Chapter 6: A Tour of the Cell

6.2 Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells lack a nucleus and membrane-bound organelles. Their DNA is located in a region called the nucleoid. Examples: Bacteria and Archaea.

• Eukaryotic cells have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.

• Comparison Table:

• Plant vs. Animal Cells: Plant cells have cell walls, chloroplasts, and a central vacuole; animal cells do not.

Surface Area-to-Volume Ratio

• The surface area-to-volume ratio (SA:V) determines how efficiently a cell exchanges materials with its environment.

• As a cell grows, its volume increases faster than its surface area, reducing efficiency of exchange.

• Formula for a sphere:

• Smaller cells have a higher SA:V ratio, allowing more efficient exchange of materials.

• Effect on Organisms: High SA:V ratios are crucial for cells and small organisms to efficiently exchange gases, nutrients, and waste.

Adaptations Affecting Surface Area-to-Volume Ratio

• Physical adaptations: Microvilli in intestinal cells increase surface area for absorption.

• Behavioral adaptations: Animals may spread out or curl up to regulate heat exchange with the environment.

• Thermoregulation: Endotherms (warm-blooded animals) and ectotherms (cold-blooded animals) face different challenges in maintaining body temperature due to their SA:V ratios.

Membranes and Compartmentalization

• Membrane-bound organelles in eukaryotic cells create compartments that minimize competing molecular interactions and increase surface area for reactions (e.g., cristae in mitochondria).

6.3-6.5: Structure and Function of Organelles

• Nucleus: Contains genetic material (DNA); surrounded by a double membrane (nuclear envelope) with pores for transport.

• Ribosomes: Sites of protein synthesis; can be free in cytosol or bound to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies drugs, stores calcium ions.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport.

• Lysosomes: Contain hydrolytic enzymes for digestion of macromolecules and recycling of cell components.

• Vacuoles: Storage organelles; central vacuole in plants maintains turgor pressure.

• Peroxisomes: Break down fatty acids and detoxify harmful substances; produce hydrogen peroxide as a byproduct.

Endosymbiosis Theory

• Proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence:

  • Both have double membranes.

  • Contain their own circular DNA and ribosomes.

  • Replicate independently by binary fission.

• Sequence of Engulfments: Mitochondria likely engulfed first, followed by chloroplasts in plant ancestors.

Mitochondria and Chloroplast Structure

• Mitochondria:

  • Outer membrane

  • Inner membrane (folded into cristae)

  • Intermembrane space

  • Matrix (contains enzymes, DNA, ribosomes)

• Chloroplasts:

  • Outer and inner membranes

  • Thylakoid membrane (forms thylakoids, stacked into grana)

  • Thylakoid space (inside thylakoids)

  • Stroma (fluid outside thylakoids, contains DNA, ribosomes, enzymes)

6.6 Cytoskeleton and Motor Proteins

• Cytoskeleton: Network of protein fibers (microtubules, microfilaments, intermediate filaments) that provides structural support, cell shape, and movement.

• Motor proteins: Use ATP to move vesicles and organelles along cytoskeletal tracks (e.g., kinesin, dynein).

• Example: Movement of vesicles from the Golgi to the plasma membrane.

6.7 Extracellular Matrix (ECM)

• ECM: Network of proteins and carbohydrates outside animal cells; provides structural support, cell signaling, and anchorage.

• Example: Collagen fibers in connective tissue.

Chapter 7: Membrane Structure and Function

7.1 Membrane Structure and Fluidity

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails; form bilayers in water.

• Hydrophobic interactions drive the formation of the bilayer, with hydrophilic heads facing outward and hydrophobic tails inward.

• Fluid Mosaic Model: Membrane is a dynamic structure with proteins and lipids moving laterally within the bilayer.

• Membrane Fluidity:

  • Saturated fats: Pack tightly, decrease fluidity.

  • Unsaturated fats: Kinked tails prevent tight packing, increase fluidity.

  • Cholesterol: Buffers fluidity; reduces movement at high temperatures, prevents solidification at low temperatures.

• Transmembrane proteins: Hydrophobic regions span the bilayer; hydrophilic regions exposed to aqueous environments.

• Other components: Glycolipids and glycoproteins involved in cell recognition and signaling.

7.2 Membrane Function and Selective Permeability

• Membranes maintain distinct internal environments by controlling the passage of substances.

• Selective permeability: Only certain molecules can cross freely (e.g., small nonpolar molecules); others require transport proteins.

• Polarity and Permeability: Nonpolar molecules (e.g., O2, CO2) diffuse easily; polar/charged molecules (e.g., ions, glucose) require transport proteins.

7.3 Transport Across Membranes

• Concentration gradient: Difference in concentration across a membrane; drives passive transport.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Tonicity:

  • Isotonic: No net water movement; cell volume stable.

  • Hypertonic: Water leaves cell; cell shrinks (crenation in animals, plasmolysis in plants).

  • Hypotonic: Water enters cell; cell swells (lysis in animals, turgid in plants).

• Plant vs. Animal Cells: Plant cells prefer hypotonic environments (turgid); animal cells prefer isotonic environments.

• Simple diffusion: Movement of molecules down their concentration gradient without assistance.

• Facilitated diffusion: Movement of molecules down their gradient via transport proteins (e.g., aquaporins for water).

• Water potential (): Predicts direction of water movement; calculated as:

where is solute potential and is pressure potential.

7.4 Passive vs. Active Transport

• Passive transport: No energy required; includes diffusion, facilitated diffusion, and osmosis.

• Active transport: Requires ATP to move substances against their concentration gradient (e.g., sodium-potassium pump).

• Role of ATP: Provides energy for conformational changes in protein pumps.

• Cotransport: Uses the gradient of one substance (often H+ or Na+) to drive the active transport of another substance.

7.5 Bulk Transport

• Endocytosis: Cell engulfs large particles or fluids by forming vesicles from the plasma membrane.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.

• Types of endocytosis: Phagocytosis (solid particles), pinocytosis (fluids), receptor-mediated endocytosis (specific molecules).

Unit 3 Vocabulary

• Organelles: Specialized structures within a cell with specific functions.

• Cytosol: Fluid portion of the cytoplasm.

• Cytoplasm: Contents of the cell between the plasma membrane and the nucleus.

• Plasma membrane: Selectively permeable boundary of the cell.

• Nuclear envelope: Double membrane surrounding the nucleus.

• Chromosomes: DNA molecules with associated proteins.

• Nucleolus: Site of ribosomal RNA synthesis in the nucleus.

• Endomembrane system: Network of membranes involved in synthesis, transport, and metabolism.

• Vesicles: Small membrane-bound sacs for transport within cells.

• Glycoproteins: Proteins with carbohydrate chains attached.

• Phagocytosis: Cellular "eating" of large particles.

• Central vacuole: Large vacuole in plant cells for storage and support.

• Cristae: Folds of the inner mitochondrial membrane.

• Mitochondrial matrix: Space inside the inner membrane of mitochondria.

• Thylakoids: Membranous sacs in chloroplasts where photosynthesis occurs.

• Granum: Stack of thylakoids.

• Stroma: Fluid surrounding thylakoids in chloroplasts.

• Plastids: Family of plant organelles including chloroplasts.

• Centrosome: Microtubule organizing center in animal cells.

• Centrioles: Structures in centrosomes involved in cell division.

• Flagella/Cilia: Structures for cell movement.

• Cell wall: Rigid outer layer in plant, fungal, and some prokaryotic cells.

• Extracellular matrix (ECM): Network outside animal cells for support and signaling.

• Amphipathic: Molecule with both hydrophilic and hydrophobic regions.

• Fluid mosaic model: Describes the dynamic nature of cell membranes.

• Transmembrane proteins: Span the entire membrane.

• Peripheral proteins: Loosely bound to membrane surface.

• Selective permeability: Allows some substances to cross more easily than others.

• Transport proteins: Facilitate movement of substances across membranes.

• Aquaporins: Channel proteins for water transport.

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a membrane.

• Tonicity: Ability of a solution to cause a cell to gain or lose water.

• Osmoregulation: Control of water balance in organisms.

• Turgid: Firmness of plant cells in hypotonic solution.

• Flaccid: Limpness of plant cells in isotonic solution.

• Plasmolysis: Shrinking of plant cell membrane from cell wall in hypertonic solution.

• Facilitated diffusion: Passive transport via proteins.

• Active transport: Movement against gradient using energy.

• Cotransport: Coupled transport of two substances.

• Exocytosis/Endocytosis: Bulk transport mechanisms.

Additional info: Where objectives or vocabulary were brief, academic context and examples were added for clarity and completeness.