Chapter 5

Chapter 5: Membrane Structure and Transport

5.1 Membrane Structure

  • Membranes are essential biological structures that serve as barriers and participate in various cellular functions.

5.2 Synthesis of Membrane Components in Eukaryotic Cells

  • Membrane components are synthesized in the endoplasmic reticulum (ER) and Golgi apparatus, and inserted into the cell membrane.

5.3 Membrane Transport

  • Membrane transport refers to the mechanisms that control the movement of substances across biological membranes.

Biological Membranes

  • Biological membranes, also known as cellular or biomembranes, have a basic framework of the phospholipid bilayer.

  • Phospholipids: Amphipathic molecules with a hydrophobic region that faces inward and a hydrophilic region that faces outward.

  • Membranes contain proteins and carbohydrates; their relative amounts vary.

Fluid-Mosaic Model

  • Membranes are described as a mosaic of lipids, proteins, and carbohydrates.

  • The fluid-mosaic model states that lipids and proteins within the membrane can move relatively to each other, allowing for fluidity in structure.

Integral Membrane Proteins

  • Integral Membrane Proteins:

    1. Transmembrane Proteins: Embedded within the hydrophobic region of the bilayer.

    2. Lipid Anchors: Covalently attached lipids that attach to an amino acid in a protein.

  • Peripheral Membrane Proteins: Non-covalently attached to integral proteins or bound to the phospholipid head groups, carrying out various functions.

Biological Importance

  • Membranes are crucial for biological and medical processes; they house proteins that may represent 20-30% of all genes across all life forms.

  • Exploring membrane proteins may enhance our understanding of biological processes and lead to therapeutic advancements.

Membrane Fluidity

  • Membranes are semifluid, with lipids capable of lateral and rotational movement, aiding their function.

  • Movement does not require energy; lipid molecules can swap positions rapidly.

  • Flip-flop movement of lipids is not spontaneous and requires ATP-driven enzymes (flippases).

Factors Affecting Fluidity

  • Fatty Acid Tail Length: Shorter tails enhance fluidity, while longer tails reduce it.

  • Presence of Double Bonds: Unsaturated fats create kinks in tails, increasing fluidity.

  • Cholesterol: Stabilizes membranes; reduces fluidity at high temperatures and increases it at low temperatures.

Lateral Transport Experiments

  • Frye and Edidin Experiment: Demonstrated lateral movement of membrane proteins in fused mouse and human cells; protein localization at different temperatures was observed (0°C vs. 37°C).

Movement Limitations of Membrane Proteins

  • 10-70% of membrane proteins may be restricted in movement due to attachments to the cytoskeleton or extracellular matrix.

Glycosylation

  • The process of adding carbohydrates to proteins (glycoproteins) or lipids (glycolipids) impacts their function and location in the cell.

  • Carbohydrates serve as recognition signals for other proteins and protect cells (glycocalyx).

Visualization of Membranes

  • Transmission Electron Microscopy (TEM): Used to observe thin biological samples, revealing the structure of membranes using heavy metal dyes.

Transmembrane Protein Insertion in the ER

  • Transmembrane proteins begin their synthesis in the ER membrane; hydrophobic segments create transmembrane regions as a polypeptide is synthesized.

Forms of Glycosylation

  • N-linked and O-linked glycosylation occur in eukaryotes, aiding in protein folding and sorting functions.

Plasma Membrane Permeability

  • The phospholipid bilayer acts as a selective permeability barrier, mainly restricting ions and hydrophilic molecules while allowing specific molecules to pass.

  • Selectively permeable membranes regulate the internal environment of the cell by facilitating the entry of essential molecules and removing waste.

Process of Diffusion

  • Diffusion: Movement of molecules from areas of higher to lower concentration to reach equilibrium, occurring passively without energy input.

Membrane Permeability to Ions and Hydrophilic Molecules

  • Biological membranes maintain distinct internal environments, allowing gradients (concentration, ion electrochemical) to establish across membranes.

Transport Mechanisms

  • Passive Transport: Movement across membranes without energy input, either through passive diffusion or facilitated diffusion.

  • Active Transport: Movement against concentration gradients, requiring energy (ATP) and carrier proteins.

Osmosis

  • Osmosis: Movement of water across selectively permeable membranes, driven by solute concentration differences, an essential process for maintaining cell turgor and pressure.

Tonicity of Solutions

  • Isotonic: Equal solute concentrations, leading to no net water movement.

  • Hypertonic: Higher external solute concentration causes cells to shrink.

  • Hypotonic: Lower external solute concentration leads to cell swelling and potential lysis.

Osmosis in Animal and Plant Cells

  • In animal cells, isotonic and hypertonic solutions cause different cell responses, while in plant cells, osmotic pressure maintains turgor, preventing wilting.

Transport Proteins in Selective Permeability

  • Membranes contain two classes of transport proteins: channels (form passages for facilitated diffusion, e.g. aquaporins) and transporters (assist molecules in crossing).

  • Transporters undergo conformational changes to move molecules across the membrane and can operate as uniporters, symporters, or antiporters.

Pumps and Active Transport

  • Pumps actively transport ions or molecules by coupling energy from ATP hydrolysis; crucial for maintaining gradients (e.g. Na+/K+ pump).

Large Macromolecule Transport

  • Exocytosis: The process of vesicles fusing with the plasma membrane to release contents externally.

  • Endocytosis: The invagination of the plasma membrane to form vesicles that ingest extracellular material, occurring through various methods such as phagocytosis and receptor-mediated endocytosis.