General Biology Study Notes: Biomolecules, Cell Membranes, and Chemical Bonds

Biomolecules: Lipids, Proteins, Carbohydrates, and Nucleic Acids

Lipids

Lipids are a diverse group of hydrophobic biomolecules essential for energy storage, membrane structure, and signaling in cells.

• Definition: Lipids are organic molecules insoluble in water due to their non-polar hydrocarbon tails.

• Types of Fatty Acids:

  • Saturated fatty acids: Contain only single bonds between carbon atoms; straight chains; solid at room temperature (e.g., butter, beeswax).

  • Unsaturated fatty acids: Contain one or more double bonds; kinked chains; liquid at room temperature (e.g., safflower oil).

• Major Types of Lipids in Cells:

  1. Fats (Triglycerides): Composed of glycerol and three fatty acids. Serve as energy storage; fatty acids store more energy than carbohydrates due to nonpolar bonds.

  2. Steroids: Include cholesterol (nonpolar), cell membrane components, vitamin D, precursors, and hormones involved in cell signaling.

  3. Phospholipids: Made from glycerol molecules, with a hydrophilic (polar) head and hydrophobic (nonpolar) tails. Key component of cell membranes.

• Amphipathic: Molecules having both hydrophilic and hydrophobic parts (e.g., phospholipids).

Example: Phospholipids form the bilayer of cell membranes, creating a selective barrier between the cell and its environment.

Proteins

Proteins are polymers of amino acids that perform a wide range of functions, including catalysis, transport, and structural support.

• Structure: Composed of 20 common amino acids; have primary, secondary, tertiary, and quaternary structures.

• Quaternary Structure: Proteins may consist of multiple polypeptide subunits held together by tertiary structure interactions.

• Disulfide Bonds: Covalent bonds that stabilize protein structure.

• Examples: Hemoglobin transports oxygen; enzymes catalyze biochemical reactions.

Example: Hemoglobin is a quaternary protein that transports oxygen in the blood.

Carbohydrates

Carbohydrates are energy-rich biomolecules composed of monosaccharide units and serve as energy storage and structural support.

• Monosaccharides: Simple sugars (e.g., glucose) that serve as building blocks.

• Polysaccharides: Polymers of monosaccharides; provide energy storage (glycogen in animals, starch in plants) and structural support (cellulose in plants, chitin in fungi and arthropods).

• Glycosidic Linkage: Covalent bond joining monosaccharides.

• Difference between alpha and beta glucose: Alpha linkage is more stable; beta linkage forms cellulose.

Example: Cellulose provides structural support in plant cell walls.

Nucleic Acids

Nucleic acids (DNA and RNA) store and transmit genetic information in cells.

• Monomers: Nucleotides, composed of a sugar, phosphate group, and nitrogenous base.

• DNA: Deoxyribonucleic acid; stores genetic information; double helix structure; backbone is directional (5' to 3').

• RNA: Ribonucleic acid; single-stranded; more reactive due to hydroxyl group; involved in protein synthesis.

• Purines: Adenine and guanine (larger bases).

• Pyrimidines: Cytosine, thymine (DNA), uracil (RNA).

• Phosphodiester Linkages: Covalent bonds joining nucleotides.

Example: DNA replication is semi-conservative, preserving genetic fidelity.

Cell Membranes and Transport

Phospholipid Bilayer

The cell membrane is primarily composed of a phospholipid bilayer, which regulates the movement of substances into and out of the cell.

• Structure: Hydrophilic heads face outward; hydrophobic tails face inward.

• Amphipathic Nature: Allows formation of bilayer and selective permeability.

• Membrane Proteins: Integral (span the membrane; nonpolar amino acids) and peripheral (attached to surface).

Example: Membrane proteins form channels, carriers, and pumps for transport.

Membrane Permeability

Membrane permeability determines how easily substances cross the cell membrane.

• Depends on:

  • Molecule and solute structure

  • Phospholipid structure (saturated vs. unsaturated)

  • Cholesterol content and temperature

  • Addition of other molecules

• High Permeability: Small, nonpolar molecules (e.g., O2, CO2)

• Low Permeability: Large, polar molecules and ions (e.g., glucose, Na+)

Example: Cholesterol modulates membrane fluidity and permeability.

Transport Mechanisms

Cells use various mechanisms to transport substances across membranes.

• Diffusion: Movement of solutes from high to low concentration; random motion; continues until equilibrium is reached.

• Osmosis: Diffusion of water across a membrane; water moves from high to low concentration of water.

• Facilitated Diffusion: Passive transport via channels and carriers; no energy required.

• Active Transport: Movement against concentration gradient; requires energy (e.g., pumps).

• Secondary Active Transport: Uses energy from one molecule moving down its gradient to transport another molecule against its gradient (e.g., sucrose/H+ cotransporter).

Example: Aquaporin channels facilitate water movement across membranes.

Chemical Bonds and Water

Covalent and Ionic Bonds

Chemical bonds are essential for the structure and function of biomolecules.

• Covalent Bonds: Atoms share electrons; can be nonpolar (equal sharing) or polar (unequal sharing).

• Ionic Bonds: Transfer of electrons; formation of cations and anions (e.g., Na+, Cl-).

• Hydrogen Bonds: Weak bonds between polar molecules; important in water and biomolecule structure.

Example: Water molecules are held together by hydrogen bonds, giving water unique properties.

Properties of Water

Water is vital for life due to its chemical and physical properties.

• High Specific Heat: Absorbs energy, helps maintain homeostasis.

• Cohesion and Adhesion: Water molecules stick to each other and to surfaces; enables transport in plants.

• Solvent Properties: Dissolves polar and ionic substances.

• pH Scale: Measures concentration of H+ ions; ranges from 0 (acidic) to 14 (basic).

Example: Water's solvent ability allows biochemical reactions to occur in cells.

Genetic Information and Nucleic Acid Structure

DNA and RNA Structure

DNA and RNA are polymers of nucleotides that encode genetic information.

• Primary Structure: Sequence of nucleotides (e.g., 5'-TCGGATGATCTGT-3')

• Secondary Structure: Double helix in DNA; hydrogen bonds between bases; hydrophobic stacking.

• Tertiary Structure: Supercoiling and folding (e.g., pseudoknot in RNA).

• Directionality: Backbone runs 5' to 3'.

Example: DNA's double helix is stabilized by hydrogen bonds and hydrophobic interactions.

Types of Naturally Occurring Ribozymes

Ribozymes are RNA molecules with catalytic activity.

• snRNPs: Catalyze breaking of phosphodiester bonds during RNA splicing.

• Ribosomal RNA: Catalyzes peptide bond formation during translation.

Example: Ribosomal RNA is essential for protein synthesis.

Scientific Process and Characteristics of Life

Characteristics of Living Things

Living organisms share several defining characteristics.

• Made of cells; highly complex and organized

• Reproduction; produce more of themselves

• Genetic information; pass hereditary information (genes)

• Evolution; populations change over time

• Carry out chemical reactions (metabolism)

• Engage in mechanical activities

Example: All living things use DNA to store genetic information.

Scientific Method

The scientific method is a systematic approach to investigating natural phenomena.

• Ask questions

• Form a hypothesis

• Make predictions

• Conduct experiments

• Analyze results

• Report findings

Example: Hypotheses must be testable and falsifiable.

HTML Table: Types of Chemical Bonds

Bond Type	Example	Polarity
Non-polar covalent	Hydrogen (H2), Methane (CH4)	Electrons shared equally
Polar covalent	Water (H2O), Ammonia (NH3)	Electrons shared unequally
Ionic	Sodium chloride (NaCl)	Electron transfer
Hydrogen bond	Between water molecules	Weak, between polar molecules

Key Equations and Formulas

• Phosphodiester Bond Formation: $\text{Nucleotide}_1 - \text{phosphate} - \text{Nucleotide}_2$

• Directionality of DNA/RNA: $5' \rightarrow 3'$

• General Formula for Water: $\text{H}_2\text{O}$

• pH Calculation: $\text{pH} = -\log_{10}[\text{H}^+]$

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the scientific method and the role of ribozymes.