Welcome to AP Biology Unit 1! This unit is the foundation of the chemical and biological bases of life, including the properties of water and the structures and functions of biological macromolecules. By developing an understanding of these concepts, we can learn how and why biological interactions occur, a critical skill for the rest of the course. 🌱

Water

Water is a polar molecule with intramolecular (within the molecule)  covalent bonding between partially positively charged hydrogens and partially negatively charged oxygen, which creates an unequal distribution of electrons in a dipole moment. When water molecules bond with each other intermolecularly (between molecules), they exhibit hydrogen bonding. Hydrogen bonds are generally weaker than covalent bonds, but they are still relatively strong and play an important role in many chemical and biological processes. 

Bonding is responsible for many molecular properties, such as the shape and function of proteins, the stability of many chemical compounds, and the formation of intermolecular interactions in crystalline solids. The chemical structure and behavior of water molecules provide water with the properties of  cohesion,  adhesion,  surface tension, specific heat, and evaporative cooling. 💧

Hydrophilic substances have an affinity for water and tend to have polar bonds to interact attractively with water molecules. Conversely, a  hydrophobic  substance avoids or "fears" water, tending to lack polar bonds. 

Macromolecules

Part of the upcoming unit focuses on the properties of macromolecules—carbohydrates, lipids, proteins, and nucleic acids—including how they interact with each other and with molecules like water. Macromolecules are exactly what their name suggests, large molecules. These macromolecules, also known as polymers, are made up of chains of smaller, linked molecular subunits called monomers. 🔗

Monomers link through the process of dehydration synthesis. Dehydration synthesis removes water and forms a covalent bond to attach monomers. In reverse, hydrolysis breaks chains of monomers apart through the addition of a water molecule, which eliminates the covalent bond. To remember these processes, remember that "synthesis" brings components (in this case, monomers) together, while the root "-lys-" means "slice" or to break something, such as a macromolecule, apart. 

All four of the main macromolecules contain carbon, a highly versatile element with four valence electrons that readily bonds with a variety of other elements. Other important elements in biological molecules include oxygen, nitrogen, hydrogen, and phosphorus. These elements form functional groups, a chemical combination of elements that contributes to how monomers assemble and dictates the function or role of macromolecules. The six main functional groups students should know for AP Biology are:

1. Carboxyl
2. Carbonyl
3. Hydroxyl
4. Amino
5. Phosphate
6. Sulfhydryl

Knowing the functional groups that are associated with each macromolecule is essential, as functional groups lead to the shape and purpose of a biological molecule. ⚙️

Carbohydrates 🧁

Carbohydrates are macromolecules composed of carbon, hydrogen, and oxygen. This group of macromolecules includes sugars and starch. Carbohydrates contribute to the structure of organisms, such as cellulose in the structural makeup of plants, or the complex carbohydrate chitin, which forms the exoskeletons of insects and arthropods. Most importantly, carbohydrates serve as short-term energy. ☀️

Lipids 🧈 

Made of a different ratio of carbon, hydrogen, and oxygen,  lipids  are  nonpolar macromolecules, which include oils and fats. Lipids are a long-term source of energy storage.

Depending on the presence of double bonds, lipids may be saturated or unsaturated. 

• Lipids have long chains of hydrocarbons, and the double bond, if there is one, is normally found in the hydrocarbon chain. 
• Saturated lipids 🍟 do not have a double bond and tend to form solids at room temperature. 
• Unsaturated lipids 🥑 have at least one double bond and will tend to stay a liquid at room temperature.

Phospholipids contain a polar and a nonpolar region, which helps in their function in the cell membrane. The head region of the phospholipid is hydrophilic and is considered "water-loving", whereas the tail region is hydrophobic and considered "water-fearing." The head faces out, and the tails face inwards, so the phospholipid bilayer that is formed in cell membranes has hydrophobic portions in the middle and hydrophilic portions that face out.

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Proteins 🥩

Proteins are made of carbon, hydrogen, oxygen, nitrogen, and sulfur. The building block of proteins is amino acids, which link together to form peptide chains. Proteins carry out a variety of functions, which include catalyzing chemical reactions through enzymes, transporting molecules, providing structural support, hormonal response, and immune regulation. 

The sequence of amino acids determines the overall shape and function of proteins. A functional protein undergoes at least three folding sequences.

• The primary structure is the peptide bonding of amino acids.  
• The secondary structure is through local folding into alpha-helices or beta-sheets.
• The tertiary structure is what gives the functional shape through R-group bonding.
• The quaternary structure arises if there is more than one chain bonded together. They are subject to denaturing and the topic of many disruptions in the body. which in return determines the function.

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Nucleic Acids 🧬

Finally, nucleic acids compose DNA and RNA. They are made of carbon, hydrogen, oxygen, nitrogen, and phosphorus. 

• The building block, or smallest monomer, of nucleic acids is the nucleotide, which is made of a five-carbon sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, cytosine, or guanine in DNA or in RNA thymine is replaced with uracil). Nucleic acids hold genetic information. DNA is structured antiparallel, with each strand running in opposite 5’ to 3’ orientation.
  

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