Cellular Structure, Function, and Processes: Study Notes for Anatomy & Physiology

Cellular Membrane Transport

Types of Membrane Transport

Cells regulate the movement of substances across their membranes through various transport mechanisms. These processes are essential for maintaining homeostasis and cellular function.

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration.

  • Simple Diffusion: Direct movement of small, nonpolar molecules (e.g., O2, CO2) across the lipid bilayer.

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

  • Facilitated Diffusion: Movement of larger or polar molecules via membrane proteins (channels or carriers).

• Filtration: Movement of water and solutes across a membrane due to hydrostatic pressure.

• Active Transport: Movement of substances against their concentration gradient using energy (ATP), often via pumps (e.g., Na+/K+ pump).

• Bulk Transport: Movement of large particles or volumes via vesicles.

  • Exocytosis: Export of materials out of the cell by vesicle fusion with the plasma membrane.

  • Endocytosis: Import of materials into the cell by vesicle formation from the plasma membrane.

Types of Endocytosis

Endocytosis is subdivided based on the nature of the material being internalized:

• Phagocytosis: 'Cell eating'; uptake of large particles or cells (e.g., bacteria).

• Pinocytosis: 'Cell drinking'; uptake of extracellular fluid and dissolved solutes.

• Receptor-mediated Endocytosis: Specific uptake of molecules after binding to cell surface receptors.

Cytoplasmic Organelles and Their Functions

Major Organelles

Organelles are specialized structures within cells that perform distinct functions necessary for cellular survival and activity.

• Mitochondria: Site of aerobic respiration and ATP production; often called the 'powerhouse' of the cell.

• Ribosomes: Sites of protein synthesis; can be free in cytoplasm or attached to rough endoplasmic reticulum.

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis.

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

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes for breaking down waste materials and cellular debris.

The Endomembrane System

Gene Expression Events

The endomembrane system coordinates the synthesis, processing, and transport of proteins and lipids within the cell.

• Transcription: The process by which DNA is copied into messenger RNA (mRNA) in the nucleus.

• Translation: The process by which ribosomes synthesize proteins using mRNA as a template.

Organelles Involved in Gene Expression

• Nucleus: Contains DNA; site of transcription.

• Ribosomes: Site of translation.

• Endoplasmic Reticulum: Processes and folds newly synthesized proteins.

• Golgi Apparatus: Further modifies and sorts proteins for their final destinations.

Macromolecules and Their Monomers

Types of Macromolecules

Cells are composed of four major classes of macromolecules, each built from specific monomers.

• Proteins: Polymers of amino acids; perform structural, enzymatic, and regulatory functions.

• Carbohydrates: Polymers of monosaccharides (simple sugars); provide energy and structural support.

• Lipids: Diverse group including fats, oils, and steroids; built from fatty acids and glycerol; function in energy storage and membrane structure.

• Nucleic Acids: DNA and RNA; polymers of nucleotides; store and transmit genetic information.

Nucleus Structure and Function

Components of the Nucleus

The nucleus is the control center of the cell, housing genetic material and coordinating activities such as growth and reproduction.

• Nuclear Envelope: Double membrane that encloses the nucleus and regulates passage of materials.

• Nucleoli: Dense regions within the nucleus where ribosomal RNA is synthesized and ribosome assembly begins.

• Chromatin: Complex of DNA and proteins; condenses to form chromosomes during cell division.

Cell Cycle

Periods of the Cell Cycle

The cell cycle consists of two main periods that govern cell growth and division.

• Interphase: Period of cell growth, DNA replication, and preparation for division.

• Mitosis (and Meiosis): Period of nuclear division; mitosis produces identical daughter cells, while meiosis produces gametes with half the chromosome number.

Subphases of Interphase

Interphase is subdivided into three phases, each with distinct events:

• G1 Phase (Gap 1): Cell grows and synthesizes proteins.

• S Phase (Synthesis): DNA replication occurs.

• G2 Phase (Gap 2): Cell prepares for mitosis; additional growth and protein synthesis.

Mitosis

Phases of Mitosis and Key Events

Mitosis is the process by which a cell divides its nucleus and contents to produce two identical daughter cells. It consists of several phases:

• Prophase: Chromatin condenses into visible chromosomes; nuclear envelope breaks down; spindle fibers form.

• Metaphase: Chromosomes align at the cell's equatorial plane (metaphase plate).

• Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell.

• Telophase: Nuclear envelopes reform around each set of chromosomes; chromosomes decondense.

• Cytokinesis: Division of the cytoplasm, resulting in two separate daughter cells. Note: Cytokinesis often overlaps with telophase.

Summary Table: Cell Cycle Phases

Phase	Main Events
G1 (Interphase)	Cell growth, protein synthesis
S (Interphase)	DNA replication
G2 (Interphase)	Preparation for mitosis
Prophase (Mitosis)	Chromosome condensation, spindle formation
Metaphase (Mitosis)	Chromosome alignment
Anaphase (Mitosis)	Chromatid separation
Telophase (Mitosis)	Nuclear envelope reformation
Cytokinesis	Cytoplasmic division

Key Equations

• Osmosis: Water movement is driven by differences in solute concentration across a membrane.

• Active Transport: $\text{Rate} = k[\text{Carrier}][\text{Substrate}]$ Additional info: This equation represents a simplified model of carrier-mediated transport rate.

Example

During facilitated diffusion, glucose enters cells via GLUT transporters, moving down its concentration gradient without energy input.

Additional info: These notes expand on the original outline to provide academic context and definitions for each topic.