Chapter 3: Cells – The Living Units (Anatomy & Physiology Study Notes)

Chapter 3: Cells – The Living Units

3.1 Cells: The Smallest Living Units

Cells are the fundamental structural and functional units of life. Understanding their structure and function is essential for grasping how the human body operates at all levels.

• Cell Theory:

  • A cell is the basic unit of life.

  • The function of an organism depends on the individual and collective activities of its cells.

  • Structure and function are complementary: biochemical functions are dictated by cell shape and subcellular structures.

  • Continuity of life has a cellular basis: all cells arise from preexisting cells.

• Cell Diversity: Human cells vary in size, shape, and function (e.g., epithelial cells, red blood cells, muscle cells, neurons, sperm cells).

Generalized Cell Structure

Despite diversity, all human cells share three basic parts:

• Plasma membrane: Flexible outer boundary

• Cytoplasm: Intracellular fluid containing organelles

• Nucleus: DNA-containing control center

Extracellular Materials

Substances found outside cells are essential for tissue function and include:

• Extracellular fluids: Interstitial fluid (bathes cells), blood plasma, cerebrospinal fluid

• Cellular secretions: Saliva, mucus

• Extracellular matrix: Substance that acts as glue to hold cells together

3.2 Structure of Plasma Membrane

The plasma membrane is a dynamic structure that separates the cell from its environment and regulates movement of substances in and out.

• Lipid bilayer: Composed mainly of phospholipids with hydrophilic heads and hydrophobic tails

• Membrane proteins: Integral (span the membrane) and peripheral (loosely attached); responsible for transport, signaling, and maintaining cell shape

• Glycocalyx: Surface sugars that function as biological markers for cell recognition

• Cell junctions: Structures that hold cells together (tight junctions, desmosomes, gap junctions)

Membrane Protein Functions

• Transport: Channels and carriers move substances across the membrane

• Receptors: Bind chemical messengers and initiate cellular responses

• Enzymatic activity: Catalyze metabolic reactions

• Cell-cell recognition: Glycoproteins serve as identification tags

• Attachment: Anchor cytoskeleton and extracellular matrix

• Junctions: Connect adjacent cells for communication and adhesion

Clinical Note: Homeostatic Imbalance 3.1

• Changes in the glycocalyx of cancer cells can prevent immune recognition, allowing mutated cells to evade destruction.

Cell Junctions

Most cells are bound together to form tissues and organs via specialized junctions:

• Tight junctions: Form impermeable barriers to prevent passage of substances between cells (e.g., in the digestive tract)

• Desmosomes: Anchor cells together like rivets, providing mechanical stability (e.g., in skin and heart tissue)

• Gap junctions: Allow ions and small molecules to pass directly between cells, facilitating communication (e.g., in cardiac muscle)

3.3 Passive Membrane Transport

Passive transport moves substances across the membrane without energy input, driven by concentration gradients.

• Simple diffusion: Movement of lipid-soluble molecules directly through the lipid bilayer

• Facilitated diffusion: Movement of specific molecules via carrier or channel proteins

• Osmosis: Diffusion of water through a selectively permeable membrane, often via aquaporins

Factors Affecting Diffusion Speed

• Concentration gradient (greater difference = faster diffusion)

• Molecular size (smaller molecules diffuse faster)

• Temperature (higher temperature increases diffusion rate)

Osmolarity and Tonicity

• Osmolarity: Measure of total solute concentration in a solution

• Tonicity: Ability of a solution to change cell shape by altering water volume

  • Isotonic: Same osmolarity as cell; no net water movement

  • Hypertonic: Higher osmolarity than cell; water leaves cell, causing shrinkage (crenation)

  • Hypotonic: Lower osmolarity than cell; water enters cell, causing swelling and possible lysis

Hydrostatic vs. Osmotic Pressure

• Hydrostatic pressure: Outward pressure exerted by fluid on cell membrane

• Osmotic pressure: Inward pressure due to water being pulled into a cell with higher solute concentration

• Equilibrium is reached when hydrostatic and osmotic pressures are balanced

Clinical Note: Homeostatic Imbalance Burns

• Severe plasma membrane damage (e.g., burns) leads to loss of fluids, proteins, and ions, disrupting cellular homeostasis.

Tonicity in Blood Cells

Solution Type	Effect on Blood Cells
Isotonic	Cells retain normal size and shape
Hypertonic	Cells lose water and shrink (crenate)
Hypotonic	Cells gain water, swell, and may burst (lyse)

Key Equations

• Osmolarity calculation: $\text{Osmolarity} = \text{Molarity} \times \text{number of particles per molecule}$ Example: 1 M NaCl solution = 2 Osm (since NaCl dissociates into 2 particles: Na+ and Cl-)

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Examples
Simple Diffusion	No	Down concentration gradient	O2, CO2
Facilitated Diffusion	No	Down concentration gradient	Glucose, ions
Osmosis	No	Down water potential gradient	Water