Cell Membrane and Transport

Fluid Mosaic Model

  • The fluid mosaic model describes membranes as:

    • Phospholipid bilayers with embedded proteins.

    • Plasma membranes are typically 5 to 10 nanometers thick.

    • Structure consists of two sheets of phospholipids (bilayer).

    • Lipid bilayers form the foundation of cellular membranes.

    • Globular molecules (proteins) are integrated within the lipid bilayer.

Membrane Composition

  • Membrane exhibits a mosaic-like appearance due to the variety of embedded proteins.

  • Analogy compares a membrane to "boats on a pond" where:

    • Phospholipid serves as the bilayer.

    • Channel proteins facilitate passage of ions and molecules.

    • Carrier proteins assist in transport.

    • Glycoproteins are involved in cell recognition.

    • Cholesterol maintains membrane fluidity.

    • Phosphate heads: polar and hydrophilic.

    • Fatty acid tails: non-polar and hydrophobic.

Types of Membrane Proteins

  1. Integral Proteins

    • Span across the bilayer and are typically transmembrane.

    • Permanently attached to the membrane.

  2. Peripheral Proteins

    • Associate temporarily with the membrane's surface through non-covalent interactions.

Amino Acid Orientation in Proteins

  • Non-polar (hydrophobic) amino acids interact with the lipid bilayer.

  • Polar (hydrophilic) amino acids are located internally, facing aqueous environments.

Functions of Membrane Proteins

  • Membrane proteins fulfill various critical functions:

    • Junctions: Connect and join two cells together.

    • Enzymes: Involved in metabolic pathways.

    • Transport: Responsible for facilitated diffusion and active transport.

    • Recognition: Serve as markers for cellular identification.

    • Anchorage: Provide attachment points for cytoskeleton and extracellular matrix.

    • Transduction: Function as receptors for peptide hormones.

Visual Representation of Protein Functions

  • Outside the cell vs. inside cell comparison:

    • Transporter

    • Enzyme

    • Cell-surface receptor

    • Cell-surface identity marker

    • Cell-to-cell adhesion

    • Cytoskeleton anchor

Red Blood Cell Structure

  • Biconcave Shape:

    • Cooperation between embedded membrane proteins and the cytoskeleton creates a biconcave shape.

    • Increases surface area for gas exchange, improving oxygen and CO2 transport efficiency.

Protein Spectrin and Cell Shape

  • Spectrin protein serves as a scaffold:

    • Connects membrane proteins to actin filaments in the cytoskeleton.

    • Facilitates the formation of the biconcave shape of red blood cells.

Phospholipid Bilayer

  • Basic structure of every plasma membrane:

    • Composed of a double layer of phospholipids (bilayer).

    • Hydrophilic heads interact with aqueous solutions outside and inside the cell.

    • Hydrophobic tails remain away from water in the interior.

Formation of Phospholipid Bilayers

  • Phospholipid bilayers form around cells and organelles:

    • Micelle formation:

    • Phospholipids aggregate in water, forming bilayers.

    • When in aqueous environments, they create planar bilayers or liposomes.

Diffusion and Molecular Movement

  • Diffusion is driven by random molecular motion (Brownian movement):

    • Molecules dissolved in water are in constant motion.

    • Leads to molecules moving from areas of high concentration to low concentration.

Characterizing Osmosis

  • Osmosis is a specific type of diffusion involving water through a selectively permeable membrane:

    • Water molecules move based on solute concentration.

Types of Solutions Affecting Cells

  1. Hypertonic Solution:

    • Higher solute concentration compared to another solution.

    • Water moves out from the cell, causing it to shrink.

  2. Isotonic Solution:

    • Equal concentration of solutes.

    • No net movement of water; the cell maintains its shape.

  3. Hypotonic Solution:

    • Lower solute concentration.

    • Water moves into the cell, causing it to swell or rupture (lysis).

Impact on Cells in Different Solutions

  • Isotonic environment: Normal cell appearance.

  • Hypotonic environment: Cells swell.

  • Hypertonic environment: Cells shrink.

Membrane Transport Mechanisms

  • Transmembrane domains control entry and exit of water and molecules:

    • Passive Transport: No energy required.

    • Includes diffusion and facilitated diffusion.

    • Active Transport: Requires energy (ATP).

Facilitated Diffusion

  • Type of passive transport using specific transport proteins to move molecules across membranes.

Active Transport

  • Active transport moves substances against their concentration gradient:

    • Energy provided by ATP.

  • Example: Sodium-potassium pump:

    • Moves three Na+^{+} out of the cell for every two K+^{+} moved inside, generating a negative charge.

Sodium-Potassium Pump Steps

  1. Three sodium ions bind inside the cell.

  2. Protein pump uses ATP to change shape.

  3. Sodium is pumped outside the cell.

  4. Two potassium ions bind outside.

  5. Potassium is transported inside the cell.

  6. The process repeats, transporting up to 300 Na+^{+} per second.

Types of Bulk Transport

  • Endocytosis and Exocytosis are inverse processes for transporting bulky materials:

    • Endocytosis: Plasma membrane engulfs food particles and fluids.

    • Exocytosis: Substance release from the cell following packaging into vesicles.

Types of Endocytosis

  1. Phagocytosis: Engulfment of large particles.

  2. Pinocytosis: Engulfment of fluids and small particles.

  3. Receptor-mediated endocytosis: Specific uptake of molecules.

    • E.g., Lipoproteins like LDL bind with specific receptors to bring cholesterol into cells.

Examples of Exocytosis

  • In plant cells, used for exporting materials to build cell walls.

  • In animal cells, insulin is released into the bloodstream by exocytosis when blood sugar levels rise.