Cell Biology: Gap Junctions and Membrane Proteins

Gap Junctions

• Molecular channels between cells allow passage of cytoplasmic molecules.
• Connexons are the protein channels that form gap junctions.
• Electrically excitable tissues, such as heart muscle and smooth muscle, utilize gap junctions for rapid communication.

Plasma Membrane Proteins

• Transport: Movement of substances across the membrane.
• Intercellular Joining: Connection between cells, such as gap junctions.
• Enzymatic Activity: Catalyzing specific chemical reactions within the cell.
• Cell-cell Recognition: Identifying and interacting with other cells.
• Receptors for Signal Transduction: Binding to signaling molecules and initiating intracellular responses.
• Attachment to ECM: Connecting the cell to the extracellular matrix.

Plasma Membrane

• Acts as a selectively permeable hydrophobic barrier between the interstitial fluid and the cytoplasm.
• Interstitial fluid is a blood filtrate containing salts, sugars, amino acids, vitamins, hormones, metabolites, and gases such as O2 and CO2.
• To maintain homeostasis and function, cells must extract necessary substances, retain valuable materials, and eliminate waste.

Passive Transport

• Three types: simple diffusion, facilitated diffusion, and osmosis.
• Diffusion: The tendency of molecules or ions to distribute evenly.
• Factors influencing diffusion rate:
  • Concentration gradient slope: Steeper gradient, faster diffusion.
  • Molecule size: Smaller molecules diffuse faster.
  • Temperature: Higher temperature, faster diffusion.
• The hydrophobic nature of the plasma membrane requires molecules to be lipid-soluble or utilize channels/transporters to cross.

Simple Diffusion

• Doesn't require carriers.
• Occurs for lipid-soluble nonpolar substances like O2, CO2, fats, urea, and alcohol.
• O2 follows its concentration gradient into the cell, while CO2 flows out.
• Movement occurs down the concentration gradient.

Facilitated Diffusion

• Requires carriers or channels to assist water-soluble substances across the membrane.
• Features:
  • Specific: Transports certain molecules only.
  • Not ATP-requiring: Doesn't use energy directly.
  • Limited by carrier/channel saturation: Maximum transport rate can be reached.
  • Down concentration gradient: Movement follows the concentration difference.

Carrier-Mediated Facilitated Diffusion

• Transports large lipid-insoluble molecules unable to pass through membrane pores/channels.
• Glucose is a prominent example of a substance transported via carrier-mediated facilitated diffusion.

Channel-Mediated Facilitated Diffusion

• Uses protein channels for transport.
• Selectivity based on pore size and amino acid charges lining the channel.
• Types:
  • Leaky channels: Always open.
  • Gated channels: Regulated opening.
• Movement is still down the concentration gradient.
• Can be inhibited, show saturation, and are usually specific.

Osmosis

• The unassisted diffusion of water from an area of higher water concentration to an area of lower water concentration across a semipermeable membrane.
• Water being polar but small allows for movement across the plasma membrane directly.
• Specialized water channels called aquaporins are present on many cells.
• In plant cells, excess water entry can lead to turgor pressure.
• In animal cells, excess water entry can lead to lysis (cell bursting).

Osmolarity and Tonicity

• Osmolarity: Total concentration of solute particles in a solution.
• Tonicity: The effect a solution has on cell volume.

Vesicular Transport

• Active transport mechanisms involving ATP-requiring vesicle movement.
• Includes exocytosis and endocytosis.

Exocytosis

• Secretes hormones, neurotransmitters, mucus, and ejects waste.
• Substance enclosed in a vesicle moves to the plasma membrane, fuses, ruptures, and releases contents outside the cell.
• Vesicle docking involves v-SNAREs (vesicle-associated membrane proteins) and t-SNAREs (target membrane-associated proteins).

Endocytosis

• Large particles enter the cell.
• A vesicle encloses the substance, pinches off, and moves into the cytoplasm.
• Contents may be digested or move to the other side of the cell for exit.
• Types:
  • Phagocytosis: Engulfing bacteria, cell debris.
  • Pinocytosis: Ingesting fluid and dissolved substances.
  • Receptor-mediated endocytosis: Concentrating hormones, enzymes, or other macromolecules within a cell.

Neurons

• Conduct electrical impulses between body parts.
• Features:
  • Extreme longevity: Can last over 100 years with proper nourishment.
  • Special structures and functions related to reception, propagation, and transmission of nerve impulses.

Neuron Regions and Functions

• Dendrites: Receive signals from other neurons.
• Cell body (soma): Integrates incoming signals and initiates action potentials.
• Axon: Transmits signals to other neurons or target tissues.
• Axon terminal: Releases neurotransmitters at synapses.

Electrical Activity of Neurons

• Resting membrane potential: The difference in electrical charge across the neuron's membrane when it is not transmitting signals.
• Action potential: A brief electrical impulse that travels along the axon.

Action Potential Generation

• Influenced by:
  • Diffusion of ions across the cell membrane.
  • Types of ion channels:
    • Sodium channels
    • Potassium channels
    • Calcium channels

Propagation of Action Potential

• Factors affecting propagation speed:
  • Axon diameter: Larger diameter, faster conduction.
  • Myelination: Presence of myelin sheath, faster conduction.

Synaptic Transmission

• Synapse: Junction between a neuron and another cell (neuron, muscle, or gland).
• Postsynaptic potentials: Changes in the membrane potential of the postsynaptic cell caused by neurotransmitter release.
• Synaptic integration: The process of combining multiple synaptic inputs to determine the overall response of a neuron.

Neurotransmitters

• Chemicals released at synapses that transmit signals between neurons.
• Examples: acetylcholine, dopamine, serotonin, norepinephrine, glutamate, GABA.

Neural Circuits

• Networks of interconnected neurons that process and transmit information.
• Examples: sensory circuits, motor circuits, reflex circuits.