microscope—This uses a light source to illuminate a specimen. A simple microscope will have one lens to magnify the image. A compound microscope has two lenses, which can greatly increase the magnification of the image. A light microscope can be reliably used up to about 1,500x magnification, and it can be used to view living specimens.

  • electron microscope—This uses a beam of concentrated, high-energy particles to create a detailed image of a specimen. They can be used to magnify an image up to 10,000,000x, depending on the microscope. However, an electron microscope cannot be used to examine living specimens.

Types of Data

When gathering data for research or experimentation, the data may fall into one or more of the following categories:

  • quantitative—expressed numerically
  • qualitative—descriptions of characteristics that cannot be measured
  • continuous—numerical data that can fall anywhere within a particular range
  • discrete—numerical data that can have only certain values

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