Membrane Structure and Function

Overview

This chapter explores the composition, behavior, and functions of biological membranes. Understanding membrane structure is foundational for topics such as cell communication, cellular respiration, photosynthesis, nervous system signaling, and immune system recognition.

SECTION I — MEMBRANE STRUCTURE

Phospholipids: The Foundation of Membranes

• Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.

• Structure:

  • Hydrophilic head: Composed of a phosphate group and glycerol.

  • Hydrophobic tails: Consist of two fatty acid chains.

• In aqueous environments, phospholipids spontaneously self-assemble into structures such as bilayers, micelles, and vesicles due to thermodynamic favorability.

The Fluid Mosaic Model

• The fluid mosaic model describes the membrane as a dynamic structure with proteins embedded in or attached to a fluid lipid bilayer.

• Fluidity:

  • Phospholipids move laterally within the layer (common) and can flip-flop between layers (rare; requires flippase enzymes).

  • Factors affecting fluidity:

    • Temperature: Higher temperatures increase fluidity.

    • Unsaturated fatty acids: Increase fluidity due to kinks in tails.

    • Saturated fatty acids: Decrease fluidity by allowing tight packing.

    • Cholesterol: Acts as a fluidity buffer—prevents solidification at low temperatures and excessive fluidity at high temperatures.

• Importance of fluidity: Affects protein function, membrane permeability, cell signaling, and membrane fusion.

Membrane Asymmetry

• Each side of the membrane has a distinct composition.

• Carbohydrates are always found on the extracellular side.

• Lipids and proteins are inserted in the endoplasmic reticulum (ER), processed in the Golgi apparatus, and delivered to the membrane via vesicles, preserving asymmetry.

Membrane Proteins

• Integral proteins: Penetrate the hydrophobic core; often span the membrane (transmembrane); amphipathic.

• Peripheral proteins: Loosely bound to the membrane surface; often attached to the cytoskeleton.

• Functions of membrane proteins:

  1. Transport

  2. Enzymatic activity

  3. Signal transduction

  4. Cell–cell recognition

  5. Intercellular joining

  6. Attachment to cytoskeleton and extracellular matrix (ECM)

Membrane Carbohydrates

• Carbohydrates are covalently attached to proteins (glycoproteins) or lipids (glycolipids).

• Functions:

  • Cell recognition (e.g., immune system, blood type antigens)

  • Embryonic development

  • Pathogen recognition

SECTION II — MEMBRANE TRANSPORT

Selective Permeability

• The membrane allows some substances to cross more easily than others.

• Easily crosses: Nonpolar molecules (e.g., O2, CO2), small hydrophobic molecules.

• Needs help: Ions, polar molecules, large molecules.

Passive Transport

• Passive transport does not require energy.

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

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

• Tonicity: The effect of a solution on cell volume.

  • Animal cells:

    • Hypotonic: Cell lyses (bursts)

    • Isotonic: Cell is normal

    • Hypertonic: Cell shrivels

  • Plant cells:

    • Hypotonic: Turgid (ideal)

    • Isotonic: Flaccid

    • Hypertonic: Plasmolyzed

Facilitated Diffusion

• Still passive, but requires transport proteins.

• Channel proteins: Form hydrophilic channels (e.g., aquaporins, ion channels).

• Carrier proteins: Bind to molecules and change shape to shuttle them across; highly specific.

Active Transport

• Requires energy (usually ATP).

• Sodium–Potassium Pump:

  • Pumps 3 Na+ out and 2 K+ in per cycle.

  • Maintains membrane potential.

• Proton pumps: Found in plants, fungi, and bacteria; create electrochemical gradients.

Electrochemical Gradients

• Ion movement is driven by two forces:

• Chemical gradient: Difference in concentration.

• Electrical gradient: Difference in charge across the membrane.

• Together, these form the electrochemical gradient.

Cotransport

• Active transport of one molecule powers the transport of another.

• Examples:

  • H+/sucrose cotransport in plants

  • Na+/glucose cotransport in animal intestines

Bulk Transport

• Uses vesicles to move large quantities of substances.

• Exocytosis: Releases materials from the cell.

• Endocytosis: Takes in materials; three types:

  1. Phagocytosis: Engulfing large particles.

  2. Pinocytosis: Engulfing extracellular fluid.

  3. Receptor-mediated endocytosis: Specific uptake using receptors (e.g., LDL receptors, clathrin-coated pits). Defects can cause diseases such as familial hypercholesterolemia.

SECTION III — WATER POTENTIAL (AP BIO EXTENSION)

Water Potential

• Water potential (Ψ) predicts the direction water will move.

• Formula:

• Solute potential (Ψs): Becomes more negative as solute concentration increases.

• Pressure potential (Ψp): Positive in turgid (firm) cells.

• Water moves from regions of higher Ψ to lower Ψ.

SECTION IV — FIGURE-BY-FIGURE EXPLANATIONS

• Amphipathic phospholipid: Shows hydrophilic head and hydrophobic tails.

• Fluid mosaic model: Proteins float in a sea of lipids.

• Fluidity factors: Unsaturated tails prevent packing; cholesterol stabilizes membrane.

• Freeze-fracture: Membrane splits along hydrophobic interior; proteins become visible.

• Protein functions: Illustrates six major roles of membrane proteins.

• Carbohydrates: Always face the extracellular side.

• Selective permeability: Nonpolar molecules cross easily; ions do not.

• Diffusion: Movement down a concentration gradient.

• Osmosis: Water moves toward higher solute concentration.

• Tonicity: Effects of hypo-, iso-, and hypertonic solutions on animal and plant cells.

• Facilitated diffusion: Comparison of channels and carriers.

• Active transport: ATP changes protein shape to move substances.

• Na+/K+ pump: 3 Na+ out, 2 K+ in.

• Cotransport: H+ gradient drives sucrose uptake.

• Endocytosis/exocytosis: Vesicle formation and fusion with the membrane.

SECTION V — COMMON MISCONCEPTIONS

• Water moves toward higher solute concentration, not specifically toward 'salt.'

• Active transport is different from facilitated diffusion (active transport requires energy).

• Hypertonic does not mean 'lots of water'—it means higher solute concentration outside the cell.

• Cholesterol does not always increase fluidity; it stabilizes membrane fluidity depending on temperature.

• Carbohydrates are never found on the cytoplasmic side of the membrane.

SECTION VI — EXAM-STYLE QUESTIONS

• Multiple Choice Examples:

  1. Which molecule crosses the membrane most easily?

  2. What happens to a plant cell in a hypertonic solution?

  3. Which transport process requires ATP?

  4. What is the role of cholesterol?

  5. Why is receptor-mediated endocytosis specific?

• Free Response Examples:

  • Explain how membrane structure results in selective permeability.

  • Describe how the Na+/K+ pump maintains membrane potential.

  • Predict the movement of water given solute concentrations.

Summary Table: Types of Membrane Transport

Additional info: This guide expands on the original notes by providing definitions, examples, and a summary table for clarity and exam preparation.