Body Compartmentalization

Main Compartments

Body compartmentalization refers to the division of the body into distinct spaces, each with specific functions and contents.

• Anatomical compartments: Body cavities (e.g., thoracic, abdominal)

• Functional compartments: Body fluid compartments (e.g., intracellular fluid, extracellular fluid)

Lumens are the internal spaces within organs, such as the hollow part of a blood vessel or the digestive tract.

Cell Membranes (Plasmalemma)

Structure and Composition

Cell membranes are essential for maintaining cellular integrity and regulating interactions with the environment.

• Composed of cell plasma membranes, membrane tissues

• Made up of cholesterol, phospholipids, carbohydrates, and proteins

Membranes act as barriers to isolate cells and provide structure and support. They also regulate cell-to-cell communication and exchange between extracellular and intracellular fluids.

The Phospholipid Bilayer

The Fluid Mosaic Model describes the structure of the cell membrane, where phospholipids form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Bilayers become more malleable at high temperatures and more rigid at low temperatures.

• Nonpolar molecules (e.g., oxygen, steroids) can pass through the membrane more easily than polar molecules.

How Cells Form Tissues

Cell Junctions

Cells connect to form tissues using various junctions:

• Tight junctions: Prevent communication between cells; major component of the blood-brain barrier.

• Desmosomes: Provide cell-to-cell anchoring; among the strongest junctions.

• Gap junctions: Allow chemical and electrical signals to pass directly between cells.

Proteins act as a skeleton and bridge connections between cells, forming tissues such as nervous and muscle tissue.

Osmosis and Membrane Dynamics

Osmosis

Osmosis is the movement of water across a membrane in response to a solute gradient.

• Water acts as a universal physiological solvent.

• 2/3 of water is intracellular.

• Extracellular fluids (ECF) act as buffers between environments and cells.

Osmosis and Equilibrium

At equilibrium, fluid concentrations are equal on both sides of the cell membrane, and net movement of water stops.

Osmolarity

Osmolarity is the concentration of solute expressed as the total number of solutes per liter (mmol/L).

Tonicity

Tonicity describes how a solution affects cell volume:

• Hypertonic: More solutes outside the cell; water leaves the cell, causing shrinkage.

• Isotonic: Equal amounts of solutes inside and outside; cell volume remains normal.

Aquaporins

Water movement between compartments is facilitated by aquaporins, specialized membrane channels.

Components of the Cell Membrane

Barrier and Transport Functions

The cell membrane's polar and nonpolar regions make it a selective barrier. It regulates the movement of chemical signals, water, nutrients, ions, and proteins.

Diffusion

Simple Diffusion

Simple diffusion is the passive movement of molecules down a concentration gradient.

• Relies on temperature and concentration

• Non-polar molecules diffuse more rapidly

Factors Affecting Diffusion

• Concentration gradient

• Surface area of membrane

• Permeability (inversely related to thickness and resistance)

Fick's Law of Diffusion

Fick's Law quantifies the rate of diffusion:

• Where:

  • = Net flux of molecule

  • = Surface area of membrane

  • = Molecule outside

  • = Molecule inside

  • = Permeability constant

Rate of diffusion is directly proportional to surface area, concentration gradient, and membrane permeability.

Facilitated Diffusion

Facilitated diffusion moves specific substrates down the concentration gradient via transporters. It does not require energy (ATP).

Passive vs Active Transport

Ion Channels and Carrier Proteins

Ion Channels

Ion channels allow rapid diffusion of ions across membranes. They can be open (leaky) or gated, and are selective for specific ions.

Types of Carrier Proteins

• Uniport: Transport one type of substrate

• Symport (Cotransport): Move two or more substances in the same direction

• Antiport: Move substances in opposite directions (e.g., sodium-potassium pump)

Mechanism of the Sodium-Potassium Pump

1. 2 K+ are released into ICF

2. 3 Na+ from ICF bind

3. ATP is phosphorylated with Pi from ATP

4. 3 Na+ are released into ECF

5. 2 K+ from ECF bind, and the process repeats

Resting Membrane Potential and Cell Communication

Resting Membrane Potential

The resting membrane potential is the electrical gradient between extracellular and intracellular fluids.

Nernst Equation

The Nernst equation calculates the equilibrium potential for a single ion:

• = valence of ion

For multiple ions, the equation is applied separately for each ion.

Membrane Potential Changes

• Depolarization: Membrane potential moves closer to zero

• Repolarization: Membrane potential returns to resting potential

• Hyperpolarization: Membrane potential becomes more negative than resting potential

Cell Communication

Electrical and Chemical Signals

Cells communicate via electrical signals (changes in membrane potential) and chemical signals (ligands).

• Neurocrines: Neurotransmitters and neuromodulators released by neurons

• Hormones: Produced by neuroendocrine cells, travel through blood

Cell Communication Methods

• Gap junctions: Create cytoplasmic bridges for signal transfer

• Cell adhesion molecules: Require cell-to-cell contact

• Autocrine signaling: Communication within the same cell

• Paracrine signaling: Communication between nearby cells

Signal Pathways

• Require cell receptor proteins

• Signal transduction converts chemical signals to intracellular responses

• Receptors are proteins found on cell membranes

Types of Receptors

• Ligand-gated/receptor channel: Ligand binding opens or closes the channel

• G-protein coupled receptors: Ligand binding activates GTP, opening ion channels or altering enzyme activity

• Receptor-enzymes: Ligand binding activates an intracellular enzyme

• Integrin receptors: Alter enzymes or the cytoskeleton

Second Messengers

Signal transduction often involves second messengers, which amplify and regulate cellular responses, frequently through phosphorylation.

Additional info: These notes expand on the original content by providing definitions, examples, and equations for key concepts in membrane dynamics and cell communication, suitable for college-level Anatomy & Physiology students.