Cell Biology: Membrane Structure and Function

The Membrane: Structure and Function

• Cell boundary - defines a cell and separates interior from exterior
• Selective barrier to molecules - impermeable to hydrophilic molecules
• Boundary of intracellular compartments
• Membranes are dynamic and fluid, allowing for cell function and communication.
• Phospholipids make up the majority of the lipid component of the membrane.
• Phospholipids create a bilayer - impermeable barrier to most molecules in the environment.
• The fluid mosaic model describes the membrane as a 2D fluid.

Composition

• Lipids account for approximately 50% of the cell membrane
• Proteins account for approximately 50% of the cell membrane
• Carbohydrates are integral to the membrane, accounting for approximately 2-10% of the membrane structure

Lipids in Membrane Structure

• Lipids create a bilayer - impermeable barrier
• The bilayer is fluid and allows for diffusion of molecules
• The transition temperature (Tm) of a bilayer determines it's transition from rigid to fluid
• Bilayer has a distinct inner and outer leaflet, each leaflet is composed of a different percentage of lipids.

Proteins in the membrane

• Proteins are embedded in the membrane and are associated with specific functions
• Variable protein content between different cell types
• Membrane proteins are frequently linked to carbohydrates
• Common Functions of Membrane Proteins:
  • Transporters
  • Receptors
  • Ion channels
  • Enzymes
  • Anchors
• Transmembrane proteins - can be single-pass or multi-pass
  • ⍺ helix: single-pass or multi–pass
    • Flexible
  • β barrel: porins
    • Stiff
• Hydropathy plots: Show the distribution of hydrophobic amino acids along a peptide sequence
• Classification of membrane proteins:
  • Transmembrane Proteins (Integral membrane proteins):
    • α-helix (20-30 hydrophobic aa) or β-sheets
    • Single-pass
    • Multi-pass
  • Lipid-Anchored Proteins:
    • Covalent bond to lipid
    • Cytosolic
    • Extracellular
  • Membrane-Associated Proteins (Peripheral membrane proteins):
    • Cytosolic
    • Extracellular

Carbohydrates in the Membrane:

• Only found in the extracellular face of membranes
• Integral components to membrane - synthesized by the cell
• Oligosaccharides (15 sugar monomers):
  • Covalently attached to protein
  • Found on the core of the protein membrane
• Main component of the extracellular matrix (ECM)

Glycocalyx

• The Glycocalyx is a carbohydrate coat that surrounds the cell.
• Functions of the Glycocalyx:
  • Cell-cell and cell-ECM adhesion
  • Protection - mechanical, chemical, pathogen damage
  • Microenvironments
  • Signaling
  • Recognition:
    • ABO blood group antigens
    • Major histocompatibility complex (MHC)

Membrane Fluidity

• Membranes are dynamic and fluid.
• Movements that lipids and proteins make within the membrane:
  • Lateral Diffusion (proteins and lipids):
    • Lipids: 1-10 μm/second
    • Proteins: 0.1-1 μm/second
  • Rotational/spin (protein and lipids)
  • Flexion (lipids only)
  • Flip-flop (lipids only)
    • Spontaneously - rare
    • Enzymatic activity - flippases

Restrictions to Lipid Mobility

• Lipids associated with proteins
• Lipid rich and protein rich domains

Restrictions to Protein Mobility

• Tight junctions: lateral diffusion limited to membrane domains
  • Apical domain
  • Basolateral domain
• Protein aggregation: Proteins can cluster together
• Interation with external complexes: (focal adhesions - ECM)
• Interation with internal complexes: (cytoskeleton - cortex)
  • Cytoskeleton fibers
  • Integrins

Membrane Faces

• Membranes form closed compartments
• Internal face is known as the cytosol
• External face is known as the exterior

Selective Permeable Membrane

• Lipids can diffuse through a phospholipid membrane
• Larger molecules and ions need to be transported by specific proteins
• The difference in concentration gradient between the outside of the cell and inside, as well as the charge of the molecule, determines the rate and the direction of transport.

Passive Transport

• Simple diffusion: Molecules pass through the membrane without the help of any protein, down their concentration gradient.
• Facilitated diffusion: Molecules pass through the membrane with the help of a protein.
  • Channels: These proteins form hydrophilic pores, allowing molecules to pass through.
  • Transporters/Carriers: These proteins bind to specific molecules, leading to a conformational change that allows transport of the molecule.
  • Saturation: The rate of transport reaches a maximum when all protein binding sites are occupied.

Active Transport

• Primary Active Transport: Molecules are moved against their concentration gradient, requiring energy, obtained directly from ATP hydrolysis.
  • P-type pump:
    • Phosphate group is added to the protein to drive transport
    • Examples: Na+/K+ pump, Ca2+ pump
  • **V-type: **
    • Not phosphorylated
    • Primarily pumps H+
    • Examples: Found in vacuoles, endosomes, lysosomes
  • F-type:
    • Function as ATP synthase or a proton pump depending on the concentration gradient
    • Examples: Found in bacteria, mitochondria, thylakoid membrane
  • ABC:
    • Transmembrane proteins with ATP binding domains
    • Examples: Sugar, amino acid and peptide transporters
• Secondary Active Transport:
  • Molecules are moved against their concentration gradients, but the energy required is obtained from an electrochemical gradient, typically created by primary active transport.
  • Symport: Two molecules move in the same direction across the membrane.
    • Example: Glucose and Na+ are transported together
  • Antiport: Two molecules move in opposite directions across the membrane.
    • Examples: Na+/Ca2+ and Na+/H+ antiporters
  • Cotransport: The movement of one molecule is coupled to movement of another.
    • This is a general term used to describe symport or antiport.

Action Potentials and Synapses

• Membrane Potential:
  • The difference in electrical charge across the membrane, created by the distribution of ions inside and outside the cell.
  • K+ Leak channels are important for maintaining the resting membrane potential, typically around -60mV.
• Action Potential:
  • A rapid change in the membrane potential, triggered by a depolarization of the membrane.
  • Voltage-gated channels, primarily for Na+ and K+, play a crucial role in controlling the action potential.
  • Depolarization: The membrane potential becomes less negative, moving closer to 0 mV.
    • Caused by the influx of positive ions, often Na+
  • Repolarization: The membrane potential returns to its negative resting value.
    • Caused by the efflux of K+
• Synapse:
  • The junction between two neurons, or between a neuron and another cell.
    • Electrical signals are converted into chemical signals at the synapse.
    • Neurotransmitters are released from vesicles in the presynaptic neuron. These bind to specific receptors on the postsynaptic neuron.
• Postsynaptic Cell:
  • Neurotransmitter binding to receptors triggers a conformational change.
  • This change can open ligand-gated ion channels on the post-synaptic cell, ultimately converting the chemical signal back into an electrical signal.
• Neuromuscular Junction:
  • The synapse between a motor neuron and a muscle fiber.
  • Acetylcholine released from the motor neuron binds to receptors on the muscle fiber, triggering an action potential.
  • This leads to a rise in intracellular Ca2+ levels, ultimately causing muscle contraction.

Explore the fascinating structure and function of cell membranes in this quiz. Learn about the components that make up the membrane, including lipids, proteins, and carbohydrates, and how they contribute to its selectivity and fluidity. Test your understanding of the fluid mosaic model and the dynamics of membrane behavior.