Biology Study Guide for the HESI Exam

Cell Reproduction

Familiarity with the cell cycle is helpful in understanding these processes. Cells do not arbitrarily reproduce, nor do they reproduce nonstop. Instead, there are triggers and signals that must be present before a cell begins reproduction.

Asexual

While sexual reproduction involves the fusion of gametes, such as sperm and egg cells, asexual reproduction occurs when one parent creates genetically identical offspring. There are two types of asexual reproduction: binary fission and mitosis.

Binary Fission

Bacteria and archaea (primitive prokaryotes) use the process of binary fission to reproduce asexually. During binary fission, the cell duplicates its DNA and then splits into two identical daughter cells. Each cell is a genetic copy of the parent cell.

Mitosis

Mitosis is the division of the nucleus and is an important process in cell division. Mitosis occurs in eukaryotic cells. Unicellular eukaryotes can use mitosis to asexually reproduce. In multicellular organisms, mitosis is used for growth, repair, and replacement of worn-out cells. The end result of mitosis is two nuclei, each genetically identical. The final process of cell division after mitosis is cytokinesis, in which the cell membrane pinches off to separate the cell into two. Mitosis can be broken down into the following stages:

• prophase—DNA winds up into structures called chromosomes. The two copies of each chromosome are called sister chromatids and are attached at the centromere. The cell now temporarily has twice as many chromosomes as a typical cell. The nuclear membrane begins to dissolve, which some sources refer to as prometaphase.

• metaphase—The chromosomes line up in the center of the cell.

• anaphase—The centromeres split and the sister chromatids separate and move toward opposite ends of the cell.

• telophase—A new nuclear membrane forms around each set of chromosomes, which creates two daughter cells. Each daughter cell has the same number of chromosomes as the original cell.

Sexual: Meiosis

Meiosis is another type of cell division. It creates gametes, or sex cells, that are used during sexual reproduction. During meiosis, one cell with a full set of chromosomes becomes four cells that contain only half of the chromosomes found in the parent cell. Meiosis shares the same stages as mitosis, but it occurs in two ordered sequences, so there is a meiosis I and a meiosis II. During sexual reproduction, two gametes will fuse together to create a new organism. Unlike asexual reproduction, where the offspring are clones of the parents, sexual reproduction creates more diverse offspring, increasing the genetic variation in sexually reproducing organisms.

Biological Molecules

All living things are made of the same essential building blocks. These organic (carbon-based) molecules can be classified into four chemically distinct groups: carbohydrates, lipids, proteins, and nucleic acids. The structure and function of each is detailed in this section.

Carbohydrates

Carbohydrates are made of carbon, hydrogen, and oxygen, usually in a 1:2:1 ratio. Simple sugars are the smallest forms of carbohydrates, while carbohydrates such as starch and glycogen are much larger. One essential function of carbohydrates is that they provide the body with a source of quick energy. Another important function is that they aid in cell-to-cell recognition.

Lipids

Lipids include fats, oils, and steroids. They, like carbohydrates, are made of carbon, hydrogen, and oxygen. Unlike carbohydrates, lipids are not in a fixed ratio, though carbon and hydrogen make up the bulk of each molecule. Lipids serve as insulation and long-term energy storage. They also make up structures like cell membranes, steroids, and waxes. Three categories of lipids are discussed below.

Fatty Acids

Fatty acids are long chains of carbon atoms with hydrogen attached and a carboxyl (COOH) group at one end. They can be saturated (no double bonds) or unsaturated (one or more double bonds). Saturated fats are usually solid at room temperature, and unsaturated fats are usually liquid at room temperature. Fatty acids aid in energy storage, cell membrane stability, and some cellular processes.

Phospholipids

Phospholipids are the main components of cell membranes. They consist of a hydrophilic (water-attracting) head, which contains a phosphate group, and two hydrophobic (water-repelling) fatty acid tails. The structure forms a bilayer in cell membranes, keeping the cell separate from its external environment and regulating what enters and leaves the cell.

Steroids

Steroids are lipids made up of four fused carbon rings. Cholesterol is a steroid and is used to synthesize other steroid hormones like estrogen and testosterone. Steroid hormones regulate processes such as reproduction, stress response, salt balance, and others.

Proteins

Proteins are made of amino acids, and are incredibly complex molecules with a wide variety of roles. Proteins in cell membranes help transport substances in and out of cells. Proteins create structural tissues such as muscle and skin. Proteins make up enzymes, which are chemicals in the body that speed up the rate of reactions by lowering their activation energy (the energy needed for a reaction to occur). Non-steroid hormones are made of protein, as are antibodies, which help fight infection.

Nucleic Acids

Nucleic acids such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are important in storing and transmitting genetic information. These molecules are made of building blocks called nucleotides, which contain a five-carbon sugar, a phosphate group, and a nitrogen base. DNA carries the genetic code, which is the instructions for creating proteins. RNA helps carry out the instructions found in DNA.

Metabolism

Metabolism refers to all of the chemical reactions that take place in the body to maintain life. Metabolic reactions are responsible for releasing and storing energy, growth, and repair. These reactions are controlled by enzymes and depend on the availability of materials such as nutrients or oxygen.

Water

Water may be a common substance on Earth’s surface, but it has several unique properties that make it special. In fact, these properties make life on Earth possible.

Hydrogen Bonding

Many of water’s unique properties are due to its chemical structure. Water is made of two hydrogen atoms bonded to one oxygen atom. They share electrons in a covalent bond. Water molecules are polar, which means that the molecule has oppositely charged ends. Because of this, water molecules are attracted to each other like little magnets and form a weak bond known as a hydrogen bond.

Water’s Specific Heat

Specific heat is a measurement of how much heat energy one gram of a substance must absorb to change its temperature by one degree Celsius. Water has a very high specific heat compared to other liquids, which means that it absorbs a lot of energy before it changes its temperature or state. This is due to the hydrogen bonds between the water molecules, which must be loosened before the water molecules can move faster, changing their temperature.

Water’s specific heat is important for living things because it keeps the temperature of aquatic ecosystems from fluctuating drastically from day to night or season to season. It also helps regulate the temperature within living organisms because water is the main liquid component of cells.

Cohesion and Adhesion

Because of water’s polarity, it exhibits both cohesion and adhesion. Cohesion occurs when water molecules are attracted to other water molecules. Adhesion occurs when water molecules are attracted to other polar or charged molecules.

Lattice Crystals

Most of the time, when a substance freezes, the molecules become packed more tightly together and the substance becomes more dense. Because of hydrogen bonding, water molecules line up with each other, positive to negative ends, and they form a crystal lattice structure. This structure causes ice to take up more space than liquid water, making it less dense. This is why ice floats on liquid water.

Water as a Solvent

A solvent is a substance that dissolves another substance, known as the solute. Water is often called the universal solvent because it can dissolve many substances, including gases, liquids, and solids.

This is essential for living things, because the biochemical reactions necessary for life are carried out in a water-based, or aqueous, environment within the cells and tissues of organisms. Water’s ability to dissolve many important nutrients, ions, and molecules makes it possible to transport these chemicals and provide an environment in which they can interact.