Chemistry Study Guide for the HESI Exam

Nuclear Chemistry

Nuclear chemistry is the branch of chemistry that focuses on reactions and processes that involve atomic nuclei. Unlike normal chemical reactions, nuclear reactions involve changes within the nucleus itself, leading to the transformation of one element into another.

Radiation

Radiation is the energy or particles emitted by unstable atomic nuclei as they undergo transformation. The process by which these unstable nuclei break down and emit radiation is known as radioactivity. This happens because some atomic nuclei are not stable due to an imbalance of protons and neutrons. This phenomenon was first discovered by Henri Becquerel and later studied by Marie and Pierre Curie.

Radiation Types

Radiation is classified into three main types based on the type of particles or energy it releases. Each type has different properties, levels of penetration, and potential effects on matter.

Alpha Radiation

Alpha radiation consists of the emission of two protons and two neutrons, identical to a helium nucleus (\(\text{}^4_2 \text{He}\)). It has a charge of \(+2\) and is relatively heavy. This type of radiation has low penetration power and can be stopped by a sheet of paper or human skin. It is highly ionizing but not deeply penetrating.

Beta Radiation

Beta radiation consists of high-speed electrons (\(\text{}^{\phantom{-}0}_{-1} \beta\)) or positrons, particles with no mass and a charge of \(+1\) (\(\text{}^{0}_{1} \beta\)) emitted from the nucleus. It has a charge of \(–1\) or \(+1\) and is much lighter than alpha particles. This type of radiation has moderate penetration power and can’t be stopped by paper, but it is stopped by plastic or aluminum. It is less ionizing than alpha radiation but penetrates deeper.

Gamma Radiation

Gamma radiation consists of high-energy photons (\(\text{}^0_0 \gamma\)) emitted from an atomic nucleus. It has a neutral charge but no mass, as it is pure energy. This type of radiation has high penetration power and can only be stopped by dense shielding like lead or concrete. It is weakly ionizing but has very high penetration. It can damage biological tissues and DNA.

Biochemistry

Biochemistry is the study of the chemical processes that occur within living organisms. It explores how biological molecules, such as carbohydrates, proteins, lipids, and nucleic acids, interact to sustain life.

Carbohydrates

Carbohydrates are organic molecules made of carbon (\(\text{C}\)), hydrogen (\(\text{H}\)), and oxygen (\(\text{O}\)), usually in a ratio of \(1\text{:}2\text{:}1\). They serve as the primary source of energy for the body and play structural and functional roles in cells.

Carbohydrate Roles in Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA)

Carbohydrates contribute to genetic material through deoxyribose (in DNA) and ribose (in RNA), as shown in the image below:

Retrieved from: https://openstax.org/books/organic-chemistry/pages/28-1-nucleotides-and-nucleic-acids?query=deoxyribose

• Deoxyribose is a five-carbon sugar found in DNA. It differs from ribose by lacking an oxygen atom at the \(2’\) carbon, making DNA more stable.

• Ribose is the sugar found in RNA with the same structure as deoxyribose but with an oxygen atom at the \(2’\) carbon.

Carbohydrate Types

Carbohydrates can be classified based on their complexity. There are three general classifications: monosaccharide, disaccharide, and polysaccharide.

Monosaccharide

Monosaccharides are the simplest type of carbohydrate, consisting of a single sugar unit. Examples include:

• glucose—the main energy source for cells
• fructose—found in fruits and honey
• galactose—found in dairy products

Disaccharide

Disaccharides are two monosaccharides linked together through a glycosidic bond. Examples include:

• sucrose (table sugar)—glucose plus fructose
• lactose (found in milk)—glucose plus galactose
• maltose (malt sugar)—glucose plus glucose

Multiple Saccharides

Polysaccharides are complex carbohydrates composed of many sugar units. Some examples include:

• oligosaccharides—These are short chains of sugars that are important for cell signaling.

• starch—This is the main energy storage in plants, consisting of amylose (unbranched) and amylopectin (branched).

• glycogen—This is the main energy reserve in animals, stored in the liver and muscles. It is highly branched for rapid energy release.

• cellulose—This is a structural polysaccharide found in plant cell walls. Humans cannot digest it, but it provides dietary fiber that supports digestion

Carbohydrates and Energy

Carbohydrates are the body’s preferred energy source, and their metabolism involves several key biochemical pathways:

• glycolysis—This process involves the breakdown of glucose into pyruvate, producing adenosine triphosphate (ATP), which provides cellular energy.

• Krebs cycle (citric acid cycle)—This is a series of reactions that starts with pyruvate and generates ATP and electron carriers for further energy production.

• anaerobic glycolysis—If oxygen is absent, pyruvate is converted into lactate instead of entering the Krebs cycle.

Gluconeogenesis

Gluconeogenesis is the process of producing glucose from non-carbohydrate sources, such as amino acids and glycerol. This is needed during fasting or intense exercise when glucose levels drop.

Proteins

Proteins are essential macromolecules that perform a wide range of functions in the body, including enzymatic activity, structural support, transport, and immune defense. They are composed of amino acids, the fundamental building blocks of proteins.

Amino acids are organic molecules containing an amino group (\(–\text{NH}_2\)), a carboxyl group (\(–\text{COOH}\)), a hydrogen atom, and a unique R-group (side chain). The R-group determines the properties of the amino acid (polar, nonpolar, acidic, or basic).

Retrieved from: https://commons.wikimedia.org/wiki/File:Alpha-Amino_Acids_V.1.png

Amino acids link together via peptide bonds, forming different protein structures:

• dipeptide—a molecule consisting of two amino acids joined by a peptide bond
• peptide—a short chain of amino acids (up to \(50\) amino acids)
• polypeptide—a longer chain of amino acids that folds into a functional protein

Lipids

Lipids are a diverse group of hydrophobic biomolecules that include fats, oils, phospholipids, and steroids. Hydrophobic means insoluble, which indicates they don’t dissolve in water. Lipids are essential for energy storage, cell membrane structure, and hormone production. Unlike carbohydrates and proteins, lipids are not polymers, meaning they are not made of repeating monomeric units.

Fatty Acids

Fatty acids are long hydrocarbon chains with a carboxyl group (\(–\text{COOH}\)) at one end. They serve as building blocks for many lipids and are a major source of energy. They can be categorized as follows, based on their length:

• short-chain fatty acids (SCFAs)—fewer than six carbon atoms
• medium-chain fatty acids (MCFAs)—six to \(12\) carbon atoms
• long-chain fatty acids (LCFAs)—more than \(12\) carbon atoms
• very-long-chain fatty acids (VLCFAs)—Over \(20\) carbon atoms

Retrieved from: https://openoregon.pressbooks.pub/mhccmajorsbio/chapter/lipids/

Neutral Fats

Neutral fats are also known as triglycerides. They are the most common type of fat in the body and consist of three fatty acids attached to a glycerol backbone. They serve as long-term energy storage in adipose tissue, insulation to regulate body temperature, and cushioning to protect vital organs. When needed, triglycerides undergo lipolysis to release fatty acids and glycerol for energy production.

Retrieved from: https://openoregon.pressbooks.pub/mhccmajorsbio/chapter/lipids/

Phospholipids

Phospholipids are the main structural components of cell membranes. They contain the following:

• two hydrophobic (insoluble) fatty acid tails
• a hydrophilic (soluble) phosphate-containing head

This amphipathic (dual) nature allows phospholipids to form the bilayer structure of cell membranes, with the hydrophobic tails facing inward and the hydrophilic heads facing outward.

Retrieved from: https://openoregon.pressbooks.pub/mhccmajorsbio/chapter/lipids/

Cholesterol

Cholesterol is a type of steroid lipid found in cell membranes as a precursor for steroid hormones. It is needed for membrane fluidity, bile acid production, and steroid hormone synthesis. Excess cholesterol can lead to plaque formation in the arteries, increasing the risk of cardiovascular disease.

Retrieved from: https://openoregon.pressbooks.pub/mhccmajorsbio/chapter/lipids/

Saturated and Unsaturated Fats

Fats can be classified based on the presence or absence of double bonds in their fatty acid chains:

• saturated fats—They contain no double bonds between carbon atoms. They are typically solid at room temperature due to their straight, tightly packed structure. Excess consumption is associated with heart disease.

• unsaturated fats—They contain one or more double bonds, which create kinks in the fatty acid chain, preventing tight packing. A diet rich in unsaturated fats is linked to reduced risk of cardiovascular disease and improved metabolic health.

Nucleic Acids

Nucleic acids are macromolecules that store and transfer genetic information. They are found in every living cell and are essential for protein synthesis and heredity.

Nucleic acids are made up of nucleotides, which consist of three components:

• a phosphate group

• a five-carbon sugar (deoxyribose in DNA, ribose in RNA).

• a nitrogenous base (adenine, cytosine, guanine, and thymine in DNA; adenine, cytosine, guanine, and uracil in RNA).

These are the two main types of nucleic acids:

• deoxyribonucleic acid (DNA)—This is the molecule that carries genetic instructions for cell growth, development, and reproduction.

• ribonucleic acid (RNA)—This plays a crucial role in protein synthesis by transmitting genetic information from DNA to ribosomes.

In DNA, adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G) through hydrogen bonds, forming a double-helix structure. In RNA, adenine pairs with uracil (A-U).