Membrane Transport and Cell Signaling Study Notes

Chapter 5: Membrane Transport and Cell Signaling

Overview:

  • Plasma Membrane:   - Separates living cells from surroundings.   - Exhibits selective permeability allowing specific substances to cross more easily than others.

Concept 5.1: Cellular Membranes as Fluid Mosaics of Lipids and Proteins

Structure of Membranes

  • Phospholipids:   - Most abundant lipids in membranes.   - Amphipathic molecules: contain both hydrophobic (nonpolar) and hydrophilic (polar) regions.   - Form a phospholipid bilayer that acts as a stable boundary between two aqueous compartments.

  • Membrane Proteins:   - Also amphipathic and embedded in bilayer.   - Hydrophilic portions protrude from the bilayer.

Fluid Mosaic Model

  • Definition: The membrane is a mosaic of protein molecules embedded in a fluid bilayer of phospholipids.

  • Protein and Lipid Interaction: Groups of proteins or lipids can associate in specialized patches.

Membrane Fluidity

  • Movement:   - Lipids and some proteins can shift sideways; phospholipid movement is rapid, while protein movement is relatively slow.   - Some proteins are anchored, while others drift.

  • Effect of Temperature:   - Cool temperatures cause membranes to transition from a fluid to a solid state.   - Membrane fluidity depends on lipid composition; unsaturated hydrocarbon tails maintain fluidity at lower temperatures.

  • Role of Cholesterol:   - Effects fluidity at different temperatures:     - Warm Temperatures: Restrains phospholipid movement, reducing fluidity.     - Cold Temperatures: Keeps membranes fluid by preventing tight packing.

Evolution of Membrane Lipid Composition

  • Adaptations in lipid composition help organisms respond to environmental temperature changes.

Membrane Proteins and Their Functions

Types of Membrane Proteins

  • Integral Proteins:   - Penetrate the hydrophobic interior of the lipid bilayer, many are transmembrane.   - Hydrophobic regions typically consist of nonpolar amino acids often coiled into helices.

  • Peripheral Proteins:   - Loosely bound to membrane surfaces.

Major Functions of Membrane Proteins

  1. Transport

  2. Enzymatic Activity

  3. Signal Transduction

  4. Cell-Cell Recognition

  5. Intercellular Joining

  6. Attachment to Cytoskeleton and ECM

Role of Membrane Carbohydrates in Cell-Cell Recognition

  • Recognition occurs via binding to surface molecules (often carbohydrates) on the extracellular surface of membranes.

  • Carbohydrates can be covalently bonded to lipids (forming glycolipids) or proteins (forming glycoproteins).

  • Vary among species and individual cell types.

Synthesis and Sidedness of Membranes

  • Membranes exhibit asymmetrical arrangements determined during synthesis by the Endoplasmic Reticulum (ER) and Golgi apparatus.

Concept 5.2: Membrane Structure and Selective Permeability

Plasma Membrane Functions

  • Regulates transport of substances across cellular boundaries.

  • Selective Permeability: Only certain substances can cross the membrane easily.

Permeability of Lipid Bilayer

  • Hydrophobic Molecules: Easily cross the lipid bilayer (examples: hydrocarbons).

  • Polar Molecules: Do not cross easily (example: sugars).

  • Water: Does not cross easily compared to nonpolar molecules.

Transport Proteins

Types of Transport Proteins

  • Channel Proteins: Hydrophilic channels allow specific molecules or ions to cross (e.g., aquaporins for water).

  • Carrier Proteins: Bind to molecules and change shape to transport them across.

  • All transport proteins are specific to the substance they move.

Concept 5.3: Passive Transport and Diffusion

Passive Transport

  • Diffusion: Tendency for molecules to spread evenly into available space, with directional movement towards dynamic equilibrium.   - Substances move down their concentration gradient (from high to low concentration) without energy input.

Osmosis

  • Definition: Diffusion of free water across a selectively permeable membrane from lower to higher solute concentration until equilibrium is reached.

Tonicity and Effects on Cells

  • Tonicity Types:   1. Isotonic: No net water movement; solute concentrations are equal inside and outside of the cell.   2. Hypertonic: Higher solute concentration outside; cell loses water.   3. Hypotonic: Lower solute concentration outside; cell gains water.

  • Osmoregulation: Necessary adaptation for organisms in hypertonic or hypotonic environments. Example: Paramecium utilizes a contractile vacuole to pump excess water out of the cell.

Water Balance in Cells with Walls

Impact of the Environment

  • Plant Cells in Solutions:   - Hypotonic: Cells become turgid.   - Isotonic: Cells become flaccid.   - Hypertonic: Leads to plasmolysis (membrane pulls away from the wall, often lethal).

Facilitated Diffusion: Passive Transport Aided by Proteins

  • Facilitated Diffusion: Transport proteins aid in passive movement across the membrane.

Types of Facilitated Diffusion Proteins

  • Channel Proteins: Provide corridors for specific molecules.

  • Carrier Proteins: Undergo shape change triggered by binding and releasing transported molecules.

  • Both types require no net energy input.

Concept 5.4: Active Transport

Definition and Mechanism

  • Active Transport: Moves solutes against their concentration gradients, requires energy (usually from ATP).

  • Sodium-Potassium Pump: An example of active transport crucial for maintaining ion gradients in animal cells.

Membrane Potential and Electrochemical Gradient

  • Membrane Potential: Voltage difference across membranes caused by the distribution of anions and cations.

  • Electrogenic Pump: A transport protein generating voltage across a membrane, such as the sodium-potassium pump in animals and proton pumps in plants.

Cotransport

  • Definition: Coupled transport where the diffusion of one solute drives the transport of another solute against its gradient.

  • Example: Plant cells utilizing hydrogen ion gradients to drive nutrient transport into the cell.

Concept 5.5: Bulk Transport Across the Plasma Membrane

Mechanisms

  • Exocytosis: Vesicles fuse with the membrane to release contents outside.

  • Endocytosis: Formation of new vesicles from the plasma membrane, allowing the uptake of molecules.

  • Types:   1. Phagocytosis (cellular eating)   2. Pinocytosis (cellular drinking)   3. Receptor-mediated endocytosis: Specific for certain substances, examples include cholesterol uptake via low-density lipoproteins (LDLs).

Concept 5.6: Cell Signaling

Role of the Plasma Membrane

  • Crucial in cell-to-cell communication within multicellular organisms and also in unicellular organisms.

Local and Long-Distance Signaling

  • Local Signaling: Involves direct contact between cells or short-distance signaling through local regulators like growth factors.

  • Long-Distance Signaling: Hormonal signaling in animals (endocrine signaling) involves hormones released into the circulatory system.

The Three Stages of Cell Signaling

  1. Reception: Detection of signal via ligand binding to receptor.

  2. Transduction: Conversion of the signal into a cellular response through intracellular signaling pathways.

  3. Response: Cellular activity resulting from the signaling pathway.

Types of Receptors

  • Membrane Receptors: Bind water-soluble signal molecules. Two main types:   1. G Protein-Coupled Receptors: Activate intracellular signaling pathways.   2. Ligand-Gated Ion Channels: Open in response to ligand binding, allowing ion flow.

Intracellular Receptors

  • Found in the cytoplasm/nucleus; specific to small or hydrophobic chemical messengers (e.g., steroid hormones).

  • Example: Aldosterone's activation and impact on renal cells.

Transduction Mechanisms

  • Often involves phosphorylation cascades where protein kinases add phosphates to regulate activity.

  • Second Messengers: Small molecules aiding signaling within the cell (e.g., cyclic AMP, calcium ions).

Response Outcomes

  • Regulation of cellular activities may involve gene transcription or enzyme activity modulation. Final activated molecules often act as transcription factors.