Structure and Function of the Cell Membrane

Lecture 1: Structure and Function of the Cell Membrane

Learning Objectives

• Understand the fluid mosaic model of membrane structure.

• Explain how the molecular structure results in selective permeability.

• Identify how composition of fluid compartments act to store energy.

• Define the laws of diffusion governing movement across membranes.

The Fluid Mosaic Model

• Physical Description: The membrane is a thin, flexible, and sturdy barrier surrounding the cytoplasm. Its thickness is approximately $8\,nm$ ($8 \times 10^{-9}\,\text{m}$).

• Composition: It is described as a "sea of lipids in which proteins float like icebergs." The composition is approximately $50\%$ lipid and $50\%$ protein.

• Stability: The structure is held together by hydrogen bonds.

• Functional Roles: The lipid component acts as a barrier to polar substances, while proteins act as "gatekeepers" to regulate traffic.

Lipid Bilayer and Membrane Fluidity

Lipid Composition The lipid bilayer consists of two back-to-back layers made of three types of lipid molecules:

1. Phospholipids ($75\%$ of lipids): Organized in a double row (bilayer).

2. Cholesterol: Scattered among the phospholipids.

3. Glycolipids: Scattered among the phospholipids.

Characteristics of Phospholipids Phospholipids are amphipathic, meaning they have both polar and nonpolar regions:

• Polar heads: Charged, hydrophilic (water-loving) surfaces facing the intracellular and extracellular fluid.

• Nonpolar tails: Hydrophobic (water-fearing) core consisting of fatty acid tails.

Membrane Fluidity Factors Membranes are fluid; lipids move within the plane of their leaflet. Movement between leaflets ("flip-flopping") is rare, leading to asymmetric lipid composition. Fluidity is determined by:

• Lipid tail length: Longer tails decrease fluidity.

• Double bonds: A higher number of double bonds increases fluidity.

• Cholesterol content: Higher amounts of cholesterol decrease fluidity.

Membrane Proteins: Types and Functions

Classification of Proteins

• Integral Proteins: Extend into or completely across the cell membrane (transmembrane proteins). These are amphipathic, with hydrophobic regions (coiled into helices of nonpolar amino acids) spanning the core and hydrophilic ends interacting with aqueous solutions.

• Peripheral Proteins: Attached to the inner or outer surface of the membrane and are easily removed. They may interact with the cytoskeleton.

Functions of Membrane Proteins Proteins serve six primary roles:

1. Receptor Proteins: For signal transduction.

2. Cell Identity Markers: For cell recognition.

3. Linkers: For structural support.

4. Enzymes: For catalyzing reactions.

5. Ion Channels: For movement of ions.

6. Transporter Proteins: For moving substances across the membrane.

Selective Permeability of the Membrane

Permeability Rules for the Lipid Bilayer

• Permeable to: Nonpolar, uncharged molecules (e.g., $O_2$, $N_2$, benzene); lipid-soluble molecules (steroids, fatty acids, some vitamins); small uncharged polar molecules ($H_2O$, urea, glycerol, $CO_2$).

• Impermeable to: Large uncharged polar molecules (glucose, amino acids); and all ions ($Na^+$, $K^+$, $Cl^-$, $Ca^{2+}$, $H^+$).

Mediated Transport Membrane proteins are required to mediate the transport of substances that cannot permeate the hydrophobic core, such as ions and large uncharged molecules.

Principles and Laws of Diffusion

Definition and Mechanism Diffusion is the random mixing of particles in solution due to kinetic energy. Net diffusion occurs from a region of higher concentration to a region of lower concentration until equilibrium is reached and particles are evenly distributed.

Factors Affecting Diffusion Rate

• Concentration Gradient: A greater difference between the two sides increases the rate.

• Temperature: Higher temperatures increase the rate.

• Particle Size: Larger substances diffuse more slowly.

• Surface Area: An increase in surface area increases the rate.

• Diffusion Distance: Increasing the distance slows the rate.

Physical Consequences

• Size Limit: The rate of diffusion limits cell size to approximately $20\,\mu m$.

• Adaptations: To increase diffusion, cells can increase membrane surface area or decrease membrane thickness (thinner membranes result in faster diffusion).

• Efficiency: Diffusion is extremely fast over small distances.

Gradients and Membrane Potential

Types of Gradients

1. Concentration Gradient: Drives non-charged molecules.

2. Electrical Gradient: Influences ions based on membrane potential.

3. Electrochemical Gradient: The combined influence of concentration and electrical gradients on ion movement.

Stored Energy and Capacitance Membranes separate and store charge, acting like capacitors. Cells utilize approximately $30\%$ of their resting energy to maintain these gradients, which represent stored energy.

Ion Concentrations (Cytoplasm vs. Extracellular Fluid)

• Extracellular Fluid: High concentration of $Na^+$ and $Cl^-$, low concentration of $K^+$.

• Cytoplasm: High concentration of $K^+$, low concentration of $Na^+$ and $Cl^-$.

Osmosis and Osmotic Pressure

Mechanism of Osmosis Osmosis is the net movement of water through a selectively permeable membrane from an area of high water concentration (low solute concentration) to an area of lower water concentration (high solute concentration).

• Conditions: Occurs only if the membrane is permeable to water but impermeable to certain solutes.

• Behavior: Water moves to eliminate the osmotic gradient.

Osmotic Pressure Osmotic pressure is defined as the pressure that must be applied to a solution to prevent the inward flow of water across a semi-permeable membrane.