Cell Structure, Membrane Transport, Genetics, and Tissue Organization: Study Notes for Anatomy & Physiology

Cell Structure and Plasma Membrane

Plasma Membrane Structure and Components

The plasma membrane is a selectively permeable barrier that surrounds the cell, maintaining the internal environment and mediating communication with the external environment.

• Lipid bilayer: Composed mainly of phospholipids, with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Integral and peripheral proteins serve as channels, receptors, and enzymes.

• Carbohydrates: Attached to proteins and lipids, forming the glycocalyx for cell recognition.

Glycocalyx Composition

• The glycocalyx is a carbohydrate-rich coating on the cell surface, important for cell recognition, adhesion, and protection.

• Composed of glycoproteins and glycolipids.

Fluid Mosaic Model Principles

• The fluid mosaic model describes the plasma membrane as a dynamic structure with proteins floating in or on a fluid lipid bilayer.

• Allows lateral movement of components, contributing to membrane flexibility and function.

Cell Junctions and Membrane Transport

Types of Cell Junctions

• Tight junctions: Seal adjacent cells to prevent leakage of molecules.

• Desmosomes: Provide mechanical strength by anchoring cells together.

• Gap junctions: Allow direct communication between cells via connexons.

Substances That Can Diffuse Directly Through the Plasma Membrane

• Small, nonpolar molecules (e.g., O2, CO2) and lipid-soluble substances can diffuse directly.

• Water can diffuse via osmosis, often through aquaporins.

Types and Characteristics of Passive Transport

• Simple diffusion: Movement of molecules from high to low concentration without energy input.

• Facilitated diffusion: Uses membrane proteins to transport substances down their concentration gradient.

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

Effects of Hypertonic, Hypotonic, and Isotonic Solutions on Cells

• Hypertonic: Higher solute concentration outside; cell shrinks (crenation).

• Hypotonic: Lower solute concentration outside; cell swells and may burst (lysis).

• Isotonic: Equal solute concentration; no net water movement.

Endocytosis Types and Mechanisms

• Phagocytosis: "Cell eating"; engulfment of large particles.

• Pinocytosis: "Cell drinking"; uptake of extracellular fluid.

• Receptor-mediated endocytosis: Specific uptake via receptor binding.

Sodium-Potassium Pump (Transport Type)

• The sodium-potassium pump is an example of primary active transport, moving 3 Na+ out and 2 K+ into the cell against their gradients using ATP.

• Maintains membrane potential and cell volume.

Secondary Active Transport Mechanism

• Uses the energy from the gradient created by primary active transport (e.g., Na+ gradient) to move other substances against their gradient.

• Examples: Symport (same direction), antiport (opposite direction).

Cell Signaling and Communication

Chemical Signaling, G Protein-Linked Receptor Signaling

• Chemical signaling: Cells communicate using chemical messengers (hormones, neurotransmitters).

• G protein-coupled receptors (GPCRs): Activate intracellular G proteins, triggering second messenger cascades (e.g., cAMP).

Genetics and Molecular Biology

Organization of DNA in the Nucleus

• DNA is packaged into chromatin, which condenses to form chromosomes during cell division.

• Chromatin consists of DNA wrapped around histone proteins.

Functions of Types of RNA in Protein Synthesis

• mRNA (messenger RNA): Carries genetic code from DNA to ribosomes.

• tRNA (transfer RNA): Brings amino acids to ribosomes during translation.

• rRNA (ribosomal RNA): Structural and enzymatic component of ribosomes.

Definition of Semiconservative DNA Replication

• Each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

Lysosome Functions

• Lysosomes contain digestive enzymes to break down waste, cellular debris, and foreign material.

Definition of a Gene

• A gene is a segment of DNA that encodes instructions for making a specific protein or RNA molecule.

Enzymes in DNA Replication

• Helicase: Unwinds the DNA double helix.

• DNA polymerase: Synthesizes new DNA strands.

• Ligase: Joins Okazaki fragments on the lagging strand.

Transcription vs. Translation

• Transcription: Synthesis of mRNA from a DNA template in the nucleus.

• Translation: Synthesis of a polypeptide at the ribosome using mRNA as a template.

Base Pairing Rules (mRNA Codon and tRNA Anticodon)

• DNA: A-T, C-G; RNA: A-U, C-G.

• Codon (mRNA) pairs with complementary anticodon (tRNA) during translation.

Genetic Code Redundancy

• Multiple codons can code for the same amino acid, providing redundancy and reducing the impact of mutations.

mRNA Processing

• Includes addition of 5' cap, poly-A tail, and splicing to remove introns.

Signal Recognition Particle (SRP) Role

• SRP directs ribosome-mRNA complexes to the rough ER for synthesis of membrane-bound or secretory proteins.

Genetic Theory of Aging

• Suggests that aging is programmed by genes, possibly involving telomere shortening and accumulation of mutations.

Endocrine System Overview

Endocrine vs. Exocrine Glands

• Endocrine glands: Secrete hormones directly into the bloodstream (e.g., thyroid, pituitary).

• Exocrine glands: Secrete products via ducts to body surfaces or cavities (e.g., sweat, salivary glands).

Tissues and Membranes

Epithelial Tissue Classifications

• Classified by cell shape (squamous, cuboidal, columnar) and layers (simple, stratified).

• Functions include protection, absorption, secretion, and filtration.

Cell Types in Each Connective Tissue

• Fibroblasts: Produce fibers and ground substance.

• Adipocytes: Store fat.

• Chondrocytes: Found in cartilage.

• Osteocytes: Found in bone.

• Blood cells: Found in blood tissue.

Connective Tissue Extracellular Matrix Composition

• Composed of protein fibers (collagen, elastic, reticular) and ground substance (proteoglycans, glycoproteins, water).

Connective Tissue Origin

• All connective tissues arise from embryonic mesenchyme.

Connective Tissue Fiber Types

• Collagen fibers: Provide strength.

• Elastic fibers: Provide elasticity.

• Reticular fibers: Form supportive networks.

Epithelial Membranes

• Composed of epithelial tissue and underlying connective tissue.

• Types: mucous, serous, and cutaneous membranes.

Tissue Repair Stages and Inflammation

• Inflammation: Initial response to injury; brings immune cells and nutrients.

• Organization: Granulation tissue forms, restoring blood supply.

• Regeneration/Fibrosis: Replacement of damaged tissue with original tissue or scar tissue.

Germ Layer Origins of Tissues

• Ectoderm: Forms nervous tissue and epidermis.

• Mesoderm: Forms muscle, connective tissue, and some epithelial tissues.

• Endoderm: Forms lining of digestive and respiratory tracts.

Summary Table: Types of Cell Junctions

Junction Type	Structure	Function	Location Example
Tight Junction	Interlocking proteins	Prevents passage of substances	Intestinal lining
Desmosome	Protein plaques and filaments	Provides mechanical strength	Skin epidermis
Gap Junction	Connexon channels	Allows cell communication	Cardiac muscle

Key Equations

• Osmosis: $J = -P (C_2 - C_1)$ Where $J$ is the flux, $P$ is permeability, and $C_2 - C_1$ is the concentration gradient.

• Nernst Equation (membrane potential): $E = \frac{RT}{zF} \ln \left( \frac{[\text{ion outside}]}{[\text{ion inside}]} \right)$