Section 3: Membrane Structure and Function

Overview of Membrane Structure

Cellular membranes are essential components of all living cells, providing structure, protection, and the ability to regulate the movement of substances. The plasma membrane is often described as a fluid mosaic of lipids and proteins, reflecting its dynamic and heterogeneous nature.

• Fluid Mosaic Model: The membrane consists of a double layer (bilayer) of phospholipids with embedded proteins, carbohydrates, and cholesterol.

• Selective Permeability: The membrane allows some substances to cross more easily than others, maintaining the internal environment of the cell.

• Key Components:

  • Phospholipids: Form the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.

  • Proteins: Serve as channels, carriers, receptors, and enzymes.

  • Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids) for cell recognition.

Membrane Transport Mechanisms

Transport across the plasma membrane is vital for cell survival and function. There are several mechanisms by which substances move across membranes:

• Passive Transport: Movement of substances down their concentration gradient without energy input.

  • Simple Diffusion: Direct movement of small, nonpolar molecules (e.g., O2, CO2).

  • Facilitated Diffusion: Movement via specific transport proteins (e.g., glucose transporters).

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

  • Example: Sodium-potassium pump (Na+/K+ ATPase) exchanges Na+ and K+ ions across the membrane.

  • Equation:

• Bulk Transport: Movement of large molecules or particles via vesicles.

  • Exocytosis: Secretion of substances by fusion of vesicles with the plasma membrane (e.g., insulin release).

  • Endocytosis: Uptake of substances by forming vesicles from the plasma membrane.

    • Phagocytosis: "Cellular eating"—engulfing large particles.

    • Pinocytosis: "Cellular drinking"—ingesting extracellular fluid and dissolved solutes.

    • Receptor-mediated endocytosis: Specific uptake of molecules via receptor proteins (e.g., LDL cholesterol uptake).

Cotransport and Coupled Transport

Cotransport is a type of active transport where the movement of one solute down its concentration gradient drives the transport of another solute against its gradient, using a cotransporter protein.

• Mechanism: ATP is used indirectly to create an ion gradient (often H+ or Na+), which then powers the cotransport of another molecule (e.g., sucrose in plants).

• Example: H+/sucrose cotransport in plant cells loads sucrose produced by photosynthesis into the veins of leaves.

• Equation (for proton gradient): (used by proton pump to create H+ gradient)

Table: Common Prefixes and Suffixes in Membrane Biology

Examples and Applications

• Insulin Secretion: Pancreatic cells use exocytosis to release insulin into the bloodstream.

• Neurotransmitter Release: Nerve cells secrete neurotransmitters via exocytosis at synapses.

• Cholesterol Uptake: LDL cholesterol is taken up by receptor-mediated endocytosis; defects in this process can lead to hypercholesterolemia.

Section 4: A Tour of the Cell

Microscopy and Cell Study Techniques

Biologists use various microscopy techniques and biochemical methods to study cell structure and function.

• Light Microscopy (LM): Uses visible light and lenses to magnify specimens; allows observation of living cells.

• Electron Microscopy (EM): Uses electron beams for much higher resolution.

  • Transmission Electron Microscopy (TEM): Reveals internal structures of thin cell sections.

  • Scanning Electron Microscopy (SEM): Provides 3D images of cell surfaces.

• Cell Fractionation: Technique to separate cell components by size and density using centrifugation, allowing study of individual organelles.

Important Parameters in Microscopy

• Magnification: Ratio of image size to actual size (e.g., 1000X).

• Resolution: Minimum distance two points can be distinguished (e.g., 200 nm for light microscopy).

• Contrast: Difference in brightness between parts of the image; often enhanced by staining.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structural features.

• Prokaryotic Cells: Domains Bacteria and Archaea; lack a membrane-bound nucleus and organelles; generally smaller.

• Eukaryotic Cells: Domain Eukarya; have a nucleus and membrane-bound organelles; generally larger.

• Common Features: Plasma membrane, cytoplasm (with cytosol), chromosomes (DNA), and ribosomes.

• Key Differences:

  • Prokaryotes: Nucleoid region, no true organelles.

  • Eukaryotes: Membrane-bound nucleus, organelles (e.g., mitochondria, endoplasmic reticulum).

Cell Size and Surface Area-to-Volume Ratio

Cell size is limited by the need to efficiently exchange materials with the environment. The surface area-to-volume ratio is a critical factor:

• Formula: (for a cube of side s)

• Formula:

• Implication: As a cell grows, volume increases faster than surface area, limiting size.

Key Organelles and Their Functions

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

• Mitochondrion: Site of cellular respiration; converts energy from food into ATP.

• Chloroplast: Site of photosynthesis in plant cells; converts solar energy into chemical energy.

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis.

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

The Endomembrane System

The endomembrane system is a group of organelles that work together to modify, package, and transport lipids and proteins.

• Components: Nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Function: Regulates protein traffic and performs metabolic functions.

Mitochondria and Chloroplasts: Energy Conversion

• Mitochondria: Convert chemical energy from food into ATP via cellular respiration.

• Chloroplasts: Convert solar energy into chemical energy via photosynthesis (in plants and algae).

The Cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, organizes cell contents, and enables movement.

• Types of Fibers:

  • Microtubules: Hollow tubes; maintain cell shape, enable movement of organelles and chromosomes.

  • Microfilaments: Thin rods; involved in cell movement and muscle contraction.

  • Intermediate Filaments: Provide mechanical support for the cell.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Additional info: Some context and explanations have been expanded for clarity and completeness, including formulas and examples relevant to General Biology students.