Cell Membranes and Transport

Structure and Function of the Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds the cell, maintaining the internal environment and mediating communication with the external environment.

• Phospholipid Bilayer: Composed of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

• Carbohydrates: Attached to proteins and lipids on the extracellular surface, forming the glycocalyx for cell recognition.

• Cholesterol: Stabilizes membrane fluidity.

Example: The sodium-potassium pump is an integral membrane protein that helps maintain cellular ion balance.

Ways Substances Cross the Membrane

Substances cross the plasma membrane via several mechanisms, each with specific vocabulary and characteristics.

• Passive Transport: Movement of substances without energy input.

  • Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) down their concentration gradient.

  • Facilitated Diffusion: Movement of larger or polar molecules via protein channels or carriers.

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

• Active Transport: Movement of substances against their concentration gradient, requiring energy (ATP).

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

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

• Vesicular Transport: Movement of large particles via vesicles (endocytosis and exocytosis).

Key Terms: Permeability, concentration gradient, carrier protein, channel protein, vesicle.

Plasma Membrane Pumps

Membrane pumps are proteins that use energy to move substances across the membrane against their concentration gradients.

• Sodium-Potassium Pump (Na+/K+ ATPase): Moves 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed.

• Calcium Pump: Moves Ca2+ out of the cytoplasm into the extracellular space or organelles.

Equation:

Endocytosis and Exocytosis

Endocytosis and exocytosis are forms of vesicular transport for large molecules or particles.

• Endocytosis: The process by which cells engulf substances into a pouch which becomes a vesicle.

  • Phagocytosis: "Cell eating"; uptake of large particles.

  • Pinocytosis: "Cell drinking"; uptake of fluids and dissolved substances.

  • Receptor-mediated Endocytosis: Specific uptake via receptor binding.

• Exocytosis: The process by which cells expel materials in vesicles that fuse with the plasma membrane.

Comparison Table:

Function of Cell Organelles

Cell organelles perform specialized functions necessary for cell survival and activity.

• Nucleus: Contains genetic material (DNA); controls cell activities.

• Mitochondria: Site of ATP production via cellular respiration.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes for breakdown of waste.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

Movement of Substances Through the Cell

Substances move through the cell via cytoplasmic streaming, vesicular transport, and along cytoskeletal elements.

• Cytoplasmic Streaming: Movement of cytoplasm facilitates distribution of materials.

• Vesicular Transport: Organelles exchange materials via vesicles.

• Microtubules and Motor Proteins: Transport vesicles and organelles along cytoskeletal tracks.

Tissues and Cell Junctions

Classification of Body Tissues

The human body is composed of four basic tissue types, each with distinct structure and function.

• Epithelial Tissue: Covers surfaces, lines cavities, forms glands.

• Connective Tissue: Supports, binds, and protects organs.

• Muscle Tissue: Produces movement.

• Nervous Tissue: Transmits electrical impulses.

Cell Junctions

Cell junctions are specialized structures that connect adjacent cells, providing communication and structural integrity.

• Tight Junctions: Seal adjacent cells to prevent passage of substances between them (e.g., intestinal lining).

• Desmosomes: Anchor cells together, providing mechanical strength (e.g., skin).

• Gap Junctions: Allow direct communication between cells via channels (e.g., cardiac muscle).

Comparison Table:

Classification of Epithelial Tissue

Epithelial tissues are classified by the number of cell layers and the shape of the cells.

• Number of Layers:

  • Simple Epithelium: Single layer of cells.

  • Stratified Epithelium: Multiple layers of cells.

• Cell Shape:

  • Squamous: Flat and thin.

  • Cuboidal: Cube-shaped.

  • Columnar: Tall and column-like.

Comparison Table:

Example: The simple squamous epithelium in the alveoli allows for rapid gas exchange.