Study-Unit-2-lecture-notes-2025

Lecture Unit 2: Cell Membranes

Overview of Cell Membranes

• Two distinct environments: the inside and outside of the cell.

Amphipathic Molecules

Definition

• Amphipathic molecules contain both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.

Structure

• Hydrophilic "Head": Contains a phosphate group, which has a negative charge, making it polar and soluble in water.

• Hydrophobic "Tail": Composed of long hydrocarbon chains (fatty acids) that are nonpolar and repel water.

Phospholipid Example

• Phospholipids: The most common amphipathic molecule in cell membranes.

  • Structurally, they have a phosphate group attached to a glycerol that connects to two fatty acid tails.

Phospholipid Bilayer

Formation in Aqueous Environments

• In water, phospholipids arrange themselves into a bilayer where the hydrophilic heads face outward toward the water, and the hydrophobic tails point inward, away from the water.

Properties of the Bilayer

• Provides structural stability and defines the boundary of cells.

• Acts as a barrier for the passage of hydrophilic substances, allowing selective permeability.

Fluid Mosaic Model

Concept

• Describes the cell membrane structure: a mosaic of diverse components such as lipids, proteins, and carbohydrates that float in or on the fluid lipid bilayer.

Components

• Lipids: Primary structure and barrier to polar molecules and ions.

• Proteins: Vary in type and presence; some are integral (spanning the bilayer) while others are peripheral (attached to the surface).

Fluidity Characteristics

• Components can move laterally within the layer.

• Variability in composition between different types of membranes (e.g., lipid-rich, protein-rich).

Membrane Fluidity

Factors Influencing Fluidity

1. Fatty Acid Chain Length: Shorter chains increase fluidity.

2. Degree of Saturation: Unsaturated fats prevent tight packing, enhancing fluidity.

3. Cholesterol Content: Cholesterol can both stabilize and fluidize the membrane, depending on its concentration.

4. Temperature: Higher temperatures increase fluidity due to increased kinetic energy.

Adaptations to Temperature Changes

• Organisms can adjust their membrane lipid composition to maintain fluidity under varying temperature conditions. For example, increasing unsaturated lipid levels in colder conditions enhances fluidity.

Membrane Proteins

Types of Membrane Proteins

• Integral Proteins: Embedded within the bilayer, facilitating transport and cellular signaling.

• Peripheral Proteins: Loosely attached to the exterior or interior surfaces, playing roles in signaling and structural integrity.

• Anchored Membrane Proteins: Covalently attached to lipids that insert into the membrane.

• Transmembrane Proteins: Span the entire membrane, allowing transport and intercellular communication.

Role in Functionality

• Membrane proteins are crucial for various functions including transport, enzymatic activity, and signal transduction, maintaining the asymmetrical nature of the membrane.

Membrane Carbohydrates

Functions

• Serve as recognition sites, facilitating cell-cell interactions and adhesion.

• Glycoproteins and Glycolipids: Carbohydrates attached to proteins and lipids that assist in identification and signaling.

Impact of Changes

• Alterations in carbohydrate composition can signal malignant transformations in cells.

Cell Junctions

Types of Cell Junctions

1. Tight Junctions: Prevent leakage of materials between cells.

2. Desmosomes: Anchor adjacent cells together, providing structural integrity to tissues under physical stress.

3. Gap Junctions: Allow the rapid exchange of ions and small molecules between neighboring cells, crucial in cardiac tissue for synchronized contractions.

Homeostasis

Definition

• The maintenance of stable internal conditions suitable for cellular function despite external environmental changes.

Mechanism

• Homeostasis requires a well-regulated cellular environment, facilitated by the selective permeability of membranes, allowing cells to maintain optimal conditions by regulating nutrient intake and waste elimination.

Selective Permeability

Mechanism Overview

• Biological membranes selectively allow certain substances to cross while blocking others, which is vital for maintaining homeostasis.

• Passive Transport: Does not require energy (e.g., diffusion).

• Active Transport: Requires energy to move substances against their concentration gradient (e.g., Na+/K+ pump).

Diffusion and Osmosis

Key Concepts

• Diffusion: Movement of molecules from regions of higher concentration to lower concentration.

• Osmosis: The specific case of water movement across membranes, following its concentration gradient.

Active vs Passive Transport

Comparisons

• Passive Transport: Includes simple and facilitated diffusion; doesn't require cellular energy.

• Active Transport: Utilizes energy to move substances against concentration gradients (e.g., antiporters, symporters).

Endocytosis and Exocytosis

Definitions

• Endocytosis: Process of taking materials into the cell by engulfing them in vesicles.

• Exocytosis: Process of expelling materials from the cell via vesicle fusion with the cell membrane.

Types of Endocytosis

1. Phagocytosis: Engulfing large particles.

2. Pinocytosis: Uptake of fluids and smaller solutes.

3. Receptor-mediated Endocytosis: Specific uptake of molecules via receptor proteins.

What kinds of chemical interactions hold some of the membrane proteins embedded in the membrane and others only on the membrane layer?

Answer: Hydrophobic interactions keep some proteins embedded, whereas ionic bonds keep others embedded on the surface.