Ch 3: Cell Membranes, Transport Mechanisms, and Cellular Physiology: Study Notes for Anatomy & Physiology

Cell Membrane Structure and Function

Selective Permeability of the Cell Membrane

The cell membrane is a dynamic structure that regulates the movement of substances into and out of the cell, maintaining homeostasis.

• Selective permeability refers to the ability of the membrane to allow certain molecules to pass while restricting others.

• The lipid bilayer is primarily permeable to small, nonpolar molecules (e.g., O2, CO2), while ions and large polar molecules require transport proteins.

• Example: Glucose requires a transporter to enter most cells, while oxygen diffuses freely.

Molecules Crossing the Cell Membrane

• Can cross: Small nonpolar molecules, some small uncharged polar molecules.

• Cannot cross: Large molecules, ions, and most polar molecules without assistance.

• Transport proteins facilitate movement of these restricted molecules.

Membrane Proteins: Classes and Functions

• Integral proteins: Embedded within the lipid bilayer; function as channels, carriers, or receptors.

• Peripheral proteins: Attached to the membrane surface; involved in signaling and structural support.

• Transporter proteins: Include channels (for passive transport) and pumps (for active transport).

• Example: Sodium-potassium pump (Na+/K+ ATPase) maintains ion gradients.

Types of Transporter Proteins

• Uniport: Transports one type of molecule in one direction.

• Symport: Transports two molecules in the same direction.

• Antiport: Transports two molecules in opposite directions.

• Example: Na+/K+ pump is an antiporter.

Body Fluid Compartments

Body fluids are distributed in distinct compartments, each with specific properties.

• Intracellular fluid (ICF): Fluid within cells (~2/3 of total body water).

• Extracellular fluid (ECF): Fluid outside cells (~1/3 of total body water), including plasma and interstitial fluid.

• Sub-compartments: Plasma (in blood vessels), interstitial fluid (between cells), transcellular fluids (e.g., cerebrospinal fluid).

Compartment	Approximate % of Total Body Water
Intracellular Fluid (ICF)	~66%
Extracellular Fluid (ECF)	~33%
Plasma	~7%
Interstitial Fluid	~26%

Major Ions in Body Fluids

• Plasma: High in Na+, Cl-; low in K+

• Interstitial fluid: Similar to plasma but lower in protein.

• Intracellular fluid: High in K+, Mg2+; low in Na+

Ion	Plasma	Interstitial Fluid	Intracellular Fluid
Na+	High	High	Low
K+	Low	Low	High
Cl-	High	High	Low
Proteins	High	Low	High

Transport Mechanisms Across Membranes

Types of Membrane Transport

• Passive transport: No energy required; includes diffusion, facilitated diffusion, osmosis, and filtration.

• Active transport: Requires energy (ATP); includes pumps and vesicular transport.

Diffusion and Osmosis

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

• Facilitated diffusion: Diffusion via transport proteins.

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

• Osmotic pressure: Pressure required to prevent osmosis.

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

Equation for osmotic pressure:

$\Pi = iMRT$

Where $\Pi$ is osmotic pressure, $i$ is the van 't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature.

Osmolarity and Osmolality

• Osmolarity: Number of osmoles of solute per liter of solution.

• Osmolality: Number of osmoles of solute per kilogram of solvent.

• Equation: $\text{Osmolarity} = \frac{\text{osmoles}}{\text{liter}}$

• Equation: $\text{Osmolality} = \frac{\text{osmoles}}{\text{kg}}$

Tonicity: Isotonic, Hypotonic, and Hypertonic Solutions

• Isotonic: Same solute concentration as the cell; no net water movement.

• Hypotonic: Lower solute concentration than the cell; water enters cell, may cause lysis.

• Hypertonic: Higher solute concentration than the cell; water leaves cell, may cause crenation.

Solution Type	Effect on Cell
Isotonic	No change
Hypotonic	Cell swells and may burst (lysis)
Hypertonic	Cell shrinks (crenation)

Transport Proteins: Examples and Applications

• Channels: Allow passive movement of ions (e.g., Na+, K+).

• Pumps: Use ATP to move ions against gradients (e.g., Na+/K+ ATPase).

• Carrier proteins: Bind and transport specific molecules.

Vesicular Transport: Endocytosis and Exocytosis

• Endocytosis: Uptake of materials via vesicles.

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

• Pinocytosis: "Cell drinking"; uptake of fluids.

• Exocytosis: Release of substances from the cell via vesicles.

Cellular Homeostasis and Feedback Mechanisms

Osmoregulation and Feedback

• Homeostasis: Maintenance of stable internal environment.

• Feedback mechanisms: Negative feedback restores equilibrium (e.g., ADH regulation of water balance).

• Example: Increased plasma osmolarity triggers ADH release, promoting water reabsorption in kidneys.

Electrical and Chemical Disequilibrium

• Electrical disequilibrium: Difference in charge across the membrane (membrane potential).

• Chemical disequilibrium: Unequal distribution of ions and molecules.

• Example: Resting membrane potential is maintained by Na+/K+ gradients.

Cell Structure and Division

Cell as the Basic Living Unit

• Cell: Fundamental unit of life; capable of metabolism, growth, and reproduction.

• Example: Human cells include muscle cells, nerve cells, and epithelial cells.

Cellular Organelles and Functions

• Nucleus: Contains genetic material (DNA).

• Mitochondria: Site of ATP production.

• Endoplasmic reticulum: Protein and lipid synthesis.

• Golgi apparatus: Modifies and packages proteins.

• Lysosomes: Digestive enzymes for breakdown of waste.

• Centrosome: Organizes microtubules during cell division.

Chromosomes and Cell Division

• Chromosome: DNA molecule with associated proteins; carries genetic information.

• Chromatid: One of two identical halves of a replicated chromosome.

• Centromere: Region joining sister chromatids.

• Spindle fibers: Microtubules that separate chromosomes during cell division.

Mitosis and Meiosis

• Mitosis: Somatic cell division; produces two identical daughter cells.

• Meiosis: Gamete formation; produces four genetically unique cells with half the chromosome number.

• Phases: Prophase, metaphase, anaphase, telophase (for both mitosis and meiosis).

• Diploid (2n): Full set of chromosomes; haploid (n): Half set (gametes).

Process	Number of Daughter Cells	Chromosome Number
Mitosis	2	Diploid (2n)
Meiosis	4	Haploid (n)

Cell Cycle

• Interphase: Cell growth and DNA replication.

• Mitosis: Division of nucleus.

• Cytokinesis: Division of cytoplasm.

Solutions and Their Effects on Cells

Definitions

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

• Facilitated diffusion: Diffusion via transport proteins.

• Osmosis: Movement of water across a membrane.

• Filtration: Movement due to pressure.

• Osmotic pressure: Pressure needed to prevent osmosis.

• Hemolysis: Rupture of red blood cells in hypotonic solution.

• Crenation: Shrinking of red blood cells in hypertonic solution.

Calculating Osmolarity and Osmolality

• Osmolarity: $\text{Osmolarity} = \frac{\text{osmoles}}{\text{liter}}$

• Osmolality: $\text{Osmolality} = \frac{\text{osmoles}}{\text{kg}}$

Effects of Solutions on Cells

• In isotonic solutions, cells retain their normal shape.

• In hypotonic solutions, cells swell and may burst (hemolysis).

• In hypertonic solutions, cells shrink (crenation).

Osmosis Across Selectively Permeable Membranes

• Water moves from the side with lower solute concentration to the side with higher solute concentration.

• This movement continues until equilibrium is reached.

Additional info: These notes expand on the original list of objectives by providing definitions, examples, and context for each major topic in cell physiology and membrane transport, as well as cell structure and division.