Biology Study Guide for the HESI Exam

General Information

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.

The Biology section of the HESI contains 25 questions, and you will have 25 minutes to answer them. The following are some key concepts you should understand in order to do well on this section.

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.

The Classification System

Scientists use 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,” in which the first letter of every word indicates the corresponding taxonomic class.

The Kingdoms Used Today

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: Eubacteria, Protista, Archaebacteria, Plantae, Fungi, and Animalia. Scientists categorize organisms within one of these kingdoms by determining the organisms’ cellular composition, methods for obtaining and using energy, genotypic similarities, and other traits.

Cells

The most fundamental unit of life is the cell. 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. Cells belong to one of two major groups. Prokaryotes lack a separate nucleus and membrane-bound organelles (substructures within cells that perform specific functions), while eukaryotes do possess these features.

Here is a prokaryotic cell:

And here is an eukaryotic cell:

Although the structure of cells differs between types of organisms, they all have a few things in common. All cells are surrounded by a flexible barrier called the cell membrane, which keeps the cell separate from its external environment and controls what enters and leaves the cell. They also have DNA, the genetic code that has instructions for building the organism’s proteins, and ribosomes, where the proteins are made. The fluid inside the cell in which organelles are suspended and chemical reactions take place is the cytoplasm.

Some organisms are made of only one cell (unicellular) and some are made of many cells (multicellular). The cells of multicellular organisms often specialize in specific tasks, such as storing energy or sending signals. The process of cellular specialization is called differentiation. In the earliest stages of development, the cells of multicellular organisms have not differentiated yet, and they are referred to as stem cells.

Organelles

These are the major organelles that function in a cell:

• nucleus—This organelle in eukaryotes contains the DNA. An organism’s DNA is divided into structures called chromosomes.

• ribosomes—These small organelles use coded instructions from DNA to create proteins.

• endoplasmic reticulum (ER)—This is a network of membranes that fold and transport proteins, make lipids, and detoxify poisons. The rough ER has ribosomes attached to it, while the smooth ER does not.

• Golgi apparatus—Also known as the Golgi body, this is a stack of flattened sacs that modify, sort, and package proteins and lipids.

• lysosomes—This is a small vesicle, or sac, that breaks down waste products.

• vacuoles— These are sacs that store food, water, or waste in plant and animal cells. Plants have a large central vacuole that stores water and helps give the cell its shape. When a cell takes in food through the cell membrane, a process known as phagocytosis, it folds inward and pinches off to create a vacuole.

• mitochondria—These break down food molecules in the cell for energy. Food molecules store chemical energy in their chemical bonds. This energy is captured by the mitochondria and stored in a molecule called adenosine triphosphate (ATP) that fuels the processes of the cell.

• chloroplasts—These are organelles in plant cells that use chlorophyll (a green pigment) to capture solar energy to create food. The process in which this happens is called photosynthesis.

The Cellular Membrane

All cells are surrounded by a semipermeable membrane, which controls what enters and leaves the cell. The cell membrane is flexible and has many components that help it perform its function. The main structure is a phospholipid bilayer. The membrane also has proteins that perform special functions, cholesterol to strengthen the membrane, and carbohydrates to aid in cell-to-cell recognition.

Some substances can travel through the membrane itself, and some need special passageways called protein channels. When substances move through the membrane without the use of ATP energy, this is passive transport. When the cell uses ATP energy to move particles across the membrane, it is active transport.

Cell Energy Production

All living things need a source of energy to fuel the processes that sustain life. Organic molecules such as glucose store energy in their chemical bonds, which is then broken down into smaller units. Some organisms, like most plants, can make their own food using energy from sunlight or chemicals (autotrophs), while some, (like most animals,) need to consume other living things for food (heterotrophs).

Cellular Respiration

Cellular respiration is the process in which cells convert glucose into usable energy in the form of ATP. The first step of the process is glycolysis, which occurs in the cytoplasm and breaks one molecule of glucose into two molecules of pyruvate, producing a small amount of ATP and nicotinamide adenine dinucleotide with hydrogen (NADH), which acts as a temporary carrier of energy. The pyruvate then enters the mitochondria and is further broken down during the citric acid cycle (also known as the Krebs cycle). The final step is the electron transport chain, in which the NADH molecules send high-energy electrons through a series of proteins in the mitochondria, producing yet more ATP.

Adenosine triphosphate is a molecule with three phosphate groups. It is produced by adding a phosphate group to adenosine diphosphate (ADP), which has two phosphate groups. ATP then moves to wherever energy is needed in the cell. The third group is removed, releasing energy that is used by the cell, and the resulting ADP returns to the mitochondria to be converted to ATP again.

Retrieved from: https://commons.wikimedia.org/wiki/File:Cellular_respiration_EN.svg

The overall chemical equation for cellular respiration is:

\[\text{C}_6 \text{H}_{12} \text{O}_6 + 6 \text{O}_2 \rightarrow 6 \text{H}_2 \text{O} + 6 \text{CO}_2\]

The equation above represents aerobic respiration, which requires oxygen. Anaerobic respiration occurs without oxygen, but it is much less efficient. Most organisms, including aerobic bacteria, need oxygen to survive.

Photosynthesis

Photosynthesis is the process by which plants transform the energy in light into chemical energy that can be used to fuel life functions. Photosynthesis takes place in the chloroplasts of plant and algal cells. The equation for photosynthesis is:

\[6 \text{H}_2 \text{O} + 6 \text{CO}_2 + \text{Light energy} \rightarrow \text{C}_6 \text{H}_{12} \text{O}_6 + 6 \text{O}_2\]

The raw materials, or reactants, for photosynthesis are water and carbon dioxide, without which photosynthesis cannot take place. The products of photosynthesis are glucose and oxygen. Notice that this equation is the reverse of the equation for cellular respiration. The products of one become the reactants of the other.

Photosynthesis occurs in two main stages: the light reactions and the Calvin cycle. In the light reactions, light energy is used to split water molecules, releasing oxygen and creating ATP and nicotinamide adenine dinucleotide phosphate with hydrogen (NADPH), which is another energy carrier. These molecules are used to power the Calvin cycle, in which sugar is produced. The sugar produced by plants is used by the plant for energy and growth.

Similarity to the Animal Process

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 glucose. Plants must then use cellular respiration to convert the glucose into usable energy.

Different Types of Photosynthesis

Some stages of photosynthesis require the presence of light, whereas others can be performed in the absence of light. The light reaction in photosynthesis, as indicated by its name, is dependent on a source of light to provide the energy needed to split a water molecule. The Calvin cycle, however, can occur in the absence of light by using high-energy molecules created during the day by the light reaction. Once those molecules are used up, however, photosynthesis cannot continue.