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Page 1 Biology Study Guide for the HESI® exam

Biology is the study of living organisms and to understand anatomy and physiology, a nurse must first grasp the science of biology. While you may not use your knowledge of biology directly every day, biology serves as a prerequisite before moving on to other sciences such as anatomy, physiology, and microbiology.

Here are some key concepts you should understand in order to do well on the Biology section of the HESI exam:

Classification of Organisms

Early biologists faced difficulty in devising a method for discerning living organisms from nonliving things. After developing rules to differentiate life from non-life, scientists then faced the struggle of organizing and classifying life. Taxonomy, or the science of classifying, resulted from these early struggles. Many years of classification have occurred since, and because of advancements in biotechnology, scientists can now categorize life based on similarities and differences at the genetic level. Scientists utilize a hierarchical system for classifying organisms. Six Kingdoms contain the taxonomic breakdown of life. These Kingdoms are further divided into Phyla, then Class, Order, Family, Genus, and Species. A common mnemonic for remembering this hierarchy is, “King Phillip Came Over For Great Spaghetti,” where the first letter of every word indicates the corresponding taxonomic class. Over the years, the taxonomic classification schemes have changed and will continue to change as we further our understanding of genetics. Currently, scientists use Six Kingdoms to subdivide life: Bacteria, Protozoa, Chromista, Plantae, Fungi, and Animalia make up these Six Kingdoms. Scientists categorize organisms within one of these Kingdoms by investigating the organisms’ cellular composition, methods for obtaining and using energy, genotypic similarities, and other techniques.

The Scientific Method

The Scientific Method is a way of devising and performing experiments that yield meaningful results. It involves a procedural approach to gaining information about the physical world that begins with a formulated question and ends with the rejection of a hypothesis and the reevaluation of the experiment.

The Scientific Method can be summarized as the following:

Question ― A curiosity about a phenomenon arises and, in response, a question is formulated. Early thinkers looked at the sky and wondered why it was blue or looked at the grass and wondered why it was green.

Research ― After formulating a question, a scientist looks for any relevant research or data already discovered and provided for the phenomenon in question. This helps give some direction in how to set up or approach the question.

Hypothesis ― The scientist then generates a hypothesis, or an educated guess, as to what could be causing the phenomenon. This step helps narrow down the possible options for experimentation.

Experiment ― Using available measuring tools and technology, an experiment is designed to provide valuable data for the scientist to investigate.

Evaluation ― The data will then be analyzed and assessed for its validity. Do the observations made support the hypothesis, or do they support a different hypothesis?

Conclusion ― Finally, the scientist will decide if the hypothesis is confirmed, in which case other scientists will then recreate the same experiment to confirm that the results hold true in a different time or place using the same methods. If the hypothesis is not confirmed, the scientist may choose to adjust some of the experimental methods or devise a new hypothesis.

Overall, the Scientific Method provides a methodical approach for investigating experiments, data, and drawn conclusions. It is worthwhile to know that developments in scientific research do not arise from haphazard guessing and checking, but rather through logical design and reasoning. Even a basic familiarity with the method will prove useful when making sense of scientific experiments.

Cells, Tissues, and Organs

The most fundamental unit of life is the cell. Organisms that exist as a single cell, like bacterium, are called prokaryotes and those that are multicellular, like humans, are called eukaryotes. The major difference between these two groups is that eukaryotes possess a nucleus and membrane bound organelles while prokaryotes do not.

Beginning with an understanding of the cell, its form and function, we can start to make sense of how life operates, and what cellular features enable this operation. Learning the components of the cell is not a difficult task, but it can be tedious.

One great way to learn about cells is to complement lists of cellular components/features with drawings of cells― this is particularly useful for eukaryotic cells and their organelles. Accompany these drawings with the name of the cell “part” and what its function or purpose is. For example, the nucleus houses genetic information and instructions for cellular operations; the mitochondrion helps generate ATP to provide energy for the cell, etc. A mini white board can be a huge asset in learning the differences between plant, animal, and bacterial cells. Repeatedly diagramming the components of the various cell types and their parts (noting similarities and differences) will lead to long-term retention.

When a group of cells function together to accomplish tasks, they are operating as tissue. Due to the differences at the cellular level, plants and animals organize into different types of tissue. Plants possess meristematic tissues, which enable them to increase in size, and permanent tissues, which enable them to maintain their form.

Animals possess connective, epithelial, muscle, and nervous tissues. Like the tissues in plants, these groups serve different functions and have different forms. Connective tissues provide structure to organisms. Epithelial tissues are those found where cells line and cover organs. Muscle tissue allows animals to move, and nervous tissue enables animals to send and receive signals to its different parts.

Just as cells combine to form tissues, tissues combine to form organs. Humans possess an extensive list of organs that all serve a particular function: some help digest food to provide energy, while others help circulate air and blood. And, like tissues, organs act collaboratively to form organ systems.

The same methods of learning the cells and their functions can be applied to tissues, organs, and then organ systems. It is most important to generate your own diagrams when learning the form and function of these different systems. It is easy to believe that one has a solid grasp of these things when reading from a book or even a page of notes; however, this is much different from being able to work from the ground up in describing the composition of organisms. Condense lists of organismal features into its basic parts, and work through repeatedly processing this information with the aid of a whiteboard and note cards.

Genetics and Heredity

Heredity is the passing on of genetic traits from one generation to the next; it is the reason that children resemble parents, and why humans give birth to other humans. Genetics is the study of the principles underlying heredity.

A workable knowledge of genetics is impossible without becoming familiar with DNA (deoxyribonucleic acid). This familiarity entails its composition [knowing that guanine, cytosine, adenine, and thymine (also known as GCAT) are its nucleotides, knowing how they pair, and knowing that its strands run antiparallel, among other features], as well as its function (to house and maintain the instructions for a cell’s operations).

An understanding of “the central dogma” of molecular biology, which states that genetic information flows from DNA to RNA to proteins, can serve as a great outline for how gene transfer takes place. This understanding can help you be familiar with DNA replication before moving on to transcription and translation, the details of all of which can be processed through learning the names and functions of the various structures and enzymes involved. As this is a systematic procedure that incorporates many different parts, drawing and redrawing diagrams can prove worthwhile for long-term retention of the operations.

When these operations are understood, a more general understanding of the genetics can be studied. A familiarity with Gregor Mendel and his laws (Law of Dominance, Law of Segregation, and the Law of Independent Assortment) can act as a solid foundation for genetic transfer beyond the microscopic scale. This will lead one to learning about parents, first and second generations, and the expression of phenotypes as predicted with Punnett squares.

Much of the difficulty in learning about genetics and heredity stems from the necessity of learning an exhaustive number of terms and definitions. Intelligent usage of flash cards and diagrams can address these issues.

Mitosis and Meiosis

Mitosis and meiosis are processes by which cells reproduce. Mitosis is a form of asexual reproduction where the resulting cell is genetically identical to the parent cell, whereas meiosis results in a cell that contains only half of the chromosomes found in the parent cell. When reviewing the two processes, it is wise to note both the similarities and the differences. Similar to the method used for recalling the various taxonomic classes, a mnemonic device can prove valuable in learning the stages of mitosis and meiosis.

While these two reproduction methods share essentially the same steps, it can be useful to think that because meiosis is involved in sexual reproduction, a method for diversifying life; it is more complicated than mitosis. Recalling this can help you remember which mnemonic goes with which process.

A familiarity with the cell cycle is helpful in understanding these two processes. Cells do not arbitrarily reproduce, nor do they reproduce nonstop. Instead, there are triggers and signals that must be present before a cell will begin reproduction. Mitosis can be broken down into four major stages: prophase, metaphase, anaphase, and telophase. But there are two additional “stages” of interphase and cytokinesis. The acronym IPMATC can be useful in recalling the order in which these stages happen.

Meiosis shares the same stages, but it occurs in two ordered sequences, so there is an IPMAT 1 and an IPMAT 2. The best method for retaining the details involved in both processes is to utilize a white board and diagrams, drawing and redrawing the steps until this can be done without the aid of any reference material. It may sound repetitive, but this method of learning is invaluable for gaining a functional knowledge of this material.


Photosynthesis is the process by which plants transform the energy in light into chemical energy that can be used to fuel life functions. A solid grasp of photosynthesis entails an understanding of what cellular structures enable the process (think chloroplasts, and other structures present in plant cells that are not in animal cells) as well as how the process happens (without carbon dioxide and water the process cannot take place).

Just as you should be familiar with the chemical equation governing cellular respiration (the energy liberating process in animal cells), you should also know the chemical equation relating the reactants and products of photosynthesis. Both of these processes rely upon the transfer of free electrons to generate chemical energy. And, just as animal cells carry out the Krebs cycle to generate ATP, plants carry out the Calvin cycle to generate energy. Analogs like this are very useful to recognize as they can reduce two distinct processes into a single concept and thus simplify the material to be learned.

A familiarity with the different types of photosynthesis is also useful to learn. Some types require the presence of light whereas others can be performed in the absence of light. An understanding of what biological purpose or function this serves can be helpful. Similarly to all of the other systems and processes, the usage of a white board and diagrams, as well as a complementary list of the cellular features necessary, can prove invaluable when reviewing involved procedures such as photosynthesis. However, when you can already generate the information without the aid of reference materials, you can be certain that you have developed a firm comprehension of the concept.