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

Cells: The Living Units

Cell Theory

The cell is the fundamental structural and functional unit of life. All living organisms are composed of cells, and the activities of an organism depend on the combined activities of its cells.

• Cell: The smallest unit of life capable of independent function.

• Structure and Function: The structure of a cell determines its function; specialized subcellular structures (organelles) contribute to specific cellular activities.

• Biochemical Functions: The shape and organelles of a cell dictate its biochemical roles.

Cell Diversity

Human bodies contain over 200 different types of cells, which vary in size, shape, and subcellular components. These differences result in a wide variety of functions.

• Types of Cells: Examples include muscle cells, nerve cells, epithelial cells, etc.

• Functional Specialization: Each cell type is specialized for particular tasks.

Generalized Cell Structure

Despite diversity, most human cells share three basic structural components:

• Plasma Membrane: Flexible outer boundary that separates the cell from its environment.

• Cytoplasm: Intracellular fluid containing organelles responsible for various cellular functions.

• Nucleus: DNA-containing control center that regulates cell activities.

The Plasma Membrane

Functions

The plasma membrane acts as a selective barrier, separating intracellular fluid (ICF) from extracellular fluid (ECF). It regulates the movement of substances in and out of the cell and facilitates communication.

• Barrier: Separates ICF from ECF.

• Cell Recognition: Contains proteins that serve as cell identity markers and receptors for chemicals.

• Transport: Controls entry and exit of materials.

• Cell Membrane: Another term for plasma membrane.

Structure

• Lipid Bilayer: Composed of membrane lipids that form a flexible double layer.

• Membrane Proteins: Specialized proteins float through the lipid bilayer, forming a dynamic "fluid mosaic model." These proteins are not rigidly fixed and can move within the membrane.

• Phospholipids: Arranged with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward.

Membrane Proteins

• Integral Proteins: Firmly embedded in the membrane; may span the membrane (transmembrane proteins).

• Peripheral Proteins: Loosely attached to the membrane surface; often provide structural support or act as enzymes.

• Functions: Transport, enzymatic activity, cell recognition, and intercellular joining.

Cell Junctions

Cell junctions are specialized structures that connect cells to each other, allowing for communication and structural integrity.

• Tight Junctions: Integral proteins fuse adjacent cells to form an impermeable barrier, preventing movement of fluids and molecules between cells. Found in epithelial cells lining the digestive tract.

• Desmosomes: Anchoring junctions formed by linker proteins; provide mechanical strength, especially in tissues subject to stress (e.g., skin, heart muscle).

• Gap Junctions: Transmembrane proteins form channels (connexons) that allow small molecules and ions to pass directly from cell to cell. Important for communication in cardiac and smooth muscle.

Plasma Membrane Permeability

The plasma membrane is selectively permeable, allowing some substances to pass while restricting others.

• Passive Processes: Do not require energy (ATP).

• Active Processes: Require energy (ATP).

Passive Transport

Passive transport moves substances down their concentration or pressure gradients without energy input.

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

• Filtration: Movement of water and solutes across a membrane due to hydrostatic pressure.

Types of Diffusion

• Simple Diffusion: Hydrophobic substances (e.g., oxygen, carbon dioxide, fat-soluble vitamins) diffuse directly through the lipid bilayer.

• Facilitated Diffusion: Polar molecules (e.g., glucose, amino acids) are transported by carrier proteins or channels. Two types:

  • Carrier-mediated: Specific carriers transport molecules.

  • Channel-mediated: Channels allow ions or water to pass; water channels are called aquaporins.

• Osmosis: Diffusion of water through a selectively permeable membrane. Water moves from areas of lower solute concentration to higher solute concentration.

Tonicity

Tonicity describes the ability of a solution to change the shape or tone of cells by altering their internal water volume.

• Isotonic Solution: Same osmolarity as inside the cell; no net water movement.

• Hypertonic Solution: Higher solute concentration than the cell; water flows out, causing cell shrinkage (crenation).

• Hypotonic Solution: Lower solute concentration than the cell; water flows in, causing cell swelling and possibly lysis (bursting).

Active Transport

Active transport requires energy to move substances against their concentration gradients.

• Primary Active Transport: Direct use of ATP to transport solutes (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses ion gradients created by primary active transport to move other substances.

• Vesicular Transport: Movement of large particles or fluids via vesicles.

Types of Vesicular Transport

• Endocytosis: Cell engulfs substances by forming vesicles; includes phagocytosis ("cell eating") and pinocytosis ("cell drinking").

• Exocytosis: Vesicle merges with plasma membrane to release contents outside the cell (e.g., secretion of hormones, waste, mucus).

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Examples
Simple Diffusion	No	Down gradient	Oxygen, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions
Osmosis	No	Down gradient	Water
Primary Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Indirect (uses gradient)	Against gradient	Glucose, amino acids
Endocytosis/Exocytosis	Yes (ATP)	Bulk transport	Hormones, waste

Key Equations

• Fick's Law of Diffusion:

$J = -D \frac{dC}{dx}$

• Osmosis: Water moves from areas of low solute concentration to high solute concentration.

$\text{Water flow} \propto \text{difference in solute concentration}$

Examples and Applications

• Example: The Na+/K+ pump maintains cellular ion balance, crucial for nerve impulse transmission.

• Application: Understanding membrane transport is essential for pharmacology, physiology, and medical diagnostics.