Study Guide: The Cell and Membrane Structure (Chapters 6 & 7)

The Cell: Structure and Function (Chapter 6)

Nucleus and Ribosomes

The nucleus and ribosomes are essential organelles involved in gene expression, forming the core of the central dogma of molecular biology.

• Nucleus: Contains most of the cell's genetic material (DNA) and is the site of transcription (DNA to RNA).

• Ribosomes: Sites of protein synthesis (translation of mRNA to polypeptide chains).

• Location: The nucleus is typically centrally located in eukaryotic cells, surrounded by the nuclear envelope. Ribosomes can be free in the cytosol or bound to the rough endoplasmic reticulum (ER).

• Nuclear Envelope: A double membrane structure with nuclear pores that regulate the passage of molecules between the nucleus and cytoplasm.

• DNA Organization: DNA is packaged as chromatin (DNA + proteins) and further condensed into chromosomes during cell division.

• Nucleolus: A dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

• Ribosome Structure: Composed of rRNA and proteins, consisting of large and small subunits.

Example: In eukaryotic cells, the nucleus transcribes DNA into mRNA, which exits through nuclear pores to ribosomes for translation into proteins.

Endomembrane System

The endomembrane system is a network of membranes within eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Components: Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and vesicles.

• Rough ER: Studded with ribosomes; synthesizes and modifies proteins.

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

• Golgi Apparatus: Stacks of flattened membranes; modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Protein Transport: Proteins are synthesized in the rough ER, transported in vesicles to the Golgi apparatus for further modification, and then sent to their final destinations.

• Lysosomes: Membrane-bound organelles containing hydrolytic enzymes for intracellular digestion.

• Vacuoles: Diverse functions; food vacuoles (digestion), contractile vacuoles (osmoregulation), central vacuole in plants (storage and structural support).

Example: Secretory proteins are synthesized in the rough ER, processed in the Golgi, and exported via vesicles.

Mitochondria, Chloroplasts, and Peroxisomes

These organelles are involved in energy conversion and metabolism.

• Mitochondria: Sites of cellular respiration; convert glucose and oxygen into ATP.

• Chloroplasts: Found in plants and algae; sites of photosynthesis, converting solar energy into chemical energy (glucose).

• Peroxisomes: Contain enzymes that break down fatty acids and detoxify harmful substances; produce hydrogen peroxide as a byproduct.

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

Comparison Table:

Organelle	Main Function	Key Features
Mitochondria	Cellular respiration (ATP production)	Double membrane, own DNA, cristae
Chloroplasts	Photosynthesis	Double membrane, own DNA, thylakoids
Peroxisomes	Breakdown of fatty acids, detoxification	Single membrane, contains catalase

Cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, facilitates cell movement, and organizes organelles.

• Microtubules: Hollow tubes made of tubulin; maintain cell shape, facilitate chromosome movement during cell division, and serve as tracks for organelle movement.

• Microfilaments (Actin Filaments): Thin strands of actin; involved in cell shape, muscle contraction, and cytoplasmic streaming.

• Intermediate Filaments: Rope-like fibers; provide mechanical strength and anchor organelles.

• Motor Proteins: (e.g., myosin, kinesin, dynein) interact with cytoskeletal elements to produce movement.

• Organelles for Movement: Flagella and cilia are extensions of the plasma membrane supported by microtubules, enabling cell motility.

Example: Muscle contraction is driven by the interaction of actin filaments and myosin motor proteins.

Extracellular Components

Cells are supported and connected by structures outside the plasma membrane.

• Cell Wall: Rigid structure in plants, fungi, and some protists; provides protection and support.

• Extracellular Matrix (ECM): In animal cells, a network of glycoproteins (e.g., collagen) that provides structural support and mediates cell signaling.

• Cell Junctions: Structures that connect adjacent cells, including tight junctions (prevent leakage), desmosomes (anchor cells), and gap junctions (allow communication).

Key Terminology

• Nucleus: Organelle containing genetic material.

• Nuclear Lamina: Protein network supporting the nuclear envelope.

• Chromosomes: Condensed DNA structures during cell division.

• Chromatin: DNA-protein complex in the nucleus.

• Nucleolus: Site of rRNA synthesis and ribosome assembly.

• Ribosome: Protein synthesis machinery.

• Endomembrane System: Interconnected organelles for protein and lipid processing.

• Vesicles: Membrane-bound sacs for transport.

• Cytoplasmic Streaming: Movement of cytoplasm within cells.

• Pseudopodia: Temporary projections for movement (e.g., in amoebas).

• Myosin: Motor protein interacting with actin.

• Centrosome: Microtubule organizing center.

• Centriole: Cylindrical structure in centrosomes.

• Flagella: Long, whip-like structures for movement.

• Cilia: Short, hair-like structures for movement.

The Cell Membrane (Chapter 7)

Regulation of Membrane Traffic

The plasma membrane controls the movement of substances into and out of the cell, maintaining homeostasis.

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

• Transport Proteins: Facilitate the movement of ions and molecules across the membrane.

Components of the Plasma Membrane

The plasma membrane is a dynamic structure composed of lipids, proteins, and carbohydrates.

• Phospholipid Bilayer: Forms the fundamental structure, with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Integral (span the membrane) and peripheral (attached to the surface); involved in transport, signaling, and cell recognition.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids); play roles in cell-cell recognition and signaling.

• Fluidity: Influenced by temperature, fatty acid composition (saturated vs. unsaturated), and cholesterol content.

• Importance of Fluidity: Essential for membrane function, protein mobility, and cell signaling.

Types of Membrane Proteins:

• Transport Proteins: Channels and carriers for specific molecules.

• Enzymatic Proteins: Catalyze reactions at the membrane surface.

• Receptor Proteins: Receive and transmit signals.

• Cell Recognition Proteins: Identify cells to the immune system.

• Intercellular Joining Proteins: Connect adjacent cells.

• Attachment Proteins: Anchor the membrane to the cytoskeleton or ECM.

Types of Carbohydrates: Oligosaccharides attached to proteins (glycoproteins) or lipids (glycolipids).

Selective Permeability and the Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a fluid combination of lipids, proteins, and carbohydrates.

• Selective Permeability: Small, nonpolar molecules (e.g., O2, CO2) cross easily; ions and large polar molecules require transport proteins.

• High Permeability: Gases, small nonpolar molecules.

• Low Permeability: Ions, large polar molecules.

• Transport Proteins: Channel proteins (form pores) and carrier proteins (change shape to move substances).

Example: Aquaporins are channel proteins that facilitate water transport across the membrane.

Summary Table: Types of Transport Proteins

Type	Function	Example
Channel Protein	Forms hydrophilic channel for specific molecules	Aquaporin
Carrier Protein	Changes shape to shuttle molecules	Glucose transporter
Pump	Uses energy (ATP) to move substances against gradient	Sodium-potassium pump

Key Equations

• Osmosis: Movement of water across a semipermeable membrane from low to high solute concentration.

• Diffusion Rate (Fick's Law):

$J = -D \frac{dC}{dx}$

• Where J is the rate of diffusion, D is the diffusion coefficient, and \frac{dC}{dx} is the concentration gradient.

Additional info: For a comprehensive understanding, students should review diagrams of cell organelles, the endomembrane system, and the fluid mosaic model, as well as practice identifying organelles and membrane components in micrographs.