Structure & Function of Plasma Membranes

Chapter 5: Structure & Function of Plasma Membranes Pt. 1: Components & Structure

Learning Objectives

1. Sketch a cell membrane according to the fluid mosaic model.

2. Indicate positions and orientations of phospholipids, cholesterol and integral and peripheral membrane proteins.

3. Explain why membranes are asymmetrical.

4. Describe at least 3 different factors that affect membrane fluidity.

Membranes

• Defined as a collage of different proteins embedded in the fluid matrix of the lipid bilayer.

• Membranes are characterized as flexible and fluid, yet they are structured enough to maintain their shape and integrity.

Plasma Membrane

• The plasma membrane functions as the boundary that separates living cells from their nonliving surroundings.

• Exhibits selective permeability, allowing some substances to cross more easily than others.

• It is much more than just a simple containment bag; it plays several critical roles:

  • Defines the outer border of all cells and organelles.

  • Acts as a “gate keeper,” managing the entry and exit of substances.

  • Receives external signals and initiates subsequent cellular responses.

  • Facilitates adhesion to neighboring cells.

The Fluid Mosaic Model

• The fluid mosaic model posits that membranes are fluid structures with a mosaic of various proteins embedded within.

• Cellular membranes consist of a myriad of lipids and proteins, creating a complex, dynamic interplay.

Breakdown of Membrane Components

Phospholipids

• Phospholipids constitute the main fabric of the plasma membrane.

• They are amphipathic, possessing both hydrophobic (water-repurchable) and hydrophilic (water-attracting) regions.

• They maintain fluidity by allowing movement within the membrane.

Sterols

• Sterols are crucial for maintaining membrane fluidity and include various types:

  • Animals use cholesterol.

  • Plants use phytosterols (a large group).

  • Algae use fucosterol.

  • Fungi and Yeast utilize ergosterol.

  • Bacteria employ hopanoids, which are not true sterols.

  • Squalene serves as a precursor for both sterols and hopanoids.

Factors Affecting Membrane Fluidity

1. Temperature:

   • Affects the movement rate and distance of phospholipids.

   • Cold temperatures lead to closer packing, restricting small molecules.

   • Warm temperatures cause gaps and larger holes in the membrane structure.

2. Fatty Acid Composition:

   • The nature of fatty acid hydrophobic tails can influence membrane characteristics.

   • Saturated fatty acids result in straight, tightly packed tails, leading to reduced fluidity.

   • Unsaturated fatty acids have kinks that create space, promoting greater fluidity.

3. Cholesterol:

   • Modulates spacing between phospholipids.

   • Has a random distribution and change in amount based on temperature changes.

   • Low temperatures increase cholesterol levels to enhance fluidity.

   • High temperatures decrease cholesterol levels, causing a reduction in fluidity.

Breakdown of Membrane Proteins

• Proteins form the second major component of membranes and fulfill various vital functions:

  • Transporters, facilitating the movement of substances.

  • Receptors, receiving signals from external stimuli.

  • Enzymes, catalyzing biochemical reactions.

  • Participation in binding and adhesion processes.

Integral Proteins

• Completely integrated into the bilayer with regions that are both hydrophobic and hydrophilic.

• The arrangement within the bilayer is determined by the number of hydrophilic and hydrophobic regions.

Peripheral Proteins

• Located only on the surfaces of the membrane (either exterior or interior).

• Typically act as enzymes or structural attachments, lacking integration into the lipid bilayer.

Breakdown of Membrane Carbohydrates

• Carbohydrates form the third major component of membranes.

• Positioned on the exterior surface of the plasma membrane, they can bind to proteins (producing glycoproteins) or to lipids (forming glycolipids).

• Their primary role is in cell-cell recognition and attachment processes.

Receptor Proteins

• Receptor proteins play a key role in viral invasion, such as the glycoprotein utilized by many viruses for attachment to host cells.

• Example: HIV employs gp120 (glycoprotein 120) to adhere to human immune cell CD4 receptors.

• CD4 acts as a cell adhesion molecule, crucial for the aggregation of immune cells during the immune response.

Asymmetric Membrane

• Plasma membranes are asymmetrical, meaning that their inner surface differs from the outer surface.

• The interior is not identical to the exterior; for example:

  • Proteins on the inner membrane anchor cytoskeletal fibers.

  • Exterior proteins engage with the extracellular matrix.

  • Glycoproteins bind to essential import substances.

Synthesis and Sidedness of Membranes

• Membranes possess distinct inside and outside faces, impacting protein movement synthesized in the endomembrane system (a group of membranes and organelles).

• The synthesis, modification, packaging, and transport of membrane proteins and lipids occur through structures such as:

  • Endoplasmic Reticulum (ER)

  • Golgi apparatus

  • Nuclear Envelope

  • Plasma Membrane

  • Lysosomes

Endomembrane System

Major Components

• Nucleus

• Golgi Apparatus

• Vesicles

• Endoplasmic Reticulum

• Plasma Membrane

• Lysosomes

• Cytosol

• Extracellular Environment

Selective Permeability

• Membranes act as selective barriers, controlling the exchange of materials with surrounding environments.

• The process, regulated by the plasma membrane, allows for varying permeability to different substances.

• Small, nonpolar molecules (e.g., O2 & CO2) pass freely and quickly without requiring proteins.

• Small, polar molecules (e.g., H2O) struggle more due to the hydrophobic tails of the bilayer, yet can cross without protein assistance.

• Large, nonpolar molecules (e.g., carbon rings) can pass through, though the process is slow.

• Large, polar molecules and ions (e.g., simple sugars, H+ ions) have difficulty crossing without help because their size and charges render them incompatible with the nonpolar phospholipid membrane.

Molecular Transport Across the Plasma Membrane

• Plasma membranes must facilitate the entry and exit of materials, ensuring the cytosol's equilibrium with extracellular fluids.

• The asymmetric nature of membranes defines the methods of transport:

Passive Transport (requires no energy input)

1. Diffusion

2. Osmosis

3. Facilitated Diffusion

Active Transport (requires energy input)

1. Primary Active Transport

2. Secondary Active Transport

3. Bulk Transport

Passive Transport

• Involves the movement of molecules from areas of high concentration to low concentration, aligned with diffusion principles.

• Diffusion is defined as the tendency for molecules of any substance to evenly spread within an available area, moving down their concentration gradient.

Factors Influencing Rate of Diffusion

Simple Diffusion

• The mechanism requires no energy or protein assistance for transmembrane movement.

Facilitated Diffusion

• Functions to move substances down their concentration gradients.

• Utilizes transmembrane proteins. There are two types:

  • Channel Proteins:

  • Structure includes hydrophilic amino acids that attract ions/polar molecules.

  • Some channels are continuously open, while others are gated and only open upon signal reception.

  • Channel size limits passage; larger molecules may be excluded.

  • Aquaporins:

  • Specific water channels that allow rapid transport of water molecules across the hydrophobic plasma membrane.

Carrier Proteins

• Function through binding specific substances and changing shape to transport the substance across the membrane.

• Many carrier proteins can function in either direction as concentration gradients fluctuate.

• Example: GLUTS (Glucose Transport proteins).

Osmosis

• Defined as the special type of diffusion marking water movement across a semipermeable membrane, driven by solute concentration.

• Water molecules predominantly travel from regions of low solute concentration to high solute concentration, creating differences in water concentration when a solute cannot permeate the membrane.

Water Potential

• Represents the tendency of water to move from one area to another, influenced by solute concentration and pressure.

Tonicity

• Refers to the influence of a solution on cellular water gain or loss through osmosis.

• Key conditions include:

  • Isotonic: Equal solute concentration with no net water movement.

  • Hypotonic: Lower solute concentration results in cell water loss.

  • Hypertonic: Higher solute concentration leads to water gain by the cell.

Conditions Comparison

• Isotonic:

  • No net movement; concentrations are equal, facilitating equilibrium.

• Hypotonic:

  • Cells gain water, potentially leading to positive pressure within the cell.

• Hypertonic:

  • Cells lose water, causing them to become shriveled due to higher external solute concentrations.

Osmoregulation by Other Organisms

• Freshwater Protists like Paramecia and Amoebas use contractile vacuoles to expel excess water and prevent bursting.

• Marine invertebrates maintain internal salt concentrations that align with their environment.

• Fish manage osmotic balance by excreting diluted urine to eliminate excess water or salts.

• Osmoreculators in the brain monitor blood solute concentrations and release hormones influencing kidney functions.

Active Transport

Core Concepts

1. Compare and contrast passive and active transport.

2. Identify conditions necessary for active transport.

3. Discuss the sodium-potassium pump and its functional sketch.

4. Define the electrochemical gradient and its significance.

5. Explain co-transport mechanisms for transporting molecules against their concentration gradients.

6. Contrast the three types of endocytosis, and differentiate it from exocytosis.

7. Identify a molecule imported via receptor-mediated endocytosis.

Conditions for Active Transport

• Requires movement against a concentration gradient (from low to high) or moving ions against their electrochemical gradient.

• Energy is necessary, often sourced from ATP (Primary and Bulk transport mechanisms).

• ATP facilitates transport through the phosphorylation of transport proteins, changing their shape, enabling solute translocation.

• The electrochemical gradient, resulting from primary active transport, can also function as an energy source during secondary active transport.

Carrier Proteins in Active Transport

• Active transport occurs through transmembrane, integral proteins known as pumps.

• Three types exist:

  • Uniporter: Transports a single molecule/ion.

  • Symporter: Transports two different molecules/ions in the same direction.

  • Antiporter: Moves two different molecules/ions in opposite directions.

Primary Active Transport: Sodium-Potassium Pump

• Moves 3 Na+ ions out and 2 K+ ions in per ATP molecule.

• Key roles include maintaining resting potential, influencing cellular volume, and facilitating signaling in pathways (e.g., calcium signaling).

• Notably, the sodium-potassium pump can consume substantial cellular energy, accounting for significantATP expenditure in neuronal tissue.

Sodium-Potassium Pump Mechanism

1. Binding Phase: Cytoplasmic Na+ binds to the Na+/K+ pump.

2. Phosphorylation: ATP is used to phosphorylate the pump.

3. Conformational Change: The pump changes shape, facilitating Na+ release.

4. K+ Binding: Extracellular K+ binds to the modified pump.

5. Dephosphorylation: Leads to the return of pump to its original conformation and K+ release.

Electrochemical Gradient

• Represents the gradient of ions across membrane surfaces, resulting from concentration and electrical gradients.

• Essential for cellular functionality; for instance, the cytoplasm generally possesses more negative ions relative to the extracellular fluid.

• Electrogenic pumps are crucial transport proteins that contribute to membrane voltage.

Secondary Active Transport

• A strategy for moving ions or molecules against their concentration gradient, leveraging the energy stored in electrochemical gradients.

• Commonly observed in the transport of amino acids and glucose into cells through two methods:

  • Cotransport: Symporter utilization.

  • Counter-transport: Antiporter utilization.

Bulk Transport

Overview

• Bulk transport is a method used by cells to import or export molecules that are too large for standard transport proteins (e.g., large proteins/molecules).

  • Endocytosis: Importing substances by forming new vesicles through membrane mechanisms.

  • Types include:

    1. Phagocytosis: Cellular eating.

    2. Pinocytosis: Cellular drinking.

    3. Receptor-mediated endocytosis.

  • Exocytosis: Exporting substances through vesicle-membrane fusion, releasing their content to the extracellular space.