Cell Structure, Membrane Transport, and Cell Junctions: Study Guide for Anatomy & Physiology

Cell Structure and Function

Definitions and Basic Concepts

• Cytology: The study of cells, their structure, function, and chemistry.

• Cell: The basic structural, functional, and biological unit of all living organisms.

• Extracellular: Refers to anything located outside the cell.

• Intracellular: Refers to anything located inside the cell.

Functions of Cells

• Obtain nutrients and other essential substances from the surrounding environment.

• Generate energy for cellular processes.

• Eliminate wastes.

• Synthesize proteins and other molecules necessary for cell structure and function.

Cell Membrane

Structure and Components

• Cell Membrane (Plasma Membrane): A selectively permeable barrier that surrounds the cell, composed mainly of a phospholipid bilayer with embedded proteins.

• Main Components:

  • Phospholipids

  • Proteins

• Phospholipid Bilayer: Phospholipids arrange themselves in two layers, with hydrophilic (water-loving) heads facing outward toward the watery environments and hydrophobic (water-fearing) tails facing inward, away from water.

Region	Composition	Properties
Polar region	Phosphate "head"	Hydrophilic (attracted to water)
Nonpolar region	Lipid "tails" (fatty acids)	Hydrophobic (repelled by water)

• Functions of the Cell Membrane:

  • Regulates the movement of substances into and out of the cell.

  • Provides protection and structural support.

  • Facilitates communication with other cells via receptor proteins.

  • Helps maintain the cell's internal environment (homeostasis).

Cell Membrane Proteins

• Receptor Proteins: Bind to specific molecules (ligands) and transmit signals into the cell.

• Enzymes: Catalyze chemical reactions at the membrane surface.

• Carrier Proteins: Bind and transport specific substances across the membrane.

• Channel Proteins: Form pores that allow specific ions or molecules to pass through the membrane.

Cell Junctions

Definition and Types

• Cell Junctions: Specialized structures that connect adjacent cells and coordinate their interactions.

• Types of Cell Junctions:

  • Tight Junctions: Seal adjacent cells together, preventing the passage of substances between them. Example location: Intestinal lining.

  • Gap Junctions: Allow direct communication between cells through channels that permit the passage of ions and small molecules. Example location: Cardiac muscle.

  • Desmosomes: (Additional info: Not explicitly listed, but commonly included) Provide strong adhesion between cells, especially in tissues subject to mechanical stress.

Membrane Transport

Passive Transport

• Definition: Movement of substances across the cell membrane without the use of cellular energy (ATP).

• Types:

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

  • Facilitated Diffusion: Movement of molecules across the membrane via channel or carrier proteins.

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

  • Filtration: Movement of fluid and solutes through a membrane due to hydrostatic pressure.

• Does passive transport require ATP? No.

Active Transport

• Definition: Movement of substances across the cell membrane using energy (ATP), often against a concentration gradient.

• Types:

  • Primary Active Transport: Direct use of ATP to transport molecules (e.g., sodium-potassium pump).

  • Secondary Active Transport: Uses the energy from the movement of one substance down its gradient to move another substance against its gradient.

  • Endocytosis: Process by which cells engulf large particles (phagocytosis) or liquids (pinocytosis).

  • Exocytosis: Process by which cells expel materials in vesicles.

• Does active transport require ATP? Yes.

Key Terms and Concepts

• Concentration Gradient: A difference in the concentration of a substance across a space or membrane.

• Solute: The substance dissolved in a solution.

• Solvent: The substance in which the solute is dissolved (usually water in biological systems).

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

• Isotonic Solution: No net movement of water; cell remains the same size.

• Hypotonic Solution: Water enters the cell; cell may swell and burst (lysis).

• Hypertonic Solution: Water leaves the cell; cell shrinks (crenation).

Membrane Transport Table

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to low concentration	Oxygen, carbon dioxide
Facilitated Diffusion	No	High to low concentration	Glucose via carrier protein
Osmosis	No	Water: high to low concentration	Water movement
Active Transport	Yes	Low to high concentration	Sodium-potassium pump
Endocytosis	Yes	Into cell	Phagocytosis, pinocytosis
Exocytosis	Yes	Out of cell	Secretion of hormones

Cell Organelles and Their Functions

Organelle	Location	Function
Ribosomes	Cytoplasm or attached to RER	Produce proteins
Rough Endoplasmic Reticulum (RER)	Cytoplasm, ribosomes attached	Protein synthesis and processing
Smooth Endoplasmic Reticulum (SER)	Cytoplasm, lacks ribosomes	Synthesizes lipids, detoxifies drugs/poisons
Golgi Apparatus	Cytoplasm, closely packed stacks	Collects, sorts, packages, and distributes proteins and lipids
Mitochondria	Cytoplasm	Produce ATP (energy)
Lysosomes	Cytoplasm	Digest and remove waste
Peroxisomes	Cytoplasm	Break down fatty acids and detoxify
Nucleus	Center of cell	Stores genetic material (DNA and RNA)
Nucleolus	In nucleus	Produces ribosomes
Centrioles	Near nucleus	Cell division
Microvilli	On cell surface, shorter than cilia	Increase surface area
Cilia	On cell surface, shorter than flagella	Move materials across cell surface
Flagella	On cell surface, longer than cilia	Move sperm

Cell Membrane Structure Diagram

• Phospholipid: Composed of a hydrophilic phosphate "head" and two hydrophobic fatty acid "tails".

• Arrangement: Heads face outward toward water; tails face inward, away from water.

• Hydrophilic region: Attracts water (phosphate head).

• Hydrophobic region: Repels water (fatty acid tails).

Equations and Key Concepts

• Simple Diffusion: $J = -D \frac{dC}{dx}$ Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

• Osmosis: $\pi = iMRT$ Where $\pi$ is osmotic pressure, $i$ is the van 't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature.

Examples and Applications

• Red Blood Cells in Solutions:

  • Isotonic: No net water movement; cells remain intact.

  • Hypotonic: Water enters cells; cells swell and may burst (lysis).

  • Hypertonic: Water leaves cells; cells shrink (crenation).

Summary Table: Tonicity Effects on Red Blood Cells

Solution Type	Water Movement	Cell Status
Isotonic	No net movement	Intact
Hypotonic	Into cell	Lysis (burst)
Hypertonic	Out of cell	Crenation (shrink)

Additional info: Desmosomes, equations, and some context for organelle functions were added for completeness.