Cell Structure and Membrane Transport: Study Notes for Anatomy & Physiology

Cells: The Living Units

Why This Matters

Understanding the structure and function of the body's cells is essential for explaining how the plasma membrane regulates permeability and affects treatment outcomes.

• Cell theory: The cell is the basic structural and functional unit of life.

• All organismal functions depend on individual and combined cell activities.

• Biochemical functions of cells are determined by their shape and subcellular structures.

• Cells arise only from preexisting cells, ensuring continuity of life.

Cell Diversity

• There are over 200 different types of human cells.

• Cells differ in size, shape, and subcellular components, leading to functional diversity.

Generalized Cell Structure

• All cells share common structures and functions.

• Three basic parts of a human cell:

  1. Plasma membrane: Flexible outer boundary

  2. Cytoplasm: Intracellular fluid containing organelles

  3. Nucleus: DNA-containing control center

Extracellular Materials

Substances found outside cells are classified as extracellular materials.

• Extracellular fluids (body fluids):

  • Interstitial fluid: Bathes cells

  • Blood plasma: Fluid of the blood

  • Cerebrospinal fluid: Surrounds nervous system organs

• Cellular secretions: e.g., saliva, mucus

• Extracellular matrix: Complex mesh of proteins and polysaccharides that provide structural support

Plasma Membrane

Part 1 - Plasma Membrane

The plasma membrane acts as an active barrier separating intracellular fluid (ICF) from extracellular fluid (ECF). It controls what enters and leaves the cell and is also known as the "cell membrane."

Structure of Plasma Membrane

• Composed of membrane lipids forming a flexible lipid bilayer.

• Membrane proteins float through the bilayer, creating a dynamic mosaic pattern.

• Glycocalyx: Surface sugars attached to lipids and proteins.

• Cell junctions help hold cells together.

Membrane Lipids

• Lipid bilayer is made up of:

  • 75% phospholipids (polar phosphate heads are hydrophilic; nonpolar fatty acid tails are hydrophobic)

  • 5% glycolipids (lipids with sugar groups on outer membrane surface)

  • 20% cholesterol (increases membrane stability)

Membrane Proteins

• Allow cell communication with the environment.

• Make up half the mass of the plasma membrane.

• Most are specialized for membrane functions.

• Types:

  • Integral proteins: Inserted into the membrane; span the membrane.

  • Peripheral proteins: Loosely attached to integral proteins; support and function as enzymes, motor proteins, and cell-to-cell connectors.

Glycocalyx

• Consists of carbohydrates sticking out of the cell surface.

• Functions as specific biological markers for cell-to-cell recognition.

• Allows the immune system to recognize "self" vs. "nonself."

Clinical Note: Homeostatic Imbalance 3.1

• Glycocalyx of some cancer cells can change rapidly, making them unrecognizable to the immune system.

• Mutated cells may evade immune detection and replicate.

Cell Junctions

Types of Cell Junctions

• Tight junctions: Integral proteins fuse adjacent cells, forming an impermeable barrier.

• Desmosomes: "Button-like" plaques anchor cells together, providing mechanical strength.

• Gap junctions: Connexons form tunnels for communication and passage of ions and small molecules between cells.

Functions and Examples

• Tight junctions prevent fluid and most molecules from moving between cells (e.g., in the digestive tract).

• Desmosomes are found in tissues subject to mechanical stress (e.g., skin, heart muscle).

• Gap junctions allow electrical signals to pass quickly (e.g., cardiac and smooth muscle cells).

Membrane Transport

Selective Permeability

Plasma membranes are selectively permeable, allowing some substances to pass while restricting others.

• Passive processes: No energy required.

• Active processes: Energy (ATP) required.

Passive Membrane Transport

• Substances move without energy input.

• Types:

  • Diffusion (simple, facilitated, osmosis)

  • Filtration (across capillary walls)

Diffusion

Diffusion is the movement of molecules from areas of high concentration to areas of low concentration, driven by the concentration gradient.

• Energy is not required.

• Plasma membranes stop diffusion and create concentration gradients by acting as selectively permeable barriers.

Types of Diffusion

• Simple diffusion: Nonpolar, hydrophobic substances diffuse directly through the lipid bilayer (e.g., oxygen, carbon dioxide).

• Facilitated diffusion: Polar molecules (e.g., glucose, amino acids, ions) are transported via protein carriers or channels.

  • Carrier-mediated: Carriers transport specific polar molecules; binding causes carrier to change shape.

  • Channel-mediated: Channels allow aqueous molecules to pass; types include leakage channels (always open) and gated channels (controlled by signals).

• Osmosis: Movement of water across a selectively permeable membrane via aquaporins or directly through the lipid bilayer.

Osmolarity and Tonicity

• Osmolarity: Measure of total concentration of solute particles.

• Water moves by osmosis from areas of low solute (high water) concentration to areas of high solute (low water) concentration.

• Tonicity: Ability of a solution to change cell shape or tone by altering internal water volume.

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

  • Hypertonic solution: Higher solute concentration than inside cell; water leaves cell, causing shrinkage (crenation).

  • Hypotonic solution: Lower solute concentration than inside cell; water enters cell, causing swelling and possible lysis.

Clinical Note: Homeostatic Imbalance 3.2 & 3.3

• If plasma membrane is damaged, substances diffuse freely, compromising cell integrity (e.g., burn patients lose fluids and proteins).

• Intravenous solutions of different tonicities are used in clinical settings:

  • Isotonic solutions: Used when blood volume needs to be increased quickly.

  • Hypertonic solutions: Used to pull water back into blood (e.g., for edematous patients).

  • Hypotonic solutions: Can cause cell swelling and lysis if not carefully administered.

Key Terms and Definitions

• Phospholipid bilayer: Double layer of phospholipids forming the basic structure of the plasma membrane.

• Integral proteins: Proteins embedded within the membrane, often spanning its entire width.

• Peripheral proteins: Proteins attached to the membrane surface, providing support and function.

• Glycocalyx: Carbohydrate-rich area on the cell surface for recognition and protection.

• Desmosomes: Anchoring junctions that bind adjacent cells together.

• Gap junctions: Communicating junctions allowing ions and small molecules to pass between cells.

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

• Tonicity: Effect of a solution on cell volume.

Important Equations

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

• Diffusion rate (Fick's Law): $J = -D \frac{dC}{dx}$ Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Comparison Table: Types of Membrane Transport

Type	Energy Required?	Example Substances	Mechanism
Simple Diffusion	No	O2, CO2, fat-soluble vitamins	Directly through lipid bilayer
Facilitated Diffusion	No	Glucose, amino acids, ions	Via protein carriers or channels
Osmosis	No	Water	Through aquaporins or lipid bilayer
Filtration	No	Water, solutes	Across capillary walls
Active Transport	Yes	Ions, large molecules	Via ATP-powered pumps

Summary

• The plasma membrane is a dynamic structure essential for cell integrity and function.

• Membrane transport processes are vital for maintaining homeostasis and responding to clinical challenges.

• Understanding cell junctions and membrane transport is crucial for interpreting physiological and pathological states.