Macromolecules, Cell Structure, and Membrane Transport: General Biology Study Notes

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Macromolecules & Functional Groups

Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical behavior and properties of those molecules.

Definition: Groups of atoms that confer specific chemical properties to organic molecules (e.g., hydroxyl, carbonyl, carboxyl, amino, phosphate, methyl).
Polarity: Hydroxyl, carbonyl, carboxyl, amino, and phosphate groups are polar; methyl is nonpolar.
Isomers: Molecules with the same molecular formula but different structures (structural and stereoisomers).
Importance of Carbon: Carbon forms four covalent bonds, allowing for diverse molecular structures.
Basic Elements of Macromolecules: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), and Sulfur (S).

Polymerization Reactions

Dehydration Reaction: Joins monomers by removing water.
Hydrolysis Reaction: Breaks polymers by adding water.

Carbohydrates

Structure and Types

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, serving as energy sources and structural components.

Monomers: Monosaccharides (e.g., glucose, fructose, galactose).
Isomers: Structural and stereoisomers exist (e.g., alpha vs. beta glucose).
Alpha vs. Beta Glucose: Differ in the position of the hydroxyl group on carbon 1.
Glycosidic Bonds: Link monosaccharides to form disaccharides and polysaccharides.

Polysaccharides

Cellulose: Plant structural polysaccharide; β-1,4 linkages; not digestible by animals.
Starch: Plant energy storage; α-1,4 linkages; digestible by animals.
Glycogen: Animal energy storage; α-linkages.

Comparison Table: Starch vs. Cellulose

Property	Starch	Cellulose
Bond Type	α-1,4 linkages	β-1,4 linkages
Function	Energy storage (plants)	Structural (plants)
Digestibility	Digestible by animals	Not digestible by animals

Lipids

Structure and Types

Lipids are hydrophobic molecules including fats, phospholipids, and steroids, important for energy storage, membrane structure, and signaling.

Structure: Glycerol + fatty acids linked by ester bonds.
Saturated Fats: Single bonds; solid at room temperature.
Unsaturated Fats: One or more double bonds; liquid at room temperature.
Phospholipids: Hydrophilic head + hydrophobic tails; form bilayers in membranes.
Cholesterol: Stabilizes membrane and regulates fluidity.

Additional info: Lipids are not true polymers because they are not formed by repetitive monomer units.

Proteins

Structure and Function

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

Monomer Structure: Amino group, carboxyl group, hydrogen, R group (side chain).
Types of Amino Acids: Nonpolar, polar, acidic (−), basic (+).
Peptide Bond: Covalent bond between amino and carboxyl groups.
Directionality: Synthesized from N-terminus to C-terminus.

Levels of Protein Structure

Level	Description	Bonds
Primary	Amino acid sequence	Peptide bonds
Secondary	α-helix or β-sheet	Hydrogen bonds
Tertiary	3D folding	Ionic, hydrogen, hydrophobic, disulfide
Quaternary	Multiple polypeptides	Same as tertiary

Folding Factors: pH, temperature, salt concentration.
Denaturation: Unfolding due to stress; Renaturation: Refolding under normal conditions.

Nucleic Acids

Structure and Function

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

Monomer Structure: Sugar + phosphate + nitrogenous base.
Bonds: Phosphodiester (between nucleotides), hydrogen (between bases).
Directionality: 5′ → 3′ synthesis.
Hydrogen Bonds: A=T (2), G≡C (3).
Chargaff’s Rule: A = T and G = C.

Comparison Table: DNA vs. RNA

Property	DNA	RNA
Sugar	Deoxyribose	Ribose
Base difference	Thymine	Uracil
Strandedness	Double	Single
Directionality	5′ → 3′	5′ → 3′

DNA Denaturation: Separation of strands by heat; higher G≡C = higher melting temperature.

Cell Structure

Prokaryotes vs. Eukaryotes

Cells are classified as prokaryotic or eukaryotic based on the presence of a nucleus and membrane-bound organelles.

Prokaryotes: Lack a nucleus and membrane-bound organelles.
Eukaryotes: Have a nucleus and membrane-bound organelles.

Organelles and Functions

Ribosome: rRNA + proteins; synthesizes proteins; found in all life forms.
Rough ER: Studded with ribosomes; synthesizes proteins.
Smooth ER: No ribosomes; synthesizes lipids and detoxifies.
Golgi Apparatus: Modifies, sorts, and packages proteins.
Mitochondria: Double membrane with cristae; site of cellular respiration and ATP production.
Lysosome: Contains digestive enzymes; breaks down waste.
Vacuoles: Storage (plants), food digestion, contractile (pumps water in protists).
Chloroplast: Double membrane + thylakoids; site of photosynthesis.
Cell Wall: Provides rigidity and protection (plants, fungi, bacteria).
Nucleus: Double membrane with pores; stores DNA.
Cytoskeleton: Microtubules, microfilaments, intermediate filaments; supports shape and movement.
Peroxisome: Single membrane; breaks down fatty acids and detoxifies H2O2.

Endosymbiotic Theory

Eukaryotes evolved from engulfed prokaryotes.
Evidence: Double membranes and DNA in mitochondria/chloroplasts.

Membrane Structure

Plasma Membrane: Phospholipid bilayer with proteins.
Fluid Mosaic Model: Membrane is a dynamic mix of lipids and proteins.
Integral Proteins: Span the membrane.
Peripheral Proteins: Attach to the membrane surface.

Cell Size and Surface Area-to-Volume Ratio (SA/V)

Higher SA/V ratio = faster material exchange.
As cell size increases, SA/V ratio decreases, limiting transport efficiency.

Cell Transport

Homeostasis and Membrane Transport

Cells maintain a stable internal environment through selective transport mechanisms.

Selective Permeability: Only small nonpolar molecules (O2, CO2) freely pass.
Diffusion: Movement from high to low concentration.
Rate Factors: Temperature, size, gradient.
Facilitated Diffusion: Uses protein channels or carriers for ions, glucose, water.

Osmosis and Tonicity

Osmosis: Diffusion of water across a membrane.
Hypotonic: Water in; cell swells.
Isotonic: No net movement.
Hypertonic: Water out; cell shrinks.
Water Potential: (water moves from high to low potential).
Osmoregulation: Contractile vacuole, kidneys maintain balance.

Active Transport

Definition: Moves materials against concentration gradient using ATP.
Forces: ATP hydrolysis, electrochemical gradients.
Na+/K+ Pump: 3 Na+ out, 2 K+ in, using ATP.
Proton Pump: Moves H+ to create gradient.
Coupled Transport: Uses one gradient to move another molecule (e.g., sucrose-H+ cotransport).

Bulk Transport

Phagocytosis: Cell eating (engulfing large particles).
Pinocytosis: Cell drinking (engulfing fluids).
Receptor-Mediated Endocytosis: Specific uptake of molecules via receptors.