Lecture 2 - Cell Membranes

Cell Membrane Structure and Function

• Components:

  • Fibers of Extracellular Matrix (ECM)

  • Glycoproteins (carbohydrate proteins)

  • Glycolipids

  • Cholesterol

• Sides of Membrane:

  • Extracellular Side: External environment of the cell

  • Cytoplasmic Side: Internal environment of the cell

• Structural Elements:

  • Integral proteins embedded in the membrane

  • Peripheral proteins associated with the cytoskeleton

  • Microfilaments supporting the membrane's structure

Key Concepts of Plasma Membranes

• Fluid Mosaic Model:

  • Membranes consist of a dynamic arrangement of lipids and proteins.

  • Exhibit selective permeability, allowing only certain substances to pass.

• Transport Mechanisms:

  • Passive Transport:

    • Diffusion and osmosis move substances down their concentration gradient without energy

    • Facilitated Diffusion:

      • Transmembrane channels aid in passive transport

  • Active Transport:

    • Requires energy to move substances against their gradients (examples: ion pumps, cotransport)

  • Bulk Transport:

    • Via exocytosis (out of the cell) and endocytosis (into the cell)

Dynamic Nature of Membranes

• Fluidity:

  • Membranes are dynamic with movement like “partygoers” within a room; phospholipids and proteins can laterally move but rarely switch sides.

  • Fatty acid saturation influences membrane permeability due to structural changes.

• Permeability Effects:

  • Small, nonpolar molecules pass readily while larger polar molecules or ions require assistance via channels or carriers.

Membrane Proteins and Their Functions

• Membrane Proteins:

  • Integral proteins serve different functions, contributing to the membrane’s mosaic nature.

  • Proteins can be added or removed, indicating membrane fluidity.

Movement of Substances Across Membranes

• Selective Permeability:

  • Diffusion: Movement down a concentration gradient (passive transport).

  • Transmembrane Proteins: Facilitate transport of ions and molecules.

• Passive vs. Active Transport:

  • Passive transport: moves down gradients; Active transport: against gradients.

Facilitated Diffusion and Gated Channels

• Channel Proteins:

  • Allow rapid diffusion for ions and large polar substances.

  • Highly selective and dynamic, can be gated to control permeability.

• Carrier Proteins:

  • Change shape to transport molecules such as glucose across membranes via facilitated diffusion.

Changes in Membrane Permeability

• Gated Channels and Ion Permeability:

  • Channels can open/close rapidly, affecting membrane potential and ion concentrations.

  • Ligand-gated and voltage-gated channels help regulate movement of ions.

Active Transport Mechanisms

• Energy Use:

  • Active transport (via ATP) moves molecules against concentration gradients, such as in the sodium-potassium pump.

  • Establishes membrane potential through active pumping of Na+/K+.

Neuron Function and Membrane Potential

• Nerve Cells:

  • Display resting membrane potentials typically between -60 to -80 mV.

  • Characteristics of ion distribution: high Na+ outside, high K+ inside.

• Action Potential:

  • Rapid changes in membrane potential due to permeability changes.

  • Phases of action potential: depolarization (Na+ influx) and repolarization (K+ efflux).

Summary of Action Potential Phases

1. Resting State:

• Non-gated channels maintain resting potential.

2. Depolarization:

• Threshold reached opens voltage-gated Na+ channels leading to a rapid influx of Na+.

3. Rising Phase:

• Positive feedback loop opens more Na+ channels.

4. Falling Phase:

• Na+ channels inactivate, K+ channels open for rapid outflow (hyperpolarization).

Electrical Signaling in Nervous System

• Signal Propagation:

  • Action potentials are generated and propagated along nerve cell membranes.

  • Basis of electrical signals essential for organismal functions (e.g., in nerves, bladder epithelium).

Bulk Transport Across Membranes

• Endocytosis:

  • Uptake of substances (food particles, fluids) into the cell.

• Exocytosis:

  • Release of substances (proteins, hormones) from the cell into extracellular space or circulation.

(SIMPLIFIED)

Cell Membrane Structure and Function

Introduction to Cell Membranes

Cell membranes are the outer layers of cells that protect them and control what enters and leaves the cell. They are made of different components that work together to maintain cell health and functionality.

Key Components of Cell Membranes

• Fibers of Extracellular Matrix (ECM): These provide structural support and help cells stick together.

• Glycoproteins: These are proteins with carbohydrates attached, playing roles in cell recognition and signaling.

• Glycolipids: Similar to glycoproteins but linked to lipids; they also help in cell communication.

• Cholesterol: This substance helps keep the membrane stable and flexible.

Structure of the Membrane

• Extracellular Side: This is the outside part of the membrane that faces the external environment.

• Cytoplasmic Side: This is the inner part that interacts with the cell’s interior.

• Integral Proteins: These proteins go through the membrane and can help transport substances.

• Peripheral Proteins: These are attached to the membrane's surface and often connect to the cell's internal support structure.

• Microfilaments: These are small fibers that help support the structure of the cell membrane.

Fluid Mosaic Model

The fluid mosaic model explains how the cell membrane works: it is made up of a mixed arrangement of proteins and lipids (fats) that can move around like partygoers in a room, allowing cells to be flexible while performing different functions.

Transport Mechanisms

Different methods allow substances to move across cell membranes:

1. Passive Transport:

   • Diffusion: Substances move from high concentration (many particles) to low concentration (fewer particles) without using energy.

   • Facilitated Diffusion: Specific proteins help larger or charged substances cross the membrane without using energy.

2. Active Transport:

   • This process requires energy (like a battery) to move substances against their natural flow, from low to high concentration (uphill).

   • Examples: Using pumps to move ions (charged particles) across the membrane.

3. Bulk Transport:

   • Exocytosis: Releasing substances (like hormones) out of the cell.

   • Endocytosis: Taking in substances (like nutrients) into the cell.

Fluidity and Permeability

• Fluidity: Membranes are not stiff. They can shift and change shape, which is important for how they work.

• Saturation: The type of fats (saturated vs. unsaturated) influences how easily substances can pass through the membrane.

• Permeability: Small nonpolar molecules pass through easily, while large or charged particles need help from proteins (like channels or carriers).

Role of Membrane Proteins

• Proteins in the membrane have various roles, such as transporting materials or sending signals into the cell. They can be added or removed, affecting the cell’s activities.

Neuron Function and Membrane Potential

• Nerve Cells: They have special electrical properties, maintaining a resting state with a particular voltage (between -60 to -80 mV).

• Action Potential: This is how nerve cells send signals - it involves rapid changes in voltage along the cell membrane, consisting of phases:

  1. Depolarization: When a nerve signal is triggered, sodium enters the cell, changing the voltage.

  2. Repolarization: Potassium leaves the cell to restore the original voltage.

Summary

• The cell membrane is crucial for protecting the cell and controlling what goes in and out, and it plays a pivotal role in communication, especially in nerve cells that transmit signals throughout the body.

• Understanding the structure and function of cell membranes is essential to grasp how cells operate and communicate in living organisms.