Chapter 3

BASIC PROCESSES OF CELLS

Overview of Cellular Functions

• Cell metabolism: Sum of all chemical reactions that a cell carries out to maintain life.

• Transport: Movement of substances into, out of, or within the cell, either produced or ingested, to various destinations.

• Communication: Exchange of information between the cell and its surrounding environment.

• Cell reproduction: Process necessary for growth, development, and replacement of old or damaged cells.

OVERVIEW OF CELL STRUCTURE

Basic Components of Animal Cells

Most animal cells have three main components:

• Plasma membrane

• Cytoplasm

• Nucleus

Plasma Membrane

• Provides structural support, means of communication, and cell identification.

• Defines intracellular space (contains intracellular fluid (ICF) or cytosol) and separates it from extracellular space (contains extracellular fluid (ECF)).

Cytoplasm

• Cytosol: Intracellular fluid where many metabolic reactions occur.

• Organelles: Specialized structures with specific functions (e.g., mitochondria, ribosomes).

• Cytoskeleton: Network of protein filaments providing structural support and facilitating movement.

Nucleus

• Contains most of the cell's DNA and is the primary location for making RNA.

• DNA and RNA control organelle functions by coding for and synthesizing proteins.

STRUCTURE OF THE PLASMA MEMBRANE

Phospholipid Bilayer

• Phospholipids have a phosphate group (hydrophilic head) and two fatty acid tails (hydrophobic).

• Forms a water-resistant barrier.

The Fluid Mosaic Model

• Plasma membranes contain lipids, carbohydrates, and proteins, creating a "mosaic" appearance.

• Components in the bilayer move, giving the membrane "fluidity"—critical for membrane functions.

• Fluid Mosaic Model is the current model describing plasma membrane structure.

Membrane Proteins

• Carry out many of the membrane's functions and give different cell types unique properties.

• Protein channels: Allow transport of substances into and out of the cell.

• Receptors: Bind chemical messengers (ligands) and trigger cellular events.

• Enzymes: Speed up chemical reactions, vital for maintaining homeostasis.

• Structural support: Give cells shape and maintain integrity.

• Linker proteins: Anchor cells to one another, allowing cell communication.

DRUGS AND MEMBRANE RECEPTORS

Ligands and Receptor Interactions

• Many drugs resemble ligands that bind to membrane receptors.

• Agonists: Mimic ligand action and activate receptors (e.g., morphine mimics endorphins).

• Antagonists: Block ligand action by blocking receptors (e.g., antihistamines block histamine receptors).

TRANSPORT ACROSS THE PLASMA MEMBRANE

Selective Permeability

• The plasma membrane is selectively permeable, allowing certain substances to pass while restricting others.

• Substances cross the membrane via passive transport (no energy required) or active transport (energy required).

Passive Transport Processes

• Diffusion: Movement of solute molecules from high to low concentration.

• Concentration Gradient: Drives diffusion; molecules move until equilibrium is reached.

• Simple diffusion: Nonpolar solutes (e.g., oxygen, carbon dioxide) pass directly through the membrane.

• Facilitated diffusion: Charged or polar solutes (e.g., ions, glucose) cross via carrier or channel proteins.

Osmosis and Tonicity

• Osmosis: Movement of water from low solute concentration (more water) to high solute concentration (less water) across a membrane.

• Osmotic pressure: Force exerted by solute molecules to move water until equilibrium is reached.

• Tonicity: Comparison of osmotic pressure gradients between two solutions (cytosol and ECF).

Solution Type	Effect on Cell
Isotonic	No net movement of water; cell remains unchanged
Hypertonic	ECF solute concentration higher than inside cell; water leaves cell, cell shrinks (crenates)
Hypotonic	ECF solute concentration lower than inside cell; water enters cell, cell swells and may rupture (lyse)

Applications: Dehydration, Sports Drinks, and Water

• Strenuous exercise causes ECF to become hypertonic; water leaves cells by osmosis.

• Sports drinks (isotonic or hypotonic) help rehydrate cells and restore electrolyte balance.

• Plain water can rehydrate but may cause cells to swell if ECF becomes hypotonic.

Active Transport Processes

• Requires energy from ATP to move substances against their concentration gradients (from low to high concentration).

• Uses carrier proteins called pumps to transport solutes across the membrane.

Active Transport via Vesicles

• Vesicles: Small sacs filled with large molecules, enclosed in a phospholipid bilayer.

• Allows fusion with or formation from plasma membrane.

• Requires energy from ATP.

Types of Vesicular Transport

• Endocytosis: Movement into the cell.

  • Phagocytosis: "Cell eating"; immune cells engulf bacteria or debris.

  • Pinocytosis: "Cell drinking"; cells engulf fluid droplets from ECF.

  • Receptor-mediated endocytosis: Uses receptors to fill vesicles with specific molecules.

• Exocytosis: Large molecules exit the cell; vesicles fuse with plasma membrane and release contents.

• Transcytosis: Molecules are brought into the cell by endocytosis, transported across the cell, and secreted by exocytosis.

ORGANELLES AND THEIR FUNCTIONS

Mitochondria

• Known as the "powerhouse" of the cell; produce ATP (chemical energy).

• Membrane-bound organelles; majority of ATP used in the cell is produced here.

Peroxisomes

• Use oxygen to carry out chemical reactions that produce hydrogen peroxide (H2O2).

• Oxidize toxic chemicals to less toxic compounds for elimination.

• Break down fatty acids and some amino acids.

• Certain phospholipids synthesized in peroxisomes are critical for plasma membranes of specific cells (e.g., nervous system).

Ribosomes

• Sites of protein synthesis (build proteins).

• Found in cytosol (free ribosomes) or bound to membranes (e.g., rough ER).

Endoplasmic Reticulum (ER)

• Large folded phospholipid bilayer.

• Rough ER (RER): Ribosomes bound to membrane; packages secretory proteins into transport vesicles.

• Smooth ER (SER): No ribosomes; involved in lipid and phospholipid synthesis.

KEY TERMS AND FORMULAS

• Diffusion equation: $J = -D \frac{dC}{dx}$

• Osmosis: $\text{Water moves from low solute concentration to high solute concentration}$

• ATP hydrolysis: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Summary Table: Membrane Transport Types

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	Oxygen, CO2
Facilitated Diffusion	No	High to Low	Glucose, Ions
Osmosis	No	Low to High solute	Water
Active Transport	Yes (ATP)	Low to High	Na+/K+ pump
Vesicular Transport	Yes (ATP)	In/Out/Across	Endocytosis, Exocytosis

Additional info: These notes expand on brief points with academic context, definitions, and examples for clarity and completeness.