Cell Biology Study Guide: Macromolecules, Cell Structure, Bioenergetics, and Enzyme Function

Chapter 3: The Macromolecules of the Cell

Overview of Biological Macromolecules

Cells are composed of four major classes of macromolecules: proteins, nucleic acids, polysaccharides, and lipids. Proteins, nucleic acids, and polysaccharides are true polymers, built from repeating monomeric units, while lipids are not true polymers but are included due to their structural and functional importance.

• Proteins: Polymers of amino acids; crucial for structure and function.

• Nucleic acids: Polymers of nucleotides; store and transmit genetic information.

• Polysaccharides: Polymers of sugars; serve as energy storage and structural materials.

• Lipids: Diverse group; important for membranes, energy storage, and signaling.

3.1 Proteins

Proteins are essential macromolecules involved in nearly every cellular process. They are classified by function and structure, and their properties are determined by their amino acid composition and sequence.

• Functional classes:

  • Enzymes: Catalyze biochemical reactions.

  • Structural proteins: Provide support (e.g., collagen, keratin).

  • Motility proteins: Enable movement (e.g., actin, myosin).

  • Regulatory proteins: Control cellular processes (e.g., p53).

  • Transport proteins: Move substances (e.g., hemoglobin).

  • Signaling proteins: Mediate communication (e.g., insulin).

  • Receptor proteins: Detect signals (e.g., GPCRs).

  • Defensive proteins: Protect against disease (e.g., antibodies).

  • Storage proteins: Store amino acids (e.g., ferritin).

• Structural classes:

  • Globular proteins: Compact, often enzymes or regulatory proteins.

  • Fibrous proteins: Elongated, structural roles (e.g., collagen, keratin).

Amino Acids: The Monomers of Proteins

• 20 standard amino acids are used in protein synthesis.

• Each amino acid has a central (α) carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group).

• Stereoisomerism: All amino acids except glycine are chiral; only L-amino acids are found in proteins.

• Ionization: At cellular pH, amino and carboxyl groups are ionized.

Table: Common Small Molecules in Cells

Type	Examples	Role
Amino acids	20 types	Protein monomers
Aromatic bases	A, C, G, T, U	Nucleic acid components
Sugars	Ribose, deoxyribose, glucose	RNA/DNA, energy, storage
Lipids	Fatty acids, cholesterol	Membranes, energy storage

Polypeptides and Protein Structure

• Proteins are formed by peptide bonds (covalent bonds between amino acids via condensation reactions).

• Polypeptides have directionality: N-terminus (amino) to C-terminus (carboxyl).

• Proteins may be monomeric (single polypeptide) or multimeric (multiple subunits, e.g., hemoglobin).

Protein Folding and Stability

• Stabilized by covalent (disulfide) and noncovalent (hydrogen, ionic, van der Waals, hydrophobic) interactions.

• Disulfide bonds form between cysteine residues, stabilizing structure.

• Chaperones assist in proper folding.

• Misfolded proteins can cause diseases (e.g., Alzheimer's).

Levels of Protein Structure

Level	Description	Bonds/Interactions
Primary	Amino acid sequence	Peptide bonds
Secondary	α-helix, β-sheet, random coil	Hydrogen bonds (backbone)
Tertiary	3D folding of a polypeptide	Disulfide, hydrogen, ionic, van der Waals, hydrophobic
Quaternary	Association of multiple polypeptides	Same as tertiary

Protein Domains

• Domains are discrete, functional units within a protein, often associated with specific activities.

• Proteins may have one or multiple domains.

Protein Structure Determination

• X-ray crystallography is used to determine atomic-level protein structures.

• Requires crystallization, X-ray diffraction, and computational analysis.

3.2 Nucleic Acids

Nucleic acids (DNA and RNA) are informational macromolecules composed of nucleotide monomers. They store, transmit, and express genetic information.

• Nucleotides: Consist of a five-carbon sugar (ribose or deoxyribose), a phosphate group, and a nitrogenous base (purine or pyrimidine).

• DNA: Double-stranded helix, stores genetic information.

• RNA: Usually single-stranded, involved in gene expression and regulation.

Table: Bases, Nucleosides, and Nucleotides

Base	Nucleoside (RNA)	Nucleotide (RNA)	Nucleoside (DNA)	Nucleotide (DNA)
Adenine (A)	Adenosine	AMP	Deoxyadenosine	dAMP
Guanine (G)	Guanosine	GMP	Deoxyguanosine	dGMP
Cytosine (C)	Cytidine	CMP	Deoxycytidine	dCMP
Uracil (U)	Uridine	UMP	-	-
Thymine (T)	-	-	Deoxythymidine	dTMP

DNA Structure

• Double helix with antiparallel strands.

• Base pairing: A-T (2 H-bonds), G-C (3 H-bonds).

• Backbone: Sugar-phosphate, bases project inward.

3.3 Polysaccharides

Polysaccharides are long-chain polymers of sugars, serving as energy storage (starch, glycogen) or structural materials (cellulose, chitin).

• Monomers: Monosaccharides (e.g., glucose, ribose).

• Glycosidic bonds: Link monosaccharides; α-linkages in storage polysaccharides, β-linkages in structural polysaccharides.

• Storage polysaccharides: Starch (plants), glycogen (animals).

• Structural polysaccharides: Cellulose (plants), chitin (fungi, arthropods).

Table: Common Polysaccharides

Polysaccharide	Monomer	Bond Type	Function
Starch	α-D-glucose	α(1→4), α(1→6)	Energy storage (plants)
Glycogen	α-D-glucose	α(1→4), α(1→6)	Energy storage (animals)
Cellulose	β-D-glucose	β(1→4)	Structural (plants)
Chitin	GlcNAc	β(1→4)	Structural (fungi, exoskeletons)

3.4 Lipids

Lipids are hydrophobic or amphipathic molecules, important for membrane structure, energy storage, and signaling. They are not true polymers but are considered macromolecules due to their size and function.

• Fatty acids: Long hydrocarbon chains with a carboxyl group; building blocks for many lipids.

• Triacylglycerols: Glycerol + 3 fatty acids; energy storage.

• Phospholipids: Glycerol + 2 fatty acids + phosphate group; major membrane component.

• Glycolipids: Lipids with carbohydrate groups; cell recognition.

• Steroids: Four-ring structure (e.g., cholesterol, hormones).

• Terpenes: Built from isoprene units; include vitamins and pigments.

Table: Main Classes of Lipids

Class	Structure	Function
Fatty acids	Long hydrocarbon chain + COOH	Energy, building blocks
Triacylglycerols	Glycerol + 3 fatty acids	Energy storage
Phospholipids	Glycerol + 2 fatty acids + phosphate	Membranes
Glycolipids	Lipid + carbohydrate	Cell recognition
Steroids	Four fused rings	Membranes, hormones
Terpenes	Isoprene units	Vitamins, pigments

Chapter 4: Cells and Organelles

4.1 The Origins of the First Cells

The origin of life involved the abiotic synthesis of organic molecules, their polymerization, the emergence of informational macromolecules (likely RNA), and encapsulation within membranes (protocells).

• Miller-Urey experiment: Demonstrated abiotic synthesis of amino acids.

• RNA world hypothesis: RNA may have been the first informational and catalytic molecule.

• Liposomes: Artificial vesicles that model primitive cell membranes.

4.2 Basic Properties of Cells

Cells are classified into three domains: Bacteria, Archaea, and Eukarya. Eukaryotic cells have a nucleus and organelles; prokaryotes (Bacteria and Archaea) do not.

• Cell size is limited by surface area/volume ratio, diffusion rates, and the need for adequate concentrations of reactants and catalysts.

• Compartmentalization in eukaryotes allows for specialized functions.

Table: Comparison of Bacterial, Archaeal, and Eukaryotic Cells

Feature	Bacteria	Archaea	Eukaryotes
Size	1–5 μm	1–5 μm	10–100 μm
Nucleus	No	No	Yes
Organelles	No	No	Yes
Cell wall	Peptidoglycan	Varies	Cellulose/chitin (plants/fungi)
DNA	Circular	Circular (histone-like)	Linear (histones)
Ribosome size	70S	70S	80S

4.3 Eukaryotic Cell Structure and Function

• Plasma membrane: Phospholipid bilayer with proteins; controls entry/exit of substances.

• Nucleus: Contains DNA, nucleolus (rRNA synthesis), nuclear envelope with pores.

• Mitochondria: Double-membraned; site of ATP production (aerobic respiration).

• Chloroplasts: Double-membraned; site of photosynthesis in plants/algae.

• Endomembrane system: Includes ER (rough and smooth), Golgi apparatus, lysosomes, and vesicles; involved in protein/lipid synthesis and transport.

• Peroxisomes: Break down fatty acids, detoxify harmful substances.

• Vacuoles: Storage and turgor in plants; digestion in protists.

• Ribosomes: Protein synthesis; free in cytosol or bound to ER.

• Cytoskeleton: Microtubules, microfilaments, intermediate filaments; structure, movement, division.

• Extracellular matrix/cell wall: Support, protection, cell signaling.

4.4 Viruses, Viroids, and Prions

• Viruses: Acellular, DNA or RNA core, protein coat; require host for replication.

• Viroids: Small, circular RNA; infect plants.

• Prions: Infectious proteins; cause neurodegenerative diseases.

Chapter 5: Bioenergetics – The Flow of Energy in the Cell

5.1 The Importance of Energy

Cells require energy for biosynthesis, movement, transport, electrical work, heat, and light production. Organisms obtain energy from sunlight (phototrophs) or chemical compounds (chemotrophs).

• Photoautotrophs: Use light and CO2 (e.g., plants).

• Chemoheterotrophs: Use organic molecules (e.g., animals).

5.2 Bioenergetics and Thermodynamics

• First Law: Energy is conserved; cannot be created or destroyed.

• Second Law: Entropy (disorder) of the universe increases in spontaneous processes.

• Free energy (G): Energy available to do work; change in free energy (ΔG) predicts spontaneity.

• ΔG = ΔH - TΔS (where H = enthalpy, S = entropy, T = temperature in K).

• Exergonic (ΔG < 0): Spontaneous; Endergonic (ΔG > 0): Non-spontaneous.

5.3 Equilibrium, ΔG, and Keq

• Equilibrium constant (Keq): Ratio of product to reactant concentrations at equilibrium.

• ΔG relates to Keq: $ΔG = -RT \ln K_{eq}$

• Standard free energy change ($ΔG^\circ$): Calculated under standard conditions (1 M, 25°C, pH 7).

• Cells maintain steady-state conditions, not equilibrium, to remain alive.

Chapter 6: Enzymes – The Catalysts of Life

6.1 Activation Energy and the Metastable State

• Even spontaneous reactions require an activation energy (EA) to proceed.

• Catalysts (enzymes) lower EA, increasing reaction rates without being consumed.

• Enzymes are highly specific protein catalysts (some RNA molecules, ribozymes, also act as enzymes).

6.2 Enzymes as Biological Catalysts

• Active site: Region where substrate binds and reaction occurs; formed by specific amino acids.

• Enzyme specificity: Enzymes are highly selective for their substrates.

• Enzyme classes (EC system): Oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.

• Enzyme activity is sensitive to temperature, pH, and other factors.

6.3 Enzyme Kinetics

• Enzyme-catalyzed reactions often follow Michaelis–Menten kinetics:

$v_0 = \frac{V_{max}[S]}{K_m + [S]}$

• Vmax: Maximum velocity at saturating substrate.

• Km: Substrate concentration at half-maximal velocity; indicates enzyme affinity for substrate.

• kcat: Turnover number; number of substrate molecules converted per enzyme per second.

• Lineweaver–Burk plot: Double-reciprocal plot to determine Vmax and Km:

$\frac{1}{v_0} = \frac{K_m}{V_{max}[S]} + \frac{1}{V_{max}}$

6.4 Enzyme Regulation

• Irreversible inhibitors: Covalently bind, permanently inactivating enzyme.

• Reversible inhibitors: Bind noncovalently; can be competitive (active site) or noncompetitive (allosteric site).

• Allosteric regulation: Effector molecules bind to sites other than the active site, altering enzyme activity (can be inhibitory or activating).

• Feedback inhibition: End product of a pathway inhibits an early enzyme, regulating pathway output.

• Covalent modification: Addition/removal of chemical groups (e.g., phosphorylation) can activate or inactivate enzymes.

• Proteolytic cleavage: Irreversible activation by removal of a peptide segment (e.g., activation of digestive enzymes).

Key Equations and Concepts

• Free energy change: $ΔG = ΔH - TΔS$

• Relationship to equilibrium: $ΔG = -RT \ln K_{eq}$

• Michaelis–Menten equation: $v_0 = \frac{V_{max}[S]}{K_m + [S]}$

• Lineweaver–Burk equation: $\frac{1}{v_0} = \frac{K_m}{V_{max}[S]} + \frac{1}{V_{max}}$

• Turnover number: $k_{cat} = \frac{V_{max}}{[E]}$

Summary Table: Macromolecules and Their Properties

Macromolecule	Monomer	Bond Type	Main Functions
Protein	Amino acid	Peptide bond	Catalysis, structure, transport, regulation
Nucleic acid	Nucleotide	Phosphodiester bond	Genetic information, expression
Polysaccharide	Monosaccharide	Glycosidic bond	Energy storage, structure
Lipid	Fatty acid, glycerol, etc.	Ester bond (varies)	Membranes, energy storage, signaling

Additional info:

• Some content, such as detailed mechanisms of enzyme catalysis, protein folding prediction, and advanced regulatory mechanisms, is summarized for clarity and brevity.

• Tables and equations are reconstructed for study purposes; refer to primary literature or textbooks for detailed structures and mechanisms.