Chapter 5

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

  • A collage of different proteins embedded in the fluid matrix of the lipid bilayer.

  • Features:

    • Flexible & Fluid: Cell membranes are dynamic structures that allow for movement and flexibility.

    • Structural Integrity: While flexible, membranes maintain a defined structure.

Plasma Membrane

  • Definition: The boundary that separates the living cell from its nonliving surroundings.

  • Selective Permeability: Exhibits the ability to allow some substances to cross more easily than others.

  • Functions:

    1. Defines the outer border of all cells and organelles.

    2. Acts as a “Gatekeeper” managing what enters and exits the cell.

    3. Receives external signals and initiates cellular responses.

    4. Adheres to neighboring cells.

The Fluid Mosaic Model

  • Description: A model that proposes that membranes are fluid structures with a mosaic of various proteins embedded within the lipid bilayer.

  • Components: The cellular membranes consist of phospholipids, cholesterol, proteins, and carbohydrates arranged in a mosaic fashion.

Phospholipids

  • Main Fabric of the Plasma Membrane: Integral components that form the basic structure.

  • Amphipathic Nature: Each phospholipid has both hydrophobic (water-repellent) and hydrophilic (water-attracting) regions.

  • Fluidity: Phospholipids can move within the membrane, contributing to its flexibility.

Components - Sterols

  • Role: Crucial for the fluidity of the plasma membrane, with many different types existing across organisms.

  • Types of Sterols:

    • Cholesterol: Predominantly found in animal membranes.

    • Phytosterols: Found in plants, representing a large group.

    • Fucosterol: Found in algae.

    • Ergosterol: Found in fungi and yeast.

    • Hopanoids: Found in bacteria (though not true sterols).

    • Squalene: A precursor for sterols and hopanoids.

    • Fossil Records: Provides insight into the evolution of sterols and hopanoids.

Fluidity of the Membrane - Cholesterol

Factors affecting membrane fluidity include:

  1. Temperature:

    • Cold temperatures lead to close packing of phospholipids, restricting small molecule movement.

    • Warm temperatures create gaps, allowing larger molecules to move more easily.

  2. Fatty Acid Composition:

    • Saturated Fatty Acids: Have straight, tightly packed tails resulting in less fluidity.

    • Unsaturated Fatty Acids: Have kinks in their tails creating space, which increases fluidity.

  3. Cholesterol's Role:

    • Creates space between phospholipids with a random distribution.

    • At low temperatures, increases fluidity.

    • At high temperatures, decreases fluidity.

Break it Down - Proteins

  • Description: The second major component of membranes.

  • Functions:

    • Transporters

    • Receptors

    • Enzymes

    • In binding and adhesion

  • Integral Proteins:

    • Span the bilayer fully, having both hydrophobic and hydrophilic regions.

    • Arrangement within the bilayer is determined by the number and location of these regions.

  • Peripheral Proteins:

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

    • Typically function as enzymes or structural attachments.

Break it Down - Carbohydrates

  • Role: The third major component located on the extracellular surface of the plasma membrane.

  • Structure:

    • Bound to proteins (forming glycoproteins) or to lipids (forming glycolipids).

  • Function: These molecules are vital for cell-cell recognition and attachment processes.

Receptor Proteins

  • Definition: Proteins that viruses use to attach to host cells via glycoproteins.

  • Example: HIV's glycoprotein 120 (gp120) adheres to the human immune cell's CD4 receptor.

  • Function of CD4: Serves as a cell adhesion molecule to keep other immune cells close during an immune response.

Asymmetric Membrane

  • Key Feature: Plasma membranes exhibit asymmetry.

  • Inner and Outer Differences: The inner surface differs from the outer surface, demonstrating a non-identical nature.

  • Examples of Asymmetric Features:

    • Interior Proteins: Anchor cytoskeletal fibers to the membrane.

    • Exterior Proteins: Bind to the extracellular matrix.

    • Glycoproteins: Bind to necessary substances for import into the cell.

Synthesis and Sidedness of Membranes

  • Membranes have distinct inside and outside faces that impact the movement of proteins synthesized in the endomembrane system.

  • Endomembrane System Components: A group of membranes and organelles involved in synthesis, modification, packaging, and transport.

    • Key Structures:

    • Endoplasmic Reticulum (ER)

    • Golgi Apparatus

    • Nuclear Envelope

    • Plasma Membrane

    • Lysosomes

Selective Permeability

  • Definition: Membranes serve as selective barriers, controlling the exchange of materials with surroundings.

  • Transport of Molecules:

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

    • Small, polar molecules (e.g., H2O) have difficulty but can cross.

    • Large, nonpolar molecules (e.g., carbon rings) can pass, but slowly.

    • Large, polar molecules & ions (e.g., simple sugars, H+ ions) find it too challenging to penetrate the nonpolar region of the phospholipids without assistance.

Molecular Transport Across the Plasma Membrane

  • Importance of membrane permeability allows differences in cytosol and extracellular fluids.

  • Asymmetrical Membrane Transport Types:

    • Passive Transport (no energy input):

    1. Diffusion

    2. Osmosis

    3. Facilitated Diffusion

    • Active Transport (energy input):

    1. Primary Active Transport

    2. Secondary Active Transport

    3. Bulk Transport

Pt. 2 Passive Transport

  1. Define diffusion, osmosis, amphipathic, and electrogenic.

  2. Describe the characteristics of molecules that can easily pass through a phospholipid bilayer.

  3. Explain the effects on animal cells in hypotonic, isotonic, and hypertonic solutions.

  4. Contrast simple diffusion with facilitated diffusion.

Passive Transport

  • Challenge of Molecule Movement: Molecules move from areas of high concentration to areas of low concentration via diffusion.

  • Diffusion Defined: The tendency for molecules of any substance to evenly distribute in available space down their concentration gradient.

Factors Affecting Rate of Diffusion

Factor

Condition

Effect on Diffusion

Concentration Gradient

Greater difference increases rate of diffusion.

Faster diffusion

Molecular Mass

Smaller molecules diffuse faster.

Faster diffusion

Temperature

Higher temperature speeds up diffusion.

Faster diffusion

Solubility

Nonpolar molecules diffuse faster.

Faster diffusion

Surface Area

More surface area increases rate of diffusion.

Faster diffusion

Distance to Travel

Shorter distance increases diffusion rate.

Faster diffusion

Solvent Density

Lower density facilitates faster diffusion.

Faster diffusion

Pressure

Greater pressure enhances diffusion rates.

Faster diffusion

Simple Diffusion

  • Description: Movement across the membrane requiring no energy or protein assistance.

  • Process: Molecules transition from high to low concentration across the lipid bilayer.

Passive Transport – Facilitated Diffusion

  • Definition: Movement of substances down their concentration gradients requiring transmembrane proteins.

  • Types of Transport Proteins:

    • Channel Proteins: Form channels for specific ions and polar molecules to pass.

    • Carrier Proteins: Bind specific molecules, change shape, and carry them across the membrane.

    • Ions and large polar molecules typically use facilitated diffusion for transport.

Channel Proteins

  • Structure: Composed of hydrophilic amino acids that attract ions and polar molecules.

  • Types of Channel Proteins:

    • Some are always open (continuous flow), while others are gated (open when signals are received).

    • Channel structure can filter based on size and charge:

    • Negative Channels: Attract positive ions and repel negative ions.

    • Positive Channels: Attract negative ions and repel positive ions.

Aquaporins

  • Function: Specialized water channels that facilitate bulk transport of water across the hydrophobic plasma membrane, allowing rapid water movement.

Muscle Cells - Gated Ion Channels

  • Mechanism of Action: Neurotransmitters bind, leading to the opening of gated ion channels, allowing ions to flow across the membrane.

  • Example: GABA-gated Cl- channel in which various neurotransmitters can influence ion passage.

Carrier Proteins

  • Function: Specific to a single substance and perform the following actions:

    1. Bind to the specific substance.

    2. Change shape to carry it across the membrane.

    3. Allow movement based on concentration gradients.

  • Important Example: Glucose Transport Proteins (GLUTs) facilitate the movement of glucose across the membrane.

Passive Transport - Osmosis

  • Definition: A special type of diffusion focused on the movement of water across a semi-permeable membrane.

  • Mechanism of Action: Water moves from low solute concentration to high solute concentration until equilibrium is reached.

  • Notable Feature: Water transport is influenced by the solute concentration of solutions that cannot pass through the membrane.

Water Potential

  • Definition: The tendency of water to move from one location to another, influenced by solute concentration, pressure, and gravity.

  • Positive Pressure: Occurs due to the inflow of water into cells, counteracting diffusion.

Tonicity

  • Definition: The ability of a solution to influence the cell's water balance and volume through osmosis.

  • Conditions:

    1. Isotonic: Equal solute concentration in and out of the cell, resulting in no net water movement.

    2. Hypotonic: Lower solute concentration outside the cell; cells gain water and can swell or burst.

    3. Hypertonic: Higher solute concentration outside the cell; cells lose water and can shrink.

  • Organisms without rigid cell walls face challenges in hypertonic and hypotonic conditions; they generally prefer isotonic environments and possess adaptations known as osmoregulation to control water flow.

Osmoregulation by Other Organisms

  • Freshwater Protists (e.g., Paramecia and Amoebas): Utilize contractile vacuoles to expel excess water, preventing cell bursting.

  • Marine Invertebrates: Maintain internal salt concentrations that match their hypertonic environments.

  • Fish: Excrete diluted urine to manage excess water and salts.

  • Humans: Osmoreceptors in brain cells monitor blood solute concentrations, influencing kidney function through hormone release.

Pt. 3 Active Transport

  1. Compare/contrast Passive and Active Transport.

  2. Determine the conditions essential for active transport.

  3. Summarize and sketch the sodium-potassium pump mechanisms as a form of active transport.

  4. Describe what an electrochemical gradient signifies and its importance in cellular activities.

  5. Explain how co-transport functions to move molecules against their concentration gradient.

  6. Compare and contrast the three types of endocytosis.

  7. Distinguish between endocytosis and exocytosis.

  8. Recognize molecules imported via receptor-mediated endocytosis.

Active Transport

  • Definition: Movement of substances against their concentration gradient (low to high) necessitating energy supply, often in the form of ATP.

  • Mechanisms:

    • Direct Energy (Primary): Uses ATP to fuel movement against the gradient.

    • Electrochemical Gradient (Secondary): Maintained by primary active transport, allowing some molecules to move against their concentration gradient using the gradient’s stored energy.

  • ATP's Role: Transfers a phosphate group to the transport protein, inducing a conformational change that facilitates solute translocation across the membrane.

Carrier Proteins in Active Transport

  • Definition: Active transport occurs through integral carrier proteins (pumps) that function in:

    1. Uniporter: Moves one type of molecule or ion across the membrane.

    2. Symporter: Moves two different molecules or ions in the same direction.

    3. Antiporter: Moves two different molecules or ions in opposing directions.

Primary Active Transport

  • Function: Moves ions or molecules against their concentration gradients utilizing energy from ATP hydrolysis.

  • Example: The Sodium-Potassium Pump, which transports 3 Na+ ions out and 2 K+ ions into the cell per ATP molecule.

  • Importance:

    • Helps maintain resting membrane potential (resting voltage).

    • Affects cellular transport and regulates cellular volume.

    • Signaling roles in cellular pathways (e.g., calcium signaling).

    • Notably, in neurons, can account for up to 75% of the cellular energy expenditure.

Sodium-Potassium Pump Steps

  1. Cytoplasmic Na+ binds to the Na+/K+ pump.

  2. The pump is phosphorylated by ATP.

  3. The pump changes conformation, causing Na+ to be released outside the cell.

  4. Extracellular K+ binds to the pump.

  5. The pump returns to its original conformation, releasing K+ into the cytoplasm.

Electrochemical Gradient

  • Definition: A gradient of ions across a membrane resulting from concentration and electrical differences.

  • Significance:

    • Essential for appropriate cell functioning.

    • The cytoplasm generally contains more negatively charged molecules compared with the extracellular fluid.

    • Electrogenic Pump: A transport mechanism generating voltage across the membrane, crucial for processes like nerve impulses.

    • Membrane Potential: The voltage differential across a membrane impacting processes like muscle contraction and neuron firing.

Secondary Active Transport

  • Function: Moves substances against their concentration gradient using the energy from the electrochemical gradient.

  • Types of Secondary Active Transport:

    • Cotransport (Symporter): Transports two different substances in the same direction.

    • Counter-transport (Antiporter): Transports two different substances in opposite directions.

Bulk Transport

  • Definition: Mechanism by which cells import or export large molecules that cannot pass through transport proteins.

  • Types of Bulk Active Transport:

    • Endocytosis: Vesicle formation from the plasma membrane to bring molecules into the cell.

    • Types of Endocytosis:

      1. Phagocytosis (cellular eating)

      2. Pinocytosis (cellular drinking)

      3. Receptor-Mediated Endocytosis

    • Exocytosis: Transport vesicles that migrate to the plasma membrane to release contents into the extracellular environment.

Bulk Transport: Exiting the Cell

  • Exocytosis Process:

    • Vesicles containing exported substances fuse with the plasma membrane, releasing their contents externally.

Bulk Transport: Entering the Cell

  • Endocytosis Process:

    • Vesicles form from the plasma membrane to internalize molecules necessary for various functions.

    • Key types discussed include phagocytosis, pinocytosis, and receptor-mediated endocytosis.