Copy of Cell Function. Water Balance & Communication Notes

Current Model of the Plasma Membrane

• Cutaway view of animal cell's plasma membrane:

  • Lipids: gray and gold.

  • Proteins: purple.

  • Carbohydrates: green.

Fluid Mosaic Model

• Key Features:

  • Selectively (semi) permeable: Allows certain substances to pass while blocking others.

  • Fluidity: Membrane components can move laterally, maintaining the flexible structure.

  • Sidedness & Synthesis: Different compositions on inner and outer layers of the membrane.

  • Cholesterol: Stabilizes membrane fluidity across temperature fluctuations.

  • Glycolipids and Glycoproteins: Critical for cell-cell recognition and adhesion.

Cell Function

• Water Balance:

  • Regulation of water movement in and out of cells.

• Communication:

  • Signaling mechanisms involving membrane receptors are crucial for cell communication.

Phospholipid Structure and Properties

• Amphipathic Nature:

  • Contains both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts.

Make Connections

• Hydrophilic and Hydrophobic Portions:

  • Circle the hydrophilic (polar heads) and hydrophobic (fatty acid tails) areas on the phospholipid diagram.

  • Interactions: Hydrophilic heads face outward towards the water, hydrophobic tails face inward, shielding themselves from water.

Membrane Fluidity

• Types of Movement:

  • Transverse Diffusion (Flip-Flop): Movement from one side of the membrane to the other, slow process due to energy requirement.

  • Lateral diffusion: Rapid movement within the same membrane layer.

Control of Fluidity

• Factors Influencing Fluidity:

  • Composition of fatty acid chains and cholesterol presence.

  • Fatty Acid Characteristics:

    • Order and Rigidity: Straight, saturated fatty acids lead to less fluidity.

    • Bends and Kinks: Unsaturated fatty acids increase fluidity by preventing tight packing.

• Role of Cholesterol:

  • Stabilizes and maintains fluidity by preventing fatty acid chains from packing tightly.

Evolution and Membrane Composition

• Adaptation Examples:

  • Antarctic Fishes: Have unsaturated lipids maintaining fluidity even in freezing temperatures.

  • Winter Wheat: Alters membrane lipid composition for fluidity advantages during cold seasons.

Key Points

1. Adaptation strategies to maintain membrane functionality in extreme conditions.

2. Understanding implications for nutrient transport and signal reception in diverse environments.

Membrane Proteins

• Types of Membrane Proteins:

  • Integral Proteins: Span the membrane, involved in transport and signaling.

  • Peripheral Proteins: Loosely attached, often act as enzymes or signal transducers.

Structure of Transmembrane Proteins

• Example: Bacteriorhodopsin

  • Structure: Contains seven transmembrane helices with hydrophilic segments.

  • Orientation: N-terminus outside the cell, C-terminus inside, facilitating function.

Membrane Protein Functions

• Cell Communication and Signaling:

  • Transmembrane proteins can bind extracellular molecules and transmit signals into the cell, a vital function for cellular response mechanisms.

Synthesis and Transport

• Protein Processing in the Endoplasmic Reticulum (ER):

  • Proteins and lipids are synthesized and modified in the ER.

  • Glycoproteins and glycolipids are formed for recognition.

Transport Mechanism

• Vesicle Formation:

  • Glycoproteins and glycolipids are packaged into vesicles for transport to the plasma membrane.

  • Exocytosis releases secretory proteins outside the cell, maintaining cell integrity and function.

Membrane Transport

• Permeability Factors:

  • Permeable: Small, non-polar solutes (e.g., O2, CO2).

  • Impermeable: Large molecules like sugars and ions.

Types of Transport

• Passive Transport:

  1. Diffusion: Movement along gradient.

  2. Facilitated Diffusion: Through specific transport proteins, including aquaporins for water transport.

• Active Transport:

  • Requires Energy, moving substances against their concentration gradients via pumps (e.g., Na+/K+ pump).

Osmoregulation in Cells

• Effects of Solute Concentration:

  • Animal Cells: Lack cell walls, sensitive to osmotic imbalances (isotonic, hypertonic, hypotonic scenarios).

  • Plant Cells: Have cell walls, able to maintain structure under varying osmotic conditions.

Key Considerations

• Water Potential Calculations:

  • Includes solute and pressure potential components, critical for understanding water movement and plant health.

Active and Passive Mechanisms of Transport

• Active Transport Mechanisms:

  • Accumulates essential nutrients or removes waste against gradients, with ATP as energy source.

• Proton Pumps:

  • Generate voltage across membranes, critical for various cellular functions.

Cell Communication Mechanisms

• Local and Long-Distance Signaling:

  • Involves specific receptors and signaling pathways, leading to cell responses.

Quorum Sensing in Bacteria

• Definition: Regulation of gene expression based on population density via autoinducers, impacting virulence and adaptation mechanisms.

Hormonal Signaling and Responses

• Hormonal Effects:

  • Vary based on signaling pathways and receptor types, leading to diverse cellular responses in target tissues.

Insights into Neurotransmission

• Membrane Potential Dynamics:

  • Importance of Na+/K+ pump in establishing gradients for action potentials.

Summary**

• Membrane Structure and Function:

  • Understanding biological membranes, their components, and functions is fundamental to cell biology—for transport, signal reception, and communication.