Plasma Membrane Structure and Components

Overview of the Plasma Membrane

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

• Phospholipid bilayer: Composed of hydrophilic heads and hydrophobic tails, forming the basic structure.

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

• Carbohydrates: Attached to proteins and lipids, forming glycoproteins and glycolipids for cell recognition.

Glycocalyx Composition

Role and Structure of the Glycocalyx

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

• Components: Glycoproteins and glycolipids.

• Functions: Cell adhesion, immune response, and protection from mechanical damage.

Fluid Mosaic Model Principles

Understanding Membrane Dynamics

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

• Fluidity: Lipids and proteins can move laterally within the layer.

• Mosaic: Diverse proteins embedded in the membrane.

Types of Cell Junctions

Cell-Cell Connections

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

• Tight junctions: Prevent leakage of extracellular fluid.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap junctions: Allow passage of ions and small molecules for cell communication.

Substances That Can Fuse Directly Through the Plasma Membrane

Simple Diffusion

Certain substances can cross the plasma membrane without assistance.

• Small nonpolar molecules: Oxygen (O2), carbon dioxide (CO2).

• Lipid-soluble substances: Steroid hormones.

Types and Characteristics of Passive Transport

Mechanisms of Passive Movement

Passive transport moves substances across membranes without energy input.

• Simple diffusion: Movement down concentration gradient.

• Facilitated diffusion: Uses carrier or channel proteins.

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

Effects of Hypertonic, Hypotonic, Isotonic Solutions on Cells

Osmotic Balance and Cell Volume

Cells respond differently to various extracellular solution concentrations.

• Hypertonic: Water leaves the cell, causing shrinkage (crenation).

• Hypotonic: Water enters the cell, causing swelling or lysis.

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

Endocytosis Types and Mechanisms

Uptake of Materials into the Cell

Endocytosis is the process by which cells internalize substances.

• Phagocytosis: "Cell eating" of large particles.

• Pinocytosis: "Cell drinking" of fluids and small molecules.

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

Sodium-Potassium Pump (Transport Type)

Active Transport Mechanism

The sodium-potassium pump is an example of primary active transport, maintaining ion gradients across the membrane.

• Transports: 3 Na+ out, 2 K+ in per ATP hydrolyzed.

• Maintains: Resting membrane potential and cell volume.

$3\text{Na}^+_{\text{in}} + 2\text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

Secondary Active Transport Mechanism

Coupled Transport

Secondary active transport uses the energy from ion gradients established by primary active transport.

• Symport: Both substances move in the same direction.

• Antiport: Substances move in opposite directions.

Cell Signaling: Chemical Signaling, G Protein-Linked Receptor Signaling

Communication Between Cells

Cell signaling involves the transmission of signals via chemical messengers.

• Chemical signaling: Hormones, neurotransmitters bind to receptors.

• G protein-linked receptors: Activate intracellular signaling cascades via G proteins.

Organization of DNA in the Nucleus

Chromatin Structure

DNA is organized in the nucleus as chromatin, which condenses into chromosomes during cell division.

• Nucleosomes: DNA wrapped around histone proteins.

• Chromatin: Can be euchromatin (active) or heterochromatin (inactive).

Functions of Types of RNA in Protein Synthesis

Roles of RNA Molecules

Three main types of RNA are involved in protein synthesis.

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

• 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

DNA Duplication Mechanism

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

Lysosome Functions

Cellular Digestion

Lysosomes are membrane-bound organelles containing digestive enzymes.

• Breakdown: Degrade macromolecules, old organelles, and pathogens.

• Autophagy: Recycling of cellular components.

Definition of a Gene

Genetic Information Unit

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

Enzymes in DNA Replication

Key Enzymes and Their Functions

• DNA helicase: Unwinds the DNA double helix.

• DNA polymerase: Synthesizes new DNA strands.

• Primase: Synthesizes RNA primers.

• Ligase: Joins Okazaki fragments on the lagging strand.

Transcription vs Translation

Gene Expression Steps

• Transcription: DNA is used as a template to synthesize mRNA.

• Translation: mRNA is decoded by ribosomes to assemble amino acids into a protein.

Base Pairing Rules (mRNA Codon and tRNA Anticodon)

Genetic Code Fidelity

• DNA base pairing: A-T, C-G.

• RNA base pairing: A-U, C-G.

• Codon-anticodon: mRNA codon pairs with complementary tRNA anticodon during translation.

Genetic Code Redundancy

Degeneracy of the Genetic Code

Multiple codons can code for the same amino acid, providing redundancy and minimizing mutation effects.

mRNA Processing

Post-Transcriptional Modifications

• 5' capping: Addition of a methylated cap for stability.

• Poly-A tail: Added to 3' end for protection and export.

• Splicing: Removal of introns and joining of exons.

Signal Recognition Particle (SRP) Role

Protein Targeting

The SRP directs newly synthesized proteins to the endoplasmic reticulum for secretion or membrane insertion.

Genetic Theory of Aging

Genetic Influences on Aging

Suggests that aging is regulated by genetic factors, including telomere shortening and accumulation of mutations.

Endocrine vs Exocrine Glands

Glandular Classification

Epithelial Tissue Classifications

Types of Epithelial Tissue

• 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 the basement membrane.

Cell Types in Each Connective Tissue

Major Connective Tissue Cells

• Fibroblasts: Produce fibers and ground substance.

• Adipocytes: Store fat.

• Chondrocytes: Cartilage cells.

• Osteocytes: Bone cells.

• Blood cells: Red and white blood cells.

Connective Tissue Extracellular Matrix Composition

Matrix Components

• Fibers: Collagen, elastic, reticular.

• Ground substance: Proteoglycans, glycosaminoglycans, water.

Connective Tissue Origin

Embryonic Source

Connective tissues originate from the mesenchyme, an embryonic tissue derived from the mesoderm.

Connective Tissue Fiber Types

Structural Fibers

• Collagen fibers: Provide strength.

• Elastic fibers: Allow stretch and recoil.

• Reticular fibers: Form supportive networks.

Epithelial Membranes

Types of Membranes

• Mucous membranes: Line body cavities open to exterior.

• Serous membranes: Line closed body cavities; secrete serous fluid.

• Cutaneous membrane: Skin.

Tissue Repair Stages and Inflammation

Healing Process

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

• Organization: Formation of granulation tissue.

• Regeneration/Fibrosis: Replacement of tissue or formation of scar.

Germ Layer Origins of Tissues

Embryonic Development