Comprehensive Biology 1330 Study Guide: Cell Biology, Chemistry, Lipids, and Membrane Dynamics

Cell Foundations, Structure, and Organelle Functions

• Three Foundational Principles of Cell Theory:

  • All living organisms are composed of one or more cells.

  • The cell is the fundamental, structural, and functional unit of life.

  • All cells arise only from pre-existing cells through cell division.

• Three Modern Additions to Cell Theory:

  • Energy flow (metabolism and biochemistry) occurs within cells.

  • Genetic information (DNA) is passed on from cell to cell during division.

  • All cells have basic chemical compositions that are fundamentally the same in organisms of similar species. Molecules, Electronegativity, and Chemical Bonds: - **Molecule:** Two or more atoms held together by chemical bonds. - **Electronegativity:** An atom's inherent capacity to attract shared electrons toward itself within a chemical bond. Differences in electronegativity determine bond polarity. - **Single Covalent Bond:** Formed when atoms share exactly $1$ pair of electrons ($2$ total electrons). Definitions of Specific Chemical Bonds: - **Nonpolar Covalent Bond:** Formed between two atoms with equal or nearly equal electronegativity; electrons are shared equally between the nuclei. - **Polar Covalent Bond:** Formed between atoms with significantly different electronegativities; electrons are shared unequally, resulting in partial positive ($\delta+$) and partial negative ($\delta-$) charges. - **Ionic Bond:** Formed when an atom with high electronegativity completely steals one or more electrons from an atom with low electronegativity, generating oppositely charged ions that attract each other. - **Hydrogen Bond:** A weak electrostatic attraction between a hydrogen atom covalently bound to a highly electronegative atom (like $O$ or $N$) and another nearby electronegative atom. Valence Electrons, Cations, and Anions: - **Valence Electron:** An electron occupying the outermost shell of an atom that participates in chemical bonding. - **Cation:** A positively charged ion created when an atom loses one or more valence electrons. - **Anion:** A negatively charged ion created when an atom gains one or more valence electrons.

• Four Shared Traits Among All Cells:

  • Plasma Membrane: An outer boundary that separates the interior of the cell from the surrounding environment.

  • Cytoplasm: A jelly-like intracellular fluid (cytosol) containing cellular components.

  • Genetic Material (DNA): The hereditary molecule directing cellular activities.

  • Ribosomes: Cellular structures responsible for protein synthesis.

• Plasma Membrane versus Cell Wall Structure and Function:

  • Plasma Membrane: Composed of a phospholipid bilayer with embedded proteins and carbohydrates. It acts as a selectively permeable barrier regulating the entry and exit of substances to maintain intracellular homeostasis.

  • Cell Wall: Located outside the plasma membrane in plant, fungal, and bacterial cells; it provides rigid structural support, protection, and shape retention, unlike the flexible phospholipid plasma membrane.

• Phospholipid Structure and Membrane Formation:

  • Polar Component: The hydrophilic (water-attracting) phosphate head.

  • Nonpolar Component: Two hydrophobic (water-repelling) fatty acid tails.

  • Importance in Membrane Formation: In an aqueous environment, phospholipids spontaneously self-assemble into a bilayer. The polar heads face outward toward the aqueous intracellular and extracellular fluids, while the nonpolar tails orient inward, shielded from water. This creates an impermeable core to water-soluble molecules.

• Nuclear Envelope and Nucleolus:

  • Nuclear Envelope: A double membrane surrounding the nucleus, perforated with nuclear pores that regulate the transport of proteins, RNA, and macromolecular complexes between the nucleus and cytoplasm.

  • Nucleolus: A dense region inside the nucleus responsible for synthesizing ribosomal RNA (rRNA) and assembling ribosomal subunits.

  • Conformation of DNA: Chromatin and chromosomes represent the exact same pieces of DNA, distinguished only by structural conformation. Chromatin is loose, uncoiled DNA present during interphase, whereas chromosomes are tightly coiled, condensed DNA structures present during nuclear division.

• The Endomembranous Network:

  • Included Organelles: The nuclear envelope, rough endoplasmic reticulum (RER), smooth endoplasmic reticulum (SER), Golgi apparatus, lysosomes, vesicles, and plasma membrane.

  • Shared Characteristics: They are directly continuous or connected via transport vesicles, sharing membrane composition and collaborating in macromolecule synthesis, modification, packaging, and transport.

• Ribosome Structure, Origin, and Function:

  • Function: Synthesizes proteins by translating mRNA sequences into amino acid chains.

  • Site of Assembly: Ribosomal subunits are constructed in the nucleolus.

  • Site of Activity: Active ribosomes function either freely suspended in the cytosol (synthesizing cytosolic proteins) or bound to the exterior of the rough endoplasmic reticulum (synthesizing membrane-bound, lysosomal, or secreted proteins).

• Endocytosis versus Exocytosis:

  • Endocytosis: An active vesicular transport mechanism by which cells internalize external substances, engulfing them via plasma membrane invagination to form an intracellular vesicle.

  • Exocytosis: An active vesicular transport process where internal secretory vesicles fuse with the plasma membrane to expel waste, proteins, or signaling molecules outside the cell.

• Smooth Endoplasmic Reticulum versus Rough Endoplasmic Reticulum:

  • Rough ER (RER): Studded with ribosomes on its cytosolic surface; primarily functions in protein synthesis, folding, quality control, and initial glycosylation.

  • Smooth ER (SER): Devoid of ribosomes; functions in lipid synthesis (phospholipids, steroids), carbohydrate metabolism, detoxification of drugs and toxins, and calcium ion ($\text{Ca}^{2+}$) storage.

• Structure and Function of the Golgi Apparatus:

  • Structure: A series of flattened, membrane-bound sacks called cisternae featuring distinct polarity: a cis face (receiving side near the ER) and a trans face (shipping side facing the plasma membrane).

  • Function: Modifies, sorts, packages, and routes proteins and lipids arriving from the ER for secretion or delivery to specific cellular organelles.

• Lysosome Function:

  • Membrane-bound organelles filled with acid hydrolase enzymes that break down cellular waste, old organelles (autophagy), foreign debris, and macromolecules ingested via endocytosis.

• Role of the Large Central Vacuole:

  • Prominent in plant cells; maintains turgor pressure against the cell wall to provide structural support, stores water, nutrients, and waste products, and handles intracellular hydrolysis.

• Mitochondria and Chloroplasts Structure and Function:

  • Mitochondria: Double-membrane organelles featuring internal folds called cristae surrounding an inner matrix. They serve as the powerhouse of the cell, generating adenosine triphosphate (ATP) through cellular respiration.

  • Chloroplasts: Double-membrane structures found in plants and photosynthetic algae containing stacks of thylakoid membranes (grana) surrounded by stroma. They convert light energy into chemical energy via photosynthesis.

• Cytoskeletal Components, Cilia, and Flagella:

  • Microfilaments (Actin): Provide cellular shape, enable cell movement, and facilitate muscle contractions.

  • Intermediate Filaments: Provide structural stability, mechanical strength, and anchor organelles within the cell.

  • Microtubules: Hollow tubes composed of tubulin that direct vesicle movement, segregate chromosomes during division, and form the structural core of cilia and flagella.

  • Differences between Flagella and Cilia:

  • Form: Flagella are typically long and few ($1$ to $2$ per cell), while cilia are short and present in high numbers across the cell surface.

  • Function & Environment: Ciliated unicellular organisms live in aquatic environments and use cilia directly to propel the entire cell through the liquid medium. In contrast, cilia present in multicellular tissues (e.g., human respiratory tract) are stationary relative to the cell and function to move liquid and particles across the tissue surface, rather than moving the cell itself.

• Predicting Organelle Abundance and Dysfunction Consequences:

  • High metabolic activity or mechanical work demands high energy output. For example, because the human heart beats constantly, $24\text{ hours a day}$, requiring continuous, repeated muscle contractions, cardiac tissue expresses a remarkably high abundance of mitochondria to constantly generate ATP.

  • Systemic Rule: Cells specialized for protein export have abundant Rough ER and Golgi; liver detoxifying cells contain high amounts of Smooth ER; immune phagocytes possess elevated numbers of lysosomes. Loss or degradation of specific organelles halts their correlated pathways (e.g., loss of mitochondria leads to cellular energy failure and cell death).

• Prokaryotic versus Eukaryotic Structural Comparison:

  • Prokaryotic Cells (Bacteria): Lack a membrane-bound nucleus or membrane-bound organelles. Their cellular components comprise a nucleoid (containing circular DNA), ribosomes, cytosol, plasma membrane, cell wall (peptidoglycan), and frequently a capsule, flagella, or pili.

  • Eukaryotic Cells: Possess a membrane-bound nucleus housing linear DNA and an array of membrane-bound organelles.

Atoms, Molecules, Water, and Chemical Bonding

• Definition and Subatomic Structure of an Atom:

  • An atom is the basic unit of matter that retains the chemical properties of an element.

  • Protons: Positively charged particles ($+1$) located inside the atomic nucleus.

  • Neutrons: Electrically neutral particles ($0$ charge) located inside the atomic nucleus.

  • Electrons: Negatively charged particles ($-1$) orbiting the nucleus within electron shells.

• Elements, Atomic Number, and Atomic Mass:

  • Element: A pure substance consisting entirely of one type of atom that cannot be broken down into simpler substances by ordinary chemical means.

  • Atomic Number: The number of protons contained within the nucleus of an atom, defining its elemental identity.

  • Atomic Mass: The total combined mass of protons and neutrons within an atom's nucleus.

  • Isotope: Atoms of the same element that share the same atomic number (protons) but differ in atomic mass due to varying numbers of neutrons.

• Electron Shell Capacities and Valence Electron Calculations:

  • Innermost Electron Shell: Holds a maximum capacity of $2$ electrons.

  • Outer Electron Shells: Typically hold a maximum capacity of $8$ electrons to achieve a complete octet.

  • Valence Electron Determination: Determined by subtracting full inner shells from the atomic number. For instance, an atom with atomic number $11$ has $2$ electrons in shell $1$, $8$ in shell $2$, leaving $1$ electron in its valence shell.

  • Inert Atoms: An atom is chemically inert (unreactive) when its outermost (valence) electron shell is completely full.

• Molecules, Electronegativity, and Chemical Bonds:

  • Molecule: Two or more atoms held together by chemical bonds.

  • Electronegativity: An atom's inherent capacity to attract shared electrons toward itself within a chemical bond. Differences in electronegativity determine bond polarity.

  • Single Covalent Bond: Formed when atoms share exactly $1$ pair of electrons ($2$ total electrons).

• Definitions of Specific Chemical Bonds:

  • Nonpolar Covalent Bond: Formed between two atoms with equal or nearly equal electronegativity; electrons are shared equally between the nuclei.

  • Polar Covalent Bond: Formed between atoms with significantly different electronegativities; electrons are shared unequally, resulting in partial positive ($\delta+$) and partial negative ($\delta-$) charges.

  • Ionic Bond: Formed when an atom with high electronegativity completely steals one or more electrons from an atom with low electronegativity, generating oppositely charged ions that attract each other.

  • Hydrogen Bond: A weak electrostatic attraction between a hydrogen atom covalently bound to a highly electronegative atom (like $O$ or $N$) and another nearby electronegative atom.

• Valence Electrons, Cations, and Anions:

  • Valence Electron: An electron occupying the outermost shell of an atom that participates in chemical bonding.

  • Cation: A positively charged ion created when an atom loses one or more valence electrons.

  • Anion: A negatively charged ion created when an atom gains one or more valence electrons.

• Properties of Water:

  • Cohesion: Hydrogen bonding between identical water molecules, allowing them to stick together.

  • Adhesion: Hydrogen bonding between water molecules and other polar surface molecules.

  • Specific Heat: The quantity of heat energy required to raise the temperature of $1\text{ gram}$ of a substance by $1^{\circ}\text{C}$. Water has a remarkably high specific heat capacity because much of the added energy must break hydrogen bonds before kinetic energy and temperature can increase.

• Hydrophobicity, Hydrophilicity, and Solvent Dynamics:

  • Hydrophilic: Polar or charged substances that form hydrogen bonds with water and dissolve readily in aqueous solvents.

  • Hydrophobic: Nonpolar, uncharged molecules that cannot form hydrogen bonds with water; they coalesce and separate from polar solvents.

• Organic Molecules and Carbon Bonding Capacity:

  • Organic Molecules: Must contain both Carbon ($C$) and Hydrogen ($H$) atoms.

  • Covalent Bonding Capacity: Directly mirrors the number of unpaired electrons required to fill its valence shell. Carbon has $4$ valence electrons; thus, carbon can form a maximum of $4$ covalent bonds, and must form at least $4$ single bonds (or equivalent double/triple bonds) to complete its octet.

Lipid Biochemistry

• Structural Effects of Saturation, Unsaturation, and Dehydrogenation:

  • Possess no double bonds in their hydrocarbon chains; saturated with hydrogen atoms. Chains are straight, packing tightly together to form solids at room temperature.

  • Saturated Fatty Acids:

  • Unsaturated Fatty Acids: Contain one or more double bonds ($C=C$), introducing double-bond kinks that prevent dense packing, making them liquids (oils) at room temperature.

  • Dehydrogenation: The chemical removal of hydrogen atoms from a lipid, which introduces double bonds (converting a saturated lipid into an unsaturated structure) and alters its physical state at room temperature.

• Steroid Structure and Functions:

  • Structure: Characterized by a carbon skeleton composed of four fused rings.

  • Functions: Regulates cell membrane fluidity (cholesterol) and acts as signaling hormones (e.g., estrogen, testosterone).

• Fat Description versus Phospholipid Structure:

  • Fat (Triglyceride): Composed of one single glycerol molecule ester-bonded to three fatty acid tails; used primarily for energy storage.

  • Phospholipid: Composed of one glycerol molecule attached to two fatty acid tails and one negatively charged phosphate group. Unlike neutral fats, phospholipids are amphipathic.

• Amphipathic Definition:

  • Describing a molecule that possesses both a hydrophilic (polar) region and a hydrophobic (nonpolar) region.

• Membrane Fluidity Regulation by Cholesterol and Temperature:

  • Temperature Impact: High temperatures increase thermal motion, making membranes overly fluid; low temperatures cause lipids to freeze and pack tightly, decreasing fluidity.

  • Role of Cholesterol: Acts as a bidirectional temperature buffer. At high temperatures, cholesterol restricts excessive phospholipid movement to maintain stability. At low temperatures, it disrupts tight phospholipid packing, preventing the membrane from solidifying.

Biological Membrane Dynamics and Transport Mechanisms

• Functions of Biological Membranes:

  • Define cellular boundaries and compartmentalize intracellular organelles.

  • Regulate the selective movement of solute molecules into and out of the cell.

  • Facilitate signal transduction via cell surface receptors.

  • Provide structural attachment for cytoskeletal fibers and extracellular matrix components.

• Phospholipid Bilayer Arrangement and Permeability:

  • Bilayer Arrangement: Hydrophobic fatty acid tails orient inward toward each other away from water, while hydrophilic phosphate heads extend outward toward aqueous intracellular and extracellular environments.

  • Highly Permeable Molecules: Small, nonpolar, uncharged molecules (e.g., $O_2$, $CO_2$, nonpolar steroids).

  • Less Permeable Molecules: Large polar molecules (e.g., glucose) and charged ions (e.g., $\text{Na}^+$, $\text{K}^+$, $\text{Cl}^-$).

• Impact of Lipid Structure on Fluidity and Permeability:

  • Membranes rich in unsaturated fatty acids with kinked tails maintain higher fluidity and increased permeability.

  • Membranes rich in saturated fatty acids pack closely, decreasing fluidity and lowering permeability.

• Fluid Mosaic Model and Membrane Proteins:

  • Fluid Mosaic Model: The membrane is a dynamic fluid structure composed of a mosaic of phospholipids, cholesterol, and proteins moving laterally within the layer.

  • Integral Membrane Proteins: Penetrate into or span the hydrophobic core of the lipid bilayer (transmembrane proteins).

  • Peripheral Membrane Proteins: Bound non-covalently to the surface of the bilayer or attached to exposed regions of integral proteins.

• Bilayer Dynamics (Lateral Movement vs. Flip-Flop):

  • Lateral Movement: Phospholipids swap positions with neighbors within the same leaflet constantly and rapidly (millions of times per second).

  • Flip-Flop Movement: Transverse movement of a phospholipid from one membrane leaflet to the opposite leaflet occurs very rarely (once per month) without enzymatic assistance (flippases) because it requires pulling a polar head through the nonpolar core.

• Diffusion, Osmosis, and Solutions Terminology:

  • Diffusion: Net passive movement of solutes down a concentration gradient from an area of high concentration to an area of low concentration.

  • Osmosis: Net passive diffusion of free water molecules across a selectively permeable membrane toward a higher solute concentration.

  • Solute: The substance dissolved in a liquid.

  • Solvent: The dissolving agent (liquid) in a solution (e.g., water).

  • Concentration: The mass or moles of solute dissolved per unit volume of solvent.

  • Gradient: A physical difference in solute concentration between two adjacent regions.

• Tonicity Relationships:

  • Hypertonic Solution: Possesses a higher solute concentration relative to another solution.

  • Hypotonic Solution: Possesses a lower solute concentration relative to another solution.

  • Water inherently moves via osmosis from a hypotonic environment across a membrane into a hypertonic environment.

• Passive versus Active Transport:

  • Passive Transport: Includes simple diffusion, osmosis, and facilitated diffusion (via channels/carriers). Driven entirely by natural kinetic energy down a concentration gradient ($\text{high} \rightarrow \text{low}$); requires zero cellular ATP energy input.

  • Active Transport: Includes primary active transport, secondary active transport, and vesicular transport (endocytosis/exocytosis). Moves solutes against their concentration gradient ($\text{low} \rightarrow \text{high}$); requires direct consumption of metabolic energy (ATP).

• Exam Strategy:

  • Approximately $25\%$ of the examination consists of applied "thought" questions. Success depends on understanding theoretical underlying mechanisms rather than simple rote fact memorization.

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Molecules, Electronegativity, and Chemical Bonds:

• Molecule: Two or more atoms held together by chemical bonds.

• Electronegativity: An atom's inherent capacity to attract shared electrons toward itself within a chemical bond. Differences in electronegativity determine bond polarity.

• Single Covalent Bond: Formed when atoms share exactly 11 pair of electrons (22 total electrons).

Definitions of Specific Chemical Bonds:

• Nonpolar Covalent Bond: Formed between two atoms with equal or nearly equal electronegativity; electrons are shared equally between the nuclei.

• Polar Covalent Bond: Formed between atoms with significantly different electronegativities; electrons are shared unequally, resulting in partial positive (δ+δ+) and partial negative (δ−δ−) charges.

• Ionic Bond: Formed when an atom with high electronegativity completely steals one or more electrons from an atom with low electronegativity, generating oppositely charged ions that attract each other.

• Hydrogen Bond: A weak electrostatic attraction between a hydrogen atom covalently bound to a highly electronegative atom (like OO or NN) and another nearby electronegative atom.

Valence Electrons, Cations, and Anions:

• Valence Electron: An electron occupying the outermost shell of an atom that participates in chemical bonding.

• Cation: A positively charged ion created when an atom loses one or more valence electrons.

• Anion: A negatively charged ion created when an atom gains one or more valence electrons.