Chapter 4: A Tour of the Cell

4.1 Biologists Use Microscopes & Biochemistry to Study Cells

Microscopy has been essential for understanding cell structure and function. Different types of microscopes provide varying levels of magnification and resolution, allowing scientists to study cells and their components.

• Robert Hooke: First to observe cells and cell walls.

• Anton van Leeuwenhoek: Developed advanced microscopes; observed 'animalcules' (microorganisms).

• Light Microscope (LM): Uses visible light to magnify specimens up to ~1,000x; limited in resolving subcellular structures.

• Key Parameters of Microscopy:

  • Magnification: Ratio of image size to actual size.

  • Resolution: Clarity of image; minimum distance between two distinguishable points.

  • Contrast: Difference in brightness between specimen and background.

• Electron Microscopes:

  • Scanning Electron Microscope (SEM): Produces 3D images of specimen surfaces.

  • Transmission Electron Microscope (TEM): Passes electrons through specimen to study internal structures; highest resolution.

Example: TEMs are used to visualize organelles like mitochondria and chloroplasts, which are too small for light microscopes.

4.2 Eukaryotic Cells Have Internal Membranes That Compartmentalize Their Functions

Cells are classified as prokaryotic or eukaryotic based on structural differences. Internal membranes in eukaryotes allow for compartmentalization of cellular processes.

• Prokaryotic Cells: Bacteria and Archaea; lack nucleus and membrane-bound organelles; DNA in nucleoid region; generally smaller.

• Eukaryotic Cells: Protists, fungi, animals, plants; DNA in nucleus; contain membrane-bound organelles; generally larger.

• Common Features of All Cells:

  • Plasma membrane (selective barrier)

  • Cytosol (semi-fluid matrix)

  • Chromosomes (genetic material)

  • Ribosomes (protein synthesis)

• Plasma Membrane Structure: Phospholipid bilayer; regulates passage of oxygen, nutrients, and waste.

• Surface Area-to-Volume Ratio: Smaller cells have higher ratios, facilitating efficient diffusion.

Example: Bacterial cells are typically 1–5 μm, while eukaryotic cells are 10–100 μm in diameter.

4.3 The Eukaryotic Cell’s Genetic Instructions Are Housed in the Nucleus & Carried Out by Ribosomes

The nucleus stores genetic information, while ribosomes translate this information into proteins.

• Nucleus: Contains DNA organized into chromosomes; surrounded by a double-membrane nuclear envelope with pores for molecular exchange.

• Chromatin: DNA and associated histone proteins; condenses to form chromosomes during cell division.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

• Ribosomes: Composed of rRNA and proteins; sites of protein synthesis; found free in cytosol or bound to endoplasmic reticulum (ER).

Example: Free ribosomes synthesize cytosolic proteins; bound ribosomes synthesize proteins for membranes or secretion.

4.4 The Endomembrane System Regulates Protein Traffic & Performs Metabolic Functions

The endomembrane system is a network of membranes involved in protein and lipid synthesis, transport, and detoxification.

• Components: Nuclear envelope, plasma membrane, ER, Golgi apparatus, lysosomes, vacuoles.

• Connections: Physical continuity or vesicle transfer.

• Functions:

  • Protein synthesis and transport

  • Lipid metabolism and movement

  • Detoxification of harmful substances

• Endoplasmic Reticulum (ER):

  • Smooth ER: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/poisons, stores Ca2+.

  • Rough ER: Studded with ribosomes; synthesizes and secretes proteins, distributes transport vesicles, membrane production.

• Golgi Apparatus: Modifies, sorts, and packages ER products; manufactures some macromolecules; consists of cisternae (flattened sacs).

• Lysosomes: Contain hydrolytic enzymes for digesting macromolecules; involved in phagocytosis and autophagy.

• Vacuoles:

  • Food vacuoles (formed by phagocytosis)

  • Contractile vacuoles (pump excess water out)

  • Central vacuole (in plants; stores water and ions)

Example: Liver cells have abundant smooth ER for detoxification; plant cells have large central vacuoles for storage.

4.5 Mitochondria & Chloroplasts Change Energy from One Form to Another

Mitochondria and chloroplasts are energy-converting organelles with evolutionary origins linked to prokaryotes.

• Mitochondria: Sites of cellular respiration; convert oxygen and nutrients into ATP; found in all eukaryotes.

• Chloroplasts: Sites of photosynthesis; convert light energy and CO2 into organic compounds; found in plants and algae.

• Endosymbiont Theory: Mitochondria and chloroplasts originated from engulfed prokaryotes; evidence includes double membranes, own DNA, ribosomes, and independent replication.

• Mitochondrial Structure:

  • Smooth outer membrane

  • Inner membrane with folds (cristae) creating intermembrane space and mitochondrial matrix

• Chloroplast Structure:

  • Thylakoids (membranous sacs) stacked into grana

  • Stroma (internal fluid with DNA, ribosomes, enzymes)

• Peroxisomes: Single-membrane organelles; carry out oxidation reactions, detoxify molecules, convert H2O2 to water and oxygen via catalase.

Example: Muscle cells have many mitochondria due to high energy demand.

4.6 The Cytoskeleton is a Network of Fibers that Organize Structures in the Cell

The cytoskeleton provides structural support, organizes cell components, and facilitates movement.

• Microtubules: Hollow rods of tubulin; maintain cell shape, guide organelle movement, separate chromosomes during division; form cilia and flagella.

• Microfilaments: Thin rods of actin; bear tension, maintain cell shape, involved in muscle contraction and cell motility.

• Intermediate Filaments: Medium-diameter fibers; reinforce cell shape, anchor organelles; more permanent structures.

Example: Microvilli in intestinal cells are supported by microfilaments.

4.7 Extracellular Components and Connections Between Cells Help Coordinate Cellular Activities

Cells produce extracellular structures and form junctions to communicate and adhere to one another.

• Plant Cell Walls: Composed of cellulose, polysaccharides, and proteins; provide protection, shape, and prevent excessive water uptake.

• Extracellular Matrix (ECM) in Animals: Made of glycoproteins (collagen, proteoglycans, fibronectin); binds cells via integrins; functions in support, adhesion, movement, and signaling.

• Cell Junctions:

  • Plasmodesmata (plants): Channels for transport of water, solutes, proteins, and RNA between cells.

  • Tight Junctions: Seal cells together; prevent leakage (e.g., intestines, bladder).

  • Desmosomes: Anchor cells together; provide mechanical stability (e.g., muscle, skin).

  • Gap Junctions: Channels for communication and transport between animal cells (e.g., heart tissue).

Example: Gap junctions allow ions to pass rapidly between heart muscle cells, coordinating contraction.

Chapter 5: Membrane Transport and Cell Signaling

5.1 Cellular Membranes Are Fluid Mosaics of Lipids and Proteins

The plasma membrane is a dynamic structure composed of lipids, proteins, and carbohydrates, forming a selective barrier between the cell and its environment.

• Fluid Mosaic Model: Membrane is a mosaic of proteins floating in a fluid phospholipid bilayer.

• Amphipathic Molecules: Phospholipids have hydrophilic heads and hydrophobic tails.

• Membrane Fluidity:

  • Lipids and some proteins move laterally; fluidity affected by temperature and lipid composition.

  • Unsaturated fatty acids increase fluidity; saturated fatty acids decrease fluidity.

  • Cholesterol modulates fluidity: more cholesterol stiffens membrane at high temperatures, prevents solidification at low temperatures.

• Membrane Proteins:

  • Integral Proteins: Penetrate hydrophobic core; many are transmembrane.

  • Peripheral Proteins: Loosely bound to membrane surface.

  • Functions: Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to cytoskeleton/ECM.

• Membrane Carbohydrates: Glycolipids and glycoproteins involved in cell-cell recognition; carbohydrate composition varies among species and cell types.

Example: Blood type antigens are determined by specific glycoproteins on red blood cell membranes.

5.2 Membrane Structure Results in Selective Permeability

The plasma membrane allows selective passage of substances, maintaining the internal environment of the cell.

• Easily Cross: Small, nonpolar, hydrophobic molecules (e.g., O2, CO2, some ions).

• Require Assistance: Large, polar, hydrophilic molecules (e.g., glucose, water) need transport proteins.

• Transport Proteins:

  • Channel Proteins: Provide hydrophilic tunnels (e.g., aquaporins for water).

  • Carrier Proteins: Bind and change shape to move molecules (e.g., glucose transporter).

Example: Aquaporins facilitate rapid water transport in kidney cells.

5.3 Passive Transport Is Diffusion of a Substance Across a Membrane with No Energy Investment

Passive transport involves the movement of substances down their concentration gradients without energy input.

• Diffusion: Movement from high to low concentration; results in dynamic equilibrium.

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

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

  • Hypotonic: Lower solute outside; cell gains water (lysis in animal cells).

  • Isotonic: Equal solute; no net water movement.

  • Hypertonic: Higher solute outside; cell loses water (shrivels).

• Plant Cells: Cell wall prevents lysis; turgid in hypotonic, flaccid in isotonic, plasmolysis in hypertonic solutions.

• Facilitated Diffusion: Transport proteins aid passive movement of molecules (e.g., aquaporins, ion channels, carrier proteins).

Example: Glucose enters red blood cells via facilitated diffusion through a carrier protein.

5.4 Active Transport Uses Energy to Move Solutes Against Their Gradient

Active transport moves substances against their concentration gradients, requiring energy (usually ATP).

• Sodium-Potassium Pump: Exchanges Na+ out and K+ in; maintains electrochemical gradients in animal cells.

• Membrane Potential: Voltage across membrane; inside of cell is typically negative.

• Electrochemical Gradient: Combination of concentration and electrical gradients driving ion movement.

• Electrogenic Pumps: Generate membrane potential (e.g., sodium-potassium pump in animals, proton pump in plants/fungi/bacteria).

• Cotransport: Coupled transport of two solutes; one moves down its gradient, driving the other against its gradient (e.g., sucrose-H+ cotransport in plants).

Example: The sodium-glucose cotransporter in intestinal cells uses the Na+ gradient to import glucose.

5.5 Bulk Transport Across Plasma Membrane Occurs by Exocytosis and Endocytosis

Large molecules cross membranes in bulk via vesicles, requiring energy (active transport).

• Exocytosis: Vesicles fuse with plasma membrane to release contents outside the cell (e.g., neurotransmitter release).

• Endocytosis: Cell takes in materials by forming vesicles from plasma membrane.

  • Phagocytosis: "Cellular eating"; cell engulfs particles (e.g., white blood cells ingesting bacteria).

  • Pinocytosis: "Cellular drinking"; cell engulfs extracellular fluid.

  • Receptor-Mediated Endocytosis: Specific molecules bind to receptors, triggering vesicle formation.

Example: Cholesterol uptake by cells occurs via receptor-mediated endocytosis.

5.6 The Plasma Membrane Plays a Key Role in Most Cell Signaling

Cell signaling allows cells to communicate and coordinate activities, often involving the plasma membrane and specific receptors.

• Types of Signaling:

  • Local Signaling: Paracrine (short-distance chemical signals), synaptic (neurotransmitters between nerve and target cells).

  • Long-Distance Signaling: Endocrine (hormones travel via circulatory system).

• Reception: Signal molecule (ligand) binds to receptor protein, causing a conformational change and activation.

• Membrane Receptors:

  • Most are for water-soluble signals; span the membrane.

  • Ligand-Gated Ion Channels: Open or close in response to ligand binding, allowing ion flow (important in nervous system).

Example: Acetylcholine binding to its receptor opens ion channels, triggering muscle contraction.

Table: Comparison of Prokaryotic and Eukaryotic Cells

Table: Types of Cell Junctions

Key Equations

• Surface Area to Volume Ratio:

• Osmosis (Water Potential): Where is water potential, is solute potential, and is pressure potential.

• Membrane Potential (Nernst Equation): Where is equilibrium potential, is gas constant, is temperature, is ion charge, is Faraday's constant.

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard biology textbooks.