A&P Chapter 3 Part 1: Cell Structure, Membranes, and Transport

Cell Theory

Cells are the smallest unit of life; all organisms are made of one or more cells; new cells arise only from existing cells.

Generalized Cell

Model cell with three main parts: plasma membrane, cytoplasm, and nucleus.

Extracellular Fluid (ECF)

Fluid outside cells, including interstitial fluid, blood plasma, and cerebrospinal fluid.

Extracellular Matrix (ECM)

Network of proteins and carbohydrates outside cells that provides support and adhesion.

Plasma Membrane

Selectively permeable barrier separating intracellular fluid (ICF) from extracellular fluid (ECF).

Fluid Mosaic Model

Describes the membrane as a dynamic bilayer of phospholipids with proteins, cholesterol, and carbohydrates.

Phospholipid Bilayer

Double layer with hydrophilic heads facing water and hydrophobic tails facing inward.

Cholesterol in Membrane

Increases stability and reduces fluidity of the plasma membrane.

Integral Proteins

Embedded in the bilayer; function as channels, carriers, receptors, or enzymes.

Peripheral Proteins

Loosely attached to integral proteins; function as enzymes, motor proteins, and cell connectors.

Glycocalyx

Sugar coating of glycolipids and glycoproteins that aids in recognition and immune response.

Tight Junctions

Form impermeable seals that prevent molecules from passing between cells.

Desmosomes

Anchoring junctions that resist mechanical stress by linking cells like rivets.

Gap Junctions

Channels that allow ions and small molecules to pass directly between cells.

Passive Transport

Movement across the plasma membrane without energy, down the concentration gradient.

Diffusion

Movement of molecules from high to low concentration until equilibrium is reached.

Simple Diffusion

Direct movement of nonpolar or lipid-soluble molecules (e.g., O₂, CO₂, steroids) through the membrane.

Facilitated Diffusion

Movement of polar or large molecules with help of carrier or channel proteins.

Carrier-Mediated Diffusion

Transport using proteins that bind specific molecules, change shape, and move them across.

Channel-Mediated Diffusion

Movement through water-filled protein channels; includes leakage (always open) and gated (signal-controlled) channels.

Osmosis

Movement of water across a selectively permeable membrane through the bilayer or aquaporins.

Osmolarity

Total concentration of solute particles in a solution.

Hydrostatic Pressure

Pressure exerted by water against a membrane or wall.

Osmotic Pressure

Tendency of water to enter a solution due to solute concentration.

Tonicity

Ability of a solution to change a cell's volume by altering water movement.

Isotonic Solution

Equal solute concentration inside and outside; no net water movement.

Hypertonic Solution

Higher solute concentration outside; water exits the cell, causing it to shrink (crenation).

Hypotonic Solution

Lower solute concentration outside; water enters the cell, causing swelling and possible lysis.

Active Transport

Movement of substances against concentration gradients using ATP.

Primary Active Transport

Direct use of ATP to pump molecules (e.g., Na⁺/K⁺ pump).

Secondary Active Transport

Uses energy from ion gradients created by primary active transport; includes symporters (same direction) and antiporters (opposite directions).

Sodium-Potassium Pump

ATP-driven pump that moves 3 Na⁺ out and 2 K⁺ into the cell, maintaining electrochemical gradients.

Vesicular Transport

Movement of large substances via membrane sacs called vesicles; requires ATP.

Endocytosis

Transport into the cell by vesicle formation.

Phagocytosis

"Cell eating" of large particles by pseudopods; forms a phagosome (used by macrophages and WBCs).

Pinocytosis

"Cell drinking" of extracellular fluid; nonspecific uptake.

Receptor-Mediated Endocytosis

Selective uptake of specific molecules using receptor proteins.

Exocytosis

Vesicular release of materials (e.g., hormones, neurotransmitters, waste) from the cell.

Resting Membrane Potential (RMP)

Voltage across the plasma membrane at rest; inside is negative relative to outside (about -70 mV).

Role of K⁺ in RMP

K⁺ diffuses out of the cell, making the inside negative, until electrical forces balance concentration forces.

Role of Na⁺ in RMP

Na⁺ diffuses in slightly; Na⁺/K⁺ pump restores gradients.

Electrochemical Gradient

Combined effect of concentration and electrical forces driving ion movement.

Cell Adhesion Molecules (CAMs)

Proteins that anchor cells, assist in movement, attract WBCs, and transmit signals.

Membrane Receptors

Proteins that bind chemical signals (ligands) to trigger cellular responses.

Contact Signaling

Cells recognize each other by touching through membrane receptors.

Chemical Signaling

Ligands (e.g., hormones, neurotransmitters) bind to receptors, altering cell activity.

G Protein-Coupled Receptors

Indirectly activate enzymes or channels via G proteins, producing second messengers like cAMP or Ca²⁺.