Chap 7 slides info

Chapter 7 - Membrane Structure and Function

Introduction

Course: BSC1010C - General Biology I

Instructor: Dr. Daniel Sanches

Focus: Understanding membrane structure and function in both prokaryotic and eukaryotic cells.

The Plasma Membrane

Definition:

The plasma membrane is the living boundary that encapsulates the cell, providing a defined environment while separating the internal cellular processes from the external environment.

Selective Permeability:

The plasma membrane exhibits selective permeability, meaning it allows some substances to cross more readily than others, while restricting the passage of certain materials. This selectivity is crucial for maintaining homeostasis within the cell. Transport proteins play a central role in regulating the movement of ions, nutrients, and waste products across the membrane.

Structure of the Cell Membrane

Fluid Mosaic Model:

The plasma membrane is best described by the fluid mosaic model, which depicts cellular membranes as dynamic structures composed of a mosaic of various lipids, proteins, and carbohydrates.

• Phospholipids are the most abundant lipids in the plasma membrane, forming a bilayer that provides structure.

• Amphipathic molecules: Phospholipids possess both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions, allowing for the formation of the bilayer where hydrophobic tails face inward, shielded from water, and hydrophilic heads face outward, interacting with the aqueous environment.

Membrane Fluidity:

Key Characteristics:

• Membranes are held together primarily by weak hydrophobic interactions, allowing for flexibility and fluidity.

• Movement of Lipids and Proteins: Lipids and some proteins exhibit lateral movement within the membrane, while flip-flopping, or the transition of lipids from one layer to another, occurs rarely.

Temperature Effects:

• Cold temperatures can shift membranes to a gel-like solid state, adversely affecting membrane function.

• Lipid Composition: Membrane fluidity is influenced by lipid composition; unsaturated fatty acids enhance membrane fluidity due to kinks that prevent tight packing, whereas saturated fatty acids lead to a more rigid structure.

• Cholesterol's Role:

  • In warm temperatures, cholesterol molecules restrict phospholipid movement, reducing fluidity and stabilizing the membrane.

  • In cooler temperatures, cholesterol prevents tight packing of the phospholipids, helping to maintain membrane fluidity and preventing solidification.

Membrane Proteins

Types of Membrane Proteins:

• Peripheral Proteins: Loosely attached to the surface of the membrane; play roles in signaling and structural support.

• Integral Proteins: Embedded within the membrane and often span the entire lipid bilayer; involved in various functions, including transport and acting as receptors.

• Transmembrane Proteins: A specific subset of integral proteins that span the membrane and facilitate communication and transport between the inside and outside of the cell; characterized by regions of nonpolar amino acids that interact with the hydrophobic core of the membrane.

Functions of Membrane Proteins:

The diverse range of functions of membrane proteins include:

• Channels: Provide pathways for specific ions and molecules to pass through the membrane.

• Carriers: Bind to molecules and facilitate their transport across the membrane through conformational changes.

• Receptors: Recognize and bind to signaling molecules, initiating cellular responses.

• Enzymes: Catalyze biochemical reactions at the membrane surface.

• Cell-Cell Recognition: Glycoproteins and glycolipids are involved in identification processes, allowing cells to recognize and communicate with one another, varying between different individuals and species.

Synthesis and Membrane Orientation:

The asymmetrical structure of membranes, which is established by the endoplasmic reticulum (ER) and Golgi apparatus during protein synthesis, results in distinct inner (cytoplasmic) and outer (extracellular) faces of the membrane, influencing functionality.

Selective Permeability of the Plasma Membrane

Key Functions:

The plasma membrane regulates the transportation of molecules into and out of the cell, maintaining cellular homeostasis through selective permeability.

• Hydrophobic Molecules: Nonpolar molecules, such as oxygen and carbon dioxide, can readily pass through the lipid bilayer due to their solubility in the bilayer.

• Hydrophilic Molecules: Polar molecules and ions face challenges in crossing the hydrophobic core of the membrane, requiring assistance from transport proteins.

Transport Proteins

Channel Proteins:

These proteins create hydrophilic pathways that facilitate the movement of specific molecules. For example, aquaporins allow the passage of water molecules across the membrane.

Carrier Proteins:

Carrier proteins bind to specific molecules or ions and undergo conformational changes to transport them across the membrane; this process can be passive or active.

Passive Transport

Diffusion:

The passive movement of molecules occurs down their concentration gradient without the expenditure of energy; dynamic equilibrium is reached when the rates of movement in both directions equalize.

Osmosis and Water Balance:

Osmosis refers to the specific diffusion of water across a selectively permeable membrane, where water moves from areas of low solute concentration to areas of high solute concentration.

• Tonicity: The potential impact of a solution on cell volume and shape includes:

  • Isotonic: No net water movement; cell volume remains stable.

  • Hypertonic: Solute concentration is higher outside the cell, leading to water loss and cell shrinkage (crenation).

  • Hypotonic: Higher solute concentration is found inside the cell, causing water influx and potential cell lysis (bursting).

Water Balance of Cells:

• Cells without Cell Walls: Such cells must utilize osmoregulation strategies to manage their water balance effectively, preventing excessive gain or loss of water, which could be detrimental.

• Cells with Cell Walls: These cells possess structural integrity due to cell walls, which help maintain turgor pressure and prevent wilting or collapse when exposed to hypertonic solutions.

Facilitated Diffusion

This mechanism involves transport proteins that assist in the passive movement of molecules across membranes, primarily in cases where molecules are polar or charged. Ion channels can open or close in response to various stimuli, including changes in electrical signals in nerve cells. Carrier proteins uniquely undergo conformational changes to specifically transport solutes across the membrane, ensuring specificity in what is moved.

Active Transport

Active transport mechanisms require the direct utilization of energy (ATP) to move solutes against their concentration gradients, which is crucial for maintaining essential ion concentrations within cells.

• Sodium-Potassium Pump: A critical transport system that helps maintain the concentrations of sodium (Na+) and potassium (K+) within the cell, essential for nerve impulse transmission and muscle contraction.

Membrane Potential

Definition:

Membrane potential refers to the voltage difference across the plasma membrane, primarily influenced by the distribution of ions, which is essential for processes such as nerve impulse conduction and muscle contraction.

Cotransport and Bulk Transport

Cotransport:

This form of active transport indirectly drives the movement of various substances through the membrane, utilizing the energy derived from the primary transport process (often the sodium-potassium pump).

Bulk Transport:

Involves the transportation of larger molecules and particles through membrane vesicles, requiring energy for processes such as:

• Exocytosis: The process in which vesicles fuse with the plasma membrane to release their contents outside the cell, important for secretion and signal transduction.

• Endocytosis: The process wherein the cell forms vesicles to import substances from the external environment; includes mechanisms such as phagocytosis (cell eating) and receptor-mediated endocytosis, which involves the selective uptake of specific molecules based on receptor-ligand interactions.