Plasma Membrane Structure and Components

Fluid Mosaic Model Principles

The plasma membrane is a dynamic structure composed of lipids, proteins, and carbohydrates. The fluid mosaic model describes the membrane as a flexible bilayer of phospholipids with embedded proteins and other molecules.

• Phospholipid bilayer: Provides barrier and fluidity.

• Integral and peripheral proteins: Facilitate transport, signaling, and structural support.

• Carbohydrate chains: Involved in cell recognition and adhesion.

Example: Glycoproteins on the membrane surface help immune cells recognize pathogens.

Glycocalyx Composition

The glycocalyx is a carbohydrate-rich zone on the cell surface, formed by glycoproteins and glycolipids.

• Protects the cell from mechanical and chemical damage.

• Facilitates cell-cell communication and adhesion.

Types of Cell Junctions

Classification and Functions

Cell junctions are specialized structures that connect adjacent cells, providing structural integrity and communication.

• Tight junctions: Seal adjacent cells, preventing passage of substances.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap junctions: Allow direct communication via channels for ions and small molecules.

Membrane Transport Mechanisms

Substances That Can Diffuse Directly Through the Plasma Membrane

Small, nonpolar molecules (e.g., O2, CO2) and some lipophilic substances can diffuse directly through the lipid bilayer.

Types and Characteristics of Passive Transport

Passive transport moves substances down their concentration gradient without energy input.

• Simple diffusion: Movement of small, nonpolar molecules.

• Facilitated diffusion: Movement via channel or carrier proteins.

• Osmosis: Diffusion of water across a semipermeable membrane.

Effects of Hypertonic, Hypotonic, Isotonic Solutions on Cells

Solution tonicity affects cell volume and function.

• 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

Endocytosis is the process by which cells internalize substances.

• Phagocytosis: Engulfment of large particles or cells.

• Pinocytosis: Uptake of fluid and dissolved solutes.

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

Sodium-Potassium Pump (Transport Type)

The sodium-potassium pump is an example of primary active transport, using ATP to move Na+ out and K+ into the cell.

• Maintains electrochemical gradients essential for nerve and muscle function.

Secondary Active Transport Mechanism

Secondary active transport uses the energy from primary transport gradients to move other substances against their gradient.

• Symport: Both substances move in the same direction.

• Antiport: Substances move in opposite directions.

Cell Signaling

Chemical Signaling, G Protein-Linked Receptor Signaling

Cells communicate via chemical signals that bind to receptors, triggering intracellular responses.

• G protein-coupled receptors (GPCRs): Activate intracellular G proteins, leading to second messenger cascades.

• Example: Epinephrine binding to GPCRs increases cAMP levels.

Genetic Mechanisms

Organization of DNA in the Nucleus

DNA is organized into chromatin within the nucleus, consisting of DNA wrapped around histone proteins.

• Chromosomes: Condensed chromatin during cell division.

Functions of Types of RNA in Protein Synthesis

Three main types of RNA are involved in protein synthesis:

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

• tRNA (transfer RNA): Brings amino acids to the ribosome.

• rRNA (ribosomal RNA): Forms the core of ribosome structure and catalyzes peptide bond formation.

Definition of Semiconservative DNA Replication

Semiconservative replication means each new DNA molecule consists of one old strand and one newly synthesized strand.

Lysosome Functions

Lysosomes are organelles containing digestive enzymes that break down waste materials and cellular debris.

Definition of a Gene

A gene is a segment of DNA that encodes instructions for synthesizing a specific protein or functional RNA.

Enzymes in DNA Replication

• DNA 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 DNA template.

• Translation: Synthesis of protein from mRNA at the ribosome.

Base Pairing Rules (mRNA Codon and tRNA Anticodon)

Base pairing ensures accurate translation:

• mRNA codon: Triplet of bases on mRNA.

• tRNA anticodon: Complementary triplet on tRNA.

Example: mRNA codon AUG pairs with tRNA anticodon UAC.

Genetic Code Redundancy

The genetic code is redundant; multiple codons can code for the same amino acid.

mRNA Processing

• 5' capping

• Poly-A tail addition

• Splicing: Removal of introns

Signal Recognition Particle (SRP) Role

The SRP directs ribosomes to the endoplasmic reticulum for synthesis of membrane-bound or secretory proteins.

Genetic Theory of Aging

Suggests that aging is influenced by genetic factors, such as telomere shortening and accumulation of mutations.

Endocrine System and Tissue Organization

Endocrine vs Exocrine Glands

Epithelial Tissue Classifications

• Simple epithelium: Single cell layer (e.g., simple squamous).

• Stratified epithelium: Multiple layers (e.g., stratified squamous).

• Pseudostratified epithelium: Appears layered but all cells touch basement membrane.

Cell Types in Each Connective Tissue

• Fibroblasts: Produce fibers and ground substance.

• Adipocytes: Store fat.

• Chondrocytes: Found in cartilage.

• Osteocytes: Found in bone.

Connective Tissue Extracellular Matrix Composition

The extracellular matrix consists of protein fibers (collagen, elastin, reticular) and ground substance (proteoglycans, glycoproteins).

Connective Tissue Origin

All connective tissues arise from mesenchyme, an embryonic tissue derived from the mesoderm.

Connective Tissue Fiber Types

• Collagen fibers: Provide strength.

• Elastic fibers: Provide elasticity.

• Reticular fibers: Form supportive networks.

Epithelial Membranes

Epithelial membranes consist of an epithelial layer and underlying connective tissue. Types include mucous, serous, and cutaneous membranes.

Tissue Repair Stages and Inflammation

• Inflammation: Initial response to injury; increases blood flow and immune activity.

• Organization: Formation of granulation tissue.

• Regeneration: Replacement of damaged tissue with new cells.

Germ Layer Origins of Tissues