Membranes and Membrane Transport

B2.1 Membranes and Membrane Transport

Syllabus Overview

  • First exam scheduled for 2025

Key Terminologies

  • Alpha-helix protein: A common structural motif in proteins characterized by a right-handed coiling of the polypeptide chain.

  • Oligosaccharide: A carbohydrate that consists of a small number of simple sugars (monosaccharides) linked together.

  • Side chain: The part of an amino acid that is not involved in the formation of peptide bonds.

  • Glycolipid: A lipid with a carbohydrate attached, typically found on the extracellular side of cell membranes.

  • Globular protein: A spherical protein that is soluble in water and plays various functions in cells.

  • Phospholipid: A lipid consisting of a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails; fundamental components of cell membranes.

  • Hydrophobic: Refers to molecules that do not interact with water, typically composed of nonpolar substances.

  • Cholesterol: A type of lipid that is a component of cell membranes and helps maintain fluidity and integrity.

Topic Objectives

  1. B.2.1.1 Lipid bilayers as the basis of cell membranes

  2. B.2.1.2 Lipid bilayers as barriers

  3. B.2.1.3 Simple diffusion across membranes

  4. B.2.1.4 Integral and peripheral proteins in membranes

  5. B.2.1.5 Movement of water molecules across membranes by osmosis and the role of aquaporins

  6. B.2.1.6 Channel proteins for facilitated diffusion

  7. B.2.1.7 Pump proteins for active transport

  8. B.2.1.8 Selectivity in membrane permeability

  9. B.2.1.9 Structure and function of glycoproteins and glycolipids

  10. B.2.1.10 Fluid mosaic model of membrane structure

  11. B.2.1.11 Relationships between fatty acid composition of lipid bilayers and their fluidity

  12. B.2.1.12 Cholesterol and membrane fluidity in animal cells

  13. B.2.1.13 Membrane fluidity and the fusion and formation of vesicles

  14. B.2.1.14 Gated ion channels in neurons

  15. B.2.1.15 Sodium–potassium pumps as an example of exchange transporters

  16. B.2.1.16 Sodium-dependent glucose cotransporters as an example of indirect active transport

Guiding Questions

  • How do molecules of lipid and protein assemble into biological membranes?

  • What determines whether a substance can pass through a biological membrane?

B.2.2.1 Lipid Bilayers as the Basis of Cell Membranes

Phospholipids

  • Consist of a hydrophilic phosphate head and two hydrophobic hydrocarbon tails.

  • Classed as amphipathic molecules due to the presence of both hydrophilic and hydrophobic properties.

Phospholipid Bilayer

  • Arranges in bilayers in the presence of water:

    • Polar heads face outward towards the water.

    • Non-polar tails face inward, away from water.

  • Fluidity: Phospholipids can exchange positions in the horizontal plane but not in the vertical plane, contributing to membrane fluidity.

  • Acts as a barrier to all but the smallest molecules (e.g., oxygen, carbon dioxide).

B.2.1.2 Lipid Bilayers as Barriers

Barrier Properties

  • Hydrophobic Chains and Permeability:

    • Composed of hydrocarbons.

    • Low affinity for water, contributing to hydrophobicity.

  • Permeability to Large Molecules:

    • Limited permeability due to hydrophobic nature of the membrane.

    • Effective barrier to large molecules.

  • Exclusion of Hydrophilic Particles:

    • Ions and polar molecules are impeded by the hydrophobic core, resulting in low permeability.

  • Cellular Integrity:

    • Membrane acts as a protective boundary for cellular components, essential for maintaining cellular homeostasis.

B.2.1.10 Fluid Mosaic Model of Membrane Structure

Fluid Mosaic Model Explained

  1. Fluidity:

    • The lipid bilayer is fluid, allowing for lateral movement of molecules.

    • This flexibility is crucial for membrane function.

  2. Mosaic Structure:

    • Composed of varied molecules creating a mosaic appearance.

    • Contains proteins, cholesterol, and carbohydrates:

      • Proteins may be embedded within the bilayer or attached externally.

      • Carbohydrates typically form glycoproteins or glycolipids on the extracellular surface.

  • Proposed by: S.J. Singer and Garth L. Nicolson in 1972.

Student Expectations

  • Ability to draw a two-dimensional representation of the fluid mosaic model including:

    • Peripheral and integral proteins.

    • Glycoproteins, phospholipids, and cholesterol.

    • Designate hydrophobic and hydrophilic regions.

B.2.1.9 Structure and Function of Glycoproteins and Glycolipids

Structure and Location

  • Glycoproteins:

    • Proteins with carbohydrate structures attached.

    • Primarily found on the extracellular side of membranes.

  • Glycolipids:

    • Lipids with attached carbohydrates located on the extracellular side.

Roles in Cell Function

  • Cell Adhesion:

    • Glycoproteins are significant for facilitating interactions between cells, promoting tissue integrity.

  • Cell Recognition:

    • Glycoproteins and glycolipids are involved in the immune responses, crucial for self/non-self recognition.

  • Importance in Membrane Function:

    • Contribute to membrane fluidity and flexibility.

    • Involved in signal transduction and cell signaling processes.

  • Carbohydrate Structures:

    • Covalent linkages to proteins (glycoproteins) or lipids (glycolipids) increase membrane diversity.

B.2.1.4 Integral and Peripheral Proteins in Membranes

Membrane Proteins

Integral Proteins

  • Embedding in Lipid Layers:

    • Embedded in one or both lipid layers, often spanning the entire membrane (transmembrane proteins).

    • Examples: channels, transporters, and receptors.

  • Functions:

    • Facilitate transport of molecules across the membrane.

    • Act as receptors for signaling molecules.

    • Contribute to structural integrity of the membrane.

Peripheral Proteins

  • Attachment to Bilayer Surface:

    • Associated with one surface of the lipid bilayer through interactions with integral proteins or lipid head groups.

  • Functions:

    • Serve as enzymes aiding in specific chemical reactions.

    • Act as signaling molecules or facilitate cell adhesion.

    • Aid in changing membrane shape during cell division.

B.2.1.8 Selectivity in Membrane Permeability

Types of Membrane Transport

  • Simple Diffusion:

    • Non-selective; determined by the properties of particles.

  • Facilitated Diffusion and Active Transport:

    • Selective processes essential for maintaining cellular homeostasis.

B.2.1.3 Simple Diffusion Across Membranes

Diffusion Explained

  • Basic Mechanism:

    • Net passive movement of particles from high to low concentration.

    • Non-selective, based on particle size and hydrophilic/hydrophobic properties.

  • Permeability Characteristics:

    • Small, nonpolar particles diffuse more readily through the membrane.

B.2.1.6 Channel Proteins for Facilitated Diffusion

Facilitated Diffusion

  • Definition:

    • Passive transport through protein channels or carriers; selective process facilitated by specific transport proteins.

  • Permeability Factors:

    • Depends on specific transport proteins.

    • Includes ion channels and carrier proteins.

    • Channels: - Passive transport forming open pores for specific ions or molecules; highly selective and regulated.

    • Carriers:

    • Undergo conformational changes to transport molecules; selective and regulated based on binding sites.

Protein Channels

Structure and Mechanism

  • Structure:

    • Integral membrane proteins forming pores in the lipid bilayer, often with specific shapes for certain ions/molecules.

  • Mechanism:

    • Facilitate passive transport through hydrophilic pathways along concentration gradients.

    • Selectivity Attribute:

    • Based on size, charge, and chemical nature of the molecules.

  • Regulation:

    • Many channels can be regulated by signals (e.g., voltage changes, ligand binding).

    • Examples: Sodium (Na+), potassium (K+), and calcium (Ca2+) channels.

Carrier Proteins

Structure and Mechanism

  • Structure:

    • Integral proteins that undergo conformational changes during transport.

  • Mechanism:

    • Includes binding, conformational change, and release of molecules across the membrane.

  • Selectivity:

    • Binding sites dictate selectivity for transported molecules.

  • Regulation:

    • Carriers are subject to regulations based on molecule concentration or signals.

    • Examples: Glucose transporters (GLUT) and sodium-potassium pumps.

B.2.1.5 Water Movement and Osmosis

Osmosis

  • Definition:

    • Passive movement of water across a selectively permeable membrane from low to high solute concentration.

  • Role of Aquaporins:

    • Specialized proteins that facilitate efficient water transport across membranes.

  • Biological Context:

    • Explanation of osmotic movement in various cells, including root hair cells and cells in the kidney.

B.2.1.7 Pump Proteins for Active Transport

Active Transport Overview

  • Mechanism:

    • Movement of particles against their concentration gradient through integral membrane proteins (pump proteins).

    • Requires energy, primarily from ATP via cellular respiration.

Pump Proteins

  • Definition:

    • Integral membrane proteins facilitating active transport.

  • Energy Use:

    • ATP hydrolysis provides necessary energy for conformational changes enabling transport.

  • Examples:

    • Sodium-Potassium Pump: Moves 3 sodium ions out and 2 potassium ions into cells against their gradients.

    • Proton Pump: Transports protons (H+) against concentration gradients.

AHL B.2.1.11 Fatty Acid Composition and Membrane Fluidity

Saturated vs. Unsaturated Fatty Acids

  • Differences:

    • Saturated fatty acids: No double bonds between carbons, typically solid at room temperature.

    • Unsaturated fatty acids: Possess one or more double bonds, usually liquid at room temperature (cis formation).

  • Impact on Membrane:

    • Unsaturated fatty acids increase fluidity whereas saturated fatty acids decrease it, affecting permeability and melting points.

AHL B.2.1.12 Cholesterol and Membrane Fluidity in Animal Cells

Cholesterol's Role

  • Distribution:

    • Randomly distributed across the phospholipid bilayer.

  • Functionality:

    • Maintains bilayer fluidity in various environmental contexts.

    • Prevents excessive phospholipid separation or compaction, facilitating substance movement across membranes.

AHL B.2.1.13 Membrane Fluidity, Fusion, and Vesicle Formation

Mechanisms of Vesicle Formation

  • Vesicles:

    • Small spheroidal packages arising from various cell membrane components, e.g., RER, Golgi apparatus.

  • Endocytosis:

    • Process of taking external substances into cells through inward membrane pouching.

  • Exocytosis:

    • The process of releasing substances from cells upon vesicle fusion with the plasma membrane.

AHL B.2.1.14 Gated Ion Channels in Neurons

Functionality of Ion Channels

  • Facilitated Diffusion:

    • Allows ions to move across membranes, with movement from high to low concentrations through ion channels.

  • Reversible Opening/Closing:

    • Channels can open/close to control ion movement and are influenced by action potentials.

  • Action Potential Propagation:

    • Sodium ions flow into axons, changing interior potential positively; consequent structural changes block further ingress of ions.

AHL B.2.1.15 Sodium-Potassium Pumps as Exchange Transporters

Sodium-Potassium Exchange Mechanism

  • Transport Protein Functionality:

    • Transmembrane proteins aiding in the movement of ions across biological membranes.

  • Sodium-Potassium Pump:

    • Exchanges 3 sodium ions out of the cell for 2 potassium ions into the cell.

  • Mechanisms involved:

    • Sodium ions bind to intra-cellular sites.

    • ATP hydrolysis induces a conformational change, allowing sodium transport.

    • Expose potassium binding sites extracellularly as the phosphate group is released, completing the ion exchange.

AHL B.2.1.16 Sodium-Dependent Glucose Cotransporter Proteins

Cotransport Mechanism

  • Transport Process:

    • Sodium ions and glucose molecules are transported into a cell simultaneously.

    • Glucose moves against its concentration gradient while sodium moves down its gradient.

  • Significance:

    • Facilitates glucose reabsorption in kidneys.

AHL B.2.1.17 Adhesion of Cells to Form Tissues

Tissues Overview

  • Tissue Definition:

    • Groups of cells collaborating for specific functions, often comprising multiple cell types.

  • Four Basic Tissue Types in Humans:

    • Connective tissue: Provides support and binds other tissues (e.g., bone, blood).

    • Epithelial tissue: Covers surfaces (e.g., skin, linings of internal passages).

    • Muscle tissue: Responsible for movement (includes striated and smooth muscle).

    • Nerve tissue: Consisting of neurons for carrying electric impulses.

Cell Adhesion and Junctions

  • Cell Junctions:

    • Facilitate adhesion and anchoring to the extracellular matrix using Cell Adhesion Molecules (CAMs).

Cancer Metastasis and Loss of Cell Adhesion

  • Cancer Metastasis Process:

    • Involves cancer cell detachment from primary tumors and new tumor formation in distant tissues.

    • Loss of CAM functionality leads to adhesion loss, allowing cancer cell mobility and spread.