Cell Biology Study Guide: Molecular Structure, Bioenergetics, Enzymes, and Membranes

Molecular Structure and Chemical Bonds

Covalent and Noncovalent Bonds

Chemical bonds are fundamental to the structure and function of biological molecules. Covalent bonds involve the sharing of electron pairs between atoms, while noncovalent interactions include hydrogen bonds, ionic bonds, and van der Waals forces.

• Covalent Bond: Outer shell electrons of two atoms are shared so that both fill their orbitals.

• Noncovalent Bonds: Weaker interactions such as hydrogen bonds and ionic bonds, crucial for maintaining the structure of macromolecules.

• Example: The double helix of DNA is stabilized by hydrogen bonds between complementary base pairs.

Molecular Hierarchy

Biological molecules are organized in a hierarchy from simple to complex structures.

• Order from lowest to highest:

  1. Nucleus

  2. Chromosome

  3. DNA

  4. Nucleotide

• Example: Nucleotides are the building blocks of DNA, which is packaged into chromosomes within the nucleus.

Bond Energies

Bond energy refers to the strength of a chemical bond, with triple bonds generally being the strongest.

• Highest bond energy: C≡C (triple bond)

• Example: The triple bond in nitrogen gas (N≡N) makes it very stable.

Valence and Functional Groups

Valence determines the number of bonds an atom can form. Functional groups confer specific chemical properties to molecules.

• Carbon valence: 4, allowing it to form four chemical bonds.

• Functional groups:

  • Sulfhydryl: Can form disulfide bonds.

  • Amino: Can act as a base and pick up a proton.

  • Carboxyl: Can act as an acid and be negatively charged.

• Example: Disulfide bonds stabilize protein structure.

Hydrophilic vs. Hydrophobic Substances

Hydrophilic substances are water-loving, while hydrophobic substances are water-fearing.

• Hydrophilic: Polar, water-soluble

• Hydrophobic: Non-polar, lipid-soluble

Macromolecules and Polymers

DNA Structure

DNA is a polymer of nucleotides, each consisting of a sugar, phosphate, and nitrogenous base.

• Primary structure: Sequence of nucleotides (adenine, guanine, cytosine, thymine)

• Double helix: Stabilized by hydrogen bonds between base pairs

Glycosidic Bonds and Polymers

Glycosidic bonds link monosaccharides to form polysaccharides.

• Greatest number of glycosidic bonds: Polysaccharides like cellulose

• Polymer-monomer matching:

  Polymer	Monomer
  DNA	Nucleotide
  Insulin	Amino acid
  Cellulose	Monosaccharide

Protein Structure

Proteins are polymers of amino acids, with structure stabilized by various bonds.

• Secondary structure: Stabilized by hydrogen bonds, ionic bonds, and van der Waals forces

• Amino acid classification: Based on side chain properties (polar, non-polar, charged)

Bioenergetics

Free Energy and Thermodynamics

Bioenergetics studies energy flow in biological systems, focusing on free energy changes in reactions.

• Gibbs Free Energy ($\Delta G$): Determines spontaneity of reactions

• Equation: $\Delta G = \Delta H - T\Delta S$

• Exergonic reactions: $\Delta G$ is negative; energy is released

• Endergonic reactions: $\Delta G$ is positive; energy is absorbed

• Standard free energy change ($\Delta G^\circ$): Calculated under standard conditions

Equilibrium and Entropy

Equilibrium constant ($K_{eq}$) relates to the concentrations of reactants and products at equilibrium.

• Equation: $K_{eq} = \frac{[\text{products}]}{[\text{reactants}]}$

• Entropy ($\Delta S$): Measure of disorder; increases in spontaneous processes

• Example: Combustion of paper increases entropy.

Enzyme Structure and Function

Enzyme Kinetics

Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• Lineweaver-Burk plot: Double reciprocal plot used to determine $V_{max}$ and $K_m$

• Michaelis-Menten equation: $v = \frac{V_{max}[S]}{K_m + [S]}$

• Saturation: Occurs when increasing substrate concentration no longer increases reaction rate

Enzyme Regulation

Enzymes are regulated by various mechanisms, including allosteric regulation and covalent modification.

• Allosteric regulation: Effector molecules bind at sites other than the active site

• Competitive inhibition: Inhibitor binds to active site, blocking substrate

• Noncompetitive inhibition: Inhibitor binds elsewhere, altering enzyme function

Enzyme Classification

Enzymes are classified by the type of reaction they catalyze.

• Major classes: Oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases

Membranes and Transport

Membrane Structure

Biological membranes are composed of a lipid bilayer with embedded proteins, providing selective permeability.

• Phospholipids: Major component of membranes; amphipathic nature

• Fluid mosaic model: Describes membrane structure as dynamic and heterogeneous

Transport Mechanisms

Cells transport substances across membranes via passive and active mechanisms.

• Passive transport: Diffusion and facilitated diffusion; no energy required

• Active transport: Requires energy (often ATP) to move substances against concentration gradients

• Example: Sodium-potassium pump maintains ion gradients in animal cells

Carbohydrates and Lipids

Carbohydrate Structure

Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides.

• Monosaccharides: Simple sugars (e.g., glucose)

• Polysaccharides: Complex carbohydrates (e.g., starch, glycogen, cellulose)

• Glycosidic bonds: Link monosaccharides in polysaccharides

Lipid Structure

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids.

• Triglycerides: Glycerol attached to three fatty acids

• Phospholipids: Glycerol, two fatty acids, and a phosphate group

• Steroids: Four fused hydrocarbon rings

• Example Table:

  Lipid Type	Structure
  Triglyceride	Glycerol + 3 fatty acids
  Phospholipid	Glycerol + 2 fatty acids + phosphate
  Steroid	4 hydrocarbon rings

Additional info:

• Some questions referenced the importance of water in biological systems, including its polarity, high specific heat, and role as a solvent.

• Protein secondary structure is stabilized by hydrogen bonds, ionic bonds, and van der Waals forces.

• Enzyme activity can be affected by temperature, pH, and the presence of inhibitors or activators.