AP BIology Unit 1

Water Polarity

Water is a polar molecule because oxygen is more electronegative than hydrogen, creating partial positive and negative regions that allow hydrogen bonding.

Hydrogen Bond

A weak attraction between the partially positive hydrogen atom of one molecule and the partially negative oxygen or nitrogen atom of another that stabilizes water, proteins, and DNA.

Biological Importance of Hydrogen Bonds

Hydrogen bonds give water its unique properties, stabilize protein secondary structure, and hold complementary DNA strands together.

Cohesion

The attraction between water molecules caused by hydrogen bonding that allows water to move as a continuous column through plant xylem.

Adhesion

The attraction between water molecules and other polar surfaces that helps water move against gravity through xylem.

Cohesion vs. Adhesion

Cohesion keeps water molecules together, while adhesion allows water to stick to other surfaces; together they enable transpiration.

Transpiration-Cohesion-Tension Mechanism

Water evaporating from leaves creates tension that pulls a continuous column of water upward through xylem because of cohesion and adhesion.

Surface Tension

The resistance of water's surface to breaking due to cohesive hydrogen bonds between water molecules.

High Specific Heat

Water resists rapid temperature changes because much of the absorbed energy is used to break hydrogen bonds before increasing molecular motion.

High Heat of Vaporization

Water requires large amounts of energy to evaporate because hydrogen bonds must be broken before molecules can enter the gas phase.

Evaporative Cooling

As the highest-energy water molecules evaporate, the remaining liquid loses heat and becomes cooler.

Ice Density

Ice is less dense than liquid water because hydrogen bonds form a crystalline lattice that spaces water molecules farther apart.

Biological Importance of Floating Ice

Ice floats and insulates the water below, allowing aquatic organisms to survive during cold temperatures.

Universal Solvent

Water dissolves many ionic and polar substances because its polarity surrounds and separates charged particles.

Hydrophilic

A polar or charged substance that readily interacts with or dissolves in water.

Hydrophobic

A nonpolar substance that avoids contact with water and tends to cluster with other nonpolar molecules.

Hydrophobic Interactions

The tendency of nonpolar molecules to cluster together in water, an important force in membrane formation and protein folding.

Water in Biological Reactions

Water is both a reactant and a product in many biological reactions, including hydrolysis and dehydration synthesis.

Dehydration Synthesis

A reaction that removes water to join monomers together by forming covalent bonds, producing larger biological molecules.

Hydrolysis

A reaction that adds water to break covalent bonds, splitting polymers into their monomers.

Biological Importance of Water

Water's polarity and hydrogen bonding make it essential for transport, temperature regulation, metabolism, and maintaining life.

Structure Determines Function

The structure of a biological molecule determines its properties and function; even small structural differences can produce major functional differences.

Carbon

Carbon forms the backbone of biological molecules because it has four valence electrons and can form four stable covalent bonds.

Organic Molecule

A carbon-containing molecule that makes up the four major biological macromolecules.

Covalent Bond

A bond formed by sharing electrons that holds together biological macromolecules.

Polar vs. Nonpolar Molecules

Polar molecules have unequal electron sharing and interact with water, while nonpolar molecules have equal electron sharing and tend to avoid water.

Macromolecules

The four major biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids.

Monomer vs. Polymer

Monomers are small subunits that join together through dehydration synthesis to form polymers; polymers are broken into monomers by hydrolysis.

Carbohydrate

A macromolecule used for short-term energy storage, structural support, and cell recognition.

Carbohydrate Organization

Monosaccharides are single sugars, disaccharides contain two sugars, and polysaccharides are long chains of monosaccharides linked by glycosidic bonds.

Glucose

The primary monosaccharide used by cells for energy and the building block of starch, glycogen, and cellulose.

Glycosidic Bond

The covalent bond formed between monosaccharides during dehydration synthesis.

Storage vs. Structural Polysaccharides

Plants store glucose as starch, animals store glucose as glycogen, and plants use cellulose for structural support in cell walls.

Starch

A plant storage polysaccharide composed of α-glucose that is easily digested by humans.

Glycogen

A highly branched animal storage polysaccharide that allows rapid glucose release.

Cellulose

A structural polysaccharide made of β-glucose that forms plant cell walls and cannot be digested by humans.

Alpha vs. Beta Glucose

α-glucose forms starch and glycogen, while β-glucose forms cellulose; the different orientation changes the polymer's structure and function.

Starch vs. Cellulose

Both are polymers of glucose, but different glycosidic linkages produce different three-dimensional structures and biological functions.

Cellulose Digestion

Humans cannot digest cellulose because they lack cellulase, while ruminants rely on microorganisms that produce cellulase.

Dehydration Synthesis

Builds polymers by removing H₂O and forming covalent bonds between monomers.

Hydrolysis

Breaks polymers into monomers by adding water and breaking covalent bonds.

Lipid

A hydrophobic macromolecule used for long-term energy storage, membrane structure, insulation, and signaling.

Triglyceride

A lipid composed of one glycerol molecule and three fatty acids used for long-term energy storage.

Fatty Acid

A hydrocarbon chain with a carboxyl group that forms part of triglycerides and phospholipids.

Saturated vs. Unsaturated Fats

Saturated fatty acids contain no double bonds and pack tightly; unsaturated fatty acids contain one or more double bonds that create bends, increasing membrane fluidity.

Phospholipid

An amphipathic lipid with a hydrophilic phosphate head and two hydrophobic fatty acid tails that spontaneously forms the phospholipid bilayer of cell membranes.

Amphipathic Molecule

A molecule containing both hydrophilic and hydrophobic regions.

Phospholipid Bilayer

A double layer of phospholipids that forms the foundation of all cell membranes because hydrophobic tails face inward and hydrophilic heads face water.

Hydrophobic Interactions in Membranes

Hydrophobic fatty acid tails cluster together away from water, driving spontaneous membrane formation.

Cholesterol

A steroid lipid that helps maintain membrane fluidity and stability over a range of temperatures.

Steroid

A lipid composed of four fused carbon rings that functions in membrane structure and chemical signaling.

Membrane Fluidity

Membrane flexibility is influenced by cholesterol and the proportion of saturated versus unsaturated fatty acids.

Macromolecule Identification

Carbohydrates contain C, H, and O; lipids contain mostly C and H with little O; proteins contain C, H, O, N (sometimes S); nucleic acids contain C, H, O, N, and P.

Protein

A polymer of amino acids whose three-dimensional structure determines its function.

Amino Acid Structure

Each amino acid contains an amino group, carboxyl group, hydrogen atom, and variable R group attached to a central carbon.

R Group

The variable side chain of an amino acid that determines its chemical properties and how it interacts with other amino acids.

Amino Acid R Groups

R groups may be hydrophobic, hydrophilic, acidic, or basic, and their properties determine protein folding and function.

Peptide Bond

The covalent bond formed between the amino group of one amino acid and the carboxyl group of another through dehydration synthesis.

Polypeptide

A chain of amino acids linked together by peptide bonds that folds into a functional protein.

Primary Protein Structure

The linear sequence of amino acids that determines every higher level of protein structure.

Secondary Protein Structure

Local folding into α-helices and β-pleated sheets stabilized by hydrogen bonds between the backbone of the polypeptide.

Alpha Helix vs. Beta Pleated Sheet

The two forms of secondary protein structure, both stabilized by hydrogen bonds but differing in shape.

Tertiary Protein Structure

The overall three-dimensional shape of a protein formed by interactions among amino acid R groups.

Interactions Stabilizing Tertiary Structure

Hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges stabilize a protein's three-dimensional structure.

Hydrophobic Interactions in Proteins

Nonpolar amino acids cluster in the interior of proteins away from water, helping stabilize protein shape.

Disulfide Bridge

A strong covalent bond formed between two cysteine amino acids that strengthens tertiary structure.

Quaternary Protein Structure

The association of two or more polypeptide chains into one functional protein.

Structure Determines Protein Function

A protein's function depends entirely on its three-dimensional shape, which is determined by its amino acid sequence.

Protein Folding

The process by which a polypeptide folds into its functional three-dimensional shape through interactions among R groups.

Denaturation

The loss of a protein's three-dimensional structure due to changes in temperature, pH, or chemical environment, resulting in loss of function.

Effects of Temperature on Proteins

High temperatures disrupt hydrogen bonds and other interactions, causing proteins to denature.

Effects of pH on Proteins

Changes in pH alter charges on amino acid R groups, disrupting ionic interactions and protein shape.

Mutation and Protein Function

A mutation that changes the amino acid sequence can alter protein folding and function, especially if the new amino acid has different chemical properties.

Hydrophobic vs. Hydrophilic Mutations

Replacing a hydrophobic amino acid with a hydrophilic one (or vice versa) can disrupt protein folding and stability.

Cysteine vs. Valine

Replacing cysteine with valine eliminates the ability to form disulfide bridges, potentially changing protein structure and function.

Protein Functions

Proteins function in catalysis, transport, structure, movement, signaling, defense, and storage.

Enzyme

A protein catalyst that speeds up chemical reactions by lowering activation energy without being consumed.

Catalysis

The process of increasing the rate of a chemical reaction by lowering its activation energy.

Activation Energy

The minimum amount of energy required to start a chemical reaction.

Active Site

The region of an enzyme where a specific substrate binds and the reaction occurs.

Substrate

The reactant that binds to an enzyme's active site.

Enzyme Specificity

An enzyme only binds substrates with shapes and chemical properties complementary to its active site.

Induced Fit Model

Substrate binding causes the enzyme to slightly change shape, improving the fit and increasing catalytic efficiency.

Enzyme-Substrate Complex

The temporary complex formed when a substrate binds to an enzyme before products are released.

Enzyme Function

Enzymes bind specific substrates, lower activation energy, convert substrates into products, and remain unchanged after the reaction.

Optimal Temperature

The temperature at which an enzyme functions most efficiently.

Optimal pH

The pH at which an enzyme functions most efficiently.

Temperature and Enzyme Activity

Reaction rate increases with temperature until the enzyme denatures, causing activity to decrease rapidly.

pH and Enzyme Activity

Enzymes have an optimal pH; values above or below this reduce activity by altering protein structure.

Competitive Inhibition

An inhibitor competes with the substrate for the enzyme's active site, reducing reaction rate.

Noncompetitive Inhibition

An inhibitor binds to a site other than the active site, changing the enzyme's shape and decreasing activity.

Allosteric Regulation

Binding of a molecule at an allosteric site changes enzyme shape and regulates its activity.

Enzyme Saturation

When all enzyme active sites are occupied, increasing substrate concentration no longer increases reaction rate.

Protein Composition

Proteins contain carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.

Proteins vs. Nucleic Acids

Proteins are polymers of amino acids connected by peptide bonds, while nucleic acids are polymers of nucleotides connected by phosphodiester bonds.

AP Protein Concept

Changes in amino acid sequence alter protein structure, and changes in protein structure alter protein function.

Nucleic Acid

A macromolecule composed of nucleotides that stores, transmits, and expresses genetic information.

Nucleotide

The monomer of nucleic acids consisting of a phosphate group, a five-carbon sugar, and a nitrogenous base.

DNA vs. RNA

DNA contains deoxyribose, thymine, and is usually double-stranded; RNA contains ribose, uracil, and is usually single-stranded.

Nitrogenous Bases

The five nitrogenous bases are adenine, thymine, cytosine, guanine, and uracil; thymine is found only in DNA, while uracil is found only in RNA.